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I need a bearing that the inner bore will stay level and balanced when the out ring is rotated. My son with autism lacks the motor function to feed himself with a spoon. He can turn his wrist, when the spoon goes to his mouth its 90 degrees off. I need to be able to insert bearing into a handle and have a spoon in the bore that will stay level.

Sketch

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  • $\begingroup$ My understanding is that you want the 'plate' of the spoon to rotate around the long axis of the spoon, with the hollow face always oriented upwards for no spillage? $\endgroup$ – mart Mar 10 '16 at 14:54
  • $\begingroup$ Plenty available commercially. Search for swivel spoon and fork and hit the Images link. $\endgroup$ – Transistor Aug 8 '16 at 13:46
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One difficulty here is that the masses involved are quite small so you need a very low friction bearing. Many industrial bearings are designed for high loads and speeds with quite close tolerances so in many cases they can be a little stiff under small forces even though this is small in the context of their intended application.

In your case you might be better off with a plain bearing with quite loose tolerances, for example a polished stainless steel shaft in a Teflon bushing as low friction is a much higher priority in this case than eliminating radial play.

One approach would be to use a teflon rod with a hole down the middle as the handle and have the shaft of the spoon attached to a ground steel rod which passes through it. Using the full length of the handle as a long bearing will support it better and give you more room to work with rather than trying to find a small bearing which fits forward of the handle.

It will also help if you can 'crank' the shaft of the spoon so that the contents of the spoon are a little lower than the axis of the handle so you get a pendulum effect, similarly you would want to keep the centre of mass of the bowl of the spoon as low as possible.

If might help if you made up a prototype in aluminium so that it is easy to bend by hand and quickly try different bend configurations to quickly work out what works best.

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I'm gonna assume you need the spoon's plate to work like a gyroscope, where whichever way your son moves & holds the spoon handle, the spoon's plate stays upright. I think you need a gimbal bearing.

The standard gimbal bearing:

A standard gimbal bearing

This ball bearing was used to stabilise a camera, and I think the lessons from the project could be applied in stabilizing your spoon:

gimbal camera stabilization

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  • $\begingroup$ If I where to use a gimbal. Wouldn't I need the center of mass perfectly in the center of bore. Also if this helps I only need the spoon to stabilize on a horizontal plane. I doesnt have to keep the spoon from tipping forward. $\endgroup$ – kiel starforth Mar 10 '16 at 17:25
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The first question any engineer, if faced with a new problem, is if it allready has been solved. I assume you talked to people with similar special needs (unable to bend wrist) or their caretakers and didn't come up with a good solution.

I think the question for what kind of bearing you want is a sideshow, the first question is if your design is viable. I think not:

I assume your son holds the spoon before him, lower arm more or less level with the table, shoves the spoon into the food. Then he tilts the lower arm upward or upward without compensating in the wrist, so the plate of the spoon ends up upright. I assume there is litte inward movement, because then you'd have spillage towards the handle. So to achieve your goal - have the meals less frustrating and messy - you need make sure the palte of the spoon stays level.

Before you think about bearings, you need to consider which forces act on the spoon. In a spoon as in your drawing, the center of gravity of spoon + food will be close to the axis of rotation. Here's my proposal: the spoon attaches to a rod that runs the length of the handle. At the back side of the handle, the rod protrudesand makes an L-bend downward, where you attach a small weight. This will turn the spoon in the desired direction.

For the actual build, I think Chris Johns approach is best:

In your case you might be better off with a plain bearing with quite loose tolerances, for example a polished stainless steel shaft in a Teflon bushing as low friction is a much higher priority in this case than eliminating radial play.

One approach would be to use a teflon rod with a hole down the middle as the handle and have the shaft of the spoon attached to a ground steel rod which passes through it. Using the full length of the handle as a long bearing will support it better and give you more room to work with rather than trying to find a small bearing which fits forward of the handle.

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I've been miming eating with spoon without wrist movement and I'm not quite sure I understand the problem correctly.

From my understanding using a gimbal bearing would more of a psychological torture device, and the solution is simple! The spoons handle only needs a dog leg in its handle,say 45 degrees, and/or the users nose length from rear of spoon to the handles upper bend of dog leg for obvious reasons, from there you could have a standard bearing fixed to either end of the remaining handle and tubing cut to desired length with inside diameter appropriate to the outside diameter of the bearings then slide/press bearings into tube.

HOWEVER!!! It will be imperative that consideration is made as to the action of loading the spoon with food... If the user targets the food with a forward trajectory it would work fine, BUT!, should the need arise to drag the spoon from the rear of plate/bowl/dish to the front or vice versa as you would for the proper etiquette of soup consumption, this design is flawed.

The key to circumventing this flaw is to limit rotation by way of two keys counter sunk into each end of tube handle immediately behind the bearings ends that face the inside of the tube and two more keys fixed to the inner shaft/axle between the tubing keys to limit rotation. I would also suggest making the rotational travel limiting keys customisable by identical multiple fixing points for two opposing outer keys.

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  • $\begingroup$ Would you mind including pictures or diagrams of what you're suggesting? I'm having a hard time imagining what you mean when you talk about the dog leg with the bearings and keys. $\endgroup$ – Chuck Aug 8 '16 at 12:39

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