Uneven Thrust Loads on Needle Roller Bearings

[kiracofe8]

Originally Posted by M. Krass

or "How I Need to Make A Turntable That Will Not Bind."

So, I am in the midst of designing a mechanism that rotates about a vertical axis and I'm worried that the enormous attached moment arm will cause binding in this vertical pivot joint.

McMaster-Carr shows needle roller bearings that are rated for 1900+ lbs. of thrust, but I assume that is assuming even thrust on the washers at each end of the bearing. How does an uneven load affect the performance of such bearings? Are they a viable solution for this problem, or is there some other method that's commonly used for supporting vertical axles under torque load?

Attached is a graphic and, by circumstance, a vague look at the arm design. I hope it makes my concerns clearer.

Well, the easiest thing to do is to support the shaft by two bearings that are spaced a good distance apart. Then, instead of one bearing that has to take a large moment, you have two bearings that each have to take a small force. This is the principle behind why doors always have (at least) two hinges. If you only had one hinge, it would have to take a tremendous moment. But if you have two hinges about 6 feet apart, each only has to support a few pounds of radial load.

[Madison]

Originally Posted by kiracofe8

Well, the easiest thing to do is to support the shaft by two bearings that are spaced a good distance apart. Then, instead of one bearing that has to take a large moment, you have two bearings that each have to take a small force. This is the principle behind why doors always have (at least) two hinges. If you only had one hinge, it would have to take a tremendous moment. But if you have two hinges about 6 feet apart, each only has to support a few pounds of radial load.

Well, I should I have mentioned that I plan to support the shaft with two bearings. Despite that, because of its design and application, the load that the arm exerts (~200 ft.*lbf.) must be cantilevered off the shaft. I can't support the shaft above and below the arm, only in two places below the arm. The separation of the bearing was planned to be only about 4", but I can investigate increasing that distance to something larger; maybe as high as 12".

Kevin -- I'll look into angular contact bearings. I'd never heard of them until now, so I don't know what they're typically used for, nor do I understand how they work Thanks for pointing out their existence.

[Kevin Sevcik]

As long as you have two bearings supporting the shaft, it won't be that much of a problem even if they aren't on opposite sides of the shaft. Of course, the more seperation the better, but 4" can't be THAT bad. wil a ~200ft-lb moment, it'd put less than 1000 lbs of radial force on a bearing. MSC actually has some angular contact bearings that look more than capable of dealing with that kind of load.
Also, think about way to counterweight the thing so it doesn't apply that kinda moment in the first place.