Specifications, Service and Repair of Farm and Lawn Tractors

Massey Ferguson 7618, 7620 hydraulic system - Electrohydraulic auxiliary spool valves

The Bosch/Rexroth SB23 LS spool valves controlling supply to the hydraulic couplers are comprised of: an hydraulic part; an electric hydraulic part.

Electric hydraulic part comprising:

- an ON/OFF solenoid valve and a 3-way pressure relief valve located on the end plate of the distribution block,
- a pilot valve fitted in the electric hydraulic unit.

MF 7618, 7620 electrohydraulic control spool valves are supplied by high flow rate, high pressure oil from the priority blocks.

When no functions are being activated, the entire flow rate is directed towards the spool valves. When certain functions are being supplied, the excess flow then remains available to the auxiliary spool valves.

The adjustment of the flow rate, expressed as a percentage, may be displayed on the onboard computer (Datatronic) by the operator.

Joystick-controlled electrohydraulic spool valves are fitted with a floating Kick-out position (automatic return to neutral). The main spool of the spool valve directs oil towards the outlet ports A or B.

Each outlet port is linked to the LS pilot line of the variable displacement pump regulating valve via the priority block(s).

The spool valves consist of both electronic and hydraulic components, the latter containing spools and valves. Some elements cannot be repaired as spare parts.

Different types of spool valve block components

A spool valve block can consist of three, four or five of the following components. It can combine components of similar type of or different types.

Component a

Electrohydraulic unit (Joystick)

Component b

Electrohydraulic component (Dual Control)

Component c

Mechanically controlled component.

Couplers



The quick-disconnect couplings are held by a bracket on the centre housing. They are sealed by covers of different colours (red, green, black or yellow) corresponding respectively to hydraulic ports A and B of auxiliary spool valves.

When the Massey Ferguson 7618, 7620 auxiliary spool valve is at rest, a lever system (1) allows existing pressure to be relieved into the hydraulic hoses (2).

These hoses link the spool valves with the concerned couplers (3). By acting on this system, the clean oil under pressure is directed to the return via a hose and pipe assembly (4) connected to the upper part of the intermediate housing.

This drop in pressure facilitates the connecting of the male coupler to the female coupler. The contaminated oil, coming from the separation of the couplers, flows into flexible and transparent tanks (5) located on either side of the third-point linkage.

Pressure relief valve and electrohydraulic unit operation

When the ON/OFF solenoid valve located on end plate is open, the flow from the pump is directed to the 3-way pressure relief valve.

The 3-way pressure relief valve supplies the pilot valve with a pressure of 21 - 24 bar via a pressure balancing valve.

The pilot valve receives a signal from the digital device and moves the main spool of Massey Ferguson 7618, 7620 auxiliary electric hydraulic spool valve according to the information received from the Joystick.

The digital device is informed of the position of the main spool by a displacement sensor. The pilot valve and the digital device are housed in the unit fixed below the spool valve.

The pilot valve is protected from possible oil contamination by a set of FP filters (25 microns). Electronic unit channels P and R are connected to channels P1 and R1 of the auxiliary electrohydraulic spool valve.



(1) Main electric hydraulic valve spool (2) Electrohydraulic unit (3) Return spring (4) Piston (5) Pilot valve (6) Digital electronic system (7) Displacement sensor, FP - Set of filters (25 microns), P - Pressure, R - Return

Operation of MF 7618, 7620 auxiliary electrohydraulic spool valve



When spool (1) is moved electro-hydraulically upwards by the pilot valve (5), oil penetrates via channel P and travels to the flow regulation control spool restrictor (5) and the grooves of the main spool (1).

The oil is then carried towards a main channel and lifts the ball of valve to enter a chamber. Displacement of the spool (1) simultaneously opens the one-way valve (3).

The oil flows from the chamber towards port B and returns to the housing through port A via the hydraulic slave device. The operating principle is identical when the spool (1) is electro-hydraulically moved down by the pilot valve (5).

The oil flows from the chamber towards port A and returns to the housing through port B via the hydraulic slave device. The electrohydraulic spool valves can be manually controlled using a lever screwed onto the reversing lever.

 

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