hydraulic bearing w pressure control

[GSC]

Hi,

A long cylindrical ram which is making axial alternating movement, and bending mostly due to its own weight. The ram is supported by two hydraulic bearings with pressure control and a slide joke at the drive side. The vertical vibration of the ram often causes problems at certain speeds.
We think of doing a modal analysis on the system, but there are very limited information on the dynamics of a hydraulic bearing to input into FE. How exactly does a hydraulic bearing work?
Does anybody have any information on how to handle such problem? Any infos are appreciated. Thanks.

GSC

 

[sreid]

Perhaps the best book having detailed information on Hydrostatic bearings is "Precision Machine Design" by Alexander Slocum. It is available through the Society Of Manufacturering Engineers. This covers opposed pad designs only.

Correctly designed hydrostatic bearings are usually very stiff (easily a million pounds/inch). The linearized stiffness calculation is quite straight forward. If you want I can FAX the pages (about 20 ) to you.

sreid@dac-intl.com

 

[GSC]

Do such bearings have significant damping or rather the material damping of the ram shall be considered in the model?

I assume that the stiffness of the bushing against rotation/bending of the shaft would also derive from the bearing stiffness, right?

GSC

 

[sreid]

There is some damping in the bearings due to oil shear but it isn't usually significant.

Bearing stiffness might need to be included in a mode analysis. The hydrostatic bearings would appear as very stiff springs supporting you ram. You could get a measure of the bearing stiffness by applying a force close to the bearing and measuring the shaft movement with a dial indicator.

If your ram moves fast it is possible to have the bearings oil starve-the oil shear from velocity counteracts to oil flow through the bearing gap at the leading edge.