Coupling torque requirements can be defined many ways, and specifiers need to decide which definition to use. We will first review the various definitions, then discuss how they are used in coupling selection.
System Torque
Normal Operating Torque
The steady state torque required by the system when operating at normal design conditions. This is usually the level at which the equipment designer certifies the equipment performance.
Starting Torque
The torque needed when the system starts its operation. This torque can be greater or less than the normal operating torque.
Peak Torque
The maximum torque required by the system. This torque is normally a one time event or limited to a specified number of occasions. In torsional vibration coupling systems it is the maximum vibratory response torque that could pass through the coupling.
Cyclic Torque
It is any torque requirement of the system that varies with time. It can be of a smooth, periodic variation like a sine wave or could be an erratic variation. It does not go through zero to a negative value, but can be equal to zero. In torsional vibrating systems it is the vibratory torque that occurs at the operating speed.
Reversing Torque
This is a cyclic torque that passes through zero and becomes negative or "reverses" to the opposite direction.
Transient Torque
A transient torque is of short duration, not necessarily expected, not happening on a regular basis but occurring when a system is upset. It may or may not be equal to or greater than peak torque.
Normal Braking Torque
It is the torque used to decelerate or reduce the speed of the equipment when the brakes are applied in a normal manner. The torque is time dependent, and moves through the system.
Emergency Braking Torque
In this case the brakes are applied to stop the equipment in a very short time. The torque will exceed the normal braking torque by the inverse ratio of the time required to stop in each case.
Stall or Lockup Torque
This is the torque that passes through the system when the system stalls or otherwise come to a stop because of some activity within the driven system.
Shutdown Torque
The torque required to bring the equipment from operating conditions to a shut down condition. This can be the normal braking torque or could be a result of friction or load in a system that is coasting to a stop.
Torque to Accelerate or to Decelerate
The torque required to increase or to decrease the equipment operating speed. In the case of acceleration, the available torque
for acceleration is the difference between the driver capability and the system requirements at the current speed. Decelerating torque comes from braking devices or from frictional drag or other energy drains within the system that cannot be overcome by the driver. A formula for calculating this torque is found at the end of this section.
Driver Horsepower (Torque) Nameplate Rated Horsepower (Torque)
The torque value is derived from the driver capability shown on the nameplate as a horsepower and a speed. It is based on specific inputs to the driver such as voltage, and amps or kVA if it is an electric motor. A formula to convert horsepower and speed to torque is found at the end of this section.
Service Factor Rated Horsepower (Torque)
Some drivers have additional capabilities beyond the name plate rating. The nameplate capabilities are multiplied by the service factor. The service factor is also shown on the nameplate.
Start-Up Torque
The driver torque capability at start-up available to accelerate the driven equipment to operating speed. Some drivers have a fixed percentage of the rated torque available at start-up. It can be greater than 100%.
Peak Torque
This is the maximum torque available from the driver, it may not be able to operate for extended time periods at this torque.
Stall Torque
This is the system torque requirement that will cause the driver to come to a stop.
While all the torque values defined previously may exist within the system at some point in time, the torque requirements of the driven equipment are the primary consideration. The driver will not supply more
torque than the driven equipment will absorb or the driver can produce. Under some conditions the maximum torque within the system may exceed the driver capability, for example when brakes are energized.
A piece of driven equipment operating at its full speed capability requires a certain amount of torque. If the driven equipment is not operating at full speed the driver will supply additional torque until equilibrium is reached for torque and speed. The driver could still have additional capability, but it will not transmit it to the driven equipment. Other than at start up, the speed variation is small and subject to driver speed limits.
Drivers have speed limits that are imposed by physics or by trip devices. The physical limits can include the effects of frequency on an electric motor, or the effects of fuel restriction on an internal combustion engine, or steam availability to a steam turbine. Trip devices can include governors and over speed switches. The speed-torque capabilities of the driver are fixed by the design of the driver and the inputs to the driver.
Coupling Selection Torque Using the Driver Torque
The coupling can be selected based on the driver capabilities, using nameplate values or start-up torque. The capability requirements can be increased by an application service factor before choosing the coupling. This method of coupling selection usually results in a coupling that is oversized for the application, even if the service factor is 1.0. This translates into high cost and other problems. The reason oversizing results is twofold. First the driven loads may include equipment service factors that have already increased the torque value. Second, the driver is usually oversized. Drivers such as electric motors, come in standard sizes. If a piece of driven equipment requires a horsepower that is in between two standard sizes, the larger is chosen. Even when the requirements are right on the nose, the designer will usually pick a larger size out of conservatism.
When the coupling is chosen by driver horsepower one must be sure there is no gear reducer between the driver and the driven load. Gearboxes are constant horsepower devices that increase the torque or decrease the torque depending on the
gear ratio input to output. Other power transmission devices may do the same. In any case those types of devices must be accounted for in the coupling selection. Couplings selected using driver torque are normally mounted with one half on the driver shaft.
