Advice on using an interference fit to secure 2 objects.

[jf1991999]

I am looking for a low cost way to fix a 200mm diameter extruded cylinder to a steel Y bar section. I can't use traditional fasteners in this application.

I have decided that a press fit should work well. I would use an injection moulded disk shape with a thickness of around 25mm. 2 of these disks would be pressed into the extruded cylinder with a spacing of about 200mm. The disk would be moulded with a cut out of the Y bar section running through it so that the cylinder could be pressed onto the Y bar and held in position via the interference fit between the 2 disks and the Y-bar.

I have not had much experience with injection moulded plastics under load. There will not be much load on this joint although I need it to be reliable for 30 years. The plastic disk will be subjected to reasonable compression to securely fit it within the cyclinder and to securely hold the Y bar. UV, chemical exposure and temperature are not an issue. Water is an issue. My questions are;

1. Will the plastic flow over time and 'relax' the interference fit between both the cylinder and y-bar.

2. Could I get some recommendations on the type of plastic to use for this application.

3. Are there any formula's that help in the design of interference fits like these between different materials.

4. Is a 30 year design life possible for this type of fixing?

 

[unclesyd]

Could you post a sketch with dimensions and materials.

Thirty year is an exceeding long time for a plastic component and creep would definitely be a problem even with filled plastic.

 

[CoryPad]

1. Yes
2. Glass-filled polypropylene for low stress applications, engineering thermoplasitcs for higher stress (polyester, polyamide, acetal, etc.)
3. Some polymer suppliers have design manuals that cover this
4. 30 years is not realistic

Regards,

Cory

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

 

[BWAppliedKinetics]

I pretty much agree with CoryPad
1. Yes, though some plastic types much less than others (ie PPS or acetal is better than cheaper alternatives)

2. I believe that a glass filled plastic (PPS, acetal, delrin) will be more stable than a non-filled alternative.

3. You can use some MODs (mechanics of deformable solids) equations to figure out about what the stress will start off as in the plastic, it won't be perfect due to the non-linear behaviour of plastic, but may give you a place to start.

4. Plastic and 30 years rarely go hand in hand. Simply being exposed to the atmosphere slowly degrades many types of plastic.

You may want to consider overmoulding a replaceable component rather than a plastic press fit, or a resdesign to remove the issue. More details on the design would help with this.

BW

http://www.akeng.com

 

[Tmoose]

I don;t think I'm visuallizing your applicatoin correctly.

radio knobs and valve stem seals often have metal clips or clamps providing long term pressure.

http://ppsc-inc.com/Valve_Stem_Seals_-_VSS.jpg

http://www.enginetech.com/2004products/valve_seals2.jpg

lip seals often have metal "garter" springs applying controlled pressure

http://www.azom.com/work/N9ILW9B679eLr0hr5a34_files/image003.gif