Belt Length Calculations

[adhall]

So I'm working on the CAD layout for my gen-set frame and I got to the point where I wanted to know how long the drive belt should be. The CAD program I'm using can spit out a belt length after I draw it in to scale, and it turns out I would have been OK with this, but I thought I had better double-check it. I didn't want to buy a belt and then have to pitch it.

A quick web search located a couple of Gates design guides that applied to micro-v belts, so it looks like my problem is solved. But, unfortunately, the formulas in them did not generate values that agreed with what AutoCAD came up with. So I did some more web searching and found even more belt length formulas and all of them give out their own different belt length values. And none of them agreed with eachother. So now I'm like the guy with two clocks who never really knows what time it is.

More web searching located a web site with a very useful belt parameter calculator, and it gives exactly the same belt length numbers as the CAD program. Then I found yet another Gates engineering guide (apparently from Britain, based on the spellings) with another formula in it and this formula gives exactly the same numbers as the web based calculator and the CAD program. So now I've got three different techniques all giving the same number. I'm starting to feel sure of myself again.

For the benefit of the forum here is the link to the web calculator:
     http://www.gizmology.net/pulleysbelts.htm

If you want to see the formula in the Gates manual, here is the link to that:
    http://mechatronica.eu/en/design-manuals/synchronous-belts/Drive%20Calculation%20Guide%20(21-27).pdf

Best regards,
Andy Hall

[jimmer]

Here is another good one:

http://www.roadlessgear.com/html/techarticles/beltlength/index.php3


jim

[adhall]

Jim:

The calculator at the Roadless Gear site (the one you point to) is one of the ones I had tried. I don't think it is accurate. For example, here is a comparison:

   Large pulley:        23.5"
   Small pulley:         8.4"
   Center distance:   25"

   Road Less Gear belt length:  99.8"
   Gizmology belt length:        102.4"

That's over 2-1/2" difference. If you have allowed enough adjusting range in your takeup, you'd be OK. However, I'm inclined to believe there will be other errors that creep in, too. (For example, using the flywheel dameter as the pitch diameter is wrong. The actuall pitch diameter is a little bigger than the flywheel--it runs somewhere just outside the top of the ribs in the belt.) Put them all together and you may end up buying a belt that doesn't fit. The "cheap Swede" (that's what my wife calls me), doesn't like to buy things he can't use.

As I said below, I've checked the Gizmology calculations against my CAD layout and the two of them agree exactly (out to the 4th decimal place). I am wondering if the other formulas are OK for situations where the two pulleys are about the same size, but start to become inaccurate when size differance becomes large.

At any rate, I think the lesson here is to be careful about canned formulas wherever you find them. It doesn't hurt to double check the numbers, especially if you can come up with a totally different way to do that. It's nice when you can measure actual parts to confirm (I sure wish I had my generator pulley on hand).

Best regards,
Andy Hall

[twombo]

Belt lengths???  I'm still trying to find a source for Poly V sheaves and hubs (SK type preferred) without having to call someone for a quote.  just a good online catalog (3D CAD drawings would be nice, but beggars can't be choosy). anybody got any favorites?? I like to do m own research where possible and I'm usually pretty good at it.  On this one, I'm batting zero!!

[jimmer]

Andy,

All I can say is that when I used that webpage (input large and small pulley and distance between centers) the recommended belt size fit right in the center of my adjustment. I could be wrong, but I don't think it was off 2.5 inches.

You can verify your answer with a string measurement once you get things physically mounted.

Good Luck,

jim

[adhall]

Jim:

When I get my gen-set together, I'll confirm the measurements and post the results. If I was wrong, everyone will find out. That will resolve the issue once and for all (as far as I'm concerned, at least).

In the mean time, let me re-emphasize my primary point which is that there is lots of bogus stuff posted on the Internet and even published in manufacturers' catalogs and manuals. It pays not to depend on a single source.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Twombo:

As nearly as I can tell, most of the guys on this forum who building generators are using automotive type belts and sheaves. I have not been able to find engineering manuals or data on this type of belt (although I haven't looked very far outside the Gates web site yet). The belts are "K" cross section. These are available from auto parts stores and also on line. The Gates part numbers are coded as follows:

   KXXYYYY

    "K" is the belt cross section
    "XX" is a two digit number that tells you how many ribs there are
    "YYYY" is the pitch length of the belt in 1/10's of an inch

So, the part number K060973 would be a "K" series belt with 6 ribs and a pitch length of 97.3".

You can buy sheaves from George at Utterpower. I also recall seeing other people selling them through this forum. If you are a machinist, you can download a manual from Gates that tells you the dimensions and you can machine your own. (Look in the "Light Power and Precision" manual for this.)

Best regards,
Andy Hall

[twombo]

Thanks Andy!!

Guess it just takes a little persistence in sorting through all the data on a web site to find the nuggets of gold, sometimes?? I found the document  giving the specs for the pulley form. I have access to all the machines and tooling to make those pulleys. Made from scrap vs 90 to 130 dollars per sheave is  a pretty easy decision for me. The QD bushings are only 14 bucks each so I'll gladly buy those. Lookes like the only tedous part will be grinding the cutter.

If I recall, the taper on the sheave and bushing is  a morse taper (3/4" in a foot) my lathe has a taper attachment so I am in like "Flynn"! Lookes like a Machine Shop 101 project!!

Many thanks, compadre!!

Mike

[adhall]

Mike:

By the way, I just came upon a nice, round piece of steel plate that looks like it has a pulley inside, so now I will be making one, too. I'm planning to make a "heavy" pulley. I won't know just how heavy until I have checked out how much radial load the generator bearings can live with happily. The disk is 14" diameter and 3" thick, so I have close to 140 lbs. of material to work with. (Why is my back so sore??)

About the cutter for making the grooves:
I have an old serpentine pulley laying around that came off an car air conditioner pump. I was just thinking of using that as a template and grinding the cutter to fit it. But I haven't looked yet to see what kind of inserts are available for this. If the inserts aren't too expensive, I may just go with that. I have no idea how precise the shape of the groove needs to be and I would rather err on the side of being too fussy.

About the taper on the hub:
I have no idea what the taper actually is. I was just going to chuck up the hub and use a dial indicator to set the taper attachment for the roughing cuts. Then I was going to use marking blue to tweek it in for the finishing cuts.

Best regards,
Andy Hall

[theboss]

here is a site where you can automatically calculate pulley ratio, size, belt lenght, rpm in & out..blah blah blah by giving a few constants and setting everything else variable..