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Stocking & Selling Drivetrain


6/20/2007

According to a recent study, the halfshaft market is showing strong unit growth and sharp price competition. This month, read about what you need to know to stay ahead of the competition.
 

According to a recent study, the halfshaft market is showing strong unit growth and sharp price competition. That should be no surprise to distributors — take a look at your store’s receipts and you’ll see that drivetrain parts are an important and profitable product line. The line includes the components downstream of the engine and transmission that deliver torque to the drive wheels. For rear-wheel drive (RWD) cars and trucks, this includes the driveshaft, U-joints, driveshaft center carrier bearing (if the vehicle has a two-piece driveshaft), differential, drive axles and axle bearings. On four-wheel drive trucks, it also includes the transfer case. With front-wheel drive (FWD) cars and minivans, the drivetrain is more compact and includes only the halfshafts. But serviceable components in the halfshaft assembly include the inner and outer constant velocity (CV) joints, boots and boot clamps.

Due to the drivetrain layout on FWD cars, two separate driveshafts are used (one for each front wheel). Halfshafts may also be used on some rear-engine, RWD import cars as well as those with all-wheel drive.

There are CV joints at each end of both halfshafts (more on those in a minute). The outboard joints have a greater amount of built-in flexibility to allow the front wheels to steer. The inboard joints have a more limited range of motion but are designed to plunge in and out so the length of the shaft can change as the suspension moves up and down.

If either CV joint fails, the halfshaft is often replaced as a complete assembly. Many technicians and DIYers prefer to replace the entire shaft because it's quicker and easier than replacing individual joints.

Replacement halfshafts must be the same length as the original. The end joints must also have the same diameter shafts as the original, and the same number of splines per shaft as the original. On some vehicles that are equipped with antilock brakes, the wheel speed sensor ring is located on the outboard CV joint. If this is the case, make sure the sensor ring on the outboard joint matches that on the replacement shaft assembly.

Replacing a halfshaft is not as easy as replacing a driveshaft on a RWD vehicle. The front suspension must be partially disassembled, and a puller of some type may be needed to separate the inner CV joint from the transaxle and/or the outer joint from the steering knuckle and wheel bearings.

On some vehicles, such as older Ford Escorts, only one halfshaft should be removed at a time. This is to prevent the differential side gears inside the transaxle from slipping out of place.
If a vehicle has locking nub nuts, new ones should be installed when a halfshaft is replaced. Some vehicles may also require new ball joint pinch bolts if the ball joints have to be disconnected to replace a halfshaft.

The transaxle oil seals may also need to be replaced at this time. Leaks here can allow vital fluid to be lost from the transmission, which can result in transmission problems or even premature failure. Your customer may also need the specified ATF or gear oil for his transaxle.

CV JOINTS AND BOOTS
CV joints are flexible couplings used on the ends of halfshafts as well as some RWD driveshafts. They are so named because there is no difference between input and output speed as the angle of the joint changes (unlike U-joints, which do cause a slight change in speed that can lead to annoying vibrations).

There are several types of CV joints: Rzeppa, crossgroove, double-offset and tripod. Rzeppa, crossgroove and double-offset CV joints have six small balls housed between an inner and outer race. A cage with windows holds the balls in position as the joint flexes.
Rzeppa CV joints are “fixed” joints. But crossgroove and double-offset CV joints are designed to plunge in and out to accommodate length changes in the halfshafts as the suspension moves up and down. Rzeppa joints are used primarily as the “outboard” joints on most domestic and import FWD applications, while crossgroove and double-offset joints are used as the “inboard” joints on most import applications.

Tripod joints are somewhat different in that they have three roller bearings mounted on a tripod inside a “tulip” or “claw” shaped housing. Tripod joints may be fixed or the plunge variety. Tripod CV joints are mostly used as the inner plunge joints on domestic FWD applications. Tripod outer joints are only used on a few import FWD cars (older Toyota Tercel, AMC Alliance and Nissan Sentra).
CV joints are designed to run many miles with no maintenance. Many CV joint failures are the result of boot failures. The rubber or plastic boots that surround the joints keep grease in and contaminants out. If a boot cracks,  tears or rips, the grease inside will quickly leak out, allowing dirt and water in, causing rapid joint wear.

Installing a new one-piece boot will require removing the halfshaft and removing the CV joint from the end of the shaft. “Split-boots” that can be installed without having to remove the shaft can save a lot of labor, but require careful installation to prevent leaks. Some split boots have a seam that must be glued. Others use screws or a “zip-lock” design to seal the boot.

CV joints require special high temperature grease. Ordinary chassis grease should never be used. The grease is usually provided with a replacement boot and should be applied to the joint according to the instructions provided.

FRONT WHEEL BEARINGS
Wheel bearings reduce friction so the wheels turn freely. Either ball or roller bearings may be used. Most are sealed construction and require no maintenance (greasing or adjustment). But many wheel bearings on older vehicles are serviceable and require periodic cleaning and repacking with wheel bearing grease (never ordinary chassis grease)!

Worn wheel bearings can cause steering wander and noise. A wheel bearing failure can be dangerous because it may cause the wheel to separate from the vehicle!

Sealed wheel bearing assemblies are replaced as a unit. The serviceable variety are replaced as matched sets (race and bearings). Inner and outer bearings are different. Old grease seals should never be reused because they can leak. 















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