With input current of positive polarity, the flow direction that fluid flows off from control port 1 and then enters control port 2 is specified as positive flow polarity. PRESSURE CHARACTERISTIC
Pressure characteristic is usually measured within a null range of 10% of rated current when control ports are blocked.
Both valve pressure gain and null bias can be estimated from pressure characteristic curve.
Pressure gain will degrade with spool null edge wear. INTERNAL LEAKAGE
Internal leakage characteristic is measured when control ports blocked.
Internal leakage flow will increase with spool null edge wear. NO-LOAD FLOW CHARACTERISTIC
No-load flow characteristic is measured at zero load pressure drop.
Rated flow, hysteresis, null bias, linearity, symmetry, etc. can be estimated from no-load flow curve.
For a zero lapped servo valve (tolerance of ±2.5%), its flow gain may be from 50% to 200% nominal gain within the null range of ±3%. FLOW-LOAD CHARACTERISTICS
Valve flow to load can be estimated by following formulas:
where： Null bias
Null bias is measured under standard valve operating conditions and exclude from valve hysteresis. It is allowed that valve null bias gets to 6% during its useful lifetime. THRESHOLD
Threshold is measured near null range (10% of rated current ).
Threshold will degrade with supply pressure decreased, spool worn, or fluid conraminated. LAP
Unless stated in order forms, the servo valves provided are of zero lap. For a servo valve with zero, its tolerance in lap is from 2.5% overlap to 2.5% underlap.
Prescribed amounts of overlap or underlao can be specified on special order.
Underlap (or open center) increases flow gain at null, reduces valve pressure gain at null, and increase valve null leakage flow.
Overlap (or closed center) reduces flow gain at null, degrades pressure gain(into a loop), and reduces valve null leakage flow.