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Alternator Belt Tension Instructions Conflict

rockitdoc

rockitdoc

Well Known Member
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The Lycoming instructions are conflicting, I think.

If using the 'fish scale' technique and applying 14 lbs as described and getting 5/16" deflection, the torque required to rotate the alternator pulley is 30 in-lbs.

According to the 'torque' method shown, the torque required to rotate the pulley is 22 ft-lbs!

I tried to get the belt tight enough to achieve 22 ft-lbs and with the belt stretched as tight as I could get with a prying crowbar, I could only get 12 ft-lbs and the belt was tight as a drum.

Does anybody have an explanation for this apparent contradiction?

I sent Lycoming a message with the same question.

Alternator Belt Tension.jpg
 
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Walt said:
Use the 3/8 values.
Click to expand...

Also, the 3/8" values are still way too high (11 ft-lbs vs about 2 ft-lbs) if the 5/16" dimension is used with a 14 lb pull at the center of the belt.

At 11 ft-lbs the belt is stretched about as tight as it can be. The amount of slack at the center is negligible. Is this the objective?

At 5/16" and 14 lbs the belt has some slack; about 5/16", actually
 
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Slip Torque Method

I had an issue with what I thought was a tension issue with the belt. Turned out that was not the problem but did a lot of tests. I found the torque method the best and the Lycoming recommendation spot on. The deflection method was not very repeatable in a gauge R&R study. It's close though but subjective. Not sure why you are getting the values that you are seeing. I assume the torque wrench is in the correct setting and calibrated.
 
dmattmul said:
I had an issue with what I thought was a tension issue with the belt. Turned out that was not the problem but did a lot of tests. I found the torque method the best and the Lycoming recommendation spot on. The deflection method was not very repeatable in a gauge R&R study. It's close though but subjective. Not sure why you are getting the values that you are seeing. I assume the torque wrench is in the correct setting and calibrated.
Click to expand...

Accurate torque wrench.

My question: should belt be as tight as possible? That’s what you get when it takes to get 11ftlbs torque at the pulley without slipping.
 
rockitdoc said:
Accurate torque wrench.

My question: should belt be as tight as possible? That’s what you get when it takes to get 11ftlbs torque at the pulley without slipping.
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Absolutely not. High side loads on the bearings can lead to early failures. You really want the minimum tension where the belt never slips.
 
BobTurner said:
Absolutely not. High side loads on the bearings can lead to early failures. You really want the minimum tension where the belt never slips.
Click to expand...

Ok. That eliminates the 11 - 22 ftlbs specified by Lyc. At those values the belt is as tight as the strings on my guitar. I thought that was too tight and would lead to early bearing failure.

So, the question is still how to adjust the belt?
 
I torqued mine to lycoming specs. It was a new belt and it stretched to a reasonable tension after a few flight hours.
 
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The posted torque figures are for a standard pully on the alternator. If you installed a larger pully on the alternator, the torque chart will not apply.

Too much belt tension will wear out the bearings. I find it hard to believe that you double the tension on a belt that is 1.5 mm wider......:eek:

I run 100 in. lbs. on a 3/8" belt, and would do the same on an 11mm belt.
 
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Belt tightness

If you are using a one-off alternator and belt combination maybe the contact points and or pulley diameter is making a difference. I used a B&C alternator and went by their guidelines, and it worked fine with about the correct amount of deflection for the belt. I say about as it is a little difficult to know how much pressure to put on the belt to measure the deflection. This is from the B&C instructions:

This method worked (Torque method) perfect for a now used belt. The belt was not as tight as guitar strings.
 

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Called Lyc for Clarification

Jeff at Lycoming says tighten the belt until it has the 5/16" slack specified when pulled at 14 lbs between the flywheel and the alt pulley. That 'feels' right to me, anyway, based on beau coup years putting alternator belts on V8 Chevys, etc.
 
rockitdoc said:
Jeff at Lycoming says tighten the belt until it has the 5/16" slack specified when pulled at 14 lbs between the flywheel and the alt pulley. That 'feels' right to me, anyway, based on beau coup years putting alternator belts on V8 Chevys, etc.
Click to expand...

Not following unless you meant "deflection" versus slack. Would also assume measurement should be specified at the midpoint.
 
Freemasm said:
Not following unless you meant "deflection" versus slack. Would also assume measurement should be specified at the midpoint.
Click to expand...

The Lycoming Installation Manual says a 5/16" movement in the belt should occur midway between the pulleys when a 14 lb pull on the belt is applied at that location on the belt.

That's what I did in accordance with my conversation with Lycoming. Belt is taught, but not overly so, as in guitar strings.

My belt is what came in the 14A kit as is my Plane Power alternator.
 
