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Project Report on Hydraulic Disc Brakes

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ABSTRACT

The current tendencies in automotive industry need intensive investigation in problems of interaction of active safety systems with brake system equipments. At the same time, the opportunities to decrease the power take-off of single components, disc brake systems. Disc brakes sometimes spelled as "disk" brakes, use a flat, disc-shaped metal rotor that spins with the wheel. When the brakes are applied, a calliper squeezes the brake pads against the disc (just as you would stop a spinning disc by squeezing it between your fingers), slowing the wheel.

The disc brake used in the automobile is divided into two parts: a rotating axis symmetrical disc, and the stationary pads. The hydraulic disc brake is an arrangement of braking mechanism which uses brake fluid, typically containing ethylene glycol, to transfer pressure from the controlling unit, which is usually near the operator of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the vehicle.

The frictional heat, which is generated on the interface of the disc and pads, can cause high temperature during the braking process. Hence the automobiles generally use disc brakes on the front wheels and drum brakes on the rear wheels. The disc brakes have good stopping performance and are usually safer and more efficient than drum brakes. The four wheel disc brakes are more popular, swapping drums on all but the most basic vehicles. Many two wheel automobiles design uses a drum brake for the rear wheel. Brake technology began in the '60s as a serious attempt to provide adequate braking for performance cars has ended in an industry where brakes range from supremely adequate to downright phenomenal.

One of the first steps taken to improve braking came in the early '70s when manufacturers, on a widespread scale, switched from drum to disc brakes. Since the

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CHAPTER-1

INTRODUCTION

A brake is a mechanical device which inhibits motion. Most commonly brakes use friction to convert kinetic energy into heat, though other methods of energy conversion may be employed. For example regenerative braking converts much of the energy to electrical energy, which may be stored for later use. Other methods convert kinetic energy into potential energy in such stored forms as pressurized air or pressurized oil. Eddy current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc, fin, or rail, which is converted into heat. Still other braking methods even transform kinetic energy into different forms, for example by transferring the energy to a rotating flywheel. Brakes are generally applied to rotating axles or wheels, but may also take other forms such as the surface of a moving fluid (flaps deployed into water or air). Some vehicles use a combination of braking mechanisms, such as drag racing cars with both wheel brakes and a parachute, or airplanes with both wheel brakes and drag flaps raised into the air during landing.

Since kinetic energy increases quadratically with velocity ( ), an object moving at 10 m/s has 100 times as much energy as one of the same mass moving at 1 m/s, and consequently the theoretical braking distance, when braking at the traction limit, is 100 times as long. In practice, fast vehicles usually have significant air drag, and energy lost to air drag rises quickly with speed.

Almost all wheeled vehicles have a brake of some sort. Even baggage carts and shopping carts may have them for use on a moving ramp. Most fixed-wing aircraft are fitted with wheel brakes on the undercarriage. Some aircraft also feature air brakes designed to reduce their speed in flight. Notable examples include gliders and some World War II-era aircraft, primarily some fighter aircraft and many dive bombers of the era. These allow the aircraft to maintain a safe speed in a steep descent. The Saab B 17 dive bomber used the deployed undercarriage as an air brake.

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Friction brakes on automobiles store braking heat in the drum brake or disc brake while braking then conduct it to the air gradually. When travelling downhill some vehicles can use their engines to brake.

When the brake pedal of a modern vehicle with hydraulic brakes is pushed, ultimately a piston pushes the brake pad against the brake disc which slows the wheel down. On the brake drum it is similar as the cylinder pushes the brake shoes against the drum which also slows the wheel down.

HISTORY OF DISC BRAKE

Ever since the invention of the wheel, if there has been "go" there has been a need for "whoa." As the level of technology of human transportation has increased, the mechanical devices used to slow down and stop vehicles has also become more complex. In this report I will discuss the history of vehicular braking technology and possible future developments.

Before there was a "horse-less carriage," wagons, and other animal drawn vehicles relied on the animal’s power to both accelerate and decelerate the vehicle. Eventually there was the development of supplemental braking systems consisting of a hand lever to push a wooden friction pad directly against the metal tread of the wheels. In wet conditions these crude brakes would lose any effectiveness.

The early years of automotive development were an interesting time for the designing engineers, "a period of innovation when there was no established practice and

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In this chaotic era is the first record of the disc brake. Dr. F.W. Lanchester patented a design for a disc brake in 1902 in England. It was incorporated into the Lanchester car produced between 1906 through 1914. These early disc brakes were not as effective at stopping as the contemporary drum brakes of that time and were soon forgotten. Another important development occurred in the 1920’s when drum brakes were used at all four wheels instead of a single brake to halt only the back axle and wheels such as on the Ford model T. The disc brake was again utilized during World War II in the landing gear of aircraft. The aircraft disc brake system was adapted for use in automotive applications, first in racing in 1952, then in production automobiles in 1956. United States auto manufacturers did not start to incorporate disc brakes in lower priced non-high-performance cars until the late 1960’s.

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CHAPTER-2

CHARACTERISTICS

Brakes are often described according to several characteristics including:

Peak force

– The peak force is the maximum decelerating effect that can be obtained. The peak force is often greater than the traction limit of the tires, in which case the brake can cause a wheel skid.

Continuous power dissipation

– Brakes typically get hot in use, and fail when the temperature gets too high. The greatest amount of power (energy per unit time) that can be dissipated through the brake without failure is the continuous power dissipation. Continuous power dissipation often depends on e.g., the temperature and speed of ambient cooling air.

Fade

– As a brake heats, it may become less effective, called brake fade. Some designs are inherently prone to fade, while other designs are relatively immune. Further, use considerations, such as cooling, often have a big effect on fade.

Smoothness

– A brake that is grabby, pulses, has chatter, or otherwise exerts varying brake force may lead to skids. For example, railroad wheels have little traction, and friction brakes without an anti-skid mechanism often lead to skids, which increases maintenance costs and leads to a "thump thump" feeling for riders inside.

Power

– Brakes are often described as "powerful" when a small human application force leads to a braking force that is higher than typical for other brakes in the same class. This notion of "powerful" does not relate to continuous power dissipation, and may be confusing in that a brake may be "powerful" and brake strongly with a gentle brake application, yet have lower (worse) peak force than a less "powerful" brake.

