Substituting a Gear Reducer With the Wrong Ratio
A bucket elevator is down. Its 15 hp, 1,765 rpm TEFC motor is direct-coupled to a 12.6:1 helical reducer that has failed; the reducer output drives the head shaft through a 1:1 chain drive (20T driver, 20T driven). At design belt speed the head shaft turns 140 rpm and requires 350 lb-ft. The stores cage holds one spare: 15 hp, 1,765 rpm input, 10:1 ratio, same shaft heights and bolt pattern, but no backstop. Production wants it running tonight. The head shaft bearings are limited to 200 rpm and the boot has 4 in of take-up left. Before the failure the motor drew 13.5 A on a 21 A nameplate FLA.
1. If the 10:1 reducer is bolted in with the existing 1:1 chain, what head shaft speed and power result, and is the motor adequate?
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Output speed of the spare = 1,765 / 10 = 176.5 rpm; with the 1:1 chain the head shaft also runs 176.5 rpm - 26 percent faster than design.
Elevator torque is set by the weight of material on the belt and the lift height, so it stays near 350 lb-ft:
- Design power = 350 x 140 / 5,252 = 9.3 hp
- New power = 350 x 176.5 / 5,252 = 11.8 hp (79 percent of a 15 hp motor)
The motor can carry it, and the head shaft bearings (200 rpm limit) are not exceeded. That is not the whole answer, though. Running 26 percent fast changes the machine: material is thrown further off the head pulley (discharge chute may no longer catch it), the boot and take-up see higher belt speed and more wear, the head pulley and bearings wear faster, and there is no margin left for start-up or a plugged boot.
Reducer capacity check at the new speed: 63,025 x 15 / 176.5 = 5,357 lb-in = 446 lb-ft against 350 lb-ft required - a service factor of only 1.28, and that assumes uniform loading. Acceptable, but it should be recorded, not assumed.
2. Re-ratio the chain drive so the head shaft returns to about 140 rpm. Give tooth counts and list what must be verified before it runs.
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Required chain ratio = 176.5 / 140 = 1.26.
Try 19T driver on the reducer, 24T driven on the head shaft: 24 / 19 = 1.263, so head shaft = 176.5 / 1.263 = 139.8 rpm. That restores the original belt speed and everything downstream of the head shaft.
Torque at the reducer output falls to 350 x 19/24 = 277 lb-ft, which improves the reducer's service factor to 446/277 = 1.61 - a better outcome than running it fast.
Verify before running:
- Chain capacity at the new pull and speed; the smaller driver sprocket raises chain tension slightly for the same torque (tension = torque / pitch radius).
- Sprocket fit - bore, keyway and hub length on both the reducer output and head shaft; a smaller driver also increases the overhung load on the reducer output bearing.
- Centre distance and chain length - changing tooth counts changes the chain length; keep the take-up adjustment within range and avoid an offset (half) link on a drive.
- Guard clearance for the larger driven sprocket.
- Confirm the new sprockets are a matched set for the existing chain pitch and that the chain is not worn past the point where it will run on new sprockets.
3. The spare reducer has no backstop. How does this affect the decision, and what would you require before energizing?
Read the worked answer
A loaded bucket elevator is a potential energy store: if the drive stops, the weight of the loaded belt will drag the head shaft and motor backwards. The backstop or holdback is a safety device, not a convenience. Running without it risks a runaway belt, a shock-loaded drive and material dumped out of the boot.
Options, in order of preference:
- Transfer the backstop from the failed reducer (if it is a bolt-on cartridge on the input or output shaft and the housings are compatible).
- If it is not transferable, install an independent holdback on the head shaft or a brake on the motor - a significant job, not a tonight job.
- Run only with the belt empty and a procedure that prevents a loaded stop, and only with written approval.
Also settle before energizing: rotation direction of the replacement reducer (a different helix hand reverses the output), oil level and grade per the reducer nameplate, coupling fit and gap, and the fact that the original reducer failed for a reason - pull the failed unit apart or send the oil for analysis before the same failure repeats on the spare.