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Discussion Starter #1
I'm adding power steering to a 1953 F250 with a straight 6. It's already had an alternator conversion done, so I'm planning on moving the alternator up for clearance and installing a p/s pump below it. I should be able to mount the p/s in a fixed position as it'll be run by the same belt as the alt. The belt could then be tightened by adjusting the alternator only. This got me to thinking- is there a minimum amount or % of the pulley that the belt needs to cover? The belt will only be running on the crank, p/s, and alt. Hopefully this makes sense? The only other option would be to scrounge up a triple crank and double w/p pullies which I'm guessing are like hen's teeth.... Thoughts?
 

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Short version:

80 degrees on the smallest pulley, (360 / (number of pulleys) = 360 / 3 = 120 Smallest is 2/3 of average: 120 * 2/3 = 80

Long Version:

The formula has a lot of components, among which are the pitch diameter of the pulleys. This is not the OD of the pulley, and it is not the ID of the pulley. It is the diameter where the top of the tension layer of the V belt rides. Think about a MONSTER V groove in a pulley. The tension part of the V belt could be very low in the groove, or very high in the groove.

Also, the Center line Distance of the pulleys is another variable, as the closer a smaller pulley is to a larger pulley, the more the larger pulley "shadows" the smaller pulley, and lessens the pitch diameter. Vision a belt running between a wagon wheel and a golf ball, 2" apart. The belt will barely touch the golf ball because of the angle of incidence relative to the wagon wheel. Now move the wagon wheel 50 yards away. The belt will basically have 180 contact on each of the them.

Tension of the components: How much "Force" is "pushing" on the belt. The higher the force, the less coverage diameter is needed to prevent slippage, but this in turn puts a torque laterally on the spinning shaft bearings, ie, your water pump and alternator bearings wear out.

Another variable is more complex, and that is, simplifying it, the delta time from rest to steady state. What that means is, kinda, how fast is the engine acceleration. Think about holding a rope in your hands. If 3 Denver Bronco linemen SLOWLY pull on the rope, assuming you can keep your footing, it will be a very large force before the rope slips. Now, same 3 linemen "jerk" the rope as hard as they can. The force that it slips (and burns) your hands is much less due to the impact loading (the rapid acceleration), and that results in

Static versus dynamic friction. Short version, once it starts to slip, it takes very little force to keep it slipping, and a lot of friction to "stop" it from slipping.

Temperature of the belt (and pulleys) is another variable.

Conclusion:

Ensure the pitch angle and width of all the pulleys is the pulleys is the same. If one is 7/16" wide and another one is 9"16" wide, you will slip. Quality new belt (not heat hardened or glazed = slippery), smallest pulley as far away from the largest as possible, good but not extreme tension on the system, ensure the pulleys are aligned (no pulley is forward or backward compared to the others) and no 1/4 mile runs, which a 6 cyl in a F250 probably is not going to happen.

Hopefully, my explanation was not just mud.
 

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Discussion Starter #3
Whoa- that's a lot to take in, but it does make sense. If I'm catching it correctly, my best option would be to mount my p/s pump low and left of the crank, but have my alternator higher and slightly out from the pump. That way I'd have more grab on the smaller pulley- correct? All 4 pulleys (w/p, crank, alt, p/s) are the same belt width, and will be aligned. Picturing a square box with a pulley at each corner, the upper right corner (alt) being a smaller pulley will be slightly further out of the square. Hopefully that makes sense? Thanks for the info Bob!!
 
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