Hydraulic cylinders are actuation devices that convert the hydraulic energy of pressurized fluids into the mechanical energy needed to control the movements of machine linkages and attachments. This energy conversion generates linear force and motion. Hydraulic cylinders are an important component overall in the field of hydraulics, a specialized form of power transmission that harnesses the energy transmitted by moving liquids under pressure and converts it into mechanical energy. Power transmission, as a general term, refers to the process of using technology to translate energy into practical, usable forms. Within the category of power transmission, hydraulics falls within the sub-category of fluid power, which depends on moving fluids (both gases and liquids) to produce energy.
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History of Hydraulic Power and Cylinders
The history of hydraulic cylinders is inextricably tied to the history of hydraulic power in general. Technically speaking, hydraulics can be dated back to ancient times, when the power of moving water was harnessed for various applications. The most basic application of hydraulics was using moving water to move wheels. Ancient Rome used this sort of hydraulics to operate mills producing a wide variety of products (e.g. flour, timber, etc).
Modern hydraulics can be traced back to 1648, when the French scientist Blaise Pascal discovered that the pressure in a confined fluid must stay constant and is exerted equally in all directions. However, this theoretical principle (known as "Pascal’s Law" or "Pascal’s Principle") was not put to any real practical use until the following century. In 1738 Daniel Bernoulli built upon Pascal’s work by describing fluid behavior under various flow and height conditions (Bernoulli’s Principle) and using his insights to power pumps and mills. In 1795, Englishman Joseph Bramah patented the first practical hydraulic machine: a hydraulically driven press. Almost half a century later (1840), William Armstrong developed more efficient applications of hydraulic power than watermills, including a hydraulically driven crane. Together, Bramah and Armstrong are often considered the "fathers" of modern hydraulics.
Cylinders played a fundamental role in both Bramah’s and Armstrong’s work. Bramah’s practical breakthrough occurred when he discovered how to power the moving plate of his press through the liquid flow between a smaller cylinder and larger cylinder. Armstrong’s crane was based on a specialized sort of cylinder known as a “jigger.” As the field of hydraulics developed and continues to develop, cylinders developed accordingly in terms of various factors such as material composition, internal architecture, mounting positions, etc.
Advantages of Hydraulic Systems and Cylinders
The use of hydraulic systems in general offer several advantages within the overall field of power transmission. Some of those advantages are described in our article on Hydraulic Pumps.
How Hydraulic Power Works
The essence of hydraulics lies in the fact that liquids are incompressible (unlike gases). Because of this fact and Pascal’s Principle, force applied at one point in a confined liquid can be transmitted efficiently to another point in that liquid and used to power various mechanisms. A fuller explanation of the principles behind hydraulic power can be found in our article on Hydraulic Pumps.
How Hydraulic Cylinders Work
It has previously been noted that “Pascal’s Law” applies to confined liquids. Thus, for liquids to act in a hydraulic fashion, it must function with some type of enclosed system. An enclosed mechanical system that uses liquid hydraulically is known as a hydraulic power pack or a hydraulic power unit. Generally speaking, these packs/units consist of a reservoir (to store unused hydraulic fluid), a pump (to supply the rest of the hydraulic system with fluid), various types of tubes (to transport the hydraulic fluid), and actuators (devices that actually convert the energy produced by hydraulic liquid flow into mechanical power).
Hydraulic cylinders form a major type of hydraulic actuator. The other major type of actuator is a hydraulic motor. The main difference between hydraulic cylinders and hydraulic motors lies in the fact that hydraulic cylinders primarily produce linear mechanical motion while hydraulic motors primarily produce rotary mechanical motion.
Although a hydraulic power pack is designed overall to harness the energy of fluid transmission, the cylinders represent the part of the pack where energy conversion truly takes place. Within the cylinder (or cylinders, as there are sometimes several) is a gear system and two valves alongside the piston. On one end is the intake check valve, with the discharge check valve being located on the opposite end. As with the cylinder, some systems have just one piston or gear cog while others have multiple.
Within a hydraulic system, tubing and a pressure vessel (or hydraulic pump) are needed to store and transport the fluid. When pressurized hydraulic fluids are introduced to the vessel, it presses upon the piston and engages the attached rod. When the pump is working, the piston is withdrawn. This creates a vacuum that draws hydraulic fluid from the reservoir, through the hosing and intake valve and finally into the cylinder. When the piston is returned to its original position and the check valve closes, the fluid becomes pressurized. This pumping action is repeated at variable speeds until enough pressure has been built up in the cylinder to force the fluid to pass through the discharge valve. This creates the energy needed to work the attached machinery and move the intended load.
