Insane Hydraulics
Bold and audacious blog about fluid power
The DFR/DFR1 controller of the ubiquitous Rexroth A10VO (sizes 18...100) hydraulic pump has the so-called "pulsations orifice" (or better - a "double orifice plug") in it:
I am calling it "pulsation orifice" because this is how it used to be referred to in the Hydromatik service manuals. The double-orifice plug can be put in the open/closed position by turning it with a flat-head screwdriver. The controller that came with the orifices closed (i.e. turned by 90ยบ) was called "DFR-SO74" (whatever this means). I don't think I ever came across one, though.
In any case, a bleed orifice in a hydraulic control is a sure way to dampen it, and thus make it more stable (less prone to self-excited oscillations), so if your A10VO is doing "wow-wow-wow" instead of the usual "z-z-z", the said orifice may be clogged, and you can fix this by cleaning it.
The "official disassembly instruction" is to plug the tank port and blow compressed air into the servo-cylinder hole of the controller, however, sometimes the orifice is so stuck that compressed air is not enough. I want to show you another way to remove it that doesn't require any special tools (almost):