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View Full Version : How tight for the alternator belt?


kevinh
04-17-2006, 09:18 PM
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?

mark manda
04-17-2006, 10:53 PM
3/4" for me. Max.

David Johnson
04-17-2006, 10:55 PM
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.

vlittle
04-17-2006, 11:24 PM
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

jarhead
04-18-2006, 12:06 AM
...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.

rv8ch
04-18-2006, 02:13 AM
Great info, Vern. I would have never found this in a million years. Thanks for posting it.

kevinh
04-18-2006, 09:35 AM
Great info Vern. Thanks!

Davepar
04-18-2006, 12:19 PM
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??

jarhead
04-19-2006, 12:42 AM
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.

gmcjetpilot
04-19-2006, 06:10 PM
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".

jarhead
04-19-2006, 09:21 PM
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.

gmcjetpilot
04-20-2006, 04:31 PM
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

jarhead
04-20-2006, 11:06 PM
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.

gmcjetpilot
04-21-2006, 12:47 AM
ref Lycoming s. i. 1129A

Lycoming SI 1129A Accessory Drive Belt Tension
Vern Little 9AI 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 its less than that, your belt its too tight.


Cheers George

Davepar
04-21-2006, 09:45 AM
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.