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Paul Alciatore
06-07-2012, 05:53 PM
I have followed the other thread on machining a crowned pulley with some interest. There are a lot of clever methods there and I always like to learn by observing.

But it did bring to my mind the question of just what is important about a crowned pulley. I mean, I do understand the idea of using a crowned pulley to steer the belt toward the center of the pulley and it is a very clever idea that works very well. But, just what is required in the shape of a crowned pulley to either maximize this effect or to promote the best power transmission or to for the best belt life or whatever?

I can see this self centering effect working with many shapes of crowned pulley. A radius naturally would work. But so would other curves: parabola, ellipse, catenary curve, a Vee shape, a faceted shape with several straight segments, etc. (Side note, would you believe a simple word like "catenary" is not in the spell checker? I had to just add it after double checking the spelling.) In my mind, all of the above would work to center the belt. So what shape would be best and why? Has any research been done on this?

In other words, what are the real world requirements for the shape of the crown on a crowned pulley?

Forrest Addy
06-07-2012, 07:47 PM
I worked briefly in a machine shop in El Paso that machined conveyor pulleys for mine equipment. These were large units some 5 ft in dia X 8 ft wide having 6 and 8 inch shafts. These were crowned with shallow tapers about 1/4 the pulley width each side. The taper was so shallow a few thousandths depth of cu one way or another would make the intersection shift up to an inch.

My boss of the time told me these pulleys ran flights of belting maybe a mile in length incorporatinng a frieght car of rolled belting spliced together. Imagine the difficulty in reeving such a belt. Then it runs for years transporting millions of tons of coal or tailings. And all it takes to make all that belt track is a shallow crown.

Wow!

In some circles, a theoretically perfect pulley crown would probably be a circular arc but I doubt if the belt would care. As long as a purturbed belt re-centers itself the drive will continue to - um - drive.

Anyone remember the Model T Ford water pump drive? Crowned pulleys, leather belt, fixed center distance. A millwrights nigthmare in mass production.

Paul Alciatore
06-07-2012, 08:08 PM
I worked briefly in a machine shop in El Paso that machined conveyor pulleys for mine equipment. These were large units some 5 ft in dia X 8 ft wide having 6 and 8 inch shafts. These were crowned with shallow tapers about 1/4 the pulley width each side. The taper was so shallow a few thousandths depth of cu one way or another would make the intersection shift up to an inch.

My boss of the time told me these pulleys ran flights of belting maybe a mile in length incorporatinng a frieght car of rolled belting spliced together. Imagine the difficulty in reeving such a belt. Then it runs for years transporting millions of tons of coal or tailings. And all it takes to make all that belt track is a shallow crown.

...<snip>

Forrest, Very interesting! I also worked in El Paso for a short time, one year or so. Was that mine the copper operation on the western edge of the city? I worked for a TV station there and had some trouble getting permission to enter their property to take field strength readings. I finally convinced them that I did not work for the News department who had done some stories on them that they did not appreciate and all was well. Engineers welcome, reporters NOT!

I imagine that the important thing on those pulleys would be to match the taper on the two sides so the belt stayed on center.

jdunmyer
06-07-2012, 09:31 PM
I've put a few wraps of duct tape in the center of a flat pulley in order to form a "crown" of sorts. It worked fine. Belt was 6" wide, handling about 12 Hp.

Electrical tape didn't work very well, if the belt slipped at all, the tape would wad up and come off.

sasquatch
06-07-2012, 09:44 PM
Re: A few wraps of Duct Tape:

Now,, i bet Red Green NEVER thought of that one!!:D

Shuswap Pat
06-07-2012, 10:00 PM
As I still try to figure out the easy way to make a true radius crown - all the literature I have read suggests 1/8" per foot on the diameter (taper) - on a 2 foot wide roll, the center would be 1/8" larger than the ends.

We used to do 'Slow down Belt' rolls, they are 5-6' wide, 12 1/2" diameter. They would get a trapazoidal crown - tapered ends, 1/3 flat in the middle.

These belts were about 8' centers, so they were almost as wide as long. We always managed to get them to track.

My challenge is always to come up with the 'Manual way' to show some of the 'CNC' guys what a real machinist can do.

Patrick

lakeside53
06-07-2012, 10:32 PM
I made 1 degree "crown" on my 2x72 belt sanders. Two linear tapers meeting in the middle and a short "flat" area. Nicely rounded in short order by the belt.

Less is better for the belts. Doesn't take much to make them track, but you don't want to put a center curve in the belt. Needing more than 1 degree is probably trying to compensate for some other tracking error.

mickeyf
06-07-2012, 10:47 PM
I would guess that an absolute minimum of crown is best. Especially after hearing about the 8' wide belts. The greater the arc, the less contact between the belt and the pulley, presumably reducing power transfer. This is my intuition speaking, not science, so may be wrong.

