This article will give an in depth discussion about hydraulic cylinders.
The article will give more detail on topics such as:
A hydraulic cylinder is a mechanical device engineered to produce linear motion using hydraulic pressure. It operates by harnessing hydraulic fluid pressure to move a piston either forward (pushing) or backward (pulling).
This relies on core principles of physical science, which include:
To devise a system utilizing these principles, one may reference the setup shown below.
Because hydraulic fluids are non-compressible, pistons A1 and A2 remain stationary until an external force is applied. Applying force to one piston results in the movement of the second piston, calculated through Pascal's Law as shown here.
The pressure on the left piston with Area A1 and Force F1 can be defined by:
Because pressure is equally communicated through the fluid, it maintains that the pressure on the right will match the left side's pressure:
Consequently, we ascertain:
Hence, the force experienced on the opposing side is the exerted force multiplied by the ratio of the areas. With the force determined, displacement can easily be calculated.
Hydraulic cylinders play an essential role in powering movements in industrial and commercial manufacturing. They find applications in various fields like:
This chapter will explore the different types and piston configurations of hydraulic cylinders, a core component in modern fluid power and motion control systems used across heavy equipment, manufacturing, construction, and automation industries.
In various applications and industries, hydraulic cylinders may also be referred to as hydraulic actuators, hydraulic rams, or hydraulic pistons. Recognizing these alternative names is important when searching for suppliers or evaluating hydraulic systems, as the terminology may vary depending on the context and sector.
While pneumatic actuators are commonly employed where fast, precise, and lower-force motion is required, hydraulic actuators are engineered for applications demanding high force output and the ability to move heavy loads with precision. In industrial automation, process control, and heavy machinery, hydraulic actuators deliver the power needed to operate equipment such as presses, excavators, loaders, and industrial valves. Hydraulic actuators are typically designed around the piston-cylinder mechanism and can be found in both linear and rotary motion systems.
Hydraulic cylinders are available in a wide range of sizes and pressure ratings, making them suitable for customized solutions in construction equipment, agricultural machinery, manufacturing presses, and marine vessels. Selecting the right cylinder size, type, and bore is crucial in hydraulic system design to ensure optimum efficiency, longevity, and safety.
Small hydraulic cylinders, often called compact hydraulic cylinders or micro cylinders, feature a stable design, ease of operation, and durability, making them suitable for long-term use in space-constrained applications. They are commonly utilized in robotics, medical devices, material handling, woodworking, and automation equipment with intricate, compact components. Despite their small size, compact cylinders can deliver significant force, depending on bore size and working pressure.
Common features users evaluate when selecting a small hydraulic cylinder include material type, maximum pressure rating, mounting style (such as flange mount or clevis mount), stroke length, and compatibility with hydraulic fluids or lubricants.
Hydraulic cylinders can also be constructed from various materials, such as:
Stainless steel hydraulic cylinders are chosen for applications where corrosion resistance is crucial, such as chemical processing, food production, pharmaceuticals, and marine environments. While most hydraulic cylinders are manufactured from alloy steels like 1045 and 1018 for their high strength, these materials are susceptible to oxidation and rust in wet or humid conditions. Protective coatings like epoxy paint can be used on carbon steel cylinders, but over time, coatings may deteriorate, exposing the metal to corrosion.
Stainless steel hydraulic cylinders, which resist pitting and corrosion even in harsh conditions, are ideal for offshore oil rigs, deck machinery, shipboard cranes, davits, and boat lifts. Their robust construction reduces maintenance costs and extends service life, making them a valuable investment for industries requiring reliable hydraulic actuation in corrosive settings.
Hydraulic cylinders are categorized as either single-acting or double-acting based on how hydraulic fluid moves within the cylinder. A single-acting hydraulic cylinder has only one port and chamber that is pressurized, producing movement in one direction. In contrast, a double-acting cylinder has two chambers and ports, allowing hydraulic fluid to move the piston both forward and backward for bidirectional operation. Understanding these types is fundamental when specifying hydraulic cylinders for power transmission, heavy lifting, or automated machinery.
