Axle Shafts


Axle Shafts

The purpose of the axle shaft is to transfer driving torque from the differential assembly to the vehicle's driving wheels. There are two types of axles: dead and drive or live.

DEAD AXLE The dead axle does not drive a vehicle. It merely supports the vehicle load and provides a mounting place for the wheels. In automotive operation, dead axles are usually on trailers and tandem-type axle vehicles.

DRIVE OR LIVE AXLE The main purpose of the drive or live axles is to transfer torque from the differential to each driving wheel. Depending on the design, rear axles can also help carry the weight of the vehicle or even act as part of the suspension. Most driving axles are enclosed in a housing. This housing is usually of a three-piece design. The two axle housings are pressed into the differential housing and then welded in place. Welding the three pieces together makes the axle housings and differential housing a single unit. A variation of the driving axle is the open axle shaft. In this design, the axle shaft is not enclosed in a housing. Instead, it is part of the suspension system and moves up and down with changing road conditions.

Three types of driving axles are commonly used: semifloating, three-quarter floating, and full-floating. All three use axle shafts that are splined to the differential side gears. The combination of the driving axles and the differential is known as the final drive. At the wheel ends, the axles can be attached in any one of a number of ways. This attachment depends on the type of axle used. The differences between the three types actually lie in the manner in which the shafts are supported on bearings and the way they are attached to the wheel hubs.

Semifloating Axle Shafts

Semifloating axles help to support the weight of the vehicle. The differential assembly is supported by tapered roller bearings and the axle housing. The semifloating axle shafts are not affected by stresses associated with the differential's operation.

The inner ends of the axle shafts are splined to the axle side gears. The axle shafts transmit only driving torque and are not acted upon by other forces. Therefore, the axle shafts are said to be floating.

The driving wheels are bolted to the outer ends of the axle shafts. The outer axle bearings are located between the axle shaft and axle housing. This type of axle has a bearing pressed into the end of the axle housing that supports the axle shaft. The axle shaft is held in place with either a bearing retainer belted to a flange on the end of the axle housing or by a C-shaped washer that fits into grooves machined in the splined end of the shaft. A flange on the wheel end of the shaft is used to attach the wheel.

When semifloating axles are used to drive the vehicle, the axle shafts push on the shaft bearings as they rotate. This places a driving force on the axle housing, springs, and vehicle chassis moving the vehicle forward. The axle shafts takes bending stresses associated with turning corners and curves, skidding, bent or wobbling wheels, as well as the weight of the vehicle. In the semifloating axle arrangement, if the axle shaft breaks, the driving wheel comes away from or out of the axle housing.

Three-Quarter Floating Axle

The wheel bearing on a three-quarter floating axle is on the outside of the axle housing instead of inside the housing as in the semifloating axle. The wheel hubs are bolted to the end of the axle shaft and are supported by the bearing. In this arrangement, the axle shaft does not support the weight of the vehicle. It is transferred through the wheel hub and bearing to the axle housing. Three-quarter floating axles are found on older vehicles and some trucks.

Full-Floating Axle Shafts

Most heavy commercial vehicles use a full-floating axle shaft to accommodate the higher vehicle loads. This design is similar to the three-quarter floating axle except that two bearings rather than one are used to support the wheel hub. These are pressed over the outside of the axle housing and carry all of the stresses caused by the torque loading and turning. The wheel hubs are bolted to flanges on the outer end of each axle shaft.

In operation, the axle shaft transmits only the driving torque. The driving torque from the axle shaft rotates the axle flange, wheel hub, and rear driving wheel. The wheel hub forces its bearings against the axle housing to move the vehicle. The stresses caused by turning, skidding, and bent or wobbling wheels are taken by the axle housing through the wheel bearings. If a full-floating axle shaft should break, it can be removed from the axle housing. Because the rear wheels rotate around the rear axle housing the disabled vehicle can be towed to a service area for replacement of the axle shaft. This is considered by many technicians to be an advantage of the full-floating axle design.

Independently Suspended Axles

This type of rear driving axle system is found mostly on European automobiles. The driving axles are usually open instead of being enclosed in an axle housing.

The two most common suspended rear driving axles are DeDion axle system and the swing axle system.

