I feel that you are missing the point a little on this one.
The question posed was in relation to a Bosch Rexroth bent axis piston pump. These pumps are unique in that they have a maximum specified inlet pressure. There many other similar pumps that operate in the same manner, both as pumps and motors and yet do not have a stipulated maximum inlet pressure.
I don't doubt your logic in terms of the tribology in piston pumps that handle aggressive or non lubricating fluids, but the pumps in question are designed to run with mineral oil. If you run them on anything else, they wont give you a warranty and the main bearing usually fails well before the slippers wear out.
For the sake of clarity, can you explain what you mean here : "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". Outside of the piston chamber is either steel or oil in the pump casing.
In my experience, running these pumps at low or negative pressures tends to separate the slippers from the pistons. So it prolongs the life of the pump to boost it and for all of the reasons list above, the higher boost the better...that is apart from the Bosch Rexroth pumps that seem to dislike excessive inlet pressures for some reason.
For the benefit of the other people that might read this...Having checked again, I see that the minimum outlet and maximum inlet are quoted together and I wonder if they is related in someway???
Call Danfoss in Denmark or email them. They have made use of axial piston theory and modified its uses and designs more than anyone I know of over the years, as hydraulic motors, pumps, water pumps, and even chemical pumps. They have some brilliant engineers who "get" fluid mechanics. One member of the founding family of Danfoss is an incredibly brilliant inventor and has been responsible for much of their innovation.
However much theoretical detail you want to get wound up in; at the end of the day I think you will find that the reason suction pressure has an upper limit is due to maintaining the fluid film between shoe and plate.
You got a response? So far all I've gotten is a catalog in the mail. I was waiting to hear from them before I added to this topic. Thanks for all the input so far.
I have placed a call with Bosch Rexroth HQ in the UK.
They are stumped at the moment, but have promised to let me know when they find out.
A point to note...They said that they only change the port plate to convert a pump to a motor. Just to increase efficiency. A motor inlet pressure will be more than 30 BAR.
Bent axis pumps are generally used for open loop applications, they cannot generally be stroked over centre so they are not much use in hydrostatic transmissions if you want to run motors forward and reverse. Many of the reasons for not putting 30bar into the inlet of an open loop pump have already been covered. The question should be what is the application or reason where you would consider the need to have 30bar on the inlet of a pump? I have seen open loop and closed loop pumps used in many innovative solutions. I am sure such companies like Eaton, Rexroth and Sauer Danfoss can provide solutions for most applications.
