A Field Guide to Sizing a Shaft-Mount Reducer

Most gearbox problems on a conveyor don't start with the gearbox. They start with a decision made somewhere else: a sheave swap to squeeze out more tons an hour, a replacement sized off a nameplate that's no longer there, or a new reducer that went into service without oil.

The fix for all of them is the same. Understand torque, and size for the job the conveyor will actually do.

Torque in one sentence

Torque goes up when horsepower goes up at the same speed, or when output speed goes down at the same horsepower. That's the whole idea. A 10 HP motor turning a head shaft at 10 RPM puts far more twist on that shaft than the same 10 HP motor turning it at 100 RPM.

Torque (in-lb) = HP × 63,025 ÷ output RPM

For sizing, only two numbers matter: motor nameplate horsepower and RPM at the driven shaft. The gearbox ratio and the motor's own RPM are just how you get there.

Finding shaft RPM when you only know belt speed

On a conveyor you usually know the belt speed and can measure the drive pulley. Work backward:

Shaft RPM = belt speed (FPM) × 12 ÷ (3.1416 × pulley diameter in inches, including lagging)

A 375 FPM belt on a 20" drum with ⅜" lagging (call it 21" overall) turns about 68 RPM. With that number and the motor nameplate, you can size a reducer standing on the catwalk.

Service factor: the 1.4 rule

Service factor is the margin between what the motor can deliver and what the reducer is rated to handle. For a standard bulk-handling conveyor, that's 1.4 (AGMA Class II). A 10 HP motor wants a reducer rated for at least 14 HP.

Severe or critical duty moves you to 2.0 (Class III). Think full-load stop-and-start all day, heavy shock loads, or a conveyor the whole plant waits on. Use 2.0 when the application calls for it, not by habit. On larger drives it gets you a much bigger gearbox very quickly.

Speed matters as much as horsepower. In one Dodge example, a 15 HP motor at 70 RPM output lands on a size 4 Torque-Arm reducer. Slow that same 15 HP to 15 RPM and the selection jumps to a size 7. Same motor, three case sizes larger, because slower output means more torque.

The speed-up trap

Every plant eventually wants more throughput, and the quickest way to get it is to speed the belt up. Change the sheaves, keep the motor, keep the gearbox.

Here's what that does. Take a loaded conveyor on a 50 HP drive turning the head shaft at 50 RPM, with about 63,000 in-lb of torque available. Speed it up to 70 RPM on the same motor and available torque drops to about 45,000 in-lb. It will run. But the day it trips out fully loaded, it may not have the breakaway torque to restart, and somebody ends up shoveling off a belt.

Getting to 70 RPM and keeping the ability to restart loaded took a 75 HP motor in that example. Sometimes it means a larger reducer too.

Before any speed change, pull the motor's real amp draw at full load. A motor running at half its rated amps may have plenty of room. One running at full-load amps all day has none.

Design for next year's speed

On a belt-driven shaft-mount reducer, the V-belt drive between motor and gearbox is where you keep your options open. Specify a slowdown drive: a smaller sheave on the motor and a larger one on the reducer, with a little less reduction in the gearbox. That leaves room to speed up later without rebuilding the drive.

Avoid speed-up drives, where the motor sheave is larger than the reducer sheave. They can work within catalog ratings on smaller units, but on larger reducers they're a short road to an early failure.

What to have ready before you call

• Motor nameplate: horsepower, RPM, frame size
• Output speed at the shaft, or belt speed plus pulley diameter with lagging
• Shaft diameter at the reducer, and bearing centers if you're upsizing
• Incline or lift, and whether you need a backstop
• Mounting position and angle, especially on screw conveyors
• Environment: high ambient heat, heavy dust, moisture, high foot traffic
• Photos of the drive arrangement and any reducer nameplate still attached

Pictures help more than anything. Often the first person to look at an old gearbox has already torn the tag off, and nobody knows what it is.

One more thing: it ships dry

Every Dodge Torque-Arm reducer leaves the factory without oil, with a yellow tag that says so. Fill to the level for your mounting position before startup. On an inclined screw conveyor the correct fill changes with the angle, and too little oil isn't much better than none.

Let's size it together

Davis Industrial is an authorized Dodge distributor. Our solutions specialists work directly with Dodge's application engineering team, so a selection can be checked while we're still on your site. This month we're walking through the whole Torque-Arm family: TXT-E, Torque-Arm II and Motorized Torque-Arm II, and where each one belongs.

Schedule a free site walk, or call 813-247-3620.