Thread: Alternator RPM?

Well the a/c pulley you're driving off is 13A and most alternators are 11mm groove. If not, swap the pulley or open it up on a lathe. Done that before today.

Yes, a lathe . . . I've been after one for about 15 years! But I don't have one yet. This pulley does use a 10A so does the extra 1mm matter that much? I've also got a few pressed steel split pulleys, but they are a bit hard to set up to stop wobbling.

I had a second alternator on my 1988 Rangie for about 10 years and then transferred it to the replacement 1990 Rangie where it has happily run for the last 2 years. Very important to get the mount square so the belt runs true, this gave me a bit of trouble til I got it sorted. Mine is mounted right where yours is but the a/c compressor is different and mine is mounted to the top of it. If you use a slotted support leg you will have 2 modes of adjustment, might help with getting a belt to fit. I run the second alt across to the aux battery, no problem at all and much simpler than rewiring the existing charging circuit. Using an alt with a built-in IC reg simplifies things too.
8,000rpm sounds like a lot to those of us used to piston engines where a piston has to accelerate up a bore, stop at the top and accelerate back down for every turn of the crank; something like an alternator with purely rotary motion has no such issues. Also, given the diameter of the rotor in the alternator, surface speed will still be less than many of the other components attached to your engine, e.g. flywheel & clutch.

The primary concern for a vee belt is that the corded layer of the belt is in the vee of the pulley. The top layer is there for extra friction but you may indeed get away with an 11mm belt because the angle of wrap is good. Try an 11mm belt and see how it goes. Keep a spare belt handy, change the pulley if required later.

What I did was just measure the Crank to Aircon belt plus alternator and got a longer belt that covers all three.
Adjustment is by the original tensioner pulley.
It's seems pretty easy to get incrementally longer belts for long belts if you get the drift.
Mine is on a 92 with Sanden Aircon but I think yours is the same in layout.
Regards Philip A

An automotive alternator should be run at 2 to 3 times the crankshaft speed. 10,000 rpm is not a problem, but they tend to "flatten out" the the output curve above 3k to 4k rpm. So depending a bit on the use of the car, and the the engine type, usual cruising speed and therefore rpm, it may be better to go a bit higher alternator rpm so that you achieve a good output at low engine rpm.

Okay, that all makes sense. 7000rpm is fine - you learn something new every day - and I see why you don't fool around too much with belt widths. I might at least be able to send a pulley away to be machined. (After a couple of decades of this stuff, I really have reached the limits of what you can do without a lathe, though.) Rewiring shouldn't be too hard since it's a carby engine and there's not much back there. It's pretty oily, too.

With the York compressor, there seems to be a lot less room and the alternator is pretty much in the only spot it will fit. Philip, I've got a photo of yours that you posted somewhere, but what belt width did you use? It looks wider, the way it fits on the aircon pulley. I also couldn't figure out how to calculate the pulley speeds once there were more than two pulleys.

Originally Posted by Davo

I also couldn't figure out how to calculate the pulley speeds once there were more than two pulleys.

There is the drive pulley (the crankshaft) and the driven pulley (either the a/c OR the alternator) No difference to a 2 pulley problem, calculate for each pair of pulleys. The belt linear speed is related only to crankshaft speed and size.