David Jenkins
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| posted on 30/1/07 at 06:33 PM |
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Fanbelt tension
I'm getting annoyed with my fanbelt! Well, water-pump & alternator belt anyway.
Ever since I've fitted a smaller steel crank pulley I've had the ignition light flickering faintly at low revs. The Bright6 light does
show flickers that you probably wouldn't notice with a filament bulb, but it never did it before the change.
I've tightened the belt to what I feel is right, but I don't know how far to take it. The Haynes Book for Boys says 1/2" deflection
when pushed - but how hard should I be pushing? A couple of grammes or 100kg?
I'm concerned that if I over-tighten it I'll knacker the water-pump bearings, or similar.
So, any clues or guidance?
cheers,
David
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BenB
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| posted on 30/1/07 at 06:37 PM |
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quote:
Originally posted by David Jenkins
I've tightened the belt to what I feel is right, but I don't know how far to take it. The Haynes Book for Boys says 1/2" deflection
when pushed - but how hard should I be pushing? A couple of grammes or 100kg?
Enough to deflect it 1/2" 
Seriously though, the belts are usually re-inforced so don't stretch much. It will only deflect so far. If you keep on pushing the deflection
won't change you'll just snap the belt or the alternator mounting!!
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BenB
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| posted on 30/1/07 at 06:39 PM |
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I'd suspect that it might not have anything to do with the belt itself. If the crank pulley is smaller the rpm of the alternator will be lower.
Alternators need a certain amount of rpm to put out any power, your idle speed could be flickering around the minimum rpm for the regulator, hence the
flickering. If you increase the idle speed a bit does the flickering go away. Probably not the belt if it does....
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David Jenkins
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| posted on 30/1/07 at 06:49 PM |
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Ha! Good point - I'd forgotten that the alternator's probably spinning slower. That could well be the problem... (Dur... sometimes it
helps if I engage the brain).
I wish they'd make a steel crank pulley in standard size, for those who aren't pushing the revs on their engines. I could risk the old
pulley, except that I have heard of them breaking at normal revs.
Maybe it's time to try making my own, on the lathe. 
[Edited on 30/1/07 by David Jenkins]
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02GF74
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| posted on 30/1/07 at 07:38 PM |
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as thingy ^^^^ said, the smaller crank pulley is spinning the alternator slower; if the lamp flickers at tidel, then don't worry. If is still
flickers above 1,555 - 2,000 rpm then worry 
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indykid
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| posted on 30/1/07 at 08:33 PM |
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new smaller alternator pulley to match?
sorry if that's what you were thinking of machining.
tom
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David Jenkins
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| posted on 30/1/07 at 11:03 PM |
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quote:
Originally posted by indykid
new smaller alternator pulley to match?
sorry if that's what you were thinking of machining.
tom
Fair comment!
I was actually thinking of the crank pulley - but the alternator one would be easier (no keyway, no timing).
DJ
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paulf
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| posted on 31/1/07 at 06:01 PM |
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Its not actually to difficult to make a crank pulley, I made an alloy one for my crossflow, the centre is steel and I made the keyway by turning a
bung to fit the bore of the centre boss and then drilling a hole on the join line of the two parts , after removing the bung I was left with a half
round hole that I filed out square and to fit the key on the crankshaft.
I then marked out the position of the missing tooth on the pulley and shrunk on the ring gear in position to give the correct timing.
Theres some pics in my archives.
Paul.
[I was actually thinking of the crank pulley - but the alternator one would be easier (no keyway, no timing).
DJ
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David Jenkins
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| posted on 31/1/07 at 08:45 PM |
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I wouldn't need to file the keyway - I have an antique Cowells hand shaper that can make perfect grooves in small work.
I'd have to pull the handle a few dozen times, while twiddling the vertical feed between pulls, but the results can be very good - as long as
the initial setup is correct (that's the tricky bit  ).
Hmm - I know someone with a dividing head - maybe I could machine 36 - 1 teeth in one edge, for my Megajolt... but then I'd REALLY need to get
the keyway in the right place!
Maybe this could be a good "occasional project" to occupy me during boring evenings...
cheers,
David
[Edited on 31/1/07 by David Jenkins]
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