Advantages of Hydraulic Control
There are many unique features of hydraulic control compared to other types of control. These are fundamental and account for the wide use of hydraulic control. Some of the advantages are the following:
1. Heat generated by internal losses is a basic limitation of any machine. Lubricants deteriorate, mechanical parts seize, and insulation breaks down as temperature increases. Hydraulic components are superior to others in this respect since the fluid carries away the heat generated to a convenient heat exchanger. This feature permits smaller and lighter components. Hydraulic pumps and motors are currently available with horsepower to weight ratios greater than 2 hp/lb. Small compact systems are attractive in mobile and airborne installations.
2. The hydraulic fluid also acts as a lubricant and makes possible long component life.
3. There is no phenomenon in hydraulic components comparable to the saturation and losses in magnetic materials of electrical machines. The torque developed by an electric motor is proportional to current and is limited by magnetic saturation. The torque developed by hydraulic actuators (i.e., motors and pistons) is proportional to pressure difference and is limited only by safe stress levels. Therefore hydraulic actuators develop relatively large torques for comparatively small devices.
4. Electrical motors are basically a simple lag device from applied voltage to speed. Hydraulic actuators are basically a quadratic reson.amcc from flow to speed with a high natural frequency. Therefore hydrauliic actuators have a higher speed of response with fast starts, stops, and spee:d reversals possible. Torque to inertia ratios are large with resulting high acceleration capability. On the whole, higher loop gains and bandwudths are possible with hydraulic actuators in servo loops.
5. Hydraulic actuators may be operated under continuous, intermit tenit, reversing, and stalled conditions without damage. With relief valwe protection, hydraulic actuators may be used for dynamic breaking. Larg
Fail-Safe Circuits
Fail-safe circuits are those designed to prevent injury to the operator or damage to the equipment. In general, they prevent the system from accidentally falling on an operator and also prevent overloading of the system. In following sections we shall discuss two fail-safe circuits: One is protection from inadvertent cylinder extension and other is fail-safe overload protection.
1. Protection from inadvertent cylinder extension: Figure 1.13 shows a fail-safe circuit that is designed to prevent the cylinder from accidentally falling in the event when a hydraulic line ruptures or a person inadvertently operates the manual override on the pilot-actuated DCV when the pump is not working. To lower the cylinder, pilot pressure from the blank end of piston must pilot open the check valve to allow oil to return through the DCV to the tank. This happens when the push button is actuated to permit the pilot pressure actuation of DCV or when the DCV is directly manually actuated when the pump operates. The pilot-operated DCV allows free flow in the opposite direction to retract the cylinder when this DCV returns to its offset mode.
2. Fail-Safe System with Overload Protection: Figure 1.14 shows a fail-safe system that provides overload protection for system components. The DCV V1 is controlled by the push-button three-way valve V2. When the overload valve V3 is in its spring offset mode, it drains the pilot line of valve V1. If the cylinder experiences excessive resistance during the extension stroke, sequence valve V4 pilot-actuates overload valve V3. This drains the pilot line of valve V1 causing it to return to its spring offset mode. If a person then operates the push-button valve V2 nothing happens unless overload valve V3 is manually shifted into its blocked-port configuration. Thus, the system components are protected against excessive pressure due to an excessive cylinder load during its extension stroke.
Speed Control of a Hydraulic Motor
Figure 1.12 shows the speed control circuit of a hydraulic motor using a pressure-compensated FCV.The operation is as follows:
– In a spring-centered position of the tandem four-way valve, the motor is hydraulically blocked.
– When the valve is actuated to the left envelope, the motor rotates in one direction. Its speed can be varied by adjusting the throttle of the FCV. Thus, the speed can be infinitely varied and the excess oil goes through the PRV.
– When the valve is deactivated, the motor stops suddenly and becomes locked.
– When the right envelope is in operation, the motor turns in the opposite direction. The PRV provides overload protection if, for example, the motor experiences an excessive torque load.
Meter-In Versus Meter-Out Flow-Control Valve Systems
A meter-out flow control system is one in which the FCV is placed in the outlet line of the hydraulic cylinder. Thus, a meter-out flow control system controls the oil flow rate out of the cylinder.
Meter-in systems are used primarily when the external load opposes the direction of motion of the hydraulic cylinder.
When a load is pulled downward due to gravity, a meter-out system is preferred. If a meter-in system is used in this case, the load would drop by pulling the piston rod, even if the FCV is completely closed.
