Pulley flank angles - help !

Well, as you all know I have been machining up some pulley sheaves.

Before setting the top-slide I hunted around on the internet to find the correct flank angle. I was advised by the belt supplier that it was a 'Z' section belt with a 10mm width at the top and a radial thickness of 6mm. (The internet suggested that this type has an included angle of 40 degs. I therefore set the top slide (with a DI) to machine 20 degs.

I have checked the finished parts by several methods) and I have no doubt that I'm within 0.25 deg of the target of 20. Unfortunately neither the new or the old belt seems to be a particularly good fit with my 40 deg angle pulley.

Now, I had assumed that the pulley flank angle would be the same as that of the belt. I had previously measured the used and worn pulleys and decided that they were pretty close to 40, so 40 they must have been when they were new ! Unfortunately, now that I check them more carefully they look to be more like 34 degs!!!

Unfortunately this would seem to tie up with more information I have gleaned from the web suggesting that the pulley angles for a Z drive can be 34 or 38 degs depending on pulley diameter.

So is this really correct or am I confused and mislead by alternative facts in the web ??? Are drive pulleys really made to a different angle to the one on the belt ? Might this be something to do with the bending of the belt causing the inner zone to compress and get fatter ?

If you experts confirm this is the case it looks like I will have to start the machining all over again

Gerry

See here Fenner Page 69

Vee angle changes on larger diameters from 34 to 38 degrees, according to the belt type. Have used these angles in the past so can vouch for them.

Another vote for 34 degrees or something close from me.

I've read that the included angle is (slightly?) dependent on the radius of the vee pulley groove - the tighter the bend the less the included angle. Whether it's significant and to what extent the default 34 degrees figure needs to be modified is very likely something Mr Fenner's drawing office keeps in a safe place.

I've made Z and A section pulleys of a variety of diameters using 34 degrees included angle and been happy with the results.

HTH Simon

I wouldn’t worry too much over the angle, for optimum power transmission and belt life it might be best to get it right but otherwise it probably won’t be critical.

Mike