17-4(H900), Weck's Lube, and Actuation Mystery. 1

[Buckshott00]

Hello All,

I was wondering if you could help me figure out something. I have an assembly I am testing. It was using rivets before and now I am trying it with grooved pins. Everything except the pins are 17-4 H900.

I assemble my piece. It actuates smoothly but slowly. Through various stages of actuating it, it would sometime have a catch, or be smoother than glass, and then go back to slow.

I worked it enough times to see where pieces were rubbing when it catches. Everything in the assembly is supposed to be in line, but it was swinging slightly off center during travel and rubbing another face of the piece. So I dipped it in weck's lube and actuated a couple times. At first it was the same, then it went to being super smooth.

I WASHED the weck's off!!! First in water, then later in Isopropyl Alcohol, and dried it. Still smooth as glass. I suspect somehow the lube worked into the pins. However, I can't think of a way to check.

Also I tried something similar with a sub component and did not achieve the same result.

Thoughts and Suggestions are appreciated. Thanks!

 

[desertfox]

Hi

When there is relative movement between similar materials friction is usually higher than rubbing two different materials, if you think lubrication got to the pins, then all I can suggest is tripping the assembly down.
Are the components of the assembly meant to rotate relative to the pins?
If you can explain more in detail or provide a picture of the assembly you might get more meaningful answers.

 

[dhengr]

Wow... Would you please read your own OP, and while you can see it, the rest of us can’t see your ‘assembled piece,’ with lube or without. Are you talking about a dump truck body or a heart valve? You just won’t get many meaningful answers or comments if you can’t describe and explain your problem any better than you have. Sketches with dimensions, sizes, loads, showing how it works, etc. truly are worth a thousand words.

 

[tbuelna]

If there is no significant external loading applied to the linkage when you cycle it then any friction or sticking is due to interference within the joints. This sticking/resistance in the joints likely has nothing to do with the hardness of the joint component materials. However, if there is some prevailing friction in the joints due to misalignment or interference, then having a lubricant present at the joint sliding interface would reduce the prevailing static or sliding friction.

Lastly, it is usually a bad idea to use rivets as pivot shafts. When a rivet is "bucked" at installation it usually causes the rivet body to expand in diameter. And this could result in loss of clearance between the rivet body and its mating bore.

 

[Buckshott00]

Thanks tbuelna

You're spot on!

I was able to look at the pins under increased magnification, and noticed some odd things. I talked with the manufacture and he confirmed that sometimes grooved pins especially ones very small (such as in this application) can become "impregnated grooves" where the massaged material lobes out.

It is essentially creating an out of round or a lobing condition resulting in extra friction / poor fit (interference) between the shaft and its mate.

This is part of the reason I am implementing switch from rivets to a grooved pin, provided that I could make a grooved pin work in the size range of the application. They would end up prematurely destroying the final product due to fatigue from lapping the item in excess to deal with the friction caused by the rivet.

Do you have any tips on how to treat the pins for even insertion? Currently, I'm just using a pin vise and starting it, and then a small press to set it. I think my next step is to try a heavier lube with the mating parts before actuating later.

Thanks!!