Siemens SINUMERIK 840D Description Of Functions page 35

02.99
"Fail-safe"
position
Asymmetrical
characteristics
P
A
P
B
A B
P T
Fig. 2-12 Asymmetrical characteristics
 Siemens AG 2000. All rights reserved
SINUMERIK 840D/SIMODRIVE 611 digital Descr. of Functions HLA Module (FBHLA) – 04.00 Edition
Directly actuated, servo solenoid valves have a "fail-safe" position which they
assume when the power supply is disconnected so as to ensure that the cylin-
der remains stationary. However, when the power supply is connected and dis-
connected or the valve is forced to slide through the crossed position, brief
reactions at the cylinder end cannot be precluded.
An absolutely safe stop with loaded cylinder (e.g. vertical mounting position)
can only be guaranteed with a "fail-safe" valve in conjunction with additional
shutoff valves.
Pilot-actuated servo solenoid valves and HRVs have no defined "fail-safe" posi-
tion. They assume a zero-overlapped mid-position (not a safe stop position)
when the supply is disconnected.
Fail-safe disabled (a)
A B
P T
Fig. 2-11 "Fail-safe" position (symbol)
Pilot-actuated servo solenoid valves with asymmetrical throttle cross-sections
are used to achieve a sufficiently powerful throttling effect when lowering vertical
differential cylinders with suspended load on the piston rod.
Q
A
v
Q
B
G
2.3 Configuring the hydraulic drive
Fail-safe enabled
A B
P T
+Q
–U
E
–Q
2 Configuring
+U
E
2-35