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3.) Understanding the Inner Workings: How Hydraulic Cylinders Work
A hydraulic cylinder jack, often referred to as just "cylinder", is identical in nature to the commonly seen hydraulic cylinder on heavy equipment. They both provide, through pressurized hydraulic fluid, linear motion with high force. The difference is that this special cylinder is used like a jack to lift or pull a load. Unlike a permanently attached hydraulic cylinder, jack cylinders provide versatility through their ability to be placed in multiple positions and configurations.
What are the basic parts of a cylinder?
The basic cylinder parts are shown in the schematic illustration entitled Hydraulic Cylinder Jack Anatomy. These are: base, cylinder body, optional return spring, and plunger. A plunger differs from a piston rod only in name; a plunger's diameter is large in comparison to the cylinder bore. Also shown in the schematic but not labelled are the hydraulic fluid ports and the piston. Optional end fittings, such as high alloy steel saddles, are often retrofitted to the plunger. Some plungers are plain-end while others have male and/or female threads to accept a range of special attachments. Some plungers have center holes devoid of threads. Some cylinders have collar threads, carrying handles, and special provisions within the base called mounting holes to allow for various fixture arrangements. Other bases have a single, relatively large, mounting hole with internal threads.
Why are there two names for the same cylinder component?
There exists no industry standard nomenclature for cylinder construction; the terms below refer to the same cylinder part:
- Plunger is sometimes referred to as piston rod or ram. Plunger is the more correct in that its definition is generally taken to mean a piston whose rod diameter is large in comparison to the cylinder‘s inside diameter, or bore.
- Locking collar is sometimes referred to as locking nut. They are one in the same. Occasionally the term mechanical load holding is used.
- Collar thread is sometimes referred to as cylinder thread.
- Center hole cylinder is sometimes referred to as a hollow cylinder. They both refer to the fact that the cylinder contains a circular opening through the entire cylinder body.
What are the basic actions of a cylinder?
Essentially the cylinder's plunger extends and retracts with the application of fluid pressure. These two actions can be used to produce an array of pushing, pulling, and lifting applications.
What are the distinguishing characteristics of cylinders?
Profile refers to the retracted height of the cylinder body. Standard and low profiles are available. Standard profile cylinders have a retracted height approximating 1.3X the stroke. Low profile cylinders are suitable for those applications where minimal clearance is available. Strokes for low profile cylinders are necessarily limited.
High Duty Cycle
General purpose cylinders are limited to the number of cycles they can sustain before maintenance considerations become an important factor. High duty is generally considered as greater than 2,500 life time cycles. High duty cylinders (also known as industrial grade) are fabricated of rugged, resilient materials.
Body & Plunger Materials
Coated alloy steel is the standard cylinder body material. Chrome plated steel is the standard plunger material. Without comprising capacity, aluminum bodies and plungers provide the advantage of lighter weight, offer some corrosion resistance, and are non-sparking in hazardous environments. Aluminum is not recommended in high duty cycle (production) applications.
Cylinders are manufactured in load capacities ranging from 2 to 1,000 tons. Plunger force is produced by fluid pressure against the piston or bottom portion of the plunger. Because the effective area for pressure application is smaller by the area displaced by the plunger, the resulting capacity can be considerably less in the return stroke of double acting cylinders.
What are the different functional variations of cylinders?
Aside from the fact that there exists an almost endless array of sizes, profiles, and capacities supplied by numerous manufacturers, cylinders fall into two basic operational categories:
The most common cylinder configuration is single acting (see the illustration). Pressurized fluid is routed to only one side of the piston causing extension of the plunger. When the hydraulic fluid is allowed to exhaust through the port, the weight of the load retracts the plunger. An optionally supplied internal spring can also provide forced retraction.
Spring return retraction of single-acting cylinder
To retract, small cylinders are outfitted with steel springs to pull the plunger back once the oil pressure is released. This is called "spring return".
Load return retraction of single-acting cylinder
Larger cylinders retract through the weight of the load pushing the plunger back. This is called "load return."
Routing pressurized fluid through two ports provides hydraulic action bilaterally to the piston. This requires the connection of a fluid circuit to the cylinder in order to accommodate the entering and leaving fluid streams. Double acting cylinders are used in applications that require force to both push and pull.
What are some of the specialized parts of a cylinder?
Most cylinders are equipped with some, if not all, of the following specialized components:
- The plunger wiper located at the cylinder top and plunger interface cleans the plunger during retraction and thereby reduces hydraulic fluid contamination and minimizes seal-damaging foreign matter.
- A stop ring located at the plunger to piston interface, or in some models at the top dead cylinder bore interior, prevents over-extension of the plunger and absorbs unintended lateral forces caused by off-center load application.
- Side bearings located at the interior top and bottom of the plunger stroke support the plunger against side forces that can be produced when eccentric (off-center) loads are encountered.
What other equipment is used with a cylinder?
Depending on the application, a host of auxiliary components can be required to complete a task. The motive force is supplied by an external pump powered by hand, electricity, air, or gasoline driven. A simple connection between port and pump or a circuit is attached to the cylinder in order to convey fluid. Normal fluid power accessories like control valves, hoses, gauges and fittings complete the hydraulic circuit. In some instances an accumulated fluid volume is necessary; this is provided by a fluid storage tank, often referred to as a reservoir. See the diagram entitled Typical Circuit for a Hydraulic Cylinder Jack to get a diagrammatic view of some of these components.
How do I pick the right cylinder to fit my job?
Choosing the right cylinder for your application boils down to answering a list of simple questions:
- Will your job require a pushing or lifting action or a pulling action, or possible both? Single acting cylinders extend the plunger when pressurized; double acting cylinders extend and also retract the plunger when pressurized thus providing force in two directions.
- How much clearance exists between the item you wish to manipulate, i.e. the load, and surrounding stationary structures such as floors or walls? If space is abundant then a standard profile cylinder will be satisfactory. If space is limited then a low profile, ultra-low profile, or even a flat (pancake) profile cylinder may be in order.
- What is the amount or distance of movement that will be necessary? This will dictate the required stroke of the cylinder.
- After lifting, will the manipulated item (load) need to be suspended for an extended period? If the ancillary hydraulic accessories will not be permanently available to the job, then a cylinder with a locking collar (nut) should be selected so that the load can be indefinitely supported mechanically.
- Will the cylinder be relatively stationary during the course of the job, or will its frequent relocation and positioning be necessary? If portability is important, then consideration should be given to lightweight model cylinders fabricated of aluminum.
- Is the insertion of objects through the center of the cylinder necessary as in tensioning or extracting operations? A center hole (hollow) cylinder should be chosen.
- Will the job require repeated cycling of the plunger as in a production application? The standard cylinder is for light duty, less than 2,500 life time cycles. Heavy duty industrial grade duty cycle cylinders are available from several manufacturers.
- And last, but certainly not least, how much plunger force is needed? How heavy is the object you wish to lift, jack, position, pull, or otherwise manipulate. Select a suitable cylinder capacity keeping in mind that an over-capacity margin should be applied.