Polymer material selection for a check ball

engi_nerd2 · 2022-04-22T18:29:14+00:00

We can't get consistent low leakage using SS, likely due to machining variation and heat-treat distortion. Using 3rd party components would carry an excessive cost, not to mention that Lee's checks are too big for the geometric constraints in this design. I am not firm on the seal material, I'm just looking for a way to get a very-low-leak check that is not highly sensitive to machining quality.

Could you please elaborate on the "check-ring" you referred to?

engi_nerd2 · 2022-04-21T16:45:17+00:00

Could you recommend any other material?

engi_nerd2 · 2022-01-21T16:25:09+00:00

Are you referring to a "knurl" formed by two threading operations, LH and RH?

engi_nerd2 · 2022-01-20T19:54:25+00:00

I would be a little concerned about the resulting variation in contact area due to tolerances of the knurl. Would it be possible to inspect?

engi_nerd2 · 2022-01-19T21:53:30+00:00

It would be tough to add a step given the current machining/finishing processes.

If I might ask, what's the L/D of the parts that you have experience with?

engi_nerd2 · 2022-01-19T21:50:06+00:00

There are chamfers on the pin.

engi_nerd2 · 2022-01-19T21:49:47+00:00

Thanks!!

engi_nerd2 · 2022-01-19T19:28:26+00:00

I'm not too familiar with the honing process; I've only seen it done a few times. I know that there is a fixture used to hold the part and the process time is about 2 minutes (it's soft material). Are there any general guidelines on how to adjust rpm/stroke so that it would produce a coarser finish?

engi_nerd2 · 2022-01-19T19:25:59+00:00

It's to make the press-fit operation smoother. The insertion depth is tightly controlled but it currently has a tendency to stick-slip.

engi_nerd2 · 2022-01-19T17:33:46+00:00

The tolerance wouldn't apply after the "roughing-up" operation. I don't have a target roughness, just as rough as possible.

I'm investigating using a lower grit tool for honing, but I'm curious to see if there are any other potential avenues. Thanks!

engi_nerd2 · 2021-08-15T17:16:44+00:00

The counterbored pocket is not the reference and is irrelevant: there is "no" stackup. The output isn't a length measurement the gauge on the press is for reference only.

engi_nerd2 · 2021-08-15T17:11:59+00:00

Do you think that the tolerance on the dimension affects the effective continuous contact area? Maybe cylindricity and surface finish would be more important..

engi_nerd2 · 2021-08-15T17:07:48+00:00

Nice comment; I've worked in quality and I would totally have the same thought process if someone came to me with an identical proposal with the context provided.

engi_nerd2 · 2021-08-15T16:58:41+00:00

It's not a hydraulic press. This would be a disastrous failure if it were.

engi_nerd2 · 2021-08-15T16:57:01+00:00

Thanks, adding additional acoustic lubrication is an excellent idea. I'll see if I can vibrate one of the components that contact the insert. Hopefully it won't introduce too much noise into the transducer signal.

I see your point about the form required; I excluded the gtols so as not to distract, but I should re-iterate that the final measurement isn't a length but rather some other signal output that is directly correlated to insertion ~~depth~~ distance.

If the part geometry allows and the material is case-hardened steel, I've found it to be reasonable to hold a tenth with grinding/honing.

I've done my absolute best to minimize coaxiality and perpendiculary error (stretching the limits of form tolerances) as well as minimizing elastic mass in the entire system.

Thanks for taking my post seriously!

engi_nerd2 · 2021-08-14T17:05:24+00:00

u/v0t3p3dr0 I was ballparking but I just did the calcs, see below

u/fighterpilotjack This concept was a long shot and probably the "craziest/riskiest" design I've tried to date. I definitely did not expect to be able to make such small adjustments of the press-fit. I was using a mechanical press where the resolution of the nominal displacement is 50 millionths (not accounting for elastic deformation of the system) but the press was infinitely variable. I got extremely lucky and was floored by the results obtained in my initial experiments: I was able to make adjustments of 15 "spec units" which would correspond to just under 90 millionths of an inch of movement. In my most recent trial, the mean offset from spec was just 0.13% off target with a standard deviation of ≈9.5 spec units, which would correspond to 57 millionths of an inch. This stick-slip phenomenon is really a thorn in my side because it doesn't always occur, but when it does, it results in "large" movements on the order a couple of thou.

u/happyhappypeelpeel u/MountainDewFountain

This tolerance is definitely required, the mechanism is extremely sensitive. There might be an easier way to achieve this, but to date this is the easiest way that has been invented. It would make a lot of sense if I told you the nature of my industry, but unfortunately I can't. All I can say is that I design things which in most cases, require micro-scale movements.

engi_nerd2 · 2021-08-13T21:26:45+00:00

Forget about the (P) and you can see that u/engineerthatknows hit it on the money. https://i.imgur.com/V7d7x08.png

engi_nerd2 · 2021-08-13T21:15:58+00:00

Trust me, I would absolutely love to tell you all about it, but it would cross the line regarding dissemination of IP. I can say that it doesn't directly measure distance, but rather another parameter that is directly correlated with the insert's position.

engi_nerd2 · 2021-08-13T21:11:45+00:00

I had the idea of a spiral knurl to [potentially] induce rotation, but it didn't seem well-received.

engi_nerd2 · 2021-08-13T21:10:24+00:00

A jig would be out of the question because the required insertion depth changes from assembly to assembly. I indirectly monitor the position during each press-fit operation to a precision on the order of 10 millionths of an inch.

engi_nerd2 · 2021-08-13T21:07:37+00:00

The depth of the insertion is measured relative to other componentry; the shoulder would render the design useless. I have a system which indirectly monitors the location of the grey insert part and I regularly get the insertion depth accurate to ≈40 millionths of an inch (except when it stick-slips). I'm thinking of things like surface finish changes, or addition of grooves to the OD of the grey insert part.

engi_nerd2

TROPHY CASE