Discussion

When the steering wheel is turned to the right, the steering wheel causes rotation of the steering column, the steering shaft, the input shaft, the cam and the directional control valve.

The cam moves the plunger against the spring and the pressure regulating valve spool. Pilot oil from the pump flows between the pressure regulating valve spool and the valve body, which function as an orifice. As the regulating valve moves down, the size of the orifice increases.

The larger orifice creates a smaller pressure drop which increases the pressure of the pilot oil to the directional control valve. Also, the pilot oil flows through the orifice in the pressure

regulating valve. The force of the pilot oil pressure between the piston and the regulating valve pushes up the regulating valve against the spring.

The flow of the pilot oil is proportional to the downward movement pressure regulating valve spool. The pilot pressure controls the steering speed. Increasing the flow of pilot oil between the body and the pressure regulating valve will increase the steering cycle time.

Vol. 5, No. 2, 2004

As the steering wheel is rotated, the directional control valve also rotates. This movement allows pilot oil from the pressure regulating valve to flow through the directional control valve port through the quad check valve to the neutralizer valve and the steering control valve directional spool.

When the pilot pressure moves the steering control valve directional spool to the RIGHT TURN position, the steering control valve directs main pump oil to the head end of the left steering cylinder and the rod end of the right steering cylinder. The machine articulates to the right.

When the steering wheel is returned to the CENTER position, the flow of pilot oil to the steering control valve directional spool is blocked. The centering spring in the steering control valve moves the directional spool to the neutral position and the machine stops articulating.

NOTE: The machine turning speed depends on the rotational position of the steering wheel.

The farther the steering wheel is turned, the faster the machine will turn. The turning speed of the machine does not depend on how fast the steering wheel is turned.

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This illustration shows the optional Command Control Steering system in a LEFT TURN.

When the operator turns the steering wheel to the left, torque is transmitted through the steering column and the steering shaft to the steering pilot valve input shaft. The pilot valve located in the loader frame directs pilot oil through the quad check valve, through the neutralizer, to the directional spool in the steering control valve.

Turning the steering wheel to a greater angle from the neutral position increases the flow of pilot oil to the directional spool in the steering control valve. Increased flow of pilot oil to the steering control valve moves the directional spool further from the neutral (blocking) position and allows greater flow of steering pump oil to the steering cylinders.

When the pilot oil shifts the directional spool to the LEFT TURN position, steering pump oil is sent to the to the head end of the right steering cylinder and the rod end of the left steering cylinder. The machine articulates to the left for a left turn.

The pressure in the steering cylinders is also sent to the shuttle valve. The cylinder pressure moves the shuttle valve up and becomes the signal pressure to the steering pump control valve.

The signal pressure is sensed in the margin spool spring chamber of the control valve. The signal pressure combines with the force of the margin spring and causes the pump to UPSTROKE.

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The supply oil from the steering pump flows over the backup relief valve. If the pressure exceeds 23500 kPa (3400 psi), the backup relief valve opens and excess oil flows to the hydraulic tank. Also, the steering control valve is equipped with a pressure reducing valve.

The reducing valve will supply pilot oil to the steering pilot valve if the pilot oil supply is lost.

The steering control valve also is equipped with a crossover relief valve. In case of a pressure spike above 25600 kPa (3700 psi) as the machine is articulating, the crossover relief valve will open and send the excessive oil pressure to the hydraulic tank.

When the machine fully articulates, the striker mounted on the loader frame contacts the neutralizer valve mounted on the rear frame. This action stops the flow of pilot oil from the pilot valve to the directional spool. The directional spool shifts to the CENTER position and the steering oil to the cylinders is blocked.

Also, when the steering wheel is returned to the CENTER position, the flow of pilot oil to the directional spool is blocked. The centering spring in the steering control valve returns the directional spool to the neutral position, and the machine stops turning.

The pilot valve is mounted on the front frame while the steering wheel and shaft are mounted on the rear frame. As the machine begins to turn, the shaft for the steering pilot valve begins to rotate back to the neutral position. As the shaft rotates closer to the neutral position, the turning speed of the machine is reduced due to lower pilot pressure to the steering control valve

directional spool. However, the machine will continue to turn until the steering wheel returns to the center position. As long as the pilot directional valve is rotated from the neutral position and the neutralizer valves are not closed, pilot oil will flow to the steering control valve

directional spool.

NOTE: The machine turning speed depends on the rotational position of the steering wheel. The farther the steering wheel is turned, the faster the machine will turn. The turning speed of the machine does not depend on how fast the steering wheel is rotated.

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