A hydraulic cylinder is a device which converts fluid power into linear mechanical force and motion proportional to the effective cross-sectional area of the piston. Their output force, or motion, is in a straight line. Their work applications may include pushing, pulling, tilting, and pressing. Cylinder type and design are based on specific applications. Cylinders usually consist of a movable element such as a piston and piston rod, plunger or ram, operating within a cylindrical bore. Common types of cylinders include double-acting, single-acting, and telescoping.
The function of hydraulic cylinder is to provide linear motion. It converts the fluid energy into mechanical energy (linear movement). Cylinders are broken down into two main categories: pneumatic and hydraulic. Pneumatic cylinders can be operated by several types of gases, however, compressed air is by far the most common. Hydraulic cylinders can be operated with a very large range of fluids. By far the most common is petroleum based hydraulic fluid. Fire-resistant fluids are also common, they may be synthetic, or water based.
A hydraulic cylinder is made up of 7 main components:
- Cylinder Barrel (Main, Body). The barrel is the cylindrical body of the cylinder. The barrel guides the piston and provides a sealing surface for the piston seal. The barrel is also the structural part of the cylinder that holds the ends in place.
- Cylinder Cap (Back End, Blind End, Blind Head, Rear End). A cylinder end closure which completely covers the bore area (opposite the rod end).
- Cylinder Head (Packing Gland, Stuffing Box, Head End, Front End, Front Face, Rod End). The head gland or stuffing box is a cylinder component that serves more than one purpose. It retains the static and dynamic seals at the head end of the cylinder, it seals off at the end of the cylinders, it is the support or guide for the rod and it is the mechanical stop to keep the piston from falling out of the cylinder barrel.
- Piston. The piston is a cylinder component that has multiple purposes. The piston retains the primary seal from the extend side of the cylinder and the retract side of the cylinder. It is the component that moves back and forth from the hydraulic oil. The piston is the mechanical means of connecting and moving the rod in and out of the cylinder as well as serves as a guide for one end of the piston rod.
- Piston Rod. The rod is also known as the cylinder shaft, plunger or ram. It is the round device that moves in and out of the cylinder. The piston rod can be solid or hollow. The rod is usually plated, or heat treated to provide better wear and corrosion resistance.
- Ports. Cylinder ports are the means in which the cylinder is able to have fluid lines or conduits connected. O-ring Boss (SAE) Port is a port that has straight or parallel threads as well as an O-ring to seal the threads. Pipe port threads are tapered threads that count on interference between the male and female threads to do the sealing. 4-Bolt Flange Port is a port that counts on 4 bolts to hold to two halves together. This type uses an O-ring to do the sealing. BSP is a British Standard Pipe Thread.
- Static seals keep the joint pressure tight.
- Rod wiper / scraper is provided to remove and prevent foreign material from entering the bearing and seal area.
- Rod seals hold pressure in the cylinder by preventing fluid from leaking out.
- Piston seals prevent pressurized fluid from leaking across the piston as the system pressure pushes the piston and rod assembly down the cylinder bore.
A ram-type cylinder is a cylinder in which a cross-sectional area of a piston rod is more than one-half a cross-sectional area of a piston head. In many cylinders of this type, the rod and piston heads have equal areas. A ram-type actuating cylinder is used mainly for push functions rather than pull.
Pneumatic cylinders rarely provide much more than a hundred pounds per square inch of pressure to the piston, whereas hydraulic cylinders are capable of anywhere from 1500 to 10,000 psi, which can be ten to hundred times the force of a pneumatic cylinder.
A cylinder in which fluid force can be applied in either direction to the moveable element. These cylinders are also classified as differential cylinders because of their unequal exposed areas during extend and retract. The difference in effective area is caused by the area of the rod that reduces the piston area during retraction. Extension is slower than retraction because more fluid is required to fill the piston side of the cylinder. However, more force can be generated on extension because of greater effective area. On retraction, the same amount of pump flow will retract the cylinder faster because of the reduced fluid volume displaced by the rod. Less force, however, can be generated due to less effective area.
A cylinder in which hydraulic energy can produce thrust or motion in only one direction. Once the pressure is released, the load (can be spring or gravity returned) on the cylinder retracts the rod. The inactive end is sometimes vented to atmosphere through a breather/filter or vented to reservoir below the oil level.
