Pneumatics
Pneumatics is a branch of engineering that deals with the study and application of compressed air to produce mechanical motion. This technology has found widespread use in various industries due to its efficiency, cleanliness, and versatility. In this section, we will explore the fundamental principles, components, and applications of pneumatics.
Gas Laws
Pneumatics relies on the principles of Boyle’s Law and Charles’s Law, two of the Gas Laws often studied in physics and chemistry. These laws describe the behavior of gases under different pressure and temperature conditions. Understanding these laws is essential for designing and operating pneumatic systems.
Boyle’s Law, formulated by Robert Boyle in the 17th century, describes the relationship between the pressure and volume of a gas at a constant temperature. This law is one of the fundamental principles of gas behavior and is often stated as follows:
“At a constant temperature, the pressure of a given amount of gas is inversely proportional to its volume.” Mathematically, Boyle’s Law can be expressed as:
Where: P1 and P2 are the initial and final pressures, respectively, and V1 and V2 are the initial and final volumes of the gas, respectively.
In simpler terms, when the volume of a gas increases, the pressure decreases. Likewise, when the volume of a gas decreases, the pressure increases, so long as the temperature remains constant. This relationship can be visualized with the example of a balloon full of air. When you try to squeeze the balloon down to a smaller volume, the pressure increases.
Charles’s Law is another fundamental principle in physics and thermodynamics that describes the relationship between volume and temperature of a gas while keeping the pressure constant. It is named after Jacques Charles, a French scientist who first formulated the law in the late 18th century. This law is often stated as follows:
“At a constant pressure, the volume of a given amount of gas is directly proportional to its temperature.” Mathematically, Charles’s Law can be expressed as:
Where: V1 and V2 are the initial and final volumes, respectively, and T1 and T2 are the initial and final temperatures of the gas, respectively.
In other words, Charles’s Law asserts that as the temperature of a gas increases, the volume of the gas should increase proportionately. Similarly, as the temperature of a gas decreases, the volume of the gas should decrease as well, so long as the pressure remains constant.
It should be noted that in order for Charles’s Law to be accurately applied, temperatures must be expressed in kelvins. The Kelvin scale is an absolute temperature scale where 0K represents absolute zero, the lowest possible temperature. The relationship breaks down if temperatures are expressed in Celsius or Fahrenheit, as these scales have arbitrary zero points.
Pneumatic Cylinder
Any device that converts compressed air energy into mechanical motion is called an actuator. A pneumatic cylinder is a cylindrical actuator that uses compressed air to exert a force, typically along a shaft that moves in and out of the cylinder. Pneumatic cylinders and other actuators are used widely in industry for countless applications to make machine parts move or twist, clamp parts down, move items one way or another, open or close doors, and so on. In Robot Gladiator League (RGL), you may use a pneumatic cylinder to operate a hammer that smashes your opponent! Understanding how pneumatic cylinders work and how to use them is (literally) powerful knowledge.
The type of pneumatic cylinder you will be using in RGL is called a ‘double acting’ cylinder. This means that it pushes and pulls. A single acting cylinder either pushes or pulls only.
Parts of a Pneumatic Cylinder
Pneumatic cylinders are very simple mechanisms. Below is a picture of the cylinder you will be using to create your weapons. The various parts of the cylinder are labelled and described.
Barrel – The barrel is the hollow cylindrical tube that makes up the body of the cylinder. This is most of what you see when you look at a cylinder. The barrel is generally closed on the bottom end, and this end usually has some sort of mounting feature to allow the cylinder to be connected to a machine, device or other structure.
Piston – Attached to the internal end of the rod is a flat disc or short cylinder called the piston, and it is just a tiny bit smaller than the inside diameter of the barrel. Rubber and metal seals on the outside of the piston touch the inside wall of the barrel to form an airtight seal.
