Gearbox failure on a critical line: the decisions in the first hour

It starts as a noise. A growl from the main drive on the line, louder on the morning shift than it was last night. By ten o'clock there's a knock in it. By half eleven the output shaft has stopped turning and the motor is still running.

The line stops. Everything behind it stops too. The production manager wants a time.

You don't have one yet. What you do have is about an hour to make four decisions. Get them right and you're running by teatime. Get them wrong and you're still waiting for a part on Friday.

Decision one: is it really the gearbox?

A motor spinning with a still output shaft looks like a gearbox. It isn't always.

Before anyone orders anything, check the cheap things. Has a shear pin gone, or a keyway sheared? Has the coupling spider broken up? Has a taper lock slipped on the shaft? Has a torque limiter done its job? Each of those stops the drive and looks like a gearbox failure from the walkway. Each is a ten-minute fix.

Then look at the gearbox itself. Is there oil on the floor? What does the drain plug magnet show? Is there play on the output shaft when you rock it? Metal flakes on the magnet and play in the shaft mean the inside is gone. Our guide to coupling failures covers what to look for on the drive train.

Decision two: make it safe to work on

A failed gearbox has some traps of its own.

The oil can be hot enough to burn. HSE's maintenance guidance says to "allow components that operate at high temperatures time to cool", and to "support parts of plant that could fall" (HSE, maintenance). A gearbox that held up a lift, a bucket elevator or an inclined conveyor may have been the only thing holding the load. Take the gearbox out and the belt runs back, or the table drops. Chock it, pin it, or run it down to its stops before a single bolt comes out.

Then think about the lift. A big gearbox can weigh more than a person can safely handle, and they're rarely in easy places. The Manual Handling Operations Regulations ask you to "avoid the need" for risky manual handling where you reasonably can (regulation 4). So that means a chain block, a beam, or a forklift with a jib. Under LOLER, every lifting operation has to be "properly planned by a competent person", "appropriately supervised", and "carried out in a safe manner" (regulation 8). Rigging a chain block off a bit of pipework at midnight is none of those.

Decision three: repair, swap or wait

This is the big one. There are usually three routes.

Swap from stock. If you hold a spare, this is almost always fastest. Check it really is the same: ratio, mounting, shaft size, hand of output, and the oil it was filled with. A spare that's sat for years may need fresh oil and a turn by hand before it goes in.

Fit something that will do for now. A gearbox supplier may have a unit with the same ratio and a different mounting. With an adapter plate, it might get you running for a few weeks. Treat it as temporary, and write it down as temporary. Our post on breakdown or bodge explains why.

Repair or wait. A rebuild or a new unit can take days or weeks, especially on older or foreign kit. If that's the only route, you need a plan for the line while you wait. See the part you need isn't on the shelf.

Here's where the cost of downtime should drive the choice. If the line costs you thousands an hour, a temporary unit and a weekend fitter's time are cheap. Have that number worked out before the day it happens. Our guide to what an hour of downtime really costs shows the sum.

Decision four: who does the work

A gearbox swap sounds simple. Unbolt, lift, lift, bolt up. The bit that decides whether it lasts is the alignment.

Shaft alignment with a laser or dial gauges is a skill. So is setting chain tension, fitting couplings without a hammer, and checking soft foot. Get it wrong and the new gearbox runs hot, chews its seals and fails early. Our guide to shaft alignment basics covers the method.

HSE's code of practice for PUWER says maintenance work should only be done by people "competent to do the work" (L22, paragraph 79). On a weekday, your best fitter is on it. On a night shift, the only person there may be an electrician or a newer fitter who has never aligned a drive. HSE's maintenance page says to "decide if the work should be done by specialist contractors" before you start. Ask the question in the first hour, not the fifth.

Here's our view. A big drive swap on a critical line is a two-person job with an experienced mechanical fitter leading it. If you don't have one on shift, get one in. Paying for a few hours of the right person costs far less than a second failure in a month.

Before it goes back

Fill it with the right oil, to the right level. Check rotation before you couple up. Run it with no load, then light load, and listen. Take a temperature reading on the case after an hour, and again after a shift.

Then keep the old one. Strip it, or have the supplier strip it, and look at what failed. Bearings, teeth, seals or oil. That tells you whether it wore out or was killed. If it was killed, something is still wrong. Our guide to a gearbox running hot is the place to start.

What to do on Monday

1. List the gearboxes on your critical lines, and next to each, write whether you hold a spare.
2. Check every spare: ratio, mounting, shaft and oil. Turn it by hand.
3. Find out where each big gearbox would be lifted from, and what with, before you need to.
4. Name who on each shift can align a drive, and who you'll call when they're not there.

* Paragraph numbers refer to L22, fourth edition (2014, amended 2018).