DeDION AXLE SYSTEM The DeDion axle system resembles a driveline. The driveline axle resembles a drive shaft, with U-joints at each end of the axles. A slip joint is attached to the inboard, or innermost, U-joint. The outboard, or outermost, U-joint is connected to the driving wheel. This allows the driving axle to move up and down as it rotates.

SWING AXLE SYSTEM On automobiles with swing axles, the driving axles can be open or enclosed. An axle fits into the differential by way of a ball-and-socket system that allows the axle to pivot up and down. As the axle pivots, the driving wheel swings up and down. This system best describes the drive axles of a front-wheel-drive vehicle.

Axle Shaft Bearings

The axle shaft bearing supports the vehicle's weight and reduces rotational friction. In an axle mount, radial and thrust loads are always present on the axle shaft bearing when the vehicle is moving. Radial bearing loads act at 90 degrees to the axle shaft's center of axis. Radial loading is always present whether or not the vehicle is moving.

Thrust loading acts on the axle bearing parallel with the center of axis. It is present on the driving wheels, axle shafts, and axle bearings when the vehicle turns corners or curves.

There are three designs of axle shaft bearings used in semifloating axles.

- Ball-type bearing

- Straight-roller bearing

- Tapered-roller bearing

The bearing load of primary concern is axle shaft end thrust. when a vehicle moves around a corner, centrifugal force acts on the vehicle body, causing it to lean to the outside of the curve. The vehicle's chassis does not lean because of the tires' contact with the road's surface. As the body leans outward, a thrust load is placed on the axle shaft and axle bearing. Each type of axle shaft bearing handles axle shaft end thrust differently.

Normally, the way the axles are held in the housing is quite obvious after the rear wheels and brake assemblies have been removed. If the axle shaft is held in by a retainer and three or four bolts, it is not necessary to remove the differential rover to remove the axle. Most ball- and tapered-roller bearing supported axle shafts are retained in this manner. To remove the axle, remove the bolts that hold the retainer to the backing plate, then pull the axle out. Normally, the axle shaft slides out without the aid of a puller. Sometimes a puller is required.

A tapered-roller bearing supported axle shaft does not use a retainer to secure it. Rather, a C-washer is used to retain the axle shaft. This C-washer is located inside the differential, and the differential cover must be removed to gain access to it. To remove this type of axle, first remove the wheel, brake drum, and differential cover. Then, remove the differential pinion shaft retaining bolt and differential pinion shaft. Now push the axle shaft in and remove the C-washer. The axle can now be pulled out of the bearing.

Ball bearings are lubricated with grease packed in the bearing at the factory. An inner seal, designed to keep the gear oil from the bearing, rides on the axle shaft just in front of the retaining ring. This type of bearing also has an outer seal to prevent grease from spraying onto the rear brakes. Ball-type axle bearings are pressed on and off the axle shaft. The retainer ring is made of soft metal and is pressed onto the shaft against the wheel bearing. Never use a torch to remove the ring. Rather, drill into it or notch it in several places with a cold chisel to break the seal. The ring can then be slid off the shaft easily. Heat should not be used to remove the ring because it can take the temper out of the shaft and thereby weaken it. Likewise, a torch should never be used to remove a bearing from an axle shaft.

Roller axle bearings are lubricated by the gear oil in the axle housing. Therefore, only a seal to protect the brakes is necessary with these bearings. These bearings are typically pressed into the axle housing and not onto the axle. To remove them, the axle must first be removed and then the bearing pulled out of the housing. With the axle out, inspect the area where it rides on the bearing for pits or scores. If any are present, replace the axle.

Tapered-roller axle bearings are not lubricated by gear oil. They are sealed and lubricated with wheel grease. This type of bearing uses two seals and must be pressed on and off the axle shaft using a press. After the bearing is pressed onto the shaft, it must be packed with wheel bearing grease. After packing the bearing, install the axle in the housing. Shaft end play must be checked. Use a dial indicator and adjust the end play to the specifications given in the service manual. If the end play is not within specifications, change the size of the bearing shim.

The installation of new axle shaft seals is recommended whenever the axle shafts have been removed. Some axle seals are identified as being either right or left side. When installing new seals, make sure to install the correct seal in each side. Check the seals or makings of right or left or for color coding.

More Drive Axles And Differentials

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