Changing alternator belts some time ago I found out that I could install the belt with Rodney Dickman's idler pulley in a little bit different way, (Yellow line):



It seemed much easier to install this way and although it does have a little less contact on the crank pulley it seems to work well.

Just curious if anyone else has tried this and what the longer term results have been...

[This message has been edited by randye (edited 12-16-2007).]

never mind - I thought that the idler was a spring loaded thingie

[This message has been edited by Phil (edited 12-16-2007).]

quote Originally posted by randye: Just curious ... what the longer term results have been...

The differences in pulley wrap are probably insignificant, but your belt routing will subject the idler pulley to higher loads, which will mean higher loads on the attach bracket, possibly faster pulley wear, and somewhat shorter bearing life. Only life cycle testing would determine whether those differences are significant or not.

Think about it. The original ("Rodney") routing places the idler pulley in the slack section of the belt loop. Of course, when the engine isn't running the tension in the belt is the same around the entire loop. Conversely, when the engine is running the highest belt tension is between the crank pulley and the water pump pulley, the next highest tension is between the water pump pulley and the alternator pulley, and the lowest tension is in the section between the alternator pulley and the crank pulley.

Given the simple cantilever design of the attach bracket for the "Rodney" idler assembly, I think it's best to stick with the original belt routing. Lower operating loads are better.

[This message has been edited by Marvin McInnis (edited 12-16-2007).]

I agree with Marvin. Every time you hit the gas pedal and/or induce a load on the alternator (like turning on the headlights or the A/C), you're going to be stressing the idler pulley. Personally, I'd rather see that stress get spread out between the water pump and crankshaft pulleys, instead of being concentrated on the idler pulley bearing.

[This message has been edited by Blacktree (edited 12-16-2007).]

quote Originally posted by Marvin McInnis: The differences in pulley wrap are probably insignificant, but your belt routing will subject the idler pulley to higher loads, which will mean higher loads on the attach bracket, possibly faster pulley wear, and somewhat shorter bearing life. Only life cycle testing would determine whether those differences are significant or not. Think about it. The original ("Rodney") routing places the idler pulley in the slack section of the belt loop. Of course, when the engine isn't running the tension in the belt is the same around the entire loop. Conversely, when the engine is running the highest belt tension is between the crank pulley and the water pump pulley, the next highest tension is between the water pump pulley and the alternator pulley, and the lowest tension is in the section between the alternator pulley and the crank pulley. Given the simple cantilever design of the attach bracket for the "Rodney" idler assembly, I think it's best to stick with the original belt routing. Lower operating loads are better.

Not sure I'm following you Marvin.
Seems like the shear loading angle on the pulley would run tangent to the center of the belt contact area, thru the centerline of the pivot axis, (green arrow, "stock Dickman set-up").

Movining the belt to the opposite side of the pulley, (shown in yellow), moves the load angle on the alternate routing as shown by the red arrow.


In both instances the load on the pulley bracket places only one of the fasteners in primarily tensile load.
I haven't thought much about the dynamic loads or differences on the static loads on the bracket yet.

In both cases the pulley turns the same direction, (but it shouldn't matter one way or the other to the bearing).

The only reason I mentioned this alternative way of routing the belt is that working from underneath the car, I found this routing to be easier to install for whatever reason.
I did send this to Rodney some time ago and he replied that he didn't think it would matter. I just was curious if anyone else had hit on this and what their experience had been.

Might need to think about this more, but for now it seems to be working well after 500+ miles.

Then again maybe I've got WAY too much time on my hands today

The rotation of the parts makes the load, or "pull" from the water pump to the alternator. Under load, it's getting tighter and tighter. With the pulley as you have it routed, the load will INCREASE on the pulley as the belt load increases. If you leave it where it was designed, it is always at the loosest point when the belt is underload. It may seem like an insignificant difference, but the load increas when the alternator starts pulling the engine down is anything but insignificant. I agree with Marvin and Blacktree, the pulley with your way of routing the belt is being loaded very heavily and it may or may not be able to withstand those loads.

