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.
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.
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.
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.
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.
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.
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.
Become a member to get immediate access to the rest of this lesson, and all the other great content on LunchBox Sessions.