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Close up on the u-joint



The most common u-joint is the "cross-and-bearing" type. It is still known by its original name by many persons in both the automotive and industrial trades: the "cardan" joint. It is named after the Italian mathematician who invented it in the 16th century. It has two yokes which are bored for four needle-bearing assemblies. One yoke is on the transmission output shaft and another yoke is on the driveshaft. The connecting link between these two yokes is the cross. The cross is a cast or forged iron or steel component which has bearing surfaces called "trunnions" machined onto its cross arms.

The u-joint has more than one axis of motion. In fact, it has two. Because it has two, it can compensate for the complex angular path of the moving driveshaft as it rotates.

The cardan type joint is an imperfect solution, however. Regardless of how popular and workable it seems (after all, 80 years of superiority is hard to ignore) designers have been compensating for its weaknesses.

Figure 5 U-Joint Assembly

This is the hard part.



Talking about u-joint movement is rather difficult. The action of a universal joint is a dynamic, three-dimensional action. A u-joint turns and oscillates, and does this over a period of time. We can discuss a single instant of its action, but in the final analysis, synthesizing these concepts into a unified "moving picture" is not easy. However, we will make it as understandable as possible.

The universal joint turns or rotates in a simple circle. In each rotation, it turns through 360 degrees of motion.

Looking at a slip yoke coming out of a transmission, you can see that it rotates in a single plane. Therefore, we can calculate just exactly how far a point on the yoke moves with every revolution it passes through.

For instance: assume the yoke is 4" from one side of the yoke to the other side. In this case, during its revolution, it describes a circle 4" in diameter. If we take a single point on its tip and calculate the circumference of the circle the yoke describes, we find the following:

4.0 x 3.1459 = about 12.5 inches.

Therefore, every time the drive yoke turns, it travels 12.5 inches.

Figure 6 Yoke Circle

Figure 7 Straight Line Driveshaft

Figure 8 Angled Driveshaft

Looking at the illustrations, you can see when the driveshaft and universal joint are in a straight line, the driven yoke also travels 12.5 inches for every revolution. It is only in this absolutely straight line situation where the driving yoke and the driven yoke travel the exact same distance when you measure the circumference of their outer edges.

When the driveshaft is at any angle other than straight, the tip of the driven yoke no longer travels 12.5 inches. There is an oscillation it goes through, twice each revolution. We'll call this oscillation a "wobble". Therefore, in order to calculate the actual travel of the driven yoke during its 360 degree revolution, we need to consider this "wobble distance" or, to be more technical, the "oscillation distance" of the yoke.

The tip of the driven yoke, when operating at an angle to the drive yoke, wobbles away from the input plane of rotation on top of its turn, and away from the plane of rotation at the bottom of its turn. This is what makes the actual distance of the yoke 12.5 inches, plus something.

Take a look at the illustration (See figure 9) which shows the tip of the driven yoke being just 1/4 inch out of plane with the drive yoke. Not much of a difference, right? Unfortunately, the 1/4 inch wobble represents a lot more than 1/4 inch of added travel. Remember, it is traveling some additional distance for every fraction of a degree it rotates above or below the point where it is level with the plane of rotation. It would take a knowledge of calculus to figure out the exact distance it travels, in addition to 12.5 inches, but let's assume it travels a total of 1/2 inch extra above the plane of rotation and 1/2 inch extra below the plane of rotation. This is a full inch of added travel, which makes the driven yoke rotational distance 13.5 inches, or almost 10% further than the drive yoke.

Figure 9 Drive Yoke Out Of Plane

Remember, both yokes travel at the same speed of rotation, taken as a unit. That is, 2,000 RPM in the drive yoke will produce 2,000 RPM in the driven yoke. Unfortunately, this 2,000 RPM in the driven yoke is not delivered at a constant speed.

Rather, because the cross-and-bearing oscillates, the output RPM is delivered at an oscillating speed. It gets faster and slower then faster then slower as it turns.

From the horizontal plane or rotation, you have to accelerate the mass of the yoke (and its cross and bearing units) from rest, to 1/4" off plane, then come to a complete stop (when the tip is at maximum wobble point) and accelerate it back to zero again repeating the acceleration/stop/acceleration cycle for the yoke tip below the plane of rotation.

At just a couple of RPMs, this variation in speed, or "velocity" (a non-constant velocity) doesn't make all that much of a difference. Also, at very low angles of variance, the difference in velocity is very small.



Difference in distance means a difference in velocity.

Figure 10 Yoke Motions Smooth rotation is shown by line (x) on graph, each 360 degrees of rotation marked by a verticle line. The wavy line or "sine wave" line shows the velocity of the wobbling u-joint yoke. Several degrees of angle are shown, and the greater angles produce the greater variations in acceleration required to turn the tip of the yoke through a full 360 degrees.

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