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Durability

– Friction brakes have wear surfaces that must be renewed periodically. Wear surfaces include the brake shoes or pads, and also the brake disc or drum. There may be tradeoffs, for example a wear surface that generates high peak force may also wear quickly.

Weight

– Brakes are often "added weight" in that they serve no other function. Further, brakes are often mounted on wheels, and unsprung weight can significantly hurt traction in some circumstances. "Weight" may mean the brake itself, or may include additional support structure.

Noise

– Brakes usually create some minor noise when applied, but often create squeal or grinding noises that are quite loud.

2.1 Types of Braking

Systems:-Brakes may be broadly described as using friction, pumping, or electromagnetic. One brake may use several principles: for example, a pump may pass fluid through an orifice to create friction:

Frictional brakes

are most common and can be divided broadly into "shoe" or "pad" brakes, using an explicit wear surface, and hydrodynamic brakes, such as parachutes, which use friction in a working fluid and do not explicitly wear. Typically the term "friction brake" is used to mean pad/shoe brakes and excludes hydrodynamic brakes, even though hydrodynamic brakes use friction.

Friction (pad/shoe) brakes are often rotating devices with a stationary pad and a rotating wear surface. Common configurations include shoes that contract to rub on the outside of a rotating drum, such as a band brake; a rotating drum with shoes that expand to rub the inside of a drum, commonly called a "drum brake", although other drum configurations are possible; and pads that pinch a rotating disc, commonly called a "disc brake". Other brake configurations are used, but less often. For example, PCC trolley brakes include a flat shoe which is clamped to the rail with an electromagnet; the Murphy brake pinches a rotating drum, and the Ausco Lambert disc brake uses a hollow disc (two parallel discs with a structural bridge) with shoes that sit between the disc surfaces and expand laterally.

Pumping brakes

are often used where a pump is already part of the machinery. For example, an internal-combustion piston motor can have the fuel supply stopped, and then internal pumping losses of the engine create some braking. Some engines use a valve override called a Jake brake to greatly increase pumping losses. Pumping brakes can dump

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energy as heat, or can be regenerative brakes that recharge a pressure reservoir called a hydraulic accumulator.

Electromagnetic

brakes are likewise often used where an electric motor is already part

of the machinery. For example, many hybrid gasoline/electric vehicles use the electric motor as a generator to charge electric batteries and also as a regenerative brake. Some diesel/electric railroad locomotives use the electric motors to generate electricity which is then sent to a resistor bank and dumped as heat. Some vehicles, such as some transit buses, do not already have an electric motor but use a secondary "retarder" brake that is effectively a generator with an internal short-circuit. Related types of such a brake are eddy current brakes, and electro-mechanical brakes (which actually are magnetically driven friction brakes, but nowadays are often just called “electromagnetic brakes” as well).

Brakes are one the key parts of any vehicle, without which it is virtually not possible to

use the vehicle for travel. Clearly, a brake, which serves to slow down the vehicle, should not be too weak. But interestingly, when designing a brake system, it should also be taken care that it’s not too efficient. A too strong a brake would expose us continuously to the ill effects of a sudden brake application in bus or car. If a vehicle is stopped abruptly or strongly, the passenger may hit the front seat or whatever is there. Hence, too efficient a brake system is not required!

The braking system is strongly relation to Newton’s laws of motion. Indeed, the above phenomenon is linked to Newton’s second law of motion, which states “A body continues to be in its state of rest or of motion unless external force acts on the same”.

On the other hand, if a brake system is too weak, the stopping distance would increase and hence may lead to accidents. Thus, a brake system should be perfect enough to stop the vehicle at minimum safe distance, without affecting the comfort of the passenger. In an endeavour to achieve this there have been a lot of developments in the brake system technology, right from Mechanical brakes to Air brakes in automobiles. In this article we would like provide the relevant information regarding the same

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WORK

ING OF A BRAKE

We all know that pushing down on the brake pedal slows a car to a stop. But we do not how does this happen, how does our car transmit the force from our leg to its wheels and how does it multiply the force so that it is enough to stop something as big as a car.

Fig 1: Braking – fundamentals

Friction and how it applies to automobiles

A brake system is designed to slow and halt the motion of vehicle. To do this, various components within the brake system must convert vehicle’s moving energy into heat. This is done by using friction.

Friction is the resistance to movement exerted by two objects on each other. Two forms of friction play a part in controlling a vehicle: Kinetic or moving, and static or stationary. The amount of friction or resistance to movement depends upon the type of material in contact, the smoothness of their rubbing surfaces and the pressure holding them together. Thus, in a nutshell a car brake works by applying a static surface to a moving surface of a

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vehicle, thus causing friction and converting kinetic energy into heat energy. The high-level mechanics are as follows.

As the brakes on a moving automobile are put into motion, rough-textures brake pads or brake shoes are pressed against the rotating parts of vehicle, be it disc or drum. The kinetic energy or momentum of the vehicle is then converted into heat energy by kinetic friction of the rubbing surfaces and the car or truck slows down.

When vehicle comes to stop, it is held in place by static friction. The friction between surfaces of brakes as well as the friction between tires and roads resists any movement. To overcome the static friction that holds the car motionless, brakes are released. The heat energy of combustion of in engine is converted into kinetic energy by transmission and drive train, and the vehicle moves.

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BRAKE BASICS

When you depress your brake pedal, your car transmits the force from your foot to its brakes through a fluid. Since the actual brakes require a much greater force than you could apply with your leg, your car must also multiply the force of your foot. It does this in two ways:

Mechanical advantage (Leverage)

Hydraulic force multiplication

The brakes transmit the force to the tires using friction, and the tires transmit that force to the road using friction also. Before we begin our discussion on the components of the brake system, let's cover these three principles:

Leverage

Hydraulics

Friction

4.1 LEVERAGE

The pedal is designed in such a way that it can multiply the force from your leg several times before any force is even transmitted to the brake fluid.

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Fig 3: Leverage

In the figure above, a force F is being applied to the left end of the lever. The left end of the lever is twice as long (2X) as the right end (X). Therefore, on the right end of the lever a force of 2F is available, but it acts through half of the distance (Y) that the left end moves (2Y). Changing the relative lengths of the left and right ends of the lever changes the multipliers.

