yield strength and hardness

[Kinsrow]

I have a plate that is mounted to an arbor press to press 2 pins into boss features on the enclosure. The pins are 1.5 inches apart. The pin is rounded at both end. The plate is hardened Rockwell C 55-60 A1 steel with thickness need to be determined

At this point I'm not sure how force it takes to press the pins in. I'm guessing it is in the magnitude of 100-200lbf. Supposed the pin reaction force is in the magnitude of 50 lbf, I can estimate the thickness needed to withstand bending and shear load. How do I make sure that the pin will not leave indentation on the plate. The pin is made out of stainless steel.

 

[JStephen]

You might check Roark's formulas for stress and strain for "contact stresses" and see if that's one of the cases covered.

If I remember right, a Brinell hardness tester uses a hardened ball, but the problem is that you want to make sure no indentation is left, and the hardness testing is only concerned with the size of the indentation, not whether there is one.

Consider putting a washer under/over the pin?

 

[CoryPad]

JStephen is sniffing in the right area.

Plane strain indentation (such as pushing a pin or Brinell ball) requires a localized stress approximately 3 times the plate's yield stress. So, the force would be that stress times the contact area of the pin(s).

Obviously testing would be a good thing to consider. Any differences between calculation and physical testing would be down to geometry (lack of infinite body, insufficient thickness to maintain only a localized subsurface stress) and material property differences.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Kinsrow]

Well,

Since the pin has a pointy tip (I'm not sure why the product people designed it that way...)I will have a very small contact area, which will yield to singular stress concentration and this produces very big stress ( assuming that the pin doesn't deformed first).

I decided to use W1 tool steel and make the plate thicker. Hopefully this will eliminate my worries.

Thanks for your input.

Kinsrow

 

[unclesyd]

How did the pin go from rounded to pointy?

 

[Kinsrow]

Unclesyd,

The pin has taper rouneded end. When I pressed this in, I will have a point contact. Sorry for the confusion.

Kinsrow

 

[CoryPad]

Even for point contact between a sphere and a flat surface, the plastic zone that develops requires a pressure that is ~ 3 times the yield stress of the material.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[handleman]

Cory,

How would one go about calculating the area for stress in this situation? Theoretically with point contact the stress is infinite, right? This is something I've always been curious about. Intuitively, stress can't be infinite, surfaces can't be perfectly round/flat, and some amount of elastic deformation is going to occur first to let the force spread out over some area. This is a bit further into MOM than I went in school.

Thanks!

 

[CoryPad]

You can use the actual contact area (2[π]r²).

You can read more about this in textbooks like Metal Forming: Mechanics and Metallurgy by Hosford and Caddell:

http://www.amazon.com/gp/product/01...103-3536243-6788626?s=books&v=glance&n=283155

or Materials Selection in Mechanical Design by Ashby:

http://www.amazon.com/gp/product/07...103-3536243-6788626?s=books&v=glance&n=283155

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Kinsrow]

In this case, I'm assuming that the pin will deform first. The pin is made out of stainless, so it is softer than the press plate. How much will it deform, not sure....

kinsrow.