Shaft Mounted Speed Reducer (SMSR): Sizing, Torque Arm, Taper Bush & Ordering Guide

How to specify a shaft mounted speed reducer for conveyors, bucket elevators and crushers — 5:1, 13:1 and 20:1 ratios, 0.5–50 kW, torque to 17,000 Nm, torque arm rules, taper bush selection and what to send for a quote.

Short answer: a shaft mounted speed reducer (SMSR) mounts directly onto the driven shaft of a conveyor or elevator, held against rotation by a torque arm. It removes the foundation, the base plate and the coupling alignment job entirely. Super Mech Industries manufactures SMSR units in Ahmedabad in nominal ratios of 5:1, 13:1 and 20:1, covering 0.5 kW to 50 kW with output torque up to 17,000 Nm, in cast iron housings with hardened and ground gears.

It is the default drive on belt conveyors, bucket elevators, crushers and screens in mining, quarrying and bulk material handling — precisely because those plants are dusty, hard to level and rarely have a good place to bolt down a foot-mounted gearbox. This guide covers how to size one, the two details that decide whether it survives (torque arm and taper bush), and what to send us to get a real quote.

Why shaft mounted, instead of foot mounted

A conventional foot-mounted gearbox needs a machined base plate, shimming, a coupling, and an alignment procedure that has to be redone whenever the structure settles. On a conveyor gantry in a quarry, that is a recurring maintenance cost. The SMSR sidesteps all of it:

• No base plate and no foundation — the reducer hangs on the shaft it drives.
• No coupling alignment — there is no coupling on the output side at all.
• Taper bush mounting — fits and comes off without a fight, and suits a range of shaft diameters.
• Torque arm reaction — one anchor point takes the reaction load into the structure.
• Tolerant of harsh sites — cast iron housing, sealed, designed for dust and weather.

The V-belt drive between motor and reducer input adds a second useful property: it is a soft start and an overload fuse. Belts slip before gear teeth break, which on a crusher feed is a meaningful protection.

SMSR specification range we manufacture

Photographs and further details are on the Shaft Mounted Speed Reducer product page. Frame dimensions, bore ranges and torque arm geometry vary by size — ask for the dimension sheet for the frame you are considering before finalising the gantry design.

Sizing an SMSR: the sequence

Step 1 — output speed sets the total reduction

Start at the conveyor. If the pulley must turn at 36 RPM and your motor runs at 1,440 RPM, the total reduction needed is 1,440 ÷ 36 = 40:1. That total is shared between the V-belt drive and the gear set. With a 20:1 reducer, the belt drive supplies the remaining 2:1. This split is the reason only three nominal gear ratios are needed to cover almost every conveyor speed — the belt pulleys do the fine tuning.

Step 2 — output torque decides the frame

Use the standard relation:

T (Nm) = 9,550 × P (kW) ÷ n (RPM)

An 11 kW motor driving an output shaft at 36 RPM gives 9,550 × 11 ÷ 36 ≈ 2,918 Nm. That is the number to compare against the reducer's torque rating — not the motor kW on its own. Two drives with identical motors need different reducers if their output speeds differ.

Step 3 — service factor for the real duty

Bulk handling is not uniform duty. Material surges, chutes block, belts start under load. Apply a service factor before selecting:

Select on the service-factored torque. Under-selecting here is what puts a reducer on a truck back to Ahmedabad eight months later.

Step 4 — check the starting condition

A loaded belt that has been standing overnight takes far more torque to break away than to keep running. Direct-on-line starting can put two to three times full-load torque through the drive for a second or two. If the belt starts loaded, say so in the enquiry — it changes the selection, and it may be an argument for a soft starter or a fluid coupling ahead of the belt drive.

The torque arm: the detail that decides survival

The reducer hangs on the driven shaft. Nothing else holds it. When torque is applied, the reaction wants to rotate the entire housing around that shaft. The torque arm is the only thing preventing it, and it is where most field failures originate.

Rules that matter

• Anchor to something structural. A main frame member or a purpose-built bracket welded to it. Never to sheet cladding, walkway grating, handrail, or a bracket that flexes visibly under hand pressure.
• Keep it in tension where possible, and roughly perpendicular to the line from the shaft centre to the anchor point. A torque arm at a bad angle multiplies the load on the anchor.
• Use the resilient mounting. A rubber-bushed or spring-loaded turnbuckle end absorbs shock and start transients. A rigid rod bolted solid at both ends transmits every jolt straight into the housing and the structure.
• Do not leave it slack. A loose torque arm lets the gearbox swing on every start and then hammers when it takes up. It fatigues the anchor and the housing lug.
• Check the direction. The arm must resist the actual reaction direction. Reversing the motor without repositioning the arm puts the arm in compression when it was designed for tension.

If the torque arm anchor point is difficult on your structure, send us a photograph of the mounting area with the enquiry — the arm length and orientation can often be adapted at manufacture, which is far cheaper than modifying the gantry on site.

Taper bush mounting, done properly

The reducer's hollow output bore grips the driven shaft through a split, tapered bush. Tightening the bush screws pulls the taper into the bore, which contracts the split sleeve onto the shaft and creates a genuine interference fit. Torque is carried by that friction grip with the key as a backup, not the other way round.

