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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 tube that produces linear actuation utilizing hydraulic pressure. Basically, the pressure of a hydraulic fluid forces a piston to move in either a pushing or pulling motion.
This takes advantage of the following laws of physical science:
\begin{equation} \ Pressure = \frac{Force}{Area} \end{equation}
To implement a system that takes advantage of the facts above, a system as shown below can be set up.
Because hydraulic fluids are incompressible, the plungers A1 and A2 will remain in the same positions if no force is exerted on any of them. But if a force is exerted on one of them, we will notice a displacement on the other end because of a resultant force calculated using Pascal‘s Law as follows.
The pressure on the left plunger with Area A1 and Force F1 is given by:
\begin{equation} \ Pressure_{1} = \frac{F_1}{A_1} \end{equation}
But if pressure is transferred equally through the fluid, then it means this pressure is also the same with pressure at the right side:
\begin{equation} \ Pressure_{2} = \frac{F_2}{A_2} \frac{F_1}{A_1} \end{equation}
Thus it‘s also true that:
\begin{equation} \ F_{1} = \frac{A_1}{A_2}F_2 \end{equation}
So the force on the other end is equal to the applied force multiplied by the proportions of the areas. The displacement can then be calculated easily going forward since the force is already given.
Hydraulic cylinders are the moving force in many commercial as well as industrial manufacturing concerns. Some of their applications are as detailed below:
This chapter will discuss the types and piston configurations of hydraulic cylinders.
Depending on the application and industry, hydraulic cylinders can be called hydraulic actuators or hydraulic pistons. These terms can be understood in the following contexts:
Pneumatic actuators are typically used in controlling processes that require an accurate and quick response. This is because pneumatic actuators do not need 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.
These hydraulic cylinders can have different sizes with unique purposes based on the size, for example:
Small hydraulic cylinders have a stable structure, are easy to operate and can be used for a much extended period of time. They are typically used in fast motion applications as well as in equipment with intricate and small components.
The hydraulic cylinders can also be made of different materials, for example:
Stainless steel hydraulic cylinders are typically used in applications where the priority is corrosion resistance. The vast majority of other hydraulic cylinders are made from alloy steel combinations such as 1045 and 1018. However, they are prone to oxidation and rusting when used in wet or humid environments. The carbon steel cylinders can still be prone to dents, surface abrasion, or harsher chemicals, even if they may be painted using epoxy. Thus, such conditions can wear away the paint and expose the carbon steel to corrosion. Therefore, in such instances stainless steel hydraulic cylinders are preferred, for example in marine environments both onshore and offshore. These can be used on maritime cranes, davits, or boat lifts.
Hydraulic Cylinders come as either Single or Double Acting. If only one chamber is pressurized by the hydraulic fluid, it is a single acting, otherwise it‘s double acting.
In a single acting cylinder, there is one chamber that receives pressurized hydraulic fluid. Which side that is will depend on the intended use of the cylinder. If it is meant for a pushing motion, the chamber opposite the cylinder rod will be pressurized. The other chamber is usually spring loaded to cater for the retraction. If the chamber with the cylinder rod is the one pressurized, it will be a pull motion. The opposite chamber will also be spring loaded to cater for the protrusion.
In a double acting cylinder, both chambers can be pressurized. Of the two chambers, the one that accommodates the cylinder rod will have little surface in contact with the hydraulic fluid, since we cannot take into account the surface area of the piston already occupied by the cylinder rod. This difference in the surface area will need less pressure to retract than the other. Thus, pressure control and direction control is important in this set up of hydraulic systems.
The three most popular hydraulic piston configurations are ram styles, tie-rod, and welded. Tie-rod cylinders utilize threaded steel tie-rods with great strength, usually on the outside of the cylinder casing, to provide additional stability. Welded cylinders incorporate a heavy-duty welded cylinder housing that has a barrel welded right onto the end caps and thus requires no tie rods. Ram cylinders usually have no piston but rather use the cylinder rod as the piston.
Single-acting hydraulic cylinders that have no pistons but have large rods are called rams. These rams operate exactly like the conventional single-acting cylinders. However, they use large diameter rods in place of pistons and piston seals in their designs. Thus, in place of pistons, rams have high-pressure cap-end ports. They also do not have any low-pressure rod-end ports.
