Steady As She Goes... Constant-Velocity Driveshaft Joints The original constant-velocity joints weren t on FWD steering axles but on RWD driveshafts. Once you see what they are for and how they work, you can also see how to repair them when they wear out. O nce a term gets carved in the stone of shop manuals and parts catalogs, that's what the thing is forever, regardless of how poor the fit is between the part and the function and the word. A "constant-velocity" joint is just such a mismatch: the joint doesn't turn at a constant velocity; it moves at roughly the speed the driveshaft does, as the motorist decides. What's more, as we'll see, different parts of the joint rotate at speeds varying from the driveshaft, and for a very good reason. Generally when we talk about CV joints here in Import Service, we're talking about the kind used on front-wheel-drive cars, Rzeppa joints, tulip-tripot plunge joints and the like. We'll give those a rest this month and look at the constant-velocity joints used on rear- wheel-drive vehicles and on the front driveshafts of some four-wheel-drives, as on the Toyota 4Runner shown. The most common, though not exclusive, use for these joints is between the driveshaft and the drive axle, if the drive axle has coil springs and various bushed links to hold it more or less in place. This joint can accommodate the larger changes of driveshaft angle resulting from the axle's twist under engine load. Heavier vehicles with the drive axle clamped to stiff multileaf springs ordinarily don't encounter much axle rotation with drive torque, so they don't need the CV's. Many pieces of machinery besides just cars and trucks use rotating shafts to convey torque from one place where it is generated to another where it does some useful work. While the origins are surely lost in antiquity, we can see interesting examples of driveshafts working machinery in classical Chinese irrigation pumps as well as the more familiar Dutch windmills of the late Middle Ages. The Chinese and Dutch machine-builders found the same problem: it is hard to arrange the equipment so there is a straight axial run between the engine generating the power (counting water wheels, windmills and treadmills as engines) and the grinding or pumping workpiece. No doubt, you've seen pictures of long belts used to turn shafts in turn-of-the-century factories, all to avoid making torque go through an angled shaft. A worse problem comes if the shaft must make an angle that changes variably. If the angle is fixed, especially if the angle is 90 degrees, it is fairly simple to just use a gear, as in a differential or steering box. But if the angle changes during use, such a gear arrangement will be very hard to build and expensive. So we have universal joints. Early joints intended to allow a driving shaft to turn a driven one through some sort of angle took several forms (see next page, lower left), eventually becoming the U-joints and CV joints we're familiar with. 8 June 1999
Some involved sliding fingers that shared a ball held in between, with the high level of friction and wear you can imagine. Some required intricate internal sliding joints reminiscent of the inside parts of those take-apart wooden puzzle balls. Some were merely large rubber donuts, flexing as the shafts did (a system still used on some vehicles with independent suspension where the differential does not move relative to the vehicle frame except to squirm a little in its mounts). Some were identical, in geometry at least, to what we think of as a universal joint. We still see these on steering columns. But there is a general problem with U-joints, a problem arising from the geometry. The design of single- Cardan U-joints is this: each mating shaft has a twopivot yoke holding opposite arms of the Cardan-cross spanning the two (By the way, people often speak of a Cardan-cross as the U-joint "spider," though the notion of a quadruple-amputee arachnid as a model for a mechanical part has always stumped me. Whatever you want to call it, that's the part I'm talking about). As long as the shafts are pointing in exactly the same direction, as long as they are coaxial, there is no problem: shafts, yokes, Cardan-cross, needle-bearing cups and all rotate at exactly the same speed. But once the shafts change so they are no longer coaxial, geometric problems occur; and the drive and driven shafts try to turn at constantly varying speeds, taking the form of a powerful torsional vibration twice per rotation. Let's suppose the driving shaft turns at a fixed, constant rate. Actually, of course, in a vehicle on the road, both shafts change their speed depending on all sorts of factors from engine smoothness to the qualities of the pavement; but we can't factor in all of that, or we'd never get anything sorted out clearly. As the shaft turns, the two arms of the Cardan-cross and the needle-bearings it holds must, because of the rigidity of the mechanical parts, turn in the circle its yoke describes at the exact RPM the shaft is turning. Various designs preceded the standard U-joint of today, and some like the rubber disk have