"twisting" force on shaft in journal bearing 1

[Allen3]

This is kind of a newbie question.

Let's say I have a horizontal shaft perfectly aligned in a journal bearing, and at running speed, the bearing is capable of carrying a load of 1000 pounds-force, and by that I mean that I can put 500 pounds of downward force on each end of the shaft, for a total of 1000 pounds of downward force, without any problems.

What happens if a "twisting" force is applied to the shaft, i.e., if a downward force of 500 pounds is applied to one end of the shaft and an upward force of 500 pounds is applied to the other end? In general, how suitable is a journal bearing for carrying a "twisting" force on the shaft? Is there a good reference that discusses twisting force capacity of journal bearings? What types of bearings would be better at handling a twisting force? BTW, what is the proper name for this type of force? Is it generally called a twisting force, or something else?

Thanks much.

 

[caoimhin1]

"Moment load" is probably the best description. Its the type of loading scenario you get with cantilevered loads. e.g. a shaft supported by two bearings with a tensioned belt pulley at one end.
For ball and roller bearing, any bearing which can take the load is fine. The direction of load is unimportant. Not sure about plain bearings.

 

[LostHippie]

if the journal bearing relies on an oil film for proper cooling or support then it is pretty essential to keep the shaft properly lined up with the bearing surface. A misaligned journal bearing will result in an uneven oil wedge (for small moment loads) and the development of a hot spot where the oil heats up to much higher temps than normally seen. This will result in premature oil degradation.

In the case of a misaligned journal bearing with high moment loads, the shaft will actually make contact with the journal, which would cause extreme heating and wear, resulting in smearing of the babbitt on both surfaces and changing the geometry of the inner bore of the bearing, affecting proper development of oil film, thus affecting proper operation of the bearing even if it is realigned after the damage has been done. If severe enough the shaft could wear through the babbitt material and into the hard supportive backing material, resulting in potential scoring of the shaft.

Moment loading of shafts is typically accounted for by installing two radial bearings of some sort to convert an applied moment to the shaft into 2 applied radial forces on the bearings, which is exactly what a journal bearing was designed for.

Hope this helps.

 

[unclesyd]

Check this link out as I think it will assist you with your problem.

http://web.njit.edu/~sengupta/met 301/Design of Shaft.pdf