CV Axles Explained
As for the personal vehicle, Nicholas Otto typically is credited with the invention of the gasoline internal combustion engine. When he got it right, he built a motorcycle around it. Otto’s engine used reciprocating piston motion to turn a crank that provided torque to do
work. Pulleys, belts, drive and driven pulleys, axles, brakes and methods of steering continually improved over time, and in the natural course of engineering, drive shaft technology has evolved repeatedly. Some of the early vehicles used a torque tube, which was a pipe-like affair that not only provided protection for the spinning drive shaft, but actually transmitted the pushing power of the drive axle to the vehicle frame.The engine and transaxle that provide the driving power is sitting on rubber mounts connected to the frame, but the wheels and the final drive have, for decades, been unsprung weight. What that means is that the wheels and the final drive assembly (the rear axle) are riding directly on the ground and have to follow the bumps and whatnot while the body and frame ride smoothly above the springs. Because the axle is mounted to the body with hinge-like rods and beams that push the vehicle as the wheels do their work, the torque tube has pretty much disappeared and the propeller shaft (driveshaft) spins in the open air. The driveshaft is relatively stationary on the transmission end as it transmits power to the final drive. But during the course of driving, it has to continually provide torque to the drive axle while absorbing angle changes related to torque, load and terrain. The changes aren’t that steep in most cases, but it goes without saying that the propeller shaft not only has to continue providing torque during those angle changes
But the real-world dynamics of suspension geometry demands that the shaft gets longer and shorter as the axle travels up and down. So there is a flexible joint on each end of the shaft, which typically is connected to the splined output shaft on the transmission with a long sleeve that is smooth on the outside (for sealing) and splined to match the tranny output on the inside. These splines are heavily lubricated so they’ll move freely to lengthen and shorten the shaft, necessarily accommodating geometry changes that come with axle movement.CV Joints and Axles The older type Cardan U-Joints that provide the necessary flexibility will originate vibration if their angles are too sharp. There are wedged leaf spring spacers to change the mounted angle of the rear
axle in cases where product variability or mods have created a problem. This angle problem stems from the fact that geometry dictates standard Cardan U-Joints, even when brand new, tend to get hard-then-easy (slow-then-fast) while being driven,sort of like the plain old 3/8 wiggler on the end of that long extension you typically use to get to transmission bell housing bolts. Anybody who has ever tried to use a regular wiggle socket on an impact wrench has experienced that, right? Or am I the only dodo who has tried that? Tool wisdom dictates that we use a specially designed constant velocity wiggle socket on an impact wrench, one that won’t get hard-then-easy (slow-then-fast) while transmitting torque. That special wiggler is, in effect, a constant velocity (CV) joint for driving sockets. It spins at the same speed through a wide range of angles while transmitting torque from the power source to its business end, and that’s the same job handled by CV joints. Double Cardan CV joints on early 1970s Impalas, Caprices and Lincolns had two regular U-joints operating in tandem at the rear of the driveshaft with a special yoke between them. This arrangement made driveshaft less vibration prone, but was a pain to service and couldn’t accommodate steep angle changes.
