For example, the 5cc bent axis pump from Bosch has a max inlet pressure that's less than 10% of the outlet (30 bar / 350 bar)
I'm a relative novice when it comes to these so excuse me if these are stupid questions. If the limitation is due to pump case strength, could that be overcome if you submerged the pump so that the pressure on the exterior matched the inlet? Also, wouldn't the piston slipper/shoe load on the swashplate face already be higher on the outlet side? Thanks for your input.
I can only think that the pressure limitation on the inlet is due to the inlet seal arrangement that is designed for predominately negative pressures. In most cases, the pumps generate a negative inlet pressure and the seal arrangement would be set up to ensure the seals are not dragged into the pump. If the pump inlet is boosted, the forces acting on the seals are reversed. I think that they are trying to protect the seal on the inlet.
Each of the pistons in the rotating group will see maximum working pressure in each revolution so the pressure under the piston cannot be a limiting factor. The pressure differential between the inlet and outlet directly effects the noise and vibration emitted by the pump, so one would imagine they would want them to be as close together as possible.
Shaft torsion and pump casing stress are a function of outlet pressure. Inlet pressure is not a consideration, except for the torsional vibration that is also a function of the pressure differential between the pump inlet and outlet.
The limiting factor on pump casing pressure is the shaft seal. Most have a max working pressure of 5 BAR and are permanently connected to the reservoir to avoid having high pressure in the casing. In any case, increasing the pressure on the outside of the pump casing would be very impractical.
A point to note, the Parker F11 and F12 bent axis pumps can also be used as motors...bi-directional motors. They all function in the same way.
It has to be something specific to the way Bosch Rexroth design their pumps.
This probably depends on whether the pump design uses the pump casing as the inlet or has a more conventional inlet and outlet valve arrangment as per a recirocating piston pump.
Given that the one of the advantages of an axial piston pump is the reduced size and weight of the valve arrangment using the casing as the inlet chamber, this could well be your answer as to mkae the casing high pressure would beef up the casing to a large extent, create an issue with the shaft seal and generally is of such a diffculty that it isn't worth it compared to a multi-piston PD pump.
Look for an axial design that has inlet valves and and outlet mounted on the piston side and you may find it changes. It is difficult to see if the Bosh pump you mention has this as there is no cutaway that I can see, but it would seem likley that the casing sees the suction pressure given its much lower max pressure.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
A boosted inlet would be similar to a hydrostatic closed loop pump with inlet charge pressure.
Many companies make open loop and closed loop vrsions of the same hardware. The closed loop version usually cannot be run open loop as the inlet ports are too small. (Open loop has only atmospheric pressure pushing fluid in, but closed loop can have 100-400 psi pushing the fluid in, so the ports can be smaller.) The case and seal connections and drains might also be different.
I have no knowledge of that pump so am guessing here, but boosting an open loop version would make the inlet conditions better. Why it could not be boosted beyond certain limits must be seal or case or mechanical design issues as others have already addressed.
These are port plate piston pumps, they have no valves in them. The pumping action is a function of the reciprocal motion of the pistons as they follow the angle of the swash.
Any pump will benefit from high inlet pressure. I still can't see that the pressure limitation on the inlet is anything other than a protection measure for the sake of the seals.
They must have simple non-return valves in otherwise the piston action wouldn't pump anything. Sorry if I confused things by talking about valves instead of NRVs, but that is what I meant.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
Ok, I'll buy that, but some types of axial flow pumps do have valves (wobble plate pumps for instance). The suction pressure issue does look to be seal related somehow and may depend on the actual interanl make-up and design of the pump
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
hydtools had it right in the beginning. Typically all of the pistons/integral shoes are held against the swash plate with one retaining plate that has a spring applying uniform, factory preset, force against the swash plate.
Excessive suction pressure will add to the springs force and hold the shoe so tightly against the swash plate that the fluid film between the two will be lost and scratching, burning, weeping, and gnashing of teeth will occur. .
I am told there is excellent animated video on youtube of Danfoss axial piston chemical pump operation; I couldn't find it though.
It's perhaps a little unprofessional to say this, but that is rubbish.
Any and all pistons that see inlet pressure will see outlet pressure too. How can you say that excessive force applied when the piston chamber is filling is more damaging than when the piston chamber is being emptied with the pressure behind the piston being equal to the system working pressure?
Boosting the pump prolongs its working life and as stated before, there are many piston pumps, bent axis or otherwise that operate as motors. They are effectively boosted to system pressure without suffering damage.
Besides all of the above, there are holes in the pistons and in the slipper to bleed oil onto the swash plate. This ensures that the hydrodynamic film under the slipper is proportional to the pressure in the piston chamber. As the pressure goes up, so the effectiveness of the film between the slipper and swash plate.
Apparently none of us know why and the Bosch Rexroth website does not make it easy to find out why the suction pressure is limited to 30bar absolute as stated. I agree that bleed holes exist to balance slipper forces with piston forces.
Here is a thought, ask Bosch Rexroth. May have something to do with their deltaP limit.
It's been ages since I saw that book...very informative it was.
I've tried really hard to think why they would want to limit the inlet pressure, there is just no obvious explanation for it other than the sealing.
Only one other thought...
Perhaps they have balanced the rotating group to reduce the vibration induced stresses in the pump. If they balance the rotating mass to compensate for a higher delta P between the inlet and outlet, having pressures above 30 BAR on the inlet may unbalance the pump. That would mean the unit couldn't be used as uni directional motor without carrying out mods to the pump?
hydromech, yes a little unprofessional, but coming from the English, not unexpected. (Now THATS Unprofessional!).
The reason I can say " that excessive force applied when the piston chamber is filling is more damaging than when the piston chamber is being emptied" is because the damaging pressure comes, not from the inside of the piston, but from outside of the piston in the chamber that sees suction pressure.
It is certainly the higher discharge pressure inside the piston that causes/initiates the film of fluid to flow from inside piston across the shoe face to the lower suction pressure chamber. This is flow is absolutely necessary for lubrication and cannot be sequestered. The higher the discharge side pressure, actually the better the lubrication! (But then other things start happening that aren't good).
If you will look at most axial piston pump's data sheets you will see that there is a MINIMUM discharge pressure; below that pressure the spring and suction pressure will hold the shoe face down and not allow the discharge pressure inside the piston to flow across the face.
So you have the pump designed for a certain maximum suction and minimum discharge pressure. This will ensure flow across the shoe face. If you add excessive suction pressure to that, then add in the spring pressure holding down the shoes, you will "clamp" the face to the swash plate and stop the necessary lubricating flow coming from within. Bad things will happen.
The excessive suction pressure problem is not as bad with hydraulic fluid as it is so friendly for lubrication, but when using these pumps for water (RO services), or heaven forbid, methanol, the problem is greatly exacerbated.
