Pipes, Hoses, and Tubing.

Pipes, hoses and tubing are the components that connect all the different components of a hydraulic circuit. Pipes are used primarily for long lasting installations. Hose are used more often for going around sharp bends and corners. One disadvantage of using a hose is that they tend to shrink while under pressure. 


One advantage to using hoses as apposed to pipes or tubing is that they are easily removed and quickly replaced. Steel is the most suitable material for hydraulic pipelines. Copper and galvanized steel shouldn’t be used due to the fact that they both react poorly with hydraulic fluids. Galvanized steel also has the potential to flake causing debris to get into the hydraulic circuit. A disadvantage to using pipe is that it can’t be bent due to the thickness of its walls. 


Tubing has thinner walls then pipes allow it to be bent instead of having to use pipefittings. When installing tubing it should always have a bend in it, this helps to reduce vibration from one component to another. When installing hoses make sure to avoid sharp bends in it. Allow for a little extra material due to the hose decreasing slightly when under pressure. If this isn’t taken into consideration the hose might fail prematurely.

Displayed below are the hydraulic pipelines in an airplane wing. 

Preventative maintenance

Proper machine and fluid maintenance is key in extending the life of any hydraulic machinery. To help accomplish this there should be scheduled preventative maintenance preformed at periodic time intervals. Preventative maintenance helps with reducing costly downtime and it also helps catch small problems that can later turn into bigger ones. 

When performing preventative maintenance it is crucial that you follow the manufactures recommendations. Contamination of any sort, be it from dirt, sand, and anything else could cause damage to the hydraulic circuit so cleanliness is imperative. As I have stated before the heat and pressures associated with hydraulic circuits and cause severe injuries if they aren’t properly dealt with. Before any maintenance is done one must be sure that all power supply to the circuit is turned off. 

The power supply is not only electrical but also hydraulic power sources such as cylinders and accumulators. Make sure the proper lock out/ tag out/ block out labels are applied to the power supply to let other personnel know not to turn it back on. You don’t want someone else unknowingly turning on the machine while you are working on it. Replacing filters and seals at a regular time interval is also a vital part of any

Safety!!

Safety is the most important component of any hydraulic circuit. If safety protocol and regulations are not followed serious injury or death could result. There are numerous examples of technicians or maintenance workers trying to take circumvent these regulations and resulting in injury. 

 Let’s examine one of these examples. A diesel mechanic was severely injured as a result of an accident he suffered while replacing a hydraulic hose assembly on a front-end loader. He was found crushed between the lift arm assembly and the front axle. He was apparently replacing the hose which transported oil from the control valve to the cap-end of the cylinder. He was positioned in the only location from which the hose could be accessed.

The reason for the accident was because the mechanic thought that if he had the bucket on the ground and the parking break on nothing could move. When he went to unscrew the cylinder to remove the hose he didn’t take into account that cylinders can move due to gravity as well, which is what happened and this intern crushed him in between the front axle and lift arm assembly. The mechanic should have supported the front end load from the underside so that the bucket could have never moved. Safety should be a constant concern on everybody's mind. 

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.