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How tight for the alternator belt?

kevinh

Well Known Member
Just curious - how tight should the alternator belt be? I've always heard press in on the belt with your thumb and it should deflect about 1/4". Sound about right?
 
kevinh said:
Just curious - how tight should the alternator belt be? I've always heard press in on the belt with your thumb and it should deflect about 1/4". Sound about right?
Yup. Sounds about right to me, depending on the span between the pulleys. On the typical lycoming alternator setup, 1/4" would be about right. Tight enough not to slip under load and not so tight as to cause undue stress on the belt or bearings. If it's too loose, you'll note glazing of the belt surfaces in relatively short order. Most auto parts stores sell a belt tensioner that will provide a more exact reference, but I think the 1/4" rule is a good one.
 
Torque the belt

Belt deflection is an unreliable method of tightening the belt.

Instead, use a torque wrench on the pulley belt and adjust the tension so that you get about 12 ft-lbs before the belt slips.

A loose belt can lead to under-voltage or over-voltage from the alternator. Overvoltage is caused by the regulator trying to force more output from the alternator while the belt is slipping. When the belt grabs, it puts a surge of voltage on the bus.

Please don't ask me how I found out!

ref Lycoming s. i. 1129A

Lycoming SI 1129A Accessory Drive Belt Tension

New 3/8? 11-13 ft/lbs 132-156 in/lbs used 3/8? 7-9 ft/lbs 84-108 in/lbs

New ?? 13-15 ft/lbs 156-180 in/lbs used ?? 9-11 ft/lbs 108-132in/lbs

Vern Little 9A
 
vlittle said:
...use a torque wrench on the pulley belt and adjust the tension so that you get about 12 ft-lbs before the belt slips.
I think you meant the alternator pulley nut, correct?
This is the procedure we were taught in A&P school, to comply with the Lyco S.I.
 
Alternator Belt Tension

Great info, Vern. I would have never found this in a million years. Thanks for posting it.
 
That does sound like a much more accurate method, but I'm not sure I get it. You put the torque wrench on the pully nut, hold the starter ring in place, and then adjust the tension until the belt slips with 12 ft-lbs of torque??
 
Davepar said:
You put the torque wrench on the pully nut, hold the starter ring in place, and then adjust the tension until the belt slips with 12 ft-lbs of torque??
Yep, that's the extent of it. Obviously, you turn the pulley nut in the tightening direction so you don't risk breaking it's torque.
 
Right-ee Tight-ee

jarhead said:
Yep, that's the extent of it. Obviously, you turn the pulley nut in the tightening direction so you don't risk breaking it's torque.
What direction is that. I think the alternator (some not sure) is opposite of "right-ee tight-ee".
 
gmcjetpilot said:
What direction is that. I think the alternator (some not sure) is opposite of "right-ee tight-ee".
When I rebuilt an alternator in A&P school, the pulley nut was right-hand threaded (righty-tighty, lefty-loosey). The alternator was off an O-235L2C from a Tomahawk, and we used Kelly Aerospace's overhaul manual. I think the TQ spec was 35-40ft/lbs, but I'm not positive...

I think the only alternators that have left-hand threaded shafts (lefty-tighty, righty-loosey) are automotive-sourced alternators from some cars with "serpentine" belts.
Maybe also a belt-driven alternator on a left-hand rotation engine (LTIO-series, for example), but I'm not at all sure about that - I haven't seen one of those in person.
 
Good and tight

jarhead said:
When I rebuilt an alternator in A&P school, the pulley nut was right-hand threaded (righty-tighty, lefty-loosey). The alternator was off an O-235L2C from a Tomahawk, and we used Kelly Aerospace's overhaul manual. I think the TQ spec was 35-40ft/lbs, but I'm not positive...
Thanks, I think my auto NipponDenso is normal thread direction also.

Another question, so with the torque method what is the "equivalent" side belt deflection. With cars you apply so many pounds force mid way and measure deflection. Using the torque method what kind of equivalent belt deflection do we get?

One reason for the torque method is the deflection method is hard to read, because the "free" belt length is so short on our planes. Also with the deflection method the amount of force needs to be known. So to do the belt deflection method properly, you need a "fish scale" measure the force to achieve the spec deflection. The torque method sounding better all the time.

With that said this makes the torque method even more cool, but it would be nice to get a read on what the equivalent belt deflection is for a given force, say 20 lbs after the torque method is used.

To be honest I have only installed plane belts with the "German mechanics" method..... "Gooo-tin tight!" :rolleyes: (but not too tight).

Checking installation for Plane-power and B&C belt alternators they give no belt adjustment info?

G
 
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gmcjetpilot said:
Another question, so with the torque method what is the "equivalent" side belt deflection. With cars you apply so many pounds force mid way and measure deflection. Using the torque method what kind of equivalent belt deflection do we get?
....

