Universal Joints.
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.
4wd.sofcom.com/4WD.html
A double Hookes Joint forms a C.V. joint - Land-Rover
Stage-1.
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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:
- fixing bolts loose - tighten to specifications
- excess grease (off-centre weight) in rubber boot, if fitted
- prop' shaft phase incorrect - reset
- worn universal joints - replace
- slip-joint worn - replace
- worn transmission or diff' bearings - repair
- prop' shaft out of balance - take to specialist
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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