Belt

When designing a pulley system, the first consideration should be how much force is going through it. Then consider what level of tension on the belt is possible. Finally, choose the belt strong enough to withstand the force created by the driving pulley and tensioning.

Some types of belts require less tension than others. In this application it is desirable to keep the design simple. For this to apply, a simple “slide to tighten” method is being utilized.

This method is not able to produce an excessive amount tension. Therefore, the application requires the use of a V-belt. This type of belt has a larger surface area allowing for 5 times more efficient power transfer. This means less tension, while still sustain good power transfer and minimal slipping.

Side | 37 3.3.4 Groove

Table 13 (Optibelt Product Catalog, 2013)

The V-belt being used is a 1000 mm long Optibelt AVX10 Marathon I. This is a 10 mm wide, cogged automotive specific V-belt which reduces energy consumption by negating the compression stress induced by the belt wrapping around the pulleys. Main reason for choosing this belt is because it fits the application well, and because it was available at nearby stores.

Due to this being a specialized belt it is difficult to find specifications on the pulley groove.

SPZ is the standard dimension for “10 mm” V-belts. The only difference between this belt and a standard SPZ belt is that the top width is 10 mm instead of 9,7 mm. The datum width B_d/W_b is still 8,5. Since they are so similar it was decided to design the pulley around a standard SPZ groove. This means it will work with the current belt, but it can also easily be replaced with a SPZ belt in the future.

Table 14 (SKF Pulley Catalogue, 2014)

Side | 38 3.3.5 Calculations

Maximum allowed torque before slipping:

The motor being used in the car during this project is an Alva X100 motor. It is capable of outputting a maximum torque of 8 Nm, which is what will be used during these calculations.

The gearing results in the output torque on the roller being 40,9 Nm. It is important to note that it will never utilize this amount of torque due to energy efficiency. The torque figure being used in the calculations will only function as a safety in case of controller failure.

Minput(roller)=8 Nm itot1=14,06 i3=0,36 𝑀𝑖𝑛𝑝𝑢𝑡(𝑟𝑜𝑙𝑙𝑒𝑟) = 𝑀max (𝑚𝑜𝑡𝑜𝑟)∗ 𝑖_{𝑡𝑜𝑡1}∗ 𝑖₃ = 40,49 𝑁𝑚

Minput(roller): Torque being applied to the roller by the wheel Mmax(motor): Maximum torque the Alva x100 can generate itot1 and i3: Gearing constants, see Gearing input

When setting the maximum allowable tensioning force, the weakest part must be

considered. In this case, it is the shaft of the measuring motor. According to the technical data sheet for the motor, the maximum radial force 15 mm from the flange is 110 N. While considering longevity and reduce wear on the motor; a safety factor of 1,5 is applied. Using this safety factor, the maximum allowable tensioning force is 73 N (N0). Whilst the system is stationary this force will be divided by the two belt sides.

Nmax=110 N SF=1,5

𝑆₁₌𝑆_{𝑟𝑒𝑠𝑡𝑖𝑛𝑔=}𝑁₀

2 =73 𝑁

2 = 36,5 𝑁

Sresting: Tension on the belt while resting Nmax: Maximum radial force on motor shaft N0: Maximum allowed radial force on motor shaft

There will be a coefficient of friction in-between the pulley and the belt. Unfortunately, the exact coefficient is unknown, due to the pulleys being made from scratch. Test could be made to determine the coefficient, but due to time restraints and the nature of the project it was neglected. Instead the coefficient was decided by using a standard chart.

Side | 39

Table 15 (Chaudhari, 2015, Table 1)

Going with the combination of a steel pulley and rubber belt the coefficient ends up being 0,3. V-belt grooves for 50 mm pulleys have an angle of 34 degrees. This results in the coefficient of friction being increased to 1,03.

