Yes you do Martyn, If you don't, then the edges of the A7 flat belt WILL chafe on the pulley flanges. Crowning the pulley also helps when pulleys are misaligned. You will find plenty of A7 camshaft pulleys with chips in the flanges – still doing sterling service without chafing the belt.

As for the crown profile, you could use a small round nosed tool and guess it by eye, clean up with a file. Unless you have a big heavy lathe, a form tool is out of the question.

BobH. 1932 RN

I do it the same way as Andrew, flat in the middle and slight angle at each side then hand blend, on wider ones I do two angles each side so total of 5 facetes

Hi Martyn, Guys,

Tension will cause the belt to climb up the slope of the crown until its running centrally on top of the crown. Have a look at some of the old line shaft pulleys. The crowning was all that kept them on the pulley.

Many years ago, beyond memory, at Tech we were given a mathematical proof that a crowned pulley caused the belt to run on the top of the crown.

There have been millions of pulleys confirming this.

Kempes Engineers Year book, 1982 in section E9/3, referring to flat belts, calls for pulleys to be cambered, with either a smooth curve or two symmetrical smooth curves with a central flat portion not exceeding half the width of the belt. It also says that flat tapers should not be used since they introduce stresses into the belt, and decreases contact between belt and pulley.

Like Jason, I would probably use two very slight tapers with a central flat portion

So the transition from the slight taper needs to be blended into the central flat.

It says "The effectiveness of the camber depends on the ratio of camber to pulley diameter, and is practically independent of belt width. It is affected by length of drive and by pulley diameter itself, but not to any significant extent".

It also says that "If the pulleys are very different in size, it is good practice to leave the smaller pulley flat and provide all the necessary camber on the larger pulley"

All the A7 fan pulleys that I have seen have been crowned

HTH

Howard

I have had success with a wide form tool on plastic using back gear and the lowest speed pulley setting. It took a lot of pressure to start cutting and it would not have worked on brass, aluminium or steel. I have a video of it somewhere.

Martin C

I've no idea what the question regarding hand turning was about, but it can't have been important.

To complete the picture the drive pulley on the crankshaft for the governor is also crowned:

SInce this pulley only has one flange the crowning was machined using a hydraulic copy attachment and a marked out and hand filed pattern; the one bottom right:

Does this count as hand turning?

Andrew

Although the OP said he did not fancy doing it by hand it is not too hard, even if you opt to do the blending by file similar methods can be used.

Draw out the profile and work out a set of co-ordinates so the work can be roughed with teh handwheels or DRO and simply blend which is much like dot to dot.

0.020" increments

Woodturning scraper with boring bar used as a rest

Works on steel too

Found this on the net, although it seems a very small amount for small pulleys :-

Crowned pulley dimensions

A convex crowned pulley usually has an increase of +/- 1/16” per foot of pulley width. Therefore, an 18” face width pulley (4” in diameter), could either have a 4” diameter measurement in the center with 3-13/16” diameter edges or a 4-3/32” diameter measurement in the center with 4” diameter edges.

If you look at any belt sander / linisher you will see a crowned pulley. It can give you an idea of scale and shape.

I've not noticed a crown on my belt sanders, maybe it's due to them all having tracking adjustment.

I think so too. I've got a Parkside cheapie belt sander from Lidl that clamps to a workbench and I use it for wood and ally flats and chamfering the edges. It hasn't got any crown I can see, the track adjustment works fine, and it's becoming one of my standard go-to tools.

It's a superb example of mediocre engineering at its very finest.