There is a difficulty in transmitting the power of the motor to the transmission or rear axle which has to be met by a special piece of mechanism. In the chain-driven vehicle the motor and transmission usually remain in alignment when the vehicle is standing still, however, road vibrations and the nature and location of the load usually cause frame deflexions which tend to destroy this alignment, while with a live rear axle the motor is protected from bouncing up and down by the chassis springs, and the road wheels are continually bouncing over the rough surface of the road. This means that at one moment the axle is in line with the motor and the next moment it may be several inches above or below the motor center.

A rigid shaft in the case of a chain-driven vehicle would bind unless the alignment was perfect and provision made to prevent frame deflexion, while with a live rear axle the shaft would bend and bind in its bearings and the whole transmission system would be put out of commission in a short time. To overcome this, the shaft which is termed the drive or propeller shaft is made flexible to a certain extent by means of universal joints, sometimes called Hooke or Cardan joints.

These universal joints serve the purpose of connecting shafts whose axles lie in the same plane, but make an angle with each other and are particularly required when the angle varies between the shafts in service. There are various types and perhaps the simplest universal joint consists of a squared block secured to one of the shafts to be connected, fitting in a square hole in a sleeve secured to the other shaft. The four faces of the block are curved in the direction of the axis of the shaft to which the block is fastened. This type of joint is shown in Fig. 85.

Fig. 86 illustrates a somewhat different type. This consists of six internal and external teeth; the external teeth are curved in the direction of the axis of the shaft and mesh with teeth cut into the housing. This joint is provided with a pressed steel cover and packing washers to retain the lubricant. The above types may be properly termed alignment joints, since they are only used between the clutch and transmission, when only slight angular movement exists, Fig. 86 being the type furnished with all sizes of the well-known Hell-Shaw universal clutch. These joints also constitute slip joints, since they permit movement for clutch disengagement.

The cross type of universal is depicted in Fig. 87 and consists of two forks, each of which is secured to one of the shafts to be connected to each of the forks by a pin. In this type of joint, the axes of the two pins do not intersect, but are at some distance from each other. However, there is an advantage in having the pins both in the same plane. This end can be attained in various ways by either using pins of different diameters and passing one through the other, by one long and two short pins, by forging the pins integral with a common center or forming them integral with the forks.

In the joint shown in Fig. 88 the cross is replaced by a split ring, which carries the pin bearings, while the pins are forged integral with the forks. The ring is divided to facilitate assembling and thus permits the pins to have their axes in the same plane. This type of joint could also be made by forming the pins integral with a central ring and providing forks with separate bearing caps.

Fig. 89 illustrates the Swenson joint, which in some respects is similar to the split-ring type. It consists of a fork with integral pins and a large pin passing through a hub and supported by square bushings in a ring. The integral pins are also provided with square bushings which fit into slots in the ring. The bushings are held in position by two discs which together with the ring form a housing.

All universal joints are of the pin type with the exception of the leather or fabric-disc type, however, there are various methods used to provide angular movement. The Hartford joint, shown in Fig. 90, is termed a slotted shell and trunnion type. This consists of a cup-shaped steel forging secured to one of the shafts with two diametrically opposite longitudinal slots milled into the shell. The other shaft is provided with a ball-shaped end, fitting the interior of the shell and provided with a pin extending into the slots. Hardened steel trunnion blocks are interposed between the pins and the walls of the slots, to distribute the bearing pressure. This type also serves as a slip joint and is easily enclosed with a tubular steel casing and a leather boot.

Since this type of joint permits endwise movements of the shaft, some provision must be made to hold the latter in proper relation to the two joints. Coil springs are used for this purpose.

One of the Simplest Universal Joints.

Fig. 85. One of the Simplest Universal Joints.

Universal with Internal and External Teeth.

Fig. 86. Universal with Internal and External Teeth.

Cross Type Universal with Forks.

Fig. 87. Cross Type Universal with Forks.

Splitting Type Universal.

Fig. 88. Splitting Type Universal.

Swenson Universal Joint.

Fig. 89. Swenson Universal Joint.

Hartford Block and Trunnion Type Joint.

Fig. 90. Hartford Block and Trunnion Type Joint.