4.2 HYDRAULIC SYSTEMS

The basic idea behind any hydraulic system is very simple: Force applied at one point is transmitted to another point using an incompressible fluid, almost always an oil of some sort. Most brake systems also multiply the force in the process.

4.3 FRICTION

Friction is a measure of how hard it is to slide one object over another. Take a look at the figure below. Both of the blocks are made from the same material, but one is heavier. I think we all know which one will be harder for the bulldozer to push.

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Fig 4 : Friction force versus weight

To understand why this is, let's take a close look at one of the blocks and the table:

Fig 5: Close look at one of the blocks

Even though the blocks look smooth to the naked eye, they are actually quite rough at the microscopic level. When you set the block down on the table, the little peaks and valleys get squished together, and some of them may actually weld together. The weight of the heavier block causes it to squish together more, so it is even harder to slide.

Different materials have different microscopic structures; for instance, it is harder to slide rubber against rubber than it is to slide steel against steel.

The type of material determines the coefficient of friction, the ratio of the force required to slide the block to the block's weight. If the coefficient were 1.0 in our example, then it would take 100 pounds of force to slide the 100-pound (45 kg) block, or 400 pounds (180 kg) of force to slide the 400-pound block. If the coefficient were 0.1, then it would

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take 10 pounds of force to slide to the 100-pound block or 40 pounds of force to slide the 400-pound block.

So the amount of force it takes to move a given block is proportional to that block's weight. The more weight, the more force required. This concept applies for devices like brakes and clutches, where a pad is pressed against a spinning disc. The more force that presses on the pad, the greater is the stopping force.

A SIMPLE BRAKE SYSTEM

The distance from the pedal to the pivot is four times the distance from the cylinder to the pivot, so the force at the pedal will be increased by a factor of four before it is transmitted to the cylinder.

The diameter of the brake cylinder is three times the diameter of the pedal cylinder. This further multiplies the force by nine. All together, this system increases the force of your foot by a factor of 36. If you put 10 pounds of force on the pedal, 360 pounds (162 kg) will be generated at the wheel squeezing the brake pads.

There are a couple of problems with this simple system. What if we have a leak? If it is a slow leak, eventually there will not be enough fluid left to fill the brake cylinder, and the brakes will not function. If it is a major leak, then the first time you apply the brakes all of the fluid will squirt out the leak and you will have complete brake failure.

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CHAPTER-5

TYPES OF BRAKES

TYPES OF BRAKES

1. DRUM BRAKES

2. DISC BRAKES (CALLIPER BRAKES)

Fig 6: Types of Brakes

5.1 DRUM BRAKES

The drum brake has two brake shoes and a piston. When you hit the brake pedal, the piston pushes the brake shoes against the drum. This is where it gets a little more complicated. as the brake shoes contact the drum, there is a kind of wedging action, which has the effect of pressing the shoes into the drum with more force. The extra braking force provided by the wedging action allows drum brakes to use a smaller piston than disc brakes. But, because of the wedging action, the shoes must be pulled away from the drum when the brakes are

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released. This is the reason for some of the springs. Other springs help hold the brake shoes in place and return the adjuster arm after it actuates.

5.2 DISC BRAKE

The disc brake has a metal disc instead of a drum. It has a flat shoe, or pad, located on each side of the disc. To slow or stop the car, these two flat shoes are forced tightly against the rotating disc, or rotor. Fluid pressure from the master cylinder forces the pistons to move in. This action pushes the friction pads of the shoes tightly against the disc. The friction between the shoes and the disc slows and stops the disc.

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CHAPTER-6

TYPES OF DISC BRAKES

The Three Types of Disc Brakes

Are:-1. FLOATING CALIPER DISC BRAKES 2. FIXED CALIPER DISC BRAKES 3. SLIDING CALIPER DISC CALIPER

6.1 FLOATING-CALIPER DISC BRAKES

The calliper is the part that holds the brake shoes on each side of the disc. In the floating-calliper brake, two steel guide pins are threaded into the steering-knuckle adapter. The calliper floats on four rubber bushings which fit on the inner and outer ends of the two guide pins. The bushings allow the calliper to swing in or out slightly when the brakes are applied

When the brakes are applied, the brake fluid flows to the cylinder in the calliper and pushes the piston out. The piston then forces the shoe against the disc. At the same time, the pressure in the cylinder causes the calliper to pivot inward. This movement brings the other shoe into tight contact with the disc. As a result, the two shoes “pinch” the disc tightly to produce the braking action

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Fig 7: Floating calliper Disc Brake

6.2 FIXED-CALIPER DISC BRAKE

This brake usually has four pistons, two on each side of the disc. The reason for the name fixed-calliper is that the calliper is bolted solidly to the steering knuckle. When the brakes are applied, the calliper cannot move. The four pistons are forced out of their calliper bores to push the inner and outer brake shoes in against the disc. Some brakes of this type have used only two pistons, one on each side of the disc

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Fig 8: Fixed Caliper Disc Brake

6.3 SLIDING-CALIPER DISC BRAKE

The sliding-calliper disc brake is similar to the floating-calliper disc brake. The difference is that sliding-calliper is suspended from rubber bushings on bolts. This permits the calliper to slide on the bolts when the brakes are applied.

Proper function of the brake depends on (1) the rotor must be straight and smooth, (2) the calliper mechanism must be properly aligned with the rotor, (3) the pads must be positioned correctly, (4) there must be enough "pad" left, and (5) the lever mechanism must push the pads tightly against the rotor, with "lever" to spare.

Most modern cars have disc brakes on the front wheels, and some have disc brakes on all four wheels. This is the part of the brake system that does the actual work of stopping the car

The most common type of disc brake on modern cars is the single-piston floating calliper.

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MAIN PARTS:

The main components of a disc brake are:

• The brake pads

• The calliper, which contains a piston

• The rotor, which is mounted to the hub

BRAKE PAD

Fig 9: Brake Pad

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CHAPTER-7

COMPONENTS OF HYDRAULIC DISC BRAKE

Now that we understand hydraulics let's take a look at the different parts which make up the hydraulic brake. The entire braking system can be broken down into the following main parts:

1. Master cylinder (Lever) 2. Lines

3. Fluid

4. Slave cylinder (Calliper) 5. Pads

6. Rotor

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Fig 11: The layout of a typical brake system

7.1 MASTER CYLINDER

Converts mechanical force from the brake pedal, power booster and push rod into hydraulic pressure

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Most reservoirs also have a low brake fluid level switch to alert the driver of a low fluid condition.