Direction is determined by what side of the piston meets with the pressurized fluid. Fluid above the piston will retract the rod, while fluid below it will cause it to extend. The introduction of various amounts of pressurized hydraulic fluid on either end controls the movement of the piston, rod and attached load.
Parts of a Hydraulic Cylinder
A hydraulic cylinder (suppliers of which can be found on IQS Directory) holds some of the most important mechanical components within a hydraulic system. Despite their impressive role of converting kinetic energy into mechanical energy, basic hydraulic cylinders are relatively simple devices. Major components of hydraulic cylinders are included in the following list:
- A round, rectangular or oval tube shaped barrel makes up the main body of the cylinder that houses and connects all of the components.
- At one end of this barrel is the cylinder cap, which closes off and seals the non-moving end of the cylinder. The cylinder head closes off the other end, but has a round seal through which the piston rod may move in and out. Double acting cylinders have a cylinder head on both ends and no end cap.
- The piston face is a metal disc-like part that fits exactly in the cross section of a cylinder barrel, dividing the chamber into two smaller compartments. The piston is essential to the generation of linear motion by means of hydraulic fluid pressurization. Cylinders of any body type use hydraulic pistons, (such as those offered by suppliers listed on IQS Directory) to lift, turn, tilt, press, steer, pull and push heavy machine components and any attached loads. This strenuous work necessitates they be made of durable materials.
- Attached to the piston is the piston rod. The rod is housed partially within the barrel, but extends beyond the body through the cylinder head and is attached to the machine components which must move through various mounting accessories.
- Each compartment within the barrel also has a port through which high pressure hydraulic fluid is introduced and through which un-pressurized fluid is returned to a reservoir.
- Several seals are placed around the piston head, flow valves and cylinder head to ensure that fluids do not leak into, out of or from one compartment to the other causing a loss of pressure and decreased functionality.
Hydraulic Cylinder Images, Diagrams and Visual Concepts
Types of Hydraulic Cylinders
- Double Acting Hydraulic Cylinders
- Use hydraulic pressure to actuate the rod to extend and retract in both directions.
- Heavy Duty Hydraulic Cylinders
- Designed for high pressures, high flows and rugged environments. Heavy duty cylinders are particularly suited to demanding industrial and mobile applications.
- High Pressure Hydraulic Cylinders
- Have significantly smaller and lighter designs than standard cylinders, which saves substantial weight and space in equipment. High pressure hydraulic cylinders are used in applications that need high forces and short or medium strokes, such as material testing and material transforming.
- Hydraulic Cylinder Manufacturers
- Make the apparatus that changes hydraulic fluid into mechanical power.
- Hydraulic Pistons
- Short, cylinder shaped discs housed within cylinder barrels in order to compartmentalize the enclosed space within hydraulic cylinders.
- Hydraulic Rams
- The large output pistons.
- Hydraulic Cylinders
- Devices that convert pressured fluid into mechanical power. Hydrolic is a common misspelling of hydraulic.
- Large Bore Cylinders
- Designed for a multitude of applications in which a large force is required in both the push and pull directions.
- Mobile Hydraulic Cylinders
- Used in many applications, such as snowplows, construction equipment, personnel lifts and material handling equipment.
- Replacement Cylinders
- Manufactured and installed in older equipment with outdated cylinders. Replacement cylinder manufacturers offer products that will update machinery with the newest cylinder technology.
- Single-Acting Cylinders
- Use hydraulic pressure to actuate the rod in only one direction.
- Small Hydraulic Cylinders
- Can have strokes of less than an inch and are used in applications that require extreme precision.
- Stainless Steel Hydraulic Cylinders
- Linear actuators designed specifically for highly corrosive environments as well as those where hygienic cleaning is essential to industrial processes.
- Stepped Cylinders
- Two-way hydraulic cylinders that provide a faster starting stroke and a subsequent, more powerful working stroke.
- Telescopic Hydraulic Cylinders
- Have multiple stages which enable longer strokes to be achieved, while utilizing less space.
- Thread Cap Cylinders
- Have their head gland threaded on and the threads are protected by an o-ring.
