Greasing the CV in its center pivot point may require removal of the driveshaft. This takes a little longer to do, but is much cheaper in the long run.
As the U-joints operate at greater angles, they'll create increasing axial loads on the retaining yokes of the driven shaft. This will cause the joint to rip out the end of the driven shaft. This is a result of the tendency of the U-joint to transmit torque in a plane that's perpendicular to the driving shaft. At the transfer case end, the driveshaft is the driven shaft. At the differential end, the driveshaft is the driving shaft. We often equate this to using a flex socket at the end of an extension. In a straight line it will work well, but at an extreme angle it will be difficult to keep the socket on the nut or bolt. The most common failures are the U-joint pulling out of the driveshaft itself at the transfer case end and breaking the retaining strap and bolts on the differential pinion yoke. This type of failure is especially common in lifted, short-wheelbase Jeeps such as the Wrangler and CJ.
Make sure that you have no binding interferences in the driveshaft at all extremes of shaft operation. It can be difficult to simulate the extent of suspension travel and frame-flexing in your garage or driveway or even on a ramp. Allow a little extra margin for things that you can't simulate. For example, you're driving down the highway and your rearend goes airborne because of the stored energy from compressed springs and their recoil effect. Your differential may fall well below what it would if allowed to hang freely from the vehicle.
The wear pattern left by the dust cap of the slip yoke on the spline stub will usually be a good indicator of whether the shaft is running in the proper position and the extent of travel from compression to extension.
Driveshaft travel, from full spring compression to full spring extension, is also an issue that will need to be addressed. On a front axle with shackles in the front and arched springs, as the springs compress, if nothing else happened, the driveshaft would have to get shorter. But, as the springs compress, shackles will swing away from the differential. Again, if nothing else happened, the driveshaft would lengthen. As it is, the two things work in opposition to each other, and the driveshaft will have a relatively stable length. The extent of travel on an existing application can usually be determined by looking at the wear pattern on the spline stub from the dust cap on the slip yoke.
When buying U-joints, look for a large/strong central body. While this isn't an advertisement for Spicer, we've found that Spicer components are about the best you can find and will afford the longest life.
There are many configurations to which the above principle does not apply. Most notably, a front shaft with a shackle reversal. In that situation, the two normally compressive and extending actions are now working in tandem. This will cause the driveshaft to compress and extend substantially more than it would otherwise. Additionally if you have flat springs, the driveshaft will tend to compress only minimally but extend substantially. A special note to you YJ owners with a shackle reversal: Make sure you have enough room for the driveshaft to compress (shorten). Salvage yards are doing a great business in NP231 transfer cases, replacing those that have broken because the front driveshaft was driven into the case on the road or trail. It's prudent to at least try to simulate the full range of movement for the driveshaft through the extent of suspension travel to make sure your driveshaft meets your needs.
Tom checks a slip yoke and spline stub for excessive lateral play. A general recommendation is to allow no more than 0.007 inch. If you have a dial-caliper and a vise, this is an easy at-home check to perform.
Breaking the driveshaft or U-joints can be the result of a number of different causes. Assuming that you're not having one of the problems outlined above, the first thing you may want to look at is the quality/strength of the components themselves. Original equipment driveshafts are not built for ultimate strength. Typically, the tube will be the weak point on a factory shaft. When you're having a new driveshaft built or an old shaft reworked, increase the tube diameter, wall thickness, and grade of tube used. The cost differential will be minimal in the short run, and the potential savings for the long haul will be substantial.
This is what a broken weld could look like. This might have been caused by a defective weld or even by inconsistent properties in the tube. Many welders, especially neophytes, like to weld too hot. That will cause the metal to become brittle around the weld and actual fail there, instead of in the weld. Once again, replace your tube with a high-quality, heavy-walled one that has been drawn over a mandrel.
There's a lot of talk about increasing joint sizes to accommodate high-horsepower engines. Due to limited parts availability, there will be upper limits to how large a U-joint you can install against any particular transfer case or differential. Additionally, a larger U-joint will usually incur a binding interference at a lower degree of articulation than a smaller U-joint. Beyond increasing their size, the quality of your components is more likely the most significant improvement that can be made for increasing the strength of your powertrain. Look for good quality parts. The U-joints should have a large central body, large trunnions, and a channel across the end of the trunnion or in the bearing cap to allow for thorough greasing of all the caps. They should also be a forged and hardened joint. Try dragging a file across the trunnion of the joint. If you are cutting away any material, the joint is too soft.
Experience has shown that Spicer components are worth hunting and asking for. The tube should be drawn over a mandrel (factory tube is cold-rolled electric-welded). This will nearly double the yield and tensile strength of a comparable size of tube. The best weld will be done with the MIG process. Additional strength will be the result of the uniform consistency of the MIG weld having a lower potential for stress risers.
Also consider that if you intend to use all that 500 hp available in your Jeep, while in low range and First gear, it would be impossible to assure that you won't break something. If horsepower is constant (which it would be at a given rpm of the engine) and you halve the speed on the output shaft of the transfer case (low range) you will double the torque on the driveshaft. Compound-low gearsets in many of today's transfer cases make things even worse.
Some applications, such as Jeeps with shackle reversals or super-travel suspensions, may demand extreme travel in the slip yoke. This shaft has nearly 2 feet of splines.
One hundred hp at 3,000 RPM equals 175 lb-ft of torque. One hundred hp at 300 rpm equals 1,750 lb-ft of torque. You'll find that if you brace your vehicles against an immovable object or place the wheels into a deep hole, the engine can develop enough torque to break anything we could conceivably build and install.
Bearing that in mind, what do we do? Drive sensibly and take it easy. When you're out climbing that rocky hill and your wheels start to bounce, do you give it gas? Most broken driveshafts, U-joints, axles, transmission, or transfer cases are the result of high-stress-impact loads. Easy, elegant four-wheeling will save your driveshaft and other equipment every time. We do realize there are those situations where pedal-to-the-metal is the only way to go and, well, we take our chances then.
Proper driveshaft setup and maintenance serves four-wheelers well. If you have questions that haven't been addressed in this feature, give Tom Wood at Six States a call. Doing it right the first time ensures that a pesky broken driveshaft won't ruin that trail ride you've looked forward to for a year. Remember, getting it right guarantees against your getting the shaft.