Master Cylinder/Lever

The master cylinder, mounted to the handlebar, houses the brake lever and together they produce the input force needed to push hydraulic brake fluid to the slave cylinder (or calliper) and cause the brake pads to clamp the rotor.

The lever stroke can be divided into 3 categories:

1. Dead-stroke - This is the initial part of the lever stroke when the primary seal pushes

fluid toward the reservoir before it goes on to push fluid on to the calliper via the brake lines.

2. Pad Gap Stroke - This is the part between the calliper beginning to push the pistons out

of their housings and the pads contacting the disc (as the dead space between the pads and rotor is taken up).

3. Contact & Modulation - The pads are now clamping the rotor and by stroking the lever

further, additional brake power will be generated. Modulation is rider controlled and not necessarily a characteristic of the braking system; however some brakes may allow the rider to better modulate or control the braking forces than others.

7.2 MASTER CYLINDER

-

OPEN OR CLOSED

Master cylinder systems can be categorized into two groups - open and closed.

An open system includes a reservoir and bladder which allow for fluid to be added or removed from the braking system automatically during use. Reservoirs are the overflow for fluid which has expanded due to heat produced by braking. The bladder has the ability to expand and contract therefore as the fluid expands the bladder will compensate without any adverse effects on the 'feel' of the brake. Reservoirs also provide the additional fluid needed as the pads begin to wear resulting in the need for the pistons to protrude further to compensate for the reduced pad material.

A closed system also utilizes a reservoir of brake fluid however the lack of an internal bladder to compensate for the expansion in brake fluid and also to compensate for pad

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wear means that any adjustments to the levels of brake fluid within the working system need to be made manually.

7.3 BRAKE LINES

Hydraulic brake lines or hoses play the important role of connecting the two main working parts of the brake, i.e. the master cylinder and slave cylinder. We've already mentioned that hydraulic systems can be very versatile in that their lines or hoses can be routed almost anywhere so let's take a closer look.

Hose Construction

Hydraulic hoses are multi-layered in their construction and usually consist of 3 layers:

1. Inner Tube - This layer of tubing is designed to hold the fluid. Teflon is usually the

material of choice here as it does not react or corrode with brake fluid.

2. Aramid (Kevlar) Layer - provides the strength and structure of the hose. This woven

layer is flexible and handles the high pressures of the hydraulic system efficiently in that it should not expand. Kevlar is also very light, which is a desirable attribute for any cycle component, and also it can be cut easily and re-assembled using standard hose fittings.

3. Outer Casing - Serves as a protection layer for both the Kevlar layer and the bike frame

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Fig 12: The layers that make up an average hydraulic brake line

Steel Braided Brake Lines

Steel braided hoses can provide some advantages over standard hydraulic hoses. Steel braided hoses are also usually a 3-layer construction, the inner most layer contains the brake fluid and there is an outer most layer which provides protection against abrasions. The key difference is in the middle layer which is made up of a stainless steel braid.

This stainless steel layer is designed to be more resistant against expansion than that of standard lines. This can be an advantage because when the brake lever is applied we want all of the force we put in to be transferred to the calliper to cause braking. Any expansion in the hydraulic line due to the pressures within will mean that some of that pressure will not be transferred to the calliper. This will be wasted effort and will require additional lever input by the rider to compensate.

Steel braided lines may also be more appealing aesthetically. Many riders believe that they look better than the standard, boring black hoses that are supplied with the vast majority of brakes on the market.

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Fig 13:2011 Formula R1 brake with braided brake lines

7.4 BRAKE FLUID

Hydraulic braking systems typically use one of two types of brake fluid - DOT fluid or mineral oil. An important thing to note before we get into the properties of each is that the two fluids should never be mixed. They are made up of very different chemicals and the seals within the braking system are suited to either fluid or not both; therefore mixing or replacing one fluid with the other is likely to corrode the internals of your brake.

On the other hand, mixing fluid from the same family is allowed but not generally advised. For example you may mix DOT 4 fluid with DOT 5.1 without harming your braking system.

DOT Brake Fluid

DOT brake fluid is approved and controlled by the Department of Transportation. It has to meet certain performance criteria to be used within braking systems and is classified by its performance properties - mainly its boiling points.

DOT 3, 4 and 5.1 brake fluids are glycol-ether based and are made up of various solvents and chemicals. Glycol-ether brake fluids are hygroscopic, which means they absorb water from the environment even at normal atmospheric pressure levels. The typical absorption rate is quoted to be around 3% per year. This water content within the brake fluid will

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The table below shows DOT brakes fluid in its various derivatives with its corresponding boiling temperatures. Wet boiling point refers to fluid with water content after 1 years' service.

DOT Fluid Dry Boiling Point Wet Boiling Point

DOT 3 205 °C (401 °F) 140 °C (284 °F)

DOT 4 230 °C (446 °F) 155 °C (311 °F)

DOT 5 260 °C (500 °F) 180 °C (356 °F)

DOT 5.1 270 °C (518 °F) 190 °C (374 °F)

DOT brake fluid is commonly used in Avid, Formula, Hayes and Hope brakes.

DOT 5 Brake Fluid

DOT 5 brake fluids (not to be mistaken for DOT 5.1) are very different from other DOT fluids as it is silicone based and not glycol-ether based. This silicone based brake fluid is hydrophobic (non water absorbing) and must never be mixed with any other DOT brake fluid.

DOT 5 can maintain an acceptable boiling point throughout its service life although the way in which it repels water can cause any water content to pool and freeze/boil in the system over time - the main reason that hygroscopic fluids are more commonly used.

Mineral Oil

Mineral oil is less controlled as a brake fluid, unlike DOT fluid which is required to meet a specific criteria, therefore less is known regarding its performance and boiling points from brand to brand.

Manufacturers such as Shimano and Magura design their brakes around their own brand of mineral oil and should never be introduced to DOT brake fluid as this will likely have an adverse effect on the brake's seals.

An advantage of mineral oil is that, unlike most DOT fluids, it does not absorb water. This means that the brake will not need to be serviced as often, but any water content within the braking system could pool and freeze/boil adversely affecting the performance of the

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brake.

