Overview of hydraulic systems

I decided to systematize information about hydraulic systems.
Let’s start with the basics, one by one.
Each hydraulic steering system used in agricultural machinery consists of:

  • hydraulic pump feeding the system
  • control valve - orbitrol
  • control actuator (piston)
  • a priority valve is also often used

Applied solutions:

Open circuit (OC)
This is the simplest system, it consists of a pump directly connected to the open-type orbitrol, i.e. when you do not turn the steering wheel, i.e. in the neutral position, the oil flows freely through the orbitrol to the reservoir.

Closed circuit type CC

In this type, in the P line which supplies the orbitrol, the pressure is constant, e.g. 170 bar, the orbitrol is of the closed type, i.e. when the steering wheel is not turned, the P line is cut off. This type is used on older JD models, Claas Lexion combines

CC closed circuit with load sensing Ls Static or Dynamic

In this type there is a priority valve which is controlled by the Ls line and redirects the flow from the pump proportionally to the demand to the orbitrol when it is needed, i.e. when you turn the steering wheel. The priority valve maintains a constant post-stroke pressure in the P-line from 10 to 50 bar (depending on the vehicle). Only the connection of the Ls line with the P line causes the pressure to increase to the required level.
Ls Static - in the Ls line (if you do not turn the steering wheel), the oil does not flow in any direction. To increase the pressure in the P line, it is necessary to connect the Ls line with the P line
This type can be found on the Fendt 700, 800, 900 tractors
Ls Dynamic - in the Ls line, oil flows towards the orbitrol all the time and there must be a connection with the T (return) line. It is enough to block the connection of the Ls line with the T line to maximize the pressure in the P line.
This type is most common in Ls systems.
It is found in all JD of Ls
All NH and Case
Massey Fergusson, Valtra, Claas, DF and others.

System type with orbitrol with built-in priority valve.
This type uses a CCLs orbitrol, but the priority valve is coupled to the orbitrol and the Ls line does not extend beyond the valve block. The EF line is then used to power other components on the tractor.
This system can be found, for example, in the Fendt 300.
It is also present in the Fendt 400, but in this model there is an additional valve reducing the pressure in the system when we put the reverse in the neutral position and this must be taken into account when designing the AS system.


Good job!

And then on the John Deere 8x20 series, the return pressure from the OC orbitol valve gets regulated to ~300 PSI and feeds the diff-lock solenoid, PTO solenoid, and the power shift solenoids. The brake cylinder has a load sense coupled to the orbitol valve that if the oil pressure drops out, the brake receives all oil, and the steering and the rest of the system receives nothing.