View Full Version : Taperlock failure

srn4 - n/a
4-May-06, 10:09 PM
I finaly managed to pull the broken taperlock out of the pulley.

I am slightly concerned with what I see...Can someone with some knowledge please comment on this..

The crystal structure in the metal looks far to large and irregular to be safe....

Its also kinda weird that they chose to put the keyway almost opposite the grub screw hole, thus making the material thinner...if they moved the keyway 5mm to the other side it would not have fractured the way it did!

I have ordered a new part (Cost 6), its just rather worrying how weak it appears to be!


Mark - n/a
4-May-06, 10:34 PM
Hi Duncan

For a broken taperlock yours looks like any other !! the most important thing is to make sure you have some clearance above the key when the taperlock is clamped on the shaft,some taperlocks have a non standard depth keyway !!

Regards Mark

jar2 - n/a
4-May-06, 10:55 PM
Hi Dunc,

You are right - the smaller taperloks with largeish bores in them (the one you are using!) are pretty dodgy IMO. Taperloks are a cheap, made-to-fit-all solution - they are not that great in some situations. The quality is also variable - and doesn't depend on the price! I tend to use Tollok (or Fenlock) bushes (http://www.tollok.com/) and no keys whenever possible - they have a far better grip, you can use smaller shaft diameters and they are made from decent quality steel and therefore don't fracture!

srn4 - n/a
5-May-06, 07:35 AM
Ok, thanks guys. I hope it wont snap again, it was rather embarising this time!

So am I right in thinking that there should be .5mm clearence above the key and none either side? (when not tight, the taperlock slides up and down the key with not too much effort?)

And also what is the best way to ensure that it is fitted correctly?


Ian Brooks - n/a
5-May-06, 07:59 AM
Very interesting....

The majority of the surface looks like ductile overload, and the appearance (at least so far as I can tell from the photo) is quite normal for such a failure.

The brighter area (bottom right) - are there some kind of parrellel lines, like wave marks left by a receding tide on the beach? If so, this would be the fatigue failure, which initiated on the outside surface, this would possibly confirm that the failure was caused by an oversize key.

Most failures (which are not 'instant') have a number of features:

1 - Fatigue initiation point, the place where the failure started. Identified by looking for:

2 - Beach marks, parrellel lines like those left by the receding tide on the shore. The beach marks emanate from the intitiation point, and are caused by the slow growth of the fatigue crack, one line per cycle.

3 - ductile overload, once the fatigue crack gets so big that the rest of the component can stand it no longer, it fails suddenly by ductile overload. This is usually the largest feature of a fracture surface.

4 - polished area. As the crack grows, the part may flex, rubbing the crack surface together, thus polishing the surface. This sometimes obliterates any fracture evidence. It is also possible for this to happen after failure if the part continues to operate.

Right, I'm going now....


srn4 - n/a
5-May-06, 08:14 AM
Thats interesting Ian. It looks like the crack has been caused at the top (wider part) of the bush and has then cracked the bottom half off..... But there are so many speculations you could make...

The new part just arrived and its quite interesting to see that it is made of a different steel (much darker in appearence rather than normal steel colour)

I am also pleased to see that the keyway has been rotated away from the grubscrew hole so the material around the circumfrence of the taperlock is much thicker and even.

I am going to go and mill a teeny bit off the key and then have another go at flying this bath tub!