Rams are generally cheaper than their conventional single-acting cylinders counterparts.
Ram types of hydraulic cylinders are typically used to give vertical motion, including lifting loads in a vertical direction. Such a cylinder is also used to provide the motion in a horizontal direction but needs attention and suitable guides to guide the motion.
A good example of a Ram cylinder is a telescopic cylinder.
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.
These are a type of linear actuator consisting of a series of tubular rods called sleeves. These sleeves, which are typically 4 or 5, sequentially decrease in diameter.
As the hydraulic pressure is introduced to the cylinder, the main or barrel, which is the largest sleeve, is extended first. Once the barrel has gotten to its maximum stroke, the next sleeve then begins to extend. This will continue until the cylinder reaches the last stage.
This cylinder holds the two caps of the cylinder barrel ends using threaded steel rods. Tie rods can number all the way up to 20 depending on the bore diameter and operating pressure. One of the big advantages of the tie rod is it is effortlessly stripped and examined for repair. Tie rod cylinders are used in a large majority of industrial manufacturing applications. Smaller bore cylinders typically have few, maybe four tie rods, whereas larger bore cylinders can have as many as 20 tie rods in order to weather the forces produced by the cylinder.
In a welded rod hydraulic cylinder, often the barrel gets welded directly to the end caps. The head cap can utilize a variety of retention approaches, such as threading or bolting down. This design is generally accepted for mobile equipment because of the compact construction, inside bearing lengths, and its duty cycle compared to tie rod construction. But, this design does make inspection and repairs a lot more difficult in the field due to requiring less common 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.
There are various components that make up hydraulic cylinders. These components will be discussed below.
This is the main case of the cylinder. Made of steel, commonly carbon steel, the barrel or cylinder tube is built to withstand hydraulic fluid pressure inside its walls throughout its lifespan. A wide variety of steel qualities are available that offer ruggedness and strength, but basically greater pressures will require thicker cylinder walls and stronger steels.
To prevent corrosion and abrasion, the surfaces are given special treatment. Either these will be coated or painted. Some applications of hydraulic cylinders like food packaging might need a material without coats to avoid peels falling into the food, thus stainless steel can be employed in such cases. The internal walls usually need no surface finishing because the hydraulic liquid is usually non corrosive and also protects it from corrosion. However, in some cases where water is used as the hydraulic fluid, coating may be required on the inside walls as well.
This is the part that protrudes out of the barrel. It is attached to the piston inside the cylinder. Because of the friction from the rapid protruding and retracting, this is the only external component that is not painted. Because of its function, not only does it need protection from corrosion, but wear and pitting. This is a very delicate component because peeling, pitting, or corrosion will most likely scrap the seals, contaminate the hydraulic fluid and eventually jeopardize the whole hydraulic systems.
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So, the materials and coating of the hydraulic cylinder are of the highest importance. Usually, the cylinder rod is machined from steel or stainless steel and then coated with Hard Chrome Plating (HCP). Nevertheless, many coatings that wear slowly have been adopted, for example COREX. This is a coating that can be up to ten times less porous than HCP, and offer a hardness of up to 1400Hv, almost twice the hardness of HCP. If the rod is working in extreme environments that are highly susceptible to corrosion, a material like Inconel can be employed to make it.
The piston is the disk that separates the two chambers of the barrel. It is the one that is pushed by the hydraulic fluid. The piston is connected to the cylinder rod. Thus, the movement of the piston is seen by the way that the rod is moving. It is very important that no hydraulic fluid passes over the piston. To make sure of this, the piston is fitted with seals, usually U-seals. It must also not wear as it reciprocates, so it also comes fitted with wear rings.
This probably is the weakest aspect of a hydraulic system. Good seals can reduce friction and wear, which then lengthens service life, while bad seals can lead to downtime and frequent maintenance.
Seals are used all over the hydraulic cylinder. These are fabricated of a wide variety of different materials, considering their intended use in the cylinder and the type of cylinder they are in. It is crucial that these be slow to wear, capable of surviving multiple repetitions of the rod moving in and out of the barrel, removing any contamination.
The cylinder designers will select the perfect seal for a specific cylinder application. This they do by taking a number of factors into consideration. Cylinders destined to function in areas with high temperatures require seals that will not melt. For such operations, a common material used is Viton. Cylinders that will be working in extremely cold conditions will need seals that will not harden and crack. Polyurethane is a popular material selected for these applications.
