Most gearboxes are not changed because they have failed. They are changed because the conveyor around them has grown - more tonnage, a longer belt, a heavier product, an extra shift - or because the case has reached an age where seals, bearings and spare-part lead times have quietly turned a reliable drive into a monthly conversation. Either way, the work happens on a line the plant is still trying to run, and the project succeeds or fails long before the crane arrives.

A retrofit is an engineering exercise with a procurement deadline attached. This guide covers what to measure before ordering, which details decide whether the new unit drops into the old space, how the mounting concept can change safely, and how to close the job so the next crew inherits a documented drive instead of a mystery.

Why Retrofits Go Wrong

The classic retrofit failure is not a mechanical one. It is an order placed from a nameplate. The old case carries a type string that tells you the family and the frame, but it says nothing about the ratio actually cut inside it, the service factor it was selected with, the shaft it was fitted to, or whether the machine has been modified twice since. An order built on that nameplate produces a gearbox that is dimensionally plausible and functionally wrong, and the error surfaces at installation, when the options are expensive.

The second common failure is a hidden dimension. Shaft height, the distance from the mounting face to the output shaft centre, is the number that decides whether the new case sits where the old one sat. Two frames from the same family can differ by tens of millimetres here, and a conveyor head station has no room to absorb that difference without moving the pulley or re-working the structure.

The third is a mounting concept change made without telling the structural side. A foot-mounted case that becomes an arm-mounted case changes what the steel sees, and the bracket has to land on something that was designed to take a reaction - not on a handrail post that happens to be in the right place.

The Survey: Six Numbers Before Any Quotation

A retrofit survey is short, and every number in it exists to answer a question the supplier will otherwise answer by assumption.

Output speed and ratio. Measure the driven shaft's speed with a tachometer at running load, or count it directly, rather than trusting the historical figure. Divide the motor's full-load speed by the measured output speed to get the actual ratio. Plants are regularly surprised: a line that "runs at 60 rpm" often runs at 54, and a ratio ordered from the assumption will change the tonnage.

Motor data. Power, full-load speed, frame size, shaft diameter, and how it is started. A motor that was replaced at some point is frequently a different power from the one in the original drawings, and the gearbox may have been living on borrowed margin ever since.

Output shaft condition. Diameter at the hub or bore position, runout, keyway width and depth, and an honest account of previous repairs - sleeving, build-up welding, a turned-down journal. This is the number that decides whether the existing shaft can be reused or has to be re-made.

Shaft height and mounting geometry. From the mounting face to the output shaft centre, plus the bolt pattern, the arm or bracket pick-up points and the direction of the reaction. Photograph all of it with a tape measure in frame; a photograph with a tape in it settles arguments that paragraphs cannot.

Available space. The gap between the case and the machine, the clearance for the coupling and its removal, the headroom for lifting gear, and the free space around the oil drain and level plug. Retrofits fail on clearance far more often than on torque.

Hours, ambient and duty. Shifts per day, days per week, the ambient inside the tower or the building, whether the drive sits under cladding, and the starting regime - empty belt, loaded belt, or the occasional restart after a trip. These set the service factor, and the service factor is what separates a frame that lasts from one that merely fits.

Ratio, Speed and the Trap of a Close Match

Suppliers work to standard ratio steps, and the old gearbox's ratio is very often not on the new list. When the closest available ratio differs by a few per cent, three consequences follow and all three should be discussed before the order is placed.

Belt speed changes with the ratio, which changes tonnage for a given belt loading and can affect a downstream process that was tuned to the old speed. On a metering or feed duty, a three per cent speed change is a process change. On a transfer conveyor it is usually invisible. Knowing which kind of line is being modified is the difference between an acceptable substitution and an expensive surprise.

Motor margin changes too. A slightly lower ratio at the same motor means slightly higher output speed and slightly lower output torque, which is usually harmless - but on a drive that was already running at its thermal limit, "usually harmless" is not a specification.

The cleanest approach is to state the target output speed and tonnage rather than the old ratio, and let the selection land on the closest step with the consequence explained. Where the process genuinely needs the exact old speed, a small change in pulley diameter or a different motor speed is often cheaper than a non-standard gear cut with a long lead time.

Changing the Mounting Concept

Many retrofits are also conversions. A foot-mounted case on a grouted pad becomes an arm-mounted case hanging from the structure, usually because the pad has settled, the floor space is needed, or the maintenance team is tired of re-shimming soft feet. The conversion is straightforward and it changes three things worth planning.

The reaction path changes from the floor to the structure. Someone who owns the steel needs to confirm that the chosen member can take the torque reaction, including the reversing case if the conveyor can run back. A bracket bolted to a brace that was sized for wind load is not the same as a bracket bolted to a column.

Alignment becomes adjustable rather than fixed. An arm-mounted drive is aligned by shimming the bracket and setting the coupling, so the bracket should be built with slotted or shimmed feet and the shim stack should be labelled and recorded. That record is what makes the next alignment check a twenty-minute task instead of an afternoon.

The coupling becomes part of the project. On a bore-mounted drive there is no coupling to specify; on an arm-mounted conversion there is, and its size, element type, bore range and installed length all have to be settled against the actual shaft and the actual gap.