John Stricker

Even after reading your last post, randye, with all of it's technical terms and pretty drawings, I still agree with Marvin. In the original routing, the idler pulley is the last component in the series, meaning it gets the least amount of load. In your modified routing, the idler is between the water pump and alternator pullies, so it's getting more of a tensioned load. Like Marvin said, there'd need to be some long-term testing to get any quantifiable results, but initially, it seems the original routing would provide the longest idler pulley bearing life. Your modified routing might be easier to install, though, so it's a tradeoff you'll have to analyze and pick which one to get what you want.

Kudos for being curious enough to come up with an alternative option! Not a lot of people take the initiative to experiment like that

------------------
1986 Fiero GT - Engine swap in progress
GM Master Tech, ASE Master Tech, ASE Advance Engine Specialist, Hendrick Master Tech

quote Originally posted by jstricker: The rotation of the parts makes the load, or "pull" from the water pump to the alternator. Under load, it's getting tighter and tighter. With the pulley as you have it routed, the load will INCREASE on the pulley as the belt load increases. If you leave it where it was designed, it is always at the loosest point when the belt is underload. It may seem like an insignificant difference, but the load increas when the alternator starts pulling the engine down is anything but insignificant. I agree with Marvin and Blacktree, the pulley with your way of routing the belt is being loaded very heavily and it may or may not be able to withstand those loads. John Stricker

OK..NOW I think I'm seeing it...
The primary dynamically loaded bearing in the "stock" set-up is the alternator bearing, and my alternative routing places the idler pulley as the primary bearing load.

quote Originally posted by randye: Not sure I'm following you Marvin. ... Movining the belt to the opposite side of the pulley, (shown in yellow), moves the load angle on the alternate routing as shown by the red arrow.

You are correct concerning the load angles on the idler pulley, but it's the load magnitudes that are different. The bearing load on any pulley can be determined from F = (Ta + Tb) * sin (alpha / 2)), where F is the bearing load, Ta and Tb are the belt tensions (static + dynamic) on each side of the pulley, and alpha is the total belt wrap angle around the pulley. It is important to note that Ta = Tb for an idler pulley, but they are unequal when a pulley is driving a load.





For the sake of simplicity, I'll use your picture and lets assume a "perfect" situation with low (~0) static belt tension, zero slippage on any pulley, and negligible belt stretch. When the engine is running, both the water pump and alternator present loads that require belt tension to turn them, and thus both present a dynamic tension load on the belt (i.e. Ta != Tb). Let T1 = the belt tension between the crankshaft pulley and the water pump pulley, T2 = the belt tension between the water pump pulley and the alternator pulley, and T3 = the belt tension between the alternator pulley and the crankshaft pulley. T1 has to be high enough to turn both the water pump and the alternator, while T2 only has to be high enough to turn just the alternator, and T3 will just equal static belt tension and doesn't have to carry any dynamic tension at all. Note that when the engine is stopped T1 = T2 = T3, and that when it's running T1 > T2 > T3.

With Rodney's belt path, all the idler sees is T3, which is just the static belt tension. But with your path, the idler sees T2, which the sum of the static belt tension plus the tension required to turn the alternator, which is always going to be greater than T3 alone. Q.E.D.

I hope this makes things a little clearer.

[This message has been edited by Marvin McInnis (edited 12-16-2007).]

quote Originally posted by Marvin McInnis: With Rodney's belt path, all the idler sees is T3, which is just the static belt tension. But with your path, the idler sees T2, which the sum of the static belt tension plus the tension required to turn the alternator, which is always going to be greater than T3 alone. Q.E.D. I hope this makes things a little clearer.

YUP

I didn't give much thought to the dynamic transients of the belt tensioning under load and no load conditions.
The question is then, (as pointed out), the duty life of the idler bearing under the increased load from my alternate belt routing.

"Note that when the engine is stopped T1 = T2 = T3, and that when it's running T1 > T2 > T3."

It appears that my alternate belt routing would actually decrease the bearing load on the alternator, possibly increasing it's lfe at the sacrafice of the idler bearing.
It would also lower the load on the alternator bracket... I think.

A longer test" of this may prove interesting, but I'd probably better order another idler pulley from Rodney as a back-up

Not having specs on the idler pulley, (bearing), Rodney uses it's impossible to know it's capacity right now

[This message has been edited by randye (edited 12-16-2007).]

quote Originally posted by Phil: never mind - I thought that the idler was a spring loaded thingie

No, that would be my bracket.

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