A single-acting hydraulic cylinder utilizes hydraulic power in one direction, either for extending or retracting the piston, depending on the design. The location of the pressurized chamber depends on the intended motion: if used for pushing, hydraulic fluid enters the chamber opposite the rod, while the other chamber includes a return spring. If used for pulling, hydraulic fluid is applied to the chamber containing the rod, while the opposite chamber’s spring provides the return mechanism.
Double-acting hydraulic cylinders feature two ports for hydraulic fluid, enabling precise movement in both extension and retraction. This configuration allows for high precision, speed, and force in applications like industrial presses, injection molding machines, and construction equipment. Since both ends of the cylinder can be pressurized independently, this design supports continuous and controlled motion in both directions. Double-acting cylinders are vital in applications requiring repetitive or bidirectional operations in hydraulic systems.
The most common hydraulic piston configurations are ram-style, tie-rod, and welded cylinders. Each design serves unique requirements in hydraulic power transmission, system maintenance, and installation:
Single-acting hydraulic cylinders without pistons but equipped with large rods are known as ram-type hydraulic cylinders. Rams function much like traditional single-acting cylinders, but instead of a conventional piston and seals, the rod serves as the moving force element. This configuration results in high-pressure hydraulic ports at the cap end, while the rod end often lacks a port. Rams are highly efficient for delivering powerful, linear movement and are especially common in equipment such as hydraulic jacks, automobile lifts, and large presses used in steel mills or construction.
Notably, ram cylinders are typically more cost-effective and have fewer wear components compared to piston-style single-acting cylinders.
Ram-type hydraulic cylinders are commonly used for vertical lifting loads, such as in automotive lifts or overhead cranes. While their design makes them ideal for vertical motion, they can also function in horizontal movement with appropriate alignment and support to prevent binding or side-loading. Industries such as mining, material handling, and heavy construction rely on ram cylinders for robust, reliable actuation where significant force is required.
A prime example of a specialized ram cylinder is the telescopic hydraulic cylinder, which optimizes the stroke length in a compact package.
Telescopic hydraulic cylinders, also known as multi-stage cylinders, provide extended stroke lengths in a collapsed, space-saving body. These cylinders are available as single-acting, double-acting, or even a combination of both, making them highly versatile for demanding hydraulic applications. Telescopic cylinders are a subset of linear actuators defined by multiple concentric tubular rods (“stages� or “sleeves�) that nest inside each other. They extend sequentially under hydraulic pressure, allowing for long reach in equipment with limited mounting space.
These cylinders are essential in applications such as construction trucks, dump trucks, refuse vehicles, vehicle trailers, and agricultural equipment where high lifting capacity and compact installation are required. Telescopic hydraulic cylinders are praised for their smooth, controlled extension, efficient force transmission, low maintenance, and ability to meet specific motion and angle requirements.
Structurally, a telescopic cylinder typically features four or five sleeves that decrease in diameter from the outermost (the barrel) to the innermost stage. As pressurized fluid is introduced, the largest stage extends first, followed in succession by each smaller sleeve, providing a long reach with precise control—an important advantage in dump trailers and lifting mechanisms. Users should consider sleeve thickness, maximum collapse length, stroke per stage, and compatibility with hydraulic fluids when specifying telescopic cylinders for their projects.
Tie-rod hydraulic cylinders are widely used in industrial manufacturing environments due to their modular design, ease of maintenance, and cost-effectiveness. These cylinders utilize externally mounted threaded steel rods, called tie rods, to secure the end caps of the cylinder barrel. Depending on bore size and operating pressures, the number of tie rods can range from four to twenty, providing robust structural integrity during demanding hydraulic operations.
Key benefits of tie-rod cylinders include straightforward disassembly for routine inspection, seal replacement, or repair—making them ideal for industries that prioritize minimized downtime. These cylinders are also highly customizable, with a range of mounting styles, port locations, seals, and cushioning options available to suit applications in packaging, plastics machinery, metalworking, and factory automation.
