Water pump fault

[graeme]

On our recent trip to Triumphest in California, after 1500 miles, we were just exiting the highway at our destination, when there was a loud mechanical knocking/clatter related to engine speed. When I lifted the bonnet I could see that the w/pump pulley was rattling around with the fan belt.

 

On inspection, the threaded part of the pump spindle had sheared and allowed the pulley to come loose and rattle around destroying itself and the woodruff key.

 

I had a spare pump/gasket and key in the boot but had to seek assistance from Moss Motors who supplied a new pulley with somebody driving the 50 miles from Goleta to Santa Maria to deliver the pulley on the same day, great service.

 

Looking at the photos, can anybody identify why the shaft failed?

 

 

 

 

Thanks

 

Graeme

 

 

[Motorsport Mickey]

Difficult to see Graeme but it looks like a stress raiser on the thread at the end. Was the shaft supplied with a thread right up to the shoulder ?

I normally like to see a small undercut at the end of the thread (no matter what part) or the thread fading out to nothing rather than ending abruptly to help prevent a fracture starting where the thread finishes. RogerH will have a pertinent view upon this.

 

Mick Richards

Edited October 7, 2014 by Motorsport Mickey

[AlanT]

I should think that the pulley was not making a perfect fit on the shaft. This sets up a kind of wobble and the bolt is being cyclically bent.

 

I did research in fracture of stub-axles many years back. Fracture runs quick when the material is cycled from stress in one direction to stress in the other.

 

I'll bet you have used this to break if a piece of metal off on purpose at some time.

[RogerH]

Hi Graeme,

the second pic shows an area of cracking in the 3 to 5-o-clock position.

 

This propagates towards the centre of the stud.

 

The crank pulses are transmitted through the fan belt and these forces are imparted into the stud - probably always in the same place on the stud.

Have a look to see where the thread 'run out' is - it may well coincide with the centre of the cracked area.

 

The plain area between 5-o-clock and 3-o-clock is where the stud snapped in one go.

 

It could be caused simply by a poor thread cut at the run out.

 

As Mick states putting an undercut at the end of the thread run out could reduce the stress raiser.

 

Do you know how old the pump is?

How tight was the belt before failure?

 

Roger

[Robin Powell]

What size is the thread at the end? I had a similar pump failure a few years ago and the replacement had a bigger thread and nut. I think from memory that the failed one only had a 7/16th and the replacement 1/2 inch nut.

[graeme]

Thanks for the feedback.

 

The Bastuck pump with larger bearings was fitted in 2005/6 and has done about 40,000 miles. The belt tension was checked before our trip and had about 1/2" movement on the long side.

 

The thread is/was 5/16" UNF with a 1/2"AF Nylok nut fitted.

 

On a couple of occasions, during the previous four months, the pump had thrown out some grease, observed on the inner wings and bonnet. I assumed that I had put too much grease into the pump during the winter service. Could this be connected?

 

Cheers

 

Graeme

[peejay4A]

Mine throws out grease after a few strokes on the grease gun Graeme.

[AlanT]

On a Bastuck speedo-drive I found poor tolerances, cheap parts, failure to understand the original design.