Reusing the Coupling, the Motor and the Guard

Retrofit budgets are usually tight, and reusing what exists is legitimate - with a condition. A coupling can be reused if it is inspected properly: element condition, hub fit, keyway wear, and whether its torque rating still covers the new duty with the service factor applied. A coupling that was marginal on the old drive does not become adequate on a new one, and it is the cheapest component in the line to change while the crane is already there.

Motors are reused more often than gearboxes are, and the checks are simpler: insulation resistance, bearing condition, shaft and keyway, and whether the starting method still suits the new inertia. A new gearbox behind an old motor is a perfectly sound project, provided the motor is measured rather than assumed.

Guards are almost always re-made. The new case's coupling position, the guard's split line and its mounting points rarely line up with the old arrangement, and a guard that is fabricated on site after the drive is installed is usually a guard that does not come off again. Include it in the drawing package.

Planning the Installation Window

The mechanical work of a retrofit is a day or two. The project takes weeks, and the calendar is where most of the risk sits. Three planning habits keep the window short.

Order the consumables with the gearbox: oil to the correct specification and quantity, seals, the coupling element, and the fasteners for the bracket. A drive that arrives complete but waits three days for the right oil has cost the plant three days for no engineering reason.

Pre-fabricate the bracket and the guard from the survey drawing, and trial-fit the bracket to the structure before the shutdown if the line can be stopped for a few hours at a weekend. A bracket that has already been bolted up once will go back up in a fraction of the time.

Write the sequence down and give it to the crew: isolation, oil drain, coupling removal, case removal, bracket fit, new case landing, alignment, coupling fit, guard, oil fill, run-in. Every step has a tool and a torque figure attached. On a shutdown, sequence beats speed.

Commissioning Checks That Pay for Themselves

The first hours of a retrofitted drive are the only chance to capture a baseline, and the baseline is what makes the next five years of maintenance interpretable.

Two of these deserve emphasis on a retrofit specifically. Motor current tells you whether the new drive is working harder or easier than the old one, which is the fastest way to catch a ratio or alignment problem while the crew is still on site. And the oil sample from the first change is the only one that will ever be free of accumulated history - everything afterwards is read against it.

When a Retrofit Beats a Rebuild

A rebuild - new bearings, seals and inspection of the gearset in the existing case - is the right answer when the case is sound, the ratio is right, and the duty has not changed. It is cheaper and faster than a new unit, and on a drive that has simply reached its bearing life it restores decades of service.

A retrofit is the better answer when any of four things is true: the duty has grown beyond the old frame's margin, the ratio no longer matches the process, the mounting concept has become a maintenance problem, or the old case's parts are no longer obtainable on a sensible lead time. In those cases a rebuild restores a drive that will keep producing the same problems, and the plant pays twice.

The decision is usually made on a comparison of two quotations and one honest sentence about the duty. Where the duty has changed, saying so plainly is worth more than any discount on a rebuild.

Getting the Selection Right the First Time

Retrofit selections are cross-checked the same way new ones are: motor speed divided by required output speed gives the ratio, the duty class sets the service factor, and the frame follows. What makes a retrofit different is that the mechanical envelope is fixed in advance, so the selection has to satisfy torque and geometry at the same time.

Our engineers run that check against the survey numbers before quoting, and the online selector gives a first-pass frame and ratio in a few minutes: gearbox selection system. Bring the measured output speed, the motor data and the shaft height - those three numbers remove most of the risk from the project.

For a worked example of a frame specified for exactly this kind of conversion - coupled output, torque arm, mid-band conveyor duty - see the MC2PLST04 solid shaft gearbox, which is commonly ordered as a foot-mounted case's arm-mounted successor on long-running lines.

Frequently Asked Questions

Can a new gearbox be fitted to an existing shaft without re-machining?

Usually yes, if the shaft is measured rather than assumed. The work is in the survey: diameter at the hub position, runout, keyway width and depth, and the history of any previous repair. A shaft that has been sleeved or built up with weld can still carry a new hub, but the fit class and the hub bore have to be chosen for what the shaft actually measures, not for what the drawing says.

How long does a conveyor gearbox retrofit take on site?

The mechanical work is typically one to two days for a mid-size unit: isolation, drain, removal, bracket, landing, alignment, coupling, guard, fill and a short run-in. The project timeline is dominated by the survey, the quotation, the manufacturing lead time and the bracket fabrication. Plants that pre-fabricate and trial-fit the bracket usually complete the site work inside a single planned shutdown.

Do I have to change the motor as well?

Not usually. Motors are commonly reused after insulation resistance, bearing and shaft checks, and the decision turns on whether the starting method still suits the new drive's inertia and whether the motor's power still matches the duty. What must not be reused is a motor whose actual rating is unknown - measure it or read the plate, but do not inherit an assumption from an old drawing.

What if the closest available ratio is not the old one?

State the target output speed and tonnage instead of the old ratio, and ask for the consequence of the closest step to be explained. A small speed change is invisible on a transfer conveyor and significant on a metering duty, and where the exact speed matters, a pulley change or a different motor speed is often cheaper than a non-standard gear.

Is it worth keeping the old gearbox as a spare?

Rarely as a complete unit, often as documentation. A rebuilt old case takes up floor space and degrades in storage, while drawings, seal and bearing references and the ratio record cost nothing and shorten every future enquiry. If the old case is sound and the duty unchanged, a rebuild as a standby is a reasonable decision - just record what was done to it, or the standby becomes another unknown.