The first CV joints I remember seeing in the flesh were on a 1969 VW Beetle (my dad ran a bug shop for a lot of years), and those oddball joints provided drive torque and miniscule plunging action at both ends, with the drive axle splined to the inner part of the joint and the outer part of the joint literally bolted to flanges on the transaxle on one end and the hub on the other. Those bolts were notorious for coming loose if not properly torqued on the older VW Beetles and Rabbits; more than a few of those little cars wound up on the side of the road with that kind of failure.Mid-1990s Lexus LS model cars have bolted CV joints of this design on their rear differentials, but they don’t typically come loose. Those same bolt-on joints were used on the inner end of the axles on front wheel drive VWs (Rabbit, Jetta, etc.), but the outer CV joints were the more conventional kind and had to provide torque during steering without creating the wobbly feel of a non-constant velocity joint. Early four-wheel drive domestic pickups had regular U-joints at the point where the drive axle connected to the front wheels, because they had to steer and pull at the same time. Foreign makes started using CV axles for the front wheels on 4WD vehicles, because, once again, they work smoother at the steering end of the axle. And with the differential pumpkins mounted to the car body feeding their power to the hubs through CV axles on so many vehicles nowadays, these are ever more prevalent, even on rear wheel drive vehicles. Some GM vehicles have an inner CV joint that snaps onto a stub axle that is supposed to stay in the final drive when the CV shaft is removed. The stub is removable, though, and tends comes out with the axle, which is fine as long as you know it isn’t part of the axle proper. The stub is outfitted with a replaceable polished seal sleeve. Servicing the Joints Just about everybody who has pulled a wrench gets to replace a fair number of CV axles and parts houses nowadays usually have them for just about any vehicle. CV joints on both ends of today’s drive axles have to be sealed with robust rubber accordion-style boots to keep grease in and road splash out. If the boots become compromised (and it happens all the time on high-mileage vehicles) the invading road grit mixes with the CV joint grease, converting it to a very effective grinding compound. In that case, the balls make dimples for themselves in their channels during straight ahead driving. During sharp turns as the balls work their way back and forth past those dimples, an annoying popping sound warns the driver that things aren’t as they should be, and the CV axles typically are replaced. In extreme cases, the balls literally will fall out of some joints and leave the vehicle immobile. Boot failure isn’t generally a problem on the plunging joints, but those can come apart internally too. If they’re sticking instead of plunging, you’ll actually feel the powertrain doing strange cyclic things while driving. On the lift with the hood open and the wheels spinning in gear it may be trying to move from side to side. If an engine mount works loose on some of vehicles, the inner joint may contact the H frame and cause a similarly odd sensation. A careful inspection will typically reveal signature marks on the outer surface of the inner CV joint hull in cases like that. Tools and Parts In the days before CV axles became so cheap, we’d replace busted CV axle boots if the joints weren’t popping by first removing the CV joint, disassembling it, inspecting it for dimples where the balls traveled, reassembling and greasing it if it was OK, and putting it all back together with new clamps. I did a lot of these at the VW dealership, and we’ve actually done some of it in my automotive department, but the clamps have to be the right kind and usually take a special tool to install. Note: A heavy tie wrap or a regular hose clamp won’t do the job. If the clamps aren’t installed tight enough, they won’t hold the boot in place on the big end, and if it pops off, the boot may as well be broken. There are special air powered tools and special universal boots that can be stretched over the CV joint and put in place, but the tool costs about $300. Renault CV axles had a big knuckle that was part of the axle shaft, and the dealership had a special cone-shaped chrome plated tool for stretching the boots over that knuckle after you popped the peculiar spring clip assembly off that retained the joint. But you had to lube the boot tool with motor oil to get the boot to slide up those fastigiated rods and stretch them over that big ugly knuckle – grease wouldn’t work. At the outer end, the spindle has to be disconnected at the lower ball joint (there isn’t usually an upper ball joint on strut-equipped cars) so as to let it swing out, allowing the drive splines to be extracted from the hub. On some CV axles, the splines slide freely out of the hub, and on others a hub puller is required to push it out. I’ve had confused automotive students who had removed several easy sliding hubs try to beat the first hard-pressed shaft they encountered out of there, and that leads to disaster, particularly if the CV axle is being removed from the hub for some other kind of service and wasn’t slated to be replaced. That is usually an hour’s work with a thread file fixing the damage gives them a new perspective when they screw up that way. It’s important to double-check the torque specs when reinstalling the nut, especially on free-sliding splines. The inside joints either snap into the axle gears in the differential or onto a stub shaft that snaps into the axle gears, and that joint is the one that plunges. So if you’re removing the CV axle for some other kind of service, don’t yank on the shaft. If those plunging rollers come out of their channels inside that boot it’s difficult to put them back in place. A pry bar works well (usually) when popping the joints out of the axle gears (sometimes it’s hard to get anything in there), but there are times when it’s a heavy duty pain getting them out. There’s a slide hammer attachment that works on about 20 percent of the joints I’ve seen. The small spring ring on the tip of the inner joint that expands as the inner joint splines seat in the axle gear can become worn over time to the point that it becomes stepped instead of smooth and round and it can really grab hold of that axle gear with a vengeance. If that happens on an axle that has a difficult to access center joint, it can be a pain. If it happens on both joints, it’s a mega pain. If one axle comes out more easily, the toughie can be knocked out of there with special tools designed for that purpose. One way or another, CV axles are here to stay, even if hybrids and EVs totally replace regular gas burners. The power still has to make it to the wheels, and CV axles are the best way to get it there. Source: http://www.searchautoparts.com/motorage/maintenance-repair/naked-axles?page=0,0