Final drive, Driveshaft, Universal and CV joint

Final drive

We dissembled a Spiral bevel type of final drive assembly of a Holden or Ford rear wheel drive. This final drive receives it torque from the transmission through a propeller shaft, which is connected to the transmission output shaft at one end. The other end of this shaft is connected to the drive pinion flange of this final drive assembly. Torque transmitted from the propeller shaft turn the drive pinion. The drive pinion then will turn the crown wheel and rotate the differential assembly. The drive shaft connected to the differential assembly through the side gear will drive the wheel and move a vehicle. The pinion gears inside the differential casing allows wheels rotate at different speeds and transmit torque evenly to both wheel. For example as a car going straight the drive shafts to the wheel are rotated by the rotation of the differential case – as there is no planetary action happening the side gears are rotating with the casing. However, as vehicle turns left or right, one wheel will move faster than the other will. The faster rotating drive shaft will cause the attached side gear to spin faster than the other side gear driven by the slower wheel. This will cause the pinion gears to walk around the slower side gear therefore a planetary action is happening. The faster wheel rotates the pinion gears walk around the slower gear as if it is free spinning. 

We dissembled the final drive as shown above.

1.         Before any dissembling carried out, I placed a mark with twin on the diff housing caps to make sure they will be reassembled back to its original state.

2.         I didn’t find any identification marks for crown wheel and pinion, therefore I put another two marks with twin, one between the crown wheel and housing and the other the one on the pinion and the flange. I also placed another mark on flange and the diff casing.

3.         After differential caps is removed, I lifted the differential case out the carrier housing out.

The position of the crown wheel is marked on its carrier, therefore we could put the crown wheel back in its original position to ensure it performs well.

4.         After undo the bolts that connect the crown wheel, a chisel is used to remove the crown wheel from the differential casing.

5.         To disassemble the differential pinion and side gears, the pin that is holding the differential pinion shaft is tapped out with chisel.

6.         Once pin is removed, tap the differential pinion shaft a little bit and slid it out.

7.         By rotating both side gears at same time and direction, the pinion gears are exposed and removed. Then other gears are taken out the differential case.

8.         The last part of disassembly is to remove the drive pinion that drives the crown wheel from the differential carrier casing. First, we have to mark the nut in relation to the pinion shaft so later we have to preload the drive pinion when reassembling. To prevent pinion shaft from rotating as we unlock the nut a special tool is shown below to hold onto the drive pinion flange to hold the pinion shaft in position.
 
9.        As the pinion nut is unlock. We tap the pinion shaft with a soft faced hammer while nut is still in position to prevent damaging to the shaft. Once the shaft is loss from the carrier casing, we remove the nut and take out the drive pinion. This completes the disassembling.

Checks and measurements were done while we disassembling the final drive as listed below to ensure that final drive is healthy. Final drive components take huge amount of force while operating and is rotating at such a high speed. If anything is noticeably damaged as visual inspection, further investigation is needed to find out the reason for such damage. Problem causing this damage need to be fixed first and then damaged components need to be fixed or replaced.

l  We visually inspected conditions of all physical components such as broken gear teeth, cracked casing and excessive wearing etc. This final drive passed the visual inspection as there is no faulty found by visual inspection.

l  We measure the run out of the case flange to make sure it is in good contact with the propeller shaft. The measured result is 0 mm which is less the max allowed specification – 0.5 mm. Excessive run out may not only cause drive line to work roughly but also cause damage to the driveline components.

Next thing we did is to reassemble the final drive assembly. It is in the reverse order of disassembly but a little bit more difficult, because not only we have to put everything back but also have to make adjustment according to specification to ensure final drive works properly.   

1.       Before install drive pinion back to the carrier casing, inspect all its components for damage. Apply lubricant to its bearing and treaded surface. Tighten the nut with a torque wrench set to 18 NM. After we reinstall the drive pinion back, we used loaded tester and spring balance to test the tightness of the drive shaft for exercise purpose. Both results show 18 NM.

2.       Check the marking we have made earlier, they should match up by now. Turn the nut about half a turn further to complete the preload process. This extra turning allows the nut to push onto the crushable sleeve. Preload needs to be done is because huge amount of torque will pass the drive pinion when operate, therefore it must be firmly attached to carrier casing otherwise damage will happen. This is the preload process of the drive pinion.

3.       Reassemble the side and pinion gears back to the differential casing. Note the pin that is holding the pinion gear shaft will only go in one way; therefore make sure it is in the right direction. Once reassembled the side clearance are check to ensure there are correct clearance between this gears. The clearance provide space for lubrication and heat expansion. However if it is too large then the gears will have too much space to move the differential wouldn’t work properly. The clearance is adjust with shims. The clearance of side and pinion gears of this final drive are both 0.1 mm which is within specification – 0.05 to 0.2 mm.

4.       Bolt crown wheel back to the differential housing with torque wrench set to 97 NM. Bolts should be tightening progressively in a specified sequence.

5.       Put the differential assembly back to the carrier with crown wheel installed. Refit the caps to the carrier to hold the differential assembly in place. Make sure the thread of the adjuster is lined up with the carrier case and cap thread. Tighten the cap bolts with a torque wrench set to 78 NM.

6.       Checked the radial and lateral run out of the crown wheel both are 0.1 mm, which is same, a specification. Therefore, the crown wheel is not distorted in anyway.

