Variable Control Methods of Closed-Loop Piston Hydraulic Pumps

Bypass valves serve as a critical speed limitation mechanism in closed-loop hydraulic systems, particularly for controlling the maximum rotational velocity of the hyd motor. These valves function by diverting excess flow around the hyd motor when predetermined speed thresholds are reached, effectively limiting rotational velocity regardless of pump output or load conditions.

The bypass valve operates based on a pressure differential principle, where the pressure drop across an orifice plate or fixed restriction is proportional to the square of the flow rate. When the hyd motor speed reaches the desired limit, the resulting pressure differential overcomes the valve's spring setting, causing it to open and redirect a portion of the pump flow back to the low-pressure side of the circuit.

This type of speed limitation offers several advantages, including simplicity, reliability, and independence from external power sources. However, it introduces some energy loss due to the pressure drop across the valve when activated. Modern designs often incorporate adjustable bypass valves that allow for precise calibration of the maximum speed limit based on specific application requirements for the hyd motor.

Inlet vacuum limitation is a critical consideration in closed-loop hydraulic systems, as excessive vacuum at the hyd motor inlet can lead to cavitation, reduced performance, and potential damage. Cavitation occurs when the pressure drops below the fluid's vapor pressure, causing the formation and subsequent collapse of vapor bubbles within the hyd motor.

Several factors contribute to inlet vacuum, including excessive fluid velocity in supply lines, restrictions in the inlet path, and inadequate replenishment flow in the closed loop. At low speeds, the problem can be exacerbated by the hyd motor's reduced ability to draw fluid effectively into its chambers.

To prevent excessive vacuum, closed-loop systems incorporate several protective mechanisms, including charge pumps that maintain positive pressure in the low-pressure side of the circuit, properly sized inlet lines to minimize velocity, and vacuum relief valves that open to admit makeup fluid when dangerous vacuum levels are detected.

The specific vacuum limitation for a hyd motor is typically specified by the manufacturer and depends on factors such as fluid type, operating temperature, and motor design. Maintaining inlet pressure within recommended limits is essential for ensuring proper lubrication, preventing cavitation damage, and maintaining consistent low-speed performance.

Dynamic braking in closed-loop hydraulic systems provides controlled deceleration of the hyd motor by converting kinetic energy into heat, which is then dissipated through the system's cooling mechanisms. This is particularly important in applications where rapid yet controlled stopping is required, or where gravitational loads might cause overspeed conditions.

The dynamic braking process typically involves restricting the flow from the hyd motor during deceleration, creating backpressure that opposes the motor's rotation. This backpressure converts the kinetic energy of the moving load into thermal energy in the hydraulic fluid. In variable displacement systems, the pump can be shifted to a negative displacement mode, effectively acting as a brake on the hyd motor.

Modern systems often incorporate proportional valves to modulate the braking force, providing smooth, adjustable deceleration characteristics. Pressure-limiting valves protect the system from excessive pressures during the braking process, while accumulators may be used to capture and reuse some of the energy that would otherwise be dissipated as heat.

Effective dynamic braking requires careful coordination between the pump control system and the hyd motor's operating parameters, ensuring that deceleration rates remain within safe limits for both the equipment and any loads being moved. The system must also account for thermal management during prolonged or frequent braking events to prevent fluid overheating.

The E1 type electro-hydraulic displacement control system for Linde HPV closed-loop pumps offers a significant advancement over purely mechanical systems, providing precise, remote control of pump output and consequently the performance of the connected hyd motor. This system converts an electrical control signal into a proportional mechanical adjustment of the pump's displacement.

The control mechanism consists of a proportional solenoid valve that regulates hydraulic pressure to an actuator, which in turn adjusts the pump's swash plate angle. The electrical input signal (typically 0-10V or 4-20mA) determines the position of the solenoid, controlling the pressure supplied to the displacement actuator and thus the pump's output flow to the hyd motor.

A key feature of the E1 system is its integral position feedback mechanism, which continuously compares the actual swash plate position with the desired position based on the input signal. This closed-loop feedback ensures precise control and compensation for any external disturbances that might affect the hyd motor performance.

The E1 control system provides several operational advantages, including programmable control profiles, remote operation capability, and the ability to integrate with automated control systems. It maintains excellent linearity between input signal and hyd motor speed, typically within ±3% of setpoint across the operating range.

The Denison P6P series employs a unique load-sensing control principle that optimizes pump output to match the exact requirements of the hyd motor and connected system. This approach minimizes energy consumption by ensuring the pump only produces the flow and pressure necessary for the current operating conditions.

At the heart of the P6P control system is a pressure-compensated flow control mechanism that adjusts pump displacement based on two primary inputs: the demanded flow rate (determined by hyd motor speed requirements) and the system pressure (reflecting the load on the motor). This dual-input control allows for precise matching of pump output to system demand.

The load-sensing feature continuously monitors the pressure at the hyd motor inlet and adjusts the pump's discharge pressure to maintain a constant differential pressure across the control valve. This ensures consistent flow rate to the hyd motor regardless of load variations, providing stable speed control even under changing operating conditions.

The P6P control system also incorporates a unique anti-cavitation feature that maintains positive pressure at the hyd motor inlet during low-pressure conditions, preventing cavitation damage and ensuring smooth operation. This is particularly beneficial during rapid deceleration or when operating with vertical loads.

The HD type hydraulic control system utilizes pilot pressure signals to regulate pump displacement, offering a robust, reliable solution for controlling hyd motor performance in demanding industrial environments. This purely hydraulic control method eliminates the need for electrical components, making it ideal for applications where electromagnetic interference, moisture, or explosive atmospheres are concerns.

The control principle relies on a proportional relationship between the pilot pressure and pump displacement. A pilot valve, typically operated manually or by another hydraulic signal, generates a control pressure that acts on a spool or piston within the pump's control housing. This force adjusts the swash plate angle, changing the pump's output flow and thus the speed of the connected hyd motor.

HD type controls often incorporate pressure compensation features that adjust the pump output based on system pressure, ensuring stable hyd motor performance even as load conditions change. This pressure feedback mechanism helps maintain consistent speed under varying load conditions without external adjustment.

A significant advantage of the HD control system is its inherent fail-safe characteristics. In the event of pilot pressure loss, the system typically defaults to a safe condition, either full displacement for maximum torque or zero displacement to stop the hyd motor, depending on application requirements. This feature enhances system safety in critical applications.