How can a driveshaft become unbalanced?

Posted by: Vstrom

How can a driveshaft become unbalanced? - 03/12/14 07:07 PM

Just went through a real trial with fixing a vibration that turned out to be the driveshaft on a 96 F350 long bed crew cab so it is a 2 piece shaft.

Vibration had been getting worse for some time...changed the center support, no difference. Replaced the U-joints, no difference. Spline joint good.
Took it to one driveshaft shop that was stumped...told me I just needed to replace the whole thing.
Took it to another one- they took off most of the factory weights, took off the rubber damped heavy steel ring on the pinion end and welded on a washer for a balancing weight on the front shaft.
Smooth as can be now..just can't understand how it can get unbalanced in the first place!
Posted by: ZeeOSix

Re: How can a driveshaft become unbalanced? - 03/12/14 07:20 PM

Good question. Maybe some factory balance weights flew off at one time? In order for the balance to change, the mass distribution needs to change.
Posted by: turtlevette

Re: How can a driveshaft become unbalanced? - 03/12/14 07:22 PM

did they check it for straightness? If it got slightly bent it'd be off then a rebalance would compensate.
Posted by: lomez

Re: How can a driveshaft become unbalanced? - 03/12/14 07:41 PM

damaged/worn u-joints....but then you said you replaced them.
Have no clue then.
Posted by: GreeCguy

Re: How can a driveshaft become unbalanced? - 03/12/14 07:41 PM

Back in the day I had a '67 Chevy half ton truck that did the same thing. The two piece drive shaft had a support bearing in the middle and the rubber bushing that held the bearing in place finally wore out and fell out which caused the shaft to "sag" in the middle. I bought a used one at a junkyard for 30 dollars - popped it in and no more problem.
Posted by: Chris142

Re: How can a driveshaft become unbalanced? - 03/12/14 08:49 PM

Usually they get bent. Maybe that rubber balancer they removed got a heavy spot or off center?
Posted by: Kira

Re: How can a driveshaft become unbalanced? - 03/12/14 09:14 PM

Your 18 year old, two-piece shaft and center bearing needed some TLC. What was the total?

There's a shaft shop near me and anytime you need to remove ANY shaft and bring it to them for service-it's a joy.
You know smoothness is in your future.

Glad you got it sorted. Kira
Posted by: Vstrom

Re: How can a driveshaft become unbalanced? - 03/13/14 02:21 AM

The first shop told me I used [censored] U-joints, they were the best Moog offers. They also told me the center bearing, an Anchor brand, was [censored] and I HAD to use the OEM Ford bearing that costs 6x as much.
OK....the vib was the same before and after the U-joints and center bearing replacement. Beside that the OEM bearing is discontinued and no longer available from Ford. The original U-joints appeared perfect when removed and had no play in them.
I did replace the center bearing again with a Timken branded one that made no difference whatsoever.
No factory weights were missing. There was no visible damage to the shaft or any components. The truck has never been in an accident.

The shop that got it right said it was severely out of balance and really had no definitive answer as to why it was. They said a lot of towing caused it....huh?
Total at the shop was $250 to fix, that didn't include the $45 I spent on Moog U-joints and one $20 center bearing followed by another $45 center bearing. This was with the shaft removed and carried to them.
There is 200,000 miles on the truck and I am the original owner. There was a TSB shortly before my truck was built in the fall of 95 adding the vibration damper ring to the rear of the back shaft to stop a "moaning" noise some trucks had. So now mine is gone-
I may be hearing a little different noise but the truck is "noisy" anyway with the diesel and stiff suspension so it is hard to tell. The vibration is gone, which was at 35 and 65 MPH.
I'm happy but puzzled.
Posted by: paulo57509

Re: How can a driveshaft become unbalanced? - 03/13/14 03:11 AM

My GMC van has a one-piece drive shaft and the R&R instructions in the FSM state to mark the drive shaft and companion flange before separating the two so that the drive shaft is replaced in the same orientation because the drive shaft and differential were balanced as an assembly.

I'm going to guess that the same might be true of your two-piece drive shaft. They might not be balanced with the differential, but their relationship and balance with each other was lost when the center support was replaced.
Posted by: Trav

Re: How can a driveshaft become unbalanced? - 03/13/14 07:01 AM

In the salt belt a lot of drive shafts have severe rust and large rusty scales.
Its common they go out of balance, i always attributed it to that but who knows.

