How do I calculate this? 3

[Noobster101]

Hello guys, I hope everyone is okay. I am having a bit of a trouble with a mechanical problem, and I was hoping someone could point me in the right direction (I am kind of bad with physics but I am trying to prove something).

Background:

I am currently inserting a pin trough 2 objects to create an interference fit. At the moment it works, however the surface on object 1 is showing deformation of the plastic. My guess is that the large size of the pin is causing this. Therefore, I plan to reduce the size of the pin and the hole to a much smaller one, while maintaining the same interference between them. But my boss wants me to prove this using physics (which i am bad at).

Basically:

I have a 2.65mm OD pin/ 2mm ID and 25mm of length.

That pin is being used to join 2 objects together, trough a 2.6mm hole to create an interference fit. both object 1 and 2 are ABS and the pin is stainless steel.

I am interested in knowing the stress generated on the surface of object 1, when the pin is inserted into both holes. For the sake of this exercise, lets assume the force used to push the pin is 100N.

Currently after inserting the pin there is some deformation observed on the surface of object 1.

Once I know how to do this, I will do the same calculations with the smaller pin, compare the two and reach some conclusions

Here is an image of what I am trying to explain

https://imgur.com/a/Rsspvhf

. Thank you for your time

 

[NRP99]

Noobster101

It looks like standard problem of press fit. You can consider the deformation of 2.65mm(shaft)-2.6mm(hole)=0.05mm equal in both components and calculate the pressure and stress required to cause this deformation. But I doubt the magnitude of deformation since you have thermoplastic and steel. Thermoplastic will behave very softly than steel. So may be better you search "press fit calculation" on google.

 

[Noobster101]

Thanks for the response. Will the press fit calculations allow me to calculate the stress on the surface of the object?

 

[NRP99]

Yes. Search in Shigley's Mechanical engineering Design book-Limits and Fits. Or refer any other book on machine design that is available or try this link.

 

[Noobster101]

Thanks man. I am confused on the part where it says about the contact area. I am not sure about that one :s

 

[Pud]

Before you get too excited, an interference fit in plastic will soon end as a transitional fit.
Read up on creep in plastics. ABS is one of the poorer plastics for creep behaviour.


Politicians like to panic, they need activity. It is their substitute for achievement.

 

[Compositepro]

A small pin has less contact area than a large pin. Forces between the two objects will cause larger contact stress with a small pin than with a larger pin. Deformation of the plastic is far more likely with a small pin under most reasonable circumstances. Your boss is probably trying to make you come to this realization.

 

[Noobster101]

Hello Compositepro,

Thank you for the explanation. Is there any way I can prove this mathematically? I am a bit new to this subject :s

 

[GregLocock]

For a given quality of fit, in a general sense, a large pin in a large hole will be less problematic than a small pin in a small hole. Keywords for your google search are Lame's equation and Hertzian contact stress.

Cheers

Greg Locock


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