Pneumatic Cylinders Chapter 3
Basic Operation • A cylinder is a device used to produce linear motion. • It is composed of 2 basic assemblies, the barrel and head assembly and the piston and piston head assembly. • As compressed air is applied to the rear of the piston and the chamber in front of the piston is open to exhaust.
Basic Operation • To retract the cylinder, the air on the rear of the piston is exhausted to atmosphere and compressed air is applied to the front of the piston. • The pneumatic cylinder is composed of several precision machined parts. • Components common to most cylinders include the barrel, the front and rear head, the piston, the piston rod and a number of seals.
Cylinder Components • The barrel • Is a carefully machined cylindrical tube. • Usually made of aluminum, brass, or stainless steal. • It forms a chamber which contains the air and provides a surface for the piston packing to seal against. • It is closed at each end by heads. • Each head may also contain provisions for mounting the cylinder
Cylinder Components • They are available in two styles: round head and square head. • The rod bearing • These give support to the piston rod and guides it during its stroke. • These are made of bronze, nylon, or other bearing material and may be pressed into the head or placed in a replaceable cartridge.
Cylinder Components • The rod seal • Prevents air from escaping between the piston rod and the front head. • They are usually of o-ring or u-cup design and are usually made of buna-N rubber. • The rod wiper • Is designed to prevent dirt and foreign material from entering the cylinder and damaging the rod bearing and seal
Cylinder Components • The piston seal is designed to prevent air from passing between the piston and barrel. • O-ring seals are the most popular and are usually standard in round line cylinders • They come in a variety of materials and provide a simple, economical means of sealing the piston to the barrel.
Cylinder Mounts • Cylinder mounting styles are: • Centerline, foot, or pivot. • Centerline Mounts • Centerline mounts provide the most secure means of mounting a cylinder. • With this mount the thrust is on the same plane as the mount thereby removing compound forces from the mounting bolts
Cylinder Mounts • This type of mount is very strong but requires accurate alignment • Foot mounting • Foot mounted cylinders are rigidly mounted, but the mounting plane does not run through the centerline of the cylinder. • This places compound forces on the mounting bolts and creates higher stresses than centerline.
Cylinder Mounts • Pivot Mounting • Some applications, such as moving a lever, require a cylinder to move through an arc as it cycles. • To move a cylinder through an arc it is necessary to pivot mount the cylinder. • The 2 types of pivot mounts are clevis and the trunnion.
Cylinder Mounts • Clevis Mount • Is attached to or is an integral part of the cylinder. • This gives a pivot point at the rear of the cylinder. • Normally a rod clevis is used with a clevis mount to provide a pivot point at the rod end.
Cylinder Mounts • Trunnion Mount • A trunnion can be attached at any point along the cylinder. • This allows pivoting from the front or middle of the cylinder which reduces some of the side load inherent in pivot mounting. • A rod clevis is recommended for use with trunnion mounting.
Cylinder Types • Cylinders are available in a variety of types to meet the requirements of the various situations in which they are applied. • They fall into 2 major categories: • Single acting and double acting.
Cylinder Types • Single acting cylinders • A single acting cylinder is any cylinder which uses air on only one stroke. • It uses some other means, usually a spring to move the piston on the other stroke. • The stroke using air is the working stroke. • They are usually classified as push type and pull type.
Cylinder Types • Push type cylinder • Push type cylinders use air to extend the piston. • When this air is exhausted the spring returns the piston to its retracted position. • Pull type cylinder • A pull type cylinder uses air to retract the piston and the spring extends the piston when the air is removed.
Cylinder Types • Single acting cylinders are often used for clamping applications and for those applications where the cylinder must move to a certain position whenever the air is removed. • A typical application would be to open and close gates in a grain elevator. • If air pressure is lost the cylinder would close the gates to prevent loss of grain.
Cylinder Types • Double Acting Cylinders • Is a cylinder designed to use air to extend and retract the piston. • Double acting cylinders are far more popular than single acting. • They are available in single rod end and double rod end, cushioned and non-cushioned.
Cylinder Types • Although double acting cylinders are designed to use air on both strokes. • They may be used as single acting cylinder with load weight, counter weights or external springs to return the cylinder.
Cylinder Types • Single Rod End • The single rod end, double acting cylinder are by far the most popular cylinder in industry. • It is used to move parts and levers, position tools, control product flow, assemble products, and package goods. • They are available in cushioned or non cushioned.
Cylinder Types • Double Rod End • The double rod end cylinder is a specialized cylinder which has a piston rod extending from the piston through each end of the cylinder. • It can be used in applications where 2 work stations are being used alternately. • This allows one work station to be loaded and unloaded while the other is being worked.
Cylinder Types • The second common application is where one end of the cylinder is used to do the intended work and the other end is used to actuate limit valves which control the work. • Cushioned Cylinders • Are designed to reduce the speed of the piston during the last fraction of an inch of the stroke. • This reduction in speed, when properly applied, reduces shock and vibration which means less noise, wear, maintenance.
Cylinder Types • However they have little effect when applied to a massive load moving at a rapid speed. • In such applications an external means of cushioning should be used. • Non Rotating Rod Cylinders • Some applications call for cylinders in which the rod will not rotate inside the cylinder.
Cylinder Types • Rodless Cylinders • These cylinders are available for applications which require long strokes and must operate in limited spaces. • A conventional cylinder takes up at least twice its stroke in linear space. • A rodless cylinder occupies little more than its stroke length. • The 2 types of rodless cylinders available are cable and the magnetic band cylinder.
Cylinder Types • Cable Cylinders • These double acting cylinder consist of a piston which reciprocates within the barrel and a flexible cable which is attached to both sides of the piston and runs out each end of the cylinder. • The load is attached to this bracket which reciprocates opposite piston.
Cylinder Types • Magnetic Band • This cylinder has a lug mounted to the side of the piston which projects through a slot in the extruded aluminum barrel. • The slot is sealed by two stainless steel bands, one inside and one outside, held in place with permanent magnets. • As the piston moves, the jug separates the bands between the piston and recloses them after it has passed through.
Cylinder Types • This cylinder has all the features of the cable cylinder and has eliminated most of the problems. • They are available in lengths up to 30 feet and generate a force up to approximately 440 pounds. • Tandem Cylinders