If you deal with hydraulic systems long enough, you may hear the term “open-loop” and “closed-loop” from time to time. These terms refer to two types of hydraulic circuits in common use today. Each of these systems has both strong points and weak points, which is why you’ll find them used in different applications across the industrial world. It’s important to understand the difference between these concepts in order to maintain your equipment correctly and make better decisions when it comes to designing or upgrading your hydraulic systems.
So, what’s the difference between these concepts and what are the benefits and drawbacks of each? In this article, we’ll take a closer look.
Closed loop hydraulic systems are also commonly known as ‘hydrostatic drives’ and is a system commonly found on mobile machines such as skid-steer loaders and dozers, and on industrial machines such as conveyors.
In a hydrostatic drive, fluid flows directly from a piston pump to a motor and returns directly to the pump without entering a reservoir. The speed and direction of the fluid controls the motor and, since this is a simple closed loop, the system can operate in either direction just as easily.
To control speed and direction, this type of system features a displacement controller (a control lever) attached to the swash plate of the piston pump. Moving the control lever forward sets the pump’s swashplate angle to deliver fluid to the motor and the motor begins to turn. If the operator pushes the lever forward more, the swashplate angle increases, which results in greater fluid displacement. Which means more fluid is delivered to the motor, which turns even faster.
If the operator pulls the control lever back to the neutral position, the flow will cease. If the lever is pulled back in the opposite direction, the displacement will be reversed. This sends the fluid in the opposite direction to turn the motor the other way. Thus, a closed loop (hydrostatic) system offers a way to finely control the speed and direction of a motor.
Unlike an open loop hydraulic system, fluid does not flow to a reservoir, but flows directly back to the pump (which is why the term “closed loop” is used to describe this type of circuit). Without the benefit of a large reservoir of fluid to dissipate heat, this means the potential for heat generation is more dramatic in closed loop systems. To compensate for this increased risk of overheating, some systems feature a hot oil purge valve. This valve discharges some of the hot oil from the loop to be filtered and cooled. In some designs, this oil is used to flush some of the working parts of the hydraulic system, to provide an added benefit.
This type of system typically features a smaller ‘charge pump’ (also called a replenishment pump), which draws cooled, filtered fluid from a small reservoir and pumps it into the loop to replace fluid losses as a result of internal leakage via case drains and / or fluid diverted by the hot oil purge valve.
To protect the system from excessive pressures, closed loop systems also commonly feature a cross port relief valve. If the pressure rises beyond the limit of the motor, the relief valve opens and “short circuits” the loop – bypassing the motor and sending the fluid flow straight back to the pump. Thus, the motor stalls, but is protected from a spike in pressure which would otherwise cause the system to over-pressurize, resulting in a blown hose, snapped coupling or other catastrophic failure mode.
Closed loop systems are compact, light-weight and offer precise motor control, which is why they are generally used in mobile power transmission applications. Closed loop systems have more control options and the direction of travel can be reversed without the use of valves. This type of system is simple and cost-effective and generally less expensive to maintain.
The open loop hydraulic circuit is a system predominantly used to power linear actuators such as hydraulic cylinders and is found on many mobile machines such as excavators and crane trucks and on industrial machinery such as hydraulic presses.
In an open loop hydraulic system the pump inlet and actuator-return ports are connected to a fluid reservoir. Unlike a closed loop circuit, in this type of system, the pump provides continuous fluid flow to the system. The fluid flows through the system and returns to the reservoir, whereupon fresh fluid is drawn from the reservoir and is pumped back into the system.
Actuator speed and direction is achieved through directional control valves and flow regulating valves positioned between the actuator and the pump. Switching the position of a spool within a directional control valve changes the direction of fluid flow to and from the actuator. Opening or closing the aperture of a variable orifice within a flow regulating valve increases or decreases the flow of fluid to and from the actuator - thereby causing the hydraulic cylinder or motor to either speed up or slow down respectively.
When the directional control valve directs flow to the actuator, the resistance of the load acting against the actuator causes pressure to rise within the circuit and thus, allows work to be done. The pressure within the circuit is regulated by a pressure relief valve which opens and directs oil to the reservoir when system pressure reaches a predetermined setting. When no work is to be done, oil flow is directed away from the actuators to the reservoir which causes no resistance to flow and therefore no pressure within the circuit.
Open loop systems generally run cooler, because the oil in reservoir helps to dissipate heat. In addition, this type of circuit has the advantage that it is possible to eliminate the presence of foreign contaminants and air bubbles, which are given an opportunity to escape from the fluid while in the reservoir - which aids in maintaining the power and efficiency of the system.
Open loop hydraulic systems can be complex and are usually more difficult to troubleshoot and maintain than their closed-loop cousins. However, they offer more versatility in system design and multiple control valves can be stacked in series to achieve more sophisticated control.
Open loop systems are less expensive to maintain due to the use of affordable fixed displacement pumps, which are less expensive than the variable displacement pumps needed by closed loop systems. However, open loop systems are slightly less efficient due to the pumps running to provide continuous flow. Marginally increasing energy and fuel costs.
Closed loop and open loop hydraulic systems are both in use in many applications across the fluid power industry so it’s helpful to understand the difference between these two systems. Berendsen Fluid Power supplies components and performs repairs to both types of systems. Our repair facilities are also equipped to perform open and closed loop testing at all of our branches, nationwide.