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the unbearable bearing question

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hboy007

Tue Jul 12 2011, 09:28PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

I've been looking for suitable ball / roller bearings for days now, but I can't seem to find a solution that fits my problem.

Here is what's going on: I have a 30mm shaft protruding 2.5cm from a surface and a tube with less than 45mm inner diameter. (I am restricted by the tube wall thickness because there is another part mounted to it with screws and the outer diameter of the tube has an upper limit of 60mm). Now I will have to stuff some bearings in between that take axial, radial forces and angular momentum.

Here are my candidates :
* two stacked and preloaded 51106 axial bearings, there will be a rim sandwiched between the two which is connected to the outer tube
* thin section deep groove bearings like the 6806-2RS ones but I don't trust them when it comes to axial / momentum loads)
* pairs of angular contact or four point contact bearings in thin section fashion but I really can't find anything suitable
* there are cross roller thin section bearings out there but I don't want to spend 200 Euro on one set of bearings.

Here are the typical loads
axial load (typ) 20N
axial load (max) 100N
radial load (typ) 10N
radial load (max) 100N
in-plane angular momentum acting on the outer tube trying to twist out the inner ring of a normal radial bearing (typ) 5Nm
angular momentum (max) 50Nm

plainly axial or radial loads could be accommodated by using one of the bearing types listed above but the momentum gives me a serious head scratch. I'm starting to get desperate with this issue, it has the potential to make or break one of my most expensive projects. Help is much appreciated!

Finn Hammer

Wed Jul 13 2011, 06:35AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

It is beyond me that you make a request like this without at least posting a drawing. Your restriction: wall thickness due to something attached with screws sounds like you have not yet discovered the flange.
A drawing is mandatory followed by a clear description of what the final product you are trying to make is.
-Finn

hboy007

Wed Jul 13 2011, 08:25AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

I always seem to fail at communicating my ideas, sorry for that. I thought that since the final design is still subject to changes, I'd present you with a description reduced to the problem itself. Now I shall try to explain what is going on:

Hopefully you may at this point have guessed that I'm working on a motorized gimbal mount (the mechanics for the second axis is hidden in these views but you can see one of the motors). The bearings hold the rotor in place and allow azimuthal movement. The mixed loads are produced by holding and rotating a video camera or telephoto lens. Usually the center of mass is not on the vertical axis. I also want the gimbal mount to survive being knocked over with all the equipment in place and it must not be damaged by operating it in a horizontal rather than a vertical position ( imagine me taking a spherical panorama picture from a building or bridge with the head mounted at the end a long, extendable steel tube

)

The total outer diameter of the toothed belt disc has to be held as low as possible because it sticks into the field of view in some situations. Looking at the section views, I should rather go for a bigger disk and increase the wall thickness of the spacer that comes on top of it, too. A custom made tube with belt ridges machined into it might be nice, too, but I try to keep the cnc work as simple as possible.
Flange connections would be nice and at least a trimmed flange is possible for the connection to the rotor. I cannot, however, see how i could manage to improve the mating to the belt disc.

Daedronus

Wed Jul 13 2011, 10:17AM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

Have you looked at harmonic drives instead of the belt transmission?

Ash Small

Wed Jul 13 2011, 10:34AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've not fully studied the problem yet (bit of a hangover this morning) but I'd try to use tapered rollers here.

Small tapered rollers should cope with the loadings, they are also pretty compact.

I'll have another look at the problem later, but maybe you can give them some consideration in the meantime. (It might require a slight re-design, though.)

hboy007

Wed Jul 13 2011, 11:23AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

Ash Small wrote ...

I've not fully studied the problem yet (bit of a hangover this morning) but I'd try to use tapered rollers here.

I'd love to - I have a pair of 20mm inner diameter tapered roller bearings here from an earlier prototype but the outer diameter of these is massive. 30mm inner diameter implies 62mm outer diameter which is beyond anything I would want to use.

I've seen a drawing of thin section tapered roller bearings once, but I cannot find any bearing numbers or sources. It seems these are exotic bearings and thus very costly. I've seen the cross roller bearings but they are even more expensive.

A nice technician from SBN WÃ¤lzlager GmbH just replied to my inquiry so I called and we discussed the issue. He approves of my construction but suggests that I try the 3806-2RS thin section angular contact bearings (30x42x10) to achieve a lower profile. These are specialized bearings for axial and radial loads and cost as much as one of the stepper motors but this design option seems really tempting. 51106 axial bearings on the other hand are dirt cheap and withstand massive axial forces.

Maybe I fork the design at this point and construct a low profile portable head for hiking tours and a more solid version for heavy video cameras and 600 f/4 - style telephoto lenses (a fellow photographer and me are planning to rent one over a weekend).

Daedronus wrote ...

Have you looked at harmonic drives instead of the belt transmission?

I haven't heard of these yet, but they appear to be similar to cycloidal gears. I've seen those at the SPS/IPC/DRIVES exhibition once. How would these fit into my design with the paraxial motor?

Ash Small

Wed Jul 13 2011, 01:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I imagine that bronze bushes would be OK for this application. They won't take up much room, axial and radial loads can be accomodated, possibly with a 'top hat' design, and some of the self lubricating ones may prove to be suitable.

While they aren't as 'friction free' as roller bearings, the stepper motors should easily provide enough torque for this not to be a problem.

Bushes are very under-rated these days, it seems, but in a lot of applications they exceed the performance of rollers.

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