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Jim Hubbell
01-17-2009, 02:50 AM
I have volentered to make two flat sheaves of 4in. diameter. One to be 6in. wide and one 10in. wide. They are to run a 6in. wide sanding belt. I believe a crown is necessary to keep the belt on track. I looked in the mach. handbook but could find nothing. If anyone remembers a good " rule of thumb " in this regard I will appreciate seeing it.

Thanks in advance.

Jim

Your Old Dog
01-17-2009, 05:10 AM
I don't think you need a crown on one that size if running two wheels. On a 2x72 square wheel (3wheel) grinder the idler pulley is ever so slightly crowned and it's the tracking adjustment. I can tell you that too much crown makes it overly sensitive when adjusting. If I really wanted to find out I'd make the wheel flat and then build it up a little with electrical tape in the middle and see how it acted.

On my 4x48 neither of the belts are crowned but one end has the tracking adjustment.

Sounds like a macho grinder you're working on :D

brian Rupnow
01-17-2009, 08:27 AM
The book recomends 1/16" on diameter per foot of face. That works out to 0.148 degrees from centerline to one side, or 0.296 degrees included angle

oldtiffie
01-17-2009, 08:45 AM
Bringing in the Sheaves:
http://au.youtube.com/watch?v=rPQsQIWJrMw&feature=related

HSS
01-17-2009, 08:55 AM
YOD, I just finished my 2X72 grinder and made it to plan guidelines. It called for 2* crown from centerline both ways and that SOB is way too sensitive for tracking. I am going to remove some crown using Brian's posted formula and try again. I was wondering why it was so hard to track that thing.:confused:
Thank you guys.

Patrick

GadgetBuilder
01-17-2009, 09:35 AM
Only the driven sheave needs a crown. If you crown both they tend to "argue", making centering unstable. Better to have less crown than too much - that causes instability too.

The axes need to be well aligned since the crown only provides fine guidance.

John

Edit: http://www.visusa.com/belt_tracking01.htm

HSS
01-17-2009, 10:06 AM
Only the driven sheave needs a crown. If you crown both they tend to "argue", making centering unstable. Better to have less crown than too much - that causes instability too.

The axes need to be well aligned since the crown only provides fine guidance.

John

Edit: http://www.visusa.com/belt_tracking01.htm

According to the link, my tracking wheel is totally wrong cause it's an apex crown. Damnit. Take it apart and stick it back in the lathe.

lynnl
01-17-2009, 10:22 AM
...
On a 2x72 square wheel (3wheel) grinder the idler pulley is ever so slightly crowned ...
:D

Doesn't that square wheel make for a sort of bumpy grinder? :)

Circlip
01-17-2009, 11:18 AM
Never heard of "knocking the corners off" ??

Jpfalt
01-17-2009, 12:53 PM
I was involved for several years in design of large bandsaws and maintenance equipment.

Most important is that the wheels not be low in the center of the tire.

Second is that you need to be careful that the structure not deflect when belt tension is appied. The belt, band or in this case sanding belt will tend to center up on the tight spot and if the frame deflects, will run off the wheels.

In setting this up, use the crown on the driving wheel and keep the rest of the wheels VERY slightly crowned.

Jim Hubbell
01-17-2009, 07:13 PM
I neglected to say what this is going to be used for. I am doing this for the local First Robotics competition group. High school kids. In my opinion it is a fine opertunity for the boys and girls.
The belt is part of the "play-ball " handling mechanism.

The 1/16th on dia. is what I have so far. The second sheave is being glued as I type this.

Some tape wound on might help to determine if more or less crown is needed.

Thanks all

Jim

jdunmyer
01-17-2009, 07:57 PM
I've heard that the R.O.T. is 1/8" per foot. Have used 1 degree on the compound and the pulleys have worked out well.

oldtiffie
01-17-2009, 08:35 PM
Thanks jdunmyer.

That's the "rule of thumb" that I've used - and was told to use over 55 years ago!! I could not find or confirm it in "Google" or else-where - still can't.

