Closed Loop Circuits
Objectives
- Understand the construction and function of components used to make up a generic closed loop system.
- Study the pressure and flow changes that occur during operation of a closed loop system.
Closed Loop Systems
Closed loop systems are easily recognized by the pair of hoses connecting a pump to a motor, as both hoses will be the same diameter.
This is very important; since closed loop systems are reversible, both hoses must be capable of carrying lots of oil to the pump’s inlet with minimal restriction.
Closed Loop Systems:
- are sometimes called “hot loops” or hydrostatic drives
- require less space and add less weight than an open loop system
The advantages of a closed loop system are that they tend to be lighter and take up less space than open loop systems, making them a good solution for mobile design.
Just like an open loop hydraulic circuit, the pump displaces fluid and sends it to an actuator (usually a hydraulic motor) to create motion.
(varies the
pump's output)
(varies the
pump's output)
The big difference in a closed loop is that the fluid that leaves the hydraulic motor does not return to tank; it is directed right back to the inlet side of the pump. In other words, the hydraulic fluid is trapped or held “static” inside the system.
The pump displacement can be reversed easily and the hydraulic motor will then turn in the opposite direction.
A pump displacement control (either a direct mechanical linkage or an electrical, hydraulic, or electro-hydraulic pilot control) will allow for variable displacement from the pump and therefore variable speeds at the hydraulic motor.
This pump control is usually the only directional and flow control mechanism needed in a hydrostatic drive.
Hot Loop
As mentioned earlier, another common name for a closed loop circuit is a “hot loop”, and for good reason!
Because the system continues to circulate the same fluid from the pump to a loaded motor then back to the pump, heat can become an issue.
One of the biggest drawbacks to hydrostatic loops is that they heat oil quickly.
As pressurized fluid leaks through small clearance passages in the pump and motor, the fluid molecules strike metal surfaces and the friction creates heat.
If the system runs continuously for many hours, heat may build up to a level that is too high for the fluid to maintain its correct viscosity range.
This will result in metal to metal friction which will create even more heat. The excessive heat will also damage shaft seals among other parts.
Heat must be controlled in a hydrostatic circuit.
This is typically done with the addition of a hot oil shuttle valve whose function is to bleed off about 10 percent of the loop flow and send it back to the reservoir for cooling.
Pump
Valve
Valve
Plate
Pump Relief
Pressure
Port
Relief
Port
Relief
Oil
Shuttle
Relief
260 psi
The shuttle valve is automatically piloted into one porting position or another depending on which side of the loop has the higher pressure.
The side of the loop that has the higher pressure is determined by which direction fluid is flowing around the main loop.
The pressure of the fluid flowing from the pump to the motor pilots the hot oil shuttle valve into position.
The fluid that is being bled off for cooling is taken from the return side of the loop. The amount of fluid that is bled off is controlled in part by a pressure relief valve that follows after the hot oil shuttle valve.
The relief valve is often set between 200 and 400 psi. The setting of the hot oil shuttle relief valve accounts for the system charge pressure value whenever the main hydrostatic pump is displacing fluid.
This pilot line shuttles (“bumps”) the valve into position. There is no flow on this line; the pressure is all that’s required to keep the valve in place.
Relief
260 psi
Oil
Shuttle
Review
What is installed in a closed loop system to prevent over heating?
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