Do you torque screws?

jcarne · #1 05-31-2017, 08:46 PM

After searching the forums with little found I was wondering if you all torque smaller screws? The reason I am asking this question is that I am installing the four screws that attach the elevator trim motor to the z brackets with 4-40 screws that have a nut on the other end. Our manual doesn't show torque values for anything less that #10. I know the torque for a 4-40 should be way down there (probably barely noticeable). What say you?

tgmillso · #2 05-31-2017, 08:58 PM

I guess it depends on whether you are a guru, geek or otherwise. The pros amongs is probably wont because they are doing this all day and have a feel for it. Given that it's my first rodeo, I do. I have a CDI 401SM torque screwdriver and I use this almost all the time because you can torque anything from a 4-40 screw all the way up to AN3. For things like inspection panels I wouldn't bother, but if it's anything structural, I do. The tanks are an example where I run the screws in with the electric drill on a low clutch setting, they quickly run around and finish them with the CDI torque screwdriver. IIRC the torque value for an 4-40 is about 4 in lb, not including nyloc drag, but you will want to check this. You are right that you have to dig around a bit to find published values as menu charts don't go below #10, but they are out there.
Tom.

terrye · #3 05-31-2017, 08:58 PM

Unless they are a flight critical application (and they shouldn't be), no, I don't torque screws, especially #10, #8, #6 and #4. One of the problems with these small sizes, is that the locknut, or locking nutplate, has a substantial proportion of the torque required to break the screw, so you have to be very careful, first to establish the running torque in the nut or nutplate, then the torque value if you can find one. If you can't find a torque value, and really think you should have one, you can tighten a test screw to breaking torque and then install to 80% of this torque.

jcarne · #4 05-31-2017, 09:07 PM

Very interesting responses so far guys.

Tom kudos to you for finding that torque value as I couldn't find anything from FAA or milspec.

tgmillso · #5 05-31-2017, 11:10 PM

Hi Jereme. I just happened to be torquing a screw on a receiver rail last week of this dimension, so I had it fresh off the top of my head. Keep in mind that this was a lubricated thread, so a dry thread should be about 30% extra torque.
I too have really struggled to find legitimate sources for torque specs. When I can't find something applicable in aircraft specs I will fall back on SAE values, however even these don't seem to be published down to the values we use. Thus what I have been doing it using torque values provided by screw manufacturers and suppliers, then comparing material types and tensile strengths against what are published for AN507/AN509 screws. Below is the best chart I have found to date. Note the dry thread/wet thread difference that you need to apply, however in most situations for us we are applying dry threads anyway. As an additional note, I'm a huge fan of that torque screwdriver I have. It is often more convenient to use than the torque wrench on AN3 bolts due to the lack of a handle at right angles. Definitely one of my most used tools.
Good luck.
Tom.
http://www.federalscrewproducts.com/torque-chart.htm

BillL · #6 06-01-2017, 05:52 AM

Timely post. I just finished some end-plates for my HS and elevators. I used standoffs and screws to attach. #6 - I wondered what they would tolerate for torque. Using a stainless stud with aluminum standoff in a nut plate I torqued it to 50 in-lb before it plateaued! that was only 1/4" of the stud in the aluminum.

Thanks for that chart, it is helpful. It looks like I overtorqued the screws at 15 in-lb. But, hopefully, they will never come loose. It did appear that after about 7 in-lb the screw did not turn much, less than 10-15 degrees. Maybe I have more testing to do.

jcarne · #7 06-01-2017, 06:45 AM

Quote:

Originally Posted by tgmillso

Hi Jereme. I just happened to be torquing a screw on a receiver rail last week of this dimension, so I had it fresh off the top of my head. Keep in mind that this was a lubricated thread, so a dry thread should be about 30% extra torque.
I too have really struggled to find legitimate sources for torque specs. When I can't find something applicable in aircraft specs I will fall back on SAE values, however even these don't seem to be published down to the values we use. Thus what I have been doing it using torque values provided by screw manufacturers and suppliers, then comparing material types and tensile strengths against what are published for AN507/AN509 screws. Below is the best chart I have found to date. Note the dry thread/wet thread difference that you need to apply, however in most situations for us we are applying dry threads anyway. As an additional note, I'm a huge fan of that torque screwdriver I have. It is often more convenient to use than the torque wrench on AN3 bolts due to the lack of a handle at right angles. Definitely one of my most used tools.
Good luck.
Tom.
http://www.federalscrewproducts.com/torque-chart.htm

Tom thanks for that chart, I have it bookmarked now.

jcarne · #8 06-01-2017, 06:46 AM

Quote:

Originally Posted by BillL

Timely post. I just finished some end-plates for my HS and elevators. I used standoffs and screws to attach. #6 - I wondered what they would tolerate for torque. Using a stainless stud with aluminum standoff in a nut plate I torqued it to 50 in-lb before it plateaued! that was only 1/4" of the stud in the aluminum.

Thanks for that chart, it is helpful. It looks like I overtorqued the screws at 15 in-lb. But, hopefully, they will never come loose. It did appear that after about 7 in-lb the screw did not turn much, less than 10-15 degrees. Maybe I have more testing to do.

Bill, very interesting test, thanks for posting. I would have thought for a #6 it would have plateaued at a lower torque.

JonJay · #9 06-01-2017, 07:14 AM

Good chart and some very good comments about running torque. Note it states this is a guide and each should be evaluated per application, which is true of any fastener. If you are going to use a tool to set torque on tiny screws and bolts, make sure it is properly used. I see lots of "I snapped off the screw(bolt) with the torque wrench" posts than I do "the screw fell out", which I rarely see.
For any small critical fasteners, like the servo application, switch terminals, etc... I use a thread locker if they are not being screwed into a locking device like a nut plate or nyloc.
Keep those tests coming Bill. Your engineering prowess is only matched by your natural curiosity and inclination to figure things out and share it with us less worthy!

rzbill · #10 06-01-2017, 08:39 AM

Nice thread with good info.

On a slightly different aspect, The only "screws" that I have found in my 600 hrs of maintenance that needed some assistance were the coarse threaded Lycoming screws in various places on the engine. I had one baffle screw come out early in the crafts life and I subsequently re-torqued all of them to the Lycoming spec (SP-1776 I think) for 1/4-20s.

After doing that once or twice with a torqueing tool, it became easy to do it by hand occasionally with a plain screwdriver when the cowl was off. Have not had a screw come out since then.