________________________________________________________________________________

Massey Ferguson 8220 Tractor steering system

The steering system used on Massey Ferguson 8220, 8250 tractors is of the dynamic type (constant flow rate of 0.5 l/min - standby pressure of 6 bar. There is no mechanical linkage between the steering wheel and the steering ram.

The Orbitrol type is marked on a plate riveted to the lower part of the unit (Fig.48). Its control principle is of the ‘Load Sensing’ type.

The MF 8220, 8250 tractors steering system comprises the following main components:

- a pressurized oil feed from the priority block(s) supplied by the variable displacement pump,

- an Orbitrol closed centre steering unit, whose capacity differs according to the tractor type, installed in by-pass (Orbitrol OSPC 125 LS Dynamic 2WD - Orbitrol OSPC 200 LS Dynamic for 4 WD),

- a double acting ram fitted either on the 4 WD front axle or on the 2 WD axle beam.

The Orbitrol unit is supplied via the priority block(s) by the high flow rate circuit. When the steering wheel is turned, the necessary flow of oil is directed to the corresponding side of the steering ram.

The flowrate in excess, not required by the ram, flows through the return ports to the selector cover of the Heavy Duty gearbox or to the righthand side of the Full Powershift gearbox (according to version).

In the case of an engine breakdown or failure of the hydraulic pump, the Orbitrol acts as a hand-operated pump so that the steering can be controlled.

Fig.48/49 Orbitrol OSPC (Parts list)

(1) Bolt, (2) Bolt, (3) O-ring, (4) Cover plate, (5) O-ring, (6) Stator, (7) Pin, (8) O-ring, (9) Rotor, (10) Spacer, (11) Link shaft, (12) Washer, (13) Centring springs, (14) Needle bearing, (15) Bush, (16) Washer, (17) O'ring, (18) Relief-valve, (19) Seal, (20) Shock valve, (21) Orbitrol unit (distribution valve), (22) Non-return valve, (23) Suction valve, (24) Bush, (25) Spool valve, (26) Non-return valve, (27) Sleeve, (28) O'ring, (29) Distributor plate

Fig.50. Layout of channels and ports

A - Steering ram, B - Location of steering column, L - Supply to left-hand side union of the steering ram, LS - Signal to variable displacement pump via the priority block(s), P - High flow rate supply from the variable displacement pump, R - Supply to right-hand side union of the steering ram, T - Return to the housing via the selection cover or the gearbox (depending on transmission version), 1 - 2WD axle, 2 - Dana front axle, 3 - Carraro fixed front axle, 4 - Carraro suspended front axle

Disassembling and reassembling the Orbitrol steering

Disassembly the Massey Ferguson 8220, 8250 steering unit

Remove the distribution valve from the tractor. Place the distribution valve in a vice fitted with plastic jaws

Take out the bolts (1). Carefully mark the location of the bolt (2) and then remove it.

Remove the cover plate (4), the O'ring (5), the stator (6) and O'ring (8).

Remove the spacer (10), the rotor (9), the distribution plate (29) and the O'ring (28). Take out the splined link shaft (11).

Unscrew the threaded bush and take out the valve ball from the non-return valve (22).

Take out the two axle pins and valve balls from the suction valves (23) of the distribution valves.

Extract the sleeve (27) and spool valve (25) assembly by pushing it out while checking that the pin (7) lies along the horizontal axis.

Remove the washers (12) (16), the needle bearing (14) as well as the bush (24) from the sleeve and spool valve assembly. Remove the pin (7), and the centring springs (13) by pressing on their ends. Separate the sleeve (27) from the spool valve (25).

Unscrew the plug from the relief-valve (18). Using an 8 mm Allen key, disassemble the threaded bush and remove the seal, the spring and the valve (the crimped seat cannot be removed).

Unscrew the two plugs from the shock valves (20) and remove the seals. Using a 6 mm Allen key, remove the threaded bushes and take out the springs, the valve balls and their seats (the crimped seats cannot be removed).

Extract the seal (19), the bush (15) and the O'ring (17). Disassemble the non-return valve (26).

Reassembly the MF 8220, 8250 steering unit

Check and clean all components, replace those defective. Lubricate with clean transmission oil before reassembly.

Reassemble the non-return valve (26). Fit the seal (19), the O'ring (17) and the bush (15).

Place the valve balls and springs in the recesses of the shock valve (20). Screw in the threaded bushes, fit the seals and tighten the plugs.

Fit the valve and spring in the recesses of the reliefvalve (18), screw in the threaded bush. Fit the seal and tighten the plug to a torque of 40 - 60 Nm.

Insert the Massey Ferguson MF 8250, 8220 tractor spool valve (25) into the sleeve (27). Position the centring springs (13) according to Fig. 48 and insert the pin (7).

Position the bush (24) on the sleeve and spool valve assembly so that the chamfer facilitates assembly in the steering unit.

Place the washers (12) (16), the chamfer of washer (12) towards the centring springs (13), by inserting the needle bearing (14) between them.

Fit the sleeve and spool valve assembly in the steering unit by oscillating it slightly. Check that the pin (7) is held horizontally.

Place the two valve balls and the two pins in the recesses of the suction valves (23).

Fit the valve ball in the recess of the non-return valve (22) and screw in the threaded bush.

Position the splined link shaft (11). Fit the O'ring (28), the distributor plate (29).

Fit the rotor (9) so that the two concave depressions "C" lie along the axis of the slot in the splined link shaft (11). Refit the spacer (10).

Place the O'rings (5) (8) on the stator (6).

While avoiding moving the rotor (9), fit the stator (6). Then move it so that its fixing holes match with those of the steering unit. The rotor (9) and the pin (7) must be in the position shown in Figure 50. Refit the cover plate (4).

Refit the bolt (2) (in the same location as marked during disassembly) and the bolts (1) fitted with their seals (3). Tighten opposing bolts to a torque of 30 - 35 Nm.

Using a test-bench or a suitable fixture, check the adjustment and correct operation of the steering unit.

Refit the steering unit on the tractor. Check the oil tightness of the unions.