retained specialized uses. Other designs successfully solved the geometric problem and kept the shafts turning at the same relative speed to one another throughout each revolution, but at a cost of a mechanical complexity that translated into high manufacturing expenses and rapid wear from the amount of surface friction. If the angles between the shafts are different, however, the geometryinduced vibration becomes significant, not only perceptible to the driver but damaging to the equipment. The centerline from the crankshaft through the transmission, down the driveshaft and into the differential pinion shaft is seldom perfectly in line because of movement of the suspension and of the engine. Ideally the angles formed between the transmission output shaft and the driveshaft and between the driveshaft and the pinion shaft will be equal. Then (but only then) the geometry-induced vibrations will cancel out. They cancel out only in a final sense, though: the transmission output shaft and the differential pinion will turn in perfect synch with the joints at equal angles on the same plane. The driveshaft itself will still vary in rpm the same way, but will match a speedup at its transmission shaft end with a slowdown at its pinion shaft end. But when the differential nose can twist up under engine torque or down under deceleration engine braking, the angles are no longer equal, and you need a constant-velocity joint between the driveshaft and the differential. While there are a few differences in detail from one RWD constantvelocity joint to another, all of them share the same basic mechanical/geometric structure; The driveshaft ends in a regular Cardan joint, pivoting at the shaft and in the front of an intermediate link, the yoke. The yoke holds Cardan joints at either end, spring-loaded apart with a pivot ball between the shaft and the yoke. The rear of the yoke U-joints to the differential pinion drive flange. This description, of course, assumes we're talking about the rear joint on a rear driveshaft. Turn the page upside down or stand on your head to understand a CV on a front driveshaft. Thanks to Moog for these graphics. June 1999 9
The point of the spring-loaded pivot ball in the middle of the yoke is to keep the joint under enough compressive force it can't flop around loose. This way it keeps the rotational plane of the yoke exactly between the rotating plane of the driveshaft and that of the pinion shaft. The result is that, while the yoke does not move exactly in time with the driveshaft, its variations in speed correspond exactly to the opposite variations in speed generated by the second U-joint s geometry. So the driveshaft and the pinion shaft do move exactly in synch. Notice the CV yoke now sustains exactly the same rotational speed variation the entire driveshaft did in our earlier example when both traditional U-joints were at the ideal equal angles on the same plane. The internal socket ball works in a fairly simple way, compression-loading the joint to keep the yoke axis at an equal angle to each shaft. But it is possible to assemble the pieces in the wrong sequence, so lay them out in order when you disassemble the joint. Notice that not all socket ball joints use a replaceable socket ball; those that do often use an identifying notch. Some socket balls use internal needle bearings just like the U-joint cups. Angle Between Shafts 25 20 15 10 5 0 5 10 15 20 25 One Shaft Rotation In an ordinary U-joint, we have A C three rotation-speed-relevant factors: the speed of the driving B shaft, the speed of the driven shaft and the angle between them. When the angle is zero, when the two shafts are exactly in line, the variation in their rotational speed is zero. As that angle increases, the driven shaft speeds and slows twice per shaft rotation, causing a torsional vibration. The greater the angle, the greater this variation, but the frequency remains constant with the speed of the driving shaft. If we have two U-joints at the right configuration, however, the variations can exactly cancel out, the basis for the driveshaft double-cardan CV. Shaft RPM Difference But if the other shaft is at an angle, the Cardan-cross flip-flops twice per revolution, oscillating about its bearing cups to allow the two shafts to share the same rigid cross. If the driving shaft is turning at a constant speed, though, the driven shaft must vary in speed by a certain amount, accelerating and decelerating each time the Cardan-cross flip-flops. The amount of speed change this involves depends on the angle of the two shafts: the steeper the angle, the greater the change of rotational speed, twice each shaft revolution. If we look at the graph on the left, we can see just how much of a variation this can be. Clearly such a variation in shaft speed would translate into not only drive harshness and vibration, but into reduced service life as the variable load put repeated and opposite stresses on the components. And we observe just this in the workbay real-world. If you are familiar with older four-wheel-drive vehicles with large single-cardan joints in the front hubs (the kind