V belt pulley groove angles question

[The Cobbler]

am I right in thinking that the v groove in pulleys changes angle according to the diameter of the pulley?
I just watched a video on you tube on a pulley repair and that raised this question

 

[36truck]

No I don't believe they change angles. They do get wider for bigger diameters.

 

[rlitman]

am I right in thinking that the v groove in pulleys changes angle according to the diameter of the pulley?
I just watched a video on you tube on a pulley repair and that raised this question

Yes, but only for the small end of the diameter range of conventional V belts, and only a tiny amount. It's all because V belts widen a little when bent, and that's one of the biggest reasons for using other belting systems (micro V, wedge, etc.).

In my own experience with overdriven belts, I've had better luck using cogged belts (like an AX instead of an A or BX instead of a B) when one side is too tightly radiused, because the conventional belts will wear the pulley groove from a V to a U more quickly (due to that bulging), and that's in cast iron pulleys at that.

This reference: https://www.engineersedge.com/v_belt_sheave.htm
shows 34 degrees for 3.4"-7.0" datum pulleys for A (1/2" wide) and B (3/4" wide) belts, but 38 degrees for larger pulleys. Note that conventional V belts themselves are constructed with a 40 degree angle.

 

[The Cobbler]

The video I watched was machining a 3 step pulley that had been broken ( actually there were 2 pulleys)
The machinist matched the angle from the larger ( less used ) groove in both cases to machine the smaller grooves. I thought there was a difference in angles, but even with wear, I thought matching the angle on the existing groove would have been more appropriate

 

[rlitman]

The video I watched was machining a 3 step pulley that had been broken ( actually there were 2 pulleys)
The machinist matched the angle from the larger ( less used ) groove in both cases to machine the smaller grooves. I thought there was a difference in angles, but even with wear, I thought matching the angle on the existing groove would have been more appropriate

I'm guessing that's an aluminum pulley, and fractional horsepower motor? So like a 3L, 4L or 5L series belt on something like a drill press (or maybe a bandsaw)? More searching from my keyboard brought me to Machinery's handbook references (sometimes the internet is easier than walking to the shop), which shows as little as 30 degrees on those really small ones. If so, it'll probably never see enough use for that couple of a degree difference to matter. And if he was smart enough to use a cogged belt, it really won't matter, because they don't expand as much in a tight wrap. The belts I mostly deal with transfer 7.5-15HP to blowers running 24x7, so the run hours really add up.

 

[The Cobbler]

yes, aluminum pulley, probably 4 L or 5 L belts... for a lathe

 

[alfadan]

Looks like in the case of a 4L, the difference under a variety of sheave diameters is only 6 degrees included angle. The fact that multispeed pulleys are extremely common, run with one, flexible, belt proves it ain't no big deal.

 

[rlitman]

Just got recommended that video, LOL. What's with all the starting and stopping that poor lathe? You can clearly hear the interrupted cut and how it changes when you're cutting more than the weld. That's what I'd be focusing on.

First pulley is cast iron, and I have real questions about the stability of that "weld". I would have sat that flat on a graphite block and brazed it. I've turned a few pulleys, and getting them to run true on a lathe can be a royal pain. His mandrel approach was good, and even then you can see how the bore wasn't concentric with the grooves. He could probably have offset it (in a 4-jaw) so that the weld blended better by being on the "high" side of the cut, but while that may look better, it may also add vibration, so I do think he did about as good a job as could be done.

Second is aluminum, and is pretty much EOL and U'd out. That's why he was having trouble indicating it, but I think I'd have approached the angle finding in much the same way as him. In the case of it being worn out, changing the angle might even help with belt contact.

 

[MBfreak]

Swedish Standard SMS 981 FWIW
The belt angle is always 39 degrees.
However
The pulley angle depends on the median diameter of the pulley "V".
Example
Up to 106 mm median dia angle 32 degrees
Over 300 mm median dia angle 38 degrees.

Ola