35/40 degree tool on pulley?

[65coupei6]

I took Rickwrench's waterpump pulley specs to a few machinist's. They all told me to use a 40 degree tool to cut the groove for the belt instead of a 35 degree in the specs. They really didn't tell my why. I just think it is what they have.

So my question is would that make a difference for the belt to seat right or slip?

 

[rickwrench]

I just looked at the pdf plan and it shows 40 degrees for the groove angle. Did you mean they said to use 35 degrees on the pulley instead?
If that's the case and they've turned custon pulleys before, I'd listen to them, as I just spec'ed the groove to what a standard automotive belt is pitched at.
Rick(wrench)

 

[Anonymous]

what is so special about the water pump pulley? is it different than the stock ones? i have a double belt pulley on mine, is the size different or something?

 

[Anonymous]

i cant edit right now...but i didnt see the article on it at first...just saw the drawing......but the 35 deg. groove makes sense for a 40 degree belt so that it would catch better in the groove and not slip...

 

[65coupei6]

Yup. I switched them by accident. But, anyways one guy quoted my $450. I think he is a little nuts! The other guy said $125. Which, is not really that bad for a single unit.

 

[Anonymous]

$450 is definatly high, since accuracy on the part is not that critical. That guy is probably busy right now and told you that so you would leave. $125 seems right on the money for a single part. Right now, material costs are a bit high, so that has to be factored in.

 

[rickwrench]

The belt pitch angle must change depending on diameter? OK, that makes sense. As the belt goes around the pulley radius, the bottom is compressed and would get fatter, while the top is stretched and would get thinner. I wonder how much the pitch angle on the belt actually changes. Hmm, that means each pulley would have a slightly different optimum pitch depending on the diameter.

OK I just went out into the garage and performed a little experiment. I measured three old fashioned (uncogged) automotive v-belts all, of them were 35 degrees not 40. I could have sworn automotive belts were 40 degrees in pitch. Hmmm, good thing I don't work for NASA...

The experiment, belt pitch angles at:
2.5" dia. pulley - 31 degrees
5.0" dia. pulley - 33 degrees
Flat belt - 35 degrees

So, the belt goes into the groove flat, and the bottom expands as it goes around the curve, gripping the pulley.

This is all coming together now in my big round melon. The wear marks on the very soft, un-heat-treated aluminum, prototype pulley are at the bottom of the belt contact area! I guess I'll (continue to) perform a destruction test on the soft prototype to see how long it lasts with the wrong groove angle. 9,500 miles and counting.
I will update the PDF Plan.
Rick(wrench)