7.       As we did with the drive pinion, we need to preload the carrier bearings to make sure the crown wheel and drive pinion are in good contact and are held firmly inside the housing. Loss one side of the cap and remove it. Put a mark with twink on the bear surface where we could see through the hole in the cap. Refit the cap and bolt it on. Now use the special tool show below to tighten the adjuster until the marking on the bearing just start to move. Now we have completed the preloading process.

8.       Once preloading is complete we need to adjust the position of the crown wheel in relation to the drive pinion, this ensures that the contact between gear teeth of crown wheel and drive pinion are correct.

9.       Bearing blue is used to mark the teeth on the crown wheel. Turn the pinion shaft allow the crown wheel teeth to mesh with drive pinion teeth. We observed from the teeth mark is heavy heel contact. We adjusted the crown away from the pinion and pinion towards the crown wheel to correct this problem. We double checked by measuring the back lash of the crown wheel with DTI the results are all within specification. This completed the resembling process.

       Drive Shafts

       As defined in Automotive Mechanics written by ED MAY – drive shaft is a shaft between the final drive and driving wheel; a shaft between the transmission and the rear axle (Propeller shaft).

The picture below is a shaft between the final drive and driving wheel

In our lab, we mainly looked at the latter one – Propeller shaft which is shown below

As shown in the picture above one side of the shaft is splined onto the output shaft of a transmission and the other end is connected to the flange of the final drive. The splined connection allow the shaft to vary in length. As vehicle is driving on uneven road and the body of the car is moving up and down the distance between the transmission and drive will vary. The yokes on the shaft is joined together by universal joints which also accommodate this problem and also allow shafts to operate at angle according to the road condition. However with this type of design, the York must be installed in correct phasing. When drive shaft is operating at angel the velocity between the front and the end are different. As they are in phase, this difference will cancel each other out otherwise as if they are out phase the car will start to vibrate badly.

In this lab we roughly looked at the following points to ensure correct drive shaft operation

l  Drive line phasing

l  Drive shaft balance

Both of these will cause vehicle to operate roughly. Drive shaft phasing is needed to check the manufacture specification and reinstall the shaft to the correct phasing. Drive shaft balance is check with DTI to look for run out. If it is certain that run out is out specification then balancing is need to be done by adding weights to the drive shaft. This should only be done by specialist.

In this lab we also look briefly at universal joints – cross and York type

The disassembling procedure is described as below

1.         Remove the snap ring under the cup

2.         With one side of the yoke rest on vice, tap the York that is adjacent. This will force the cup to come above the York.

3.         Remove the cup and beware of the needle rollers in it.

4.         Repeat the same for the opposite cup.

5.         Now one yoke is able to come off the joint.

6.         Put remaining York on the vice with freed joint journals rest the vice and tap the York with hammer and force the remaining cap out and remove it.

Th   reassembly is the opposite, just be very careful with the needle rollers as they are very easy to come out. inspections should be done on joint surface condition, caps surface condition , roller condition, seal condition and driveshaft yoke surfaces. For the universal joint we inspected failed the inspection because needle rollers missing. With properly functioning rollers the joint will be worn out in no time. The whole joint need to be replaced. As new joint fitted special care is needed to not damage or loss the needle rollers.

CV   CV joint

       Is the constant velocity joint as in compare the Cross and York type joint, CV joint will provide constant velocity all across the shaft. there are three basic types of CV joint

Birfield Joint

Double offset joint

Tripod joint

General check with CV joints

l  Inspect for split, tears and deterioration in the CV boot. CV joint need to be protected by these boots from dried up or foreign objects. It needs to be lubricated very well in order to functioning properly.

l  Joints must should be checked for play, roughness or binding

l  CV joint components must be checked if there any physical damage.

On a drive shaft, the joint that connected to the wheel is called outer CV joint and the inner joint that connects to the final drive is called the inner joint. Outer joint is normally the birfield type and the inner joint is Tripod type.

The outer CV joint could move up, down, left and right direction but it could not move in back and forth direction. The inner joint could however move back and forth quite a lot. The inner joint allows the drive shaft vary in length when car is moving on an uneven surface.

The following is what I did in this lab to disassemble the outer CV joint

1.         Remove the bands that’s hold the boot on to the joint and drive shaft. Move the boot and expose the joint.

2.         This joint didn’t have a snap ring inside therefore I just with a soft faced hammer the joint off the drive shaft

3.         Push the inner ring and cage on one side, remove the ball that’s exposed with a flat head screw driver. Repeat this until all balls are removed.

4.         Turn the inner ring and cage together about 90 degrees. Lift inner ring and cage out of the outer ring.

5.         Turning the inner ring about 90 degree then lift it out of the cage.

6.         Disassembly of the outer CV joint is complete.

The reassembly procedure is the opposite. The tricky part when reassemble is that a pair of special window on the cage needs to be identified before the cage could be put back into the outer ring. This pair is slightly larger than the other ones. Once the larger windows are found, hold the cage vertically and slide the cage into the outer ring. The cage and inner ring should be able to rotate freely inside the outer ring. When try to install the joint back to the drive shaft remember to put the CV boot on. Line the inner ring splines up with the shaft and hit on the shaft on the outer ring to push the joint to go over the clip ring. Care should be taken not to jam the clip ring in between the inner ring and drive shaft. The rest of installation process is quite simple.

To disassemble inner CV joint is quite easy

Remove the bands that is holding the CV boot

Remove the boot

Slide the drive shaft of the housing – spider should be mark in relation to housing, so we could reinstall inner correctly. The disassembling process is completed.

The reassembly is the exact reverse of disassemble.