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Hydraulic Circuit Controls?

  1. Feb 25, 2016 #1
    I don't know if this is the place to ask this, but I am a mechanical engineering student about to finish a degree, and I have recently been researching hydraulics. After watching a rear loading garbage truck eat bag after bag going down my street, I can't help but wonder a few things. I understand how hydraulic circuits work, such as power steering systems, but how does the engine of the garbage truck know to increase speed (rev up) when the operator opens the valves to the circuit? And I have seen the operator pull both handles for the sweep and slide mechanisms together, and walk away from the truck. How does the truck know to run the sweep cycle first when both levers are pulled at the same time, deactivate the sweep lever when the slide phase begins, and automatically shut the system down when complete? I've searched and searched but cannot find what I am seeking. I appreciate any responses, thank you.
  2. jcsd
  3. Feb 25, 2016 #2
    Having worked on garbage trucks, and some other hydraulic systems, there's a couple of things that could be going on with the truck when it revs up. Depending on how new the truck is, it could either have an always on pto with a bypass valve built in. Basically the Hydraulics are always at full pressure, and the bypass valve has them shut down and the fluid freely circulating through the truck (this is common on systems that assist the transmission or steering of a truck), when he opens the bypass valve the and pressure drops the bypass valve closes and the fluid runs back into hydraulic arm, in turn the pump puts a load on the engine and with any type of throttle control the motor will in turn rev to stay at a preset rpm range depending on the type of throttle control.
    Some of the brand new ones that the county I'm in just bought (due to the automatic sweep control you mentioned i think this is more like it) some of your newer hydraulic systems are electronic, and controlled by a bunch of relays, and off/on switches and actuators. In this mode usually the hydraulic system is independent, and it seems strange to me, but has a bunch of smaller hydraulic motors, most of them electric instead of pto driven from the engine.
    since you hear the truck rev it sounds like a hybrid system, one where the pump is always on, but when he hits the switch, it's actually an electric actuator that operates a relay bank attached to a bunch of electric valves attached to pressure side of the hydraulic cylinders that operate the different systems. A lot of the newer john deer and bobcat skid loaders use this. (It looks like a bad science fiction movie in the electrical panel of these)

    I hope this helped, if you can get an approximate year and make on the body and name off of the switching valve, I can pinpoint it for you. I've probably worked on a similar system and maybe even have a book about it.
  4. Mar 9, 2016 #3
    there are alot ways to run the system if you look at the front of your truck it may have a hydraulic pump couple directly to the engine,running load sensing pump, though valving made up of joystick controller using travel limit switch cylinder control so pull lever engine with be sent signal via load sensing putting pump on stroke
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