Deflection method

That's the nice thing about experimental builders we are free to do what we want but the method that will protect the bearings and ensure no slippage is probably the torque method. Sometimes I wonder with the number of premature alternator failures if this is not a cause of premature failures in our alternators. I'm sure the heat under the cowling does not help. Also, the method calls out a new belt vs a used belt (over 50 hours) which seems to be close. As I stated before I had an issue with alternator output and spent a lot of time going over the proper tension and the torque method lines up well with the specified requirements. It was also close to the deflection method once I torqued it. I did start with the deflection method and by changing the deflection even an 1/16 of an inch at a time the torque numbers where all over the place. Possibly after you start flying you might want to try again.
 
Since the goal is not having the belt slip, it seems the torque method should work best. I put a larger pulley on my PP alternator, which therefore has a bit more belt contact area, and thus more friction.

It really does not require a lot of belt tightness like the 1970s cars that required a long pole to get leverage to pull the alternator tight enough to keep the belt from slipping. Of course, we were probably doing it wrong then, too.
 
Torque Method vs 'Stretch' Method

I like the 'stretch method' best. Reason?:
It does not depend on the 'slipperiness' of the pulley or the belt. I believe in my case the new pulley and belt caused the belt to slip on the new pulley at significantly lower torques than might have happened for less slippery components.

The 'stretch method' is not dependent on these variables. True; it is more difficult to accomplish accurately, but not impossible. Hopefully, my alternator bearings will thank me.
 
rockitdoc said:
I like the 'stretch method' best. Reason?:
It does not depend on the 'slipperiness' of the pulley or the belt. I believe in my case the new pulley and belt caused the belt to slip on the new pulley at significantly lower torques than might have happened for less slippery components.

The 'stretch method' is not dependent on these variables. True; it is more difficult to accomplish accurately, but not impossible. Hopefully, my alternator bearings will thank me.
Click to expand...

That's why the torque method is preferable in my opinion. Better to find out that the belt is going to slip when torquing it that some dark wet night. the deflection method doesn't account for belt slippage.
 
Jetmart said:
That's why the torque method is preferable in my opinion. Better to find out that the belt is going to slip when torquing it that some dark wet night. the deflection method doesn't account for belt slippage.
Click to expand...

True. The question then is, what torque is enough but not too much? For my setup, the 22lbs for the 11 mm belt makes the belt guitar string tight. Too tight, imho.
 
Something is wrong......

1. Are you using a beam torque wrench?..... you should be.

2. Are you turning the pully nut clockwise?...you should be.

3. Is the belt the same one that came from Van's with the PP alternator?
That alternator kit comes with a 3/8 belt...... https://store.vansaircraft.com/plane-power-60a-alternator-kit-es-alternator-deluxe.html

A simple way to measure belt deflection with a PP kit installed.
Place a scale against the bolt head and touching the belt at the
flywheel - belt intersection. Push up firmly with one hand and take a reading.
20221013_145607.jpg
 
gasman said:
Something is wrong......

1. Are you using a beam torque wrench?..... you should be.

2. Are you turning the pully nut clockwise?...you should be.

3. Is the belt the same one that came from Van's with the PP alternator?
That alternator kit comes with a 3/8 belt...... https://store.vansaircraft.com/plane-power-60a-alternator-kit-es-alternator-deluxe.html

A simple way to measure belt deflection with a PP kit installed.
Place a scale against the bolt head and touching the belt at the
flywheel - belt intersection. Push up firmly with one hand and take a reading.
View attachment 32188
Click to expand...

1. Are you using a beam torque wrench?..... you should be.

Not a beam, but this Snapon has served me well over the years.

Torque Wrench.jpg

Set at 22 lb-ft

Torque Reading.jpg

2. Are you turning the pully nut clockwise?...you should be.

Affirmative.

3. Is the belt the same one that came from Van's with the PP alternator?
That alternator kit comes with a 3/8 belt....

Yes, but mine measures 11 mm wide. 3/8" = 9.5 mm

The deflection in my belt was measured exactly as you indicated; 5/16"
 
For what it's worth I followed the same guidance (using a beam-type torque wrench on my B&C alternator) and like you the belt seems tremendously tight to me. I'm hoping it will loosen up with time...
 
mburch said:
For what it's worth I followed the same guidance (using a beam-type torque wrench on my B&C alternator) and like you the belt seems tremendously tight to me. I'm hoping it will loosen up with time...
Click to expand...

No doubt it will. Those bearings are prolly plenty tough.
 
I have the same Dayco 15355 top cog belt as in your photo. I have 5/16 deflection as measured in photo above. With a beam torque wrench it slips at 10 ft lbs. I think your problem is the torque wrench.... try a beam wrench.

With 5/16 deflection, your belt is not too tight.

The two belts that Plane Power lists for your application is a Gates 7355XL and it is 10mm. And Dayco 15355 and it is 11mm.
The extra 1 mm sits above the pully surface and does nothing for grip..... Do use the 5/16" deflection to set the tension and
expect the the slip to end up at 11 ft lbs after a few hours.
 
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