Pneumatics in a nutshell

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In a pneumatic circuit the air is controlled and directed using different mechanical and electrical components. To control the pressure in the circuit a pressure regulator is used. A pressure regulator is a valve that restricts or stops the air from moving past it.  The component that controls the direction of airflow is called a directional control valve. Like in a hydraulic circuit all pneumatic direction control valves have an internal spool that controls the flow of air. Most directional control valves are two or three position valves. To the right is a picture of a pressure relief valve.

One component that pneumatic circuits have that hydraulic circuits do not is a lubricator. A lubricator is a device that injects pressurized oil into the pneumatic circuit periodically to keep the components lubricated. If the wrong pressure is being pumped into the circuit it could cause serious damage to the different  components.                              

The way pressure is controlled in a pneumatic circuit is with a pressure switch. The pressure switch will sense a high or low pressure situation and then send an electrical signal to the compressor motor to switch on or off depend on the situation. Another safety component is a safety relief valve which will open up if the pressure pushing against the spring becomes greater then the spring force. 

Alternatives to hydraulic circuits

 A good alternative to using a hydraulic circuit would be using a pneumatic circuit. Pneumatics is the use of pressurized gas in an enclosed circuit to perform a specific function. Because of the benefits that pneumatic circuits offer over hydraulic circuits they are becoming more widely used today in different industries. 

These benefits include initial low cost, cleaner then hydraulics, and easy storage of air because it is compressed. Another benefit of a pneumatic circuit is that the motor that powers the compressor doesn’t always have to be running; it only runs when the compressor is on. 


The basic components that a  pneumatic circuit needs are a compressor to compress air, a tank to store the compressed air, a pressure switch to turn off the compressor when the desired amount of air pressure is reached, a check valve to stop air from getting back into the compressor, a safety valve to stop any over pressure getting into the circuit, a pressure gauge to observe circuit pressure, and lastly a circuit to perform a desired function. Some of the different actuators that use pneumatics are air motors and air cylinders. Air motors produce rotary mechanical motion while air cylinders produce linear mechanical motion.

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.