The front axles are designed for two purposes, one to transmit the weight of the vehicle from springs to the front wheels and the other to permit the front wheels to turn right or left as required. The front axle assembly is a complete major component of a vehicle, which includes the axle beam, stub axles with brake assemblies, swivel pins and track rod.
It is usually made of steel forging usually of “I” or “H” section. It is heat treated for extreme toughness and is machined to close limits. It serves the dual purpose of transmitting the weight of the vehicle from the springs to front wheels and an anchorage on which the stub axles pivot. The stub axles are mounted to the ends of the axle by pivot pins so that they can swivel about their pins in the horizontal plane. Two machined surfaces on the axle beam, equidistant from its centre known as spring pads (chairs) carries the road springs. Each pad is drilled with four holes so that springs can be rigidly bolted to the axle. The spring centre bolt mating with a dowel hole on the spring pad ensures positive location of a road spring. The axle beam can have either yoke or plain end depending upon the method of mounting the stub axles.
There are two stub axles made of steel forgings, fitted to a front axle and mounted to the axle beam by pivot pins so that they can swivel as required for steering. Each stub axle is made up with an arm, the stub axle arm, are connected to each other by a track rod. In addition one of them has a steering arm for connection to the pull and push rod. Thus any movement of the pull and push rod swivels the stub axle. There are two types of stub axles known as Elliot type and Reverse Elliot type Stub Axle.
(a) Elliot Type Stub Axle. In this design the swivel pin is fixed in the stub axle, forging. The ends of the swivel pin can turn freely in the bushes fixed in the axle beam fork.
(b) Reverse Elliot Stub Axle. This design is one of the most commonly used and has the swivel pin fixed in the end of axle beam and locked by a cotter pin, so that the forked stub axle can turn on the swivel pin ends.
Swivel Pins/King Pins/Pivot Pins
These pins are generally made of good quality steel, case hardened and used to secure the stub axle to the axle beam. They are locked in position by cotter pins. The king pin holes in each end of the axle “I” beam are bored at an angle so that top end of the king pins are inclined towards the centre line of the vehicle. This is known as “king pin inclination”.
The brake backing or support plates carry the brake shoes and wheel cylinders and are bolted to the steering knuckles. The stub axle arms have tapered ends which fit into taper hole in the knuckles. A key seated in the track arm fits into a key way into the knuckle, which properly positions the stub axle. The arms are held secured to the knuckle by a castle nut and cotter pin. The right and left track arms are connected with each other by the tie rod (Track rod), which is adjustable and controls the toe-in of the front wheels. The stub axle arm is forced with the right track arm and is connected to the Pitman arm (Drop arm) through the steering connecting rod (Pull and Push rod).
The track rod connects two stub axle arms to each other through knuckle or ball joints. These joints are known as track rod ends. The ends are invariably screwed on to the track rod and permits track rod adjustments. The track rod is therefore threaded at either end, one end being with left hand thread and the other end having right hand threads. Track rod adjustments is carried out by slackening the locking devices at either end and turning the rod in the desired direction for lengthening or shortening as required. When the rod is lengthened the “toe –in” is increased.
Pull and push rod or drag link
The connection between the steering arm of the front axle and the drop arm of the steering gearbox assembly is made by a pull and push rod some times called a Drag link. It is usually made of steel tubular constructions with spring-loaded ball at each end. One end fits on the steering arm of the stub axle and the other end on the steering drop arm. On the majority of the vehicles the pull and push rod lies parallel to the chassis, with the steering arm carried on the right hand side stub axle. On other vehicles, the pull and push rod lies across the chassis and connects the steering arm mounted on the left hand side stub axle.
The live axles are those axles, which transmits the drive from gearbox to the road wheels. In case of front axle it transmits the drive and manoeuvre the wheels through constant velocity joints during steering operations.
Constant velocity joints
The joints which maintains the same speed of the in put and out put shaft throughout each revolution, irrespective of the angle between the both are known as C.V joints. These types of joints are used in a front wheel drive vehicle to allow the front wheels to turn steeringg. There are different types of CV joints. These are Tracta, R-zeppa, Bendix and Spider Cardon Cross C.V joint.
Tracta CV Joint
Made up of slotted portion with integral forks on the hub shafts and the inner shaft. A flat is provided on the trunion of each portion to facilitate assembling and the dismantling of the joint. The slotted and the spigot ends are matched together as laid and marked accordingly for assembly purpose. The housing of the joint forms part of the axle casing and is secured by bolts. The steering swivel pins are mounted in each housing. The outer end of the axle shaft is supported by roller bearings as for fully floating axle.
The R-zeppa type joint
This type of joint is housed in the axle casing. It resembles a large ball bearing in which the inner race can swivel in the outer race. It includes an outer member, an inner race with six steel balls and a ball cage, a shaft retaining ring and a piloting extension comprising a housing pin, cup and a spring assembly. The outer member is in the form of a spherically grooved cup with an integral splined shaft on which the wheel hub is mounted. The inner race is splined internally and grooved externally, the groove corresponding with those in the outer member. The spring assembly, which is comprised of a small coil spring and plunger in a recess in the shaft of the outer member. The pilot housing is located within the cup of the outer member and the rim of the housing abuts one side of the ball cage Intermediate bearings are not fitted in the outer end of floating type. The driving shaft as for the Tracta type joint because, the steel balls of the Rzeppa provide the necessary support. Birfield joint in Maruti Gypsy is the further development in the R-zeppa CV joint.
Spider Universal Joints
It is a cordon cross-joint with needle bearings similar in design to the propeller shaft universal joint. On spider axle joints, the snap rings slots are in the bearing retainer. The driving axle shaft is splined at both ends and fits in the corresponding bevel gear wheel of the differential and the other end fits in the splined inner race which is surrounded by retaining ring and set screws. The axle is of the fully floating type.
Initially this joint was manufactured by Bendix Products Corporation of America. It consists of two members each with two fingers or arm in the side of which are formed semi circular grooves. When the two members are assembled the fingers of one fit in between the fingers of the other and balls are inserted in the groove of the fingers and form the driving correct in between them. Four balls are placed in side grooves of the fingers where as one ball is placed correctly in the groove provided on the both members. The formation of grooves is such that the balls always lies in a plane making equal angles with the axis of the shafts.