Based on ISO 1219-1 and 2
The valve shown is a 4 way two position valve. The '4 way' means it has 4 pipe connections, generally Pressure, Return, Port A and Port B. The 'two position' means that it has two switched positions i.e. it can sit in the A or B position.
Nex,t we have a 2 position 2way valve.
Finally, we show 3 way two position valve.
The valves shown above are all drawn with the de-activated position e.g. pushed over by the spring, not the solenoid.
Here we have a 4 way 3 position valve shown in the centre position. This means it has four pipe connections to the valves but 3 different switched positions possible.
Note how this valve now has two springs instead of one. With no signal to the valve, it will sit in its central position blocking flow to all ports.
When the electrical solenoids are operated the spool position moves to either the right or left-hand position.
The second symbol shows a 2 position detented valve. This valve has no springs but instead, the spool is held in position by a mechanical detent when electrical power is removed.
Finally, the third symbol shows another 4 way 3 position valve but with a different centre position layout. In this valve, the A and B lines are connected to tank instead of blocked, as shown above.
Hydraulic valves can be activated by in many different ways. The symbols here show an electrical solenoid, spring, electrical with hydraulic pilot, hand emergency, proportional solenoid, manual lever and foot operation.
Note how the hydraulic pilot is shown as a solid triangle although if this was a pneumatic pilot it would be shown as solid triangle.
This symbol shows a complete pilot operated directional valve. This would probably be a large, high flow, hydraulically operated bottom valve with a smaller electrically operated pilot valve.
The solenoids show the hydraulic pilot as well as an external pilot pressure supply X and external pilot pressure drain Y.
The symbol also shows and open centre P to T spool.
The proportional valve symbol includes several key differences from the ON/OFF type directional valve. Firstly the outside of the spool section contains lines either side. The solenoids also have arrows running through them to show they operate gradually rather than just on and off.
The square box with the triangle inside represents internal electronics and the box with the G and upturned U represents closed loop control of a particular valve stage.
The first symbol shows a directly controlled proportional valve.
The second is a proportional directional control valve, pilot-operated, with closed-loop position control of the main and pilot stages, with integrated electronics.
The third is a servo-valve, pilot-operated, with closed-loop position control of the main and pilot stages, with integrated electronics.
The forth is a servo-valve, pilot-operated, pilot stage with electrical control mechanism with two coils, continuously controlled in both directions, with mechanical feedback of the valve-spool position to the pilot stage, with integrated electronics.
The final symbol shows a servo-valve with a preferred position in case of a power failure, electrical feedback and integral electronics.