There are two types of lift fitted on the Super Major, Dexta, and Super Dexta tractors. The first type is the single acting lift fitted to all Super Major tractors up to 1963 and Dexta and Super Dexta tractors throughout the life of the marquee. The second is the double acting lift fitted to New Performance Super Major tractors after 1963.

The wording single acting and double acting refers to the operation of the draft control linkage rather than the output of the hydraulics. On all top link sensing tractors, in draft control (Qualitrol), pressure on the top link is used to measure the load being applied to the tractor by the implement. When an implement, mounted on the third point linkage, is working in the ground, a force (draft) is trying to rotate the implement around the lower link connection. This causes a pressure or tension on the top link that is transmitted to the control valve through a series of external and internal springs. The main control lever is not connected directly to the control valve but is so arranged that it can change the spring force required to move the control valve from the external linkage or override this linkage and cause the control valve to move into the raise position. As the lift reaches the fully raised position, the piston protrudes clear of the ram cylinder bore and contacts on a pin fixed in the linkage that causes the control valve to move to the neutral position. When the lift is lowered, the lever moves in such a way as to remove all spring pressure from the control valve linkage and allows the spring mounted on the front of the control valve to push the valve into the lowering position.

In operation, the load (draft) is set by the position of the main control lever in its quadrant. The nearer the top of the quadrant, the less pressure is required on the top link for the lift to raise. As the lever is moved down the quadrant, a greater pressure is required before the lift raises. If a tractor and implement is working in light soil, the lift control lever may have to be close to the top of the quadrant to hold the implement at the required depth in the soil. This is because the draft forces in light soil are low and the lift needs to be sensitive to small changes in the force. In heavy soil the lift lever will have to be further down the quadrant for the implement to retain the same working depth as above. This is because the heavier soil imposes greater draft forces on the top link. These corrections can be further controlled by a flow control valve fitted to some tractors, which adjust the flow of oil to the ram cylinder, and causes the lift to raise more slowly when in work. This avoids the implement being lifted completely out of work but allows it to lift enough to reduce the load on the tractor.

Draft control also allows the wheel slip on the tractor to be controlled. As the tractor’s wheels start to slip, soil is removed from beneath them by the tyre treads causing the rear of the tractor to sink. As the rear sinks, pressure is increased on the top link by the sinking as the tractor changes its position in relation to the implement. The lift senses the increase in pressure and raises the implement causing the weight of the implement plus draft to be applied to the rear wheels. This action reduces wheel slip and the tractor moves on. If wheel slip is a problem, weight can be added to the tractor to overcome this. On tractors operating three point linkage equipment, this weight should be added to the front of the tractor to stop the front from lifting, and to the implement so that tractor and implement pivot on the rear wheels. Weight added directly to the rear wheels is dead weight and is only of use when dragging implements from the drawbar.

It is important to note that draft control (Qualitrol) is NOT a depth control system. It is a LOAD control system controlling the load placed on the tractor by an implement within the parameters set by the operator, by the position of the control lever.

On the early Super Major, Dexta and Super Dexta tractor the hydraulic lift, in draft control, responds to pressure on the top link. This causes the implement to raise. Once the pressure on the top link is released, the implement is lowered. On the New Performance Super Major, pressure on the top link causes the lift to raise and when the pressure is reduced the lift will lower. But with longer or heavier implements working in light soils, compressive force on the top link may not be generated. The lift is designed to work with both pressure and tension forces acting on the top link hence the double acting designation.

Setting of the lift differs between the 1958 to 1964 Dexta and Super Dexta, the 1960 to 1963 Super Major and the 1963 to 1964 New Performance Super Major.

Draft Control Setting for: 1958 to 1964 Dexta and Super Dexta; 1960 to 1963 Super Major
Draft Control Main Spring

• Select draft control.
• Lower the lift arms and set the lever at the bottom of the quadrant.
• Remove the pin that connects the top link yoke to the spring link.
• Turn the spring link on the thread until all movement is taken out between spring and spring link and spring pressure is felt.
• Re-connect the top link yoke.

Draft Control Setting for: New Performance Super Major 1963 to 1964
Draft Control Main Spring

• Follow all steps as described above.
• Turn the spring link until all movement is taken out of spring; then tighten until movement returns.
• Back off to position of minimum/no movement.
• Re-connect the top link yoke.

Pressure Test

The simple way to pressure test is to connect the pressure gauge into the external services port. A pressure gauge reading at least 4,000 psi (285 kg/sq.cm) will be required.

• Connect the gauge.
• Set engine revs at 1,600 rpm (note: if at all possible, drive the tractor beforehand to warm up the engine).
• Run the tractor until the oil is hot (this can be done by dead heading the auxiliary service output for a short period).
• Lower lift arms about 1/2” (13mm).
• Pull out the auxiliary service knob.
• Read the pressure gauge. Pressure should read 2,450 to 2,550 psi (172.24 to 175.77 kg/sq.cm). These are the figures for a new pump. For an older tractor, 2,250 to 2,500 psi (160.71 to 178.5 kg/sq.cm) is within limits. Do not allow pressure to exceed 2,550 psi (175.77 kg/sq.cm).
• To set the pressure, shims are fitted or removed from beneath the spring in the pressure relief valve. Remove the valve from the hydraulic pump (on the Dexta/Super Dexta) or from the front of the lift housing near the auxiliary service knob (on the Super Major).
• Remove the sealing “O” ring and grip the end of the valve in a soft-jawed vice.
• Unscrew the end and remove. In the end cap is the valve seat and the valve plunger. Check for damage on the seat and on the valve. The seat will be clearly seen.
• Remove the spring from the housing. If no shims come out with the spring, sharply tap the hollow end of the housing on a piece of wood or soft surface so as not to damage it. Any shims should fall out. These shims are .010” (0.254 mm) and .025” (0.635mm) thick. Shims of these thickness can be added to increase the system pressure but should be used with caution and in conjunction with a pressure gauge as damage to the hydraulic system and to the operator could occur if the pressure is set too high. If the pressure does not increase after shims have been added, the fault is not in the pressure relief valve and any extra shims fitted should be removed before the problem is rectified.