with 'notchy' steering at left and right steering stops), you know they had a useful life of about 50,000 miles, even though most of them were not engaged or powered more than a small percent of their driving time (estimates based on offroad club measurements are that even 'done-gone-muddin' yahoos use four-wheel drive no more than about four percent of the time). Even at that, the considerable stress from the steering, braking and engine torque wore through these joints very quickly, even though they were of much more robust construction than ordinary driveshaft U-joints. The double-cardan, the driveshaft CV joint, solves this problem with a compression-spring-loaded center ball and socket, as we'll see taken apart in the photos. Its geometry keeps the angle of the shafts on either side of it equal with respect to the rotational axis of the yoke. Then the CV's Cardan joints flipflops cancel one another out, and the only rotational variation occurs in the yoke casting itself. Pretty neat, eh? Enough for the geometry lesson. Now let's look at how the RWD constant velocity joint comes apart and goes back together again. It's like a pair of U-joints, but it's unlike them as well and a bit more difficult. As you can see from the parts laid out, there may be more parts in such a joint than in some simple transmissions more if you count bearing needles as individual parts. But the work is not that complicated if you do it in the right sequence with the right tools. You know the joint needs repair when it shows the ordinary symptoms of a failed U-joint, but there is an additional symptom if the internal ball and socket has worn out. You can push the joint from side to side. A worn joint will almost always makes that familiar clunking sound, or you can see the rust and metal chunks when a needle bearing perishes. By Joe Woods June 1999 11
Constant-Velocity Driveshaft Joints There are enough parts in a constant-velocity double-cardan joint to equal the number in a small transmission, and their assembly order and sequence are critical to their proper function, keeping drive- and driven shafts at exactly the same speed. While it is possible to remove and replace ordinary U-joints in a vise or even with a hammer and a couple of sockets against a concrete floor, you need the right tool to do the CV version. There is much less clearance on each side than on the plain vanilla joint and more opportunity to bend the socket ball protective cup. Thanks to Snap-on for the use of their tool. Some constant-velocity joints hold their Cardans not with snap rings but with a thermoplastic bond driven in under great pressure at the factory. These are identifiable by the small peg-shaped protrusions on each side. Before you can disassemble these joints, heat the yoke adjacent to the cups with a propane or other low temperature torch until the thermoplastic squirts out the peg holes. Apply heat around the entire ring of the yoke bore, because the plastic grips the cup all the way around. There is considerably more in the joint than you might think, and it is well worth continuing the heat long enough to remove as much as possible. It will be very difficult to remove the cup if you don't expand most of the thermoplastic out with the heat. You might as well burn out all eight plastic rings before you start pushing the joints out in the interest of time and propane economy. The thermoplastic will ordinarily catch fire once it's out of the joint, so be alert to where it falls. Ordinary C- clips, not just plastic, will hold the replacement cups. One difficulty with the driveshaft CV joints including a socket ball is the protective sleeve around the ball joint. If you press the joint as far as you can with a U-joint tool, you can crush the sleeve, which is not included in the replacement parts. Heads-up! Worry the old cup out the last hair s breadth with your locking pliers. 12 June 1999
Constant-Velocity Driveshaft Joints How could there be any kind of CV joint work without an envelope of mystery grease? Each center ball kit comes with a bag of gick to pack into the cavity into which you'll shortly insert the spring and other parts. Make sure to displace all the air pockets. There are many sequences in which you can install the parts of the center socket and ball. Only one of them will work, however, so if you're not familiar with the job, lay out the parts in sequence as you take the old one apart. New U-joints go in the traditional way, but again be sure not to damage the socket and ball protective sleeve. Zerk fittings face outward from the yoke. Don t forget the center ball when greasing the joint. Putting the last needle-bearing cup in the CV joint requires overcoming the considerable force of the compression spring to line up the Cardan-cross shafts and the needle-bearing cups. The technique I found works best is to drive the first side in farther than you ordinarily would, and then back it up when you drive the opposite one in. This allows you to fit the last needle-bearing cup securely around the shaft and only have the yoke opening to fight when you're lining things up. 14 June 1999