Delrin: ability to accept pressed bronze bushings?

[J0J0]

Idle rollers (1" dia. x 1.25" length) on a small 20-year old conveyer system are made of nylon running on steel shoulder bolts turning < 100 rpm. The nylon rollers wear over a year or 2 such that the bearing clearance is no longer acceptable and the belts that run over these rollers start to wander.

Replacement rollers made from delrin don't last much longer.

So we've decided to try inserting sintered bronze bushings into the rollers. When drilled, the nylon bushings are not holding the bushings. They press in (-0.003 interference fit) but then loosen immediately. (The machinist says the nylon is cracking, but I can't see evidence; it looks to me like scoring, not cracking.)

Does delrin have better properties such that it will be better at maintaining the pressed bushings? A better, commonly-available plastic we should look at?

Any observations, suggestions that help toward the goal of getting these rollers rolling again would be greatly appreciated.

Thanks.

 

[TVP]

Delrin is a tradename for POM (polyoxymethylene, also called acetal). It will not be substantially different from nylon with respect to accommodating a press-fit bushing. If you want to use plastic, look at a "bearing grade" of nylon (PA 6,6) or polyphthalamide (PPA), which will be reinforced with glass and/or carbon fibers and are generally internally lubricated, meaning they are compounded with PTFE, silicone, or some other lubricant. The LNP Lubricomp range is where I would start:

http://www.sabic-ip.com/gep/Plastics/en/ProductsAndServices/ProductLine/lnplubricomp.html

Alternatively, have you looked a plastic plain bearing instead of sintered bronze? There are a number of products from GGB that may be suitable.

http://www.ggbearings.com/products.php

 

[J0J0]

Thanks TVP.

Do you think the issue could be not the properties of material but the age of the nylon? Would the delrin (simply because it's new) be better suited to receiving the bronze than old nylon will?

I guess the question is: do these plastic brittle with age?

Thanks again.

 

[DesignerGuy16]

Try machining a hub and screwing the "bushing" to the wear material instead of a press fit. Don't know the size, but I know it'd work.

James Spisich
Design Engineer, CSWP

 

[unclesyd]

i would take a quick look at MoS2 filled Nylon a first effort. Nylatron is one but there are others around. We have used this to resolve a lot of problems with friction and wear.

i would give someone like Alro a call for a specific recommendation.

http://www.alro.com/DIVPlastics/plastic_product_nylatron.htm

 

[TVP]

J0J0,

Yes, plastics can become embrittled over time, due to exposure to UV radiation, as well absorption of oil and other organic fluids.

 

[Spurs]

JOJO

What you are facing is an issue where plastic elongate undger stress. Unfilled nylons and acetals can elongate 80% or more before rupturing.

If you use glass or other special reinforcements in the acetal or nylon, the elongation to break may drop to the 2-4% range.

I suspect that you just dont have enough of an interference fit for your design to work.

See the following link

http://plastics.dupont.com/plastics/pdflit/americas/design/DCI385.pdf

Another possibility to help with wear is to change the rollers to a high molecular weight nylon. For example - DuPont Zytel E51HSB would be a good choice. The higher molecular weight material has better toughness than the standard nylon 6.6 materials.

A 3rd possibility is to insert mold over the metal inserts with the plastic materials

 

[Ross0684]

Jojo
Another trick that I use is to leave a small "internal" ridge in the bore of the nylon at the start of the bore(that is the internal bore of the ridge is slightly smaller than the bore of the nylon)-hope that makes sense,
a groove is then machined in the outside of the bronze bush which corresponds in size with the "internal" ridge.
When the bronze bush is pressed into the nylon, the nylon expands at the ridge, but then snaps back into the groove of the bronze bush, thus applying a crude type of circlip arrangement which conteracts any end thrust which causes the bronze bush to move outwards.
Trust that you can follow my example.
I also do similar to hold precision roller/ball bearings into nylon/delrin/ UHMW components, in these cases the bearings are pushed completely passed the ridge and to date have not had any problems with this method.
Ross

 

[DesignerGuy16]

Another idea is to screw in the metal insert and have it designed like a keen-sert with a wedge that will deform the threads and lock the two pieces together.

James Spisich
Design Engineer, CSWP

 

[diskullman]

Try retrofitting Thompson Nyliners instead of bronze

Russell Giuliano

 

[plasgears]

Try diamond knurling the bushing prior to press. A straight knurl would not work. The interference should be more than the depth of knurl.