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CV joint tech -- A primer on constant velocity drive joints and diagnostic tips

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CV joint tech -- A primer on constant velocity drive joints and diagnostic tips

I realize that much of this introductory text will seem very basic, but let’s start by explaining CV joint technology. A CV (constant velocity) joint is a torque/drive mechanical coupling in which the rotational speed of the output shaft (inner joint) matches that of the input shaft (outer joint), regardless of the shaft angle. This allows torque output from the transaxle to the driven wheels to remain unchanged from the transaxle to the wheels even as the suspension (and the CV shaft) changes angles during up/down travel and during turns. CV joints don’t cause restrictions during angle changes, as compared to universal (U-type) joints.

A U-type joint works fine when there is a very limited shaft angle variation (primary driveshaft on a RWD vehicle, for example). However, a U-type joint will cause the driven shaft to slightly change length and will create a speed (rotation rate) change between the input and output side of the joint when the shaft must change its angle by more than a couple of degrees. Given the angle movements required for both suspension travel and steering angle changes, a U-type joint will create a vibration as the two sides (input and output) begin to “argue” during shaft angle movement. For this reason, a CV joint is required for FWD and independent RWD systems.

One factor of CV joint design that permits this freedom of movement without bind or differences in output/input is that the inboard CV joint features a “plunging” movement which allows the shaft to move in/out during suspension travel without restricting suspension travel.

Outboard joints

Outboard (wheel side) CV joints are usually the “fixed” type, also often referred to as a Rzeppa joint.  This features a bearing race with slightly arced and offset longitudinal grooves. A series of ball bearings run along these grooves. The balls are “trapped” in a cage that keeps the balls aligned to their grooves. The offset groove/caged ball design allows the joint to articulate to follow suspension travel and steering angles. These are also referred to as AC (angular contact) joints. Depending on the specific manufacturer, a typical outboard CV joint will allow a maximum articulation angle of about 47 to 50 degrees (naturally, a higher maximum angle will allow a smaller turning circle).

NOTE: Because of the greater operating angles and their greater exposure to road hazards (rocks, dirt, moisture, etc.), outboard CV joints tend to wear sooner than inboard CV joints.

Inboard CV joints

In order to accommodate suspension travel, the inboard joint is designed to allow in/out or “plunging” movement in addition to articulating for suspension travel angles.  This plunging movement allows the shaft assembly to change its length during suspension travel, compensating for control arm up-down/angles and to prevent restricting lower control arm movement.

Several styles of inboard CV joints are in use (although they all accomplish the same task). Tripod style CV joints use a three-legged design of three equally spaced roller bearings (instead of balls) that glide along track grooves inside a “tulip” style housing.  A DO (double offset) is also a plunging type joint, but features a series of ball bearings.

Typical inboard CV joints provide a plunge movement of about 50mm and a maximum articulation angle of about 22 to 31 degrees (depending on make and model).

Diagnosing CV joint problems

Popping/clicking noise during turns

A clicking or popping noise is indicative of a worn or damaged outer CV joint. One way to confirm this is to drive the vehicle in reverse (in a circle). If the noise is more pronounced, this confirms the need to replace the joint. If you’re driving backwards with the wheel cranked full right and the noise is louder, suspect the right outer joint.

If driving backwards with the steering wheel cranked full left, suspect the left outer joint. NOTE: Clicking/popping can also be caused by worn or damaged rack & pinion steering inner tie rod ends.

Shudder/clunking noise

If you hear/feel a clunking noise when the transmission is placed into drive gear, or during acceleration or deceleration, this may point to a worn or damaged CV joint. Keep in mind that the same type of noise/feel can result from excessive backlash in the differential gears.

If the vehicle features FWD (front wheel drive), suspect an inner CV joint. If the vehicle features RWD (rear wheel drive) with an independent rear suspension that features CV joints, suspect either the inner or outer joints. Naturally, the same condition may be caused by primary driveshaft CV or U-joints, worn/damaged inner tie rods or other worn suspension parts. If you suspect inner CV joints on the drive axle (FWD or RWD), try driving the vehicle in reverse while accelerating and decelerating. If the problem becomes more noticeable, the cause is likely one or both inner joints.

