Chapter 8 - The pneumatic cylinder &#; part 2
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In chapter 7 we looked at the most important features and characteristics of a pneumatic cylinder:
In this chapter we will cover:
6. Cushioning
Compressed air can enter the cylinder at a very high speed. If the piston hits the cap or head at high speed, it can lead to damage. In order to avoid that, most cylinders are equipped with end-of-stroke absorbers / cushioning&#;s, which reduce the piston&#;s speed shortly before it reaches the cap and thus reduce shock.
There are two ways to reduce the shock:
Flexible Shock Absorbers
The easiest way to avoid a hard shock is to assemble some kind of soft material between the piston and the cap/head. It comes in form of a ring which is usually made from polyurethane and thus offers very good shock-absorption. This design is often times used for cylinders with rather small diameters where the strain is not that high. The same kind is used for compact cylinders where the small dimensions do not allow any larger devices. The flexible shock absorbers are highlighted with red arrows in the graph below.
Adjustable (pneumatic) Cushioning
The adjustable pneumatic cushioning is used if it comes to stronger forces due to higher speeds or bigger sizes.
This way of reducing shock is efficient as well as wear-free. During the last 10 to 50 mm of its travel (depending on the size of the cylinder), the piston builds an air cushion inside the cylinder. The degree of cushioning (= speed reduction) can be adjusted at both ends of the cylinder for both sides respectively. The mechanism is a flow-regulating valve.
Both tie-rod and profile cylinders offer that feature, according to ISO . Rodless cylinders often offer it, too. Round cylinders (ISO ) of larger diameters, as well as special cylinders, can be equipped accordingly.
The components of the cushioning are marked with red in the graph below.
Below you can learn more about the function of the adjustable pneumatic cushioning:
During the movement of the piston, the compressed air exhausts through the port (8) (graph 1).
Before the piston reaches the head, the brake piston (4) &#; which is part of the cylinder piston (5) &#; prevents the air within the braking chamber (7) from exhausting through the port (8) (graph 2). The air trapped there can only exhaust through a much smaller orifice. The orifice can be adjusted with the throttle screw.
Inside the braking chamber (7) pressure goes up and generates a temporary air spring. Its resistance remains there until the air has completely exhausted through the throttle screw (2).
CAUTION! The throttle screw can only adjust the degree of cushioning / speed of the cylinder piston for the last 10 to 50 mm of its movement.
How to adjust the speed of a cylinder in general will be covered later in this chapter.
7. Detection of cylinder position (magnetic)
Sensors are absolutely necessary elements when it comes to industrial automation. Sensors generate information and control entire processes.
In order to detect the position of the piston, sensors which are triggered by a magnetic field are assembled onto the pneumatic cylinder. The cylinder piston is equipped with a magnet so that the sensor can receive a signal.
There are two different types of sensors / switches:
REED switch
REED switches consist of two ferromagnetic nickel-iron wires. They are packed together in a glass tube filled with a noble gas and are made from a magnetic material.
Inductive, PNP switch
The function of the inductive PNP switch is based on the principles of a bipolar junction transistor. When the magnet that is built into the piston gets close to the switch, it gives a well-defined signal. The switch can be used &#;normally open&#; as well as &#;normally closed&#;.
An LED displays the condition of the switch.
PNP switches have generally 3 wires. They work within a voltage range of 5 to 30 V DC.
Advantages of the PNP switch in comparison to the REED switch:
8. Speed control of cylinders
To control the speed of a pneumatic cylinder (actuator) over the entire stroke, flow regulators or flow-regulating silencers can be used.
To control the speed, we regulate the exhaust air flowing out of the cylinder chamber. We thereby avoid an immediate exhaust. The air is in both chambers available as long as the piston has reached the end position. The movement is therefore very smooth.
CAUTION! In order to set the speed of the cylinder and to get a smooth movement, it is always the exhaust air that has to be controlled.
To do this there are different products available for speed regulation:
Uni-directional flow regulator = one-way flow control valve
In order to allow compressed air to flow into the cylinder at full speed, while allowing it to exhaust slowly, we use a one-way flow control valve.
There are many different designs of flow-regulators available and their sizes differ depending on the manufacturer.
Function fittings are screwed either directly into the cylinder or into the valve. Therefore there are two different types, one for each area of application.
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Sample circuits for cylinders with speed control
Below you see three different ways of speed control for a double-acting cylinder, with air supply via a joint FRL.
