Which type of bearing to use?

[bertterbo]

I'm wanting to make an "articulated" arm type thing (although it will only rotate in one plane). As shown in the image below, the grey bars will be 3030 aluminium extrusion, and there will be an M8 bolt going through the 3 pieces as shown by the black line.

The grey pieces on either end should be able to rotate freely about the bolt, however, I'm trying to build it with as little play as possible. In my test, I just used penny washers between each connection, and then using a lock nut, I tightened it as accurately as I could so that there was zero play, but the pieces could still rotate. As you might imagine, I couldn't achieve that.

I appreciate that ideally, I would drill large holes in the grey pieces to take bearings, but I don't have the equipment to do that accurately. Just a bench drill press. So I then looked into thrust bearings, that could replace the penny washers and do a better job of zero play, but still allowing things to rotate. However, they're very expensive (£5 a piece) and have a very small outside diameter of around 16mm, half the width of the 3030 extrusion, so not sure if they're right for the job? what do you think? are they worth a gamble?

Is there anything else I should be considering?

 

[Spectric]

All you need are spacers that sit in the parts you want to rotate that are marginaly longer than the thickness of the metal, now you just fully tighten the fixings and the parts will rotate. These spacers are like tubes which the fixing passes through the centre.

 

[bertterbo]

Isn't that going to require extremely accurate drilling to get it to work with zero play?

wouldn't it also make it easier to twist the ends? as now you have very little area that the pieces are resting on?

 

[Inspector]

Plastic flanged bushings (bushes in UK speak) would do the trick and are self lubricating. Delron, Tufnol, UHMW, Nylon etc can be bought ready made or made to order. If you can find stock flanged bushings then they could be easily and quickly replaced if the wear. You could use brass or bronze bushings but they would probably cost more and need lubrication of some sort. Example below.
https://www.alibaba.com/trade/searc...earchText=plastic+flanged++bushings&viewtype=
Pete

 

[bertterbo]

Plastic flanged bushings (bushes in UK speak) would do the trick and are self lubricating. Delron, Tufnol, UHMW, Nylon etc can be bought ready made or made to order. If you can find stock flanged bushings then they could be easily and quickly replaced if the wear. You could use brass or bronze bushings but they would probably cost more and need lubrication of some sort. Example below.
https://www.alibaba.com/trade/searc...earchText=plastic+flanged++bushings&viewtype=
Pete

I might be missing something, but aren't I going to have the same problem I had with using washers? Where once you tighten down enough to remove any play, you've essentially clamped the spacer tight between the two pieces of aluminium and it won't rotate? Or will the lubrication hopefully avoid that issue?

 

[Inspector]

The plastic bushings don't need lubrication. That is what the "self lubricating." part of the sentence implied. You haven't said if the arm will be working in a horizontal plane? Vertical? Or something in between? Nor have you said how long it is or how much it will be holding up. All of which has a, no pun intended, bearing of how it should be made. I gave you a reasonable suggestion based on the little information you provided and what I know about bearings, bushings etc, admittedly minimal that you could do further research on. I'll leave it up to you to figure things out going forward.

Pete

 

[bertterbo]

The plastic bushings don't need lubrication. That is what the "self lubricating." part of the sentence implied. You haven't said if the arm will be working in a horizontal plane? Vertical? Or something in between? Nor have you said how long it is or how much it will be holding up. All of which has a, no pun intended, bearing of how it should be made. I gave you a reasonable suggestion based on the little information you provided and what I know about bearings, bushings etc, admittedly minimal that you could do further research on. I'll leave it up to you to figure things out going forward.

Pete

I didn't say they needed lubrication.

As for the what plane it will be on, horizontally, and no extra weight, it just needs to support itself.

 

[ChaiLatte]

Bushings, bearings, spacers, it does not matter what you use. What causes the play is the fit between the parts. If the bolt wiggles around in the hole (whether that hole is direct in the extrusion, through a bush or through a bearing), the assembly will not be tight.

An 8mm drill will not drill an 8mm hole. An 8mm bolt is not 8mm dia.

If you want it not to wobble about, you would have to drill something like a 7.8mm or 7.9mm hole and then ream it with an 8mm reamer. You would then have to put an 8mm pin through that hole.

The hole drilling and reaming is easy and the drill press is good enough to do that. Clamp the upper and lower parts together and drill and ream as a pair (match drill/ream). Do the same for the two middle parts.

Finding a suitable pin to fit your accurate 8mm hole is less easy. You could use a shoulder screw (sometimes called a shoulder bolt), but you would need M6 (8mm plain section) x 90mm long. They are not at all easy to find in that length and in quantity two (closest I found was 25 pieces for over £33).

