Shaft-mounted gearbox backstop selection should begin with the reverse load, not the motor nameplate. Prove that the stopped machine can back-drive, calculate the maximum credible reverse torque at the shaft where the backstop acts, and then verify the device’s torque rating, speed range, lubrication, mounting and free-running direction. Treat the backstop as a directional holdback—not as a service brake or an energy-isolation device.
The WS shaft-mounted gearbox is cataloged in the WS50–WS350 range and is selected from input speed, output speed, torque and shaft dimensions. The catalog does not assign a backstop capacity or direction to every application. Those details need a project-specific proposal and the selected manufacturer’s data.
Decide whether the machine can reverse
A backstop is relevant when gravity, process pressure or another stored load can rotate a stopped drive in the unwanted direction. Common examples include inclined belt conveyors, bucket elevators and some lifting or inclined process machines. A level conveyor whose resistance always exceeds its reverse-driving force may not need one, but that conclusion must follow a system assessment rather than appearance.
Map the stopped state:
| Question | Evidence required |
|---|---|
| What drives reversal? | Lifted material, belt tension, elevated buckets, counterweights or process force |
| What is the worst inventory? | Maximum credible load distribution at trip, not average throughput |
| Where does torque act? | Head shaft, low-speed gearbox shaft, intermediate shaft or high-speed shaft |
| What else restrains motion? | Brake, second drive or process resistance, with failure assumptions stated |
| What happens if it rolls back? | Personnel exposure, spillage, mechanical damage and restart difficulty |
CEMA’s belt-conveyor design chapter warns that a backstop is a safety device and that friction opposing rollback should not be overestimated. A clean, warm conveyor may have less resisting friction than the condition used in a casual calculation. Define the credible low-resistance case.
Calculate reverse torque at the mounting shaft
Use a conveyor or machine model that separates the force needed to lift the load from rolling, flexing and mechanical resistance. For an inclined belt, the vertical component tends to reverse the drive while some resistance opposes it. Tsubaki’s backstop selection method explicitly separates no-load, horizontal-load and vertical-load power before calculating reverse power and torque. Its equations and service factors belong to that manufacturer’s method; use the current instructions for the selected device.
Once credible reverse power at the mounting shaft is known, torque follows from:
T = 9,550 × P / n
where T is N·m, P is kW at the shaft and n is shaft speed in rpm.
Calculation example: assume a system study—not a WS product rating—finds 6.0 kW of maximum credible reverse power at a 48 rpm low-speed shaft. The static reverse torque is:
T = 9,550 × 6.0 / 48 = 1,194 N·m
If the chosen backstop supplier requires a 1.5 application factor for the stated duty, the selection torque would be:
Tselection = 1,194 × 1.5 = 1,791 N·m
The 1.5 factor is hypothetical here. Do not combine it with another motor-stall or service factor unless the manufacturer explains how the factors interact. The final device must be selected from its published permissible torque at the actual shaft speed and operating conditions.
Avoid the motor-torque shortcut
Motor rated torque can be a screening input, but it is not automatically the reverse holding load. A motor can be oversized, current-limited by a VFD, connected through a reducer, or subject to a breakdown torque that differs from gravity-driven rollback. Conversely, a high reduction ratio can make modest high-speed torque correspond to substantial low-speed torque.
Keep every value on a declared shaft basis:
- motor shaft torque and speed;
- gearbox input torque and speed;
- gearbox output torque and speed;
- driven-shaft reverse torque;
- backstop shaft torque and speed.
If the backstop is on a high-speed shaft, convert the reverse load through the actual gear ratio and the manufacturer’s approved efficiency method. Do not assume forward-drive efficiency applies unchanged during back-driving. Also verify that the gearbox itself can transmit the resulting locked load.
The related gear reducer ratio selection guide covers ratio, continuous torque, peaks and shaft loads. The backstop review is an additional load case, not a substitute for the gearbox rating check.
