Help with an interference fit between plastic and metal 2

[JonGD]

I am calculating the interference fit between two components, but all the limits and fits data i have fails to mention the effect of materials on the tolerances. What allowances should be made when pressing a plastic (injection moulded actile resin) bush into a metal (aluminium) component?

 

[sprintcar]

Most plastic bushing for a press fit will have a 'barb' or other positive retention feature on the leading edge - for example, strain reliefs for power cords. In your case, the temperature will important as Al expands much faster than plastic, so your fit at room temp may be too loose at operating temp.
Contact commercial bushing mfrs and check their catalogs for fit recommendations.

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein

 

[arto]

look at it as thick walled cylinders [Budynas "Adv Strength & applied Stress Analysis" p.143]
Interface pressure is proportional to 1/(1/Ei + 1/Eo)


check throughout the temp range

 

[btrueblood]

Um, you have that backwards, sprintcar -- plastic has a much higher expansion coefficient than aluminum, or aluminium.

 

[GregLocock]

...except for those plastics that have a negative coefficient of expansion. or zero.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ccw]

Hi JonGD,
you did not describe the function of the insert. This could impact deliberations on the fit and method of retention. I concur with sprintcar about the retention barb or flange. My experience has shown that many unreinforced thermoplastics exhibit a large creep or cold flow rate compared to metals (I suppose that is why it is called plastic instead of elastic). A proper functioning fit today may be a failing fit a week from today. It might be a better situation not to depend on friction from residual compression stress along a cylindrical contact area in metal with a thermoplastic like "moulded actile resin".

 

[JonGD]

Or injection moulded acetile resin even!

It may help more if i specify how the components are used. The bush contains an orifice to restrict the flow of pressurised air from the main inlet into another chamber (i'm a development engineer for a pneumatics company). The parts are quite small - nominally a Ø4.28±0.02mm bush pressed into a Ø4.20±0.05mm hole. The main reason for the interference fit is to avoid leaks as the bush is largely held in place by positive air pressure.

 

[sprintcar]

Dyslexic fingers.....

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein

 

[ccw]

Hi JonGD,
Acetal (Dupont Delrin) seems to be a good choice for your application. I am curious about the orfice, if it is molded with the part? If more sealing pressure is created by increasing interference, there may be a point where the orfice configuration is adversely affected over time. If more sealing pressure is needed, then the plastic part could be redesigned with a two step diameter pin in the molding process. The large diameter could be in the shank and the controlling small diameter could be in the mushroom head of the part that is practically stress free. If current part works fine as is, I would venture an opinion that your part dimensions are about right. Are you reaming or burnishing the hole in the aluminum, or is it die cast?

 

[israelkk]

JonGD

Ø4.28±0.02mm bushing pressed into a Ø4.20±0.05mm hole is not a press fit. What happens when the bushing is Ø4.26 and the hole is Ø4.25? 0.01 mm is not a real press fit for plastic any imperfection/small bump in the plastic hole can cause the measuring tool to give Ø4.26 while it is actually Ø4.25.

Take into account that plastic can not hold a constant stress without relaxation therefore any predesigned press fit will relax in time leaving a different actual press fit.
Consult Dupont Plastic design literature where they reference this issue.

 

[JonGD]

Thanks everybody for your help (so far!)

In answer to your questions/comments...

The orifice itself is a synthetic ruby component pressed into the plastic bush.

The components that the orifice bushes are being forced into are machined (reamed/drilled) and die cast (actually Zinc not Aluminium).

Using standard "limits and fits" tables, An interference fit (H8/s7) for components between Ø3mm - Ø6mm would be toleranced to provide an interference fit of 0.001mm - 0.013mm which seems like nothing to me, especially with plastic - hence my original question!!

 

[ccw]

Hi JonGD,
that is the reason I asked about the hole construction. Just drilling would typically not suffice for the surface quality needed to have the assembly seal at minimum material conditions. Any parting lines, ejector marks or flash on the Delrin OD would also be detrimental. Since you have the actual orifice protected by the ruby, you could kick it up another notch if still concerned about the fit. If the Delrin creeps, it will slightly reduce the gas path diameter or more likely extrude axially. Have you considered switching over to a bonefide elastomer material?