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Cylinder Materials and Tolerances

01/14/2009 6:33 AM

Hello everyone,

I need to manufacturer a cylinder of approximately 80mm OD that will be rotating inside a chamber at up to 1000 RPM. I would like to keep the gap between the two parts to a minimum. Does anybody have any suggestions what the two materials could be and tolerances? My main worry is siezing of the cylinder if the tolerance is too tight. The cylinder will be supported on bearings. My first thought is for the Chamber to be Aluminium and the cylinder Nylon, but the materials could be different.

Thank you in advance.

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#1

Re: Cylinder Materials and Tolerances

01/14/2009 11:03 PM

Hello Worrier:

Welcome! I hope you do not live up to your name! No need to worry here we don't bite! Very friendly lot and there is some very clever Engineers here! Not all are Engineers But, it depends what your definition of an Engineer is I suppose?

To get do to business............You want a cylinder to rotate inside another cylinder or pipe?

Can you tell me what you are building please? Is one cylinder inside another just as a guide or, is it to produce power, and if so how?

If you can explain if the cylinders are flat on the ends or have some kind of connecting rod, and where does that go etc?

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#2

Re: Cylinder Materials and Tolerances

01/14/2009 11:14 PM

Is it meant to transmit power (of course there may be no other reason of rotating ) then the material of the shaft (the cylinder) and the housing must be of the material that is designed to transmit the power . As far as the gap between the cyl and the hsg is concerened it may be minimised in various ways (teflon bushing/ graphite bushing/ brass with graphite impregnation - fitted into the hsg may be OK or now several ceramics bushings are available. What is the meaning of minimum ? what is that you want to avoid leaking out? Why not use plain bearings with lip seals Pl understand when you are working with this type of dissimilar materials, the problems are high (as the components go into operation, the coeff of expansion may make either excessive clearance or sieze the shaft (depending on who has more) and both are not OK as per the OP Tht is why it may be better to have ths shaft and housing with near by coeff and seal / guide (bearing) by a thin shell of proper material A brief description/outline may be helpful.

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#3

Re: Cylinder Materials and Tolerances

01/15/2009 1:34 AM

Worrier --

Your problem is a classic case in precision mechanism design. You leave out too many of the necessary descriptions of your project to permit anything but a abstract discussion or the design process, an effort that would take perhaps 8 or 10 pages minimum to present even at an undergraduate level of complexity. Missing are issues of temperature and its effect on expansion or contraction of the materials. Also you make no mention of forces and moments acting on the components, the properties of any fluid present, the manner in which the rotating cylinder is to be supported, the tolerances that your manufacturing process can hold in making the parts and a few other lesser issues.

OK, given the small amount of information you have provided I'll make some comments that might help you better frame the requirements of your design. Nylon is a poor choice for precision parts . In addition to its thermal coefficient of expansion it is notorious in the common 6-6 form as an absorber of moisture which causes it to swell appreciably. It is difficult to machine to close tolerances. And it can be expected to be a poor bearing material at the 5 meters/second relative velocity of your application. Therefore after completing your tolerance analysis and determining the absolute worst case allowance you would be well advised to add 2 or 3 tenths of a millimeter diametral clearance.

Aluminum is a poor wearing material. If I had to use it for this application I would put a wear resistant coating of a minimum or .05mm of a type 3 Anodize (so called "hard anodize").

As mentioned above this pair of concentric cylinders will be held in relationship to each other by some kind of "bearing" capable of supporting the loads and moments. This bearing could be the cylindrical surfaces themselves or separate purpose constructed bearings.

These bearings can have actual clearance or none depending on the type and the way they are fitted. In all cases they will have some form of elasticity in addition, though in the case of properly preloaded anti-friction bearings that elasticity will be essentially negligible for this size mechanism. Simpler (and generally less expensive) plain bearings or bushings will have some clearance in order to function with reasonable minimum friction. This clearance will have to be added to the clearance between the cylinders with a safety factor to allow for eventual bushing wear.

I've gotten as deep as I can into detailed commentary at this point save for one further observation. This application is similar to the rotor and housing clearances that have to be fitted in a moderate sized positive displacement rotary pump. In that situation precision machined metal components tend to be used and rotor to housing diametral clearances will run in the range or .05 to .1 mm. Metals usually used are cast iron, steel and bronze for noncorrosive oils and such and bronze, stainless steel and high grade filled engineering plastics for water based and slightly corrosive fluids. Aluminum tends to be avoided except in rare vehicle mounted applications where light weight is a premium.

Ed Weldon

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#4

Re: Cylinder Materials and Tolerances

01/15/2009 10:47 AM

A couple rules of thumb to get you thinking of possibilities:

With ordinary, moderate precision machining, your cylinder could be 80.00 mm (+/- .01 mm) and the bore into which it fits could be 80.05 mm (+/- .01mm). Given moderate precision ball bearings, and ordinary machining care, the two pieces could live in harmony, with no collisions, if temperatures were similar for the inside and outside pieces. To maintain constant clearances under varying temperatures, it would be best to keep both materials the same, so as one expands, the other expands at about the same rate.

With plastics, additional clearance must be provided, because humidity can also affect size. You'd also need to allow for the differences in thermal expansion between the aluminum (or other metal) and plastic.

The longer the cylinder, the greater the possibility for flex affecting the clearance.

Another alternative, depending upon the intended function of your two pieces, would be to fill the gap between pieces with a lubricant, with the two pieces operating as a plain bearing, like those in an engine crankshaft. Depending upon materials and temperature differences, a clearance of about .02 mm would be typical. Surface finishes would have to be smoother than for a cylinder supported on ball bearings (in which the two pieces never touch).

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