Open Loop vs. Closed Loop Hydraulic Systems
There are two main types of hydraulic systems: open loop and closed loop. Let’s explore the differences and how they might affect you.
Definitions and Differences
Closed Loop
Automotive Lift Repair Orlando Hydraulic systems are categorized into closed loop and open loop systems. Closed loop circuits, also known as hydrostatic systems, involve a pump connected to a motor. The fluid circulates between these two components through connecting hoses without returning to a tank. Although closed loop systems do include an oil reservoir, it is smaller compared to those used in open loop systems.
Closed loop circuits are favored in high power, high pressure mobile equipment because they are more compact, lightweight, and cost-effective. Examples of closed loop systems include skid steers and refrigeration systems, where precise motor control is essential.
Open Loop
In contrast, open loop systems consist of a pump, motor, and reservoir tank. The fluid is pumped from the reservoir, moves through the components, and returns to the reservoir.
One advantage of open loop systems is better heat dissipation compared to closed loop systems. In an open loop system, the fluid cycles back to the tank, where it loses heat before being recirculated. Closed loop systems lack this heat dissipation mechanism.
Open loop systems also tend to be less contaminated. Contaminants have time to settle in the reservoir, whereas in closed loop systems, contaminants continue to circulate. This results in a lower risk of damage due to contamination.
If your Automotive Lift Repair Orlando system includes cylinders, an open loop system is preferable. This avoids pressure spikes that can occur in closed loop systems with unequal fluid volumes on either side of a cylinder.
Choose a closed loop hydraulic system for precision and compactness. If your priority is longevity and these factors are less critical, an open loop system may be more suitable.
What is a Closed Loop Hydraulic Circuit?
Closed loop hydraulic systems, also known as ‘hydrostatic drives’, are commonly used in mobile machinery such as skid-steer loaders and dozers, as well as industrial machines like conveyors.
In a hydrostatic drive, fluid flows directly from a piston pump to a motor and back to the pump without entering a reservoir. The speed and direction of the fluid control the motor, and since this is a closed loop system, it can operate in either direction with equal ease.
This type of Automotive Lift Repair Orlando system uses a displacement controller (control lever) attached to the swash plate of the piston pump to manage speed and direction. Moving the control lever forward adjusts the swash plate angle to deliver fluid to the motor, causing it to turn. Pushing the lever further forward increases the swash plate angle, resulting in greater fluid displacement and faster motor speed.
Pulling the control lever back to the neutral position stops the flow. Pulling it further back reverses the displacement, sending fluid in the opposite direction and reversing the motor’s rotation. This closed loop system allows for fine control of motor speed and direction.
Unlike open loop hydraulic systems, fluid in closed loop systems does not flow to a reservoir but circulates directly back to the pump, hence the term “closed loop”. Without a large reservoir to dissipate heat, these systems are more prone to heat generation. To address this, some systems include a hot oil purge valve to discharge hot oil for filtering and cooling. In some designs, this oil also flushes certain hydraulic components, providing additional benefits.
Automotive Lift Repair Orlando Closed loop systems often feature a smaller ‘charge pump’ or replenishment pump, which draws cooled, filtered fluid from a small reservoir and pumps it into the loop to replace fluid lost due to internal leakage or fluid diverted by the hot oil purge valve.
To protect against excessive pressure, closed loop systems typically include a cross port relief valve. If pressure exceeds the motor’s limit, the relief valve opens, bypassing the motor and sending fluid directly back to the pump. This prevents over-pressurization, which could otherwise result in a blown hose, snapped coupling, or other catastrophic failures.
They provide more control options, allowing for direction reversal without valves, and are simple, cost-effective, and generally less expensive to maintain.
What is an Open Loop Hydraulic Circuit?
Open loop Automotive Lift Repair Orlando hydraulic circuits are primarily used to power linear actuators, such as hydraulic cylinders, and are commonly found on mobile machinery like excavators and crane trucks, as well as industrial machinery like hydraulic presses.
Unlike a closed loop circuit, the pump in this system provides a continuous fluid flow. The fluid circulates through the system and returns to the reservoir, where fresh fluid is drawn and pumped back into the system.
Actuator speed and direction are controlled by directional control valves and flow regulating valves situated between the actuator and the pump. By switching the spool position within a directional control valve, the direction of fluid flow to and from the actuator changes. Adjusting the aperture of a variable orifice within a flow regulating valve increases or decreases the fluid flow to and from the actuator, causing the hydraulic cylinder or motor to speed up or slow down, respectively.
When the directional control valve directs flow to the actuator, the load resistance against the actuator causes pressure to rise within the circuit, allowing work to be done. A pressure relief valve regulates circuit pressure by opening and directing oil to the reservoir when system pressure reaches a set level. When no work is required, oil flow is redirected to the reservoir, causing no resistance or pressure within the circuit.
Automotive Lift Repair Orlando Open loop systems generally run cooler because the oil in the reservoir helps dissipate heat. Additionally, the reservoir allows foreign contaminants and air bubbles to escape from the fluid, maintaining the system’s power and efficiency.
Automotive Lift Repair Orlando Open loop systems are less expensive to maintain due to the use of affordable fixed displacement pumps, which are cheaper than the variable displacement pumps needed for closed loop systems. However, open loop systems are slightly less efficient since the pumps run continuously to provide fluid flow, marginally increasing energy and fuel costs.
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