Circuit 1 (Cylinder C1):
The double-acting cylinder C1 is controlled by the single solenoid 5/2-way valve S1. As soon as the valve is actuated, air flows into the Plus chamber of the cylinder via F1.1. The air flow is not restricted by F 1.1. Simultaneously, air needs to exhaust from the Minus chamber of the cylinder via F1.2. This flow is restricted by F1.2. Via the directional valve S1 (port 3), the air is finally exhausted. As soon as the electric actuation of the directional valve S1 is taken away, the valve switches back to normal position. Air flows at full speed via F1.2 into the Minus chamber of cylinder C1, while the air in the Plus chamber exits via F1.1 and S1, exhausting at port 5 of S1. This flow is restricted by / adjusted at F1.1. The positive movement of the cylinder is regulated at F1.2, the negative movement is restricted at F1.1. F1.1. and F1.2. can be screwed either into the cylinder or the valve, but the correct version has to be selected either way!
Circuit 2 (Cylinder C2):
The double-acting cylinder C2 is controlled by the single solenoid 5/2-way valve S2. The speed of the cylinder is adjusted at flow control silencers F2.1 and F2.2. , which are screwed into the valve. For positive movement, the directional valve S2 needs to be actuated. Air streams at full speed from port 1 to port 4 of the valve and into the Plus chamber of cylinder C2. At the same time air needs to exhaust from the Minus chamber of the cylinder. The air streams into port 2 of the valve S2. Its flow is restricted by F2.2 before leaving the valve S2 at port 3. For negative movement, the valve switches back into standard position. The Minus chamber is supplied via port 2 of valve S2. The exhaust from the Plus chamber is restricted by F2.1 in port 5 of the valve S2.
Circuit 3 (Cylinder C3):
The double-acting cylinder C3 is controlled by the single solenoid 5/2-way valve S3. For an extremely quick positive movement after actuating the valve, the Minus chamber is exhausted by the quick exhaust valve F3.2.; the exhaust air does not pass the directional control valve S3. For negative movement, the valve S3 needs to switch back into normal position. The exhaust air of the Plus chamber has to go through the one-way flow regulator F3.1
9. International Standards
The most common cylinders in pneumatics have been standardized, the target being maximum compatibility between products of different manufacturers.
The most common cylinder standards are:
The HAFNER ISO cylinders
The standard ISO is valid since . The previous and very closely related standard was ISO ( - ).
The standard defines piston diameters (ø32...ø320 mm), maximum pressure (10 bar), and distinct features and dimensions, as well as standard accessories.
Thus any accessories are also mostly interchangeable.
The ISO cylinders in the HAFNER range have the type number DIL and DIP (for products with a run- through piston rod: DBL and DBP)
Materials used in an ISO cylinder of the D-series:
Due to high speeds, long strokes and high frequencies the seals can wear out faster than other parts. Therefore we offer spare-part kits for the cylinders. The spare-part kits contain all potentially worn-out parts.
Repair kits for cylinders types DIL, DIP, DBL and DBP have the order code DIR.
Piston and Piston Rings
A piston is a cylindrical engine component that slides back and forth in the cylinder bore by forces produced during the combustion process. The piston acts as a movable end of the combustion chamber. The stationary end of the combustion chamber is the cylinder head. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity. Thermal conductivity is the ability of a material to conduct and transfer heat. Aluminum expands when heated, and proper clearance must be provided to maintain free piston movement in the cylinder bore. Insufficient clearance can cause the piston to seize in the cylinder. Excessive clearance can cause a loss of compression and an increase in piston noise.
Piston features include the piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings. The piston head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal engine operation.
A piston pin bore is a through hole in the side of the piston perpendicular to piston travel that receives the piston pin. A piston pin is a hollow shaft that connects the small end of the connecting rod to the piston. The skirt of a piston is the portion of the piston closest to the crankshaft that helps align the piston as it moves in the cylinder bore. Some skirts have profiles cut into them to reduce piston mass and to provide clearance for the rotating crankshaft counterweights.
A ring groove is a recessed area located around the perimeter of the piston that is used to retain a piston ring. Ring lands are the two parallel surfaces of the ring groove which function as the sealing surface for the piston ring. A piston ring is an expandable split ring used to provide a seal between the piston an the cylinder wall. Piston rings are commonly made from cast iron. Cast iron retains the integrity of its original shape under heat, load, and other dynamic forces. Piston rings seal the combustion chamber, conduct heat from the piston to the cylinder wall, and return oil to the crankcase. Piston ring size and configuration vary depending on engine design and cylinder material.
Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring. A compression ring is the piston ring located in the ring groove closest to the piston head. The compression ring seals the combustion chamber from any leakage during the combustion process. When the air-fuel mixture is ignited, pressure from combustion gases is applied to the piston head, forcing the piston toward the crankshaft. The pressurized gases travel through the gap between the cylinder wall and the piston and into the piston ring groove. Combustion gas pressure forces the piston ring against the cylinder wall to form a seal. Pressure applied to the piston ring is approximately proportional to the combustion gas pressure.
A wiper ring is the piston ring with a tapered face located in the ring groove between the compression ring and the oil ring. The wiper ring is used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil. Combustion gases that pass by the compression ring are stopped by the wiper ring.
