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     Here you have it  - a Rexroth A7VO55LR3E/61L open circuit hydraulic pump. So far all of these pumps that passed through our shop came from Volvo rotary excavators, but I have no information if this is a protected control or not. The main reason I am putting this pump into the back-engineering section is because I couldn't find the complete description of this control option in the standard Rexroth catalogue, and also because it was this very pump that led me to wring the article on control self-oscillations. I also think that this is a nice example of how these A7VOs manage torque-limiting and high pressure cut-off function - by means of "pushing a single spool". From now on, whenever I am asked a question about how these controls work - I will be able to redirect the "wondering" to this page.

     There is nothing too complicated here - it is basically the standard LRD control "enhanced" with override options, both for the high pressure cut-off and the torque limiter (see the hydraulic diagram).

  The torque limiter function is based on the good old rocker principle, the description of which you, again, can find in any of the many A7VO standard model catalogues available for download all over the internet. The torque setting override assembly is nothing but a small spool with two slightly different diameters, that works between the torque limiter adjustable spring and the rocker. The pilot pressure (which is the outlet pressure of a similar pump, working next to this one) is acting on the small annular area, "helping" the torque limiter piston to compress the spring. This feature assures limitation of the sum of torque of two pumps driven by the same engine.

   It is the cut-off function that I would like to describe in more detail. Since the symbol schematics doesn't give you clear understanding on how this control is designed, it is best to study the operation principle of the high pressure cut-off function on this cut-view. The pressure cut-off function is performed by means of the small two diameter spool, placed inside the main spool, with the annular area connected to the pump outlet. A steel pin between the rocker and the combination of spools transmits the momentum, created by the torque limiter piston to the main spool.

    Although the simplified symbol schematics can give you a wrong idea that this control operates with two spools, it actually operates by means of one single spool, that can be shifted with two forces - one resulting from the system pressure acting on the torque limiter piston, and transmitted to the main spool by means of the L-shaped rocker and the pin, and the other resulting from the system pressure acting on the annular area inside the main spool (between the main and the small spool, sitting on top of the connecting pin - cut-view). Due to this clever placement, the two forces do not add up, which means that at any time the stronger one prevails. However the pre-adjusted springs were also strategically placed so that  the  force created by the annular area has but to balance the cut-off spring, while the torque-limiter piston has to "fight" both of the springs. It must be said, that although increasing the cut-off setting does affect the torque limiter setting, the increase is not that significant, since the spring is much weaker than the one in the "torque-limiting department".

    To the left you can find the complete control spools layout, and also the close-ups of the torque limiter override and the presusre cut-off override
Rexroth A7VO55LR3E
Main Spool and Orifice in T1
Torque Limiter Pin
Torque Limiter Assembly
Servo Piston Orifice
A7VO55LR3E Control Spools Arrangement
Torque Limiter Setting Override
High Pressure Cut-Off Override
A7VO55LR3E Hydraulic Diagram
A7VO55LR3E  Control Spool Detail