1. The Field Problem: Rated Torque Is the Wrong Starting Point

A 1,200 mm wide inclined yard conveyor at an aggregate plant was fitted with a backstop bearing selected on a simple rule: "take the motor rated torque, add 30 percent." Eight months later the belt rolled back 4.5 m during a power failure, the loading chute plugged, and the gearbox input shaft backstop showed heavy sprag marking. The bearing had not been overloaded by the motor. It had been grossly undersized for the reverse holding torque, which has nothing to do with motor power and everything to do with gravity acting on a loaded inclined belt.

This is the single most common selection error we see in conveyor backstops. Motor torque governs driving; gravity governs holding. On a loaded incline, the holding case is frequently larger than the driving case, and it exists at zero speed — precisely where a backstop must do its only job.

2. Mechanism: Where Reverse Holding Torque Actually Comes From

When an inclined conveyor stops with a full belt, the material and belt mass create a force component acting down the slope:

F_rev = (m_belt + m_material) × g × sinθ

where masses are per metre of conveyor, g = 9.81 m/s², and θ is the incline angle. Friction in idlers and pulleys slightly reduces this load, so ignoring friction is the conservative and correct choice for holding-torque sizing.

This linear force is converted to torque at the drive pulley:

T_pulley = F_rev × R_p

and referred to the shaft where the backstop is mounted. Because most backstops sit on the gearbox high-speed (input) shaft, the torque is divided by the gear ratio and multiplied by gearbox efficiency:

T_hold = (F_rev × R_p) / (i × η)

Three multipliers then have to be applied, and skipped multipliers are what destroy bearings in service:

• Shock factor K_a = 1.3–1.8 — emergency stop with a loaded belt produces elastic recoil; the belt stretches, then snaps back against the locked sprags. Fast stoppages and long, high-tension belts push toward the upper end.
• Load distribution factor — an unevenly loaded belt, or discharge plough, can concentrate material on one side.
• Degradation margin 1.2 — sprag/roller contact surfaces lose capacity as surface finish and lubrication degrade over years of service.

Metal-to-metal sprag backstops must also be checked for centrifugal lift-off speed: above a certain freewheeling speed the sprags lift off the inner race and the unit no longer holds. A backstop sized purely on torque can pass every static calculation and still fail dynamically.

3. Worked Example: 320 m Incline, 8°

Design data: conveyor length 320 m, incline 8°, belt mass 22 kg/m, material load 60 kg/m, head pulley diameter 800 mm (R_p = 0.40 m), gearbox ratio i = 25, efficiency η = 0.95, backstop on the gearbox input shaft.

The selected unit therefore needs a holding torque of at least 1,084 N·m at the input shaft, not the 240 N·m that the "motor torque + 30 percent" rule would have produced. That is a 4.5× discrepancy, and it explains most field failures on this type of conveyor.

4. Engineering Solutions

• Do not upsize blindly. A larger sprag unit needs a larger oil flow and has higher drag torque; on long freewheeling runs that means heat.
• Consider mounting position. Moving the backstop to the low-speed shaft removes gearbox backlash from the holding path and eliminates sprag lift-off risk, but torque rises by the gear ratio — usually a roller-type, large-bore backstop becomes the practical answer.
• Limit reverse speed. A controlled brake that brings the belt to rest in a defined ramp reduces K_a from 1.8 to about 1.3 and is often cheaper than the next bearing size up.
• Check the torque arm. The reaction from the holding torque must reach the frame through a defined load path; a torque arm in bending rather than tension will deflect and preload the sprags unevenly.

5. Series Selection and Verification

For the worked example above, an RSBW unit on the input shaft is the natural fit; if the customer insists on low-speed mounting, the specification moves to an FRHD roller design sized for the full pulley torque of 14.3 kN·m plus shock.

What to demand from the supplier: reverse holding torque certificate at the rated value, the shock factor the rating is based on, centrifugal lift-off speed, drag torque at maximum freewheeling speed, torque-arm reaction load and direction, oil flow/grade requirement, and the material and hardness record for sprags and races.

6. FAQ

Q: Can I size the backstop from the motor nameplate torque?
A: No. Motor torque sizes the drive; gravity on the loaded incline sizes the backstop. On steep or heavily loaded belts the two differ by a factor of three to five.

Q: Is a higher holding torque always safer?
A: Not automatically. Oversizing raises drag torque and heat in freewheeling duty, and can push the sprags below their minimum engagement load, causing skidding rather than locking.

Q: Should I add the shock factor to the catalogue rating?
A: Yes — compare your shock-inclusive, degradation-adjusted torque with the catalogue holding torque, not with a nominal "peak" figure whose definition you cannot verify.

Q: How do I confirm the calculation is right before shipment?
A: Ask for a reverse locking test at rated torque with the test record signed, and request the torque-versus-slip curve so you can see margin, not just a pass/fail stamp.