Accumulators are devices that store energy in the form of fluid under pressure. A vessel which uses compressed gas and fluid to store energy. Accumulators are normally cylindrical and use either a piston or diaphragm to separate the fluid and the gas.
Telescopic hydraulic cylinders, sometimes called multi-stage cylinders, are a type of linear actuator consisting of a series of tubular rods called sleeves. The sleeves (usually 2 to 6) sequentially decrease in diameter and are nested inside of each other.
Once hydraulic pressure is introduced to the cylinder, the largest sleeve is extended first. Once the largest sleeve has reached its maximum stroke, the next sleeve begins to extend. This process continues until the cylinder reaches its last stage, called the plunger.
There are three common types of telescopic cylinders: Single-acting, Double-acting, and a combination of single and double-acting.
A telescoping cylinder is a cylinder employing several pistons which telescope into each other. This cylinder is used when a relatively long working stroke is needed for a short cylinder length. Telescopic cylinders are used in a variety of applications that require the use of a long cylinder in a space-constrained environment.
Generally, no. Speed is a factor of flow. You need more flow to make things move faster.
Side loading: Side loading is caused from the cylinder being misaligned which creates an unusual force on the piston rod. A side load of enough magnitude can result in tube scoring, piston rod wear, rod bearing wear, and seal failure.
Contaminated fluid: Contaminated fluid can cause premature cylinder failure. Abrasive particles in the fluid can damage seals, piston rod surface and the ID of the cylinder barrel. Airborne contamination can be drawn into a cylinder by a faulty wiper seal. Contamination occurs in numerous ways. The most common type is drawn in from the pump.
Rough or scored rod: Rough places on the rod damage the seals and reduce their normal life resulting in the necessity for frequent replacement.
Over-Pressurization: Subjecting a cylinder to operating pressures greater than those for which it was designed. Over-pressurization creates extreme forces against various internal components and can result in premature failure.
Mis-staging is a telescopic cylinder extending or retracting in the wrong sequence. When the cylinder tries to correct its sequence, rapid movement and violent slamming between moving stages can occur.
Hydraulic cylinder drift is caused by internal leaks in the cylinder across the piston. The fluid physically moves from one side of the piston to the other which creates an uneven balance and causes the cylinder to move or “drift”.
It’s a sudden, large, and unstable lateral deflection. It can be associated with only a small increase in compressive load above a critical level, known as the buckling load. The corresponding stress can be far less than the yield strength of the rod material.
Piston rod column failure (buckling) can occur if the rod diameter is not sized to match the stroke and load. The manufacturer should be consulted for application assistance.
Column strength considerations: Standard size rods are recommended for use in cylinder applications where column strength, rod sag, or rate of cylinder return do not require an oversize rod. Being more flexible, standard rods absorb shock loads and minimize bearing loads caused by misalignments. For long push stroke cylinders, an oversize rod may be required to prevent column failure and rod bending. Total cylinder length when extended is considered in column strength.
Whenever possible, we highly recommend using SAE O-ring or JIC fittings. Both of these provide a highly reliable, reusable connection. Since these fittings don’t rely on mechanical deformation to create a seal, the risk of a broken fitting or port is virtually eliminated.
SAE O-ring (O-Ring Boss) are straight thread fittings that seal using an O-ring between the thread and the wrench flats of the fitting. The O-ring seals against the machined seat on the female port.
Fittings with O-rings offer advantages over metal-to-metal fittings.
O-ring fittings can either be adjustable or non-adjustable. Non-adjustable fittings are screwed into a port where no alignment is needed. Adjustable fittings can be oriented in a specific direction.
JIC 37° flare fittings seal with metal to metal contact between the flared nose of the fitting and the flared tube face in the female connection.
NPT, National Pipe Thread fittings seal using a metal-to-metal connection. The metal of the male and female threads deforms during installation to create this seal. As a result, pipe thread connections tend to leak after a connection is made, disassembled, and re-assembled. If the connection leaks after re-assembly, you may need to replace one or more of the fittings. Continuing to tighten the connection will not necessarily eliminate the leak and can easily result in a split fitting or port.
Over-tightening the connection can easily split the female portion of a pipe thread connection. This is especially true when installing male pipe thread fittings into cast iron ports on valves, motors, and cylinders. Split ports are not covered by manufacturer’s warranties!
Under or over-tightening any fitting can allow leakage, but all-metal fittings are more susceptible to leakage because they must be tightened to a higher and narrower torque range. This makes it easier to strip threads or crack or distort fitting components, which prevents proper sealing.