An oil ring is the piston ring located in the ring groove closest to the crankcase. The oil ring is used to wipe excess oil from the cylinder wall during piston movement. Excess oil is returned through ring openings to the oil reservoir in the engine block. Two-stroke cycle engines do not require oil rings because lubrication is supplied by mixing oil in the gasoline, and an oil reservoir is not required.
Figure 4 - Piston Rings
Figure 5 - Piston Ring Gap
Piston rings seal the combustion chamber, transferring heat to the cylinder wall and controlling oil consumption. A piston ring seals the combustion chamber through inherent and applied pressure. Inherent pressure is the internal spring force that expands a piston ring based on the design and properties of the material used. Inherent pressure requires a significant force needed to compress a piston ring to a smaller diameter. Inherent pressure is determined by the uncompressed or free piston ring gap. Free piston ring gap is the distance between the two ends of a piston ring in an uncompressed state. Typically, the greater the free piston ring gap, the more force the piston ring applies when compressed in the cylinder bore.
A piston ring must provide a predictable and positive radial fit between the cylinder wall and the running surface of the piston ring for an efficient seal. The radial fit is achieved by the inherent pressure of the piston ring. The piston ring must also maintain a seal on the piston ring lands.
In addition to inherent pressure, a piston ring seals the combustion chamber through applied pressure. Applied pressure is pressure applied from combustion gases to the piston ring, causing it to expand. Some piston rings have a chamfered edge opposite the running surface. This chamfered edge causes the piston ring to twist when not affected by combustion gas pressures.
Another piston ring design consideration is cylinder wall contact pressure. This pressure is usually dependent on the elasticity of the piston ring material, free piston ring gap, and exposure to combustion gases. All piston rings used by Briggs & Stratton engines are made of cast iron. Cast iron easily conforms to the cylinder wall. In addition, cast iron is easily coated with other materials to enhance its durability. Care must be exercised when handling piston rings, as cast iron is easily distorted. Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring.
Compression Ring
The compression ring is the top or closest ring to combustion gases and is exposed to the greatest amount of chemical corrosion and the highest operating temperature. The compression ring transfers 70% of the combustion chamber heat from the piston to the cylinder wall. Most Briggs & Stratton engines use either taper-faced or barrel-faced compression rings. A taper faced compression ring is a piston ring that has approximately a 1° taper angle on the running surface. This taper provides a mild wiping action to prevent any excess oil from reaching the combustion chamber.
A barrel faced compression ring is a piston ring that has a curved running surface to provide consistent lubrication of the piston ring and cylinder wall. This also provides a wedge effect to optimize oil distribution throughout the full stroke of the piston. In addition, the curved running surface reduced the possibility of an oil film breakdown due to excess pressure at the ring edge or excessive piston tilt during operation.
Wiper Ring
The wiper ring, sometimes called the scraper ring, Napier ring, or back-up compression ring, is the next ring away from the cylinder head on the piston. The wiper ring provides a consistent thickness of oil film to lubricate the running surface of the compression ring. Most wiper rings in Briggs & Stratton engines have a taper angle face. The tapered angle is positioned toward the oil reservoir and provides a wiping action as the piston moves toward the crankshaft.
The taper angle provides contact that routes excess oil on the cylinder wall to the oil ring for return to the oil reservoir. A wiper ring incorrectly installed with the tapered angle closest to the compression ring results in excessive oil consumption. This is caused by the wiper ring wiping excess oil toward the combustion chamber.
Oil Ring
An oil ring includes two thin rails or running surfaces. Holes or slots cut into the radial center of the ring allow the flow of excess oil back to the oil reservoir. Oil rings are commonly one piece, incorporating all of these features. Some on-piece oil rings utilize a spring expander to apply additional radial pressure to the piston ring. This increases the unit (measured amount of force and running surface size) pressure applied at the cylinder wall.
The oil ring has the highest inherent pressure of the three rings on the piston. Some Briggs & Stratton engines use a tree-piece oil ring consisting of two rails and an expander. The oil rings are located on each side of the expander. The expander usually contains multiple slots or windows to return oil to the piston ring groove. The oil ring uses inherent piston ring pressure, expander pressure, and the high unit pressure provided by the small running surface of the thin rails.
The piston acts as the movable end of the combustion chamber and must withstand pressure fluctuations, thermal stress, and mechanical load. Piston material and design contribute to the overall durability and performance of an engine. Most pistons are made from die- or gravity-cast aluminum alloy. Cast aluminum alloy is lightweight and has good structural integrity and low manufacturing costs. The light weight of aluminum reduces the overall mass and force necessary to initiate and maintain acceleration of the piston. This allows the piston to utilize more of the force produced by combustion to power the application. Piston designs are based on benefits and compromises for optimum overall engine performance
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