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Service Manual
Adjustments
MDC Neutral start switch (NSS) adjustments
Series 90 Pumps
The Neutral Start Switch (NSS) provides a means to prevent the system prime mover from starting while
the pump control handle is in-stroke. When the control input shaft is in its neutral position, the switch pin
engages a slot on the eccentric cam attached to the control shaft. This allows the spring loaded NSS to
close, completing the electrical starting circuit for the prime mover. When the control input shaft is not in
its neutral position, the eccentric cam opens the NSS switch.
The neutral start switch is threaded into the special lock nut for the eccentric plug. Turning the NSS
clockwise (CW) moves the NSS closer to the switch cam on the control shaft, and narrows the NSS
deadband. Turning the NSS counterclockwise (CCW) moves the NSS farther from the switch cam on the
control shaft, widening the NSS deadband. The switch pin is located in an eccentric plug which is turned
to move the center of the NSS deadband.
MDC with neutral start switch / View at section A-A
A
Control shaft
Switch cam
Eccentric plug
Switch pin
Special lock nut
for eccentric plug
Orifice check valve seat
Control spool assembly
Control link assembly
View A-A
Adjust the Neutral Start Switch to meet the following three requirements:
1. The distance you can move the control handle without opening the NSS is the NSS deadband. The
distance you can move the control handle without moving the control spool enough to port
hydraulic fluid to the pump displacement control cylinders is the control deadband. These deadbands
must be concentric.
Since you cannot adjust the position of the control deadband, you must adjust the position of the
NSS deadband to match it.
2. The NSS deadband must be wide enough so the NSS will not open within the loose area of control
handle movement caused by normal operating clearances in the control linkage (control shaft free-
play).
By setting the NSS to open outside this area, the control spool springs or control shaft centering
spring can always return the handle to neutral and re-close the NSS.
3. The NSS deadband must be narrow enough so the NSS will open before the unit builds 7 bar [100 psi]
differential system pressure in either direction.
520L0818 • Rev 0101 • August 2015
A
Switch lock nut
Neutral start switch
P104 146E
35
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