# Difference in clamping force between bolts and screws

• chessguy103
In summary,-For screws, the clamping force is greater than the tension generated by a bolt due to the shank's greater torque.-A threaded hole is necessary in certain cases due to access restrictions, in which case a threaded stud can be tensioned by a nut.-For bolts, the tension generated by the turning of the nut is greater than the clamping force of a screw, but there is a compromise in that a threaded hole is needed.
chessguy103
TL;DR Summary
Is there a difference between clamping force calculation for screws (threaded holes) vs bolts (clearance holes + nut)?
Hi everyone,

I’m trying to figure out the clamping force of a set of screws that thread into a plate.

My question is:
• Would this calculation differ from calculating the clamping force of a bolt with a nut?
• What would be the difference in how the fasteners/members would react to a tensile load?
I was reading through Shigley’s mechanical engineering design chapter 8, but the chapter really focuses on bolts and not screws. Any guidance will be appreciated.

I’ve attached a picture of the basic design I’m working with.

Thanks

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For similar materials, clamping force should be similar.
It is important that the threaded hole is perpendicular to the clamping surface of the plate and deep enough to accommodate the bolt with plenty clearance at the bottom.

chessguy103
chessguy103 said:
Would this calculation differ from calculating the clamping force of a bolt with a nut?
When reading the bolt calculation chapter, look carefully at the assumptions. To oversimplify:

Something is turning against something else that is stationary.
There is friction between the turning part and the stationary part.
That friction is in the threads.
That friction is also under the bolt/screw head or nut, depending on which is turning.

If you look at it that way, the calculation for a bolt with nut is identical to the calculation for a screw into a tapped hole.

Lnewqban said:
It is important that the threaded hole is ... and deep enough to accommodate the bolt with plenty clearance at the bottom.
This is important. I once had a startup failure because of exactly this.

Lnewqban and chessguy103
Lnewqban
chessguy103 said:
Summary: Is there a difference between clamping force calculation for screws (threaded holes) vs bolts (clearance holes + nut)?

I’m trying to figure out the clamping force of a set of screws that thread into a plate.
A screw is defined by the "turning of the head to increase the tension". Bolt tension is increased by turning the nut. The bolt clamping force is greater than the screw because the shank of a screw is subjected to a higher torque while being tightened than is a bolt.

There is a compromise. If a threaded hole is needed due to access restriction, then a threaded stud in that hole can be tensioned by a nut, giving the greater tension of a bolt.

Lnewqban
My understanding of jonting methods is that we can either rely on friction grip bolting, where torque is very high and shank tension is somehow measured, or we assume a pin joint, where less tension is used and we provide sufficient shear strength from the bolts. I the latter case, friction plays a part but we assume pin jointing for design purposes. An advantage of this method is that critical measurements during bolt tightening are avoided, and a course thread can be used, which allows for galvanising if required. I notice for the present case that for the threaded plate, the bolt shank is subject to torque during tightening, whereas with a nut being rotated on a bolt it is not. I also notice that a nut can be provided more easily with a locking device.

## 1. What is the difference between clamping force in bolts and screws?

The main difference between clamping force in bolts and screws is the way they are tightened. Bolts are typically tightened with a wrench or socket, while screws are tightened with a screwdriver. This difference in tightening method results in a difference in the amount of force that can be applied, with bolts typically having a higher clamping force than screws.

## 2. Why do bolts have a higher clamping force than screws?

The higher clamping force in bolts is due to their design. Bolts have a larger surface area where the force is applied, resulting in a greater amount of force being distributed over a larger area. In addition, bolts have a thicker and stronger shaft compared to screws, allowing them to withstand higher levels of torque.

## 3. Can screws be used in place of bolts for applications that require high clamping force?

In some cases, screws can be used in place of bolts for applications that require high clamping force. However, it is important to consider the type of screw being used, as some screws may not be able to withstand the same amount of force as bolts. It is always best to consult with a professional or refer to manufacturer recommendations before making any substitutions.

## 4. Are there any advantages of using screws over bolts for clamping force?

One advantage of using screws over bolts for clamping force is their versatility. Screws come in a variety of sizes and types, making them suitable for a wide range of applications. They are also easier to install and can be tightened with a simple screwdriver, whereas bolts require additional tools and equipment.

## 5. How can I determine the appropriate clamping force for my application?

The appropriate clamping force for an application will depend on various factors, such as the materials being clamped, the level of stress the joint will experience, and the type of fastener being used. It is best to consult with a professional engineer or refer to industry standards and guidelines to determine the appropriate clamping force for your specific application.

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