Difference in clamping force between bolts and screws

[chessguy103]

TL;DR

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

 

[Lnewqban]

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.

 

[jrmichler]

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.

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.

 

[Bystander]

..., and see https://www.omniamfg.com/mechanical...ate-fastener-minimum-thread-engagement-length

 

[Baluncore]

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.

 

[tech99]

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.