Easy way of de-tensioning belt for alternator adjustment

[Breakin Away]

Here's my easy way of de-tensioning and re-tensioning the alternator belt for adjusting the alternator position (i.e., increasing the belt tension). It works great for my M35B with original 50 amp alternator. Not sure about other motors or other alternators:

Loosen bolts attaching pulley to coolant pump. Remove the one furthest away from the belt, if needed. The pulley pivots, fully de-tensioning the belt. Then you can loosen the alternator bolt and easily nudge it out 1/4"-1/2", and re-tighten everything to re-tension it. If the resulting tension is too much or too little, repeat and re-adjust. It's really quick and requires no special tools:

[pbyrne]

It's like you read my mind.  I was staring at the engine this weekend debating how to approach this very topic.

Much appreciated!

[Breakin Away]

Another trick: Take a 1x3 or 1x4 piece of wood (about a foot long) or similar. When you loosen the bolt on the alternator, the easiest way to move it is to put the end of the wood against it and gently tap with a hammer. then re-tighten the bolt before re-tensioning the belt.

I'm finding that a new belt loses its tension pretty quickly as it runs in. I've gone from 60-70 lb down to 40 lb a couple of times with my new belt. I'm still re-adjusting it to get it to stay at 60-70. Next time I have a new belt, I'd start at around 90 lb and watch to see if it drops to 60-70.

[pbyrne]

Another trick: Take a 1x3 or 1x4 piece of wood (about a foot long) or similar. When you loosen the bolt on the alternator, the easiest way to move it is to put the end of the wood against it and gently tap with a hammer. then re-tighten the bolt before re-tensioning the belt.

I'm finding that a new belt loses its tension pretty quickly as it runs in. I've gone from 60-70 lb down to 40 lb a couple of times with my new belt. I'm still re-adjusting it to get it to stay at 60-70. Next time I have a new belt, I'd start at around 90 lb and watch to see if it drops to 60-70.

Good idea!  How are you measuring the pressure?

[glennd3]

I always try to get the belt tension to the point that I can twist the belt 90 degrees.

Another trick: Take a 1x3 or 1x4 piece of wood (about a foot long) or similar. When you loosen the bolt on the alternator, the easiest way to move it is to put the end of the wood against it and gently tap with a hammer. then re-tighten the bolt before re-tensioning the belt.

I'm finding that a new belt loses its tension pretty quickly as it runs in. I've gone from 60-70 lb down to 40 lb a couple of times with my new belt. I'm still re-adjusting it to get it to stay at 60-70. Next time I have a new belt, I'd start at around 90 lb and watch to see if it drops to 60-70.

Good idea!  How are you measuring the pressure?

[KWKloeber]

a new belt loses its tension pretty quickly as it runs in.

Next time I have a new belt, I'd start at around 90 lb and watch to see if it drops to 60-70.

Covered here for 3/8" a belt:
https://c34.org/bbs/index.php/topic,11728.msg94969.html#msg94969

Install a NEW belt at 130 lbs. then run it in and re-tension

[Breakin Away]

a new belt loses its tension pretty quickly as it runs in.

Next time I have a new belt, I'd start at around 90 lb and watch to see if it drops to 60-70.

Covered here for 3/8" a belt:
https://c34.org/bbs/index.php/topic,11728.msg94969.html#msg94969

Install a NEW belt at 130 lbs. then run it in and re-tension

I installed my new belt in April. I saw your linked post yesterday. I also saw where you said, "Besides the max tension isn't what to use -- use the minimum tension that does the job," which is why I'll try 90 next time and hope it settles in around 60. If not I'll go higher.

[Breakin Away]

How are you measuring the pressure?

I move the alternator out 1/8"-1/4" at a time, then use the Krikit gauge after re-tensioning the belt. I use an ultra Sharpie to put marks on the alternator bracket to keep track of how far I've moved it.

[KWKloeber]

"Besides the max tension isn't what to use -- use the minimum tension that does the job," which is why I'll try 90 next time and hope it settles in around 60. If not I'll go higher.

You might be missing the point re: "Max tension" -- I was referring to the tension discussed before that the bearings could tolerate.  NOT the tension recommended by Gates. 
Just below cooking the bearings is not the tension on what Gates bases its operating recommendation (85# - 115#.)

If less than 60# works fine, even better.

Hoping is one approach to reach 60.  But why not just the mfgr's procedure? 

Stretch the belt (@130) and then set it for 60.  No guessing, no hoping, no "I'll try a higher (or lower) starting point the next time."

Kinda begs the question -- Why the heck don't they just pre-stretch their belts? 
Remember pre-shrunk clothing?

-kk

[Ron Hill]

Breakin : To loosen the belt the last thing that I'd ever do is to screw with the nuts on the water pump pully!?! 

I just loosen the adjustment on the alternator and pivot it!!

A thought

[Breakin Away]

Breakin : To loosen the belt the last thing that I'd ever do is to screw with the nuts on the water pump pully!?! 

I just loosen the adjustment on the alternator and pivot it!!

OK, so let's tension that new belt at 130 lb. In order to move the alternator I'll need 225 lb of force. [=2*130*cos(30°)]  I'll continue to take the extra minute it takes to first loosen the screws (not nuts) to release the tension.

[Breakin Away]

Kinda begs the question -- Why the heck don't they just pre-stretch their belts? 
Remember pre-shrunk clothing?

Perhaps you can ask them.

However, in my experience (which is admittedly more limited than yours) there is much more than stretching happening. Specifically, as the belt wears its cross-section changes. My old belts are considerably narrower than new ones that replace them. The v-shape of the pulleys causes this, and the belts are pulled further into the pulleys. This happens gradually over the lifetime of the belt, until they sink so far into the pulley that the alternator is close to the end of the bracket. Then it's time for a new belt. I measured my last belt and it had lost about 1/16" of width at the top (i.e., went from 3/8" wide to 5/16"). Holding the new belt next to the old one made it really obvious. I threw out my old belt, but next time I do a replacement I'll snap a picture and take some caliper measurements.

I previously thought that the rubber was abrading off the sides of the belt. But since there is little or no no black powder in my engine cabinet, it's possible that the cross-sectional shape might actually be changing (getting skinnier but also taller).

I'm sure there is stretch happening also, but the belt sinking further into the pulleys as it gets narrower is definitely a significant factor. Next time (in a few years) I'll take some more pictures and do some more measurements.

[Ron Hill]

Breakin : You are correct.  What happens to the belt is that it compresses in width and wears on the edges moving down in the pully "V" giving the impression that it is "stretching"

A thought

[KWKloeber]

Perhaps you can ask them.

We know the answer to that -- $$$ !!!

there is much more than stretching happening. Specifically, as the belt wears its cross-section changes.

Yes, wear over time will certainly change the X-section.  But that's different than belt installation/stretch that happens "immediately" during run-in. 
Wear and change in X-section is subsequent.

The first situation (run-in stretch) is install, then run-in, then verify/re-tension if needed.  For that, it's irrelevant why the tension reduces, it's just so easy to follow the recommendation.

The second (subsequent wear) situation is also simple to follow the recommendation (verify tension every 250 hrs.)
If initially installed/stretched at 130#, then subsequent operating stretch will be minimized.