Welded rod hydraulic cylinders feature a seamless barrel welded directly to the end caps, resulting in a compact, high-strength construction that is commonly specified in mobile equipment and heavy-duty operations. The head cap can be retained by various means, such as threading or bolting, to ensure stability under high operating pressures. While their welded, non-tie-rod design increases strength and seal integrity, it may complicate field-servicing or in-place maintenance, often requiring specialized tools or expertise.
Welded hydraulic cylinders are equipped with advanced seal packages, enhancing service life and performance—particularly in environments exposed to abrasive contaminants, moisture, or extreme weather. Due to their narrow profile and sleek appearance, welded body cylinders are often preferred in construction equipment, agricultural tractors, and lifting machinery where installation space is at a premium. Welded hydraulic cylinders are an excellent solution when compact design, rugged performance, and minimal maintenance are required.
When selecting between tie-rod and welded hydraulic cylinders, engineers and equipment designers must evaluate installation space, serviceability, application environment, and pressure requirements. For assistance with choosing the best hydraulic cylinder type for your specific industrial or mobile hydraulic application, contact a reputable hydraulic cylinder manufacturer or supplier.
Hydraulic cylinders are composed of several key components, which will be detailed below.
The barrel, or cylinder tube, is the primary casing of the hydraulic cylinder, typically made from steel, often carbon steel. It is designed to endure the pressure of the hydraulic fluid throughout its operational life. Various grades of steel are available to offer different levels of ruggedness and strength; higher pressures generally necessitate thicker walls and stronger steels.
To protect against corrosion and abrasion, the barrel's surface is treated, either through coating or painting. In some applications, such as food packaging, a coating may not be used to prevent flakes from contaminating the food, so stainless steel might be employed. While the internal walls usually do not require finishing because the hydraulic fluid is generally non-corrosive and provides protection, coatings might be necessary for applications using water as the hydraulic fluid to prevent internal corrosion.
The cylinder rod is the component that extends out of the barrel and is connected to the piston inside. Due to the friction from its movement, the rod is not typically painted. However, it requires protection against corrosion, wear, and pitting, as damage to the rod can lead to seal failure, hydraulic fluid contamination, and overall system failure.
Therefore, the material and coating of the cylinder rod are crucial. The rod is usually made from steel or stainless steel and is coated with Hard Chrome Plating (HCP) for durability. Alternatives such as COREX coating are also used, which is up to ten times less porous than HCP and provides a hardness of up to 1400Hv, nearly twice that of HCP. In environments with extreme corrosion risks, materials like Inconel may be used to enhance durability.
The piston is a disk that separates the two chambers within the barrel and is pushed by the hydraulic fluid. It is attached to the cylinder rod, so the rod's movement reflects the piston's motion. To prevent hydraulic fluid from bypassing the piston, it is equipped with seals, typically U-seals. Additionally, to minimize wear during reciprocation, the piston is also fitted with wear rings.
Seals are often the most vulnerable component in a hydraulic system. High-quality seals help reduce friction and wear, extending the system's service life. Conversely, poor seals can lead to increased downtime and frequent maintenance.
Seals are used throughout the hydraulic cylinder and are made from various materials, depending on their application and the type of cylinder. It is essential that these seals are durable, able to withstand repeated rod movement in and out of the barrel, and effectively prevent contamination.
Cylinder designers choose the appropriate seal for each application by considering several factors. For cylinders operating at high temperatures, seals made from materials like Viton are used to prevent melting. Conversely, cylinders in extremely cold conditions require seals made from materials like polyurethane to avoid hardening and cracking.
For applications involving rapid movements, such as in factories, Zurcon and PTFE seals are commonly selected. Specialized seals with backup rings are used for high-pressure environments. Additionally, for thin hydraulic fluids that might bypass the piston or cylinder end caps, seals with exceptionally tight closures are employed.
