A gear reducer that reversed when the motor stopped. The reducer was a helical-worm unit, 20:1 ratio, driving a inclined conveyor. The conveyor carried boxes up a 15-degree incline. When the motor stopped, the boxes rolled back down. The conveyor reversed. The boxes piled up at the bottom. The customer thought the brake was on the motor — but the motor didn’t have a brake. The worm gear should have self-locked. It didn’t. At 20:1 ratio, a worm gear isn’t self-locking. This is about overhauling loads and why backstops are required on inclined conveyors.
Self-locking vs overhauling
A worm gear is self-locking when the lead angle is smaller than the friction angle. For a typical worm, the lead angle is 2-4 degrees. The friction angle is about 3-5 degrees (for steel on bronze with oil). At low ratios (under 20:1), the lead angle is small. The gear self-locks. The load can’t drive the worm backward. At higher ratios (over 20:1), the lead angle increases. Wait — actually, higher ratio means more starts on the worm. The lead angle decreases. A 40:1 worm has a smaller lead angle than a 10:1. So the 40:1 is more self-locking. But the failed reducer was 20:1. Let me recalculate.
The lead angle for a 20:1 worm: the worm has 1 start (single thread), the gear has 20 teeth. The lead angle λ = arctan(lead / (π × worm diameter)). For a 40 mm worm pitch diameter, lead = π × 40 / 20 = 6.3 mm. λ = arctan(6.3 / (π × 40)) = arctan(0.05) = 2.9 degrees. The friction angle for steel on bronze is about 3.5 degrees (μ=0.06). The lead angle (2.9°) is less than the friction angle (3.5°). The gear should self-lock. But it didn’t. Why?
Because the friction coefficient drops when the oil is warm and the surfaces are worn. A new worm gear has μ=0.06. After a year of wear, μ drops to 0.03. The friction angle drops to 1.7 degrees. The lead angle (2.9°) is now larger than the friction angle. The gear overhauls. The load drives the worm backward. The conveyor reverses. The boxes roll down.
What was changed
1. Installed a backstop on the high-speed shaft. A sprag backstop (one-way clutch) mounts on the motor side of the reducer. It free-wheels in the forward direction. It locks instantly if the reducer tries to reverse. The backstop engages in milliseconds. The conveyor doesn’t reverse. The boxes don’t roll. The backstop costs $150. The cleanup from a pile-up costs $500.
2. Specified a brake motor. For new inclined conveyors, the motor is a brake motor (spring-applied, electrically released). The brake holds the load when the motor stops. The brake is rated for 1.5x the load torque. The conveyor stays put. The brake motor costs $200 more than a standard motor. It’s the standard for inclined conveyors over 5 degrees.
3. Used a helical gearbox (non-self-locking). A helical inline gearbox doesn’t self-lock at any ratio. The load reverses through the gears. For inclined conveyors, a backstop is mandatory. A worm gear that self-locks when new might overhaul after wear. The only reliable solution is a mechanical backstop or brake. Never rely on a worm gear’s self-locking property for safety-critical holds.
The backstop selection
| Type | Holding torque | Engagement | Use for |
|---|---|---|---|
| Sprag backstop (high-speed) | Up to motor torque | Instant (0.01 s) | Conveyors, elevators |
| Ratchet backstop | High, but with lash | 0.1 s (clicks) | Crushers, heavy loads |
| Motor brake | Motor rated torque | 0.05 s | Hoists, vertical axes |
| Counterbalance valve (hydraulic) | Adjustable | Instant | Hydraulic cylinders |
The safety angle
An inclined conveyor that reverses is a safety hazard. Boxes fall. People slip on spilled product. The backstop is a safety device, not a convenience. OSHA requires a hold-on device for inclined conveyors over 10 degrees. The backstop must be inspected quarterly. A sprag backstop wears out — the sprag springs fatigue. After 5 years, the backstop may slip. Inspect it: manually turn the high-speed shaft backward. It should lock instantly. If it slips, replace it. A backstop that doesn’t lock is worse than no backstop — it gives false confidence.
The backstop rule: every inclined conveyor over 5 degrees gets one, regardless of worm gear ratio. The reversing conveyor wasn’t a worn worm — it overhauled as the friction coefficient dropped. Install a sprag backstop on the high-speed shaft or use a brake motor. Never trust self-locking. Inspect the backstop quarterly. A pile-up of boxes is expensive; a backstop is cheap.
