Belt tension preflight check?

[toto]

At my last annual, the $125 alternator belt was replaced and the belt was *very* tight when it was returned to service. The belt has gotten looser since then, and maybe gives a half inch, but still feels “normal” to my fingers on preflight. Is there a rule of thumb for belt tightness? I.e., without putting a tensioner or a torque wrench on it?

[PT20J]

It should be pretty tight. Spec is 5/16" deflection for 10 lb. applied force. I usually check it with a torque wrench at annual and just go by feel in between. SI 1129D Alternator Belt Tension, Methods of Checking.pdf

EDIT: FWIW, I haven't needed to adjust mine since we installed the new one with the rebuilt engine 500 hours ago.

Edited October 2 by PT20J

[M20F-1968]

I looked into this with Lycoming.  

Their specification is based on slippage of the belt measured with a torque wrench on the nut which holds the pulley onto the alternator.  The specification is different for a new v. a minimally used belt.  Belt tension is adjusted so that the belt slips at the appropriate torque setting.  I have the specifications written down somewhere.  Probably easier to call Lycoming or Continental and ask depending upon the engine you have.   

[N201MKTurbo]

The belts don't really stretch. The check is mostly to see if something has come loose. Every time mine has gotten loose, one of the mounting bolts was loose, or worn or something. In your case, I wouldn't be too concerned, but it would be worth pulling the cowl to check things out. I have seen the mounting bolts wear out, the bushings on the alternator mounting ears wear out, the ears on the mount wear out. Just take a look and make sure everything is nice and tight.

[N201MKTurbo]

BTW, at the last IA symposium, I specifically asked the FAA inspector about the legality of using the Gates Belt as opposed to the Lycoming belt. He said according to the latest interperation of the regs, it was legal to use the Gates belt. So you don't need the $125 belt, you can use the $20 belt. They are both the same BTW, You are paying Lycoming $105 to put a stamp on it.

[Skates97]

2 hours ago, N201MKTurbo said:

The belts don't really stretch. The check is mostly to see if something has come loose. Every time mine has gotten loose, one of the mounting bolts was loose, or worn or something. In your case, I wouldn't be too concerned, but it would be worth pulling the cowl to check things out. I have seen the mounting bolts wear out, the bushings on the alternator mounting ears wear out, the ears on the mount wear out. Just take a look and make sure everything is nice and tight.

I asked my CFI when I was doing primary training what I was looking for when checking the belt. He said "Check it every time and you'll know when it's not right."

Years later I was checking it on my Mooney and thought, "That's not good." It was significantly looser. I got out the flashlight and started looking around. The bolt on the bottom of the bracket had sheared off.

[toto]

34 minutes ago, Skates97 said:

I asked my CFI when I was doing primary training what I was looking for when checking the belt. He said "Check it every time and you'll know when it's not right."

Years later I was checking it on my Mooney and thought, "That's not good." It was significantly looser. I got out the flashlight and started looking around. The bolt on the bottom of the bracket had sheared off.

Well that's fair enough

I'll get my flashlight and start looking around..

[Hank]

I found a bolt in the bottom of my cowl once, and didn't fly until the A&P put it back. Now I pull on the alternator with my right hand and the belt with my left hand before every flight.

Both should be tight. I think the belt was tighter after it got replaced a few hundred hours ago; still pretty tight.

[bigmo]

I've always checked with the pad of my pinky finger - if I can get a little movement, it's good. I think most folks tend to put way to much pull on them. As @PT20J stated, 10# is not that much force. 

[Slick Nick]

Twist it. Anything below about 90* is a good rule of thumb.