Views: 0 Author: Site Editor Publish Time: 2024-01-31 Origin: Site
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…
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:
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)
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:
But if pressure is transferred equally through the fluid, then it means this pressure is also the same with pressure at the right side:
Thus it‘s also true that:
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:
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
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.
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
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. 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.
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
Costlier than single pneumatic cylinders
They need a bigger housing if used for example as a feed cylinder because of the coupling required.
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.