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Vibration during acceleration

If the customer complains about a vibration or “shudder” during acceleration, this may indicate a worn or damaged inboard plunge joint. Other possible causes include excessive play in inboard or outboard CV joints, or a problem with engine and/or transmission mounts or torque strap bushings (inspect for damaged, loose missing rubber mounts or deteriorated engine/trans mount).

NOTE: If the vehicle has experienced a “pullout” failure (CV shaft assembly separates at the inner CV joint or pulls out of the transaxle), this may be the result of a previous collision. Take a hard look at the engine and trans mounts, and inspect for distorted or broken suspension parts. Naturally, if any severely worn mounts or suspension components are present, these issues should be corrected before installing a replacement CV shaft assembly.

Constant vibration at speed

If the customer complains of a vibration that begins at a certain speed, and increases at higher speeds, the problem is most likely not CV-joint related or CV joint shaft imbalance. More likely causes include wheel imbalance, tire radial runout, bent wheel, etc.

Constant growling noise

If the complaint is about a constant growl or hum while driving (even with wheels straight ahead), the most likely cause is worn/dry/damaged wheel bearings. Other possible causes include dry CV joints (where lubrication has been lost), or other driveline issues, such as worn transmission shaft bearings or a worn/damaged intermediate shaft bearing.

Always, always check the boots!

The most common cause of CV joint problems relates to the protective joint boot. If a CV joint problem is suspected, or for that matter, whenever a vehicle that features CV joints enters the shop for any service procedure, always inspect the CV joint boots. Carefully check the boots for holes, splits, cracks and tears, and check the boot clamps for looseness. Even if a boot is not already torn or otherwise compromised, the boots (depending on vehicle design) may be subjected to high heat levels, in which case the boot material may become heat-hardened and brittle. When the boot reaches the point of being brittle, it’s only a matter of time before the boot cracks and splits.

Also inspect for signs of grease being slung from the boots. Any excuse for grease to exit, or for water and road debris to enter the joint, spells long-term disaster for the joint (and by “long term,” I mean that a CV joint that is allowed to be contaminated can begin to destroy itself in a short amount of time). If you find a compromised boot, the problem must be addressed immediately.

A compromised boot is almost always the root cause of CV joint problems. If a bad boot is caught early, “simply” replacing the boot (and cleaning/re-greasing the joint) may save the day. However, it’s very difficult if not impossible to accurately judge how long the boot problem has existed, and whether or not the joint’s cleanliness has been compromised. Often this is a judgment call on your part. If the joint appears to be functioning properly and is making no noise, a boot replacement may be the answer. If any doubt exists, or especially if the joint (with the bad boot) is making any abnormal noise, don’t even think about boot replacement. Instead, replace the joint. As we all know, it’s far quicker and easier to simply replace the entire CV shaft assembly as opposed to disassembly, rebuilding and reassembly.

 If a boot replacement alone is your best course of action, you have two choices: remove the bad boot and its joint, and reassemble using a new boot; or cut the bad boot off the shaft and replace with a split boot.  It should be obvious that replacing a one-piece boot is not as quick or simple as the typical customer might think. The CV shaft assembly must be removed from the vehicle (which entails removing the brake caliper, brake rotor and hub, disconnecting the anti-sway bar link, disconnecting the lower ball joint and disconnecting or removing the strut). The CV joint must then be removed from the shaft assembly in order to remove the old boot and install the new boot. And of course, after reassembly, the front wheels should be re-aligned.

Considering the labor time involved in performing a boot replacement, it makes sense that the customer should consider a replacement of the entire CV shaft assembly. Yes, this involves a higher product expense, but shaft assembly replacement will eliminate the time needed to remove/reinstall the joint as needed for boot replacement. And, common sense tells you that since the CV joint has likely been compromised by the damaged boot anyway, it’s safer to replace the joint (either by replacing the offending joint or replacing the entire shaft assembly). Replacing the entire shaft assembly reduces the customer’s labor cost and eliminates concerns for a potentially compromised joint.

