How tight for the alternator belt?

[jarhead]

Originally Posted by gmcjetpilot

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]

Originally Posted by jarhead

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!" (but not too tight).

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

G

[jarhead]

Originally Posted by gmcjetpilot

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]

Originally Posted by vlittle

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

[Davepar]

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.