Alternating ram w hydraulic bearings

[GSC]

Hi,

We have 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.
What would be the best way to approach the problem? Modal analysis? How to measure the material damping then? I have seen some discouraging discussion on this topic. How to consider the effect of the hydraulic bearings without overstiffening the system? Are there any experimental methods what may help? I am an FE analyst with limited knowledge on vibration analysis. Any ideas would be appreciated. Thanks.

GSC

 

[GregLocock]

Wow, that is a horrible problem to work on.

I wouldn't worry about the material damping, the damping in the various bearings will be far more important.

Try doing an impact test on the running machine to get an idea of the overall damping in the system.



Cheers

Greg Locock

 

[btrueblood]

GSC,

The ram is stroking horizontally, but vibrating vertically?

Sounds like the bearings are "sticky" and transmitting some torque to the ram - maybe replace/rework the bearings and/or check that they aren't scuffing. If they are scuffing, then use ones rated for higher load? Can you replace the ram, or rework it to tighter tolerances to limit the amount of bending it can undergo? Also, using a larger/longer ram might help (having more of the rod stay inside the cylinder at full stroke, so there is more support & hopefully stiffness).

 

[GSC]

Yes, the ram is stroking horizontally and has dynamic vertical vibration due to its own weight. I don't say it has no horizontal vibration excited by the machine's own drive system, but problems arise from the vertical vibration only... e.g. the ram hits the cylindrical tool around it near the end of the stroke at certain speeds.
Clearance is around 400 microns which is I think big enough, similar rams has around 100 microns dynamic deflection so something is very wrong.
Good idea indeed to have a look for scuffing... the bushing stiffness was well over 1.000.000 pounds/inch in the past, but the ram has been changed since...
GSC