624 Double Hookes type constant velocity joint

One approach to achieve very near constant velocity characteristics is obtained by placing two Hooke's type joint yoke members back to back with their yoke arms in line with one another (Fig. 6.31). When assembled, both pairs of outer yoke arms will be at right angles to the arms of the central double yoke member. Treating this double joint combination in two stages, the first stage hinges the drive yoke and driven central double yoke together, whereas the second stage links the central double yoke (now drive member) to the driven final output yoke. Therefore the second stage drive half of the central double yoke is positioned a quarter of a revolution out of phase with the first stage drive yoke (Fig. 6.32).

Consequently when the input and output shafts are inclined to each other and the first stage driven central double yoke is speeding up, the second stage driven output yoke will be slowing down. Conversely when the first stage driven member is reducing speed the second stage driven member increases its speed; the speed lost or gained by one half of the joint will equal that gained or lost by the second half of the joint respectively. There will therefore be no cyclic speed fluctuation between input and output shafts during rotation.

An additional essential feature of this double joint is a centring device (Fig. 6.31) normally of the ball and socket spring loaded type. Its function is to maintain equal angularity of both the input and

Fig. 6.31 Double Hooke's type constant velocity joint
Fig. 6.32 Double Hooke's type joint shown in two positions 90° out of phase

output shafts relative to the central double yoke member. This is a difficult task due to the high end loads imposed on the sliding splined joint of the drive shaft when repeated suspension deflection and large drive torques are being transmitted simultaneously. However, the accuracy of centralizing the double yokes is not critical at the normal relatively low drive shaft speeds.

This double Hooke's joint is particularly suitable for heavy duty rigid front wheel drive live axle vehicles where large lock-to-lock wheel swivel is necessary. A major limitation with this type of joint is its relatively large size for its torque transmitting capacity.

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