Insane Hydraulics
Bold and audacious blog about fluid power
This is a Rexroth A7VO55LR3E/61L hydraulic pump from a Volvo excavator. It has a very interesting control combo, which is not described in the standard A7VO catalog - the main reason I am putting it in the back-engineering section - as a nice example of how torque-limiting and high-pressure cut-off function can be achieved by "pushing a single spool".
Function-wise there is nothing too complicated here - it is basically the standard LRD control "enhanced" with override options for the high-pressure cut-off and the torque limiter (for the second pump torque summation).
The torque limiter function is based on the good old L-shaped rocker principle (a good simulation of hyperbolic controls can be found here, by the way). The torque setting override is done by a small two-diameter spool, that works between the torque limiter 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 high-pressure cut-off function that I would like to describe in more detail. The control schematic doesn't offer a clear explanation of how it's designed. It can even give you an idea that the control operates with two spools, which would be wrong. I think the operation principle is best seen on the cutaway view (note: torque summation is not shown):
The pressure cut-off function is achieved by the small two-diameter spool that is 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.
As you can see, the control operates through a single spool that can be shifted by two forces:
Due to this clever placement, the two forces do not add up, which means that at any time the stronger one prevails.
The force created by the annular area has to balance the cut-off spring only, 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 weaker than the one in the "torque-limiting department".
I like it when design engineers are brave enough to use what I call "serial force placement" in control valves. My head starts to hurt as soon as I even begin to think about it!
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