# Answer: Calculate Clamping Force to Prevent Bar Rotation

• gomerpyle
In summary, a force of 900 pounds is exerted on the flat bar approx. 19" away from the center of where it is bolted. A clamping force of 6,195 pounds is required to prevent the bar from rotating.
gomerpyle
Thread moved from the technical Engineering forums, so no Homework Template is shown
Say you have a piece of flat bar stock bolted to the end of a 3 x 3 x .25 HSS. A force of 900 pounds is exerted on the flat bar approx. 19" away from the center of where it is bolted. What clamping force would be needed to prevent the bar from rotating?

My attempt at this was to first calculate the applied moment around the bolt, so 19*900 = 17,100 in-lbs. Secondly, to assume that a force acts along each side of the HSS walls resisting the moment, and that the coefficient of static friction for steel-steel contact is 0.5.

The overall resisting frictional torque would be mu*N*d. If d is the same, and there are four walls of the tube, then it would be 4*mu*N*d.

Thus: N = 17,100/(4*0.5*1.38) = 6,195 pounds clamping. Is this methodology correct? Then I began thinking, what happens if instead of a hollow piece of structural tubing, the flat plate is bolted to another flat plate? Technically if you pick any point of contact between them, there are an infinitesimal amount of frictional forces with varying moment arms, so how would you approximate that?

What is an HSS? I have trouble envisioning a tube with four walls. Can you supply a diagram?

Hollow Structural Steel. I'm mostly trying to figure out how you calculate the resisting frictional torque? I can't find any information about this, only stuff about sliding friction or rotational friction (I.E., stopping a spinning disk).

Where is the frictional force acting here? Its probably not along the tube wall because the pressure of a bolt is not going to distribute out that far to make a difference, so I'm guessing its actually acting right around the surface that it bolts into, but how to calculate it? What is the moment arm?

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Any calculation would be so complex and in the end so meaningless that you may as well just guess an answer . Reality is that you will always have a potential pivot .

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The frictional force that resists the turning moment is due to tightening of nut ,

That doesn't seem like a very good design. Why aren't there two bolts spaced a bit apart to resist the torque?

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gomerpyle said:
Where is the frictional force acting here? Its probably not along the tube wall because the pressure of a bolt is not going to distribute out that far to make a difference, so I'm guessing its actually acting right around the surface that it bolts into, but how to calculate it? What is the moment arm?
It's not clear in the diagram what the rectangular box looks like with the bar removed. If it were hollow (i.e. open ended) then it wouldn't be too bad. But from your remarks above I'm assuming it ends in a flat plate.
That being so, it is extremely difficult to say what the distribution of the normal force across it will be. I suggest most of it would be concentrated in the area around the head of the bolt.

haruspex said:
I suggest most of it would be concentrated in the area around the head of the bolt.

Indeed - almost all under the bolt head and a very small distance around . Situation can be improved a little by modifying the geometry of the mating surfaces .

Better to use more bolts as Berkeman suggests (set at max practical radius from c/l) and/or use positive keying .

Better still though would be to do a basic rethink on the joint design and eliminate most of the the problem .

Putting two bolts apart or keyed joint is the best solution, in these cases the moment is resisted by shear force in the bolt or key

malemdk said:
Putting two bolts apart or keyed joint is the best solution, in these cases the moment is resisted by shear force in the bolt or key
Sure, but it is not clear from the original question whether those are options. Maybe the physical arrangement is a given and the requirement is to calculate on that basis.

## 1. How do I calculate the clamping force needed to prevent bar rotation?

To calculate the clamping force needed to prevent bar rotation, you will need to know the material properties of the bar, the length of the bar, and the desired degree of rotation prevention. From there, you can use the formula: Clamping Force = Moment / Distance from Clamping Point to Center of Rotation. This formula takes into account the torque applied to the bar and the distance from the point of rotation to the clamping point.

## 2. What is the equation for calculating clamping force?

The equation for calculating clamping force is Clamping Force = Moment / Distance from Clamping Point to Center of Rotation. This formula takes into account the torque applied to the bar and the distance from the point of rotation to the clamping point.

## 3. How does the material of the bar affect the clamping force needed?

The material of the bar directly affects the clamping force needed. Different materials have different properties, such as yield strength and modulus of elasticity, which will determine their resistance to rotation. A bar made of a stronger and stiffer material will require a higher clamping force to prevent rotation compared to a bar made of a weaker and more flexible material.

## 4. Is there a standard clamping force for preventing bar rotation?

There is no standard clamping force for preventing bar rotation as it depends on various factors such as the material properties of the bar, the length of the bar, and the desired degree of rotation prevention. It is important to calculate the clamping force specific to your application to ensure proper prevention of bar rotation.

## 5. What other factors should be considered when calculating clamping force?

In addition to material properties and length of the bar, other factors that should be considered when calculating clamping force include the surface condition of the bar (smoothness, roughness), the type of clamping mechanism being used, and the angle of the applied force. These factors can affect the friction between the bar and the clamping point, which ultimately affects the clamping force needed to prevent rotation.

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