Ports – Most cylinders have two holes drilled into the side of the barrel called ports. These threaded holes lead into the barrel on each side of the piston and allow for airflow into the barrel. Air entering the bottom port pushes on the bottom side of the piston thus pushing the rod out of the cylinder barrel. This is how a cylinder exerts a push. When air enters the other port, the one on the other side of the piston, the rod is pushed into the barrel and the cylinder exerts a pull. A double acting cylinder has 2 ports, while a single acting cylinder has only one port.
Fittings – Although not technically part of the cylinder proper, fittings are necessary parts for the cylinder to function. Fittings are small socket like mechanisms that screw into the ports and allow hoses to be attached to the cylinder. Without fittings, hoses cannot be attached to the cylinder.
There are several other components not part of the cylinder that are necessary for the cylinder to function:
Pneumatic Hose – This flexible tubing carries the compressed air to the cylinder. Pneumatic hoses insert into the fittings to create an air tight seal. Hose come in various sizes, lengths and materials.
Pneumatic Valve – A pneumatic valve controls the air flow into and out of a cylinder. There a many types, sizes, and configurations of valves, and not all valves work with all cylinders. In our case, to operate a double acting cylinder, we need what is called a 5-way valve. Below is a simple diagram of a 5-way valve.
Valves can be controlled either electronically by a switch or a computer, or mechanically by pushing a lever or button. The valves on your robot chassis are electronically controlled by activating a switch on the robot transmitter.
Compressor- A compressor is a powered machine that creates and delivers compressed air. Compressors usually have a tank that they fill with the compressed air, and fittings on the outside of the tank to which hoses are connected. Most compressors also have a ‘regulator’ connected between the tank and the output hoses that controls the pressure being delivered. Compressors also have pressure gauges that tell what the tank pressure and the regulated output pressure are.
Pneumatic Circuits
Below is a diagram of a pneumatic circuit. It consists of the parts arranged in a certain order so that a cylinder will function as intended.
The circuit begins with a compressor which is the source of compressed air which comes directly out of the compressor tank and passes through a regulator that controls the output pressure of the air. After passing through the regulator, air travels to the valve via an air hose. The valve directs the air into one of two hoses that exit the valve and go to the cylinder. When the valve is activated, either by a person, a computer or some sort of manual input, the cylinder will actuate (push or pull).
One characteristic to understand about pneumatic cylinders is that when they actuate, they push or the entire length of the stroke. Unlike a hydraulic cylinder, they cannot be stopped part way through their stroke unless some outside mechanical stop forces them to do so.
Pneumatic Cylinder Specifications
There are several characteristics of a pneumatic cylinder that must be known in order to properly use a cylinder for a specific task. They are:
- Single or Double Acting – A single acting cylinder either pushes or pulls only, while a double acting cylinder pushes and pulls.
- Bore – The bore of a cylinder is the internal diameter of the cylinder barrel. The bore partly determines how strong the cylinder is or how much force it can exert. All pneumatic have a round bore, and the diameter of the bore is usually specified in inches or millimeters. Knowing the bore of the cylinder along with the operating pressure allows for the strength of the cylinder to be calculated. Cylinder bores can range from ¼ inch or smaller to several inches.
- Stroke – The stroke of a cylinder is how far the rod extends out of and into the cylinder. This is specified in inches or millimeters. Some cylinder strokes are as short and ¼ inch and others are as long as 5 feet or more.
- Mounting style – Cylinders can be attached at both ends by a variety of ways, with the most common way being at the lower end of the cylinder as seen in the picture below. This style of mount is called a ‘rear pivot mount.’
- Port size – The ports that allow the hoses to be attached to the cylinder come in different sizes and are usually referred to a ‘npt’ which stand for national pipe thread. Pipe threads come in various sizes such a 1/8th, 1/4, 3/8th, and so on.
The specifications of the cylinder you will be using on your robot are:
- Double Acting
- Bore: 25 mm
- Stroke: 75 mm
- Rear Pivot Mount
- 1/8’’ npt x ¼ fittings
Working with a Pneumatic Cylinder
To make a cylinder do what we want it to do, let’s first look at how an air cylinder works. To make a cylinder push, compressed air is directed by the valve into the lower port of the cylinder, as seen below. The air pushes on the bottom of the piston disc and causes the rod to move outward. The air will push the piston and rod all the way to the end of the cylinder.
