18/05/2011 Inspecting, dismantling, cleaning and greasing the universal joint

The Universal Joints

I. On the theory side         

 1. What is it? 

First of all, it's a joint type of a drive-shaft, which transfers the torque/drive from the transmission to the final drive. The universal joint is one of the most popular drive-shaft joint type that mostly used in RWD vehicle. Many are one-piece construction, but longer longitudinal length cars use 2 pieces, with a support bearing mounting the front shaft to the body. 

Some shafts have a sliding spline( which is quite common) that allows the drive shaft to extent and retract on its own length.

Hooke's joint /Cross joint are universal joint, they function the same way that there is a trunnion/or cross credited the two joined yokes 90 degree out of phase with each other, allowing both horizontal, vertical angular movements of the shafts.

2. Characteristics:

Universal joint has a "variable velocity" characteristic, that the drive shaft angular speed varies between the input yoke and the output yoke, but from the transmission to the rear axle, the speed remains constant throughout. This is because the yokes and the trunnion which are responsive for bisecting the angle at which the shaft spins are limited in their own ability to completely transferring the torque without the affection of any fixed angle. Therefore the drive varies between different angles particularly that when the angle displaces further from straight condition, the transferred speed varies more. Hence, it is designed so that the change of speed in output shaft always eliminates the change in the input shaft. So, although the wheels are driven at a constant speed, the drive shaft itself is alternately changing speed.

 II. On the practical side.

  Visual check for clearance: cracks or torsion or bents etc... help identifying the problem straight away if it's too obvious.

  One of the most critical step I should remember when servicing a universal joint drive shaft after cleaning and preparing for inspection is phasing. Phasing is the alignment of the cross yokes in the center section of the drive. With the incorrect phasing it will result in severe vibration when driving because the output yoke is 90 degrees out-of-phase with the input yoke, hence when driving, it does a painful job of lumping over 90 degrees to catch up with the input yoke, this can seriously causes heavy twisting. But shafts that have a sliding spline attached to the input yoke don't suffer from any incorrect phasing because they can correct the error by sliding back and forth. 

  In my situation, my both yokes on the shaft are welded with the shaft, so there is only one way to reassemble the phasing. However, when putting it back, we still have to distinguish where is the input(to transmission) and output(to final drive). I notice that on the 2 ends of the shaft, there are 2 different sized emerged pad of weight. So I guess they are balancing method of the shaft, which can also be used as match marking.

  Next, i have to check the drive shaft runout. By putting it on a V bench and set up the dial test indicator, i have to make sure that the probe is at right angle to the drive shaft. Then, identifying the runout by slowly turn the shaft 360 degrees to inspect the change in the probe meter, then collect the highest reading and compare it to manufacturer specification. A higher reading indicates that the balance is not good hence when driving we'll feel abnormal vibrations, since its not straight enough.

  When disassembling the cross joint, remember that the trunnion needs to be knocked out of the yoke first. So I have to remove the bearing cap retain ring first to free the caps from locking, by using a hammer and a long, flat tip screw driver. Once a rollers cap is taken out, the other can be disassembled easily with out any tool. As for the input shaft, the trunnion is disengaged with the driven yoke first, then we disconnect the trunnion from the driving yoke. As for the output shaft, the trunnion is disengaged with the driving yoke first, then the driven yoke.

  When everything is dismantled, before putting back, they need to be re-greased. All moving parts, surfaces are cleaned, removed from the old grease and re-greased. The roller bearings need grease to maintain the cap pressurized shut with the trunnion, so i pay attention to these.

  It is practical to match marking all alignment before disassembling. And thanks to this, our shaft were put back in no time. In the instruction, instead of hammering the trunnion back into the yokes, a relevantly smaller bolt can be used with the bench to press the trunnion cap back into the yokes, at a point where both the retain rings can fit into their trenches. Phasing is also checked when performing reassembly.