Hydraulic & Pneumatic Cylinder Control: Operation, Speed, and Pressure
Single-Acting Cylinder Operation
When the cylinder valve is activated, the piston extends. It remains extended as long as the valve is active. When the valve is deactivated, the piston retracts. Valves can also be bistable, meaning that activating one control signal maintains a position until a different control signal is activated. Monostable valves can also be interlocked to maintain a given position indefinitely.
Double-Acting Cylinder Operation
When control signal 1.1 is activated, the plunger extends. When deactivated, it retracts. Under these conditions, the extension stroke is performed more slowly because it takes longer to fill the chamber during this forward movement. Additionally, the extension stroke results in greater labor force due to the larger piston face area exposed to pressure in the anterior chamber.
Indirect Cylinder Control
Indirect control involves using a pilot signal (e.g., 1.2) to activate a main control valve (e.g., 1.1). When the pilot signal is active, the cylinder extends; when deactivated, the cylinder retracts.
Accelerating Cylinder Movement
A common method for increasing the speed of a double-acting cylinder’s stroke is to install a rapid exhaust valve. This prevents the air flow distributor from restricting the exhaust during the advance stroke.
Cylinder Speed Reduction
Several methods exist for reducing cylinder speed. The most common involves using flow control valves, which regulate the passage of compressed air in both directions, thereby controlling the speed of both strokes. Typically, speed regulation is achieved with one-way flow control valves. Regulators need to act on the exhaust air to ensure the speed is independent of the cylinder’s load. This type of regulation is particularly effective for the extension of single-acting cylinders where the load opposes the motion.
Control of Single-Acting Cylinders
The control system for a single-acting cylinder typically includes a motor and pump that generate the necessary oil pressure for the hydraulic installation. A single-acting cylinder is often controlled by a 3/2-way valve with mechanical actuation and spring return, which is normally closed. When this valve is operated, it switches position, allowing fluid to pass into the cylinder, causing it to extend fully. When the valve is again switched out, the oil pressure is released from the cylinder.
Double-Acting Cylinder Control with 4/3 Valve
Similar to single-acting systems, a motor and pump provide the required oil pressure. A 4/3-way valve offers the unique feature that, in its central position, oil flows directly back to the reservoir. When actuated to position 1, the valve connects the inlet pressure (P) with the working duct (A) and duct (B) with the exhaust (R), causing the cylinder rod to advance. If it is operated to position 3, the opposite occurs. If at any time you turn the valve to position 2, the cylinder motion is interrupted.
Cylinder Feed Speed Regulation
Cylinder feed speed regulation is achieved by placing a flow control valve at the entrance of the cylinder. Additionally, this setup ensures that the returning oil from the cylinder has a lower pressure.
Input Flow Regulation
Input flow regulation, particularly for controlling cylinder speed, can be achieved by using a pressure relief valve placed at the cylinder’s outlet. This valve creates back pressure, which can influence the effective flow into the cylinder and thus its speed.
Pressure Regulation for Cylinder Advance
When a 4/3-way control valve with mechanical drive is in its normally closed position, oil cannot flow through the main valve. Instead, it is required to pass through a pressure regulating valve, thus ensuring a constant pressure level during the cylinder’s advancement. Otherwise, during the cylinder’s stroke, oil from the piston’s chamber side passes through a check valve.