To make the cylinder pull, the valve directs compressed air into the forward port where it pushes on the other side of the piston, as seen below. The compressed air pushes the piston and rod all the way back to the bottom of the cylinder. By operating the valve, this in and out motion can be repeated as long as there is a supply of compressed air.
So, how does air push something? We have all felt the wind push on us when we are outside or when we stick our hand outside a moving car window, but there is no wind inside pneumatic cylinders. When air is squeezed into a small space, it exerts a force on the inner walls of the container it is in. The more it is squeezed, or compressed, the harder it pushes. A measure of how hard the air is pushing on the inner walls of its container is called pressure, and this is defined as the force with which the air is pushing divided by the area of the surface on which it is pushing. In equation form, it looks like:
or Equation 1
Pressure has units of pounds per square inch, or ‘psi.’ Pressure is not just air pushing on a surface. It is any push or pull acting on an area. For example, when you stand upright there is pressure exerted on the bottom surface of your feet. Tires exert pressure on road surfaces. Water exerts pressure on the inside of a water bottle. Likewise, compressed air introduced into a pneumatic cylinder pushes on the inside walls of the cylinder and it also pushes on the round surface of the piston causing it to move.
We can rearrange Equation 1 so that we can calculate the force compressed air at a certain pressure exerts on a surface of a known size.
or Equation 2
Being able to calculate force given pressure and area is quite useful, as we shall see below.
Calculating the Force Exerted by a Cylinder
One of the most important considerations when working a pneumatic cylinder is the force (push or pull) it exerts. Calculating the force of a cylinder requires knowing two pieces of information: the area of the cylinder and the air pressure.
So, how do we find these two pieces of information? Let’s first look at how we find the area of the cylinder which is actually the cross-sectional area of the piston. To find this, we need to know the bore of the cylinder. The bore is a specification usually found on a label on the cylinder, and some cylinder manufactures even stamp the bore on the barrel. Remember, the bore of a cylinder is the inner diameter of the barrel, a number given in either inches or millimeters, and the piston is circular disc that is the same diameter as the bore.
To calculate the area of the piston, we use the equation
Equation 3
where is 3.14, and r is the radius of the bore, or one half of the diameter:
Equation 4
So, we simply divide the bore diameter of the cylinder by 2, square it and multiply it by 3.14 to get the area.
Example: Calculate the area of a cylinder whose bore is one inch.
We will use Equation 3 to find the area. To use Equation 3, we need to know the value of and the value of r. We already know the value of is approximately 3.14. We can use equation 4 to find r:
r = bore/2
r = 1 inch/2
r = 0.5 inch
Now, insert this value for r and the value of 3.14 into Equation 3:
Area = x r2
Area = (3.14) x (0.5inch)2
Area = 0.785 in2
Now that we know the area of the cylinder (or piston), we need to know what the air pressure is. The pressure of the air coming out of the compressor is set by the regulator and can be controlled by turning a small dial. A gauge on the regulator indicates what the output pressure is. Usually, to get the air pressure, we simply read the dial on the regulator.
Now that we now know how to find the area of the cylinder and the air pressure, we can calculate the force a cylinder produces. To do this, we use Equation 2 from above:
We simply multiply the air pressure we read on the regulator by the area of the cylinder and this will tell us the force the cylinder exerts.
Example: We have a cylinder with a 2-inch bore. If we attach this cylinder to a compressor whose regulator is set at 100 psi, how much force will it exert?
To answer this, we use Equation 2:
The pressure is given at 100 psi, so the equation becomes
All we need to do is find the area of the cylinder. We use Equation 3:
We know = 3.14, so now we need to find , and we use equation 4 to do this.
The bore is given as 2 inches, so the equation becomes
We now know everything we need to use Equation 3 to find the area.
Since we now know the area and the pressure, we can use Equation 2 to find the force:
This cylinder could lift two of you! Knowing how to determine the strength of a pneumatic cylinder is one of the most important things to know when working with cylinders.