LeeRay,
First, I don’t know your background, the exact application, or the status of your colleagues. The following is merely suggested to assist you in forming your own ideas to ensure the best situation so please don’t take offence to any of my comments.
My reference to internal leakage being generated for lubrication is directly related to pump speed, which is why I made it! Believe it or not hydraulic pumps are more often than not hydro-dynamically lubricated, if the speed is too slow there is insufficient leakage for the internal lubrication to function correctly, or the other possibility is that the speed is insufficient for the correct hydrodynamic film to form, and this seriously increases internal wear rates! To lubricate them hydrostatically would require TOO much leakage and thus lost “power”.
At a nominal 1kpsi it is doubtful you are looking at hydrodynamic bearings, as these are almost always at low pressure, a few bars at most, you almost certainly have hydrostatic bearings.
A hydrodynamic bearing builds up its own fluid wedge by surfing the low pressure supplied oil, I have never seen one yet running at this sort of pressure in 20+ years of machine tools. Remember it is the hydrodynamic principle that lubricates anti-friction bearings, car crankshafts etc. Does your car oil pressure run at 1kpsi? I don’t think so, the oil filter canister would surely burst, and how could you generate this without the engine running on a normal car! As an analogy, hydrodynamic bearings are surfers, hydrostatic bearings are hovercraft.
Power press at 240spm... Hmmm... Pressure failure could be a problem, as obviously over pressure. Very short stroke then? Also this is infrasound and can have very curious human responses!
I suspect that if as you state the pumps are on full load at all times then there is no benefit in having a VSD as the relief will never be open for any length of time in normal operation.
If the system is designed to pump against an open relief then this is by design, to ensure constant bearing pressure.
Do the vendor of the VSD actually understand the hydraulic system to which they have recommended this device, or are your engineers doing this off their own backs? IF the former then I suspect they don’t, well most don’t in my experience.
Hydrostatic bearings rely on constant pressure, the orbit (& I MEAN ORBIT) of the shaft is self regulating due to the throttling action of the bearing jets and the opposing forces generated by the fluid system.
Is the electronic system quick enough to respond to changes in fluid pressure to prevent metal to metal contact in the hydrostatic bearings, as it is very doubtful that the manufacturer will have considered this as a possibility when undertaking their design RA. The purely mechanical system will have been designed to ensure that this situation can not occur. However, response time reductions in the fluid pressure will have a serious impact on the shaft orbit.
Does the system have an accumulator? Is there a pressure switch, if so what is that set at?
EN292 is very well, but there will be other standards to consider. IMHO if you remove the pressure relief from being the control medium, you are changing the fundamental design of the pressure control system, thus the operating principles of the machine, thus the machine will require re-assessment against current EU Directives, the fact that you have done this as the end user and only are using it for yourself is irrelevant. You will become the manufacturer, and the user, thus subject to several bits of legislation. Oh, by the way I agree that PUWER98 guides you to harmonised standards for compliance, well done on that one, most don’t seem to get it!
Please remember that you will have fluid pulses in the system from the pump, is it vane, piston etc? Superimposed on this you will have load forces at 4 Hz.
Take a 1480 rpm pump with 7 pistons this will give you a 172 Hz pressure ripple at the bearing, check it, it will be there, unless you have a smoothing system in place, in which case it will be reduced, simple pressure reservoir, or accumulator, between the pump and the bearing, on top of this you will have a superimposed 4 Hz load pressure change, depending on the detail design of the bearing, this could change, position of jets etc. You won’t see this with a pressure gauge, try a “Hydrotecnik” & a good vibration analyser on the same time frame then you’ll see it! Even better add a shaft displacement transducer with them all synchronised, you’ll have a field day!
Superimpose these two pressure fluctuations and assess the response time of your electronic control system against the shaft orbit and the forces required to minimise this and you may find that the shaft support foce goes out of spec?
Yes the bearing pressure will self regulate to a certain extent by the external forces changing the shaft orbit and this in turn changing the reactionary forces on the fluid system, however, response times may still be too slow.
Pressure is an analogue quantity, I have not seen an analogue VSD for over 20 years!
Thus the pressure will have to be A to D’d fed into the control loop around the control loop and out to the motor, the mechanical inertia will have to be overcome for the motor to slow, can the VSD respond quickly enough and is it capable of braking the motor? When the pressure increases, and the converse is required on the pressure reduction can the VSD open up quickly enough? This can be slow, I’m sorry to say, slower than a pressure relief can operate, thus…
The pressure relief can act much faster then the full control/PID loop in the VSD controller system!
Remember the PR is subject to exactly the same pressure fluctuations as the bearings, source and load and at fundamentally the same time frame, there is not any A/D, D/A conversions in place. The fluid column has the same response wherever it is, OK there is possibility a difference in pipework length creating time delays or damping, but this should have been considered at the design stage to build in the correct delay/damping factor to minimise the orbit and pressure fluctuations.
Got to stop now, this is getting TOO heavy for a Sat night after a beer!
IMHO you really need to have a second very careful think about what is going on here.
This may not be quite as simple as saving a few quid on electricity by fitting a VSD to reduce motor energy consumption!!!
Paul