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Crowning a pulley for a flat belt Question

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Michael Gilligan04/03/2021 10:29:50
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Posted by JasonB on 04/03/2021 07:26:10:

[…]

This has always been the traditional model engineers way of doing it so no need for large radius ball turning tools and you were right in your first post about getting your files out.

.

Fair point, Jason, but the truth is: There is no need for most of the things that are discussed on this forum.

It’s a hobby, a pastime, an intellectual stimulation [or whatever]

Suggestions [hints] were invited, and I made a suggestion.

Personally, I am more interested in problem-solving than in following ‘the traditional model engineers way’.

MichaelG.

Blue Heeler04/03/2021 11:00:14
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Posted by JasonB on 04/03/2021 07:26:10:

having spent enough time toning down the language used I not go through it again, if you search "crowning jasonB" on the homepage search box you will find several threads like this about crowning pulleys & flywheels and also fish bellies on conrods.

This has always been the traditional model engineers way of doing it so no need for large radius ball turning tools and you were right in your first post about getting your files out.

Thanks for the link Jason

Douglas Johnston04/03/2021 11:19:20
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When I built a belt sander I faced the same problem and solved it by keeping the pulley parallel and then wrapping some insulating tape round the central third of the width to create the crown. Worked a treat and is still in use after 20 odd years.

Doug

Martin Kyte04/03/2021 11:35:00
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In general the 'traditional' or common way of doing things is the optimum. Methods that work and are reasonably simple get established as the norm until superceeded by something new like everyone having CNC lathes. Where 'Traditional' goes wrong is when a craft has stopped evolving (which Model Engineering hasn't) such as flint knapping for example. I'm sure you can think of other examples.

regards Martin

Andrew Johnston04/03/2021 11:55:19
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Posted by JasonB on 04/03/2021 07:26:10:

This has always been the traditional model engineers way of doing it...........

Just as well I'm not a model engineer. For the crankshaft governor pulley I used a hydraulic copy unit with a hand filed pattern:

governor pulley keyway.jpg

To keep the traditionalists happy the governor end pulleys were done with files:

governor pulleys.jpg

The pictures of full size engines that I have show that both ends of the governor belt have crowned pulleys.

Andrew

Martin Kyte04/03/2021 12:30:14
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Once most people have a copying attachment or a CNC lathe that becomes the 'Traditional" way. Nice bit of turning as usual Andrew.

regards Martin

JasonB04/03/2021 13:30:37
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Posted by Michael Gilligan on 04/03/2021 08:58:31:
Posted by JasonB on 04/03/2021 08:49:48:
Posted by Chris Evans 6 on 04/03/2021 08:41:17:

Faced with this type of calculation again do it in steps as suggested. The formula is the square root of radius squared minus distance moved (step over) squared. Hope this makes sense.

is that not just the radius?

.

... but you only emboldened part of the text !

Try adding some mathematically relevant parentheses to what Chris wrote.

MichaelG.

yes that is what I was trying to make sense of, there are several ways it could be written which would give a different outcome, I would actually have expected some trig in there, hopefully Chris will come along with the details as I can't make any of those 3 options work. But r - square root of (r squared - d squared) does give the infeed for a given stepover

20210304_132236[1].jpg

Out of interest for a 25 x 25 pulley using the table you linked to the radius that a ball turner would need to swing would be 98mm and the tangent angle in the region of 4deg, quite different to the 1.5deg that looks right.

crown.jpg

Edited By JasonB on 04/03/2021 13:38:02

Edited By JasonB on 04/03/2021 13:43:29

JasonB04/03/2021 13:53:59
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Just to try it out with the formula that made sense to me for a stepover at 7mm from ctr line

crown coordinates.jpg

20210304_134921[1].jpg

Howard Lewis04/03/2021 14:42:24
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1.5 degrees looks to be not too far from correct

Kempes Engineers Year Book, in section E9/3 advises:

That pulleys should be cambered to keep the belt tracking properly, in spite of errors in alignment of shafts and lack of truth in the belt.

The profile of the pulley should be a smooth curve, or of two symmetrical two smooth curves with a flat central portion not exceeding half the width of the belt.

The advice is that where the pulleys differ markedly in size, it is good practice to leave the smaller pulley flat, and to apply camber to the larger.

Pulley width should be 15% greater than that of the belt

Maximum camber should be about 1% on diameter., or 0.2 in per foot of pulley width, whichever is the lesser.

By my calculations this suggests an angle of 1 Degree 8.7 Minutes, so I degree would probably suffice.

Flat tapers should not be used because they cause local strains in the belt, and impair contact between belt and pulley. .

For simple souls like me, the calculation shown can be made a little easier..

The square root of ( R^2 - d^2 ) involves the difference of two squares, so the calculation becomes the

Square Root of (R + d ) * (R - d )

HTH

Howard

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