If the universal joint is operating at an angle the rotational speed on the output varies, increasing and decreasing twice per revolution. If this is bad depends on your requirements and operating conditions. Heavy duty steering axles often utilize universal joints because of the higher torque loads they can handle.

http://en.wikipedia.org/wiki/Universal_joint

ISZ

Yeah, unless the UJ's are in out-of-phase pairs, you will get vibration at twice per rev of the shaft.

M

--
Dr Michael F Platten

Very good information -- fortunately this is a rear axle so I do not need to worry about steering.

Michael, how severe could the 2 per rev be -- would it be enough to destroy the u-joint? I suppose that vibration is coming from the imbalance of the yoke -- can you balance the shaft?

Thanks,
Mike

Something else to point out is that the shaft will between the CV joint and the u-joint will only be ~8".

Thanks again,
Mike

It's not a balancing problem.  It has to do with the transfer function of a u-joint, which has a sinusoidal component proportional to the angularity.  When you use two u-joints properly phased, and the angles are ~equal, the second one subtracts what the first one added.

With just one u-joint, something is going to see a sinusoidal torque.  The u-joints may or may not break before the passengers get aggravated enough to kill the assembler.

 

Mike Halloran
Pembroke Pines, FL, USA

To put it into perspective. I am using a pair of *in phase* UJs as part of a torsional excitation system. The angle of each UJ is 12 degrees. I put constant torque in at one end of about 2 Nm. I get a  fluctuating torque output of about 100 Nm amplitude.

M

--
Dr Michael F Platten

In my younger days I put a Chrysler Hemi in a 55 Plymouth. I cut and welded the drive shaft out of phase and you could not do more than 35MPH without it rattling your teeth out.

Mike wouldn't a 6 pin CV joint used with a 4pin U joint create an out of phase problem?

I don't know anything but the people that do.

My '88 Ranger prop shaft had two cardan joints contained in one housing--very compact.

Awesome info, thanks guys! Makes pretty good sense -- thanks for saving from making a pretty good mistake.

Mike

Dual cardan joints within a common housing is very common on agricultural equipment. The Power Take Off shafts used to connect implements to tractors have been using 2 cardan joints with some type of slip joint from day one. The problem is you can't guarantee equal joint angles due to the many degrees of freedom in the hitch joint. The results is that in a tight turn the unequal angles create a sigmificant amount of vibration. The next best design is a dual cardan joint on the tractor side and a single cardan joint on the implement side.

I have also seen them used on off-road trucks. Note the phasing of the joints.
http://www.pirate4x4.com/tech/billavista/PR-shaft/index2.html

One other bit of advise for non-automotive use. The joints should never be run in straight alignment for excessive periods of time. When the joint is straight the bearings do not need to rotate, which means the needle bearings don't rotate, thus all of the wear will happen on only one portion of the needles.

ISZ

Actually, even in automotive drive shafts, the bit about not running the shaft perfectly straight still applies.

Traditional Chrysler vehicles had the drivetrain shifted slightly to one side to make sure this never happened. Fords traditionally have the input shaft of the rear-end slightly off-center. Not sure what GM did.

As I recall, the original BMC Mini had a conventional hardy spicer type at the transmission output shafts, and a CV joint at the driving axle.

Thanks again guys, I put some pictures up so you could see what I am working on:

http://picasaweb.google.com/michael.e.friedman/Tr6Rear?authkey=Gv1sRgCMijtNv7s6SRpgE&feat=directlink

I have been driving the car with the 302 for over 14 years -- but have gone through 3 triumph rears.  The pinon gear can't take the additional hp.  Therefore, the ford 8.8 with tracloc should solve that problem.   

I thought it was possibly a Triumph car issue!

The Triumph Spitfire series had an awful rear suspension deign and used a single UJ on the inner end of each (swing axle) driveshaft, running from the fixed differential. The other end of the driveshaft was unjointed and went directly into the wheel hub.

I could never understand how this was suppposed to work properly. Presumably the resulting vibrations / wheel acceleration & decelerations were supposed to be damped out by the tyre walls.

After three years of ownership of my Mk3 Spitfire I could strip the rear suspension, change the ruined UJ and rebuild it again in less than an hour. I used to buy them two at a time because if one went, it was always when the shops were shut and the next one was never far behind.

"Actually, even in automotive drive shafts, the bit about not running the shaft perfectly straight still applies."

Interesting. I assumed that this wasn't a major issue since the suspension, and thus drive shaft angles, are moving at least a little all the time.

As for the Chrysler and Ford references I had read/been told/assumed that the various offset designs were to solve clearance issues or keep the axle shafts the same part number. Of course the answers are almost always more complex.

ISZ

This image's info is confusing to me.
http://www.victoriabritish.com/icatalog/sg/images/0128.jpg
I thought both the GT6 and later Spitfires (MK2?) stopped using swing axles.

Anyhow, according to this even GT6s used the same u-joint?

The way i remember it early spits were live rear axle, it was only the Mk IV that got the much maligned IRS.

I think that must be right, the Spitfire was originally just a Herald with a new body.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

Redact that, initially they had a transverse leaf spring swing axle, according to the pimply-nerd-o-pedia.

The Mk IV got outboard CV joints, ameliorating the tuck under/jacking tendency.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

If you want worn u joints, look at the Lotus Europa series, the half shafts carry the suspension loads.

I never understood why they would use a needle  bearing for a u joint.
I would think a plain oil lite bearing would work just fine.
The same for roller rockers with needle bearings making a quarter turn each time. Makes me go hmmmm...
 

I don't know anything but the people that do.

You'd see an even worse problem with false brinelling if you used a plain bearing - all the oil would get squeezed out from under the contact patch and would never get back in.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

Plain bearings need continuous rotation to build up a dynamic oil film. That doesn't happen in a U-Joint.

Plain bearings have a much higher friction coefficient than needle bearings. They would actually run hot in an automotive application.  

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