General Instructions; Keep It Clean; Inspect For System Contamination; Replace The O-Rings And Gaskets - Danfoss 51 Series Service Manual

Service Manual
Introduction

General Instructions

General Description

Series 51 Hydraulic Proportional Controls Service Manual

Keep it Clean

You can complete many repairs or adjustments without removing the unit from the machine, if the unit is
accessible and you can thoroughly clean it before beginning any procedures.
Cleanliness is a primary means of assuring satisfactory motor life on either new or repaired units. Clean
the outside of the motor thoroughly before disassembly. Take care to avoid contamination of the system
ports. Cleaning parts with a clean solvent wash and air drying is usually adequate.
As with any precision equipment, keep all parts free of foreign materials and chemicals. Protect all
exposed sealing surfaces and open cavities from damage and foreign material. Cap all hoses after
removal, and plug all open ports. Cover any unattended parts with a protective layer of plastic.

Inspect for System Contamination

Inspect the motor for signs of system contamination. If you find contamination, fully disassemble, clean
and inspect all components of the motor.

Replace the O-rings and Gaskets

Replace all O-rings and gaskets. Discard them only after you make certain that you have the correct
replacement parts. Lightly lubricate all O-rings with clean petroleum jelly before assembly.

Lubricate all Moving Parts

During reassembly, coat all moving parts with a film of clean hydraulic oil. This helps lubricate the
surfaces during start-up.
For fluid quality requirements, refer to 520L0463 Hydraulic Fluids and Lubricants, Technical Information.

Torque Procedure

During reassembly, cross torque all retaining screws to the given value. Do not overtorque.

Overview

Hydraulic proportional controls infinitely vary the motor displacement between maximum and minimum
by feeding a variable hydraulic signal pressure to the end of the 4-way valve directly, or to a piston that
moves the 4-way valve As signal-pressure shifts the 4-way valve, it ports pressure to the ends of the servo
piston, changing motor displacement. A threshold spring and a ramp spring act on the opposite end of
the 4-way valve.

Threshold and Ramp Springs

The threshold adjustment screw varies the threshold spring force required to move the 4-way valve and
start the change in displacement. The ramp spring(s)—2 used in 160cc and 250cc motors, and one used
in the 80 cc and 110cc motors—increase the force on the 4-way valve as the servo piston moves toward
minimum displacement. This provides a motor displacement proportional to the input signal pressure.
The control operating threshold (the signal pressure when the motor starts to shift) is adjustable. Adjust it
using the adjusting screw in the end cap.
Changing ramp spring force requires replacing the springs. There are several spring rates available.
Optional orifices may be installed at several locations to regulate shift speed. Refer to the Model Code for
your motor for details.
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