I was out driving my 1976 F250 4x4 after replacing the front and rear driveshaft ujoints. I was having some drive line vibration between 30-40mph with it being at its worse at 35mph. Above 40mph the vibration went away.

On the way back from one of my test drives, the rear drive shaft fell out from the front (side that connects to the transfer case). I'm not sure if I installed it wrong or if the ujoint failed. I did reused the nuts and lock washers that were on there before. I suppose its possible they loosened up over time.

If anyone has any ideas as to why it fell out I'd love to hear them. I want to fix this and do it right. I'm glad I keep tools with me most of the time and was able take the driveshaft off and put it in four wheel drive so that I could limp home.

 

Was it a sealed u joint or a grease zert type? Did you ck for grease in the sealed one, (remove the cap on a sealed one, look for grease or add grease.)

And or install the zert and grease it. Do you have the mini u bolt straps over the end caps? Are you following the torque specks? Is the d/s "in phase" when you put the 2 pieces back together and the slip splines?

Take a flat edge carpenters framing level and check main d/s to be bent.

Are your yoke ears flared from hammering in and out of the u joints?

 

Review tension vs compression
Grease fittings need to be set up in compression rather than in tension.
- Find the direction that the driveshaft turns when going forward. Put the grease fitting in compression in the rotating driveline.

The grease fittings are voids in the metal - easier to rip apart under tension.
When you compress them - it's all good.

 

Doe,s the truck have a lift in it? There might be too much angle on the drive shaft.

 

I'd Check for bent shaft, worn or frozen splines on shaft, damage to yoke on t-case, correct 1310-1330 combo u-joint. If you have much of a lift a longer driveshaft is needed.

 

FordBarn, can you explain tension versus compression on the d/s fittings a little more? I haven't had my coffee. Who knew there's right way to lube zerk fittings?

I found a pretty good webpage on some things for the OP to check.

http://www.machineservice.com/techni...e-lubrication/

Here's an interesting point about why lowering a truck may not be such a great idea:

http://www.overdriveonline.com/how-t...lace-u-joints/

 

Never been under or maybe even in a Ford 4X4, other than a friend's '08 Super Duty so I could be barking up not only the wrong tree but may even be in the wrong forest but...


U-joints need to be phased or clocked correctly or the output speed will be cyclic and the joints over stressed.


See Proper U-Joint Installation TipsBehling Racing Equipment

 

Thanks everyone for the links and insight. There is no extra lift on the truck and I wasn't having any issues prior to removing the drive shaft to install new ujoints. As you can see from the picture below the ujoint appears to still be in good shape. I can't find any real damage to anything besides a small piece of the yoke has been rubbed off and there is a small dent in the glasspack muffler. I ordered up a new Moog 275 greasable ujoint and I'm going to carefully install it. I would also like to know more about the tension vs compression and how it relates to the proper way a ujoint is installed.

I'm going to get some new lock washers while I'm at it just in case that was the reason the drive shaft fell out.

 

Those u-joint cap straps should of all ready had lock washers on them. And make sure you torque by the book.

Also ck the outside of the yoke for a small tab that holds the u joint cap "in".

 

This is my thought as well. On my truck I found the front driveshaft yoke had the tab broken off and the u-joint can then slide out.

From the picture, I see the tab appears to be present on one side, but the u-joint probably slid out the other side based on the position of the remaining cap. I can't see because of the u-bolt, but check to make sure the tab is there.

Another possibility is you might have an incorrect u-joint. When installing the cap should fit very snug between the two tabs on the yoke. If there is a gap you might want to measure to make sure you don't have a 1310 u-joint where a 1330 is required.

Most of my u-joints don't match what the books say since I have swapped around parts from different years.

 

Also .... measure the width across the outer ends of the caps .... BOTH WAYS. Measure the diameter of the CAPS as well in BOTH DIRECTIONS.

Some Ford U-joints that get retained by U-bolts have thicker caps in those positions to resist crushing when U-bolts are over tightened. Those U-bolts only need 15-17 ft lbs.

I always use new self locking nuts when they loose their grip on the threads.

U-Joint series identification
U Joint Measuring Guide

 

Compression versus extension; Imagine the torque on the drive shaft and what it's doing to the grease fitting hole, is it smashing it or stretching it. When I first heard of this maybe 40 years ago, it was a drag racers tip in a magazine. The ujoint is several time more suceptable to grenade if the zerk hole is being ripped apart by the force as opposed to being compressed by it.
I would get new hold down straps.
I always pack the caps full of good grease when I put a U joint together, leave the zerk out until it's all assembled. It not only helps keep the needles in place but provides a good prelube.
From the description of the vibration, it almost sounds like you had a needle down and got the joint in a little off center which would also cause one cap to not be seated and come loose.

 

Clean the yokes out till it's bare metal to make sure they seat and what the OP's have said.

 

Does anyone know the thread for the zerk on the rear drive shaft? I will hopefully have some time this weekend to fix this but I noticed the zerk broke off when the drive shaft fell out.