In general:
For control elements with a piston design, this states the seal length between the allocated control edges (Figure U 1). in rest position, together with the size of the gap, it determines the leakage (Figure U 2). During the switching process, the overlap results in a characteristic progression of the characteristic curve.
For directional valves the length of the control piston in comparison with the ring nuts results in three variations:
- Positive overlap:
The piston is longer than the ring nut. When switching, the overlap must first be passed through before a flow rate can flow (Figure U 1 a).
- Negative overlap:
The piston is shorter than the ring nut. In rest position, all ports are connected (open-centre position), i.e. there is a flow rate even without a switching movement (Figure U 1 b).
- Zero overlap (zero cut):
The control edges of the piston and ring nut meet with a high degree of accuracy. In this position, there is no flow rate (apart from leaked oil), but it is released even in the case of very small piston movements (Figure U 1 c).
Positive overlap is used in the case of accumulator supplies, in barrier functions and in speed and flow rate controls. Negative overlap is preferred in pump drives, while zero cut overlap is used for control and servo valves (e.g. in the case of position and pressure controls).
Figure U 1: Types of overlap
Figure U 2: Influence of overlap on leakage