Mineral oil is also non-corrosive meaning handling of the fluid and spillages are less of a concern.

7.5 SLAVE CYLINDER/CALLIPER

The brake callipers reside at each wheel and respond to the lever input generated by the user. This lever input is converted to clamping force as the pistons move the brake pads to contact the rotor. Callipers can be fixed by a rigid mount to the frame or floating. Fixed callipers are combined with a fixed rotor which offers the only way of achieving zero free running drag one drawback of this design is that it is much less tolerant of rotor imperfections. Floating callipers slide axially and self-centre with each braking application.

Construction

Calliper construction can fall into two categories - mono-block and two piece. The difference here is the 'bridge' design, the bridge is the part of the calliper above the pistons which connects the two halves together and provides the strength to endure the clamping forces generated by the pistons.

1. Mono-block - A mono-block calliper is actually a one piece design formed from one

piece of material. This can offer a unique design and usually a lighter calliper as there is no need for steel bolts joining both halves as in a two piece design. Also the lack of a transfer port seal means there is one less opportunity for fluid leaks at the half way seam. Servicing a mono-block calliper can be tricky however and manufacturing and assembly are usually more difficult.

2. Two piece - These two piece callipers are constructed as two separate halves and are

then held together with steel bolts which can provide additional strength over a mono-block design. Servicing, manufacturing and assembly are simplified. Steel bolts and additional seals are a means of additional weight and can be problematic during servicing.

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Fig 14: Exploded view of an Avid two-piece calliper design

Pistons

The pistons are the cylindrical components housed within the calliper body. Upon lever input they protrude to push the brake pads which contact the rotor. The number of pistons within a calliper or brake can differ. Many hydraulic mountain bike brakes have 2 piston callipers, some may have 4 pistons. Whereas some automobile brake callipers have 6 or even 8 pistons. It is an important note that brake power is not determined by piston quantity. A more reliable indicator would be total piston contact area, e.g. 4 smaller pistons can be just as powerful as 2 larger pistons.

Pistons can be either opposed or single sided. Opposed pistons both protrude with lever input to push the brake pads equal amounts to meet the rotor at both sides. Whereas single sided calliper pistons stroke on one side and float the rotor to the opposite pad.

7.6 BRAKE PADS

Choosing the right brake pads can mean the difference between a great and a poor performing brake. With the sheer diversity of brake pad materials out there it is quite easy to get it wrong when the time comes to replace the pads.

Let's jump right in and take a look at the different pad materials available and their properties.

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Organic

Organic brake pads contain no metal content. They are made up of a variation of materials which used to include asbestos until its use was banned. These days you will commonly find materials such as rubber, Kevlar and even glass. These various materials are then bonded with a high-heat-withstanding resin. An advantage of organic pads is that they're made up of materials that don't pollute as they wear. They are also softer than other brake pads and as a result quieter. Also they inflict much less wear upon the brakes' rotor. However organic pads wear down faster and they perform especially poorly in wet gritty conditions (UK readers take note).

Organic pads then are probably more suited to less aggressive riding in mostly dry conditions

.

Fig 15: Organic Brake pads

Semi-metallic

The metallic content of semi-metallic pads can vary from anything between 30% and 65%. The introduction of metal content into the friction material changes things slightly. It can improve the lifespan of the pad quite significantly as metal wears slower than organic materials. Also heat dissipation is improved as it is transferred between the pad material and the backing plate. Some disadvantages can include increased noise during use and the

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Fig 16: Semi-metallic pads

Sintered

Sintered brake pads are made up of hardened metallic ingredients which are bound together with pressure and high temperature. The advantages of this compound are better heat dissipation, a longer lasting pad, better resistance to fading and superior performance in wet conditions. The trade-offs are more noise, longer bed-in time and a poor initial bite until the friction material has chance to warm.

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Ceramic

Ceramic brake pads are now seen more and more as an alternative/upgrade mountain bike brake pad. Traditionally ceramic brake pads would only be seen on high performance racing cars with brakes which need to perform under intense heat. Heat like that is not usually a problem for the average mountain bike brake and therefore for most people ceramic pads would be overkills however they might have other desirable properties. The advantages of a ceramic material then is one which can cope with extreme heat and keep performing strongly; this is in part down to its great dissipating abilities. They also last longer than other pads and noise is less of an issue. They're also easier on brake rotors and produce a lot less dust that other brake pad compounds.

Fig 18: Ceramic Brake pads

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Rotor Design

Important specifications of rotor design include hardness, thickness and rub area.

The material used to manufacture rotors must be hard and durable due to the aggressive forces inflicted upon them from the pad friction material. This has a direct impact on rotor wear.

Rotors must also have no thickness variations. Differences in thickness around the circumference of the rotor can have undesired effects on the braking system including pulsing as thicker and thinner sections pass between the pads. Rotors also need to run true. Any lateral wobble in the rotor during use can cause the brake to contact the pads intermittently during riding.

Fig 19: Left to right: Formula Lightweight, Avid G3 Clean Sweep, AshimaAiRotor A rotor's rub area can take the form of many different designs. The three rotors above show this in detail. Rub area design can affect the weight and strength of the rotor. It also has a direct effect on pad lifetime.

TYPES OF ROTORS

The two types of rotor on the market today are ISO standard 6-bolt rotors and CenterLock rotors. Both have their pros and cons.

6 Bolt

- Readily available and interchangeable between many brake models, this is the most common rotor fixing system in use today and was adopted by all manufacturers in the late 1990's. With no shortage of hub options, cross-compatibility with other products is rarely a problem. However installation of six fixing bolts can be cumbersome and there is always the risk of stripping a thread on fixing bolts and hub mounting points.

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CenterLock

- The Shimano CenterLock system eliminates the risk of stripping threads as there are no bolts to worry about, just one centre locking ring. Installation and removal is also simplified, although you will need a CenterLock tool. Lack of mass-market adoption means that hub choices are limited and brake choice may also be limited due to odd sized rotors. CenterLock rotors are also generally slightly heavier and can come at a price premium.

Fig 20: Left to right: ISO standard 6-bolt, Shimano CenterLock

2-Piece Rotors

2-Piece rotors are supplied as standard with some higher priced brake sets and can also be bought separately as an upgrade.

