[JimB]

Hi guys, when torqueing a AN3 bolt (25inch pounds) I've noticed it takes almost that to overcome the resistance of a locknut. So do i add another 25 inlbs when the nut bottoms out.

Thanks Jim

 

[apatti]

Jim,

I use a regular ratchet to get the nut started. When I get close to bottom I switch over to the torque wrench. You are right that 25 inch lbs doesn't feel like much! The first time I torqued an AN3 I was sure that couldn't be right. Also, make sure you have a washer under the nut you are torquing. I hope this helps.

 

[JimB]

Thanks Tony, I just wasn't sure how or if I should account for the drag from the locknut.

 

[alpinelakespilot2000]

In my experience, the run-on torgue for an AN3 locknut is only about 7-in# or less in most cases. If you're adding 25in# to the initial 25 in# on your torque wrench I'd have to wonder whether or not you are severely over-torquing them. If you're unsure, a dial or beam torque wrench is the only way to know for sure how much run-on you need to add to the standard 20-25 in# specs.

 

[az_gila]

Not the preferred method...

Jim,

I use a regular ratchet to get the nut started. When I get close to bottom I switch over to the torque wrench. You are right that 25 inch lbs doesn't feel like much! The first time I torqued an AN3 I was sure that couldn't be right. Also, make sure you have a washer under the nut you are torquing. I hope this helps.

AC 43.13-1b (aka "the Bible") is now quite clear on this.

d. Add the friction drag torque to the
desired torque. This is referred to as “final
torque,” which should register on the indicator
or setting for a snap-over type torque wrench.


A copy of the hardware section is on the FAA web site at this link.

The entire AC is here.

Earlier versions of the AC were not quite as clear on the subject.

As Steve said above, while I was writing, the task is much easier with a beam type torque wrench, like this Park TW-1 -

 

[Bob Brown]

The last thing that hasn't been said is calibration, I used a friends uncalibrated torque wrench against my calibrated one and there was two turns on the flat of the nut difference...

 

[az_gila]

Another plus for...

The last thing that hasn't been said is calibration, I used a friends uncalibrated torque wrench against my calibrated one and there was two turns on the flat of the nut difference...

..the beam type torque wrenches.

They are much harder to get out of calibration. You would essentially have to change the temper of the beam to cause a major change in calibration.

The "click" types have a lot more internal moving parts that have to work correctly to stay calibrated....

 

[DanB]

The last thing that hasn't been said is calibration, I used a friends uncalibrated torque wrench against my calibrated one and there was two turns on the flat of the nut difference...

So how does one determine if your torque wrench is calibrated? Can calibration be done on your own, or does it need to be sent back to the manufacturer? (speaking of a click over type)
Thanks

 

[jsharkey]

High quality torque wrenches come with a calibration certificate - a bit like a compass card. In industry they whould be rechecked every year by a quality lab or the manufacturer. Big companies have their own calibration labs with calibration equipment. Leave them set on a low torque so that the spring is not preloaded during storage - I believe that keeps them in calibration for longer. Also accuracy is best in the middle of the range for a particular size of wrench.

AN3s are probably best tightened by Torque Screw Drivers that can be set in very small increments. It's ever so easy to over torque AN3s - I know After some experience with broken bolts I experimented by measuring running torque and then adding it to the preload torque to get the proper feel - it doesn't take much for an AN3. Now I generally just snug them down by that "feel". They are rarely used in torque critical areas where clamping load is important. The important thing is not to over load them but have them snug enough to close the joint and not loosen.

1/4 inch and larger are easier to torque and for things like propellor bolts it's essential.

Jim Sharkey

 

[Phil]

Since the range for a AN3 is 20-25 lbs, I always torque them to 25 lbs. Then I figure the hardware will eventually relax back into the middle of the range.

Don't know if any of it is true or not, but that's what I'm doing to stay within the spec.

 

[Lycosaurus]

Pasta boxes

So how does one determine if your torque wrench is calibrated? Can calibration be done on your own, or does it need to be sent back to the manufacturer? (speaking of a click over type)
Thanks

My airplane build is referenced to packaged pasta boxes. (you can use other reference materials as well..... just weigh them to double check).

I place a socket horizontally in a vise The torque wrench is installed with the arm horizontal. I then dial up near the required torque on the torque wrench, and place a plastic bag with know weight on the handle, close to its end. Place just enough "calibrated" weight in the bag so you can make your torque wrench click. You can move the bag out until you are just able to make it click. Now compare the inch-pounds setting to the amount of weight (lbs) in the bag times the distance (inches) that the bag is.

Mine was off by about 3 in-lbs, so I placed a sticker on the wrench to add that amount on the dial. (e.g for 25 in-lbs, dial 28 in-lbs). You can do this for several popular settings.

(flame suit on)

 

[airguy]

I've calibrated my own torque wrenches like that many times - just be aware that it is usually not a "constant" offset - it will vary non-linearly with the amount of torque applied, so you should select several points on the torque range of the wrench and plot a correction curve.