Pulley material

I have a small 2 stroke engine which has a V-belt driven cooling fan. The OE belt pulleys are pressed steel affairs which are protected from corrosion by that yellow chromate (?) coating given to car parts. The problem is it wears through quite quickly from belt rubbing. Over every winter the exposed surfaces corrode and this causes the belt to wear rapidly. I'm getting fed up with the regular disassembly, cleaning up the pulley flanks and fitting new belts. so I plan to make some new pulleys in a material that wont corrode. Stainless steel might be the obvious choice but might be difficult to machine so I'm considering aluminium or even brass also.

I don't want them to fail by fatigue and fracture so the machined replacements can be thicker then the OE. Any thoughts anyone ?

Some 6061 aluminium should be just fine. Aluminium pulleys are used to drive most hobby lathes and compressors etc so should handle the fraction of a hp for a cooling fan. YOu can probably buy standard ally pulleys ready made and bore out the middle to suit your shafts etc.

Or would a spray of belt dressing resin on the belt and pulley before winter lay up prevent rusting?

Edited By Hopper on 02/10/2019 23:42:45

Aluminium is often used for V belt pulleys. It does wear more rapidly than cast iron but can still give many years of use. In this case it's a low load application, so you shouldn't have any problems at all.

Where is the engine stored, if you are getting corrosion on the coated steel you may also get an oxide layer on the aluminium that also where you belt as aluminium oxide is use dto make abrasives.

If normal steel pulleys rust over one or two winters to the extent that they significantly shorten belt life then there surely there must be some other factors involved here.

Millions of vehicles have pressed steel pulleys and not all of them are in sunny climes and used all the year round, many get laid up over winter and presumably any rust that does form soon polishes off when the engine is running. A fan driven by a small 2 stroke engine should not impose a serious load on a V belt drive and I expect should last 20 years.

Are the pulleys in alignment? is the belt the correct section for the pulleys? is the engine totally exposed to weather conditions? probably other things that might be involved. Might help if you could post a picture.

Ian P

Is the 'yellow coating' in fact passivated cadmium plating? Not the best anti-rust treatment, unfortunately.

Al-alloy pulleys may be OK, but I have seen enough cases where moisture in the belt has caused the metal to corrode, leaving white material on/in the belt surface - presumably abrasive aluminium oxide - and pitted pulleys. This may be less of a problem with modern 'cut' V belts. Just make sure you get a corrosion-resistant alloy, if you go down this route.

There's not alot wrong with making a V pulliey from ply wood. Keep it dry and in line. Ian S C

Thanks all for your useful and thought-provoking comments. I decided that other than machinability, brass didn't have much going for it. Today I was able to get some stainless for my small 70mm pulley and a big lump of aluminium for the larger drive pulley. So next year I'll be able to tell you which is better, at least in my particular application anyway.

Ian P - I agree its puzzling we don't see this issue more often in cars. The pulleys (polyvee) on my recent cars, (one of which is stored over winter) never seems to have the shiny groove flanks even with regular use that I see on my 2 stroke engine. I wonder if automotive pulleys have some kind of treatment to resist corrosion. One thing I have noticed on my 2-stroke engine pulleys is that the corrosion occurs where the belt rests on the pulley. You'd expect the rusting would occur only in the exposed areas. So is the belt cording material a little bit hygroscopic I wonder ?

Ian - plywood isn't such a bad idea. 70 years ago when materials were short, some large bearing cages were made in plywood. If my application was less important, I might have been tempted to give it a try.

Gerry

A tip for making V-pulleys I have seen on commercial items, and helping ensuring a symmetrical result, is to machine them as a pair of discs faced at the appropriate angle, rather than trying to cut a hefty V-groove. The two discs are then riveted or screwed together.

The one likely problem with turning a very shallow cone, as here, is the compound-slide not rotating sufficiently far. On the Myford ML7 you are lucky to reach the 45º limit; on my Harrison the slide will probably turn past 90º if it were not for the hand-wheels meeting!

Ok chaps, I'm back to pick your brains again !

My smaller pulley is comprised of two sheaves. I have thought long and hard about how to hold the stock in order to create the required shape. But this stainless (303 grade) is relatively tough stuff to machine, not to mention the swarf that comes off it, so I'm quite keen to do it the right way first time.

