Backstop Clutch Selection Guide
INFORMATION FOR SELECTION
BACKSTOP SELECTION Backstop clutches by definition are required to hold back a load from moving in a reverse direction. Care must be taken in calculating the torque requirements and should be based on maximum or worst case conditions and not averages or normally seen loads. Because the failure of a backstop or holdback clutch might result in dam- ages, take time in considering all the possible loadings and select appropriate service factors. Below is more than one selection formula; it is generally advised to select the Cam Clutch that provides the largest safety factor.
General Selection Method: A) Calculate the static torque reverse motion based upon the maximum load expected and multiply it by the service factor. Selection is based on the formula to the right. B) Select the clutch by:
Required Torque x Service Factor = Design Torque
The torque capacity of the selected Cam Clutch must be greater than the design torque requirement, must accept the maximum overrunning speed, and be suitable for the bore and installation method required.
1) Design torque requirement 2) Maximum overrunning speed 3) Bore size and installation method
Motor Stall Torque Selection Method: Another method commonly used to select the proper backstop clutch size for conveyors is to use the motor name plate ratings plus the motor's ability to produce excess torque. Depending on the motor size, it may develop over 300% of rated torque. After stalling an overloaded conveyor can overload the backstop. For proper selection of the backstop, all facets of the mechanical system should be considered to ensure that the backstop is not the weakest link in the conveyor drive. If the motor breakdown torque is not known, refer to the motor manufacturer.
Selection is based on the following formula:
Motor power hp x 5250 Shaft speed N (r/min)
S < T max _
Motor stall torque T(lb.ft.) =
x
or
Motor power kW x 9550 x Shaft speed N (r/min)
< T max _
S
Motor stall torque T(N • m) =
S = Service Factor
T max = Torque capacity of the Cam Clutch and must be greater than the motor stall torque
Select service factor from table below: % of Normal motor rating
Service factor
175% 200% 250% 300%
1.30 1.30 1.67 2.00
NOTE Always allow for the maximum possible load in your calculations, since backstopping often occurs when the conveyor is loaded above its normal loading capacity.
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