In contrast to standard stainless steel rotors, 2-piece rotors combine a stainless steel rub area with an aluminium carrier (or spider). The advantage of the alloy carrier is a cooler running disc as aluminium has superior heat dissipation qualities to that of stainless steel. This will also help to keep your pads, calliper and fluid cooler. Aluminium is also lighter than stainless steel so a reduction in weight can be expected.

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Fig 21: Formula 2-Piece Stainless Steel / Aluminium Rotor

Reason why Brakes Fail

Hydraulic brakes can fail or temporarily stop working for numerous reasons such as a simple (but potentially catastrophic) fluid leak or eventual brake fade after prolonged use. Knowing the causes of brake failure can be valuable knowledge in curing the problem and preventing future episodes.

As we know there are a couple of important principles behind hydraulic brakes. Hydraulics relies on pressure within the system and brakes rely on friction. Absence of either will result in failure of the system. For example, a loss of brake fluid will decrease the pressure within the system as the lever has nothing to transfer the input forces to. On the other hand if brake fluid contacts the brake pads or rotor, a loss of friction will occur due to the lubricating nature of brake fluid.

The above examples should be obvious to most but what about the less obvious causes of brake failure? Earlier we mentioned brake fade, a term which I bet many of you have heard, however did you know that there are multiple types of brake fade? Below is an overview of the three different types.

Pad Fade

All friction material (the stuff your pads are made of) has a coefficient of friction curve over temperature. Friction materials have an optimal working temperature where the coefficient of friction is at its highest. Further hard use of the brake will send the friction material over the optimal working temperature causing the coefficient of friction curve to decline.

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This high temperature can cause certain elements within the friction material to melt or smear causing a lubrication effect; this is the classic glazed pad. Usually the binding resin starts to fail first, and then even the metallic particles of the friction material can melt. At very high temperatures the friction material can start to vaporize causing the pad to slide on a layer of vaporized material which acts as a lubricant.

The characteristics of pad fade are a firm, non-spongy lever feel in a brake that won't stop, even if you are squeezing as hard as you can. Usually the onset is slow giving you time to compensate but some friction materials have a sudden drop off of friction under high temperatures resulting in sudden fade.

Green Fade

Green fade is perhaps the most dangerous type of fade which manifests itself on brand new brake pads. Brake pads are made of different types of heat resistant materials bound together with a resin binder. On a new brake pad these resins will cure when used hard on their first few heat cycles and the new pad can hydroplane on this layer of excreted gas. Green fade is considered the most dangerous as it can catch users unaware given its quick onset. Many people would consider new brake pads to be perfect and may be used hard from the word 'go'.

Correct bedding-in of the brake pads can prevent green fade. This process removes the top layer of the friction material and keys the new pad and rotor together under controlled conditions.

Fluid Fade

Fluid fade is caused by heat induced boiling of the brake fluid in the callipers and brake lines. When used under extreme conditions heat from the pads can transfer to the calliper and brake fluid causing it to boil, producing bubbles in the braking system. Since bubbles are compressible this results in a spongy lever feel and prevents the lever input from being sent to the calliper.

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CHAPTER-8

WORKING OF HYDRAULIC DISC BRAKES

Fig 22: Working of hydraulic disc brakes

The master cylinder is where the brake fluid starts. The pedal is attached to the master cylinder plunger. When the pedal is depressed it pushed the plunger which pushes the brake fluid down the brake lines. The brake lines are connected to the slave cylinders. When the brake fluid reaches the slave cylinders it presses out a piston to which is attached a brake pad. The brake pad then clamps against the rotor. All air must be bled from the system. (Air is compressible and if you have any in the system you will have a soft pedal.) As oil is virtually uncompressible it works as a solid link from pedal to brake.

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8.1 THE MASTER CYLINDER IN ACTION

As you can see in figure there are two pistons (primary and secondary) and two springs inside the master cylinder.

When the brake pedal is pressed, a push rod moves the primary piston forward which begins to build pressure in the primary chamber and lines. As the brake pedal is depressed further, the pressure continues to increase.

Fluid pressure between the primary and secondary piston then forces the secondary piston forward and pressurizes the fluid in the secondary circuit.

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Fig 24: Brakes applied

If there is a leak in one of the brake circuits, that circuit will not be able to maintain pressure. Figure shows what happens when one of the circuits develops a leak. In this example, the leak is in the primary circuit and the pressure between the primary and secondary pistons is lost. This pressure loss causes the primary piston to mechanically contact the secondary piston and the master cylinder now behaves as if it has only one piston. The secondary circuit will continue to function correctly, however the driver will have to press the pedal further to activate it. In addition, since only two wheels now have pressure, the braking power will be reduced.

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Fig 25: Functioning of the brakes

8.2 COMPENSATING PORTS

Small holes those are located between the master cylinder reservoir and the front side, or pressure side, of the master cylinder pistons.

When the master cylinder pistons are in the at-rest position (no braking-figure 9), the piston seals uncover the compensating ports and open the passages between the reservoir and the wheel brake channel.

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Fig 26: Compensating Ports

Assist in fluid return after brake release (See Bypass Port section below).

Note: When the brakes are released, the piston seals on both the primary and secondary

pistons are located between the compensating port and the bypass port. During braking, the piston seals close the compensating port passages to the reservoir which prevents high pressure fluid from entering the reservoir.

8.3 BYPASS PORTS

The bypass ports, like the compensating ports, are passages that are open between the reservoir and the master cylinder chambers (fig. 10). However, the bypass ports are open to the low pressure or back side of the pistons.

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Fig 27: Bypass Ports

During brake release, the following occurs:

• Strong springs in the master cylinder force the pistons back to the at-rest position faster than the brake fluid can return through the hydraulic channels. The pistons must return rapidly so they can be ready for another forward stroke, if necessary. This rapid piston return movement could create a vacuum in the master cylinder high pressure chambers, which would delay brake release.

• The bypass ports allow brake fluid from the reservoir to fill the low-pressure piston chambers.

• Brake fluid from the low pressure chambers then passes through holes in the pistons and bypasses the piston lip seals. The pistons can then return without any “dragging”.

