A gearbox is rigidly mounted in relation to the engine and chassis, but when coupled to a rear axle by a propeller shaft, allowance has to be made for the variation due to the action of the road springs in the position of the rear axle relative to the chassis. Universal joints are, therefore, fitted for connecting two shafts inclined to one another at a small angle and at the same time to permit the transmission in the rotary motion from the engine to the road wheels throughout the variations in position of the rear axle relative to the gearbox and chassis. These joints are also used on vehicles, which have coupling shafts between clutch and gearbox, between main gearbox and auxiliary gearbox, and on the driving shafts of the driven front axles.
Types of Universal Joints
Various types of universal joints are in common use is:
(a) The Flexible Hardy Coupling. This consists of a specially constructed fabric disc in layers of fabric; sometimes the whole is rubber impregnated. Six equally spaced holes are formed in the disc, and each hole has a stiffening steel plate on each side. A three armed spider is fastened to each side of the disc with the arms bolted to the holes not used by the arms of the other spider. The bolts securing the spider arms to the fabric, disc must not be tightened excessively as this damages the fabric, resulting in fraying and rapid collapse of the disc. Coupling of this type are usually fitted between a clutch and gear box, and on some vehicles between gearbox and propeller shaft and rear axle. The advantages of the fabric coupling are that it is flexible in all directions.
(b) The Hook’s or Ring Type. This joint consists of a pair of fork fitted one to each of the two splines shafts to be coupled. Each fork carries two pins projecting outwards on the same centre line and when fitted together as a coupling the pins lay at right angles to each other. Each pin is carried in a phosphor bronze bush, the bushes being mounted between two steel discs, which are bolted together to complete the assembly. The telescopic movement is obtained by splines formed in one or both of the forks hubs and provides a much free knuckle effect than the hardy fabric coupling.
(c) The Muff Coupling. In this type, two pins are formed diametrically opposite to each other on the end of a driving shaft at a right angle. Two squares dies of hardened steel fit on these pins and into a muff, which has two channels of square section to suit the dies. One shaft carries the dies and the muff forms the end of the other shaft. It has got the advantage of both knuckle and telescopic action (e.g. Dodge cars).
(d) The Yoke Type. The yoke coupling is made of two fork shaped yokes and a star centre piece carrying four pins with phosphor bronze bushes. When assembled, one yoke is secured by caps to one pair of pins and the other yoke to the remaining two pins, so that the yokes are at right angles to each other. An extended hub of one yoke is internally to allow for telescopic movement.
(e) The Hardy Spider Needle Roller Bearing Type. This is the most commonly used universal joint. It is similar in construction to the yoke type but the phosphor bronze are replaced by needles roller bearings which are either packed in grease or work in an oil reservoir. Each needle bearing assembly consists of a hardened cage carrying the needle rollers. When fitted to the trunion (star Piece) and yokes, a circlip or locking ring shaped into a groove in each of the yoke ends, retains the whole assembly in position, alternatively they are fitted with caps bolted to the yoke. End thrust of the trunion is taken on the hardened face of the inner end of the needle-bearing cage.
(f) The Lay Rub Coupling. This type is used on short tubular propeller shafts where the inclination between the gearbox and rear axle is very small. It consists of two pressed steel dished castings welded or riveted together. The four pressed out on the platform housing for special rubber blocks. Each rubber block is fitted with a steel sleeve to carry a bolt to secure the coupling to the forked ends of the shafts to be coupled. No provision is made for telescopic movement. The advantage is that it does not require any servicing and when worn the complete coupling is to be changed.
Hardy Spicer Universal Joint
- Remove the four circlips from each of the two yokes. Remove the grease nipple from the cross.
- Remove all four bearing caps lug, tapping with a hide face hammer taking care not to drop the needle rollers.
- Take out the spider cross from the yokes taking care of the seals on each of the cross pins.
- Examine the rollers, caps and spider for any wear out.
- Assemble rollers in the bearing caps with grease.
- Insert the spicer cross in the yoke and assemble the caps on the spider cross pins through the yoke eyes.
- Secure the bearing caps by means of circlips inserted in the yoke grooves.
- Check the grease nipple for serviceability and replace it on the spider cross.