𝜇 = 0,3 𝛼́ = 34

𝜇_{𝑠(𝑒𝑛ℎ)}= 𝜇

𝑠𝑖𝑛𝛼́

2

= 0,3 𝑠𝑖𝑛34

2

= 1,03

µ: friction coefficient between belt and pulley

µs: enhanced friction coefficient between belt and pulley due to v groove α: angle of pulley groove

Figure 27 Pulley dimensions

The pulley setup consists of two pulleys, one 150 mm, the other 50 mm. They are 341 mm apart. To calculate the tension during load it is necessary to know the smallest angle of contact.

Side | 40 𝑠𝑖𝑛𝛽 =(𝐷 − 𝑑)

2𝑎 =(150 𝑚𝑚 − 50 𝑚𝑚)

2 ∗ 341 𝑚𝑚 = 0,1466 𝛽 = 8,43°

𝛼 = 180° − 2𝛽 = 180° − 2 ∗ 8,43° = 163,14°

𝛼 = 2,85

α: Angel of contact β: Input angle

The maximum input torque allowed can now be calculated:

𝑀max (𝑟𝑜𝑙𝑙𝑒𝑟) = (𝑆_{2𝑚𝑎𝑥}− 𝑆₁) ∗ 𝑟𝑟𝑜𝑙𝑙𝑒𝑟 𝑝𝑢𝑙𝑙𝑒𝑦

𝑆_{2𝑚𝑎𝑥} = 𝑆₁∗ 𝑒^𝜇𝑠𝛼 = 36,5 𝑁 ∗ 𝑒^1,03∗2,85= 687.33 𝑁 𝑀max (𝑟𝑜𝑙𝑙𝑒𝑟) = (687,33 𝑁 − 36,5 𝑁) ∗ 0,075 𝑚 = 48,1 𝑁𝑚

𝑀𝑖𝑛𝑝𝑢𝑡(𝑟𝑜𝑙𝑙𝑒𝑟)= 40,9 𝑁𝑚 < 𝑀max (𝑟𝑜𝑙𝑙𝑒𝑟)= 48,1 𝑁𝑚

According to these calculations the maximum torque on the roller (Minput(roller)) does not exceed the maximum allowed torque (Mmax(roller)). This means that even with the car’s motor accelerating with maximum torque, the belt will not slip.

3.4 Double Lift System 3.4.1 How it works

The workstation has a double lift system. The system consists of two modified pre-manufactured universal lifts.

These lifts’ function of a threaded rod that is connected to four arms on each lift by two points. One of the two points are threaded and as the rod turns the points are being pulled towards each other which then lifts the car.

This style of lift has been chosen because of its simple design and the capacity which ensures a large factor of safety based on the load applied during operation (max 200 kg).

Side | 41

Figure 28 “Miniløfter” (Biltema, 2017)

The two lifts run synchronized of one electric motor.

To enable this each lift is modified with identical sprockets welded to the ends of the threaded rods. A 520-motorcycle chain runs between these sprockets. Due to the low load on the system it is possible to connect the motor directly on to the rod without any gearing.

The ratio between the sprockets is 1:1. This always ensures that the lifts run synchronized and stay at the same level.

3.4.2 Car Braces

To maximize stability, the lifts are equipped with wooden beams resting on top of the top plates to increase surface area. These are secured by two recessed bolts. Wood is a softer material than carbon fibre and wont scratch the underbody of the car. It is also more vibration absorbent than metal. To ensure there is enough friction between the car and the holders, the beams have a layer of rubber to prevent the car from sliding when lifted.

These holders are one of the adjustable parts of the system which can be swapped out or modified for future cars.

3.4.3 Lift assembly

The lifts are mounted 500 mm apart on two steel square tubes (3x40x40 mm). From the factory, the lift comes with two mounting brackets, one on each side. These sit flush with the underside of the lift to give stability. Since the lifts are mounted of the ground on the workstation the underside of the lift will not provide any stability and therefore, a second pair of brackets are added to compensate.

The lift system is adjustable, and the lifts are mounted with four through bolts each. These bolts are 8.8 because this quality is easy to acquire and strong enough

If the system is to be adapted to a new car, new holes must be made.

Side | 42 3.4.4 Safety

Whenever there are parts in motion and load being applied it is important to look at safety.

