Press fitting a Ball plunger 1

[eichman]

Hello,

I'm calculating the hole dimensions to press fit a 0.38" OD x 0.865" SST Ball Spring Plunger (

http://www.mcmaster.com/#standard-spring-plungers/=xznir4)

Part number 8262A17.

If I go with FN2 fit, i get the Hole Dia to be 0.38060 Max and 0.3800 Mins as per the calculator at Link

Is 0.38060 be machined and inspected? Isn't 6/10000" too small of a interference?

Please excuse my inexperience.

Thank you all

 

[imcjoek]

If the hole is 0.3806, you have clearance, no?

Frankly who knows how good their 0.38 dimension is. Maybe it's really 0.375. They don't give you any tolerance. Get one in your hands and mic it, but the purpose of the knurl is to make it less sensitive to needing the perfect interference fit. It'll kinda broach itself into things.

Just by feel I'd throw 3-7 tenths interference in steel and 5-10 in aluminum. You never say what material you're smashing these things into.

 

[drawoh]

eichman,

Your .38" diameter is knurled. Interference fit calculations require an accurate, documented tolerance. You have neither. If they do not have a recommended hole diameter for this thing, I recommend you find another vendor and catalogue.

--
JHG

 

[Tmoose]

I'd give McMaster a call to get the recommended hole size and any other technical info.
They have always been very helpful and forthcoming with just exactly that type of info for me in the past.

 

[eichman]

imcjoek, the material of the body I'll be fitting the Ball Plunger into is 17-4 PH. It will not experience any elevated temperatures and the problem is that I can't get my hands on one. I'm remotely supporting which makes my life that much harder

drawoh, I wish I could switch suppliers but unfortunately that's not an option

Tmoose, great idea. I'll send them a request for more detailed info.

 

[eichman]

So, I got a response from McMaaster. they replied that the hole needs to be 0.38" with tol of +/- 0.01"

 

[ornerynorsk]

Yikes, +/- 0.010" means it's either going to drop in with no resistance, or you will severely deform the plunger body when you attempt to drive it in. Is this a one-off, or do you have a lot of these to install? Your best bet, like others have said, is to mic the plunger body, and ream slightly smaller, with how much smaller based on the material it is being installed in.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[Tmoose]

In the past McMAster has shared the manufacturer's name.

I'd double check with the manufacturer.

 

[MikeHalloran]

Mc's page states that the parts are intended for press fit into wood, plastic, or aluminum.
NOT 17-4PH.
The axial knurl will probably peel right off.

You need a different part.
The ball plungers normally used in hard material have threaded ODs and a wrenching slot, which allows them to be threaded into a blind hole, with the possibility of deforming the gozinta with a prick punch for retention, or buying the plunger with a plastic locking pellet in the thread.

I know, the design is already cast in concrete.
It's going to be a problem.
Send up a flare now, or prepare to be stuck answering the phone for the product's service life, or until the sh!tstorm comes.




Mike Halloran
Pembroke Pines, FL, USA

 

[eichman]

I'm sorry for not getting back on this post. I was in training.

So, I got back in touch with McMaster telling them that hole tol of +/- 0.01" is not right and they responded that they actual hole dimension is 0.375" +0.002"/-0.
This is the REAL dimension for the hole I believe. Please let me know your thoughts on it.

And thank you for all your valuable feedback and suggestions.

 

[MikeHalloran]

You're still screwed.

Mike Halloran
Pembroke Pines, FL, USA

 

[eichman]

How so? MikeHalloran

 

[MikeHalloran]

A proper press fit at .375" nominal into 17-4 is less than a 'tenth', as is the tolerance.
The knurl will be shaved off, and is not likely to retain the part,
AS IS CLEARLY STATED IN MACARCO'S CATALOG PAGE LISTING THE IDEAL SUBSTRATES.

If failure of the ball plunger or its assembly puts you in court, YOU WILL LOSE, since you or your company seem determined to use the part in an unapproved and unintended way.

If there's no possible way for this design undecision to land you in court, well, you haven't thought about it enough.







Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

This is where shaft-locking compound comes in handy. Though it seems a little Red-Green to me.

 

[JNieman]

I was going to chime in but I think MikeHalloran pretty adequately covered the bases.

I've used both items, threaded and pressed, but never would dream of using that pressed plunger in metals. Threaded are a pretty good no-brainer piece of hardware, and can be quite adequately locked in place with appropriate goop or by just getting the nylon stud type.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[tbuelna]

MikeHalloran is correct that the spline teeth on the OD of the 18-8 plunger body will simply get squashed if it is press fit into a 17-4 part, even with just a couple thou interference fit. It was not explained what application this spring plunger was being used for, but if a press fit in a plain hole is required to retain the spring plunger, then one using a formed sheet metal housing with a flange would seem to be a better choice.

One issue with using these ball spring plungers for detent mechanisms is ensuring a consistent force at each installation. This is one reason why adjustable threaded bodies are often used. The self-locking types seem to perform quite well.

 

[chicopee]

You could explore the idea of shrinking the ball in dry ice but do the calculations first.