Can steel or brass be successfully pressed into Delrin?

[tony ennis]

Or nylon, PVC, etc?

The metal piece would be an electrical contact with a tiny bit of pressure on it. It would be faced flush to the Delrin it is pressed into, so there'd be no moment arm or other type of leverage.

[NickH]

I think if you go for slight negative clearance and a knurled part pressed in you might be laughing,
Regards,
Nick

[Spin Doctor]

Do you mean as in a bushing or pressed into the surface? If into the surface, pretty much no

[Evan]

Delrin has about ten times the coefficient of linear expansion with temperature that steel does. If it warms up the steel part will fall out.

[SGW]

It might take some experimentation, but you ought to be able to press something with a good sharp, straight knurl into a properly-sized hole so it would stay.

Another option would be to thread the hole and the part and screw it in.

[barts]

Delrin has about ten times the coefficient of linear expansion with temperature that steel does. If it warms up the steel part will fall out.

Normally this would be an issue, but the Young's modulus of Delrin is only
4.5x10^5 as opposed to steel's 30x10^6, or roughly 60 times as flexible.
This implies that a lot of interference can be used, enough so that it will
swamp any temperature effects.

- Bart

[nheng]

If you make a more definitive tongue and groove in the parts, the delrin will press over it like a tire onto a rim. Even if it loosens, it should stay on. Den

[lugnut]

So, can he use a little loc-tite to help hold the piece in place?
Mel

[Evan]

Normally this would be an issue, but the Young's modulus of Delrin is only
4.5x10^5 as opposed to steel's 30x10^6, or roughly 60 times as flexible.
This implies that a lot of interference can be used, enough so that it will
swamp any temperature effects.

Delrin or acetal isn't the best choice for this sort of fit. Is anybody else actually speaking from experience? I am. I use acetal a lot. I built a camera ball mount and used a press fit of aluminum over acetal. It worked fine until it was cooled below freezing and just fell apart. Acetal isn't very flexible which is why it is used for gears and cams in lawnmower engines. 60 times more flexible than not very flexible at all is still not very flexible.

[gellfex]

I do a fair number of delrin press fits, usually a piece of precision stainless rod acting as an axle to another component, with no knurling or other treatement of the rod. The trick, since delrin is way more elastic than metal, is to aggressively undersize the hole, but not so much the rod will ream itself a new hole when pressed. I often do >.005 under. I will qualify this with the fact that my stuff is never in extreme conditions.

I just shipped a piece with a 1" dia hex broached AL bushing pressed into a 1/2" thick delrin sheave.

[K Barton]

In the comanay that I work for we used to press delrin busshings over .3125 dia stainless rods. there was about .015" press in this appliaction, once pressed into place they would hold positon. These bushing/rod assemblies were used as cup locateing positioners on fruit handling weight sizers. On products that we cuttently manufacture we are using UHMW belt tensioning rollers which have a press of about .060" to hold position of part onto a bearing o.d. of 2.047"

Ken

[barts]

Delrin or acetal isn't the best choice for this sort of fit. Is anybody else actually speaking from experience? I am. I use acetal a lot. I built a camera ball mount and used a press fit of aluminum over acetal. It worked fine until it was cooled below freezing and just fell apart. Acetal isn't very flexible which is why it is used for gears and cams in lawnmower engines. 60 times more flexible than not very flexible at all is still not very flexible.

The yield strength of acetal is 10ksi, E is .45 mpsi, and CLTE is 5x10^-5
The yield strength of 6061T6 35ksi,E is 10 mpsi, and CLTE is 13x10^-6

For long term stability, press fit should be limited to 2.5 ksi or below to avoid creep. As nheng pointed out, a tongue and groove ("snap fit") in the mating parts will prevent loosening; in general press fits in plastic should have some sort of mechanical keying. Knurling works, since localized creep occurs in the Delrin when the high spots are causing the parts to lock together.

http://www2.dupont.com/Plastics/en_US/assets/downloads/design/DCI385.pdf

http://www.kmsbearings.com/pdf/Delrin%20Acetal%20Design%20Guide.pdf

[tattoomike68]

Sounds like he needs a gheto shock mount of some kind.

yea you can shove a sleave in a plastic bush tight as hell and it will stay.

what are you working on? a pickup truck or snowmobile?

[Evan]

If it works it works then. Delrin is more compliant but the elasticity of Delrin is non linear over time. Delrin will withstand a certain amount of deformation when the stress is applied slowly but apply the same stress quickly and it may shatter. It's also temperature dependent.

In my milling machine I used acetal springs I designed to preload the Z leadscrew with a very high spring rate coaxial spring that goes from zero to 100lbs over 1/8 inch of compression. Very low compliance, about the same as a steel spring in the same volume.

Most delrin gears and similar items that have bushings are injection molded over the bushing.

[darryl]

Another issue to consider is the conductivity of the contact material. Brass is not particularly a good conductor, so it can and will heat as current passes through it. The contact point in particular is subject to becoming a relatively high resistance, so if any considerable current must pass through it, it will become an issue. As soon as the contact heats and loosens in the hole, the problem will become worse.

I've made several switching systems where the contacts were brass, and in hindsight copper would have been better. One switch setup was on my lathe, and both sections basically burned up. The pressed-in contacts got hot and loosened up, then the plastic actually melted away from the contact.

In almost every case I've seen where brass is used in contacts, there's a coating on it. Maybe it's silver, I don't know, but I would advise against using brass by itself for contacts. Even in the lowly wall switch where the conductor material is brass, there's a contact point of some other material spotted onto the brass pieces. Not so bad where you make a secure direct connection to the brass via a tightening screw, as in switches and outlets, but you're suggesting light contact pressure, so beware.

On the choice of plastic material, I would not recommend pvc or any of the other softer and/or low plastic temperature characteristic plastics to hold contacts. I do use pvc a lot for this type of thing, but mostly because I have lots of it. It's not ideal.