The lift system is modelled in SolidWorks and through simulations in the program it is possible to get a good idea about the strength. When the max operation load is applied the model has high factor of safety and small displacements.

Figure 29 SW Simulation: Car brace, stress

Figure 30 SW Simulation: Car brace, displacement

Max stress: 24,6 MPa

Max displacement: 0,6 mm

Side | 43

3.5 Adjustable Solution

The solution is to use wide rollers in the back and two-dimensional adjustable steering discs in front. SEM have rules that dictates the dimensions of the cars. The current car is on the outer limit of width and length. The adjustability is therefore made so that the maximum value would match up with the maximum of the SEM rules.

3.5.1 Width

This value is limited by the total width of the bench, roller placement and disc size.

Figure 31 Maximum width

Figure 32 Minimum width

Maximum outer edge of wheelbase width: 1340 mm

Minimum inner edge of wheelbase width: 540 mm

Side | 44 3.5.2 Length

This value is determined by the total length of the frame, lift placement and steering assembly.

Figure 33 Maximum length

Figure 34 Minimum length

3.6 Brake Simulation

On the workstation one of the main goals are to measure the performance of the drivetrain during load. The tractive torque is the torque that must be applied on the wheels for the car to move. It consists of several elements which largely depends on the current velocity and acceleration of the car. By knowing the tractive torque, one may simulate it by applying the translated braking torque at the given speed/rpm on the measuring motor.

The tractive torque is defined by the following equation:

𝑇_𝑤 = 𝐹_𝑡∗ 𝑟_𝑤 𝐹_𝑡 = 𝐹_𝑟+ 𝐹_𝑔+ 𝐹_𝑑− 𝐹_𝑖𝑒

Tw: Tractive torque at the wheel Ft: Tractive force

rw: Radius of the wheel

Maximum length of wheelbase: 2250 mm

Minimum length of wheelbase: 1500 mm

Side | 45 Fr = tyre rolling resistance

Fd = aerodynamics drag

Fie = equivalent inertial force (during acceleration) - (including linear and rotational inertias, due to vehicle mass and rotating component of gear train and wheels)

Fg = forces due to gradient

Since it is the roller that will be applying the tractive force, it is the torque used on the roller to achieve the tractive force on the wheel that must be clarified. The same formula may be used with minor tweaks:

𝑇_𝑅 = 𝐹_𝑡∗ 𝑟_𝑅

TR: Torque at the roller rR: Radius of the roller

After this has been clarified, all that must be done is to convert the torque necessary at the roller to the motor.

𝑇_𝑅= 𝐹_𝑡∗ 𝑟_𝑅∗ 𝑖₃ i3: Ratio in between the roller pulley and motor pulley

The tractive force

For the equation above to function, the individual components of the tractive force must be defined.

First the rolling resistance will be defined. In this case the rear tyres are still experiencing rolling resistance. Therefore, the only rolling resistance that must be added are from the front tyres. Considering there are two measuring motors, each motor will apply the brake for one front tyres. To calculate the rolling resistance of the wheels it is necessary to, either do extensive testing with the tyres in question, or use a provided data sheet from the tyre’s manufacturer. In this case Michelin 90-80 R16 tyres are being used, which the data sheet is provided for. This makes it possible to make an expression for the rolling resistance using the following equation:

𝐹_𝑅 = 𝐶_𝑅∗ 𝑁 𝑁 = 𝑚𝑔 = 𝑚𝑎𝑠𝑠 ∗ 9,81𝑚

𝑠²

Cr: constant defined by the manufacture through thorough testing. It is a constant specific to the speed.