With that said this makes the torque method even more cool, but it would be nice to get a read on what the equivalent belt deflection is for a given force, say 20 lbs after the torque method is used.
I didn't measure with a ruler, but the good ol' Mk1 calibrated-eyeball saw quite a bit of "slack" in the belt before the TQ wrench clicked; much more than I expected. I imagine the amount of belt deflection would vary at 12ft/lbs TQ due to belt wear (stretches before slipping), pulley glazing (belt has no "traction" in the pulley grooves), temperature... I'm guessing that's one of the reasons Lycoming came up with the torque method; it's repeatable, and it doesn't matter (much) how much "slack" is in the belt - if the pulley doesn't slip until 12+ ft/lbs, there won't (well, shouldn't) be belt slippage in-flight.
 
Quest, nay thirst for knowledge

vlittle said:
ref Lycoming s. i. 1129A

Lycoming SI 1129A Accessory Drive Belt Tension
Vern Little 9A
I tried to get the Service instructions and found an Index on Lycoming's site,
but they only post a select few. I found index Ref. to SI-1129B, SB-536 and
SI-1254, but could not find a copy. I did find a Piper SL-636 which is the
same as Lycomings SB-355. It gives both methods of belt tension. This gives
A little more detail to Mr. Little's excellent post. Thanks for posting, nice
nugget of info. Another down side of the tension method is you need a Belt
Tension Meter to do it, where the torque method just needs the good old torque wrench.

(Notice below it gives info on new/used and 1 hour of use before checking. It the
torques match what Vern Little posted, so that is good. There is a SI-1129B.
I don't know how that differs from SI-1129A Vern referenced. )



1. SLIP TORQUE METHOD: This method consists of installing a
torque wrench on the pulley retaining nut and measuring the amount of
torque required to make the pulley slip. Turn the torque wrench in a
clockwise direction, as viewed from the pulley end, and adjust belt
tension accordingly.

SLIP TORQUE SLIP TORQUE
BELT WIDTH.......NEW BELT........USED BELT
.....3/8 Inch .....11 to 13 Ft.....Lbs. 7 to 9 Ft. Lbs.
.....1/2 Inch .....13 to 15 Ft.....Lbs. 9 to 11 Ft. Lbs.

If a new belt is being installed the slip torque should be checked to the
used belt specification after 1 hour operation, at 25 hours, and each
100 hours thereafter. See Aircraft Manual or call Aircraft
Manufacturer.

NOTE: The higher torque value for the new belts is to compensate for
the initial stretch of the belt that occurs as soon as it is operated. Do
not use the higher torque value for a belt that has been previously used.

2. BELT TENSION METHOD: This method consists of installing a
belt tension meter on the fan belt at mid point of the longest
unsupported section of the belt and adjusting the alternator to obtain
the specifications listed below.
The following specifications are for a 3 to 1 pulley ratio with a belt
wrap of 140? on the alternator pulley.

TYPE................OUTPUT..BELT LOAD
ALE,.................40 Amp....50 Lbs.
ALH, ALT, ALZ....50 Amp....75 Lbs.
ALY, ANG...........60 Amp....75 Lbs.
ALU, ALX...........70 Amp....75 Lbs.
The meter used for these specifications was a Borroughs Belt Tension
Meter Model #BT-33-73F.

The above specifications are for a used belt, or a new belt after 1 hour
operation.



I found this Nugget that references SI-1129A and explains how to use a spring (fish) scale:

Check Alternator Belt Tension

Service Instruction 1129A gives the methods:

1. Torque method for 3/8 " belt; 11 to 13 ft. lbs. torque at the nut that holds the pulley on the alternator for a new belt and 7-9 ft. lbs for a used belt.

2. Deflection method; attach a small spring scale to the belt 1/2 way between the ring gear and alternator pulley and pull 14 lbs for a new belt and 10 lbs for a used belt. The deflection should be 5/16". If it?s less than that, your belt it?s too tight.


Cheers George
 
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The rebuilt 60amp I have from Vans actually has a hex head on the shaft itself. Handy, except that it's metric. I think the alternator is the only metric thing on the whole plane.
 
Belt Slippage ??

Hopefully someone can offer some input to an old problem. I have now around 55 hours on a I/O 390 with a B&C alternator 60 amp. I started having issues around the 30 hour mark when my alternator output would go down to 7 or 8 amps and the voltage would start to drop from 14.5 to 13.5. This would start after about 30 to 40 mins of flying. On the decent down or possibly on the runway after landing the amps would pop back up to 50 to 55 amps. Thought easy enough belt slipping and when cooled contracts and starts working. Used the pull on the belt with X pounds and measure the deflection method and it measured loose so tightened it. Works fine for 15 hours, then same thing. This time tried the fix the prop from turning and measure the torque method which seems more accurate. It was a little loose so again tightened it. Again after about 15 hours same problem. I set the tightness right to the middle of the spec. Can't be this difficult to tighten an alternator belt. What am I missing?
 
