Checking free play

Part 1. Preparation and safety

Objective

• Check a final drive unit for wear and free play.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

• Work clothing - such as coveralls and steel-capped footwear
• Eye protection - such as safety glasses and face masks
• Ear protection - such as earmuffs and earplugs
• Hand protection - such as rubber gloves and barrier cream
• Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

• Make sure the vehicle is safely supported on a suitable vehicle hoist. The wheels need to be free to turn.
• Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

• While final drive components must be held firmly in position by bearings and thrust washers, there must be operational clearance between meshing components. This check determines the amount of free movement between the crown wheel and pinion.
• The alignment relationship of the crown wheel and the pinion is very important. Misalignment will cause noisy operation and uneven wear.
• Misalignment will produce a high-pitched whine. It will be evident during one of three driving conditions:
  • Drive: where the vehicle is under power
  • Float: where the vehicle is being driven at a constant speed, under very little power
  • Deceleration: the vehicle is slowing down under engine braking
• If the noise is evident in an older vehicle, it will be caused by wear between the crown wheel and pinion. If the noise is evident on a new vehicle, it is caused by misalignment.

• Once the gears have set a wear pattern, the noise can be only be removed by re-machining the crown wheel and pinion gear teeth, and correctly reinstalling the components.
• When taking measurements, make sure the dial indicator is firmly attached to a solid component.
• If the measurements are outside the manufacturer's specifications, refer to your supervisor for further instruction.

Part 2: Step-by-step instruction

1. Mark corresponding points
   With the vehicle raised and the wheels in a condition where they can be rotated by hand, mark a point on the axle casing and also make a corresponding mark on the flange.
2. Mount a dial indicator
   Mount a dial indicator onto the axle casing and clip a metal tag onto the pinion flange. Different manufacturers have ways of doing this, so follow the vehicle service manual procedures for the vehicle you are working on.
3. Check amount of movement
   Hold the road wheel by putting one hand on it, and then put your other hand around the drive shaft, so you can twist it back and forth. Feel and note the amount of movement before the road wheel begins to turn.
4. Zero dial indicator
   Move the drive shaft to one end on the movement limit, and zero the dial gauge. Now rotate the drive shaft until it reaches the other end of the free movement limit. That is before the road wheel starts to move again, and note the distance moved reading on the dial gauge.
5. Check specifications
   Check the reading against the allowable movements specified in the vehicle service manual. If the reading is outside specifications, report it to your supervisor. On four wheel drive vehicles; repeat this procedure on the front final drive unit.