This twisted, broken tube shows what happens when excessive load is applied to an undersized driveshaft. Horsepower or stress from the trail could be the culprit here. Regardless of whatever caused the break, stock tubes are usually quite weak.
Although many of us have had a driveshaft failure, few of us have a real understanding of exactly what's causing our driveshafts to vibrate, shudder, or break. We talked with Tom Wood, builder of driveshafts at Six States Distributing, and he filled us in on the basics.
First, make sure that the driveshaft in your Jeep is in sound working order. This is for your own safety and comfort, as well as for the safety of the guy behind you on the freeway going 70 mph. It certainly is no fun having your weekend outing or the trip to the corner market interrupted by a driveshaft failure, which can be an inconvenience at best and deadly at worst.
Here's a clean tear in a tube. Something was rubbing on the shaft and scored a line in the tube, creating stress risers. When installing a custom driveshaft, or even checking your old stock one, make sure nothing touches the tube that shouldn't.
Driveshaft and U-joint failures can be attributed to one or a combination of factors. These factors are maintenance, driver abuse, external damage, improper installation, poor driveline geometry, or the quality and strength of the components.
On stock applications, poor maintenance is the number one killer of driveshafts. Unfortunately for the consumer, most domestic stock driveshaft components have little or no provision for lubrication. The vehicle manufacturers are saving a couple of bucks per vehicle by not putting grease fittings on the wearing components. Because of this, there isn't much that can be done to prevent a stock driveshaft from wearing out. We can give the driveshaft a quick visual checkup to ensure our safety, though. It doesn't take much time and will never hurt to check things out any time you're under the vehicle.
To properly grease the slip yoke and spline stub, it may be necessary to plug the relief hole in the end of the slip yoke. Note also how grease is coming out through the dust cap and all four of the seals on the U-joint. Don't worry or listen to all the old wives' tales; the caps are designed to do this.
Inspection of the driveshaft requires that the vehicle's transmission or transfer case be put in neutral. Be sure to set the parking brake. This is important because if there's any pressure on the driveshaft, you can't detect the minimal clearances that will be the first indicators of an impending problem.
Try twisting in opposite directions each of the yokes that attach to the U-joint and try to move them side to side, checking for movement independent of the U-joints. If you have any detectable play in any direction, you have too much. Many times you can have a U-joint that's beginning to seize up, and you won't be able to observe any play in the joint. When that happens, you may see a rusty oxidization on the U-joint around the bearing cap seals. While driving the vehicle, you might also hear a squeaking noise that will start out slow and cycle faster as the vehicle moves faster. The squeaking may disappear at high speeds.
Check to make sure the U-bolts or strap and bolts are tight. Also look to see that the bearing caps have retained a tight fit in their respective yokes on the driveshaft. A cap that has lost its press fit will typically have a clean, polished area on the end from spinning in the bore of the yoke. This is also a good time to look for things like a dented or twisted tube, missing balance weights, crud on the driveshaft, and anything else that could cause a vibration problem.
This broken spline stub could have happened because of excessive load, shock load, fatigue failure, or a defective part. Make sure the parts you use can stand up to the intended use you plan to put your Jeep to.
You'll want to check the attaching yokes. Are they securely attached to the transfer case or differential? Often, the nut on either of these yokes will begin to back off. If that's the case, you'll be able to see the yoke move independently of the output shaft or the pinion shaft. Be careful in your diagnosis here, since the symptoms may be a result of bearing wear. To arbitrarily tighten the nut will usually accelerate the failure of a worn bearing. If you discover the yoke is loose, it may be a good time to drain the gearbox to thoroughly inspect the oil for excessive metal contamination. In the event of a bearing failure, the oil will typically have a glittery appearance.
If you are lucky enough to have grease fittings in your U-joints, use them. You should always grease until you see clean grease coming out of all four of the seals on the universal joint. It's a myth that this purging will damage the seal. They're designed to purge. We recommend that you do this every time you change your oil and when you get back from that wet, muddy four-wheeling adventure. You'll be flushing the water and grit out of the bearings, for longer U-joint life.