Vibration of a Retaining Ring / E-Clip 2

[cbk14]

I am working a design that has jack screws - basically a grooved screw with an E-clip retaining ring for captive purposes. The design will undergo pyro shock and random vibe, so there is some worry that the E-clip will vibrate out of the groove on the screw.

Has anyone experienced E-clips coming out of their grooves due to shock or vibe?

I believe a circlip / external snap ring might stay in place better, but that is only an intuition. Also the screw that is retained is quite small (#4 = 0.112 in major diameter) and I haven't found any circlip style retaining rings down to that size.

Thoughts?

 

[hendersdc]

I have no experience with small E clips in shock and vibe but from the description of your application it sounds like you will be pushing against the E clip to adjust spacing of something. It makes me wonder if something like these adjustment screws from Misumi might work out for you. They come is sizes down to M3 which is almost a #4.

Doug

 

[Jboggs]

So, here I go again: I see no evidence that you have discussed this with the people that design, test, manufacture, and sell retaining rings. Why not? There are several, and they all have done tests to determine exactly the information for which you are looking. No offense to anyone here, but I doubt if you can learn anything on this forum of greater value than coming from direct contact with the manufacturers of the products you're using.

 

[cbk14]

Jboggs, good thinking. I always forget to just call the vendor directly. Only problem is the vendor is McMaster (for ease of obtaining 3D models) and I don't think they design/test/manufacture.

 

[mcgyvr]

McMaster.. If you ask nicely.. Will tell you the manufacturer of the parts you buy if not evident from packaging,etc...

 

[cbk14]

So I did call up McMaster earlier today and explained the concern. They got back to me by email, with the response essentially "the manufacturer doesn't have shock or vibe test data." McMaster said that the manufacturer claimed a circlip style ring would be more likely to vibrate off a shaft than an e-clip retaining ring, which I find hard to believe.

They recommended I do my own testing, which basically has me back at square one. I think I will adjust my design to try to prevent the retaining ring from being able to come off the shaft by reducing the gap between the clip and the surrounding clearance bore.

 

[Jboggs]

OK. So you didn't even look into retaining ring manufacturers at all? Geez. For years the main name was Waldes Truarc, but they were bought by Rotorclip. They're probably the biggest. But there are many others. Try ThomasRegister.com. Do some leg work. Look them up. Call them. If you want retaining rings that will meet a certain spec, buy them from a company that will at least say theirs will. Do you want me to look up their phone numbers for you?

 

[Tmoose]

Some applications provide a shallow counterbore step or well with a diameter close to the E clip installed OD, and a shaped wire or cover plate and a spring to keep the clip and the retained device from moving out of the c-bore.

Needle retention in Slide type carburetors come to mind, I think.

Item 5, 6, 7, 8, and 9 here -

http://www.sudco.com/Diagrams123/expvm018.gif

or items 6 > 10 here -

http://www.sudco.com/Diagrams123/expvm.gif

view of clip on the needle 2:10 here -

http://www.bikebandit.com/aftermark...uni-round-slide-vm-series-2-stroke-carburetor

for some reason all the pictures of the slide I found on line show the underside, not the top where the shallow counterbore is. ( If I recall correctly )

 

[tbuelna]

With the aircraft mechanical systems I design it would not be acceptable to use any type of retaining ring as a travel stop on a lead screw. If the ring is being used simply as a retaining device on the lead screw shaft it should have a thrust washer next to it. Spiral retaining rings are usually acceptable for these situations, but an E-ring is not. The reason is that E-rings, especially small diameter ones, are prone to working loose due to the difficulty in ensuring a proper fit.

 

[cbk14]

Jboggs: That would be great if you looked up their phone numbers for me. Better yet, would you mind calling them for me and just post what they tell you? Actually, I might just upload my model and you could finish it up for me. That would be super! Why the crappy attitude? Nobody required you to respond, so why bother huffing and puffing? Lighten up mate.

tbuelna: Thanks for your input. This is an aerospace application so your experience is relevant. I think a spiral ring is in order, but let me ask you for some clarification - do you mean that a spiral retaining ring would be acceptable for a travel stop on a lead screw? or do you mean only that a spiral ring would be acceptable for retaining the screw in place within the assembly?