Some commonly used seals are detailed are.
The rod seal is the most important seal in hydraulic cylinder application. It is subjected to the harshest conditions in its service. It often sees the most pressure variations and spikes in the system. Its failure can result in fluids leaking into the working environment and can endanger both performance and safety.
Its purpose is to:
Buffer seals are often used in conjunction with another rod or piston seal, such as a U-Cup style seal. They are designed to absorb pressure variations, especially under high load conditions, thereby extending the life of the rod seal. Essentially, the functions of a buffer seal include:
Piston seals are designed to form a seal against the inner cylinder wall, preventing fluid from leaking over the piston head into the adjacent chamber. By keeping pressurized fluid contained on one side of the piston, these seals enable the rod to either extend or retract.
These seals can be categorized as single-acting, where pressure is applied from one side only, or double-acting, where pressure is applied from both sides.
The wiper seal prevents external contaminants from entering the hydraulic cylinder and ensures that the lubrication film is reintroduced into the cylinder when the rod retracts. Despite its critical role, the wiper seal is often undervalued compared to its importance in hydraulic systems.
Hydraulic fluid is a non-compressible liquid used to transmit power within hydraulic machinery and equipment.
Hydraulic fluid can consist of various components but is primarily mineral or petroleum-based, water-based, or synthetic.
For a hydraulic system to operate efficiently, the fluid must be incompressible. These fluids can be categorized as follows:
Petroleum-based or mineral-based fluids are widely used due to their cost-effectiveness and availability. The properties of mineral-based hydraulic fluids are influenced by additives, the quality of the base crude oil, and the refining process. Common additives include rust and oxidation inhibitors (R&O), anti-corrosion agents, demulsifiers, anti-wear (AW) and extreme pressure (EP) agents, viscosity index (VI) improvers, and defoamers. Additionally, these fluids may contain dyes to aid in leak detection. This feature is crucial for identifying leaks, reducing maintenance costs, and extending equipment lifespan.
Water-based fluids are less cost-efficient than petroleum-based fluids and generally offer lower wear resistance. However, they provide the advantage of fire resistance due to their high water content. These fluids are commonly available as oil-in-water emulsions, water-in-oil emulsions, or water-glycol blends. While they can offer suitable lubrication properties, they require careful monitoring to prevent issues. High temperatures in fire-resistant applications can cause water to evaporate from these fluids, increasing viscosity. To maintain proper fluid balance, distilled water may need to be added periodically.
Synthetic fluids are engineered lubricants designed to perform exceptionally well under high pressure and high temperature conditions. They offer several advantages, including:
However, synthetic fluids have some drawbacks. They tend to be more expensive than conventional fluids, may have slight toxicity requiring special disposal methods, and are often incompatible with standard seal materials.
Hydraulic fluid flows into and out of the cylinder through ports located at each end of the cylinder tube, with the hydraulic piston positioned between these ports. It is crucial that these ports are secure, as any weakness can lead to hazardous fluid leaks under high pressure.
Cylinder mountings are typically classified into three categories:
The hydraulic cylinder requires mounting interfaces at both ends: one at the base and the other at the head.
Centerline mounts are the preferred method, as they apply tensional or shear forces against the mounting bolts. These mounts are rigid and require precise alignment with the load. Properly aligned centerline mounts reduce rod bearing and piston loads, extending the cylinder's lifespan. Head mountings are recommended for pull stroke applications, while piston rod end mountings are suitable for push stroke applications.
Foot mounting attaches the cylinder along its side. Because the mounting surface plane is offset from the line of force, the mounting bolts experience considerable shear stress.
The cylinder must be pinned or keyed to handle shear stress and ensure the mounting bolts stay in tension.
Key mounts with keyways, which can be cut into a machine, help accommodate shear loads. They offer precise alignment of the cylinder and make installation and servicing easier.
Only one end of a cylinder needs to be keyed to the machine. Keying both ends can lead to uneven distribution of internal stress and deformation, particularly in long stroke cylinders, potentially reducing performance and lifespan.
