Which fluid is right for you?

There are many factors that one will need to take into account when selecting which hydraulic fluid to use. The primary use of the fluid is to produce power, however there are other characteristics one might want out of the fluid as well. These characteristics might include how well the fluid transfers power, the environmental impact, fire resistance, how well of a lubricant it is, high temperature applications.

One of the first fluids to be use in hydraulics was mineral oil. Today mineral oil is use mainly as a base fluid for most hydraulic fluids because it’s a great lubricant and it has a high boiling point. Other specialized hydraulic fluids include fire resistance, high temperature applications, and biodegradable fluids. The biodegradable fluids use a vegetable oil as the base lubricants so that if a hose or fitting were to erupt it would not have a great environmental impact.

Great care must be taken to keep hydraulic fluid free from contaminant. The use of filters in the reservoir is the main component to accomplish this. These filters must be changed regularly or contaminants might get into the hydraulic circuit, which could cause other components to fail resulting in costly repairs and down time. Because these hydraulic oils are at extreme pressures caution and safety are of the utmost importance. 

Where did the fluid go?

Now that we know how the oil is pumped and directed in a hydraulic circuit lets take a look at where it is stored. The hydraulic reservoir is the storage place of the fluid in a hydraulic circuit. The storage capacity of the reservoir should be at least 2 to 3 times the need of the hydraulic circuit. 

The fluid level is always rising and falling in the reservoir when the circuit is in use. The reservoir serves also as a place to catch any contaminates that may have ended up in the fluid. Most reservoirs consist of a fluid gauge, a breathing cap, baffle plates, and drain, return, and suction lines. 

When filling the reservoir one wants to make sure not to fill it completely full, this space allows for foaming which helps get the air out of the oil. The baffle plates help with reducing the heat of the fluid. Depending on the load and the use of the hydraulic system the oil can get very hot. If the hydraulic circuit requires the system to be pressurized then there will be no breathing installed. The included cut out diagram of a reservoir shows where the different components are located in the reservoir.

excuse me, what direction should i take?

We talked about what creates the flow in a hydraulic system, now I would like to get into what controls that flow.  A valve is the device that controls the direction, pressure, or the speed of the hydraulic fluid.  There are several different types of valves. Hydraulic circuit valves are grouped by the function they perform. 

The route by which the fluid takes through a valve is called the way. There are one, two, three and four way valves A pressure relief valve is a one way valve. When the pressure starts to get to high in the circuit the fluid pushes against a poppet inside the valve. This poppet is connected to a spring which intern gets depressed and opens up a port for the fluid to flow through thus reducing the pressure in the circuit.

an example of a electrical
directional control valve.

Directional valves control the direction of the fluid through the circuit. These can be two, three, or four way valves. The valves have an internal spool that changes position thus opening and closing certain pathways for the fluid to flow though. The mechanism to change the position of the spool can be manual, mechanical, hydraulic, or electric. The most common mechanism are electric solenoids.

Pump, pump, pump it up

In hydraulic circuits, the pump is the device that creates the flow of hydraulic fluid. The pump converts energy supplied to it from a motor or engine into rotational force. Hydraulic pumps are either positive displacement pumps or variable displacement pumps. Displacement refers to the flow of fluid through the pump per rotation. In a positive displacement pump, the flow rate cant be changed. 

In contrast, the flow rate in a variable displacement pump can be changed. This change allows for the saving of power and energy which otherwise would be lost doing normal changes in work loads. Different types of pumps include gear, vane, piston, rotary vane, and bent axis pumps. Bent axis pumps are the most efficient pumps, however, the gear pumps are the simplest and cheapest type. 

Flow rate of the pump is proportionally related to the actuator speed; the actuator speed depends on the flow rate of the pump, faster pump faster output. Most pumps are used in an open system, which means the reservoir is not pressurized. If the reservoir is pressurized then it’s a closed system. Examples of machines that couldn't operate without the use of hydraulic pumps are cranes, loaders, backhoes. Power steering in cars also use hydraulic pumps.