Volvo 240/740/760/780/940/960 1990-1998

Understanding the Rear Axle


See Figures 1, 2, 3 and 4

The rear axle is a special type of transmission that reduces the speed of the drive from the engine and transmission and divides the power to the rear wheels. Power enters the rear axle from the driveshaft via the companion flange. The flange is mounted on the drive pinion shaft. The drive pinion shaft and gear carries the power into the differential. The gear on the end of the pinion shaft drives a large ring gear the axis of rotation of which is 90 degrees away from the of the pinion. The pinion and gear reduce the gear ratio of the axle, and change the direction of rotation to turn the axle shafts which drive both wheels. The rear axle gear ratio is found by dividing the number of pinion gear teeth into the number of ring gear teeth.

The final drive is of the hypoid design, with the drive pinion lying below the ring gear. On solid axle models, each axle shaft is indexed into a splined sleeve for the differential side gears, and supported at its outer end in a tapered roller bearing. Bearing clearance is not adjustable by use of shims, but instead is determined by bearing thickness. Both sides of the axle bearings are protected by oil seals.

On vehicles with a multi-link suspension, the axles are actually halfshafts, bolted to the differential. Each halfshaft has a constant velocity (CV)-joint at each end, allowing a full range of motion as the vehicle passes over bumps and depressions.

The ring gear drives the differential case. The case provides the 2 mounting points for the ends of a pinion shaft on which are mounted two pinion gears. The pinion gears drive the 2 side gears, one of which is located on the inner end of each axle shaft (beam type axle) or inner driveshafts (multi-link axle).

By driving the axle shafts through the arrangement, the differential allows the outer drive wheel to turn faster than the inner drive wheel in a turn.

The main drive pinion and the side bearings, which bear the weight of the differential case, are shimmed to provide proper bearing preload, and to position the pinion and ring gears properly.

The proper adjustment of the relationship of the ring and pinion gears is critical. It should be attempted only by those with extensive equipment and/or experience.

Limited-slip differentials include clutches which tend to link each axle shaft to the differential case. Clutches may be engaged either by spring action or by pressure produced by the torque on the axles during a turn. During turning on a dry pavement, the effects of the clutches are overcome, and each wheel turns at the required speed. When slippage occurs at either wheel, however, the clutches will transmit some of the power to the wheel which has the greater amount of traction. Because of the presence of clutches, limited-slip units require a special lubricant.

Type 1041 rear axle (beam type) is attached to the body by support arms, a torque arm and a torque arm frame. A Panhard rod is installed between the rear axle and crossmember. An anti-roll bar is also mounted between the support arms. The 1041 rear axle can be easily identified by its aluminum inspection cover.

Type 1035 and 1045 rear axles (multi-link) is attached to the body by a member consisting of an upper and lower section. The upper links extend from the upper member. The lower links and 2 track rods are mounted between the lower section of the axle member and the wheel bearing housings. The wheel bearing housings and body are connected by the support arms.

Certain rear axle variants are equipped with Automatic Differential Lock. The locking mechanism is controlled by a centrifugal governor. This device operates automatically when one of the drive wheels is spinning and the speed of the vehicle is less than 25 mph (40 km/h). When the vehicle is driven at a steady speed and both driveshafts are rotating at the same speed, the differential functions exactly as a conventional type.

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Fig. Fig. 1: Example of a solid beam type of rear axle

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Fig. Fig. 2: Example of a multi-link (independent) suspension rear axle

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Fig. Fig. 3: Differential case of a multi-link rear suspension

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Fig. Fig. 4: Cutaway view of an Automatic Differential Lock rear differential