One drawback of a meter-out system is the excessive pressure build-up in the rod end of the cylinder while it is extending. In addition, an excessive pressure in the rod end results in a large pressure drop across the FCV. This produces an undesirable effect of a high heat generation rate with a resulting increase in oil temperature.
Speed Control of a Hydraulic Cylinder
The speed control of a hydraulic cylinder circuit can be done during the extension stroke using a flow-control valve (FCV). This is done on a meter-in circuit and meter-out circuit as shown in Fig. 1.11. Refer to Fig. 1.11(a). When the DCV is actuated, oil flows through the FCV to extend the cylinder. The extending speed of the cylinder depends on the FCV setting. When the DCV is deactivated, the cylinder retracts as oil from the cylinder passes through the check valve. Thus, the retraction speed of a cylinder is not controlled. Figure 1.11(b) shows meter-out circuit; when DCV is actuated, oil flows through the rod end to retract the cylinder.
Cylinders in Parallel
Figure 1.10 shows a hydraulic circuit in which two cylinders are arranged in parallel. When the two cylinders are identical, the loads on the cylinders are identical, and then extension and retraction are synchronized. If the loads are not identical, the cylinder with smaller load extends first. Thus, the two cylinders are not synchronized. Practically, no two cylinders are identical, because of packing(seals)friction differences. This prevents cylinder synchronization for this circuit.
Locked Cylinder Using Pilot Check Valves
A check valve (Fig. 1.9) blocks flow in one direction but allows free flow in the opposite direction. A pilot-operated check valve permits flow in the normally blocked opposite direction when pilot pressure is applied at the pilot pressure port of the valve.
Pilot-operated check valves are used to lock the cylinder, so that its piston cannot be moved by an external force. The cylinder can be extended and retracted by the DCV. If regular check valves are used, the cylinder could not extend or retract. External force acting on the piston rod does not move the piston in either direction thus locking the cylinder.
Automatic Cylinder Reciprocating System
The hydraulic circuit shown in Fig. 1.8 produces continuous reciprocation of a double-acting cylinder using two sequence valves. Each sequence valve senses the completion of stroke by the corresponding build-up pressure. Each check valve and the corresponding pilot line prevent the shifting of the four-way valve until the particular stroke of the cylinder is completed.
The check valves are needed to allow pilot oil to leave either end of the DCV while the pilot pressure is applied to the opposite end. This permits the spool of the DCV to shift as required.
Hydraulic Cylinder Sequencing Circuits
Hydraulic cylinders can be operated sequentially using a sequence valve. Figure 1.7 shows that two sequence valves are used to sequence the operation of two double-acting cylinders. When the DCV is actuated to its right-envelope mode, the bending cylinder (B) retracts fully and then the clamp cylinder (A) retracts.
This sequence of cylinder operation is controlled by sequence valves. This hydraulic circuit can be used in a production operation such as drilling. Cylinder A is used as a clamp cylinder and cylinder B as a drill cylinder. Cylinder A extendsand clamps a work piece. Then cylinder B extends to drive a spindle to drill a hole. Cylinder B retracts the drill spindle and then cylinder A retracts to release the work piece for removal.
Counterbalance Valve Application
A counterbalance valve (Fig. 1.6) is applied to create a back pressure or cushioning pressure on the underside of a vertically moving piston to prevent the suspended load from free falling because of gravity while it is still being lowered.
Valve Operation (Lowering)
The pressure setting on the counterbalance valve is set slightly higher than the pressure required to prevent the load from free falling. Due to this back pressure in line A, the actuator piston must force down when the load is being lowered. This causes the pressure in line A to increase, which raises the spring-opposed spool, thus providing a flow path to discharge the exhaust flow from line A to the DCV and then to the tank. The spring-controlled discharge orifice maintains back pressure in line A during the entire downward piston stroke.
Valve Operation (Lifting)
Asthe valve is normally closed, flow in the reverse direction (from port B to port A) cannot occur without a reverse free-flow check valve. When the load is raised again, the internal check valve opens to permit flow for the retraction of the actuator.
Valve Operation (Suspension)
When the valve is held in suspension, the valve remains closed. Therefore, its pressure setting must be slightly higher than the pressure caused by the load. Spool valves tend to leak internally under pressure. This makes it advisable to use a pilot-operated check valve in addition to the counterbalance valve if a load must be held in suspension for a prolonged time.