62 The need for constant velocity joints

Universal joints are necessary to transmit torque and rotational motion from one shaft to another when their axes do not align but intersect at some point. This means that both shafts are inclined to each other by some angle which under working conditions may be constantly varying.

Universal joints are incorporated as part of a vehicle's transmission drive to enable power to be transferred from a sprung gearbox or final drive to the unsprung axle or road wheel stub shaft.

There are three basic drive applications for the universal joint:

1 propellor shaft end joints between longitudinally front mounted gearbox and rear final drive axle,

2 rear axle drive shaft end joints between the sprung final drive and the unsprung rear wheel stub axle,

3 front axle drive shaft end joints between the sprung front mounted final drive and the unsprung front wheel steered stub axle.

Universal joints used for longitudinally mounted propellor shafts and transverse rear mounted drive shafts have movement only in the vertical plane. The front outer drive shaft universal joint has to cope with movement in both the vertical and horizontal plane; it must accommodate both vertical suspension deflection and the swivel pin angular movement to steer the front road wheels.

The compounding of angular working movement of the outer drive shaft steering joint in two planes imposes abnormally large and varying working angles at the same time as torque is being transmitted to the stub axle. Because of the severe working conditions these joints are subjected to special universal joints known as constant velocity joints. These have been designed and developed to eliminate torque and speed fluctuations and to operate reliability with very little noise and wear and to have a long life expectancy.

6.2.1 Hooke's universal joint (Figs 6.29 and 6.30) The Hooke's universal joint comprises two yoke arm members, each pair of arms being positioned at right angles to the other and linked together by an intermediate cross-pin member known as the spider. When assembled, pairs of cross-pin legs are supported in needle roller caps mounted in each yoke arm, this then permits each yoke member to swing at right angles to the other.

Because pairs of yoke arms from one member are situated in between arms of the other member, there will be four extreme positions for every revolution when the angular movement is taken entirely by only half of the joint. As a result, the spider cross-pins tilt back and forth between these extremes so that if the drive shaft speed is steady throughout every complete turn, the drive shaft will accelerate and decelerate twice during one revolution, the magnitude of speed variation becoming larger as the drive to driven shaft angularity is increased.

Hooke's joint speed fluctuation may be better understood by considering Fig. 6.29. This shows the drive shaft horizontal and the driven shaft inclined downward. At zero degree movement the input yoke cross-pin axis is horizontal when the drive shaft and the output yoke cross-pin axis are vertical. In this position the output shaft is at a minimum. Conversely, when the input shaft has rotated a further 90°, the input and output yokes and cross-pins will be in the vertical and horizontal position respectively. This produces a maximum output shaft speed. A further quarter of a turn will move the joint to an identical position as the initial position so that the output speed will be again at a minimum. Thus it can be seen that the cycle of events repeat themselves every half revolution.

Table 6.2 shows how the magnitude of the speed fluctuation varies with the angularity of the drive to driven shafts.

Table 6.2 Variation of shaft angle with speed fluctuation Shaft angle (deg) 5 10 15 20 25 30 35 40 % speed fluctuation 0.8 3.0 6.9 12.4 19.7 28.9 40.16 54

The consequences of only having a single Hooke's universal joint in the transmission line can be appreciated if the universal joint is considered as the link between the rotating engine and the vehicle in motion, moving steadily on the road. Imagine the engine's revolving inertia masses rotating at some constant speed and the vehicle itself travelling along uniformly. Any cyclic speed variation caused by the angularity of the input and output shafts will produce a correspondingly periodic driving torque fluctuation. As a result of this torque variation, there will be a tendency to wind and unwind the drive in proportion to the working angle of the joint, thereby imposing severe stresses upon the transmission system. This has been found to produce uneven wear on the driving tyres.

To eliminate torsional shaft cyclic peak stresses and wind-up, universal joints which rotate uniformly during each revolution become a necessity.

7BO 270

Angular movement of joint (deg)

Fig. 6.29 Hooke's joint cycle of speed fluctuation for 30° shaft angularity

6.2.2 Hooke's joint cyclic speed variation due to drive to driven shaft inclination (Fig. 6.30) Consider the Hooke's joint shown in Fig. 6.30(a) with the input and output yokes in the horizontal and vertical position respectively and the output shaft inclined 0 degrees to the input shaft.

Let wi = input shaft angular velocity (rad/sec) wo = output shaft angular velocity (rad/sec) 0 = shaft inclination (deg) R = pitch circle joint radius (mm)


Linear velocity of point (p) = wiy and

Linear velocity of point (p) = woR. Since these velocities are equal, woR = wi y y but

If now the joint is rotated a quarter of a revolution (Fig. 6.30(b)) the input and output yoke positions will be vertical and horizontal respectively.


Linear velocity of point (p) = woy also

Linear velocity of point (p) = w;R. Since these velocities are equal, woy = w;R R

but y cos 0

Fig. 6.30(a and b) Hooke's joint geometry

Example 2 A Hooke's universal joint connects two shafts which are inclined at some angle. If the input and output joint speeds are 500 and 450 rev/ min respectively, find the angle of inclination of the output shaft.

N0 cos 0

Fig. 6.30(a and b) Hooke's joint geometry

Ni cos 0 No Ni 450

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