Split boots

Split boots offer a handy fix, since joint removal isn’t needed. Cut the old boot off, clean the shaft and inspect/clean/grease the joint as needed, and install the split boot. The critical aspect involves sealing the two halves of the split boot. The adhesive included with a split boot kit must be applied carefully, and the adhesive must be allowed to cure (this could take 30 to 60 minutes). Some split boots feature small screws that secure the halves together instead of requiring adhesive. Basically, you should only consider using a split boot if time, budget (or both) are critical factors. A split type boot, regardless of how well it was designed, will not provide the durability of a one-piece boot. But, it’s a handy alternative, depending on the individual situation. As with various aftermarket quick-fixes, they may not be ideal, but split boots have their place.

[PAGEBREAK]Removing the CV shaft assembly from the transaxle

The old-school term “we don’t need no stinking instructions” does not apply here. If you’re not already familiar with the vehicle at hand, always refer to the appropriate service manual for CV shaft removal. Some inner CV shaft ends will pop out by simply applying leverage between the transaxle housing and a dimple/recess on the inner joint housing, while others are secured with a retaining clip (which may require removal of a transaxle cover plate for access).

NOTE: DO NOT pull on the shaft in order to dislodge the inner joint splined stub axle from the transaxle. This can result in pulling the inner joint apart. Apply pulling force only at the inboard joint housing. Also, once the outer CV joint has been disconnected from the hub assembly, be sure to support the shaft assembly prior to and during inner joint disconnection. Allowing the shaft to hang down can also result in pulling the inner joint apart.

CAUTION: NEVER pound the end of the outer CV shaft’s stub with a hammer in order to dislodge it from the hub. Even if you temporarily install the old nut to protect the stub splines, you can cause damage to the inner joint and/or the outer joint, the differential gears and the wheel bearing. If the stub axle is stuck in the hub and won’t push out by hand, use a puller. Even if nobody is looking, don’t cheat. Leave the BFH in your tool chest drawer.

Servicing a CV joint

If the joint is to be disassembled for inspection, the CV joint will likely be secured with a snap/lock ring (although some joints may not feature a snap ring and are designed to be pulled/nudged off). Tilt the inner race of the joint fully to one side (a slender drift will aid in this). Begin removing the bearing balls (tiling the race as needed until all balls are removed). Bear in mind that each ball establishes a distinctive wear pattern in its groove, so it’s critical to keep all balls organized per their respective grooves. DO NOT MIX UP THE BALLS. Once all balls have been removed, rotate the ball cage sideways in order to remove the inner race.

Inspect the inner and outer races, balls and cage. Look for any defects such as nicks, burrs, flaking, etc., on the balls and races. Inspect the cage for cracks, wear, burrs, etc. If any defects are found, replace the CV joint. Even if no abnormalities are found (no cracks, burrs, wear marks, etc.), check each ball in its original track for fit. The balls should fit snugly with no slop. Loose-fitting balls can be responsible for popping or clicking noises.

If rebuilding a CV joint, ALWAYS use ONLY the CV-joint-specific grease recommended or supplied with the new boot kit. The amount of grease is also critical. If a grease packet is supplied with a new boot kit, use the entire amount of grease supplied. Generally speaking, apply about 33% of the grease directly to the balls, tracks, cage and races, and apply the remaining grease to the inside of the boot. However, be sure to read and follow the lubrication instructions supplied with the kit.

When installing the new boot, it must be installed in a “relaxed” state, avoiding any collapse or twisting/distortion. Be sure to position the boot with its sealing lips seated into their respective grooves.  Do not tighten the boot clamps until you are certain that the boot is positioned properly.

If you’re installing new boots, you’ll need a clamp tightening tool. Earless type clamps feature interlocking tabs that allow the band clamp to snap together during tightening. An earless type wrench allows you to easily pull the overlapping sections of the band together in a ratcheting type action. Ear-type clamps join together by pinching the mating ends together in a metal forming/die action. Pinch type pliers snug the mating ends together and form the ears in a folding action. This requires a bit more muscle. Specialty tightening wrenches are available that feature square drive holes, allowing you to use a torque wrench to achieve proper tightening while providing added leverage.

As stated earlier, this article is very basic in terms of CV joint inspection and service (a seasoned tech is certainly already aware of these issues), but should prove helpful to newbie techs who are just starting out.   ●

 

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