Alternatively, you could use 8mm. dia. silver steel (as that is ground to an accurate diameter. 13" length is £4.92 from Chronos - buy the reamer at the same time from them) but you would either have to thread the ends for nuts or drill and tap the ends for retaining bolts.

 

[bertterbo]

Bushings, bearings, spacers, it does not matter what you use. What causes the play is the fit between the parts. If the bolt wiggles around in the hole (whether that hole is direct in the extrusion, through a bush or through a bearing), the assembly will not be tight.

An 8mm drill will not drill an 8mm hole. An 8mm bolt is not 8mm dia.

If you want it not to wobble about, you would have to drill something like a 7.8mm or 7.9mm hole and then ream it with an 8mm reamer. You would then have to put an 8mm pin through that hole.

The hole drilling and reaming is easy and the drill press is good enough to do that. Clamp the upper and lower parts together and drill and ream as a pair (match drill/ream). Do the same for the two middle parts.

Finding a suitable pin to fit your accurate 8mm hole is less easy. You could use a shoulder screw (sometimes called a shoulder bolt), but you would need M6 (8mm plain section) x 90mm long. They are not at all easy to find in that length and in quantity two (closest I found was 25 pieces for over £33).

Alternatively, you could use 8mm. dia. silver steel (as that is ground to an accurate diameter. 13" length is £4.92 from Chronos - buy the reamer at the same time from them) but you would either have to thread the ends for nuts or drill and tap the ends for retaining bolts.

The problem isn't horizontal play in the holes (for my use case). It doesn't matter if the holes are slightly too big.

The problem is play in the vertical direction (in the diagram I showed). Which was why I mentioned the thrust bearings.

Basically, I want to have zero(as close to) up/down play, whilst still being able to rotate the pieces.

 

[baldkev]

But as inspector said, wouldn't plastic flanged bushes work? Youd have, in effect, 6 per bolted joint. The bush holes through the extrusion need to be accurate ( no play ) and can be tightened up fairly well. The fact they are self lubricating would indicate they'll still allow the parts to swivel horizontally....

 

[bertterbo]

I've already ordered some to try

 

[Bob Chapman]

I have built something like your design for deep hollowing on my lathe. I used some very thin bearings almost as thin as washers but with tiny roller bearings in them. I can't remember what they are called but if you go to a bearing specialist they would know. They were not expensive.

 

[bertterbo]

I have built something like your design for deep hollowing on my lathe. I used some very thin bearings almost as thin as washers but with tiny roller bearings in them. I can't remember what they are called but if you go to a bearing specialist they would know. They were not expensive.

That's exactly what it's for ..although not for "deep deep" hollowing .. just small project hollowing. It's more to hold the tool for me as I suffer from a bad back (I'm copying the Trent Bosch Stabilizer). I'd be happy to buy one, but he doesn't ship to the UK, so am trying to make one.

The bearings you're referring to sound like thrust bearings, which is what I mentioned in the first post.

Can you post some pictures of your setup?

 

[niall Y]

Just been looking at some thrust bearings on Ebay. i/d 8mm, o/d 16mm - at £ 1.74 each

 

[flh801978]

I dont think aluminium sections will be strong enough and will vibrate in use
why not mak eit in steel sections as your picture?

Ian

 

[bertterbo]

Just been looking at some thrust bearings on Ebay. i/d 8mm, o/d 16mm - at £ 1.74 each

These were the ones I ordered AXK Axial Needle Roller Thrust Bearing AXK00619- AXK1730+2AS Washers Choose Size | eBay

 

[bertterbo]

I dont think aluminium sections will be strong enough and will vibrate in use
why not mak eit in steel sections as your picture?

Ian

I had considered it, but figured Aluminium would be easier to work with?

I thought 3030 was really strong? or do you mean it will vibrate because it's fairly light?

 

[flh801978]

Be far superior in steel and just as easy to drill if you have a bench drill

 

[Phill05]

These were the ones I ordered AXK Axial Needle Roller Thrust Bearing AXK00619- AXK1730+2AS Washers Choose Size | eBay

Wrong ones those sit on the top and bot you need these that fit within the middle bar then the top and bottom bar will revolve around the bearing.
Have a look at these: HK0808 INA Drawn Cup Needle Bearing - BearingBoys

Edit: And I would go with steel bar less flex you don't want the ally flex to rip out chunks of your turning.

 

[Sideways]

Look at a picture of the cross section of a 3030 extrusion.
I think it's a bad choice for this application.
Most of the strength that matters to your use is only in the middle part of the extrusion and you are going to put a huge hole in that.
Crank up the nuts and the extrusion might distort and crush.
You are probably best off with solid strip / bar like the photograph