Very thin bearings ?

[Bob Shaw]

Hi All,

I'm wondering if I could pick your collective brain power for a few moments...

I am looking for a single or pair of very thin bearings to fit a 30mm (+/- 2mm) outer bore and a 26mm (+/- 2mm) inner bore.

The bearing will be subjected to virtually zero load. It is used as a levelling device for a miniature camera mounted in the inner bore. It will never be rotated any faster than can be achieved by hand. The camera is in a weighted 25mm long, 26mm diam cylindrical housing. One half is heavier than the other to provide a self righting capability. A bearing at each end or a 20mm long single bearing would do. I imagine that some sort of instrument bearing(s) may be suitable.

I've looked at ultrathin bearings but most are either the wrong size (start way too big) or are way too thick. 'Standard' ultrathin bearings seem to be about 8mm thick but are clearly designed to bear significant loads which isn't an issue for me.

Currently I'm using the inner race of a roller bearing (26 X 30 X 20) which has a 2mm thickness but has a disadvantage of having no thrust capacity. It works great until the camera is tipped off the horizontal and it runs into the back (or front) of the housing and stops. I could solve that with the inner races from two thrust bearings but 1) this is starting to get messy and 2) overall device length is at a premium and even the width of two races may tip me over my maximum.

My ideal solution is a 26 X 30 X 20 ultra low friction sealed bearing with a smidgeon of axial load bearing capacity.

I have bought a 26 X 30 X 20 piece of that dry liner material (Iglidur) from IGUS. Haven't tried it yet. Not very hopefull....

Cheers!

[BillTodd]

Hi,

Could you use small loose balls, retained in a groove at the end of the inner and outer pieces by a plate on each end (for instance)?

The bearing has almost no load, so you would not need a hard surface, just a good surface finish.

[Superman]

Try looking up "NEEDLE ROLLER BEARING" 25 x 32 x 20
The outer race to suit your bore, rollers would run directly onto your shaft, your shaft would need extra fine finishing or grinding to a toleranced size

and
one that I cannot find is like a plastic sleeve or cage with ball bearings, no inner or outer shells, the bearings run directly on your parts
something like this

[rowbare]

Why not a simple bushing made of Delrin or UHMW?

[Bob Shaw]

Hi Guys,

Thanks for 3 really good suggestions. I'll probably try them all out.

I'll let you know how it goes.

Thanks!

[Bob Shaw]

Hi All,

I made a (very) short video to illustrate the problem:

http://www.tesla-turbines.com/movies/self-levelling.mpg

I have been talking to some bearing manufacturers on how to get a bearing (or bearings) that will do the job. The race I currently have is nearly but not quite there. As the camera rotates almost back to level the unscrewing force reduces below the force required to overcome the needle roller friction. A better (read "way more expensive") bearing is required. In order to get the right one(s) I need to know the unscrewing force.


This is some sort of torque calculation but as I had three goes at first year math I am not going to attempt this. I imagine its some sort of integration as the carrier is in the shape of a bucket. Dimensions are:



External radius = 15.5mm



Internal radius = 11.2mm



height = 20.6mm



bucket floor thickness = 3.3mm



Total volume = 9.31 cc



volume of each half = 4.65cc



top half (Al 2.7g/cc) = 12.57g



bottom half (W 19.3g/cc) = 89.83g



Effective mass = 77.26g



Unscrewing torque = ???




If you can work this out all well & good, if not, pointers to how to work it out will be gratefully received.


Cheers!

[BillTodd]

However calculated, the torque is going to approach zero at near vertical, so you're going to need a very low friction bearing for that mechanism to work reliably.

Friction free bearings can be produced by floating the device (e.g. a ship's compass) or by using complex magnetic suspension.

If you just want to keep the friction as small as possible then you need to reduce the diameter of the rolling surface (i.e. to reduce the contact area). Could you re-design your camera housing, so that it is suspended from the rear on a couple of small diameter bearings?


[edit] BTW if you have to use a peripheral bearing, balls will have lower friction than needle rollers.

[BillTodd]

Looking at the video, it appears you are going to use a lens cap of some description and, if the bottom half of the camera housing is to be filled with a levelling weight, the camera will look out above the centre line.

How about a small bearing in the middle of the lens cap and a matching bearing at the rear?

BTW are there any wires from the camera? Or are you powering with batteries , slip rings or rotary transformers?

[Bob Shaw]

Hi Bill,

I am going to attach my drawing and an XLS I did of my attempt at calculating the unscrewing torque.

I think I've got something badly wrong as I have a torque of over 400 oz/in which cannot possibly be correct?

BTW I am leaning towards a bearing at the front and back as suggested.

I've been looking at Kaydon KAA10LX0's. Only available in imperial. Bore 1", Outside diam 1.375", starting torque 0.25 oz/in.

The rotating camera holder element is a bucket shape. The camera itself is circular at 16mm for the first 15mm or so and then it has a square section for the final depth of 21mm. Wiring passes through the back of the alloy section of the bucket to a pogo pin arrangement which runs on concentric brass rings to feed power & signal in.


Cheers!

[BillTodd]

starting torque 0.25 oz/in.

For your purposes, this sort of info is almost meaningless. The starting torque will have been measured on proper bearing surfaces with bearing finishes, something your application will not have (nor does it need).

If you can keep the bearing diameter down, i.e. by using two small precision races mounted behind the camera cup, then the greatest friction in your application is likely to come from the slip rings ( not a problem as far as I can see - a bit of friction will help damp the motion) ; Again keep the diameter of the slip rings as small as possible.

Make the camera mounting cup from steel or brass (something heavy) then cut off the top half (or more) , since it does not need to be circular if it's not going to be a bearing surface.

[Bob Shaw]

Hi Bill,

Your solution looks good (very good!) but I can't utilise the back of the shell as a location point. Thats where the camera connects to the outside world. The cable connection is a series of concentric brass rings (total diam 18mm) which I need to mate onto with my pogo pins (see pics attached).

The bearings will have to be mounted outside the circumference of the camera shell.... maybe.

It might still be possible to do it like you describe if I extend the length of the camera internaly by the width of the required bearing. Have a look at the DXF I posted and you'll see the dilema I think.

BTW I'm thinking of using tungsten (or a tungsten powder, epoxy mix) for the camera holder.

Cheers!

[Bob Shaw]

Hi Bill,

You were talking about slip rings... I would prefer to use a slip ring but I'm having trouble finding one sufficiently small. They're usually too big. Airflyte do one that 1/4" wide but 3/4" long. I'm going to contact them and see if they anything more suitable.

Cheers!