Servo and proportional valve symbols

Understand how to read servo and proportional valve symbols

Proportional directional valve

proportional valve symbols

At first glance the proportional valve symbol looks very similar to that of a directional valve. However, the symbol includes several key differences from that of the ON/OFF style directional valves. 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 top 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 bottom symbol shows a servo-valve with a preferred position in case of a power failure, electrical feedback and integral electronics.

Proportional flow control valve


A proportional flow control valve is one that increases its orifice opening area gradually rather than instantly. These proportional valve symbols show a simple two way, two position spool valve with the proportional spool shown by the two additional lines surrounding the valve element. The first symbol is electrically controlled and the second symbol is controlled by a roller that will be in mechanical contact with another object. The third symbol is a electrically controlled variable orifice device.

Proportional pressure control valves


All pressure control valves operate in proportion to their pilot pressures, so the symbol does not include the extra lines around the box that designates the proportional version of a directional valve. The key difference in the symbol between the standard on/off electrical control and a proportional electrical control is the arrow through the solenoid and possibly the dotted triangle symbol to show internal electronics.

Proportional pressure reducing valves are very common on mobile control block as they are used to apply pressure on the ends of the spools that control the proportional directional valves.

Mobile proportional slice valve

mobile proportional slice

The majority of mobile monoblock and slice valves have proportional spools. The control of speeds and flows is critical for these applications so good control of speeds and accelerations is important. It's worth noting that each spool will be individually specified to suit each load. The symbols don't show any differences in cut but the spools mustn't be replaced in different bodies.

Learn more about proportional valves