LES A W HARRIS
06-07-2012, 11:10 PM
From: Text Book of Advanced Machine Work. Robert H Smith, M.I.T. 1910~1912~1915, Google download.

Section 4, Pg,28.

TURNING PULLEYS.
60. The taper or crowning on the face of pulleys, ranges from
per foot for large pulleys to per foot for small pulleys.

2 pages further into the section, a drawing for a 5" pulley, gives 1/2" per foot,
with a 1-3/8" face width.

Cheers,

Shuswap Pat
06-08-2012, 12:19 AM
So once you guys figure out how much is enough for 'Crown' I will be watching. My next question -I am building a 2 x 72 belt sander - similar to a KMG. It has one 'FLAT' drive wheel, one'CROWNED' tracking wheel, and 2 'FLAT' idler wheels. I fire it up, set the tracking (belt speed 4000 sfpm), start to grind and the belt wanders to one side. Relive the pressue, and it returns to the center.

I was looking at the 'Radius Master' site - (Australian company) - they have a beautifull machine, and claim no belt drift!

What could their secret be - Kangaroo Snot?

Patrick

lakeside53
06-08-2012, 02:41 AM
Both belt sanders I built track beautifully with a single 1 degree crown tracking wheel. Tiny crown... and mine are 0 to 6000sfm.

Tighten your belt tension. I can push mine off track if I man-handle it. Unlike KMG, no springs involved in mine so tension can be set to whatever I want.

Forrest Addy
06-08-2012, 07:50 AM
How much crown? Shouldn't one of the factors be the elasticity of the belt's tension material?

Leather is flexible and thus may require largish crown proportions to track properly. Synthetic belt materials with glass or steel tension (so-called "high modulus") elements may track well enough on pulleys intended for leather belts but may suffer wear or degration if mated to pulleys having significant crowns. In effect only the center portion of suck a synthetic belt is actually under tension. Therefore: recognizing industry practice as a product of evolution and experiment it would seem a subtler pulley crown be employed for synthetic belting.

I can't recall any dimentional details of the conveyor pulleys I machined back in the day in El Paso (um, 1971, I think. I was visiting my sister then and took a short job to keep me occupied Sis and Bro were at work.) I took over a running job so I had nothing to set-up. Just run parts. There wasn't much crown but it may have been 1/8" taper in a couple feet.

I do remember the lathe was a relic with the head and tailstock built up a couple of feet to increase the swing to maybe 5 ft. The chuck had a groove machined in it so the 1" roller chain wrapped in even pitches to mesh with a couple dozen dowel pins driven and spaced to approximate sprocket teeth. The chain drive was driven from a worm gear that was in turn driven with a cone pulley from a 10 HP motor. Everthing was bolted to timbers and scrap iron. Gawd! It was crude! But it worked.

j.bain87
06-08-2012, 08:50 AM
It's real simple to work out how much crown you need, divide the length of the pulley by 3.
Centre section is left flat, taper each end about 1/8" to the foot, done hundreds and never had one come back because it didn't do what it was meant to do.
Works for any size, from a foot long to 6 foot long, just do them all the same.

Jim Stabe
06-08-2012, 05:33 PM
You are making more of this than you need to. I just put the pulleys for my 2 x 72 in the lathe and hit the sides with a file and tried to make both sides the same. The belt seemed to move a little easier than I wanted so I hit it again and it was fine. Don't need to make a science project out of it.

jdunmyer
06-08-2012, 09:33 PM
Jim,
You're correct, it isn't that big a deal.

I found through bitter experience that a half-decent crown will keep the belt on and tracking UNTIL you load it enough to make it slip a bit. She'll pop right off then. If you get it aligned properly, it'll stay on, even if you do push things a bit.

sasquatch
06-08-2012, 09:50 PM
Jim, great build on the MGB, very nice fabrication,,a lengthy build for sure, but looks like you have it all covered.

Thanks for that interesting build site!!

gwilson
06-08-2012, 10:46 PM
Re: Harris's post; I have noticed that the pulleys on my toolpost grinder are a LOT more crowned than the pulleys on my Elgin bench lathe.

Jim Stabe
06-13-2012, 05:33 PM
Jim, great build on the MGB, very nice fabrication,,a lengthy build for sure, but looks like you have it all covered.

Thanks for that interesting build site!!

Thank you! Seems like most of the big fabrication tasks are done but the littany of medium and small stuff goes on forever. Maybe when I retire at the end of the year...

Jim