Belt tension?

I was told by a alternator guy that the belt should be as tight as it takes to turn the alt and no tighter. The load from a very tight belt will put radial stress on the bearings and fail the alt prematurely. I have done it that way ever since and have had not problems. It makes sense.

I agree. When you can no longer "slip" that alternator fan by hand, it should be tight enough.

Allan

When you get the old belt off use paint thinner or spray brake cleaner and thoroughly clean both sheave grooves to remove any trace of grease, antifreeze, etc which will make even a new belt slip.

It doesn’t take much tension to for a good belt to grip – even if it’s pulling an alternator. About 1” to 1-1/2” of deflection with finger pressure applied midway between the sheaves is right. (Or if you can grab and turn the fan blade a bit by hand and the crankshaft follows, the belt's plenty tight.)

Don’t overtighten the belt! It only causes rapid wear on the fan hub bearings, and is hard on the belt too.

Alternators put a lot of tug on that belt. The belt has to be the exact same size as the pulley and tightern' a fiddlestring.

Even moreso, if it is mounted on the "off" side of the rotation.

Allan

Don't know why (not smart enough), but if the alternator is mounted on the left side of the engine and is driven directly from the crankshaft, the tension on the belt does not need to be as great.

Most guys mount these things on the off side of rotation (right side of the engine) and they are either driven via another pulley in a "chain" arrangement or worse yet, are driven by another pulley, which is itself is solely driven by the crankshaft. When they are mounted on the right, they gotta be tight.

Remember the old days?

We didn't hear much about slipping alternator belts when they were mounted on the left engine side. Only when they were moved to the right side, did this really start to be a problem.

Allan

But.....you are indeed correct about in the “old days” belt slippage was less of a problem. The reason for this however is alternator output capacity, not which side of the motor the the alternator happened to be positioned.

Explanation: Back in the early 60’s when they first appeared on cars and tractors, most alternators had a maximum output of 25 - 30 amps or so. This required about 1/2 HP to drive at full load – well within the capacity of a single 1/2” V-belt.

Fast forward a few years and you’ll find alternator capacities have increased. The 80’s era, internally regulated Delco alternators (the preferred unit for 12 volt conversions) are rated usually 55 or 61 amps. This alternator needs 1-1/4 HP or more to drive at full output – about the limit of a single V-belt. Therefore if the belt is not tight, or it’s oily or wet, it’s gonna want to slip under heavy load - like immediately after starting the engine.

Today cars are now routinely supplied with 90 to 120 amp (and larger) alternators. And these are universally driven by multigroove serpentine belts to deliver the 2-1/2 HP+ it takes to spin ‘em at full load. Further, unlike V belts, serpentine belts are “threaded” around a series of accessory, tensioner and idler pulleys at the front of the engine to achieve a 180+ deg belt wrap around each pulley in order to enhance the belt’s grip. And there’s always a spring loaded tensioner to keep the belt tight as is stretches.

Bottom line is with regard to belt slippage it doesn’t matter where the alternator is positioned on the engine – it’s the layout of the belt drive that counts! ...Bob

Naw, ain't goin' there. I've lived thru this one.

But, maybe I shouldn’t have used the terms "left side" and "right side" of the engine. I should have said 'taunt' side and 'slack' side of the belt.

All things being equal, what you say is true "if" and only “if” you have but one drive pulley and one driven pulley.

Add another pulley to the mix and all bets are off, 'cause it indeed does make a difference where that alternator (or any other load for that matter) is located in it's relationship to the drive pulley.

Any time a belt is used to transfer power, and during rotation, there is a "taunt" side of rotation and there is a "slack" side of rotation. One side is working and the other side isn’t; it is just “returning” to the task.

Picture two driven pulleys and one drive pulley; a triangle if you will.

Then, drive the alternator from the pull side of rotation and it will take less "tension" on that belt, for any given load, than it would if it were being driven from the slack side of rotation, because the "pull", the “rotation” or the "tension" of that belt is always trying to "tighten" on the pull side against it’s load.

Consequently, and because of the load on the taunt or primary side, it is also at the very same time, trying to "loosen" across the 'slack' or secondary pulley. The secondary load indeed will suffer from a "traction defect". See what I mean?

The only reason I even bring this up is because I was working on the line when GM changed their alternator mounting in the 1969 model year and this is when we began noticing this phenomenon.

Same belts, same pulleys, same alternators and same outputs. Everything identical except the darned location of the alternator in relationship to that crank pulley. Belts started to slip.

If an alternator is driven secondary to another load, the belt simply has to be tighter than it would have to be if that alternator were the primary load.

Allan

Incidentally I worked at GM (RPD engineering) at the same time as you! Bob

So you're the one! There's an army of us 'Grunts' out here that have been lookin' for you, Buddy! LOL!

Tell me you were gone before the advent of that headbolt popin', wristpin snappin' iron maiden, or the the integral 250 intake, the ever infamous 700-so called tranny, and who can ever forget what lovely times we had with that damned Quad Four.

Please tell me you didn't have your fingers in the design of those little sweethearts.? :>)

Allan

I worked at the Rochester carburetor plant and had I NOTHING to do whatsoever with product design! Spent my entire career there in facilities construction & maintenance. ...Bob