Advice on mechanical properties please... 1

[Hilux]

I need to make a few insulators for mobile radio transmitter antennas from 'plastic' rod sections each about 100mm long and 30mm diameter. The insulator will mount to the vehicle with a 3/8" x 24 thread on the rod centreline at one end and the antenna element likewise at the opposite end. Some fibreglass covered antennas can be 8ft (2.4m) in length and perhaps 5/16" (8mm) in diameter so there is a significant load for these threads to support while in motion, both wind-loading and occasionally shock loads from hitting branches etc. Your advice is needed to decide which of the common materials might be most suitable mechanically. Electrical insulation is unlikely to be a problem and with solid rod I'm not too concerned with the effects of UV in the short term. What I do need is to be able to determine which parameter is most important in deciding the strength of the thread. If I have to choose between say Acetal, Nylon 6, Delrin, polyurethane etc. what would you suggest?
Thanks in advance,
John

 

[Demon3]

I would go for glass fiber reinforced nylon 6 for the good combination of strength and impact resistance. You can get lubricated nylon in case that helps you screw the thing together smoothly.

Is there a comparable part on the market? It would not be expensive to have that analyzed to see what is tried and true.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

If you are mounting by sitting the base over a flat surface with one hole only and a bolt up through that plate into the centre of the rod you are setting a reasonably difficult task.

Plastics do not hold threads designed for metals well and strip easily. You need the bolt to be as long as possible to help overcome this.

For readily available in bar stock materials at a reasonable price acetal homopolymer is the best of the unfilled materials. Delrin is simply DuPonts trade name for acetal homopolymer. Acetal co-polymer is also very good, but not quite as good as homopolymer at holding threads.

Glass filled nylon will be somewhat better than unfilled acetal, but the glass makes turning and threading more difficult and quickly wears the sharp edge off tools.

A better way would be to change the design so there is a metal base with vertical tabs and the plastic part is retained by two horizontal nuts and bolts at a decent distance apart so the bolts are in shear, not tension.

Polyurethane will not be suitable as it will nothold a thread well enough and will be to difficult to machine as it will push away from the cutter rather than cut accurately.

Glass filled PET or acetal are other options, but I think mylon is best if glass filled. Acetal homopolymer is best unfilled. Acetal is quite poor to UV, but carbon black colour helps somewhat. Nylon is a lot better to UV than is acetal, and the glass fibre further helps preserve strength after UV attack. Heat stabilised nylon is better still to UV.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[Hilux]

Thanks for the information gentlemen.

It seems the answer to my question is to use Glass-filled Nylon 6 whereas the solution to my problem is to adjust the design to avoid the inherent weakness completely! That of course is what engineering is all about.

Thanks again for your input, it is much appreciated.

John

 

[stewart123]

Pat in his statement about threads in plastics is spot on.
If threads are to be used in plastics the threads should be a bull nose type thread. This type of thread will not lead to crack growth. We found this was true in vibration testing and high stress enviornments. There are a number designs for bull nose threads but they all have rounded type of threads.

We found this out when we were making high pressure PEEK vessels. These cells go to 10000 psi. We also found this out when we bolting large plastic parts together in high vibrational situation. Also make sure your plastics have been annealed properly to remove any stresses from the manufacturing process.

If you plan to glue to a non polar plastic like PP etc. you will need to surface treat the surface so that you get a functional group attached to the surface. After surface treating you need to glue fairly rapidly because the polar groups will migrate and be replaced with nonpolar groups.

If you decide not to surface treat then you need a roughness Ra50 for good mechanical adhesion. It will not be as good as a surface treatment but it may work in your application.

 

[unclesyd]

Aside from the sound advice given above here is a design guide for using metal screws in plastics from Ticona.

http://tools.ticona.com/tools/documents/literature/B.3.2 Fastening with metal screws, farbig.pdf