N: normal force due to gravity

Side | 46

Figure 35 Rolling Resistance vs. Speed for Loads (Michelin, 2015)

Since the graph for the tyre is provided, one may find an estimated equation for the rolling resistance throughout the rpm range by using interpolation:

v@0,0015=35 km/h=9,72 m/s

CR@9,72m/s=0,0015 CR@0m/s=0,00075 m=143 Kg (Car=75 Kg, driver=78 Kg)

𝐹_𝑅(𝑣) = ((𝐶_𝑅@9,72^𝑚

𝑠 − C_R@0m

s) ∗ 𝑣

9,72𝑚 𝑠

+ C_R@0m

s) ∗ 𝑚𝑔 = 1,05 N ∗ 𝑣 9,72m

s

+ 1,05 𝑁

This is the rolling resistance for all four wheels. Each motor will only have to simulate the resistance caused by one front wheel. This results in the following equation:

𝐹_𝑅1(𝑣) =F_R(v)

4 = 1,05

4 N ∗ v

9,72m s

+1,05

4 𝑁 = 0,263 𝑁 ∗ 𝑣 9,72𝑚

𝑠

+ 0,263 𝑁

FR1: Rolling resistance for a single wheel

Side | 47 The next force to consider is the equivalent inertial force. Since the purpose of the dyno system is to measure the car’s performance, the equivalent inertial force caused by the rollers and pulleys must be removed. Both of which functions as a flywheel, braking the rotation of the car’s wheel.

To simplify the expression, only the barrel of the roller is considered. More in depth calculations may be done, but they are not included in this report.

The expression that will be used is defined accordingly:

𝑇 = 𝐼 ∗ 𝛼

I: Moment of inertia for solid cylinder T: Newtons second law

α: Angular acceleration m: mass of roller

The equivalent inertial force is largely dependent on the acceleration of the car. The

acceleration will have to be real time monitored so that the equivalent inertial force can be subtracted from the total tractive force.

Then the forces due to aerodynamic drag needs to be calculated. This force is variable, meaning that the motors will need to apply more braking power the faster the car drives.

The aerodynamic drag is defined by the following equation:

𝐹_𝐷 = 𝐶_𝑑∗𝜌 ∗ 𝑣² 2 ∗ 𝐴

Cd: Drag constant ρ: Air density

v: Speed measured in m/s

A: Reference area being affected by the drag

For the equation to function there are a couple of constants that must be known. The approximate drag coefficient and reference area of the current car is defined in the backbone of DNV GL Fuel Fighters. Which is based on simulations made in openFOAM.

Side | 48

Figure 36 (Trefall, 2019)

Depending on where SEM will be held, the density of air will vary. In this instance the

density at sea level and 15C will be utilized. This is a normal setting which also functions as a worst-case scenario.

Using the formula, it is clear that the only variable will be the speed. The speed variable can be scaled accordingly to a rpm sensor attached to the measuring motor. By using the rpm sensor and a gearing constant the speed of the car can be calculated.

The final force to consider are forces due to gradient. This will however be neglected during this phase of the subsystem. The reason for this is because the track simulator is not

finished yet, so there is no way of knowing the gradients the car will experience.

Additionally, the different gradients and turns would cause the angel of the car, relative to the flow of air, to change. This would ultimately cause the reference area to change, causing the need for more aerodynamic testing.

Result

Now that the individual tractive forces have been defined, it is important to consider that there are two motors delivering the braking power. Therefore, the final tractive force for each wheel must be defined like this:

𝐹_𝑡 =𝐹_𝑟 4 +𝐹_𝑑

2 − 𝐹_𝑖𝑒

Now that the basis for the tractive force is established, that can be used to make an expression for the torque needed on the roller to achieve this braking force:

Flau 3000=Flow 3000 (The type of grid refinement which gave the most accurate results), snappyHexMeshDict used

Side | 49 𝑇_𝑅 = (0,263 𝑁 ∗ 𝑣

9,72𝑚 𝑠

+ 0,263 𝑁 + 0,0792𝐾𝑔

𝑚 ∗ 𝑣²−1

2𝑚𝑎) ∗ 𝑟_𝑅

Finally, the gearing constant in between the roller pulley and motor pulley can be used to calculate the needed braking torque on the measuring motor at the given speed and acceleration:

𝑇_𝑀 = 𝑇_𝑅∗ 𝑖₃ = (0,525𝑁 ∗ 𝑣 9,72𝑚

𝑠

+ 0,263 + 0,0792𝐾𝑔

𝑚 ∗ 𝑣² −1

2𝑚𝑎) ∗ 𝑟_𝑅 ∗ 0,33 𝑆𝑒𝑒 𝐺𝑒𝑎𝑟𝑖𝑛𝑔 𝑖𝑛𝑝𝑢𝑡

This is the formula that must be inputted into a motor controller to achieve the correct amount of tractive torque on the wheels.

