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topic 669

Coating materials for motorscrews


Q. Hello,

I have a motorscrew that is threaded. The screw is made of stainless steel but we could also get it in brass. This screw is moving inside a hollow glass fiber reinforced polymer axis. The detail is 8 mm diameter and 12 mm long. I need some type of coating that could be applied to the screw. The coating must fulfill the following spec.

- High static coefficient of friction against the axis
- Withstand wear from the axis
- Working temperature 0-100 °C in moist air

I've tried various metal coatings, TiN, TiCN, and polymers. I've also tried to exchange the screw material for other metals and polymers. What I've found is that essentially only PA (polyamide) and HDPE works (friction is thus a tougher requirement than wear). However replacing the screw creates problems with bonding the screw to the motor axis. Thus I would like to have a coating.

I have sent for some RILSAN coated screws.
I have not yet found any suitable coating method for HDPE.

Does anyone know of a commercially available HDPE coating process that produces a coating withstanding the requirements above ?

Does anyone have some other coating to suggest?

Thanks in advance

Hannes Medelius
- Stockholm, Sweden


A. This is tough to visualize. This is only a little more than a quarter of an inch wide. It would seem that made out of UHMW PE would satisfy the need. Have you tried Teflon coating? My experience with Teflon coatings was not that good, but that could have been the applicator. Would plastisol where you baked it at a lower temp for a longer time possibly work. You would probably have to trim or machine it back to a proper size. A dip vinyl might possibly work if you experimented long enough. Vinyl has a fairly high coefficient of expansion but your part is small so it might work. UHMW PE is not sprayable or dippable to the best of my knowledge. I have only heard of one person claiming to be able to weld it and they said that it had little strength but did effect a seal.You might be able to get some powder, heat the screw and dip it in the powder and then bake it. I have repaired tanks with crosslinked PE powder and that might be a way. Please let me know what you find that finally works. Jim

James Watts
- Navarre, Florida

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