Using a liquid thread sealant for NPT connections is recommended over using Teflon tape. If using Teflon tape, only use one-and-a-half wraps around the male portion of the thread. Start two threads up from the end of the male portion of the fitting. Keep in mind that liquid thread sealant and Teflon tape are potential contaminants to the hydraulic system.
Leaks can also result from vibration, thermal cycling and from loads being supported by the connection (i.e. using the fitting in the connection to support mechanical loads).
Bolted joints are typically used to locate and/or affix two or more components that might be subjected to a tensile (‘pulling apart’) and/or a shear (‘sliding apart’) loading. Let us consider a bolted joint, where the nut has been turned on the bolt till all the components just come together. When such a bolted joint is tightened further, a torque is applied to the nut which, (after the initial friction is overcome), causes the nut to turn and stretch the bolt. The body of the bolt acts like a stiff spring. The stretching of the bolt causes a predictable preload to exist in the bolted joint. In simple terms, the preload causes the bolt and the nut to not just ‘hold’ the components together, but squeeze the components together with a ‘clamp load’. This clamp load is the key to the functioning of the bolted joint under different, loading conditions.
An accordion shaped boot or bellow is installed to protect the piston rod and prevent the ingestion of contaminant across the rod seals. It is a flexible part, usually made of leather, elastomer, plastic, or metal.
The purpose of a cushion is to decelerate the piston and rod assembly as it nears the end of stroke, preventing excessive mechanical stresses. Cushions may either be a fixed or an adjustable design. Both designs function by providing a bypass passage to remove the pressurized fluid trapped between the piston and the cylinder head or end cap when the cushion sleeve has entered the cushion bore. Varying the orifice opening with an adjustable cushion screw allows the user to select the best cushion rate for the system. A check valve in the cushion assembly allows for the free flow of fluid back to the piston face for quick acceleration when the rod is withdrawn. Heavy loads may also require an external device to stop the motion. Cushions are available on either or both ends of the cylinder. Consult manufacturer for the conditions under which cushions should be used.
Rephasing cylinders are two or more cylinders plumbed in series or parallel, with the bores and rods sized such that all rods extend and/or retract equally when flow is directed to the first or last cylinder within the system.
The term “smart cylinder” has been in use for decades. It refers to a hydraulic or pneumatic cylinder containing some sort of measuring device that provides electronic feedback of the cylinder’s piston position.
Proximity and Reed switches are used to indicate the passing of the piston within the cylinder. To activate the switches, the cylinder must be modified by placing a magnet in the piston head.
A hydraulic fuse is designed to provide excess flow protection. It automatically shuts off any line in which failure (component rupture or fracture) has occurred and where hydraulic fluid spews out of a system. It is analogous to an electrical safety device called a “fuse” in which a strip of metal melts and interrupts the circuit when the current exceeds a particular amperage. These devices consist of various types of excess-flow valves. Such devices are used to block flow and not to relieve pressure as performed by a hydraulic system protector.
Pilot operated check valve: Because of slight spool leakage on standard directional control valves, we must add a check valve to the circuit if we need to hydraulically lock a cylinder. This type of check valve is referred to as a pilot operated check valve. Unlike a simple check valve, reverse flow is required through the valve to extend or retract the cylinder. This is accomplished by allowing pilot pressure to act on a pilot piston, thus opening the check valve and retracting the cylinder. To extend the cylinder, the check valve allows fluid to flow freely in one direction and blocks flow in the opposite direction. Pilot operated check valves may be pilot to open or pilot to close. This is determined by the application.
Counterbalance valve: A pressure control valve that maintains back pressure to prevent a load from falling. A counterbalance valve is a normally closed pressure valve used with cylinders to counter a weight or potentially overrunning load. Without a counterbalance valve, the load would fall uncontrolled or overrun, and pump flow would not be able to keep up. To avoid the uncontrolled operation, we place a counterbalance valve just after or integrated into the cylinder. The pressure setting of the counterbalance valve is set slightly above the load-induced pressure. This counters the load. As we extend the cylinder, pressure must slightly rise to drive the load down.
Counterbalance valves may also incorporate external piloting for smoother, “non hunting” performance. When the manufacturer utilizes both internal and external pilots, you have the best of both worlds. The internal pilot lowers the load with counter pressure, while the external pilot drops all back pressure when performing work.
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