Hydrostatic Bearing Clearances

Andy, you will need to supply the basic dimensions of the bearing block and the pressurised zone and its material type to calculate the growth of the diameter under pressure. Also the side loading on the shaft that must be resisted. Instron in the UK or Instron Schenck Testing in Germany have a lot of experience in hydrostatic bearings and vibration testing.

I managed and performed the engineering functions a vibration test facility for a large environmental testing laboratory, and this is not an uncommon problem. The clearances should be as small as possible in order to avoid issues with crosstalk, but there has to be enough clearance to allow the oil film to be complete, even it there might be some tiny misalignment. The clearances you use will depend upon the size, design, and loads upon the linear bearings as well as the characteristics of the oil you are using. Since you have .0015" clearance in the undamaged unit, that is a good place to start.

As for your concerns with wear on the remaining device, any sort of wear should be obvious to the naked eye. I would be willing to bet that you won't discover much wear because there is no or very little surface-to-surface contact due to the presence of the oil film. If it is possible, try to set the clearance in the other units to .001"; this will be a good compromise between allowing sufficient oil for a good film and limiting lateral motion to a reasonable amount. If the clearances are too large, the oil film will fail, and the damage will look like there was no oil when, in fact, there was a great deal. A lot of research has been done on this subject, most of it relating to bearing clearances in automotive crankshafts.

What is the maximum frequency used in this system? As long as it is below 2000 Hz, .001" should be just fine. Don't worry about that additional .0005" on the third unit; it's not enought to notice if the other two are at .001". Don't forget that a difference in diameters of .001" leaves only .0005" for the oil film. This is very good, because a thin film will be very "stiff" in compression due to dynamic loading. Although the hydraulic power supply will give you 2500 psi, the resistance of the oil film to compression will cause the pressure in the area that is trying to compress to be many times the pressure supplied by the pump. Note that this is true only for dynamic loads; as time passes, the pressure in the oil film will reduce to that of the power supply.

Good luck.

Oh, yeah... Put together some sort of interlock to shut down the vibration system if you lose hydraulic pressure.

Hi,

if there are any orifices or other restrictors where the oil enters the bearing surface you should check these for being equal to the intact ones.

(These can be located up to 10" upstream).

If so then the clearance should be as you indicated.

Else you should get new restrictors or make new ones by yourself.

RHABE

Thanks for the great info. Does anyone know if the bore through the bearing block is linear, or if it may have had a slight profile (ie tighter at the ends, looser towards the oil channel in the centre)? I may remove the remaining un-failed bearing and carefully gauge the bore from front to back to check this out.

The table apparently does have a low pressure cut-out that was never wired up. Call it 27 years of lucky operation prior to this?

AndyC.

Hi AndyC

if there is an orifice then you don't need a taper. (Inlet restricted bearing).

If there is no orifice you need a taper or grooves. This is an outlet restricted bearing:

Typically a step between two cylindrical bearing sections will give good results and is easy to manufacture.

Gap should be 2:1 if length is 1:1.

Quality can be better if length is more near 4:1 (first gap wider and firsat length longer), but I forgot which gap ratio at 4:1. I can look into my old calculations for this.

Limitations will be existing as oil flow is higher if first part is longer and gap there wider.

Operation with air is similar but with different relations.

RHABE