The universal joint, or Hookes joint, is used to carry drive from one shaft to another where the two shafts are not perfectly in line and particularly where they can move relative to each other. It is used in propellor shafts at both the transmission output and the differential input ends, in steering swivels on part-time four wheel drive vehicles, in some steering columns, and in PTO winch drive-shafts.
The joint consists of a cross or spider which carries needles roller bearings at the four extremities. The bearing cups are fitted into yokes, two on the input shaft and two on the output shaft. The arrangement allows drive to be taken through an angle, particularly where that angle can change as it does in a prop' shaft due to suspension movements.
The universal joint is not a constant velocity joint: If there is a non-zero angle of deflection between the input and output shafts and if the input shaft is driven at a constant rate then the output shaft will accelerate and decelerate twice per revolution. This effect can be negligible for small angles of deflection but it increases with the angle of deflection and can cause vibration.
It is for this reason that full-time four wheel drive vehicles fit constant velocity joints, not universal joints, in the steering swivels. There would be vibrational feedback through the steering on full lock if they did not.
Many applications involve three shafts and two universal joints - e.g. transmission output shaft, prop' shaft and differential input shaft. If the angles of delection between shafts one and two and between shafts two and three are of equal magnitude (the sign, +/-, does not matter) then the acceleration / deceleration effect is cancelled out. Almost invariably the transmission output shaft and the differential input shaft are parallel to achieve this. The intermediate prop' shaft still does accelerate and decelerate twice per revolution but the differential is driven at a steady rate. This does assume that the phase angle between the two universal joints is zero. The phase angle is the relative angle of rotation between them. The axes through the yokes at each end of the (intermediate) prop' shaft are parallel when the phase is zero. Prop' shaft slip joints are commonly marked with arrows so that the phase can easily be checked on reassembly.
In some vehicles the transmission output shaft and the differential
input shaft are not parallel -
the engine and transmission may be tilted up for clearance
or the diff' may be tilted for some reason.
If the discrepancy in deflections is large,
the manufacturer may have fitted
constant velocity joints at one, or ideally both, ends
of the prop' shaft to prevent vibration.
NB. Some vehicles, notably the
Range Rover Classic,
specify a non-zero phase angle; the reason is a mystery!
If your vehicle starts to experience drive-train vibration and the prop' shaft is suspect, check the following:
If you have a browser that can use Java, the following applet shows the difference, in degrees, of the angular rotation between input and output shafts of a system with three shafts and two universal joints. The smaller the magnitude of the difference through a cycle the better. Click on the + or - boxes to vary the settings of the `phase' and the two deflection angles `a' and `b'.
The correctness of the applet and its results cannot be guaranteed.
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