Hydraulic Cylinders
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Introduction
This article will give an in depth discussion about hydraulic cylinders.
The article will give more detail on topics such as:
- What is a Hydraulic Cylinder?
- Types and Piston Configurations of Hydraulic Cylinders
- Design of Hydraulic Cylinders
- Considerations for Choosing a Hydraulic Cylinder
- And Much More&#;
Chapter 1: What is a Hydraulic Cylinder?
A hydraulic cylinder is a device that generates linear movement through hydraulic pressure. Essentially, the pressure from a hydraulic fluid causes a piston to move in either a pushing or pulling direction.
This leverages the principles of physical science, including:
- Hydraulic fluids are incompressible.
- In a fluid at rest in a closed container, a pressure change in one part is transmitted without loss to every portion of the fluid and walls of the container. (Pascal&#;s Law)
-
\begin{equation}
\ Pressure = \frac{Force}{Area}
\end{equation}
To create a system that utilizes the principles mentioned above, the setup illustrated below can be employed.
Since hydraulic fluids are incompressible, plungers A1 and A2 will stay in their positions unless a force is applied. When a force is applied to one plunger, the other will experience displacement due to a resultant force calculated using Pascal's Law, as described below.
The pressure exerted on the left plunger with Area A1 and Force F1 is expressed by:
\begin{equation}
\ Pressure_{1} = \frac{F_1}{A_1}
\end{equation}
Since pressure is uniformly transmitted through the fluid, it implies that the pressure on the right side will be equal to the pressure on the left side:
\begin{equation}
\ Pressure_{2} = \frac{F_2}{A_2} \frac{F_1}{A_1}
\end{equation}
Therefore, it follows that:
\begin{equation}
\ F_{1} = \frac{A_1}{A_2}F_2
\end{equation}
The force on the opposite end is therefore the applied force multiplied by the ratio of the areas. Once the force is known, displacement can be easily calculated.
Hydraulic cylinders are crucial for generating motion in both commercial and industrial manufacturing. Their applications include:
- Aerospace: Landing gears and wing flaps
- Automotive: Earth Moving Equipment
- Agriculture: Tractors
- Civil works: Excavators, Bulldozers, and attachments
- Oil and gas industries
- Power Generation: Flow controls for the water gates
- Motorway repairs and maintenance
- Mining: Excavators
Chapter 2: What are the types and piston configurations of hydraulic cylinders?
This chapter will explore the different types and piston configurations of hydraulic cylinders.
In various applications and industries, hydraulic cylinders may also be referred to as hydraulic actuators or hydraulic pistons. These terms are understood in the following contexts:
Hydraulic Actuators
Pneumatic actuators are commonly employed in processes that demand precise and rapid responses due to their ability to operate without requiring large motive forces.
In instances where large amounts of force are needed to operate a valve e.g., valves of a mainstream system, hydraulic actuators are the preferred choice. Hydraulic actuators come in various orientations but the most common is the piston type.
Hydraulic cylinders come in various sizes, each designed for specific functions. For example:
Small Hydraulic Cylinders
Small hydraulic cylinders feature a stable design, ease of operation, and durability, making them suitable for long-term use. They are often utilized in fast-motion applications and equipment with complex, compact components.
Hydraulic cylinders can also be constructed from various materials, such as:
Stainless Steel Hydraulic Cylinders
Stainless steel hydraulic cylinders are chosen for applications where corrosion resistance is crucial. While most hydraulic cylinders are made from alloy steels like and , these materials can be susceptible to oxidation and rust in wet or humid conditions. Although carbon steel cylinders can be painted with epoxy to mitigate these issues, the paint can deteriorate, exposing the steel to corrosion. Therefore, stainless steel cylinders are preferred in environments prone to corrosion, such as marine settings both onshore and offshore. They are commonly used in maritime cranes, davits, and boat lifts.
Types of Hydraulic Cylinders
Hydraulic cylinders are categorized as either single-acting or double-acting. A single-acting cylinder has only one chamber that is pressurized by hydraulic fluid, while a double-acting cylinder has two chambers, allowing it to operate in both directions.
Single Acting Hydraulic Cylinder
A single-acting cylinder has one chamber that receives pressurized hydraulic fluid. The location of this chamber depends on the cylinder's intended function. For a pushing motion, the chamber opposite the cylinder rod is pressurized, with the other chamber typically spring-loaded to facilitate retraction. Conversely, if the chamber with the cylinder rod is pressurized, the cylinder produces a pulling motion, and the opposite chamber is also spring-loaded to enable extension.
Advantages of Single Acting Cylinders
- Simple to design, manufacture and easy to install
- Low cost of initial purchase
- Single port (Input) and small housing
- Reduces the costs in valve and piping costs
Single Acting Disadvantages
- The thrust generated by the cylinder is reduced due to the spring that is supplying an opposite force
- When the spring wears off, the strokes of the cylinder will become inconsistent.
In a double-acting cylinder, both chambers can be pressurized. The chamber that houses the cylinder rod has less surface area in contact with the hydraulic fluid, as the rod occupies part of the piston&#;s area. Consequently, this chamber requires less pressure to retract compared to the other chamber. Therefore, precise pressure and direction control are crucial in this hydraulic system configuration.
Advantages of Double Acting Cylinders
- It has a lot of control over movement considering pressurized air moves both ways
- Faster, stronger and use less energy
- It offers a variety in design options for stroke and bore sizes
- It offers a force in all directions, that is push and pull motions
Double Acting Disadvantages
- Costlier than single pneumatic cylinders
- They need a bigger housing if used for example as a feed cylinder because of the coupling required.
