Crowned pulleys

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Electrojim
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Crowned pulleys

Post by Electrojim » Tue Mar 07, 2017 10:34 pm

I've read the postings here about crowned, flat-belt pulleys, and get the impression that, while not rocket science, crowning with a fixed radius is not the easiest thing for a home machinist to do.

Well, I am not even a home machinist, but have a need to get an existing pulley crowned. The guy who made it for me held a file against the OD, but it looks more chamfered than actually crowned, with about the center 2/3 of the face flat as a pancake. The pulley is 3" OD and 3/4" thick, and the belt that it will be driving is 1/2" wide. Since I don't do machining myself, any ideas where I should go to get this done properly, and how much of a job it actually is? Many thanks!

spro
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Re: Crowned pulleys

Post by spro » Tue Mar 07, 2017 11:24 pm

The earlier were more a gentle arc than you describe but they were leather belts. What type of belt are you using? It depends. The one constant is that the shafts be parallel to each other, in both planes. If one shape of crown meets to another at speed, that could be a problem.

GK1918
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Re: Crowned pulleys

Post by GK1918 » Wed Mar 08, 2017 7:24 am

Pretty easy and fast if you have a lathe. The crown isn't very much, like 2 degrees so set the compound to
2 degrees may have to come up with a simple mandrel to chuck it up. Its called taper turning by the compound rest method......do one side flip pulley around and where the two tapers meet in the middle just soften that
with file or some emery cloth.......

John Hasler
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Re: Crowned pulleys

Post by John Hasler » Wed Mar 08, 2017 7:55 am

GK1918 wrote:Pretty easy and fast if you have a lathe. The crown isn't very much, like 2 degrees so set the compound to
2 degrees may have to come up with a simple mandrel to chuck it up. Its called taper turning by the compound rest method......do one side flip pulley around and where the two tapers meet in the middle just soften that
with file or some emery cloth.......
I've done just that with pulleys for belt sanders. If you want to get fancy you can set the compound to one degree after doing the two degree taper and knock off the corner to make a two-step taper. Works well.

Inspector
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Re: Crowned pulleys

Post by Inspector » Wed Mar 08, 2017 10:08 am

From a discussion on large radius attachments in the lathe section.

https://www.youtube.com/watch?v=vXLXDj-L8aw&t=2s

Pete

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Re: Crowned pulleys

Post by Harold_V » Wed Mar 08, 2017 4:48 pm

Inspector wrote:From a discussion on large radius attachments in the lathe section.
Note that the video suggests a .005" press fit for the bearings. That is NOT acceptable, as it collapses the outer race beyond reason. A half thou is more than enough press, and should NOT be exceeded.

Harold
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BigDumbDinosaur
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Re: Crowned pulleys

Post by BigDumbDinosaur » Wed Mar 08, 2017 6:19 pm

Harold_V wrote:Note that the video suggests a .005" press fit for the bearings.
I caught that as well. Wonder where on earth they came up with that number.
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spro
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Re: Crowned pulleys

Post by spro » Wed Mar 08, 2017 7:58 pm

I get the impression that Electrojim has an issue with one crown not mating to another by using a belt which favors one more than the other. Then by using a using only a file, upon one, there is no clear reason why the belt slips off. I think that is the situation. The shafts could be perfectly parallel and then there is a bias at one pulley. Which crown is right for what type belt? We don't know this yet.

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rudd
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Re: Crowned pulleys

Post by rudd » Thu Mar 09, 2017 10:49 pm

I've been reading "Echoes from the Oil Field" written by a machinist in Pennsylvania oil county in the early 1900's - it was a column in American Machinist and has been reprinted as collections.
He has one column where he talks about the "new improved" radius crowned pulleys that appear, he had been using pulleys with a simple taper on each side. The radius was said to increase belt life and transmit more power. Osborn's final response after trying the "new improved" pulleys is to chuck one up on a lathe and cut the radius into a simple double taper so the belt will stay on. He said he had not had problems with belt life with the taper crowned pulleys.
My little experience with flat belt power all had the radius crown type pulley, or flat for the small motor pulleys. Never had a too much problem with them till you wanted some serious power, then they would crawl off a pulley sometimes.

Glenn Brooks
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Re: Crowned pulleys

Post by Glenn Brooks » Fri Mar 10, 2017 1:48 am

rudd wrote: My little experience with flat belt power all had the radius crown type pulley, or flat for the small motor pulleys. Never had a too much problem with them till you wanted some serious power, then they would crawl off a pulley sometimes.
I've always wondered about this. Now I wonder if small misalignments in parallel shafts aren't magnified under heavy loads, enuf to cause the belts to run off the pulleys? Could this happen when applying high power to a pulley, causing the shaft to compress or twist slightly, setting up an out of balance alignment, hence the belt comes off?

I experienced multiple occasions where the belt slipping off the crowned pulleys on my Dalton yesterday when taking a slightly to deep cuts. The bit jammed into the work and stopping the spindle from turning. This invariably popped the belt off the pulley before I could shut off the machine.

At the time I thought this was due to pulley misalignment under stress.

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Russ Hanscom
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Re: Crowned pulleys

Post by Russ Hanscom » Fri Mar 10, 2017 9:39 am

One should also consider the belt; any non uniformity as might be expected in a hide, or other worn belting material, would make the belt stretch more on one side when loaded, thus encouraging it to move off center.

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SteveHGraham
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Re: Crowned pulleys

Post by SteveHGraham » Fri Mar 10, 2017 1:48 pm

I made a crowned 3"-diameter pulley for my belt sander. Nothing to it. The crown doesn't have to be a perfectly rounded radius. When you're down around a degree, two straight cuts that meet in the middle of a pulley look almost exactly like a round crown. You can file down the middle or sand it if it makes you feel better.

I took a shortcut many people won't agree with. I crowned one side of the pulley, turned it around in the chuck, and crowned the other side. With such a light cut and shallow crown, it didn't matter that the chuck was gripping a taper. I was even able to bore it while holding it this way, but if I had had a choice, I would have done the boring and everything else BEFORE the crown.

Interesting thing: while I was researching crowned pulleys, I learned that you only need one of them in a three-pulley belt sander. The others can be flat.

If you look around, you can find a page that gives recommended crown heights for flat belts. It's very, very little. My crown was cut with the compound at 0.5 degrees.

Okay, I found it for you: http://www.visusa.com/belt_tracking01.htm

You can make a crown with a file very quickly, and it's less aggravation. You can rest a straightedge on the pulley to see where the crown is, and if it's off, just move it.
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