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Closed Loop (Hydrostatic) Systems

Closed loop systems are a sub-species of hydraulic system. In this lesson we'll describe a typical closed loop system, and include some tips on spotting them.

We'll examine the pros and cons of using a closed loop over an open loop system.
We'll take an in-depth look at some of the components you'll find in many closed loop systems, and the way they work together to build a robust hydraulic system.
And finally, we'll look at one of the best solutions for energy efficiency in closed loop systems.

Spotting A Closed Loop System

Are there any closed loop systems around you? They're not too hard to find! Just look for two hoses of equal diameter connecting a pump to a hydraulic motor in a loop.

Is it always a hydraulic motor?

Not always, but usually. While there are examples of hydrostatic loops using cylinders, they are relatively uncommon. Hydraulic motors are a very natural fit in loops, while the differential volume requirements of most cylinders complicates the loop design. We'll stick to referring to loops using hydraulic motors in this lesson, but the principles would be the same if you were controlling a cylinder instead.

A closed loop system will still include an oil reservoir, but it may seem a bit smaller than what you are used to for similar sized open loop pumps.

Natural Habitat

One of the most common places to find a closed loop system is in small, mobile machines, like skid-steers and zero-turn mowers.

In the case of this zero-turn mower, each rear wheel is powered by its own closed loop.

Closed Loop Pros and Cons

Pros

Speaking of the small reservoir, that's a clue to one of the advantages of a closed loop hydraulic system. Not only do closed loop systems require a smaller reservoir than open loop systems, but they don't need flow or directional control valves other than the hydraulic pump's own controller. Because of this, closed loops can be designed to be smaller, lighter, more efficient, and less expensive than similar open loop systems.

Many system designers prefer the fine motor motion control* and integrated pressure controls that come built right into the pump. In comparison to an open loop system, these built-in controls represent design work done and solved in advance.

*Whether control in closed loop systems is vastly superior to open loop is up for debate, especially as open loop system control continues to be refined and improved.

Cons

There's always a cost, right? One of the benefits of the large reservoir found in open loop systems is heat reduction. Oil, after cycling through the system, returns to the tank and dissipates heat while waiting to be cycled again. But in a closed loop system, that heat dissipation capability is drastically reduced. Closed loop systems do have heat controlling adaptations, but these systems are nicknamed "hot loops" for a reason.

Another con is that there is less opportunity for contaminants to leave the main system. Instead of settling, any contaminants in the system will continue to circulate and cause damage.

Why is it bad to heat hydraulic oil?

In short, because oil has a prescribed operational temperature range. If you exceed it, oxidation will increase and lubrication performance will be compromised. Check it out in Fluid Power Basics Part 2.

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We hope you enjoyed Closed Loop (Hydrostatic) Systems

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