It doesn't look feasible to machine the flanks holding the part by its OD in the chuck so Im thinking of this... Make an expanding mandrel to hold the part by its bore. Machine the inside face and OD. Remove turn around and machine the outside. What worries me is that the mandrel wont be able to cope with the cutting torque when Im working on the outer edges so I might need a clamping nut to make it secure. Then I have to think about how to hold the part it for removal the metal that will be left under the nut.

Perhaps if got it all completely wrong ?! Would be interested in your ideas on this one ?

Incidentally, Ive been using the aluminium cutting carbide tips cleaning up the offcuts. I was looking for a speed where the swarf would come off in fine chips but between 100 and 2500 all I get is nasty razor wire spirals. I think I will try some of the tips that are designed for SS and see if they perform better. The 'standard' gold coloured tips didn't seem to work well at all on this material. Lots of screaming and at the higher speeds an interesting red glow at the junction of the tip and the workpiece

If you are worried about the mandrel withstanding the cutting torque, would it be feasible to make up an arbor, with a flange welded on and trued up, with a spigot for the pulley blank, before being tapped for a couple of setscrews?

The setscrews could then clamp the blank against the flange, passing through holes in the blank, and located by the spigot, to allow the blank to be machined,

Howard

I dont see enough corrosion on your old pulley to cause rapid belt wear on a low-power application like a fan drive. Maybe try buying better quality belt and make sure the fan bearing is not nipping up under load?

Back to the machining: I would not faff about with an expanding mandrel. It's not going to hold well on a thin job like this one. Instead, drill a smaller hole in the centre of your blank and make a stepped stub mandrel with a bolt to hold the blank in place.

Then hold the finished piece in the three jaw chuck as last step and final machine the hole up the middle and the surrounding hub area to final size. If you have already done the hole in the middle to full size, make your shouldered mandrel to fit that hole and then finish off the centre section with job held in chuck by outer diameter.

You are making a chore for yourself by using stainless though. There's a reason commercial pulleys are pressed steel or machined aluminium etc. . Take it steady and you should get there eventually though.

Fit toothed belt and pulleys? No slippage (rubbing) problem. Cam belts last 50,000miles or more on cars.

Is it essential to make the pulley in two halves? The original may be made that way for cheapness; a couple of stampings placed back to back with almost zero machining needed to make them. Is it possible to replace it with a conventional pulley made from a flat disk, thinned out in the centre as necessary to fit. The advantage is the straight line machining is much simpler.

The choice of Stainless Steel is making life difficult too. Many (most?) stainless steel alloys don't machine at all well because they work-harden in a blink and are tough. Nasty, unless a machineable stainless is bought. However, try increasing the depth of cut and feed-rate to produce chips if your lathe is man enough. (My WM280 just stretches into carbide chipping zone with 2500rpm and a 1.5kW motor. ) But I rarely push it that hard, preferring to put up with ribbons rather than overwork a relatively light hobby machine. Apart from anything else, the chips come off in a burning hot spray I find too exciting for comfort!

I'd use Aluminium rather than Stainless.

Dave

I have seen scores of examples of pulleys manufactured using two back to back sheet metal pressings, but never seen one (of such a small diameter) pressed out of such thick material!

The only reason I can think of for it needing it to be heavy gauge is if the two halves are clamped together at the centre and have shims between them to adjust the belt tension. I am getting really curious about this 'small' two stroke engine that would have such a (relatively) expensive and robust cooling fan drive. I think of 50 or 100cc as being small but even at 250cc a cooling fan should not take a lot of driving and belt load would not be very high.

As has been mentioned the corrosion does not look severe enough to have any serious effect on belt life, in any event rough surfaces on the driving faces act to increase the grip (until worn off) and increase the amount of power that can be transmitted. For the rough surface to cause premature belt failure this belt must surely be driving a heavier load than just a cooling fan?

Regardless of all this I am still of the opinion that some other factors are involved in this installation, I mentioned car engine belt drives earlier as an example of pressed steel pulleys having a long life but that is just one example out of thousands where belts and especially pulleys have extremely long lifetimes.

Ian P

Thanks again all for your comments.

Ian P, yes you are absolutely correct. The belt adjustment is by shimming between the pulley sheaves on the fan shaft. The other side is thinner material though 1.6mm as opposed to the 2.6mm of the one in the picture. It gets away with this because its backed up by the plastic fan hub.