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Fig 28: Master Cylinder Return Operation: applied (left); releasing (right)

8.4 SELF ADJUSTMENT OF DISC BRAKES:

Disc brakes are self adjusting. Each piston has a seal on it to prevent fluid leakage. When the brakes are applied, the piston moves toward the disc. This distorts the piston seal. When the brakes are released, the seal relaxes and returns to its original position. This pulls the piston away from the disc. As the brakes linings wear, the piston over travels and takes a new position in relation to the seal. This action provides self-adjustment of disc brakes.

8.5 EMERGENCY BRAKES:

In cars with disc brakes on all four wheels, an emergency brake has to be actuated by a separate mechanism than the primary brakes in case of a total primary brake failure. Most cars use a cable to actuate the emergency brake.

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Fig 29: Emergency Brakes

Some cars with four-wheel disc brakes have a separate drum brake integrated into the hub of the rear wheels. This drum brake is only for the emergency brake system, and it is actuated only by the cable; it has no hydraulics.

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CHAPTER-9

BRAKE FADE

Vehicle braking system fade, or brake fade, is the reduction in stopping power that can occur after repeated or sustained application of the brakes, especially in high load or high speed conditions. Brake fade can be a factor in any vehicle that utilizes a friction

braking system including automobiles, trucks, motorcycles, airplanes, and even bicycles.

Brake fade is caused by a build-up of heat in the braking surfaces and the subsequent changes and reactions in the brake system components and can be experienced with both

drum brakes and disc brakes. Loss of stopping power, or fade, can be caused by friction fade, mechanical fade, or fluid fade. Brake fade can be significantly reduced by appropriate equipment and materials design and selection, as well as good cooling.

Brake fade occurs most often during high performance driving or when going down a long, steep hill. Owing to their configuration fade is more prevalent in drum brakes. Disc brakes are much more resistant to brake fade and have come to be a standard feature in front brakes for most vehicles.

9.1 BRAKE MODIFICATION TO REDUCE FADE

High performance brake components provide enhanced stopping power by improving friction while reducing brake fade. Improved friction is provided by lining materials that have a higher coefficient of friction than standard brake pads, while brake fade is reduced through the use of more expensive binding resins with a higher melting point, along with slotted, drilled, or dimpled discs/rotors that reduce the gaseous boundary layer, in addition to providing enhanced heat dissipation. Heat build-up in brakes can be further addressed by body modifications that direct cold air to the brakes.

The "gaseous boundary layer" is a hot rod mechanics explanation for failing self-servo effect of drum brakes because it felt like a brick under the brake pedal when it occurred. To

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counter this effect, brake shoes were drilled and slotted to vent gas. In spite of that, drum brakes were abandoned for their self-servo effect. Discs do not have that because application force is applied at right angles to the resulting braking force. There is no interaction.

Drum brake fade can be reduced and overall performance enhanced somewhat by an old "hot rudder" technique of drum drilling. A carefully chosen pattern of holes is drilled through the drum working section; drum rotation centrifugally pumps a small amount air through the shoe to drum gap, removing heat; fade caused by water-wet brakes is reduced since the water is centrifugally driven out; and some brake-material dust exits the holes. Brake drum drilling requires careful detailed knowledge of brake drum physics and is an advanced technique probably best left to professionals. There are performance-brake shops that will make the necessary modifications safely.

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A moving car has a certain amount of kinetic energy, and the brakes have to remove this energy from the car in order to stop it. How do the brakes do this? Each time you stop your car, your brakes convert the kinetic energy to heat generated by the friction between the pads and the disc. Most car disc brakes are vented.

Brake fade caused by overheating brake fluid (often called Pedal Fade) can also be reduced through the use of thermal barriers that are placed between the brake pad and the brake calliper piston. These reduce the transfer of heat from the pad to the calliper and in turn hydraulic brake fluid. Some high-performance racing callipers already include such brake heat shields made from titanium or ceramic materials. However, it is also possible to purchase aftermarket titanium brake heat shields that will fit your existing brake system to provide protection from brake heat. These inserts are precision cut to cover as much of the pad as possible. These Titanium Brake shims are an easy to install, low cost solution that are popular with racers and track day enthusiasts.

Another technique employed to prevent brake fade is the incorporation of fade stop brake coolers. Like titanium heat shields the brake coolers are designed to slide between the brake pad backing plate and the calliper piston. They are constructed from a high thermal conductivity, high yield strength metal composite which conducts the heat from the interface to a heat sink which is external to the calliper and in the airflow. They have been shown to decrease calliper piston temperatures by over twenty percent and to also significantly decrease the time needed to cool down. Unlike titanium heat shields, however, the brake coolers actually transfer the heat to the surrounding environment and thus keep the pads cooler.

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CHAPTER-10

ADVANTAGES

10.1 ADVANTAGES OF DISC BRAKES OVER DRUM BRAKES

As with almost any artifact of technology, drum brakes and disc brakes both have advantages and disadvantages. Drum brakes still have the edge in cheaper cost and lower complexity. This is why most cars built today use disc brakes in front but drum brakes in the back wheels, four wheel discs being an extra cost option or shouted as a high performance feature. Since the weight shift of a decelerating car puts most of the load on the front wheels, the usage of disc brakes on only the front wheels is accepted manufacturing practice.

Drum brakes had another advantage compared to early disc brake systems. The geometry of the brake shoes inside the drums can be designed for a mechanical self-boosting action. The rotation of the brake drum will push a leading shoe brake pad into pressing harder against the drum. Early disc brake systems required an outside mechanical brake booster such as a vacuum assist or hydraulic pump to generate the pressure for primitive friction materials to apply the necessary braking force.

All friction braking technology uses the process of converting the kinetic energy of a vehicle’s forward motion into thermal energy: heat. The enemy of all braking systems is excessive heat. Drums are inferior to discs in dissipating excessive heat:

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The conventional disc brake, on the other hand, consists essentially of a flat disc on either side of which are friction pads; equal and opposite forces may be applied to these pads to press their working surfaces into contact with the braking path of the discs. The heat produced by the conversion of energy is dissipated directly from the surfaces at which it is generated and the deflection of the braking path of the disc is very small so that the stressing of the material is not as severe as with the drum."

The result of overheated brakes is brake fade...the same amount of force at the pedal no longer provides the same amount of stopping power. The high heat decreases the relative coefficient of friction between the friction material and the drum or disc. Drum brakes also suffer another setback when overheating: The inside radii of the drum expand, the brake shoe outside radii no longer matches, and the actual contact surface is decreased.

