Universal Joints (Automobile)

Universal Joints

Universal joints are capable of transmitting torque and rotational motion from one shaft to another when their axes are inclined to each other by some angle, which may constantly vary under working conditions. 

Universal joints are incorporated in the of vehicle's transmission system to perform three basic applications : 

(a) Propeller shaft end joints between longitudinally front mounted gearbox and rear final drive axle. 

(b) Rear axle drive shaft end joints between the sprung final drive and the unsprung rear wheel stub axle. 

(c) Front axle drive shaft end joints between the sprung front mounted final drive and the unsprung front wheel steered stub axle.

Universal joints have movement only in the vertical plane when they are used for lon­gitudinally mounted propeller shafts and transverse rear mounted drive shafts. When these joints have been used for front outer drive shaft they have to move in both the vertical and horizontal plane to 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 large and varying working angles even when the torque is being transmitted to the stub axle. Due to the severe working conditions, special universal joints known as constant velocity joints are employed. These joints have been designed to absorb torque and speed fluctuations and to operate reliably with very little noise and wear having long life.

Basic Types of Universal Joints Cross-type Joint.

This type of joint is also called a Hooke-type coupling as it was developed from the joint invented by Robert Hooke in the seventeenth century. This joint is commonly used today. The joints in Fig. 26.8A and B represent the basic and developed forms respectively. They use two yokes set at 90 degrees to each other and a cross-shaped trunnion block joins these yokes. In more developed joints like Hardy Spicer type, contact between the two parts is made by needle roller bearings held in a hardened steel cup retained in each arm of the yoke. For the alignment of the trunnion, the bottom of the cup forms a contact with the end of the block. 

Fig. 26.8. Types of universal joints. A. Hooke-type joint. B. Cross type joint (Hardy Spicer).

C. Cross-type with rubber bushing. D. Layrub.

E. Doughnut rubber coupling.

A special viscous oil, similar to that used in a final drive, is used for bearing lubrication, which is contained in a reservoir formed by drilling out the centre of the trunnion arms. The oil is introduced by a lubrication hippie or is prefilled once for entire life. An oil seal, retained on each arm of the block, prevenjts the escape of the lubricant. The cups are held in the yoke either by circlips or staking. The replacement of worm parts in the joint becomes more difficult due to peening over the edge of the yokes to stake the cups. Therefore replacement of the complete shaft assembly is recommended when the joint is worn. These joints offer several advantages such as they (i) are compact, (££) have high mechanical efficiency, (Hi) have ability to drive through a large occasional 'bump' angle (maximum about 25 degrees), and (iv) due to accurate centring of shaft, are suitable for high speed operation. One major disadvantage of the cross-type joint is its inadequate flexibility to absorb torsional shocks and drive-line vibrations, especially when a comparatively rigid transmission system is used.

Lubrication failure, especially when a grease nipple in the trunnion block is missed, causes the needle rollers to indent the bearing surfaces. This type of wear causes a slight angular movement and produces a noise, commonly described as a clonk, during the change over from drive to over-run and vice versa. If this fault is not rectified in time, the rate of wear accelerates leading to misalignment and severe vibration.

Rubber Joints.

A smoother and less harsh drive is obtained by incorporating one or more rubber joints in the transmission driveline. Three types of rubber joints in use include moulton, layrub and doughnut.

Moulton Joint

This rubber trunnion type joint (Fig. 26.8C) is based on a hooke type coupling. It uses moulded rubber bushings for the transmission of drive between the trunnion and yokes. These synthetic rubber mouldings require no lubrication and due to high flexibility they damp the torsional shocks produced when the drive is transmitted through an angle.

Layrub Joint

This type joint (Fig. 26.8D), originally made by the Laycock company, was constructed of a series of rubber bushings. The name layrub is used to describe this joint. It uses a number of moulded rubber blocks, with specially shaped cavities at the ends. These blocks are sandwiched between two steel pressings. Each shaft is connected by means of a fork to alternate rubber blocks. This arrangement permits the rubber blocks to deform making the drive possible for transmission through a small angle. Also the blocks accommodate small axial and angular movements for shaft length alteration and torsional damping. This coupling is relatively large in diameter. The layrub type joint offers several advantages, such as (i) it does not require lubrication,

(ii) it is capable of driving through bump angles up to about 15 degrees, (Hi) it allows for axial movement, requiring no splining of the shaft,and (iv) its resilience damps shocks and insulates vehicle from transmission noise.

