Siemens SINAMICS S120 System Manual page 41

Asymmetrical flow rate characteristic
It is a good idea to use valves with asymmetrical restriction cross-sections for differential
cylinders or for cylinders that are not arranged horizontally and move large loads.
Figure 3-3
Fail-safe position
Directly-controlled servo solenoid valves have a fail-safe position, i.e. the control spool
moves to a safe position when the valve is disconnected from the power supply. The fail-safe
position is either "closed" (A, B, P, T disabled) or "open" (A, B and T connected and P
disabled). It should be noted that the "crossover" switching position must be passed through
when the valve is switched on and off, and there can be temporary responses from the
cylinder at these instants in time. Separate shutoff valves are therefore required in order to
implement safety functions, such as a safety-related cylinder stop.
Precontrolled servo solenoid valves, precontrolled HR servo solenoid valves and directly-
controlled HR servo solenoid valves do not have a fail-safe position and thus do not have a
safe basic position when switched off. Any safety functions must therefore be implemented
via separate shutoff valves.
①
②
Figure 3-4
Hydraulic Drive
System Manual, 04/2015, 6SL3097-4BA00-0BP1
Asymmetrical characteristics
Fail-safe closed
Fail-safe open
Fail-safe position in the valve symbol
Configuration
3.3 Configuring the hydraulics
41