Using Driven Equipment Torque and a Service Factor
The coupling can be selected by using the normal operating torque of the driven equipment, adjusted for coupling location, multiplied by a service factor. Service factors are used to account for unknowns in the driven equipment system.
Service Factors
Sometimes "Service Factors" are called "Application Factors" or "Experience Factors". They have been empirically developed for most applications, or are known by their designer based on experience with their systems. Coupling manufacturers publish Service Factors based on their experience with their couplings on various systems. The factors are listed in coupling catalogs. Manufacturers may publish different service factors by product line. The catalog service factors will include factors for the application and the type of driver. Elastomer couplings sometimes include an environmental temperature service factor, and if intended for dampening vibratory torque, will have a frequency service factor as well.
AGMA Standard 9922-A96 lists service factors for many different applications. Service Factors are not the same as design Factors of Safety. Service factors deal with the unpredictable nature of the application, not with unknowns in the design of the coupling.
Depending on the selection of service factor this method also could result in an oversize coupling. Oversize couplings cost more and can result in bearing overload, excess inertia, premature wear and more maintenance.
Using System Torque with Little or No Service Factor
A coupling can be selected based on the exact requirements of the system. In this case the requirements must include all the torque values to be transmitted through the coupling. That can include starting requirements, braking requirements, peaks, transients and any others listed at the beginning of this section. Check the coupling manufacturers catalog as the coupling can have various torque capabilities. It may have one rating based on normal operation with another simultaneous rating for low cycles of peak torque. It may be acceptable to compare the peak system torque with a 1.15 Service Factor, to the yield strength of the coupling, and allow that as part of the acceptable selection. The coupling manufacturer should be consulted when the coupling selection is based on peak torque, emergency torque or a high transient that comes along only once. The coupling may already have sufficient reserve to satisfy those requirements on a limited number of occurrences.
If the coupling has only one published torque value, the coupling would have to be selected to meet the maximum torque expected in that part of the system. However, some types of couplings have torque ratings based on wear life, maximum misalignment combined with torque, or conservative considerations.
If the coupling is subject to cyclic torque or reversing torque, the selection should be based on those torque values. In the case of cyclic torque, use the high value. In the case of reversing torque, it will be necessary to check the coupling's fatigue life against the torque peaks and the acceleration/deceleration requirements associated with reversing operation.
The most economical selection will be based on the exact torque requirements of the system including peak, transients, breaking, or other expected torque values. Of course, this approach requires that all of the torque values be known with certainty. When the coupling has been sized to meet torque, it must also be checked for bore capability. Some bore-limited couplings might have the needed torque capacity but not enough bore capability to accept the shaft that will deliver it.
Likewise, some torque-limited couplings might have sufficient bore capability to accept the shaft but not be able to carry all the torque that the shaft will deliver.
Using Torque Information Coupling Catalogs
Coupling manufacturers have several methods of listing the coupling capabilities in their catalogs. These capabilities vary on each manufacturer's experience, design requirements, and testing capabilities. Torque capabilities found in the catalogs may have to be factored or reduced for misalignment, vibration frequency, temperature (elastomers), life (including elastomeric shelf life), or maximum torque. Such factors, if they are to be used, should be shown in the same catalog.
Most often the value shown in the catalog is the normal torque capability that the coupling can transmit over its design life.
Some couplings have a listed maximum torque. Usually that maximum value is used when the application might involve short cycle fatigue on couplings that have infinite life. Couplings that wear over time, such as a gear coupling, may have maximum capabilities that are quite large as long as their application keeps wear low. Couplings that wear may also offer alternate materials for reduced wear and longer life or for higher torque.
Because some coupling torque capabilities are limited by wear of the flex element and others limited by on fatigue of the flex element, it is best to understand the type of coupling that is to be used in the system before selecting the size. In addition to the flex element, coupling torque capability is affected by the method of securing shaft to hub, and any other joint in the unit, bolted or otherwise. Usually it is the flex element that is the limiting factor for the catalog torque values.
Useful torque equations:
Converting horsepower to torque T = BHP x 63025 / RPM Where
T = the torque in inch-pounds BHP = the motor or other horsepower
RPM = the operating speed in revolutions per minute
63025 = a constant used for inch-pounds, use 5252 for foot-pounds, and 7121 for Newton-meters Converting kW to torque
Where
T = BHP x 84518 / RPM T = the torque in inch-pounds kW = the motor or other kilowatts
RPM = the operating speed in revolutions per minute
84518 = a constant used for inch-pounds, use 7043 for foot-pounds, and 9550 for Newton-meters Determining the acceleration or deceleration torque
T = (Wk^2 x N) / (307 x t)
T = the torque to accelerate or decelerate in foot-pounds
Wk2 = the inertia of the piece to be accelerated or decelerated in pound feet squared
N = the absolute change of speed in RPM t = the time for the speed change in seconds
307 = a constant that allows the speed to be in RPM, the time to be in seconds and the torque and inertia to be in pounds and feet. It is a common form of the equation.