Bachmann drive coupling repair

One very common problem is with the drive couplings on the Bachmann range of locomotives. Symptoms for this fault are that the locomotive will run OK at slow speeds but as the speed increases the motor spins faster and the loco actually slows down. Initial symptoms could be that this fault occurs in only one direction. There may also be a screeching sound as the engine slows down with more power being applied.

The fault is caused by a plastic sleeve that takes the drive from the motor or driveshaft into the final drive assembly. This small plastic sleeve is a push fit over a splined shaft. The plastic sleeve cracks, opens slightly and can no longer grip the splined shaft to transmit the drive power. The plastic sleeve simply spins on the splined shaft.

Below are a series of pictures showing a repair of the drive unit from a faulty Bachmann Class 221 DMU 221122 Virgin Trains model.

This is the drive unit of the Bachmann Class 221 DMU 221122 Virgin Trains model. It contains the motor, drive assemblies and the electrical power pick-up.

With the body removed you can see the motor in the centre of the chassis and the two power bogies. Two driveshafts with universal joints take the drive from the motor to the power bogies.

This is a close up showing the grey plastic drive coupling in location at the top of the drive bogie. This grey plastic sleeve connects the drive shaft at the bottom of the picture to the worm gear shaft in the drive bogie. It also acts as a universal joint allowing the bogie to rotate and float a little.

There's the fault. The grey plastic moulding has clearly cracked and no longer grips the steel driveshaft to transmit the drive power. It will grip at slow speeds but slips at higher motor speeds. This causes the train to go slower and not faster as more power is applied. Also, there is a screeching sound as the plastic moulding spins on the splines of the steel shaft.

...and just in case you thought this is an easy repair. This is the bogie drive unit taken apart to carry out the necessary work.

At least it's a great opportunity to clean and re-grease everything.