It seems to be a traditional "word of mouth" hand-me-down "shop/job wisdom" thing that just "works" - which it certainly seems to do.

I think it was a curve/crown with a rise of 1/8" per foot and a chord equal to the pulley/sheave width. I have done them with tapers instead of curves/arcs and they seemed to work OK but my "gut feeling" is to use the curve.

Teenage_Machinist
01-17-2009, 09:34 PM
This seems like a good thing for Human Numerical Control. A graphing calculator or spreadsheet = easy, can use parabolic or what ever.

lazlo
01-17-2009, 09:40 PM
I've heard that the R.O.T. is 1/8" per foot. Have used 1 degree on the compound and the pulleys have worked out well.

To state the obvious, that gives you a linear taper/crown. I was told that linear crowning (with a flat spot in the center) was recommended over a radius, but according to the article GadgetBuilder posted, the big-boys use a radius crown on most pulleys, and save the linear crown for wide, thin pulley like a conveyor pulley.

Lane once posted a cool picture of a radius jig that was a long bar on a pivot.

JS
01-17-2009, 09:42 PM
machinery's 5th 1920

Crown / center of pulley

1/ 20th the width of pulley for leather

`1/ 150th the width of pulley for cotton

1/ 16 or 1/8 per foot for high speed

1/4 per foot for low speed

S_J_H
01-17-2009, 09:47 PM
I have 2 flat belt machines besides my belt sander. My Artisan and SB9. The Artisan is much older and has heavily crowned pulleys except for the motor pulley which is flat. The SB9 has almost flat pulleys. The SB9 has great belt tracking. The Artisan is very touchy and alignment must be dead on or the belt will throw.
If there is one big flaw in my Artisan lathe I would say the pulleys have to much crown in them.

These pics were never taken to show the pulley crown but I think you can make out the differences and the flatter SB9 profile works a lot better.
Artisan lathe-
http://i109.photobucket.com/albums/n48/S_J_H/vintage%20Artisan%20lathe/Artisanlathe009.jpg

SB9 lathe-http://i109.photobucket.com/albums/n48/S_J_H/southbendheadstock014.jpg

Steve

garyphansen
01-18-2009, 12:35 PM
I set the compound to 1 degree and tapered one side of the pulley then moved the compound to put a one degree taper on the other side. Then I sanded the center of the crown a little. All these pulleys track fine. I have been using this lathe for about a year and a half now and so far no ware on the pulleys that I can see. Gary P. Hansen

http://img.photobucket.com/albums/v371/garyphansen/IM000119.jpg

P.S. This LeBlond turns 100 this year. Wish her happy birthday.

jdunmyer
01-18-2009, 01:23 PM
Gary,
I make my pulleys exactly like you do, a simple taper to the middle. A radiused crown might be better, I don't know, but I don't think it's that critical. See this:
http://www.oldengine.org/shows/semta97/jul19_13.jpg

I ran this mill for several years; the pulley on the mill's arbor had no crown, and the belt would not stay on. We'd wrap a few layers of tape around the middle and all was well, but we found that duct tape worked much better than electrical tape. The latter would suffer a lot when I pushed things a bit to the point of belt slippage. If both pulleys are crowned, the belt will track pretty well even if there's really bad misalignment. However, if the belt slips at all, it'll pop right off. If the pulleys are aligned near perfectly, there can be some slippage and the belt will stay on.

FWIW: this sawmill was being powered by a John Deere Model 'H', which is about 12 Hp. We were using a 4" wide belt, and had both the tractor and the mill tied down with stakes in the ground to get & keep the belt tight enough. Naturally, we used belt dressing liberally. When I was sawing, I'd make 'er bark.

lane
01-18-2009, 06:13 PM
Here are a couple of photos of cutting a 12 inch crown on pulley for belt sander.
http://i178.photobucket.com/albums/w277/lane5263/Picture003.jpg

http://i178.photobucket.com/albums/w277/lane5263/Picture004.jpg

http://sites.google.com/site/machinistsite/TWO-BUDDIES/sander-project/LSander2.jpg?attredirects=0