Fitting checklist

1. Clean and dry every surface — shaft, bush bore, bush taper, and the reducer bore. No oil, no grease, no anti-seize on the taper faces. Lubricant on a taper is what makes bushes slip under load.
2. Check the shaft. The shaft must be to size and free of burrs, weld spatter and old key damage. A worn shaft will not grip.
3. Tighten the screws progressively and evenly, in rotation, to the specified torque — not one screw at a time to full tightness.
4. Run briefly, stop, and re-check the screws. Bushes settle on the first load cycle. This one step prevents most early slips.
5. To remove, back the screws out and use the jacking hole provided. Do not attack it with a hammer and drift; that damages the bore and often the shaft as well.

Give us the driven shaft diameter and keyway with your enquiry. If the shaft is a non-standard or worn size, tell us the measured diameter — we machine bushes to suit rather than sending a bush that "nearly" fits.

Backstops on inclined and elevator drives

On any inclined conveyor or bucket elevator, a power failure with a loaded belt means the load tries to run the drive backwards. Buckets empty into the boot, belts avalanche, and people near the tail get hurt. A backstop is a one-way clutch that prevents reverse rotation.

Two things to get right at order time:

• Specify it up front. Retrofitting a backstop is more expensive than ordering it fitted.
• State the direction of rotation, viewed from a defined side — normally looking at the output shaft. A backstop assembled for the wrong hand locks the drive completely and cannot simply be turned around on site.

Where SMSR units are used

• Belt conveyors — the classic application, drum-driven with the reducer on the pulley shaft.
• Bucket elevators — head shaft drive, almost always with a backstop.
• Crushers and screens — high shock, high service factor, and where the belt drive's slip protection matters most.
• Mining and quarrying plant — dust, weather and no convenient foundation.
• Cement, aggregate and bulk material handling — long runs of identical drives, where a standard three-ratio range keeps spares simple.

For the drive-side connection between motor and reducer input on higher-power installations, see our gear couplings, and for the pulleys and belts on the input drive our V-belt pulleys. On elevator and inclined conveyor heads, a hydraulic thruster brake is often specified alongside the backstop for controlled stopping.

Common failures and their real causes

• Housing cracked at the torque arm lug — rigid torque arm with no resilient end, or an anchor that flexes. Fatigue, not overload.
• Reducer spinning on the shaft — taper bush fitted with lubricant on the taper, or screws never re-checked after the first run.
• Output seal leaking — misaligned torque arm loading the output bearing sideways, or a shaft that has worn a groove under the seal lip.
• Gear tooth pitting inside a year — under-selected for the actual duty, usually because the service factor was skipped or the starting condition was not declared.
• Belts burning out repeatedly — the belt drive is doing its job as an overload fuse. Look upstream at what is jamming, before uprating the belts.

What to send us for an accurate quote

1. Motor power in kW and speed in RPM.
2. Required output (pulley or head shaft) speed in RPM.
3. Driven shaft diameter and keyway size.
4. Nominal ratio you want — 5:1, 13:1 or 20:1 — or let us pick from the speeds above.
5. Backstop required? If yes, direction of rotation viewed from the output side.
6. Driven equipment and duty — conveyor, elevator, crusher; hours per day; loaded or unloaded starts.
7. Do you also need the motor mount, V-belt drive and guard supplied?
8. Quantity and delivery location.

A photograph of the existing unit's nameplate, plus one of the mounting area showing the torque arm anchor, usually answers half of the above in one message.

Why Super Mech Industries

We are an ISO 9001:2015 certified manufacturer in Ahmedabad, Gujarat, exporting to 29 countries, making gear units, couplings, brakes, pulleys and sprockets in one facility. For a bulk-handling drive that means the reducer, the V-belt pulleys, the coupling and the brake can be quoted and shipped together, matched to each other, with one point of contact if anything needs attention afterwards.

Frequently asked questions

What ratios does a shaft mounted speed reducer come in?

We manufacture the three standard nominal ratios — 5:1, 13:1 and 20:1 — covering 0.5 kW to 50 kW with output torque up to 17,000 Nm. Intermediate output speeds are set with the V-belt drive between motor and reducer, not by changing the gear set.

Why does an SMSR need a torque arm?

The gearbox hangs on the driven shaft with no base, so reaction torque would rotate the housing around the shaft. The torque arm anchors that reaction into the structure. It must be fitted, correctly oriented and anchored to something rigid.

What is a taper bush and why is it used?

A split, tapered sleeve between the hollow bore and the driven shaft. Tightening the screws draws the taper in and grips the shaft with a true interference fit — so one bore suits a range of shaft sizes, torque is carried by friction rather than the key alone, and removal later is straightforward.

Do I need a backstop on a bucket elevator?

Yes, on any inclined conveyor or elevator where a loaded belt would run backwards on power loss. Specify it with the order and state the required direction of rotation — a backstop fitted for the wrong hand locks the drive.

Can you supply the complete drive, not just the reducer?

Yes — reducer, motor mount, V-belt drive, pulleys, guard, backstop and brake. Tell us what you want included and we will quote it as one package.

Do you supply outside Gujarat?

Yes. We despatch across India from Ahmedabad and export to 29 countries. Include the delivery pincode or port and we will quote freight and lead time with the unit.

Super Mech Industries — Odhav, Ahmedabad, Gujarat 382415, India. Phone: +91 63510 70577. Email: smigvs@gmail.com. ISO 9001:2015 certified.