Custom bushings

[highlander]

Originally Posted by Adrian Clark

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

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

[asid61]

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.

[Michael Hill]

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

[Tyler2517]

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.

[Bruce Newendorp]

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.

[asid61]

Originally Posted by Tyler2517

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.

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.

[Gdeaver]

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.

[David Guzman]

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.

[jman4747]

Originally Posted by David Guzman

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.

I would say +1 to UHMW, Delrin (Acetal Resin), or Nylon 6. All inexpensive all great bearing surfaces.

Another thing to consider if you can wait I would research different suppliers, Mc Master Carr is the fastest and arguably the best but not always the cheapest.

PS "All inexpensive" is a relative statement.

[asid61]

The thing is, the bushing looks pretty strange, and I can't really change the design at this stage (it follows an ultra-space-saving design that requires its weirdness). Basically I changed the design so it can use a smaller bearing, which is like 7/8" inner diameter and 1.125" outer diameter. However, it has a very large flange that's around 2" outer diameter. I'll look into using plastics for machining and cost without sacrificing strength. I've used UHMW in the past, but we tend to have a lot of acetal so I'll try that on aluminum too.

[highlander]

Originally Posted by asid61

The thing is, the bushing looks pretty strange, and I can't really change the design at this stage (it follows an ultra-space-saving design that requires its weirdness). Basically I changed the design so it can use a smaller bearing, which is like 7/8" inner diameter and 1.125" outer diameter. However, it has a very large flange that's around 2" outer diameter. I'll look into using plastics for machining and cost without sacrificing strength. I've used UHMW in the past, but we tend to have a lot of acetal so I'll try that on aluminum too.

In my experience Delrin machines really easily, while UHMW has a much lower coefficient of friction. However, UHMW is annoying to machine.

[Adrian Clark]

Originally Posted by asid61

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.

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

[Clem1640]

Originally Posted by Tyler2517

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.

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

[hrench]

Originally Posted by asid61

I looked into ball bearings in the 3" range, but they all cost more than $100. turret.

Ball bearings are just better than bushings and they're not expensive if you know where to look. just went to ebay and entered '75mm bearing' and here's two for $6.50 each.

http://www.ebay.com/itm/$@#$@#$@#-6215-2RSR-C3-Deep-Groove-Ball-Bearing-75mm-Bore-130mm-Outside-Diameter-New-/390838773352?pt=BI_Heavy_Equipment_Parts&hash=item 5affce6668

[asid61]

Originally Posted by hrench

Ball bearings are just better than bushings and they're not expensive if you know where to look. just went to ebay and entered '75mm bearing' and here's two for $6.50 each.

http://www.ebay.com/itm/$@#$@#$@#-6215-2RSR-C3-Deep-Groove-Ball-Bearing-75mm-Bore-130mm-Outside-Diameter-New-/390838773352?pt=BI_Heavy_Equipment_Parts&hash=item 5affce6668

Interesting. I never thought to check for metric sizes.
The problem is there are some holes machined in the flange of the bearing. The main bearing is 2" OD, but the flange is 3". I need the huge flange for mounting holes to other stuff.