Surface area and pressure relation to friction and wear

[Jynx18]

not sure if my title is the most accurate but here is what I am asking. I will give general dimensions and weights in case it makes any difference. **EDIT** This is a pnuematic gun if you couldn't tell and if it makes any difference.**

You have a hollow aluminum tube ~7/8" I.D. Inside the tube slides a plastic (delrin) bolt approximately 3" long. The bolt slides back and forth about 1". The bolt is lightweight at less than 1 oz. The bolt fits in the sleeve with minimal wiggle and can freely slide back and forth. The configuration is in the horizontal position.

So the question is would there be less wear on the aluminum sleeve if the bolt retains full surface area and contacts the breech at all points around it and for its whole length or would reducing the surface area to 2 to 3 small rings around the bolt at different points reduce wear. Intuition would say reduced surface area would reduce wear but wouldn't the reduced surface area mean increased pressure on the contact points leading to higher wear? Main concern is on wear of the aluminum tube but insight on wear of the bolt would be nice too.

Is there some sort of general rule on this or is it very specific to the problem? The reason for the concern is it is possible for the bolt to undergo 100,000+ cycles and minimizing wear on the aluminum tube would be a plus.

If you need any other info let me know. Thanks

 

[Q_Goest]

Hi Jynx, welcome to the board. There's no easy way of determining the rate of wear that I know of. However, most plastics or wear resistant materials have PV curves that give wear rate as a function of pressure and velocity. Those PV curves however, are not particularly accurate in my experience. They're strongly affected by the material they're rubbing up against, how much water or other 'lubricants' might be available, and even what kind of gasses are involved.

I design dry, reciprocating compression and pump equipment that requires dry running rider bands. From your description, I'd say the Delrin bolt will easily hold up for 100,000 cycles, and will probably last pretty much forever, regardless of what you do. From your description, there's no significant contact pressure between the Delrin and aluminum (the P in the above PV equation). The lower the contact pressure, the longer it will last. You can reduce P by increasing the contact area, so I'd suggest looking to do that. Just a smooth cylinder sliding inside a smooth bore (surface finish on the ID of the aluminum is key) will give you the best life. The one thing you don't mention is how the Delrin bolt is being stopped and how hard it's impacting the ends. That may be the one thing that hurts your design most of all. High impact loads may damage the bolt.

 

[Jynx18]

Thanks for the reply. I figured it would be pretty difficult to get an exact answer. My concern was not whether it would last as I know it would last hundreds of thoudsands of cycles (the application is not something groundbreaking or new). The thing is some people have come forward with modifications claiming that they reduce the surface area of the bolt to decrease wear. It seemed a little counter intuitive to me so i figured id ask. unfortunately there is probably not a guaranteed way to prove one way or the other. Also like you state there is so little contact pressure it really makes no difference.

As far as how the bolt is being moved i can give a little more detail. It has a bolt pin going through it that is connected to a pneumatic ram below. The ram is actuated by a solenoid that sends air in front of or behind the ram to move the bolt forward and backwards.