Doughnut Joint

Although large in size, the great flexibility of this joint provides soft cushioning. This absorbs the majority of torsional shocks generated by the action of other joints or by vibration from either the engine or road wheel. The synthetic rubber coupling shown in Fig. 26.8E is near-circular in shape and is moulded around cylindrical steel inserts, which are bolted alternatively to the three-arm forks fixed to the shafts. The merits of this coupling are similar to that of layrub joint.

26.2.2.

Speed Variation of a Hooke-type Joint due to Drive and Driven Shaft Inclination

When a hooke-type coupling transmits a drive through an angle, the output shaft does not rotate through 360 degrees at a constant speed. Instead the speed varies every 90 degrees of rotation, and the rate of movement for one revolution is fast, slow, fast, slow (Fig. 26.9). This cyclic speed variation, and its associated vibration, is insignificant when the drive angle is less than about 5 degrees, but becomes much more intense as the angle is increased.

A simplified sketch of a Hooke's joint is shown if Fig. 26.10, in which the driving shaft A is connected to the arm YY of the central cross-piece through the driving yoke, and the remaining arm, XX of the cross-piece connects to the driven shaft B through driven yoke. The driving yoke lies in the vertical plane with its axis YY vertical, and the driven yoke lies in a plane inclined at an angle 0 to the horizontal with its axis XX horizontal.

Fig. 26.9. Speed variation with Hooke-type joints.

Let the driving shaft is turned through an angle a so that the point Y moves to Yi as shown in end view in Fig. 26.10. The point X moves about axis OB, through an angle 4> subtended by an arc Xd and it moves in a vertical plane to point Xi through an elliptical path.

Fig. 26.10. Simplified diagram of a Hooke's joint

Due to the above variation of the speed of the driven shaft for various positions of the driving shaft, a single Hooke's joint becomes unsuitable for the power transmission in automobiles. But a constant velocity ratio can be obtained by the correct use of a double joint. The acceleration of the driven shaft, as may be obtained by differentiating the equation for velocity ratio with respect to time.  

To achieve a constant speed output from the propeller shaft two Hooke-type couplings can be mounted either back-to-back or positioned in a certain way at each end of the propeller shaft. In both the configurations the relative positions of each coupling must be such that the speed

Fig. 26.11. Phasing of Hooke-type couplings.

change of one coupling is counteracted by the other. The phasing of Hooke-type couplings, as applied to two separate driveline layouts, is illustrated in Fig. 26.11.

From this diagram it can be seen that to obtain a constant speed,

(i) yokes at each end of the propeller shaft must be placed in the same plane, and

(ii) drive angle of each coupling must be equal.

A constant velocity (CV) joint imply that when two shafts are inclined to one another at some angle and are coupled together by some sort of joint, then a uniform input speed transmitted to the output shaft produces the same angular output speed throughout one revolution. There are no angular acceleration and deceleration as the shafts rotate. Various CV joints in use have a construction, which is based on either the twin hooke-type coupling arrangement or the angle bisects principle. The CV joints in use include :

• Tracta • Rzeppa

• Weiss • Tripode

o

Example 26.3. Two shafts, the axles of which intersect but are inclined at 20 to each other, are connected by a Hooke's joint. If the driving shaft has a uniform speed of1000 rpm., find from first principles, the variation in speed of the driven shaft. The driven shaft carries a rotating mass, which weighs 147 N and has a radius of gyration of 0.25 m. Find the accelerating torque

on the driven shaft for the position when the driven shaft has turned 45 from the position in which its fork end is in the plane containing the two shafts.

Different types of universal joints are:-

1.Double Hooke's Type CV Joint 

2.Tracta Constant Velocity Joint

3.Rzeppa Joint

4.Birfield Joint

5.Carl Weiss Constant Velocity Joint

6.Tripode (Tripot) Type CV Joint