Proper RC tolerance choice for concentric sliding pipe mechanism

[Danmccarter]

Hello all. I was hoping for some input on an appropriate choice for a clearance fit. The application is a electromagnetic jacking system consisting of 3 layers of concentric pipes. The different layers must be able to slide freely inside one another, so I obviously need a clearance fit. The material is probably a harder plastic like PVC, with a small portion of iron for the magnetic part. I'm not planning on a robust lubrication system, since the mechanism is low duty (its more a proof-of-concept). I was leaning towards something like RC7 or 8, but I'm worried this may be too much clearance, thus allowing the components to wobble within one another.

Your comments are much appreciated.

 

[byrdj]

why would you consider a "running" fit for a "sliding" application? dependeds a lot on the sliding surface finish, a loose transition might be the best choice

 

[stanier]

Using a combination plastic and metal design raises the aspect of differential expansion of the materials due to temperature rise. This will affect your tolerances. Coefficient of thermal expansion of PVC is 10 times that of steel.

PVC can easily be damaged by debris so seals/boots/hoods are necessary and it is difficult to maintain a surface finish.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[Danmccarter]

Thermal expansion is a non-issue for the application. Like I said, it will operate in low duty.

 

[racookpe1978]

Look at the interior of a "pipe" ... The weld seam inside will prevent sliding.

You need to plan on using a machined inside and outside member - which could start out as a "pipe" but standard pipe lengths will not be tight enough nor smooth enough for your application.

 

[Oldhydroman]

Bear in mind that even if you used a "tube" instead of a "pipe" you will have problems with the concentricity of the bore to the OD. With a "tube" the OD is controlled to a very close tolerance and so is the bore diameter, but the usual (economic) manufacturing methods fail to maintain such a close tolerance on the concentricity of the bore. This isn't a problem when the tube is used to convey pressurised fluids because the tolerance on wall thickness is taken into account in the calculation of maximum permissible working pressure. It IS a problem if you try to use the tube as a mechanical component: rocoohpe1978 is right - you probably need to look at a component which is final machined both inside and outside.

I know this doesn't answer your question about clearances but I just thought I'd add my two cents.

DOL

 

[Jboggs]

You haven't said anything about your loading (axial, transverse, moment). That will be the most important factor. We also don't know about the geometry (length, width, etc.) Its hard to recommend a material or manufacturing process until you know the application a little better than just "electromagnetic jacking". What does that mean? You might need specially machined and honed OD's and bores, or maybe just some off-the-shelf tubing, in which case your choice of sizes and fits is very limited. Or maybe just use standard tubing with custom machined flanges on each end (like rod caps and pistons in pneumatic cylinders). Its all hard to say until we know more.

 

[BrianE22]

Have you looked at buying telescoping tubing? As I write this, I'm getting a pop-up ad on this site for

http://www.DXengineering.com

 

[drawoh]

Danmccarter,

I like to use the ANSI fits. I would think that they were worked out for metal to metal contacts. Plastics are relatively compressible, and have low friction coefficients. I am not sure the behaviour would be consistent.

I would also assume that the ANSI fits apply when you do not put massive external loads on them.

If your external loads are substantial, then I do not think the ANSI fits are a good model. You know the loads, you can work out the forces, and the friction and wear. Read up on plain bearings.

--
JHG