Clevis, spherical bearings, and trunnion mounts are common configurations for pivot mountings. These mounts are used when the load needs to travel along a curved path, allowing for such motion and helping to mitigate load misalignment.
Trunnion pins are designed solely for shear loads, so only trunnion bearings with a tight fit that support the entire length of the pin should be used.
Some of the considerations when choosing hydraulic cylinders below:
The first step is to determine the mass you need to move. Once you know the weight of the mass, you can assess the force required to move it. For example, lifting a load straight up requires a force equal to its weight, but moving a load on the ground necessitates overcoming both friction and acceleration. It's also advisable to consider a force that is 120% greater than the calculated requirement for added safety.
Next, examine the geometry involved in moving the mass. For machines like a hydraulic press, which moves up and down, the geometry is straightforward and requires no further adjustments.
However, when the center of the load being moved is offset from the lifting point and perpendicular to it, the force required by the cylinder changes. For instance, in a crane, the cylinder pushes on the boom, which is often positioned far from the load. Typically, the distance from the load to the fulcrum can be ten times the lift force or more. Thus, the closer the lift point is to the fulcrum, the more force the cylinder needs to lift the load.
Flange mounting is optimal for transferring the load along the cylinder's centerline. Non-centerline mounting requires additional support to prevent misalignment.
The next step is to determine the bore size for the cylinder. The force generated by the cylinder is the product of the system pressure and the area of the internal piston surface that the pressure acts upon. This formula is used to calculate the necessary bore size to achieve the desired force.
The bore size will also be influenced by the maximum pressure range of the application. Pressures can vary significantly depending on the specific task of the system. Cylinders are available for test pressure and nominal standard pressure, accommodating different requirements. The system pressure should never exceed the cylinder's nominal rated design pressure.
The next step in selecting a hydraulic cylinder is to determine the appropriate rod size. Most standard cylinders offer either one or two rod options. Selecting the right rod size requires careful consideration of the necessary stroke length, which affects the rod's buckling strength. Additionally, bearing loads on the rod are an important factor in cylinder selection. An increase in stroke length will also increase the bearing loads on the piston rod.
When choosing between push or pull, or both in a double-acting cylinder, the decision might necessitate a specific double-acting cylinder if the hydraulic system performs dual functions. Single-acting cylinders extend the piston under hydraulic pressure, whereas double-acting cylinders extend and retract the piston under pressure. For push applications, it is crucial to properly size the rod diameter to prevent rod buckling. For pull applications, it is essential to accurately size the annulus area, which is the piston diameter area minus the rod diameter area, to ensure the load moves at the cylinder's rated design pressure.
When selecting from standard rod options, it is advisable to use a smaller rod only for small stroke push loading or low pressure applications, while opting for a larger rod to ensure maximum reliability and fatigue resistance. If the required rod diameter exceeds the largest available for the selected cylinder bore size, it may be necessary to reevaluate the design parameters.
For determining the stroke length, if the ideal length cannot be accommodated, consider a telescopic or radial configuration that allows the cylinder to operate along multiple axes. Long stroke cylinders are often prone to twisting or misalignment and may require additional support.
Once the bore, rod, and stroke sizes are established, another consideration is whether internal cushions at the end of the cylinder stroke are needed. Cushions are recommended for decelerating high-speed rods to mitigate the impact energy of the piston assembly against the cylinder end cap. Implementing cushions will not alter the cylinder's envelope or mounting dimensions.
Determining the necessary support for the piston and cylinder depends on the stroke length. A stop tube may be required to prevent excessive wear and jackknifing, though it will not prevent rod bending; an oversized rod might be needed based on Euler calculations. A common mistake in hydraulic design is underestimating the piston rod specifications, which can increase the risk of stress, wear, and failure.
As much as hydraulic cylinders are rugged when they are working, they require great attention to detail when selecting one for use. An understanding of all components and their functionality is imperative in the design or selection of a hydraulic cylinder.
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