Help designing bearing influenced by spring

[Mech_Engineer]

I think your problem might be more easily solved with a 4-bar mechanism of some kind, have you considered this approach? Gears with missing teeth and violent stop/starts will not have a very long lifespan, whereas a 4-bar linkage could provide more effective utilization of your motor and would be relatively easy to design and analyze.

https://en.wikipedia.org/wiki/Four-bar_linkage

You can test and simulate multiple options for the linkage using a free simulation tool like this one: http://www.mekanizmalar.com/fourbar.html

 

[JBA]

If there is a very high ratio gearbox involved, I would be concerned about back driving the motor in my proposed solution, that is the reason that I suggested a belt drive instead.
As an alternative, in place of a conventional high rpm motor drive and high ratio gear assembly; a Torque Motor, that is either directly mounted on the bottom driving shaft or mated with a very low ratio gear assembly could be easily back driven by your spring.

 

[Baluncore]

Let's approach this problem again.
I think the OP design has been used before somewhere and this is reverse engineering based on a poor picture or unreliable memory.

If we eliminate friction drive and rearrange the bits we have a motor with a reduction gearbox driving a cam.

A swing arm attaches to the two wires symmetrically above a fixed vertical axis bearing. A cam follower is mounted on the arm. A return spring is attached to the arm.

The cam follower mounted on the arm is a roller or wheel, the OP significantly calls it a “bearing”, running on the outside of the cam. It would not surprise me if the cam follower originally used was a standard deep groove ball bearing, bolted to the arm. The arm rotates through 90° and then falls back for each turn of the cam. The position of the cam axis, follower and arm fulcrum set the rotation angle.

There is one modification that I would make to reduce the stress. I would spring-load the cam follower on the arm so as to allow it to advance on the cam by half the follower diameter as it goes over the step on the cam. I would also consider using a polyurethane skateboard wheel as the follower as that would help silence the mechanism.

The speed of the repeat cycle is decided by the choice of motor speed and gearbox ratio. It could be adjusted if necessary with a VFD.

 

[CWatters]

I agree that's a better approach but when I looked at that I couldn't immediately see how to get the full 90 degree output swing.

I know your drawing isn't to scale but if it was I think the cam has to overlap the fulcrum? Perhaps I didn't spend enough time playing with the geometry?

 

[Baluncore]

Perhaps I didn't spend enough time playing with the geometry?

Maybe.
I believe 90° is possible. There is no problem if the arm fulcrum is under part of the cam since the arm could be made from two flat bars, one above the other. When needed the cam could pass between the two arm bars for part of the cycle. The cam follower can also be mounted offset from the arm.

 

[Haim1404]

Yes it kinda the same.
but now I return to my original question - how to design / determine the size and shape of the Cam so it will make only 90°

 

[CWatters]

I had a play around using sketchup to try draw Baluncore's proposal...

I'm no expert on Sketchup but I was able to produce this in about 30mins using the "Curve Maker" extension from the Extension Warehouse to draw an Archimedes spiral. I'm not suggesting you use this drawing but it's one way to approach it.

The 100mm long output arm rotates through a 90 degree arc. I offset the centre of the cam by 50mm along the 45 degree diagonal. Cant really justify this layout technically other than it looks about right and I think the forces on the cam follower should be reasonably uniform with this general layout? Although perhaps not optimal.

I've not allowed for the diameter of the cam follower wheel. The spiral is also over 360 degrees not the 270 degrees as requested.

 

[Haim1404]

Well guys thanks for all your suggestions and ideas.
I will keep you updated.

 

[JBA]

During my period of being a product development engineer the company had us attend a seminar on creativity and problem solving; and, although I remember little of that seminar, one statement has always stayed with me and that statement was " When brain storming for a solution to a problem you not through until you have visualized at least eight ways to solve it, even if some of them are not practical or possible they will open up your mind to possible routes to a solution"