tthomson Posted August 2, 2016 Report Share Posted August 2, 2016 Has anyone else had this happen? Driving my TR4A from Marlow to Plymouth last Wednesday evening to attend a funeral on Thursday, I was at Kennford (30 miles to go) when the front of the crank pulley sheared off. My concern is this: Is Aluminium a suitable material for this pulley, when it does not handle cyclic stress as well as it might? If the fan belt is even a little tight the stresses can cause this kind of failure. The shape of the pulley seems to be a copy of the pressed steel one, but I don't see any reason for that. The place where the shear occurred is probably less than 2mm thick. If anyone else has had a similar experience, please let me know. It may be that I was just unlucky, but I am considering getting one made in stainless. If any of you would like a stainless one too, a combined order is likely to be at a lower unit cost. Meanwhile I have my TR in the garage awaiting a suitable replacement. TT Quote Link to post Share on other sites
RogerH Posted August 2, 2016 Report Share Posted August 2, 2016 (edited) Hi Tony, any chance of some pics - the crack surface would be good. I could see that the bottom of the groove, if square cut, could be a serious stress raiser. No reason for it not to be radius'd. at the bottom. Very tight fan belt could cause such a problem. Roger Edited August 2, 2016 by RogerH Quote Link to post Share on other sites
tthomson Posted August 2, 2016 Author Report Share Posted August 2, 2016 (edited) Hi Roger, Thanks for your response. The bottom of the groove does not seem to be radiused It is a standard narrow belt kit pulley (undamped) Photos when I get a chance to do them. Tony Edited August 2, 2016 by tthomson Quote Link to post Share on other sites
peejay4A Posted August 2, 2016 Report Share Posted August 2, 2016 Plain or damped pulley Tony? Quote Link to post Share on other sites
Paul Hogan Posted August 2, 2016 Report Share Posted August 2, 2016 Hi Tony, I have had several alloy pulleys made for various TR's and have never had a problem with them. I'm currently running stock off the shelf alloy items on my GTR4A and the racing TR3S with no sign of trouble. Maybe you had a duff one but in any case Roger is correct; there shouldn't be a hard edge to the valley walls. they should be rounded to avoid stress cracks forming. hoges Quote Link to post Share on other sites
McMuttley Posted August 3, 2016 Report Share Posted August 3, 2016 out of intetest, how tight is very tight ? i tightened and changed my fan belt to stop squeal on my new revvy pulley i have less than 1" play more like 1/2 ? Quote Link to post Share on other sites
peejay4A Posted August 3, 2016 Report Share Posted August 3, 2016 (edited) 1/2" is good. For a narrow belt. Edited August 3, 2016 by peejay4A Quote Link to post Share on other sites
stuart Posted August 3, 2016 Report Share Posted August 3, 2016 Dont think we have ever heard of one shearing off like that, sounds like a bad casting to me, I doubt you could get a fan belt tight enough to do that. Do you know the supplier Tony? Stuart. Quote Link to post Share on other sites
monty Posted August 3, 2016 Report Share Posted August 3, 2016 Presumably a damped item would have more 'meat' on it to perhaps reinforce the pulley? Quote Link to post Share on other sites
tthomson Posted August 3, 2016 Author Report Share Posted August 3, 2016 Hi Stuart, The supplier was Moss in 2003, but the rebuild was completed two years ago. I have not done many miles since then so It has not had much use. It may be that they have improved the design since then, but the replacement Aluminium one I have bought looks very similar in design, including the lack of radius Roger alluded to. I hope you are right about me having a duff one, but I will check the cost of having a Stainless one made. TT Quote Link to post Share on other sites
RogerH Posted August 3, 2016 Report Share Posted August 3, 2016 Hi Tony, 1000's of these ali pulleys have been sold and I would suggest if a couple have failed we would have heard by now. The one that failed may have a problem that is out of the ordinary. Don;t bother with stainless go for another Ali pulley. Roger Quote Link to post Share on other sites
stuart Posted August 4, 2016 Report Share Posted August 4, 2016 Like Roger says that is the first failure I have heard of so possibly just unlucky. Stuart. Quote Link to post Share on other sites
tthomson Posted August 4, 2016 Author Report Share Posted August 4, 2016 Image of failed pulley: Here Quote Link to post Share on other sites
RogerH Posted August 4, 2016 Report Share Posted August 4, 2016 Hi Tony, any chance of a pic directly down from above without the flash. Roger Quote Link to post Share on other sites
