Maximum Contact Stress To Prevent Clamp Marks 1

[rockyhammer]

Hi,

I need to clamp AISI 1018 Steel, cold drawn, round bar between a pair of v-blocks, which will most likely be made from A2 tool steel. The round bar needs to be clamped while the end of the bar is chamfered and end faced. However, the round bar can not have any marks on it. I can calculate the hertzian contact stress between the round bar and the v-blocks. I need to minimize the length of the v-blocks. At what contact stress or shear stress will I begin to see clamp marks on the round bar?

Thanks

 

[Lcubed]

Keep Hertzian stress under 1.8 x Tensile Yield stress of the part.
Regards,
Lcubed

 

[rockyhammer]

According to Matweb for AISI 1018 Steel, cold drawn, the tensile yield strength is 53,700 psi and the ultimate tensile strength is 63,800 psi. So, if I keep my hertzian contact stress below 1.8 x 53,700 psi = 96,660 psi, I will not see any surface defects or plastic deformation from the v-block clamps?

 

[prohammy]

Rocky,

Take a look at this for Hertzian Stress calculate

http://www.tribology-abc.com/default.htm

Kevin Hammond

Mechanical Design Engineer
Derbyshire, UK

 

[deanc]

http://www.bendtooling.com/index.htm

This for tube,but you should find clamp info. here.

 

[jbel]

You are assuming that if you calculate the Hertzian stress and then back that out to a torque value for the clamp that the ham fisted operator will torque the clamp to your spec's.

Why not just add a soft copper shim between the round bar and the v-block? The copper will deflect and the round bar will look great. And it will compensate for any overtightening caused by the operator.

 

[BobM3]

Put a radius on the edge of the clamp and v-blocks so the edges don't dig in.

 

[Lcubed]

Rockyhammer,

That is correct. Reference Nadai and Wahl, PLASTICITY. Theoretically, permanent indentation occurs at 1.81 x Fty. Setting your maximum clamping pressure so as to hold maximum Hertzian stress to 1.8 x Fty should keep you out of trouble; less would be even better.

Regards,

Lcubed