- Tie Rod Hydraulic Cylinders
- Use one or more steel rods, which are installed on the outside diameter of the cylinder housing, to provide extra stability. Cylinder tie-rods commonly bear a large portion of the applied load.
- Welded Hydraulic Cylinders
- Made of a heavy duty, smooth welded housing for increased stability. Most housings of hydraulic cylinders are made of multiple parts, but not in the case of welded cylinders.
Applications of Hydraulic Cylinders
Hydraulic systems and their use are abundant in a wide variety of fields including construction fields, agricultural fields, industrial fields, transportation fields (e.g. automotive, aerospace), various marine work environments, etc. Lifts, material handling equipment, snowplows, brakes, power steering, excavators, dozers, back hoes, cranes, elevators, forklifts, jacks, dump trucks, spacecraft, ships, planes and even advanced robotic arms utilize the power of hydraulics.
Hydraulic cylinders are extremely variable, allowing their use in a number of different industries. Agriculture, construction, oil and gas, manufacturing, military, machining, automotive, aviation, robotics, aerospace and waste removal industries all make use of these devices which greatly increase efficiency and mechanical capabilities as the output force is much higher than the initial force applied.
As industries continue to grow, so do the required capabilities of hydraulic cylinders and the industrial equipment and machines of which they are a part.
Things to Consider When Purchasing Hydraulic Cylinders
While hydraulic power transmission is extremely useful in a wide variety of professional applications, it is generally never wise to depend on one form of power transmission. Each different type of power transmission (electrical, mechanical, and fluid) works best when integrated into an overall power transmission strategy. You should invest in finding honest and skilled hydraulic manufacturers / suppliers who can assist you in developing and implementing an overall hydraulic strategy.
With specific regard to hydraulic cylinders, it is important to note that all cylinder components must be made of durable materials that can withstand the friction and heat created by the use of the hydraulic cylinder.
Stamping or extrusion processes are used to produce seals which are made of nitrile rubber, viton, polypropylene, brass or stainless steel depending on the application.
The pistons are made of brass, steel, stainless steel, aluminum, cast iron, or bronze.
Piston rods and cylinders are made of these same materials, though are produced by different manufacturing processes. Cold rolling is used to manufacture the rods, which are often hard chrome plated to provide protection from corrosion and wear.
The interior surface of the barrel must have a micro-smooth surface, allowing the piston to move cleanly through the body with minimal energy lost to friction.
The above components must also be compatible with the hydraulic fluid which is generally a mineral, oil, ether or water composite. However, selecting the proper hydraulic cylinder for a specific application involves more than manufacturing technique, body material and fluid. Further considerations include maximum operating pressure, stroke, bore size and rod diameter among others. As the work force generated by pressurized hydraulics can vary significantly, it is important to understand system requirements before choosing a specific model.
Maintenance for Hydraulic Cylinders
Hydraulic cylinders are a source of energy, just like commonplace air cylinders, for a large number of pumps and motors. If your industrial machinery runs on hydraulic equipment, then you can understand how problematic the cost of their repair and maintenance can become. Repair and replacement are two aspects that undeniably stress a manufacturing business. This cost adds to the final production cost and decides the ultimate market price of a product. Therefore, if you want to keep your costs or expenditure down and set your MRP per the expectations of your consumers, you ideally need to contain the costs of repair and replacement.
- Maintenance Because of Design
- According to industry research, as many one in ten industrial machines does not operate how it should—specifically because of design factors. To get the best out of your machines, you need to make sure that the machine that you have chosen fits into your production and capacity requirements. Additionally, the source of energy, i.e. hydraulic cylinder, should also be chosen according to the specifications of the appliance.
- To keep machine repair and replacement costs under control, it is necessary to perform maintenance activities as per the scheduled time and whenever required. On time and accurate upkeep is the only way to increase the efficiency and endurance of your industrial machines. However, careful handling should never be overlooked. This article in the following subsections suggests a few tips on minimizing the expenses of machine handling during maintenance.
- Check for Twisted Rods
- Twists in the rods of an air cylinder are a result of poor quality materials and construction. In addition, these abnormalities can also be associated with an inappropriate rod diameter or the improper installation of the cylinder or the rod. Bent rods promote poor load balancing which can result in further problems like leakage and unexpected downtime in the application’s performance.
- You need to check if rods and cylinders have been mounted with the application accurately, as per the recommendations of your hydraulic cylinder supplier.
