Custom bushings
 

What are some good materials for a large custom bushing?
I was experimenting with some interesting turret ideas, however, a few of them require large custom bronze bushings (~2.625" OD, 2" ID) in order to be geometrically correct.
The turret only weighs around 30lbs, so would aluminum bronze be an okay bushing material? Do machining sponsors hate working with bronze enough to refuse or is it different from copper in that respect?
What are some other bushing materials?

Also, this is assuming that we don't just buy bushings. This is pretty theoretical right now, so I wanted some opinions on fabrication and materials of custom bushings.

Re: Custom bushings
 

Bronze and brass are fairly easy to machine but unless your bit is nice and sharp it can leave a terrible finish. So just use some sharp bits and take your time and you will get a nice part.

Re: Custom bushings
 

Could you elaborate on what your building exactly? And what sort of load would the bushing be under? Axial? radial? both? I ask because you're talking about using a massive bushing for a very small load, and I think your design could be improved. And given that this is theoretical, I think that your time would be much better spent figuring out how to design your turret so that it doesn't require an expensive custom bushing.

My first thought is to suggest delrin. I say this hesitantly because I don't exactly know what this bushing is doing, but I think a delrin bushing would be able to handle 30lbs load with ease. Not to mention it would cost about a third as much.

If your heart is set on using a bronze bushing picking out the alloy should actually be pretty easy. From what I understand this is a low load (for the size of the bushing) and low speed mechanism, which means you don't need to use any of the harder more expensive bronze alloys. I suggest you look for a cheap bearing grade bronze with good machinability. Something along the lines of 932 multipurpose bearing grade bronze or machinable 936 bearing grade bronze. Both are available from mcmaster, onlinemetals also has a nice selection of 932 bronze tube. Your sponsor shouldn't have any problems working with either of these alloys. Bronze doesn't get hard to machine until you get into the harder high strength alloys, and even those aren't too hard to machine.

-Adrian

Re: Custom bushings
 

What about a 'lazy susan' type bearing?

Re: Custom bushings
 

Many plastics make good bushings if not compressively loaded much. Sintered bronze is the classic material, plain bronze and brass are OK. Aluminum is not that good as it tends to gall under load. Steel isn't as bad as you might think, but must have lubrication.

For the 3" range ball bearings are not a bad choice (and not as expensive as you think), and if there are axial loads a tapered roller bearing would be good. Just look for low-speed versions if price is an issue.

You could cast some Babbitt, but that's way old school.

Re: Custom bushings
 

+1. We need to know what stresses you have.

Re: Custom bushings
 

It's a turret for a t-shirt cannon. Basically, the actual barrel rotates on something like what the arms this year had (stright piece up + angled claw) to be able to aim and shoot better. It's also practice for future years.
It doesn't weigh that much but due to the design which incorporates bevel gears, there is a large bushing on the bottom coaxial with a shaft going to a bevel gear to rotate the barrel up and down independent of the module's rotation. So the turret experices very slow radial loads in the 100rpm range but around 30lbs axial.
The load is on the flange of said bushing.

I looked into ball bearings in the 3" range, but they all cost more than $100. I also cannot design for this bearing to be smaller due to some interesting design issues that have to deal with a cim mounted to the bottom of this turret.

Re: Custom bushings
 

Could you modify the design to use this? http://www.andymark.com/product-p/am-2714.htm

Re: Custom bushings
 

You could try using a set up like 1640 uses for their swerve drive. It can support huge amounts of weight and take a good hit. Its also pretty compact. Just mount it upside down? It uses a thrust bearing to handle the axial loads and a very large bearing to handle radial loads.

Re: Custom bushings
 

The igus slewing rings would work for this. They are just so intriguing - someday I'm sure we'll find the perfect use for the one we have.

They have been available from FIRST Choice some years.

Re: Custom bushings
 

Sounds interesting. As I said though, because this is pretty low-budget (we have other stuff to work on too) I wanted to avoid ball bearings.

Slewing rings sounds interesting, but they are a little big.

Re: Custom bushings
 

I've used 2 tapered roller bearing for turntables before. You could probably go down to 3/4" shaft size. Weight I assume is not an issue.

Re: Custom bushings
 

A great material for bushings is UHMW (Ultra High Molecular Weight Polyethylene).

We used this extensively, not only on the robot but at work for many bearing applications. It is very easy to machine, has a very low coefficient of friction and can withstand significant loads for a plastic.

You can get rods from McMaster fairly cheap.

Re: Custom bushings
 

Using bearing for this application might be a little easier on your wallet than you think. 2'' ID thrust bearings are around $5, all you need is one on the top to handle the weight of the turret, one below to lock the whole thing together and some kind of bearing surface in between to handle radial loads. If you do this, since the top of the bushing is no longer a bearing surface you would only need for the inside of the bushing to be slick. This means that you could make a tube (aluminum, delrin, whatever strength is required for mounting) and put a sleeve bushing inside. A 2'' ID sleeve bushing can be bought from mcmaster for around $10. Or simply make the tube out of delrin and use that as your inner bearing surface.

This approach would change your design the least. Any timken or ball bearing for a shaft of that size would have a very large OD, and would require you to modify your design accordingly.

-Adrian

Re: Custom bushings
 

We use a 1" 6061 Al 1/4" wall tube for our swerve pivot with two sets of bearings 1.75" apart. The bottom (main load) bearings consist of a 1" flanged ball bearing race (McMaster 6384K373) to manage radial loads and a 1-1/2" thrust bearing (McMaster 6655K25) for axial load. At the top, we use an 1-1/4" Igus bushing with a bronze sleeve around the 1" Al tube (because of aluminum's softness and tendency to gall).

These handle competition loads really well (we have had no service needed to our original 4 pivots during the entire competition season comprising 70 matches overall), but the diameters are a lot smaller than what you were looking for in your original posting.

BOM's and CAD files are available: http://wiki.team1640.com/index.php?title=Swerve_Central