Prop-shaft Howl - cause and cure

Like many RP owners, I have had a bad experience with the fabric couplings on the standard Box Saloon prop-shaft, to the extent that I had to limp home 200 miles from Beaulieu having torn up the fabric just after setting off. 

So, I made the common modification of moving the handbrake pivot and fitting a Ruby prop-shaft.  It has been a good mod for the last two thousand miles, but of late the car has been excessively noisy at hurrying speeds.  Between 45mph and 50mph, there was a sudden dominant howl, that appeared to be coming from the prop-shaft.  It comes in suddenly, and builds up to quite a resonance, making driving the car on dual carriageways quite unpleasant.

Having investigated underneath, I seemed to have gained quite a lot of slack in the splines. Trying to push the prop-shaft sideways near the grease nipple, there was much more movement than I would have liked.

But now I have found the cause. 

When I bought the prop, I split the shaft at the splines, gave both halves a clean up and a paint, and fitted a pair of new Universal Joints.  Then I simply popped the two halves together, tightened the collar, fitted the prop to the gearbox and torque tube flanges and the job’s done.

propsh1


What I didn’t know is that I had built in a major source of wear and vibration by not paying attention to the relative positions of the yokes onto the Universal Joints. After all, the splines fit in several orientations.

 

 

I have since discovered that the positions of the two UJs with respect to each other MATTERS, and matters a lot.  The reason is all to do with the fact that a Cardan joint (named after its inventor Gerolamo Cardano,1545), or UJ, is NOT a constant velocity joint.  

The whole purpose of the prop-shaft is to cater for the minor alignment errors resulting when the gearbox output shaft and the Torque Tube are not absolutely exactly in line.  If they were, then the UJs and the prop-shaft would also be directly in line, both flanges and the prop-shaft would rotate at the same speed.  

However, when the UJs are bent at a small angle, the prop-shaft has an angular oscillation superimposed on its rotation. The AVERAGE rotational speed of the shaft is the same as the drive flange, but it rotates fast-slow-fast-slow every rotation. 

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As the gearbox rotates, the bigger the misalignment , the more the prop-shaft rotates further/less far/further/less far with every revolution.  Hence, the more important it is that the prop-shaft is correctly assembled.propsh2

The mathematics for all this is a bit hard, but the graph shows what happens for a large angle between the input of a UJ (e.g. the gearbox output shaft) and the output of a UJ (e.g. the prop-shaft).  As the gearbox rotates, the prop-shaft goes faster/slo wer/faster/slower, twice per rotation.  

The parameters in the equation are:

ω1 =input angular velocity (i.e. the speed at which the gearbox attempts to turn the front UJ)
ω2 = output angular velocity (i.e. the speed at which the rear UJ attempts to turn the back axle)
γ1 = the absolute angle at which the gearbox flange is at an instant in time
γ2 = the absolute angle at which the back axle flange is at the same instant in time
β  = angle at which the centreline of the prop is with respect to the output shaft of the gearbox, or the centreline of the torque tube. In an ideal world, this is zero, but in an Austin, they are never perfectly aligned. Worse still, the alignment changes with car payload, as the torque tube hinges up and down as the rear suspension compresses. 

The graph shows γ2 rather than ω2 as this more easily demonstrates the effect of twist torque.  It is easier to understand that if input and output are at different angles, you must have twisted the shaft. 

Of course, angles for the Austin are small, but they are not zero.  

The diagrams below show the two possible extremes for assembling the prop-shaft. 

When driving along the road at a constant speed, the gearbox end flange is trying to rotate at constant speed, due to the flywheel, and the torque tube end flange is trying to rotate at constant speed due to the momentum of the car. However, the prop-shaft itself will be speeding up and slowing down during the rotation. 

Upper Diagram: When the UJ yokes are aligned as shown below, the same oscillation appears at both ends.  The prop-shaft is said to be “in phase”.  Whilst its speed is varying, both ends are trying to do the same thing, both speeding up or both slowing down, so that there is no twisting vibrational torque along the shaft itself.

propsh3

Lower Diagram: When the UJ yokes are aligned as shown above, the gearbox end is trying to go faster AT THE SAME INSTANT AS the torque tube end is trying to slow down, and vice versa.  The shaft is said to be “out of phase”. Now there is a strong vibrational torque along the shaft, irrespective of the net torque to the axle.  This vibrational torque will set up a loud howl at the resonant frequency of the prop tube, and will put a cyclic stress on the splines along the shaft.  At 3000 rpm, this means that  the prop-shaft is being twisted and untwisted 100 times every second!

Needless to say, when I checked, the yokes were not in the correct alignment - they were exactly 90 degrees out, just like the lower diagram.  By removing one end from its flange, undoing the collar, aligning the yokes and refitting, I got the prop back in phase.  The difference has been amazing, there is now much less prop noise and the resonant howl at 45mph has all but gone.

 

This article, written by Geoff Hardman, originally appeared in CA7C Seven Focus in Sept 2008 pp 22-24.

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