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| Illustration 1 | g00810391 |
(1) Hydraulic oil cooler (2) Fan drive motor (3) Relief valve (4) Tank (5) Brake and fan drive pump (6) Priority valve (7) Check valve (8) Accumulator shuttle valve (9) Brake control valve (10) Line to the rear brake (11) Line to the front brake (12) Accumulator (13) Spool for the cut-in pressure and the cut-out pressure (14) Relief valve (15) Line to the secondary steering pump (16) Hydraulic tank breather (17) Line to the pump for the work tools and the steering pump (18) Line from the relief valve and from the manifold (19) Hydraulic oil filter | |
The brake and fan drive system is an open center hydraulic system. The brake and fan drive system contains a fixed displacement hydraulic pump. The brake system has priority over the fan drive system.
When the engine is started, hydraulic oil is drawn from tank (4) by the brake and fan drive pump (5). Hydraulic oil flows from pump (5) to priority valve (6). The spring holds the spool for the cut-in pressure and the cut-out pressure (13) to the right. The relief valve (14) is closed. Spool (13) and relief valve (14) move. The priority valve directs hydraulic oil to check valve (7). As the hydraulic oil pressure increases, the check valve opens and the hydraulic oil flows to the accumulator shuttle valve (8) .
The accumulator shuttle valve (8) directs hydraulic oil to the accumulator with the lowest pressure first. As the hydraulic oil pressure to the first accumulator increases, the accumulator shuttle valve moves. This allows both accumulators to charge at equal pressures. Hydraulic oil also flows to the spool for the cut-in pressure and the cut-out pressure (13) and oil flows to the right side of the spool. Most of the hydraulic oil flows through the spool for the cut-in pressure and the cut-out pressure (13). Then, the hydraulic oil flows to the right side of the priority valve (6) and to the relief valve (14). The hydraulic oil pressure and the spring on the priority valve counter the pressure of the pilot oil that flows to the left side of the priority valve. This holds the priority valve to the left as the accumulators charge. Excess hydraulic oil flow that is not needed for the brake system flows to the fan drive system.
As the accumulators charge, hydraulic oil pressure increases on the right side of the spool for the cut-in pressure and the cut-out pressure (13). When the hydraulic oil pressure overcomes the spring pressure on the left side of the spool for the cut-in pressure and the cut-out pressure, the spool for the cut-in pressure and the cut-out pressure moves to the left. This is the cut-out pressure. The cut out pressure is 14100 ± 1025 kPa (2050 ± 150 psi). This blocks the flow of hydraulic oil to the right side of the priority valve. The spool for the cut-in pressure and the cut-out pressure opens a passage that drains the hydraulic oil from the right side of the priority valve to the inlet side of the brake and fan drive pump (5). Then, the pilot oil pressure on the left side of the priority valve moves the priority valve to the right. This directs the flow of hydraulic oil to the fan drive system.
When the priority valve (6) moves to the right, hydraulic oil pressure below check valve (7) decreases. Then, the check valve seats in order to maintain hydraulic oil pressure in the brake accumulators (12). The accumulator shuttle valve (8) will move to the left or to the right. This directs the lower accumulator pressure to the spool for the cut-in pressure and cut-out pressure (13) .
The flow of hydraulic oil from the brake accumulators (12) is blocked at the brake control valve (9). When the brake pedal is pressed, the brake control valve routes hydraulic oil to the rear service brakes (10) and to the front service brakes (11). This engages the service brakes. Pressure feedback is sensed by each spool in the brake control valve. This allows the operator to feel the application of the service brakes. The pressure in the brake lines varies. As the brake pedals are depressed, the hydraulic oil pressure in the brake lines increases. Due to the spring below the bottom brake valve, the maximum pressure to the front brakes is lower than the maximum pressure to the rear brakes.
When the accumulator charge pressure falls below the spring pressure on the left side of the spool for the cut-in pressure and the cut-out pressure, the spool for the cut-in pressure and the cut-out pressure shifts to the right. This is the cut-in pressure. The cut-in pressure is 9650 ± 345 kPa (1400 ± 50 psi). Then, the accumulator charging cycle begins again.
When the engine is in operation, the fan rotates at a minimum of 1250 rpm. The maximum fan speed is 1500 rpm. The maximum fan speed is limited by the relief valve in the hydraulic circuit for the fan drive motor. The hydraulic oil exits the fan drive motor and flows to the oil cooler (1). From the hydraulic oil cooler, the hydraulic oil flows to the hydraulic oil filter (19). Then, the hydraulic oil flows to the hydraulic oil tank. (4).