 

[tbuelna]

cbaker14- I have never seen any type of retaining ring used as a travel hard stop in a mechanical system design intended for flight. I have only seen retaining rings used to hold components in place due to dynamic forces or vibration. Multi-turn spiral wound retaining rings are preferred and I don't recall ever seeing an E-ring used. Even when using an external spiral wound ring as a retaining device on your shaft, it is standard design practice to ensure no rotational moments are applied to the ring face. To prevent this you would use something like a tabbed thrust washer.

Ultimately, the main consideration when deciding what type of arrangement to use for your shaft stop is the level of fault tolerance required for your application. For aircraft mechanical systems the situation you describe would usually require a design providing single fault tolerance. This means the shaft retaining device must have two independent methods of retention. This is the only type of retaining ring I know of that would fulfill this requirement, but I don't think it is available in the small size you need.

 

[cbk14]

tbuelna: I've attached a cut-away image of the design. Thrust washers are used on both sides to protect the PCB and to prevent moments being exerted on the E-clip during unscrewing.

 

[CoryPad]

There are multiple mentions of lead screw and jack screw in this thread. These types of parts rotate during use to transmit motion. The image in the previous post shows what appears to be screws - stationary fasteners used to clamp parts together. Can you explain your design? The retaining ring does not appear to have any function.

 

[cbk14]

Corypad: The overall purpose of this design is to pull two PCBs together and accurately mate their connectors without cocking one in the other. The clips enable the screws to also act to unmate the PCBs.

The screw is inserted through the PCB (green) and secured in place with an e-clip. The PCB is mated to the other PCB by turning the screws which pulls the boards together and mates their connectors. When the screws are reversed, the clips pull the cards apart. If the clips weren't there, the screws would come out and the boards would still be mated.

Does that make sense?

 

[3DDave]

Like this ...

https://www.google.com.ar/patents/US4929185

? (This is from Google Argentina, for some reason, but a US patent.)

I don't know if it is a good idea, but I do know that it is not a frequently used one. At least suppliers like Keystone don't mention the screws and snap rings that would be used, and Keystone would have them if there was a demand.

If the separation loads are high, then the should only be one screw in the center of the connector to prevent binding the connector on install or removal. I tried to find some examples, but didn't spend more than a few minutes with the Amphenol Aerospace connectors website, one place I expect to find self-jacking connectors.

 

[cbk14]

3DDave: yes that is the exact idea. We actually adapted the jack screw idea (with E-clips) from TE, the manufacturer of the connectors.

I think the solution has been converged on. The E-clip was changed to be a "heavy duty" version which has a larger OD and higher spring force keeping it in place on the groove. The larger OD of the E-clip reduces the clearance between the C'bore and the clip. The tight C'bore physically prevents the clip from working itself out of the groove. See the attached image. Note that in the image, the PCB and washers are hidden and the receiving piece with the tapped hole and C'bore is cut-away to show the detail.

Case closed. Thanks for the input everybody!

 

[tbuelna]

cbaker14- You mentioned this jackscrew device is for an aerospace application which must pass a qual test environment that includes pyro shock and random vibe. If you are using the screws as the primary method of retaining the circuit board, then each screw should probably have two independent forms of locking in order to provide functional single fault tolerance. Normally this is achieved with fastener preload plus a self-locking thread feature. It looks like you can easily add a self-locking feature to the mating internal threads (such as an insert), but you would still need some secondary method for securing the screws.

Since the extraction function provided by the screws is not necessary for normal operation, it does not require functional fault tolerance. The only safety issue is making sure all parts of the device (such as the E-ring) are captured so they cannot escape and create a jamming or shorting hazard. In this regard, the arrangement shown in the first sketch would probably be acceptable.