Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

When cutting my "V" grooves I always work back and forth, side to side, as just jamming the cutting bit straight in is prone to produce serious chatter once you get a ways in. I always finish my groove with a side to side cleanup, and so I do end up with something of a small flat in the bottom of the groove. However, I have never done the math to figure exactly how wide that flat should be, in order to fit whatever diameter belting is intended to be used, so I suspect that I'm not getting the max traction that the graph suggests is optimal.

I would suggest that a perfectly cut "U" groove, sized to match the belting up to, but not beyond its sectional equator, would give the absolute best traction as it would provide the greatest realized surface area of contact, without the added friction of partial capture.

I was going to suggest the Perfect U cut 49% deep or less as well.

  As I looked at the drawing Jim posted, it occurred to me the 3 points of contact pulley is also very much the same as the V-belt pulley we all know often used on lawn tractors and small engines.

I thought: would it not be cool if we could get extruded rubber cord like we use to make up custom length O-ring belts for our tiny engines BUT made in V-belt cross section (vs round) say 2mm width for example! There is something about the V shape that as you pull tension on belt it really grabs as it is pulled deep into its matching pulley.
  We likely don't need the fabric molded into the bigger V-belts as the stretch of pure rubber is quite forgiving on belt length.
  I googled micro V-belt and got this multi groove shown in the photo. I suppose you could purchase a belt like this and cut all the V's with a Really sharp knife and get several single V belts from one multi micro groove....but I like the idea of rubber cord in a V shape that we can super glue to custom length better...just food for thought. There may be a micro single groove v-belt maker out there.

In the end, round cross section O-ring type belts are really handy, but a V-belt option might be nice for that one specific implement that draw a lot of power.

[ Guests cannot view attachments ]

Hi Jim,

I believe I saw your video on gluing belts.

The melt to weld belts while strong, are tricky to get right. So I want to try CA gluing belts.

What is the exact stuff you recommend?

3M Scotch-Weld Plastic & Rubber Instant Adhesive PR40, Clear??

A perfectly sized U only fits a belt at a non-tensioned state. Once tensioned and loaded, a round belt will elongate and its cross section will shrink. The shrink is dynamic with load.

A trapezoid can provide relatively constant contact patch under a somewhat wider variability of load tension.

I would also note that round belts often slip when looked at cross eyed, even in nicely fitted grooves, for the above reason. A big part of that problem stems from packing oil fogged on newly manufactured items. The oil and water pump belts on the four cylinder inline I have are o rings, and folks complain they slip. Indeed mine slipped when new. I removed the belts, cleaned pulley grooves and the belts with windex, got them dry, end of problem once the o ring had some grip. Once dry, the reduced requirement for belt tension will make beatings and belts happier, and reduce parasitic losses. There is no longer a need to preload the belt much. 

Thanks Bill, appreciated.