Rough size for hydrodynamic bearing. 1

[iainuts]

I need a very rough calculation for the size of a hydrodynamic bearing capable of supporting a load of 12,000 pounds, radially. Rotational speed is 700 - 750 RPM but I'd like to ensure this bearing is sufficiently large for rotational speeds down to ~ 400 RPM. Feel free to recommend oil type or other details if they are pertinant. This is for a reciprocating compressor, and I won't be doing the design myself for this bottom end but need some up front sizing to allow for sufficient room in the crankcase and for between cylinder centerlines for a preliminary study. So just looking for an aproximate diameter and length for this bearing. Thank you!

 

[MikeHalloran]

It mostly depends on the system pressure you select for the bearing's oil supply (and the setting for the hydraulic power supply's relief valve). The bearing normally runs at ~half that pressure, depending on the balance (that you select) between the metering orifices for each bearing pad and the clearance between the shaft surface and the periphery of the bearing pad.

Let's say that you select the system pressure at 2000 psi, usually a reasonable number because you need fancier fittings and more backup rings at higher pressures.

That would make the bearing cavity pressure ~1000 psi.
For estimation purposes, you can take that as the pressure on the projected area of the bearing. That would mean you would want a projected area of 12 square inches, e.g. a 3" diameter bearing x 4" long, or a 4" diameter bearing by 3" long. Actually you would make the bearings a little bigger than that because:

Actual hydrostatic bearings typically comprise 3 or 4 'pads' around the periphery of the bearing shell, each comprising a routed groove surrounding a land to which the pressurized oil is supplied through a metering restrictor. The pressure over the land and the groove should eventually stabilize at ~half the system pressure. The leak rate, i.e. the flow that must be supplied to each bearing pad, is computed using the radial gap and the periphery of the outside of the routed groove. There is another land outside of that routed groove, over which the pressure will be a gradient from the pad pressure to atmospheric pressure. Outside of that outer land around the routed groove is typically another routed groove axially, a rabbet around the periphery, and an oil seal, defining a low pressure channel that collects the pads' flow for recirculation through the bearing hydraulic power supply.

(Remembered from a design handbook for hydrostatic bearings, published in the 60s by the Nonferrous Foundes' Society)


Mike Halloran
Pembroke Pines, FL, USA

 

[iainuts]

Thanks Mike. That's just what I was looking for.

So is the bearing size required independant of the rotational speed?

 

[MikeHalloran]

Yes.

Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

I was thinking the OP was looking for hydrodynamic bearing info, not hydrostatic.

I'll drink another cup and try again.

 

[MikeHalloran]

Oh, crap, you're right.

Iainuts, forget what I wrote; you'll have to go back to your ME text and run through the hydro>dynamic< bearings section. Yes, you wrote the correct word; I read the wrong one.
Sorry.



Mike Halloran
Pembroke Pines, FL, USA

 

[iainuts]

Ok, thanks. Which bearing type can handle a higher load per unit area? I take it the hydrostatic one can?

Perhaps someone else can take a whack at an estimate?

 

[loki3000]

doubt that, since hydrostatic is only as good as the pump is (brute force basically), while hydrodynamic has the geometry in favour (oil film, oil wedge)

 

[iainuts]

Ok, so the hydrodynamic one is smaller... Any estimate on size?

 

[loki3000]

nope, and it has to be a very precise calc, involving the viscosity of the oil for example.
find a book like this somewhere:

http://www.amazon.de/Decker-Maschinenelemente-Funktion-Gestaltung-Berechnung/dp/3446417591