tthomson Posted August 5, 2016 Author Report Share Posted August 5, 2016 (edited) Hi Roger Here (Ignore date on image - Camera has wrong date setup) TT Edited August 5, 2016 by tthomson Quote Link to post Share on other sites
RogerH Posted August 5, 2016 Report Share Posted August 5, 2016 Hi Tony, the English language is a wonderful (but complicated) thing. It would have been nice to have the pic directly down from the previous pic. But don;t panic. Looking at the broken edge to the left of the screen you can see a very sharp upward edge. Followed by a crumbly grey surface, Followed by another sharp edge pointing downwards This is where the rim was ripped away. I can;t see it from the pic but if you look very very carefully around the crack surface you will almost certainly find an area that look different. It may even have lines radiating out from a point like the waves in sand when the tide goes out.- this is the starting point of the crack. Roger Quote Link to post Share on other sites
Paul J Posted August 5, 2016 Report Share Posted August 5, 2016 Is it at all possible the pulley vee angle is machined slightly incorrect, a tightness at the top of the vee could cause such a fracture. I have just recently fitted a narrow alloy pulley kit, now I'm concerned. Paul Quote Link to post Share on other sites
RogerH Posted August 5, 2016 Report Share Posted August 5, 2016 Hi Paul, don't panic. This looks like the first failure in 1000's sold/used. Roger Quote Link to post Share on other sites
tthomson Posted August 5, 2016 Author Report Share Posted August 5, 2016 Hi Paul, I think Roger is correct with this. I have seen enough fractures both with and without microscopes (electron or light) to recognise this type of fracture. It has all the hallmarks of a stress fracture, but I have not yet had an opportunity to look at the complete fracture surface. I have purchased a replacement Alloy pulley and I'll see how that performs. However, in case this failure was caused by the way the belt and pulleys are aligned, I will be looking to get a stainless version to keep in the car. TT Quote Link to post Share on other sites
Alec Pringle Posted August 5, 2016 Report Share Posted August 5, 2016 If it's any reassurance, this is only the second alloy pulley failure I've come across - the first was so far out of alignment it's no wonder it failed, but that was down to the installer, not the pulley ! Cheers Alec Quote Link to post Share on other sites
littlejim Posted August 6, 2016 Report Share Posted August 6, 2016 (edited) Hope there is a metallurgist on the forum. Bizarre that the whole side has departed, not just part of it. (flaw) Think I'd be double checking alignment just in case that has contributed. Edited August 6, 2016 by littlejim Quote Link to post Share on other sites
Waldi Posted August 6, 2016 Report Share Posted August 6, 2016 Not a metallurgist, just a mechanical engineer... The effect of geometry on strength of a component is significant, a sharp radius will result in stress concentrations and so do surface imperfections. In this case like a sharp (but small) groove from machining can be sufficient to initiate a crack. Aluminium has lower allowable stresses than steel, so the component should be thicker dor same load. SS300 series also has a lower allowable stress than normal (carbon) steel. Quote Link to post Share on other sites
RogerH Posted August 6, 2016 Report Share Posted August 6, 2016 (edited) Hi LJ, not surprising really that the whole thing came off. Imagine a small crack starting in the bottom of the 'V' on the outer rim side. The belt is tensioned with rotation - the crack grows. Because of the fast rotation there is almost an even load on the circumference. When the crack gets big enough the tensile load will become too great on the remaining metal and 'pop' off it comes. If the bottom of the 'V' was rounded or had rounded corners it almost certainly wouldn't happen. A one off is bad luck but not a disaster. Roger it makes changing the belt easier Edited August 6, 2016 by RogerH Quote Link to post Share on other sites
Lebro Posted August 6, 2016 Report Share Posted August 6, 2016 Seems to me from the photo that the thickness of the circular part which broke is rather low. I think my pulley (a damped one from TR shop - probably MGB type) has the full width from top of "V" (ouside dia of pulley) all the way down to the centre. It is not undercut like the one in photo. Bob. Quote Link to post Share on other sites
Eddie Cairns Posted August 6, 2016 Report Share Posted August 6, 2016 Why go with stainless as every one seems to do these days. It is not as good as carbon steel for a number of mechanical properties. Cannot think of a reason myself to use it in that application. Quote Link to post Share on other sites
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