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.
collar is sometimes
referred to as locking nut. They are one in the
same. Occasionally the term mechanical load holding is
thread is sometimes
referred to as cylinder thread.
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
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
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
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 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.
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
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
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
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