As regards the corrosion, the pics don't show the state it came out of hibernation in. They have been cleaned up with emery and wire brush, then operated for about 10 hours. Unfortunately, even in this condition, still black dust being generated indicating belt wear. But the bigger issue is due to the material removed from the pully flanks in clean-up (and the belt of course), I run out of adjustment. I can remove all the shims and still the belt is too loose. Of course I could just start machining off material from the centre portion of the flanks but then the 1.6 thick sheave isn't going to be happy about that. As you might have guessed by now Ian, the engine is from a small aircraft so my solution has to be robust rather than 'experimental'.

Hopper - I think your 'process sequencing' is the right one and have nearly finished the nutted mandrel and will let you know how it goes.. I thought it might slip because for some reason I imagined the torque was going to unscrew the nut. Its actually going to tighten it ! Also, I have to put a notch in the bore of the sheave to get the pulley over a key on the fan shaft so if necessary I can use this feature for manufacturing also.

Gerry

Thanks for the clarification. If you have run out of adjustment because of machining the original steel flanges then making new ones makes a lot of sense and using something that is less likely to corrode should solve the problem almost completely.

I used 'almost' because I suspect that having visible black dust (rubber off the belt presumably) indicates that considerable wear or some other action is taking place.

Belts do wear out which I suppose is indicated by the black dust, what is surprising in your setup is that it seems so severe. Since your application has to be robust it might be prudent to investigate further jsut so you have full confidence.

Is the belt doing a lot of work?
Is or was the belt tension correct?
Are the pulleys and shafts exactly aligned?
Is the belt the correct section for the pulleys?

Ian P

First piece finished...…

The biggest challenge firstly was the swarf. Horrible razor sharp stuff and you have to keep stopping to clear it or it gets caught in the rotation and flails around dangerously. I discovered that cutting slowly keeps it in more manageable spirals and later with a big pair of leather gloves you can crush it up nicely and reduce 3 carrier bag-fulls of the stuff into one.

Secondly the 20 degree flank angle meant the cross slide and top slide knobs tended to interfere and I had to work with the topslide almost at its positional limits. This meant that the slide was getting a bit loose in its gibs and caused the 1mm-pitched grooving you can see on the periphery. Its not really as bad as it looks in the picture so I'll run with this imperfection. But anyway I think I can avoid this on the next one with a bit more jiggery pokery.

The bolted mandrel was a fantastic success Hopper and worked perfectly. Not even a hint of slip at any time so.

The CCMT aluminium cutting tips worked beautifully at the lower speeds of 300~500RPM. At 2000~2500RPM they frequently chipped. I couldn't work out at first why the cutting quality became progressively worse until I looked at the cutting edges with a microscope and found the reason. This issue wasn't really obvious by eye.

Ian P, I think the reason for the severe wear might be the speed. I didn't think about this earlier but the engine runs almost continuously at a 5000RPM so the fan will be doing about twice that. I do know from the past when I've fitted new pully sheaves the system/belt is almost wear-less. So I don't think there is any other defects in the set-up like misalignment etc.

Now I just have to decide if I tackle the 2nd small pulley or have a go at making the large one.....

Gerry

For the second of the small pulley sheaves I got some tips designed for stainless steel cutting. They look like the one on the right, with the aluminium cutting tip on the left.

Really pleased with them. They seem to cut just as well at any speed and leave a nice finish but usefully don't have that rather fragile sharp edge which is so easily chipped.

I have also leant to get along with the swarf which I really didn't like initially. Stainless steel swarf is quite crushable, unlike aluminium. With a pair of tough gardening cloves on you can collect it and compress it into quite manageable volumes.

Altogether a great learning exercise.

Well that was a welcome Xmas break as I finally managed to finished my crankshaft pulley in stainless steel.

The issues I had with making this one was putting in the corner radii. I have a brazed carbide tool with a tip radius of about 1.5mm. I thought this would do nicely. Unfortunately once the cutting chip-width exceeded about 1~1.5 mm seriously bad structural vibrations started. I tightened everything, reduced overhangs and even tried cutting on the opposite side but failed to make any significant improvement in the cuttable chip width. I guess the machine is simply not rigid enough to plunge this size tool into the workpiece.

In the end I had to form the radius by lots of small bites that blended into each other then clean up with emery cloth. (You can still see a little bit of a chattered surface though.)

Perhaps the material I was using wasn't the best in machinability but I see indexable lathe tips for sale with radii of up to around 5mm for wheel profiling and wonder how others get on with these ? Or perhaps there's some tricks I don't know about ?