Also, for situations that require quick repetitions, like applications in factories, Zurcon and PTFE seals are often picked. Specialist seals are also common, like ones that get back up rings if these are going to be working under intense pressure. For thin hydraulic fluid that can easily pass over the piston or cylinder end caps, seals with extremely tight closure will be employed.
Some commonly used seals are detailed below.
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.
It is used to:
Buffer Seals are commonly used together with another rod or piston seal. Typically, this will be a U-Cup style seal. In many rod applications, they are designed to absorb pressure variations when working under high load conditions. This serves to lengthen the lifespan of a rod seal. Basically the buffer seal functions are to:
Piston seals create a sealing force against the inner wall of the cylinder and inhibit fluid from flowing over the piston head into the opposite cylinder chamber. In holding the pressurized fluid from escaping into the other chamber, pressure builds up on one side of the piston, this makes the rod extend or retract.
They are either single-acting piston seals, which means the stress acts only on one side, or they can be double-acting piston seals, meaning the stress acts on both sides.
Eliminate external pollutants from entering the Hydraulic Cylinder and accept the lubrication film back into the cylinder when the rod pulls back. The wiper seal is the most underrated seal type in the hydraulic cylinder relative to its significant use.
A Hydraulic fluid is a non-compressible oil or liquid which will be used to transfer power within hydraulic machinery and equipment.
This hydraulic fluid can be made of many elements but it is mainly mineral or petroleum based, water based or synthetic.
It is very important that the fluid be incompressible to have an efficient hydraulic system. These fluids can be:
Petroleum-based or mineral-based fluids are the commonly used fluids today. They commonly provide a low-cost, high quality, readily available selection. The properties of a mineral based hydraulic fluid are dependent on the additives, quality of the virgin crude oil, and the process that refined it. Common hydraulic fluid additives include rust and oxidation inhibitors (R&O), anti corrosion agents, demulsifiers, anti-wear (AW) and extreme pressure (EP) agents, VI improvers, and defoamants. Additionally, the lubricants can contain colorful dyes, which help in identifying leaks. Because leaks in hydraulics are so costly, this minor characteristic is important in extending the life of the equipment and saving money and resources.
Water-based fluids are not as cost-efficient as petroleum-based fluids. They have drawbacks which include the lowering of wear resistance. This must be weighed in contrast with the advantage of fire-resistance. Fluids based on water are used for fire-resistance due to their high-water content. They are usually available as oil in water emulsions, water in oil emulsions, and water glycol blends. Water based fluids can provide appropriate lubrication characteristics but need to be monitored closely to avoid problems. Since water based fluids are used in applications where fire resistance is desirable, these systems and the atmosphere around the systems can be hot. High temperatures can cause the water in the fluids to evaporate, which raises the viscosity. Sometimes, distilled water will have to be added to the system to rectify the balance of the fluid.
Synthetic fluids are man-made lubricants that offer excellent lubrication characteristics in high pressure situations as well as high temperature systems. Synthetic fluids have advantages like:
The drawback to these types of fluids is that they are usually costlier than conventional fluids, they may be slightly toxic and need special disposal, and they are usually not compatible with standard seal material.
Hydraulic fluid enters and exits the cylinder through ports, with one port at either end of the cylinder tube and the hydraulic piston between the two ports. These have to be secure, as a weak port can cause a dangerous hydraulic fluid leak under intense pressure.
Cylinder mountings are generally grouped into three categories:
The two ends of the hydraulic cylinder require a mounting interface; the one at the base and the other one at the head.
Centerline mounts are the preferred mounting method. These mountings exert tensional or shear force against the mounting bolts. Centerline mounts are not flexible and require precise alignment with the load. A well aligned mount cylinder has little rod bearing and piston loads, making it have a longer life. The head mountings are recommended for use in pull stroke applications. Push stroke applications can use the mountings on the piston rod end of the cylinder.
Foot mounting secures the cylinder along its side. Since the mounting surface plane is offset from the line of force, mounting bolts are subjected to a significant amount of shear stress.
The cylinder needs to be pinned or keyed to weather the stress of shear loads and allow the mounting bolts to remain in tension.
Key mounts with keyways can be cut into a machine. The mounting accommodates shear loads. They provide accurate alignment of the cylinder and simplify installation and servicing.