Specify direction from the correct viewpoint
“Clockwise” is incomplete unless the viewer’s position is defined. SEW-EURODRIVE’s X..e backstop instructions define rotation while viewing the low-speed output shaft and warn that starting the motor in the blocking direction can destroy the backstop.
Put these items on the purchase order and drawing:
- required direction of driven-machine travel;
- free-running direction at the backstop shaft;
- CW or CCW viewed from a named shaft end;
- gearbox mounting side and shaft orientation;
- a direction arrow on the delivered unit.
Do not reverse motor phases during commissioning until the backstop arrangement is understood. For Flender’s belt-conveyor gear units, the operating instructions require the phase sequence and defined direction to be checked before connection; changing blocking direction also requires manufacturer involvement for the described design.
Check speed, lubrication and release behavior
Backstops use different roller, sprag or centrifugal-release arrangements. Selection therefore needs more than torque:
- maximum permitted overrunning speed;
- minimum speed needed for centrifugal lift-off, if applicable;
- duty cycle and starts per hour;
- ambient and lubricant temperature;
- approved oil type, viscosity and fill arrangement;
- shaft fit, key or spline and axial retention;
- permissible orientation and sealing;
- inspection interval after an actuation.
A backstop that never reaches its specified lift-off speed may continue to rub and heat. A device integrated in a gearbox oil circuit cannot be treated as a dry external accessory. Use the backstop and gearbox manuals together, and do not add grease or change oil viscosity without supplier approval.
Resolve multiple-drive load sharing
Two drives do not guarantee that two backstops share reverse torque equally. Shaft twist, coupling stiffness, belt stretch, installation tolerances and engagement clearance can make one device take load before the other. One drive may also be out of service while the conveyor remains loaded.
For dual drives, send the supplier the complete arrangement and require a written selection basis. State whether one backstop must hold the full load, whether torque-limiting backstops are used, and how load sharing is verified. Do not divide the calculated torque by two without an approved analysis.
Commission the backstop as a safety function
Before coupling to the loaded machine, verify model, orientation, lubricant condition, rotation arrow and mounting fasteners. With the drive isolated and the manufacturer’s procedure available, confirm free rotation in the permitted direction without forcing the mechanism. Then verify motor phase sequence and jog only under an approved low-risk condition.
Run the conveyor in stages:
- no-load rotation and stop;
- controlled partial load;
- normal load;
- an approved stop test that demonstrates no unintended rollback;
- inspection for temperature, noise, leakage and fastener movement.
Record load, speed, stop sequence and observed reverse movement. Never create a live rollback test by defeating guards or placing people near the drive.
OSHA’s hazardous-energy guidance identifies mechanical and stored energy as service hazards. A backstop can remain loaded after electrical isolation. Block or lower the gravity load, isolate all energy sources, verify zero-energy state and follow the site procedure before removing the gearbox, torque arm or backstop.
The shaft-mounted gearbox installation guide covers hollow-shaft fit, torque-arm reaction, lubrication orientation and guarding. Those installation checks remain necessary after the backstop is sized.
Shaft-mounted gearbox backstop selection checklist
Require the proposal to state:
- maximum credible reverse torque and its shaft basis;
- applied service factor and source;
- rated backstop torque at the specified speed;
- free-running direction and viewing convention;
- maximum and minimum operating speed where applicable;
- mounting position, lubricant and temperature range;
- single- or multi-drive load-sharing assumption;
- inspection method and interval after engagement;
- backstop and gearbox model identifiers on drawings.
Reject a proposal that lists only motor power and “backstop included.” The acceptance record should connect the system load case to the exact supplied device.
Conclusion
Reliable shaft-mounted gearbox backstop selection proves the reverse load, converts it to torque at the actual mounting shaft, and checks rated torque, speed, lubrication, direction and multi-drive behavior. The backstop prevents rollback; it does not provide controlled braking or safe maintenance isolation. Submit the complete stopped-load case and shaft arrangement for supplier confirmation before ordering the WS drive.
