|Michael Gilligan||04/03/2021 10:29:50|
17658 forum posts
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’.
|Blue Heeler||04/03/2021 11:00:14|
262 forum posts
Thanks for the link Jason
|Douglas Johnston||04/03/2021 11:19:20|
738 forum posts
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.
|Martin Kyte||04/03/2021 11:35:00|
2313 forum posts
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.
|Andrew Johnston||04/03/2021 11:55:19|
5972 forum posts
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:
To keep the traditionalists happy the governor end pulleys were done with files:
The pictures of full size engines that I have show that both ends of the governor belt have crowned pulleys.
|Martin Kyte||04/03/2021 12:30:14|
2313 forum posts
Once most people have a copying attachment or a CNC lathe that becomes the 'Traditional" way. Nice bit of turning as usual Andrew.
20283 forum posts
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
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.
Edited By JasonB on 04/03/2021 13:38:02
Edited By JasonB on 04/03/2021 13:43:29
20283 forum posts
Just to try it out with the formula that made sense to me for a stepover at 7mm from ctr line
|Howard Lewis||04/03/2021 14:42:24|
|4678 forum posts|
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 )
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