Side | 50

4 Result

Figure 37 Workstation

4.1 State of Progress

The project was abruptly halted on March 12. When the government announced a nation-wide lock down due to the COVID-19 pandemic. The project was 78 % completed at that point and only 60 % of the workstation was built.

Table 16 Progress map

Frame status

The frame is the part that is closest to completion. All the welds are completed, both wheels and lifts are bolted on. At 95 %, the only thing remaining is bolting on the beam for the front wheels, grinding down the outer welds, prepping the whole frame for paint, and then apply paint.

Side | 51 Lift system status

The lifts are bolted on to the frame and two planks are bolted to the lifts, making out the car brace. However, the planks must be shaped and fitted with rubber. The sprockets must be welded on to the lifts, as well as a rod to connect one of the sprockets to the electric engine.

Rollers status

Both sets of rollers are complete The PVC pipe for both rollers are cut to the correct length, and the end caps are machined to the correct specifications except for the bolt holes.

Figure 38 Rollers

4.2 Evaluation

It is impossible to give a thorough evaluation of the workstation given that it is not finished and in no way can be tested. We will however evaluate the parts that have been made and take a closer look at some of the challenges we faced.

The welding process was a challenge at first. We needed practice before we could weld the frame, and even then, it proved difficult. Some mistakes were made, but we got

considerable help from staff at the workshop and members of DNV Gl Fuel Fighter, so the process got smoother as it went along.

Working at Verkstedteknisk workshop also proved challenging at times. We were unable to use the band saw for most of the build and were not allowed to use an angle grinder. We had to resort to using a reciprocating saw for most of the cutting of steel. The cuts got inaccurate and we had to use a belt grinder to try and get the correct length.

The result ended up being less accurate and less pretty than imagined. However, the most critical part of the frame, which is the back end where the rollers sit, became accurate and level.

DNV GL Fuel Fighter will continue working on the workstation and the fact that it was not finished in this project does not matter to much.

Side | 52

5 Further Development

5.1 Enclosure

To prevent the car from being damaged during transportation an enclosure that covers the workstation has been considered. Due to the time frame of the project this concept has not been prioritized.

The idea is to make a light wooden frame which covers the workstation and the car. The enclosure will be plated with thin plywood. This way the car is protected against the environment and objects that can potentially scratch or damage the car. To secure the enclosure to the frame mounting points can be welded to the frame. A quick release system is desirable to make mounting and dismounting the enclosure as easy as possible. The enclosure is light but has a lot of surface area. This can be utilized as commercial space for sponsors.

In addition to this, handlebars should also be added to the frame to make it easier to move.

5.2 Track Simulator

As mentioned earlier in the report, one of the future goals for the workstation is to use it as a track simulator. The rollers, electrical motor and front wheel rack are the three most important components to enable a track simulation. The rest of the development relies on simulation software and data input from the workstation.

A potentiometer measures the steering angle of the front wheels, the resistance in the rollers are determined by the electrical motor which is controlled by the software to match the situation in the simulation.

5.3 Electrical Equipment

The electrical equipment falls under the responsibility of the electrical team. At the time of writing, none of the equipment is fitted. electric motors, circuit boards and various other electrical equipment must be fitted. Plywood panels will be used as a platform for most of the equipment and brackets mounted to the frame will be used for the electric motors.

5.4 Braking System

During the design phase it was early on decided that there would be a need for an emergency brake. This has to a certain degree been designed and drawn in CAD.

However, there are still uncertainties regarding how well it would function. Additionally, there has not been made

However, there are still uncertainties regarding how well it would function. Additionally, there has not been made

In document Development of Multifunctional Workstation (Page 49-0)