What are open-circuit piston pumps and closed-circuit piston pumps? What are their shortcomings?

What Is an Open Loop Hydraulic Circuit?

An open loop circuit is where the inlet to the hydraulic pump and the return flow on the hydraulic motor or actuator are connected to the hydraulic fluid reservoir. The pump supplies a continuous flow of fluid through the system. This type of circuit often includes using directional control valves to regulate the flow and system pressure to actuators.

• Advantages of an Open Loop Circuit
• Ideal for hydraulic systems that operate at lower pressures
• System runs cooler since heat is dissipated in the reservoir
• Can use multiple control valves in a series
• Provides more flexibility in system design
• Reduced risk of fluid contamination
• Only option for circuits that contain actuators other than motors
• Cost less to configure and design
• Easier to maintain

Disadvantages are:

Could create heat in the system if working pressure exceeds the relief valve setting when using fixed displacement pumps.
Reservoir size has to be larger for adequate cooling of the fluid.

What Is a Closed Loop Hydraulic Circuit?

A closed loop circuit system for hydraulic pumps is also called a hydrostatic drive. The fluid flows in a closed circuit through a piston pump, and then the motor, and it is returned to the piston pump without passing through the reservoir.

There is often a pressure control relief valve where hot fluid can escape when needed. Most closed loop circuits also use a gear pump as a charge pump to draw cooler fluid from the reservoir, to make up for case drain leakage, directly into the piston pump.
Quantitative Pros and Cons analysis concept.

• Well-suited for drive applications and hydrostatic transmissions
• Works well with higher pressures and pumps
• Systems run at much higher pressures than most
• Less fluid required; majority of fluid contained inside
• Direction of can be reversed
• Lost fluid is replaced by fluid supplied to the main pump via the charge pump
• Allows system designers more flexibility in designing system controls
• Provides precise motor control in a light-weight, compact design

Disadvantages are:

More expensive components are used.
May require high-pressure filtration.
More difficult to diagnose and repair.