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Discussion Starter · #1 ·
So I've been studying how our hydrostatic transmissions work. There's one thing I'm confused about and am curious. There's an implement relive valve and a charge relief valve. The charge pump supplies pressure to both of these, right? So how can there be a lower pressure on the charge side, and higher pressure on the implement side? Wouldn't the lower valve dump the pressure?
 

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Ideal,

Not an expert either, but this is what I find from the literature:

Below is the hydraulic schematic for the 318. It looks like the 'charge relief valve' #5 passes pressure to the external pressure port P, whereas the 'implement relief valve' #6 dumps all pressure to the reservoir. By this diagram, the charge relief valve needs to be open to have any fluid going under a MINIMUM pressure out of the transmission unit (to the steering and to ports for implements...) so think of it as the minimum pressure to get anything but wheel motion, whereas the implement relief valve sets the MAXIMUM pressure for the implements. Any help?? Post your thoughts, and other forum members chime in here as well...
Font Rectangle Parallel Schematic Diagram

Chuck
 

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See attached schematic of the 330 hydraulic schematic. The only reason I used this schematic is that it was easy to capture.
Think of the difference between the two relief valves (and the relief settings) as a way to establish the hydraulic system priorities. The first priority is to always insure the hydrostatic system is fully charged between the hydro pump and motor. With the hydro lever in the "Neutral" position (no pump flow/pump at 0 stroke), the Charge Pump flow fills the loop between the Hydro Pump and Motor. Once the loop is filled, pressure builds and the Charge Relief Valve senses that increase. Once the pressure exceeds the Charge Pump setting, then the remaining charge pump flow is directed to the secondary circuit (since the primary circuit has been satisfied). The secondary circuit in the case of the 330 is the hydraulic lever to raise/lower the mower deck. Since this an open-center hydraulic system, if the hydraulic lever is in "Neutral" position, then the secondary flow is directed through the hydraulic valve assembly and back to sump and the implement relief valve is not used/needed.

If the hydraulic lever is moved into either the "Raise" or "Lower" position and the hydraulic cylinder for the mower deck is fully extended or retracted, then the Implement Relief Valve opens, to direct the secondary flow back to sump.
That's why the Implement Relief Pressure is higher than the Charge Relief Pressure, see TM info below.

Charge Pump Pressure . . . . . . . . . . . . 620—1240 kPa (90—180 psi)
Implement Relief Valve Pressure . . . . .5861—6722 kPa (850—975 psi

That's also why the ol girl grunts when the hydraulic lever is held when the deck cylinder is fully raised/lowered (or the 322/332 steering wheel is held in the fully turn position). The charge pump flow has no where to go, so the pressure builds until the Implement Relief Valve pressure setting is reached.
 

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Discussion Starter · #4 ·
Yes, I believe that makes sense to me. The part about the charge valve being there to give priority to the piston pump helped a lot.
So, when a implement is being used, or steering is held at full stop, the pressure to the piston pump shoots to 850-975 also?
I like understanding how things work.
 

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If/when the tractor is moving (either forward or reverse), the Charge Pump flow is directed to the low pressure side of the hydrostatic loop that is returning oil from the Hydro Motor to the inlet of the Hydro Pump. This is accomplished through one of the items #7, depending on which direction the motor is turning. The Charge Pump flow is used to make up any leakage that occurs as the Hydro Motor builds pressure to turn the transmission and help cool the circuit.

As I mentioned earlier, the remaining (i.e. secondary) flow, is then directed to either the Steering Control Valve (on 322's /332') and/or to the Hydraulic Valve Block (330/322/332's). System pressure only builds to what is required to move the Steering Cylinder or Hydraulic Lift Cylinder. If the required pressure is lower than the Implement Relief Valve setting, the Implement Relief Valve stays closed (since the circuit has sufficient pressure capabilities to move the load). If/when the required pressure is higher than the Implement Relief Valve setting (steering cylinder and/or mower deck cylinder fully extended/retracted - i.e. can't move further), then the Implement Relief Valve opens protecting the system.

Or to say it another way, the Charge Pump pressure will vary depending on the pressure requirement to move the secondary circuit load, up to the Implement Relief Valve setting (which is when either the Steering Cylinder or mower deck cylinder is fully extended/retracted).

Since this is an open center system, the available Charge Pump flow (after satisfying the primary circuit, i.e. the hydrostatic circuit) being directed to the secondary circuit has to go somewhere. If a secondary circuit cylinder is actuated, then the pressure (ie resistance to flow) builds until it is sufficient to overcome the load and the Implement Relief Valve stays closed. If the load cannot be overcome, or the cylinder is at full stroke (either fully extended/retracted), the Implement Relief Valve opens to direct the flow to sump (so the Charge Pump flow has somewhere to go).
 
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