I have a local metal company steel shot blast them, then etching primer and paint, new joints and have them balanced by a reputable drive shaft company.
Cost is about $70 for the balance, $10 to blast them and whatever the paint and joints cost.
I usually have about $150- $180 into a totally rebuilt and balanced shaft.

The etching primer and high quality paint pays back in spades! One has been 7 yrs in the salt belt doing plow duty and still looks like new.
Posted by: rockydee

Re: How can a driveshaft become unbalanced? - 03/13/14 07:30 AM

Originally Posted By: Trav
In the salt belt a lot of drive shafts have severe rust and large rusty scales.
Its common they go out of balance, i always attributed it to that but who knows.

I have a local metal company steel shot blast them, then etching primer and paint, new joints and have them balanced by a reputable drive shaft company.
Cost is about $70 for the balance, $10 to blast them and whatever the paint and joints cost.
I usually have about $150- $180 into a totally rebuilt and balanced shaft.

The etching primer and high quality paint pays back in spades! One has been 7 yrs in the salt belt doing plow duty and still looks like new.


That's a good idea. Can you pull the drive-shafts on a new rwd vehicle, mask off the joints and spray them with spray bomb primer and paint? Then chuck it all back together again. I'd rather do it before rust attacks rather than after if I can.

I like reading your posts, you are quite in the know when it comes to anything automotive. I've read your posts for a long time now, I figured it was about time to say thanks for what I learned from you.
Posted by: Trav

Re: How can a driveshaft become unbalanced? - 03/13/14 07:50 AM

I wouldn't do it. It doesn't take much to put it out of balance and there is no way to guarantee the thickness of the paint film is the same over the whole shaft.
You can use something like 389 rust blocker or Bullfrog before winter then drive the car, it wont throw it out of balance.

http://www.theruststore.com/Cortec-ECOSp...pYrAXhs1UVcYuSU

http://www.theruststore.com/Bull-Frog-Ru...pYrAXhs1UVcYuSU
Posted by: rockydee

Re: How can a driveshaft become unbalanced? - 03/13/14 08:06 AM

Good point. I didn't think a spray bomb might be able to throw a DS out of balance.
Posted by: Trav

Re: How can a driveshaft become unbalanced? - 03/13/14 08:23 AM

Figure there is about 8oz of product in the spray can that actually ends up on the piece, prime and paint could be almost a pound of material.
Half an ounce out of balance can be noticeable.

http://www.rldial.com/Ultra%20High%20Speed%20Driveshaft%20Balancing.html
Posted by: zzyzzx

Re: How can a driveshaft become unbalanced? - 03/13/14 09:36 AM

I would think that things like dirt or rust could throw it off balance.
Posted by: rockydee

Re: How can a driveshaft become unbalanced? - 03/13/14 09:48 AM

Originally Posted By: Trav
Figure there is about 8oz of product in the spray can that actually ends up on the piece, prime and paint could be almost a pound of material.
Half an ounce out of balance can be noticeable.

http://www.rldial.com/Ultra%20High%20Speed%20Driveshaft%20Balancing.html


That makes plenty of sense. It just sucks crawling under a new rig and seeing a rusty drive shaft. Mine has no rust yet, and I want to keep it that way. Those water based products work?
Posted by: Trav

Re: How can a driveshaft become unbalanced? - 03/13/14 02:14 PM

Yes they work good but its a seasonal application. The stuff is harmless to rubber and plastics and does not stain.
It is environmentally friendly also which i like better than petroleum based products.
No point in damaging the Earth unnecessarily right.

Hit the whole underside of the car every autumn liberally with these products and it will be fine even in the salt belt. great if you don't mind the yearly investment.
If the question were phrased, "are they as good as traditional rustproofing"? the answer is yes but only short term.

These are perfect also if you have an old car that only sees summer use but you still don't want rust to get a start.
They are not a spray and forget product. Here is some info..

http://www.cortecvci.com/Publications/Reports/reports.php

http://www.cortecvci.com/Publications/Reports/powerboat-article/Powerboat.pdf
Posted by: Trav

Re: How can a driveshaft become unbalanced? - 03/13/14 04:55 PM

Originally Posted By: zzyzzx
I would think that things like dirt or rust could throw it off balance.


Dirt for sure but rust is another story. If the rust remains undisturbed like minimal surface rusting no real harm but when it develops into flakes and starts coming off then yes absolutely.