- Check the Quality of the Rod
- Material quality has been discussed in the previous subsection, but the quality of the rod finish is also very important. If you want the rod to work seamlessly with the load or the object, you will need to ensure that it has a superior finishing. The surface finishing should be neither very smooth nor very rough. The finishing should also suit the object for which the rod is being used. Sometimes, industry experts suggest using alternative finishing or coating for a rod in order to extend the life and strength of the rod.
- In addition to these factors, the bearing or wearing area should also be chosen carefully. The wearing area, when given insufficient support to bear the load, causes distortion of the seal and impacts the performance of the machine adversely.
Care for Hydraulic Cylinders
Even though hydraulic systems are simpler when compared to electrical or mechanical systems, they are still sophisticated systems that should only be handled with care. For hydraulic cylinders, it is especially important that they are applied to their intended usage – i.e. linear pushing or pulling operations. Generally speaking, it is unwise to extensively use hydraulic cylinders in situations involving bending motions and sideways pressure. Even when outfitted with proper accessories that enable more than just linear motion (e.g. a clevis), best practices include using a hydraulic cylinder for non-linear motion only on occasion.
It has been emphasized previously that hydraulic cylinders must be composed of durable materials due to the large amount of stress they undergo. However, even cylinders such as stainless steel hydraulic cylinders can eventually corrode or become damaged. Thus, replacement cylinders are also available to increase the longevity of hydraulic systems. Also called redesigned or re-engineered cylinders, replacement components can be used for more than repairs and are often integrated as operational upgrades for pre-existing machinery.
Accessories of Hydraulic Cylinders
There are several accessories that are used in conjunction with cylinders themselves. Flanges are metal rims or collars that strengthen the position of a cylinder. Trunnions refer to types of protrusions that serve to mount devices in particular positions.
One of the most important accessories is a forked device known as a clevis. Although hydraulic cylinders can provide force in only a linear motion, clevis fittings attached to the end of the piston rod allow the attached devices to move in an angular motion. This significantly broadens the usability and applicability of hydraulic cylinders.
Hydraulic Cylinder Terms
- A container in which fluid is stored under pressure. Accumulators have some type of limit in the loading mechanism for maintaining pressure.
- Also called a bleed valve, or a device commonly used on hydraulic cylinders to remove pressurized fluid and air from the system.
- The inside diameter of hydraulic cylinders.
- End closures of hydraulic cylinders, that completely covers the bore area.
- Cap, plug or cover for the fluid passage of hydraulic cylinders.
- Depth Control Cylinder
- An adjustable, mechanical or hydraulic device that is used to limit the stroke of hydraulic cylinders.
- Flow Control
- A device which meters the rate of fluid used in hydraulic cylinders.
- The cavity of a stuffing box used within hydraulic cylinders.
- The end closure of hydraulic cylinders that covers the differential area between the bore area and the piston rod area.
- A type of seal consisting of an elastomer in the shape of a doughnut. O-rings are usually mounted in a groove on hydraulic cylinders for sealant purposes.
- A cylindrical member forming the internal element of assemblies in hydraulic cylinders that transmits or receives motion by a connecting rod. Fluid acts with the piston to convert pressure energy into linear motion within hydraulic cylinders.
- Psi (Pounds Per Square Inch)
- A unit of measurement of pressure within hydraulic cylinders.
- A device found within hydraulic cylinders that turns mechanical energy into either fixed or variable hydraulic energy.
- The large output piston of a hydraulic cylinders press.
- Regenerative Circuit
- A device that directs the rod end discharge to the piston side of hydraulic cylinders, increasing the speed of hydraulic cylinders. Regenerative circuits can be incorporated into a directional control valve as the fourth position on hydraulic cylinders.
- Rephasing Cylinder
- A design that allows the use of two or more hydraulic cylinders in a series. This design automatically synchronizes the position of hydraulic cylinders at the end of every stroke within the hydraulic cylinders.
- A device that connects the bore area to the head of hydraulic cylinders.
- Any cylindrical-shaped part of hydraulic cylinder components that controls the flow passing through the component in accordance with its movement.
- The linear movement of a valve spool or hydraulic cylinder rod that establishes the limits of motion.
- Stuffing Box
- A small chamber is found within hydraulic cylinders that is compressed around a reciprocating shaft or piston to form a seal.