Just one end of a cylinder must be keyed to the machine. When ends are keyed, it will result in an unequal distribution of internal stress and deformation. This mainly applies to long stroke cylinders, where performance and life may be dramatically reduced.
Clevis, spherical bearings, and trunnion mounts are common configurations in pivot mountings. Pivot mounting can be utilized when the application requires the load to travel in a curved path. Thus, clevis and trunnion mounts allow this motion. Pivot mounts can help mitigate the load misalignment.
Trunnion pins are made for shear loads only. So, only trunnion bearings that have a tight fit and support the whole pin length must be used.
Some of the considerations when choosing hydraulic cylinders are:
The initial step is to determine the size of the mass you need to move. As soon as you know how heavy the mass is, you then consider the effect the mass has on the force required to move it. For instance, if a load is being pushed straight up, it will only require a force equal to its weight to lift it, but if it's pushing a load on the ground, you will have to overcome friction and acceleration. Also note that it is good practice to assume a force 120% bigger than the calculated result for safety.
Next, you then need to study the geometry involved in moving it. Machines like a hydraulic press, which reciprocates up and down, the geometry is simple and requires no further consideration.
However, when the center of the load being moved is offset to the point of lift force and at perpendicular angles to that point of lift force, the force required by the cylinder changes. For a crane, for instance, the cylinder pushes on the boom, usually very far from the load. In most cases, distance from the load to the fulcrum can be ten times the lift force and sometimes more. So the closer your lift point is to the fulcrum the more force is needed by the cylinder to lift the load.
Flange mounting is best suited for transferring the load along the centerline of the cylinder. The non-centerline mounting needs support otherwise they misalign.
The next step is to calculate the bore size for the cylinder. The force that the cylinder produces is the product of the system pressure multiplied by the area of the internal piston surface upon which that pressure acts. This is the formula that is used to calculate the bore size required to achieve that force.
The maximum pressure range for the application will also vary the bore size. Pressures can vary greatly depending on the specific job that the system is doing. Cylinders are applicable in test pressure and nominal standard pressure, thus catering to variations. The pressure of the system must never be more than the nominal rated design pressure of the cylinder.
The following step to selecting any hydraulic cylinder should be to choose an appropriate rod size. Most standard cylinders come with either one or two rod options. Choosing the required rod size needs careful consideration of the stroke length necessary which in turn affects the rod buckling strength. On top of rod buckling, bearing loads also is another consideration in the selection of a hydraulic cylinder. Increasing the stroke length of a cylinder also increases the resultant bearing loads on the piston rod.
When deciding on push or pull or both, is double acting, the answer may require a particular double-acting cylinder if the hydraulic system is doing double duty. Since single acting cylinders extend the piston under hydraulic pressure a double acting cylinder will extend and retract the piston under pressure. For a push application, it is very important to size the rod diameter correctly to avoid rod buckling. For pull application, it is vital to size the annulus area or piston diameter area minus the rod diameter area correctly so as to move the load at the rated design pressure of the cylinder.
If selecting from standard cylinder rod options, it is recommended that a smaller rod for a given bore only be used for small stroke push loading or reduced pressure applications, the larger rod given to be used when wanting to obtain maximum dependability and fatigue life of the rod. If it is determined that the necessary rod diameter surpasses that of the biggest available within the selected cylinder bore size, it could then be essential to reconsider design parameters.
When deciding on the required stroke length, if space is not available for the ideal length, a telescopic configuration may be necessary, or even a radial configuration that allows the cylinder to move in more than one axis. Long stroke cylinders often are at risk for twisting or misalignment and need additional support.
If the bore, the rod, and the stroke sizes have been determined, the other aspect to be considered is whether internal cushions at the end of the cylinder stroke are necessary. Usage of cushions is recommended for the deceleration of high speed rods to reduce the energy of the impact of the piston assembly against the cylinder end cap. Usage of cushions will not affect cylinder envelope or mounting dimensions.
When deciding on how much support the piston and cylinder need it all depends on the stroke length, a stop tube which are necessary to stop excessive wear and jack knifing. However, a stop tube will not prevent rod bending; an oversize rod may be required, based on Euler calculations. The most common error in hydraulic design is underestimating the specification of the piston rod, making the cylinder more prone to 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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