Proper screw torques.

D

Dixie Gunsmithing

Moderator Emeritus
I received some good engineering info from K.O. Allen about the recommended screw torque to use on steel machine screws. This is hardly published anywhere, so below is everything up to 1/4"-20.

The below is for steel, higher grade, machine screws, and may not be correct for scope ring screw torque due to tube strength.

#0-80: 2.6 In. Lbs.
#1-64: 4.5 In. Lbs.
#1-72: 4.8 In. Lbs.
#2-56: 7.5 In. Lbs.
#2-64: 8.0 In. Lbs.
#3-48: 11.0 In. Lbs.
#3-56: 12.0 In. Lbs.
#4-40: 16.0 In. Lbs.
#4-48: 18.0 In. Lbs.
#5-40: 24.0 In. Lbs.
#5-44: 24.0 In. Lbs.
#6-32: 30.0 In. Lbs.
#6-40: 34.0 In. Lbs.
#8-32: 55.0 In. Lbs.
#8-36: 58.0 In. Lbs.
#10-24: 79.0 In. Lbs.
#10-32: 90.0 In. Lbs.
1/4-20: 200.0 In. Lbs.
1/4-28: 230.0 In. Lbs.
 
There are some qualifiers as I calculate how much to torque.
I drilled and tapped 10-32 into a Mosin Nagant receiver.
If the receiver were thick enough to get 1 to 1.2 times the root of engagement, I could do full torque.
The root [minor diameter of the male thread] of 10-32 is .159" ... so .159" to .191" for full strength = 5 to 6.1 threads.
So instead of the typical for 10-32, we can installation torque 1/2 the standard torque.
So if high grade bolts should get us 51 inch pounds lubed, then we should be able to get 25.5 inch pounds.

To test this:
I milled a piece of .15" 1020 steel down to .10" thick and drilled and tapped 4 holes with a #21 drill = .159" = typical screw minor diameter for 100%.


The point of yield of the female threads was:
a) 34 inch pounds clean and dry
b) 16 inch pounds waxed Johnson's Wax
c) 15 inch pounds waxed Johnson's Wax
d) 28 inch pounds Pennzoil axle grease

Which is what we would expect.
If the dry torque is 100% torque, then oil or grease increases the axial tension, so reduce to 75%. Wax reduces it further, so reduce to 50%.

What does it all mean?
We reduce torque if the female threads are not deep enough.
We reduce torque per the lubricant used.
 
Though the info I have doesn't show it, I think those ratings, which are the maximum torque, is for threads at two times the screw diameter. A minimum measurement is generally a thread engagement thickness that is equal to the screw diameter. When I sold for Huck Mfg. years ago, I think that is how they did their tests. I would have to dig out the training tape to watch it again, but if I recall, the maximum torques are for a thread engagement of two times the diameter. A thinner amount of thread engagement, of course, will not hold as much, and is also why you use fine threads on thin materials, instead of course. The chart I have, shows the clamping pressure, for each, at that dry torque, but I didn't include it. They stop it there, as the screw itself starts to yield and stretch, which is normal.

Another thing to remember, is that tap drills in most charts are for 75-80% thread engagement. They use this so the screw or bolt will thread in easily, as one close to 100% will turn tight as it goes in. The chart does not say what percent of threads was measured. I did check this chart with another from Holo-Krome, and it was pretty close. Also, keep in mind that most of these are grade eight fasteners, so a grade two would snap at that torque. This is also why the charts are not generally published, so use common sense, as Clark mentioned, in determining how much torque to apply.
 
^^"the screw itself starts to yield and stretch"

Has anyone measured thread stretch in any of these bolt torque tests? I know it's impossible to measure when threading in receiver to stock bolts, but for the sake of testing, I would be curious. In automotive applications thread stretch is more important than actual torque. When torqueing connecting rod bolts I use a thread stretch gauge.
Also, as mentioned, a dry or oily bolt will torque differently (or if there is contamination on the threads). Many manufacturers give a light torque spec (35ftlbs for example) then add +90 degrees. So you torque it lightly then twist it an additional 90degrees, thus setting the correct amount of thread stretch.
 
Sierra280,

I don't think I have any published info on thread stretch. With Huck and Cherry, they gave a certain torque on their threaded products, and the cut off point was where the tension was enough for the fastener to just start to yield, thus giving a maximum clamp pressure without going over the safe stress.

About about all fastener manufacturers are the same when it comes to testing. They put the fastener in a hydraulic tensioner, and watch the gauges for yield and failure. Then, they do a torque test on a load cell, and match the safe pressure from the tension test, and see how much torque it takes to actually get it there dry and or oily. Generally, they publish the torques as dry, as that is the way most are installed. The Huck video showed all the testing, as I was to show the video when calling on railroad and tractor/truck manufacturers. I would say that they have the stretch numbers recorded in house.
 
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