Electrical solenoids
Electrical solenoids are used to switch hydraulic valves ON or OFF. The top symbol shows a solenoid that pushes in the direction of the valve element (shown by the dotted lines). This is by far the most common type of solenoid actuation.
The middle solenoid symbol shows a direction of actuation away from the valve element.
The bottom symbol shows an electrical control mechanism with two coils that will both push and pull the valve spool.
Caution must always be shown when analysing a circuit to this detail. It is likely the designer has shown the symbol drawn incorrectly but you should always double check especially if it does not match the part code or components supplied.
Mechanical operation
These symbols show mechanically operated valves.
The top symbol shows a manual lever operation
The second symbol shows a roller activate valves.
The third symbol shows a manual push button
The bottom symbol shows a foot pedal activation.
Valve control options
There are generally a wide range of valve actuation types available and they are usually not all shown in the manufacturers standard datasheets.
The top symbol here shows a push/pull mechanism with a detent to hold it in one position.
The middle symbol shows a turning mechanism with 4 fixed detent positions.
The bottom symbol shows an electrical solenoid that is continuously controlled. The actuation is towards the valve element.
Two-stage valves
The black triangle shows a two stage, pilot operated valve. The direction the triangle is pointing represents the direction of fluid flow e.g. either remote pilot pressure coming in our fluid drain leaving the valve.
The top symbol shows a continuously controlled, bi-directional, electrical solenoid, with mechanical spool position feedback. Typical of a traditional servo valve.
The middle symbol shows a two-stage valve with an external pilot drain line.
The bottom symbol shows an electric stepping motor driving the valve.