# B Torque of two screws at 180 degrees

#### Some new guy

Two screws set 180 from each other. They are being used to clamp something between them. The object is able to move freely before clamping but is not able to compress. Tighten one screw (a) until it touches the object. Torgue the other screw (b) to 45 in-lbs. What is the resulting torque to a?

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#### berkeman

Mentor
Zero.

That was too easy.

#### Riaan_123

If the object is held only between the screws the torque will be the same.

#### A.T.

Two screws set 180 from each other. They are being used to clamp something between them. The object is able to move freely before clamping but is not able to compress. Tighten one screw (a) until it touches the object. Torgue the other screw (b) to 45 in-lbs. What is the resulting torque to a?
If the object is static, the net torque on it is obviously zero.

Did you mean torsion?

#### jrmichler

The object is able to move freely before clamping but is not able to compress.
If the object is rigid, the screw tip cannot penetrate. If we assume that the screw tip is a point, it cannot apply a torque to the object.

If the screw tip was flat, it could apply a torque. If the screw tip penetrated the object, it could apply a torque. But that assumes friction between the screws and the object.

If the screw threads have friction, some or all of the screw torque is applied to the friction. If the screw threads have zero friction, the opposite screw will spin out as soon as a force is applied to the object.

This problem has multiple answers, with each different answer depending on assumptions that were not in the original problem statement.

#### Some new guy

I kind of wanted to see the overall consensus in a general way before I got too specific.
A friend of mine has been complaining about these mounts in his rifle (picture below). The heads of the screws do the actual clamping. He says he tightens one side to the required torque and when goes to tighten the other the screw becomes stripped as if there is far too much torque. However, screw (a) torqued to 45 in-lb does not result in screw (b) having the same amount of torque as the torque wrench does not indicate that high of torque.

#### Riaan_123

If the object is rigid, the screw tip cannot penetrate. If we assume that the screw tip is a point, it cannot apply a torque to the object.

If the screw tip was flat, it could apply a torque. If the screw tip penetrated the object, it could apply a torque. But that assumes friction between the screws and the object.

If the screw threads have friction, some or all of the screw torque is applied to the friction. If the screw threads have zero friction, the opposite screw will spin out as soon as a force is applied to the object.

This problem has multiple answers, with each different answer depending on assumptions that were not in the original problem statement.
Agreed too many undeclaered variants.

#### 256bits

Gold Member
I kind of wanted to see the overall consensus in a general way before I got too specific.
A friend of mine has been complaining about these mounts in his rifle (picture below). The heads of the screws do the actual clamping. He says he tightens one side to the required torque and when goes to tighten the other the screw becomes stripped as if there is far too much torque. However, screw (a) torqued to 45 in-lb does not result in screw (b) having the same amount of torque as the torque wrench does not indicate that high of torque.View attachment 247958
Are you holding the gun stock rigid and applying the torque wrt to that.
Or
Are you using two wrenches and applying the torque wrt tof one screw o the other screw.

#### Some new guy

Are you holding the gun stock rigid and applying the torque wrt to that.
Or
Are you using two wrenches and applying the torque wrt tof one screw o the other screw.
He just said “I do one side to proper torque but the other side strips when I try to torque it.”

Sounds as though he is using one wrench and doing one side at a time.

#### 256bits

Gold Member
He just said “I do one side to proper torque but the other side strips when I try to torque it.”

Sounds as though he is using one wrench and doing one side at a time.
That is the only scenario where I could see a difference in the results all other things being equal.

There is friction between the female/male threads which indicates part of the reading on the torque wrench.
The friction is what holds the screw in place after being torqued, as well as the friction between screw and the object being clamped.
The torqued screw will be under torsion, as well as compression.
The opposite screw is mainly under compression, with some threads near the clamped object under some torsion as it attempts to screw out depending upon the finesses of the threads. In fact it could be that only the few threads near the object part become active and have to take up all the stress.

It is kind of like when changing a tire on your car. One rotates the turning of the nuts from one to another mildly, skipping the a nut in the succession, and going around again, and again, rather than completely tightening one before moving on to the next.

#### Baluncore

He says he tightens one side to the required torque and when goes to tighten the other the screw becomes stripped as if there is far too much torque. However, screw (a) torqued to 45 in-lb does not result in screw (b) having the same amount of torque as the torque wrench does not indicate that high of torque.
There is confusion in the communications. Since the head is doing the clamping, and the thread strips before the second head is tight, there must be something unexpected happening.
What could that be? Maybe the screws are too long and meet in the middle before both heads can do their clamping. Measure the screw lengths and the hole depth available. Put a washer under the head of each screw, and see if that makes sufficient difference to fix the problem.

"Torque of two screws at 180 degrees"

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