Hydrodynamic or Hydrostatic Bearing?

[juhop]

I am working on a machine with a hydrodynamic bearing that is located in an an area that has a varying pressure drop across it depending on the situation, in other words the pressure of the fluid going into the bearing varies, while still being higher than the pressure of the fluid going out of the bearing. Despite originally being a hydrodynamic bearing, would this turn the bearing into a hydrostatic bearing.
What kind of difference would variation in pressure drop make on the friction coefficient of the bearing?

 

[MikeHalloran]

Let's back up a little and get the definition right.

A hydrostatic bearing relies on an external high pressure pump to push oil into it at high pressure. The oil is distributed to several 'pads' or cavities, by metering orifices, usually nonadjustable. The orifices are typically set up so that the pressure within the pad is roughly half of the supply pressure. In other words the pressure drop across the metering orifices is about equal to the pressure drop across all the leakage paths. The leakage is usually collected and recycled.

A hydrodynamic bearing generates _all_ the pressure required to support the load solely by rotation, using oil that is supplied to a low pressure area of the annulus by a wick, reservoir, or, as in a car engine, a low pressure lube oil pump. That pump has _nothing_ to do with supporting the load. Its only job is to put an adequate flow of oil into a location where it can be sucked into the hydrodynamic bearing and used.

Now, with respect to friction, the hydrostatic bearing has relatively little friction, so long as the pump is on.

The hydrodynamic bearing has approximately the same, relatively little, friction once it's up to speed. At low speeds, the bearing surfaces contact each other, whether there is a pump or not.

Now, exactly what kind of bearing are you talking about?



Mike Halloran
Pembroke Pines, FL, USA

 

[juhop]

Thank you for you reply. Basically, I am describing a hydrodynamic bearing, which is running in an environment where there is a pressure drop across it. Hence, I am wondering what would be the correct way to model it. I am aware that usually hydrodynamic bearings have oil supplied at a low pressure, which is why I am slightly confused as what to do when the oil is supplied at a high pressure, which as far as I can see, should have some effect on the load carrying capacity and the friction of the bearing.

 

[MikeHalloran]

So far, you haven't revealed enough information about the problem for anyone to offer more help.

E.g., what do you mean by a 'pressure drop across' the bearing? In what direction is 'across'?

E.g. one defining characteristic of hydrostatic bearings is that the high pressure works against a defined area, usually a substantial fraction of the bearing's projected area, such that the product of the pad area and the pressure exceeds the radial load rating of the bearing. If you've just got a higher than usual pressure feeding into a small hole in a journal bearing, the pressure is not going to affect what's going on in the bearing.

E.g. if your shaft passes through two chambers at different pressures, such that there is a pressure difference along the axis of the bearing, then that axial flow.. may interfere with development of a hydrodynamic film, and may not be going where you think it is, because the fluid dynamics in an annulus get funky, and favor an offset of the shaft, etc.



Mike Halloran
Pembroke Pines, FL, USA

 

[romke]

if it is a hydrodynamic bearing the pressure of the oil in the supply line is of no importance. what is important however is that the bearing is supplied with sufficient oil so that in the bearing itself a lubrication film can be build up. the pressure in the bearing is much higher then in the supply line (up to 200 bar and more) and is generated in the bearing because the shaft in the bearing will find an excentric position such that the fluid film will be able to support the load on the bearing. the pressure in the bearing will vary with the load applied. as long as the shaft turns with sufficient speed and enough oil with a suitable viscosity is supplied, all will be fine.

 

[juhop]

Sorry, I forgot to mention that it is a thrust(axial) bearing. Basically the fluid enters through center of the bearing and is pushed out radially through slots between the pads of one side of the bearing.

The maximum pressure difference that the bearing experience is about 1.03 bar (15PSI). In the last message it was said that the internal pressure in bearing could be up to 200 bar. Does this mean that instead of seeing this as the pressure force trying to keep apart the two sides of the bearing, it should instead be modeled as the flow into the bearing, providing it with the right volume per unit of time of oil?

So could reduced oil supply to the bearing be the reason for an increase in friction with the axial bearing instead of a decrease in pressure drop.

 

[MikeHalloran]

Uh, that's an important omission.

Please describe your bearing in more detail, and tell us exactly what problem you are trying to solve.





Mike Halloran
Pembroke Pines, FL, USA