Final Drive - Differential

The assembled diff.

For this week, we are studying final drives in practical class. We have Gary for a tutor.
The differential that we are are taking apart and measuring is a banjo type diff. This type is where the diff gears come out of the body of the diff. As opposed to a salisbury type where we have to spread apart the body to get the gears out.
It is the hypoid type, where the centre line of the pinion gear is below the centre line of the crown wheel. This is opposed to the spiral bevel type, where the centre line of the pinion gear is on the same line as the centre line of the crown wheel.
It is also a non limited slip type, which opposed to the limited slip type which has clutch packs and springs which are used to lock the two axles together.
There was no visible identification on the final drive.

Marking the teeth with the
teeth marking solution.

There are centre punch marks on the diff housing caps for alignment, to ensure they go back in the correct way.

There is also a white painted mark on the crown wheel and carrier to ensure that gets lined up the correct way. Starting to dismantle the differential, we first removed the diff case assembly. While pulling the carrier out, be aware that the bearing caps will fall out.
Next we removed the bolts off the crown wheel and removed the crown wheel itself.
Then we were to remove the pinion gear. We first had to remove the pinion nut and flange, and then push the pinion out through the case. We then used the handle of a hammer to remove the seal and the pinion bearing.

Now comes the inspection part of the process.

The crown wheel, adjusting nuts,
and bearing caps.

We checked the condition of the crown wheel teeth. It was relatively good, apart from a very small chip in one tooth, and very slight wear on the teeth due to the wrong type of oil used. The chip on the tooth could have been caused by the cross shaft coming out and hitting it.
The condition of the crown wheel bolts are all good for service.
The condition of the threads in the case were also all good and ready for service.

Next was to remove the cross shaft, spider and side gears and thrust washers.
We tapped the roll pin out and then removed the cross shaft. We were then able to remove the spider gears, and then remove the side gears. Also the thrust washers came out.

The diff housing with the
crown wheel carrier removed.

We then had to inspect these parts.
The side gears had pitting on the teeth. This is where there gets rust spots on the teeth. This oxidises and turns blackish and it wears through the hardening of the teeth and then excessive wear is caused.
The side gear thrust washers are in good condition and ready for service.
The spider gears and spider gear thrust washers are in good condition and ready for service.
The cross shaft has a bit of where on the shaft where the spider gears run. This is a black colour. This is caused by the diff sitting around not being used and water in the oil.
The roll pin had a slight bend in it. However this is alright as the bend can apply tension to help hold the pin in, however a bent roll pin can indicate that the cross shaft has moved.

The crown wheel bolts removed.

The next step in this task was to calculate the final drive ratio. The pinion has 9 teeth and the crown wheel has 41 teeth so the calculation is 41/9 which gives an answer of 4.56 and a ratio of 4.56:1. This means that the crown wheel will turn once for every 4.56 turns of the pinion.

We then had to measure the side gear backlash clearance. We measured this with a DTI  and got .18mm and .09 mm for the two gears. This is clearance is good for service as it is within the specifications of .05 to .2mm.
If this clearance was to great or small, this could indicate that new thrust washers would be needed. This backlash is there to allow for the metal to expand under heat.
Note 1: When clamping the carrier in the vice, do not clamp the mating surfaces as the cross hatching on the vice jaws  can dig into the surface and cause it to expand minimally and then cause the mating part to be off balance.

Undoing the nut to get the
pinion gear out.

Note 2: When assembling gears, damaged thrust washers can cause the gears to jam up and not turn.

We then refitted the crown wheel, being sure to align the wheel to the carrier by paint marks. We torqued down the bolts in a star sequence to the manufacturers specs of 97Nm. (actually 35 because its half to protect threads).

We then inspected the condition of the pinion gear. The teeth, splines, thread and bearings were all in good condition. But because this diff has a collapsible spacer, it can only be used once and would need replacing, but for the purpose of the exercise, that one was not replaced.
The next task was to re assemble the pinion into the diff housing. We oiled the bearings and put it in place with the collapsible on the shaft. We then put the washer and seal in and wound up the nut. We then had to set the pre load on the pinion bearings using the inch pound torque tester. The pre load was set to 7.8 inch pounds, which is manufacturers specification. If we were using a new collapsible spacer, we would not have had to wind the nut up so much, as it would be a fresh, new spacer.

The driveshaft flange.

Preload on the pinion bearings, is there to stop the pinion gear from moving in and out of mesh.
Once the pre load was set, we were able to replace the crown wheel carrier into the housing.
We refitted the bearing caps along with the adjuster nuts. When fitting the bearing caps, it is essential that the adjuster nuts are free to rotate in their threads, otherwise they will not rotate when the caps are torqued down.
We then tightened up the adjusting nuts so that were was no sideways play in the crown wheel.
Next we had to check the backlash of the crown wheel. At the four different points around the crown wheel, we got readings between .38 and .40 mm. The specifications are between .13 to .18mm. Our readings were to high, so we adjusted the adjusting nuts, moving the crown wheel further into mesh. We then took the readings again, and got readings between .13 and .16mm. This is within specs so this is ok and good for service. If there was not enough backlash, there would not be enough clearance for the gears to expand, and the teeth would bind up and stop turning.

The pinion bearing, washer and spacer.

We then had to do a tooth marking test. We found that the pinion was meshing too close to the toe, so we needed to put thinner shims, or less shims to move the pinion out of mesh.

The pinion.

The last task was to check the driveshaft flange for runout. We set up the DTI and got a reading of .08 mm. We did not have any specs for that so that is the differential fully inspected and assembled.