Hydraulic Piston Configurations in Cylinders
The three most common hydraulic piston configurations are ram style, tie-rod, and welded. Tie-rod cylinders use strong, threaded steel rods on the outside of the cylinder to enhance stability. Welded cylinders feature a robust housing with the barrel welded directly to the end caps, eliminating the need for tie rods. Ram cylinders typically do not have a separate piston; instead, the cylinder rod functions as the piston.
Hydraulic Ram Cylinders
Single-acting hydraulic cylinders without pistons but with large rods are known as rams. Rams function similarly to traditional single-acting cylinders, but they replace pistons and piston seals with large-diameter rods. Consequently, rams have high-pressure ports at the cap end and lack low-pressure ports at the rod end.
Rams are typically more cost-effective compared to conventional single-acting cylinders.
Ram-type hydraulic cylinders are commonly used for vertical motion, such as lifting loads. They can also be employed for horizontal movement, though this requires careful guidance and appropriate supports.
An example of a ram cylinder is the telescopic cylinder.
Telescopic Hydraulic Cylinders
Telescopic hydraulic cylinders are also known as multi stage cylinders. Their huge advantage is that it can be a single acting hydraulic cylinder or a double acting hydraulic cylinder or a combination of both. They are a variant of a linear actuator with stages operated in a straight line rather than circular. Telescopic cylinders are typically used in construction trucks, dump trucks, vehicle trailers, and agricultural equipment. The telescopic hydraulic cylinders can be operated with ease, cost effective, space saving and can meet specific angle requirements.
Telescopic cylinders are a type of linear actuator composed of a series of tubular rods, or sleeves, that progressively decrease in diameter. Typically, there are four or five sleeves in this arrangement.
When hydraulic pressure is applied, the largest sleeve, known as the barrel, extends first. Once it reaches its full stroke, the next sleeve starts to extend, and this process continues through all the stages until the cylinder is fully extended.
Tie Rod Type Cylinder
This type of cylinder uses threaded steel rods, known as tie rods, to secure the end caps of the cylinder barrel. Depending on the bore diameter and operating pressure, the number of tie rods can range up to 20. One significant advantage of tie rod cylinders is their ease of disassembly for inspection and repair. Tie rod cylinders are widely used in industrial manufacturing applications. Smaller bore cylinders typically have four tie rods, while larger bore cylinders may have up to 20 to withstand the forces generated during operation.
Welded Hydraulic Cylinder
In a welded rod hydraulic cylinder, the barrel is typically welded directly to the end caps. The head cap may be secured using various methods, such as threading or bolting. This design is commonly used in mobile equipment due to its compact construction, internal bearing lengths, and duty cycle advantages compared to tie rod cylinders. However, the welded design can make field inspections and repairs more challenging, as it often requires specialized tools and equipment.
The welded rod cylinders are welded and also have loftier seal packages. These help to increase the life expectancy of the cylinder and are helpful when the cylinder will be used in locations that include contaminants and weathering. Visually, these welded body cylinders tend to have lower profiles than tie rod cylinders which improves the appearance of the equipment they are mounted on. Because they are narrower than tie rod ones, welded hydraulic cylinders work well in situations where space is a factor.
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Chapter 3: What is involved in the design of hydraulic cylinders?
Hydraulic cylinders are composed of several key components, which will be detailed below.
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Hydraulic Cylinder Tube or Barrel
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.
Hydraulic Cylinder Rod or Piston Rod
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 Hv, nearly twice that of HCP. In environments with extreme corrosion risks, materials like Inconel may be used to enhance durability.
Hydraulic Cylinder Piston
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.
Hydraulic Cylinder Seals
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.
Rod Seal
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:
- Keep the fluid inside the cylinder as it acts as a pressure barrier
- Regulate the fluid film to inhibit rod corrosion and to lubricate the rod seal and wiper seal
- Take back in, the lubrication film into the cylinder during the retracting of the rod
Buffer Seal
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:
- Defend the rod seal from fluid pressure extremes in system pressure.
- Weaken the variations in system pressure, thus improving rod seal performance by allowing the rod seal to handle more constant or gradually altering pressure.
- Behave as an interior excluder to keep system pollutants, such as metal particles, from damaging the rods.
Piston Seal
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.
Wiper Seal
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
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
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
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
Synthetic fluids are engineered lubricants designed to perform exceptionally well under high pressure and high temperature conditions. They offer several advantages, including:
- They are fire-resistant e.g phosphate esters
- They lower friction
- Thay have thermal stability
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 Cylinder Ports
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.
Hydraulic Cylinder Mounting
Cylinder mountings are typically classified into three categories:
- Center line mountings that absorb force on the centerline of the cylinder.
- Foot mountings that absorb force eccentric to the centerline of the cylinder.
- Pivot mountings that allow the movement of the cylinder body while the piston rod is in motion.
The hydraulic cylinder requires mounting interfaces at both ends: one at the base and the other at the head.
Center Mountings
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 Mountings
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.
Pivot Mountings
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.
Chapter 4: What factors should be considered when choosing a hydraulic cylinder?
Some of the considerations when choosing hydraulic cylinders below:
Mass of a Hydraulic Cylinder
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.
Geometry Involved
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.
Bore Size
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.
Rod Size
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.
Cushions in Cylinders
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.
Conclusion
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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