Hopefully someone can offer some input to an old problem. I have now around 55 hours on a I/O 390 with a B&C alternator 60 amp. I started having issues around the 30 hour mark when my alternator output would go down to 7 or 8 amps and the voltage would start to drop from 14.5 to 13.5. This would start after about 30 to 40 mins of flying. On the decent down or possibly on the runway after landing the amps would pop back up to 50 to 55 amps. Thought easy enough belt slipping and when cooled contracts and starts working. Used the pull on the belt with X pounds and measure the deflection method and it measured loose so tightened it. Works fine for 15 hours, then same thing. This time tried the fix the prop from turning and measure the torque method which seems more accurate. It was a little loose so again tightened it. Again after about 15 hours same problem. I set the tightness right to the middle of the spec. Can't be this difficult to tighten an alternator belt. What am I missing?

I suspect you may not be tightening the bolts enough. It is critical that both the top and bottom bolts be torqued appropriately to hold the alternator. If they are good and tight, you may have a defective belt.

With a good belt and good torque on the bolts, you should get 100's of hours without the need to tighten, after the first 50 or so hours on the new belt, where some relaxation may be seen.

Larry
 
On a related note, what is a safe and effective way to apply tension to a pulley driven alternator belt on a 3xx Lycoming? Is there a specific tool for that?

Also, is there an reason to choose a smooth vs. ribbed belt?

George
 
Do you have the correct belt? Is it in the v properly? (Not riding high). Is the belt glazed from slipping? Are the pulleys aligned? Any cracks in the arm or bracket?

Go to the high end of the slip torque. Most “by eye” tensioning is WAY to high, so using the slip method should keep the bearings safe.
 
With a good belt and good torque on the bolts, you should get 100's of hours without the need to tighten, after the first 50 or so hours on the new belt, where some relaxation may be seen. Larry

If you go to re-tighten after only 50 hours do you use the "new or "used" belt torques?

George
 
If you go to re-tighten after only 50 hours do you use the "new or "used" belt torques?

George

George,
The Lycoming service instruction seems to say to use the used belt specifications if it has been run even once. "A belt is considered used if it has been installed on the engine and the engine has been operated."

Here is a link to the current version of the service instruction.

https://www.lycoming.com/sites/defa...ernator Belt Tension, Methods of Checking.pdf

-Ivan
 
Belt Slippage

Previous the bolts where not slipping as it is easy to see the "wear mark" the bolt makes in the adjustment bracket. The first time I moved the wear mark out about an 1/8 in and the second about a 1/16 in and both marks are still in the "moved" position. The top nut is still in the original position and both are safety wired. I'll check the belt for glazing this weekend. Thanks to all !!
 
Alternator belt p/n

Thought might be a good idea to have an extra belt around (In case the one installed was glazed or faulty) and after review of the parts manual list 3 different belt numbers for the I/O390-A3B6:
37A19773-376
BELT, Alternator drive, 3/8 wide
37A19773-390
BELT, Alternator, 3/8 wide, 39.000 long
37A19773-470
BELT, Alternator drive 3/8 wide

I understand no two Lycomings are the same but 3 different belt options? Any idea or help?
 
Previous the bolts where not slipping as it is easy to see the "wear mark" the bolt makes in the adjustment bracket. The first time I moved the wear mark out about an 1/8 in and the second about a 1/16 in and both marks are still in the "moved" position. The top nut is still in the original position and both are safety wired. I'll check the belt for glazing this weekend. Thanks to all !!

If the alternator is not moving in its bracket, next suspect is a defective belt. Two changes equalling 3/16" in a short amount of time seems excessive if you are sure it is the belt that is stretching and not the alt slipping in it's bracket. A witness mark with a sharpie is probably the best way to insure this.

Larry
 
Belt tension

Forgot to mention (and probably should have) it’s a Gates belt, proper width, length and cogged. 7460 - 11A0915.
 
Per Gates application engineers from my historical memory.

You should know that these belts shrink in length at elevated temperature, so be careful when you check them- use 70F ish room temps or being consistent is a good idea. Typically, this kind of belt has high power transfer efficiency. If it slips it will heat up and become self limiting (to a point), but will shorten the life.
 
Self Inflicted

It appears the issue was since I have a 2 alternator 2 battery system and an electrically dependent engine I decided to set the essential bus up as high as reasonable (14.3 volts) and the main at 14.4 volts. The panel builder (large commercial producer) has the alternator fields on the essential bus for both alternators. Since I was told by B&C that the "field" terminal and "since" terminal (3&6) could be jumped I did. That means when the essential bus alternator turned on it turned off the primary alternator. Setting the essential bus alternator down to 13.8 volts has fixed the issue. Now the questions. I was told that Earth X likes to be charged at 14.4 volts. If I set the essential bus to 13.8 volts that battery only gets charged to 13.8 volts. Is this ok for this type of chemistry? I think I could un-jumper 3&6 and and run a wire to a primary bus position and then the primary bus would get it's control voltage there? I am using a bus monitor and to date has worked well. Jumping 3&6 sure seems like an easier and very workable technique why wouldn't everyone wire it this way especially for a one bus one battery system? Any advise?
 
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