Siemens SINAMICS S120 System Manual page 238

Appendix
A.3 Hydraulic
Zero overlap at the middle position
A continuous valve must have zero overlap around its middle position if it is to be used in a
position control loop.
A positive overlap will be perceived negatively in the form of a dead zone of the final control
element.
In contrast, a negative overlap results in a marked increase in oil leakage.
To achieve zero overlap, valve spools, spool housings and spool sleeves must be
manufactured with extreme precision and made of wear-resistant materials. The production
costs incurred are correspondingly high.
To maintain the zero overlap over prolonged operating periods, it is essential to ensure that
a clean pressure medium is used (to prevent erosion of control edges).
Figure A-4 Zero overlap at the middle position
238
System Manual, 04/2015, 6SL3097-4BA00-0BP1
Hydraulic Drive