8883 forum posts
My 3D-printed engine design is stumbling towards being printed, but as it will take about 24 hours and use lots of plastic filament, I want to improve it as much as I can first.
Friction is worrying me because plastic running on plastic is more like a brake than a slippery bearing.
When I did Meccano, it was recommended to reduce friction by fitting a loose washer on the axle to separate the wheel hub and frame, as in the silvery example just visible in the photo:
I suppose the idea is the brass hub slips against the washer, which is free to rotate and slips against the frame as well.
Attractive to think the arrangement reduces friction, but does it work? Another way of looking at it is it doubles the braking surfaces and wastes energy spinning the mass of the washer. The arrangement seems likely to increase friction, not reduce it.
Slightly different case maybe because the contact pressure is much higher, but I think bolts tightened on to a washer are less likely to work loose, not more!
I'm thinking of using a plastic printed washer to reduce friction on my plastic engine. Worth doing if it decreases friction, otherwise a waste of printing time and plastic, perhaps adding friction to the engine as well. What do the team think?
Edited By SillyOldDuffer on 21/06/2022 13:00:45
|Michael Gilligan||21/06/2022 13:01:15|
20289 forum posts
PTFE washers are available
8883 forum posts
Which is a strong hint washers do reduce friction, thanks!
Should have mentioned I'm trying to print the entire engine from PLA if I can make it work, so PTFE isn't entirely welcome (yet). I'm looking seriously at hook clips and bayonet fittings at the moment as a way of avoiding glue!
Except it's obvious my pure-plastic approach makes life difficult! Allowing other materials would simplify and improve the engine in many ways, for example the design could easily be changed to use standard metal roller bearings, which are far better than plane bearings made of ripply plastic.
23042 forum posts
If you are going to start adding non printed materials they a steel shaft would reduce friction far more than a washer between to smaller areas. The only thing a printed washer may do is give a smoother surface depending on which way the "layer grain" runs on the other parts
As for fixings, if they are through fixings you could have something like a rivit with a washer on the other end and then fix it in place by heating the end with something hot
Edited By JasonB on 21/06/2022 16:12:55
8883 forum posts
Great minds think alike! Rivets are under investigation, though I hadn't thought of melting them - good idea. My version isn't permanent. Need to make some and experiment to see if the plastic is springy enough and how tight they hold.
|Tim Stevens||23/06/2022 18:15:43|
1622 forum posts
Answer = yes, but not always. If the washer surface is nicely flat, and attracts rather than repels lubricant, and is not rubbing on a surface which has a low melting point (such as many thermo-plastics (the clue is in the name), and the environment does not attack the surface leaving abrasive products, the answer is more likely to be yes.
Hope this helps
|Nigel Graham 2||24/06/2022 00:28:58|
|2281 forum posts|
The general principle with metal assemblies that except in a few situations like cast-iron slideways, the bearing surfaces are of different materials, so presumably the same applies with plastics?
I don't know the type of plastic used for 3D printing but some plastics are not as slippery to each other as might be expected, even with a glossy surface.
If I were to make a largely-plastic assembly, irrespective of how the parts are made, I would think which material to use where, and if that means some small parts are of a different plastic, or are metal, that would be by natural design principles. For example, I had to make an adaptor-plate to fit a new motor to my jig-borer; and though that is carved out of PVC plate stock, the fasteners are all steel.
|Howard Lewis||24/06/2022 01:35:03|
|6306 forum posts|
Be wary of running two surfaces of the same material together where relative motion is involved .
That usually results in galling.
A metal on plastic should be OK.
Cast iron gets away with it because of the graphite acting as a lubricant, but steel on steel, or Ali on Ali is risky, especially without lubrication.
8883 forum posts
Thanks for the replies gents, even though they confirm my fears about running plastic on plastic. At least the washer idea isn't completely bonkers.
However, the answers tipped me into a rethink about the engine: I need to test how the plastic I have in mind (PLA) performs in a bearing, so something simple is in order. Back to the drawing board!
6653 forum posts
ISTR from billycart days that a washer either side of the rotating assembly reduced friction, be it on the wheels or the pivoting plank that supplied steering via a loop of rope.
|lee webster||24/06/2022 14:55:18|
|115 forum posts|
I don't know if this applies to your question but, the stub axle on an Austin Seven front axle has a hardened steel thrust washer between the stub axle and main axle beam. This washer is kept lubricated by grease from the kingpin. In many cases, the washer rotates with the stub axle wearing out the top face of the axle. To get over this I ground a small half round into the edge of the washer and drilled a small hole that lined up with the half round into the top face of the axle. A small pin was then Loctited in place that didn,t protrude above the bearing face of the washer. Which, I think, is a long winded way of saying glue the washer to the PLA!
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
Sign up to our newsletter and get a free digital issue.
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.