Another advantage of disc brakes over drum brakes is that of weight. There are two different areas where minimizing weight is important. The first is unsprung weight. This is the total amount of weight of all the moving components of a car between the road and the suspension mounting points on the car’s frame.

Auto designs have gone to such lengths to reduce unsprung weight that some, such as the E-type Jaguar, moved the rear brakes inboard, next to the differential, connected to the drive shafts instead of on the rear wheel hubs. The second "weighty" factor is more of an issue on motorcycles: gyroscopic weight. The heavier the wheel unit, the more is gyroscopic resistance to changing direction. Thus the bike’s steering would be higher effort with heavier drum brakes than with lighter discs. Modern race car disc brakes have hollow internal vents, cross drilling and other weight saving and cooling features.

Most early brake drums and discs were made out of cast iron. Current OEM motorcycle disc brakes are usually stainless steel for corrosion resistance, but after-market racing component brake discs are still made from cast iron for the improved friction qualities. Other exotic materials have been used in racing applications. Carbon fibre composite discs gripped by carbon fibre pads were common in formula one motorcycles and cars in the early 1990’s, but were outlawed by the respective racing sanctioning

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organizations due to sometimes spectacular failure. The carbon/carbon brakes also only worked properly at the very high temperatures of racing conditions and would not get hot enough to work in street applications.

A recent Ducati concept show bike uses brake discs of selenium, developed by the Russian aerospace industry, which claim to have the friction coefficient of cast iron with the light weight of carbon fibre.

Another area of development of the disc brake is the architecture of the brake calliper. Early designs had a rigidly mounted calliper gripping with opposed hydraulic pistons pushing the brake pads against a disc mounted securely to the wheel hub. Later developments included a single piston calliper floating on slider pins. This system had improved, more even pad wear. Most modern automobiles and my 1982 Kawasaki motorcycle use this type calliper. Current design paradigm for motorcycle brakes have up to six pistons, opposed to grip both sides of a thin, large radius disc that is "floating" on pins to provide a small amount of lateral movement; two discs per front wheel.

Improvements in control have been made available with the application of Anti-Lock Brake technology. Wheel sensors convey rotation speed of each wheel to a computer that senses when any of them are locked up or in a skid, and modulates individual wheel brake hydraulic pressure to avoid wheel skidding and loss of vehicular control.

The use of exotic materials for additional weight savings would be likely for the future of motor vehicle braking. Discs mounted to the wheel’s rim gripped by an internally located calliper are not necessarily a new design (Porsche, 1963) but could be a futuristic looking option for motorcycle wheels. Electric vehicles of the future will likely utilize regenerative braking, the electric motors become generators to convert kinetic energy back

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10.2 REASON FOR HIGH EFFICIENCY OF DISC BRAKES

 Flat brake disc (axial brake) under high pressure versus round brake drum (radial brake) during braking

 Full friction surface of the brake pad on the plane brake disc.

 No loss of brake power due to overheating or partial contact from brake drum parts expansion.

 Disc brakes can withstand higher loads and its efficiency is maintained considerably longer even under the highest stresses

 Higher residual brake force after repeating braking  Brake discs can withstand extremely high temperatures  Full contact of brake pads achieves maximum effect.

 No verification of brake pads. Dangerous fading or slipping is almost completely eliminated.

10.3 BETTER BRAKING BEHAVIOUR OF HYDRAULIC DISC

BRAKES

 Driver friendly braking behaviour. Sensitive braking in all situations and better  Sensitive brake application and better brake feeling

 Uniform braking from small fluctuations in brake forces  Retardation values retained even under heavy stresses  Minimal "pulling to one side" due to uneven brake forces

 Disc brake axial arrangement permits a simple and compact design  Linear characteristics lead to an even progression of brake force  Basic design principle makes for higher efficiency

 Low hysteresis is particularly suitable to ABS control cycles

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 Minimal braking effect from high temperatures and extreme driving requirements Minimal heat fading

 No brake disc distortion from extreme heat due to internal ventilation with directional stability and large power reserve under high stress

 The decisive safety aspects of the disc brake design are shorter braking distances  High power and safety reserves for emergencies

 Constant braking power under high stresses

 Shortened braking distance under emergency braking with considerably improved directional stability

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CHAPTER-11

LIMITATIONS

The limitations of hydraulic disc brakes:

Braking systems fails if there is leakage in the brake lines.

The brake shoes are liable to get ruined if the brake fluid leaks out.

Presence of air inside the tubing ruins the whole system.

Pad wear is more.

Hand brakes are not effective if disc brakes are used in rear wheels also. (Hand brakes are better with mechanical brakes).

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CHAPTER-12

APPLICATIONS

The applications of Hydraulic Disc Brakes are:

Hydraulic Disc brakes are used primarily in motor vehicles, tanks, but also in machinery and equipment, and aircraft, bicycles, carriages and railway. The disc brakes have been widely used in cars and trucks, especially in the premium sedan. The disc brakes on the new mine hoist brake. The disc brake inertia is small, fast action, high sensitivity, and adjustable braking torque. The multi-rope friction hoist all use disc brakes.

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II.

CONCLUSION

Many trucks and buses are equipped with hydraulic actuated disc brakes. The high contact forces are transmitted mechanically via needle mounted actuating device.

In view of the fact that the air can circulate freely between the disc and the brake shoe, disc brakes are cooled much better, especially since it is possible to do so ventilated discs extra holes. The gases resulting from friction, dust, dirt, do not stay on the working surfaces. These brakes are not sticky.

The disc brakes have been widely used in cars and trucks, especially in the premium sedan. The disc brakes on the new mine hoist brake. The disc brake inertia is small, fast action, high sensitivity and adjustable braking torque.

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III. REFERENCES

 TechCenter By Karl Brauer, Editor in Chief, Edmunds.com

 http://cars.about.com/od/thingsyouneedtoknow/ig/Disc-brakes

 http://en.wikipedia.org/wiki/Disc_brake

 http://www.kobelt.com/pdf/brochure_brake.pdf

 http://auto.howstuffworks.com/auto-parts/brakes/brake-types/disc-brake.htm

 http://www.sae.org/search?searchfield=brake%20system

 http://www.hinduonnet.com/thehindu/2000/05/25/ceramic brake disc

References

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