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Understanding
Types: Specifications and Terminology When Choosing a Cylinder
Types of Hydraulic Cylinder Jacks
All cylinders are used for lifting of heavy loads, plus all
cylinders can be used horizontally for pushing. In addition to these functions,
"hollow" (also called "center hole") jacks can be used for
pulling. And "Lock nut" jacks can be used for long periods (days and
months) of load holding. Cylinder jacks are divided into several types:
Standard (also called
"General Purpose")
These hydraulic cylinders are used for versatile lifting & pushing.
Low
Profile
These hydraulic cylinders are used for lifting under low
clearance. Low profile cylinders include "flat" jacks and
"pancake" jacks. A jack labelled "low profile" has the same
shape as a standard cylinder; it is just shorter. A jack labelled
"flat" jack has more of an oval shape and a "pancake" jack
has a standard cylindrical shape but with a larger diameter making look more
like a pancake. Usually the flat jacks and the pancake jacks have the lowest
overall height as low as 1.28 in consequently these are also referred to as
"Ultra-low profile" jacks.
Regular Size vs.
Low Profile
Regular size cylinders are tall & thin.
Low profile cylinders are short & stubby.
Aluminum
These hydraulic cylinders are more lightweight for easy
transport and they have excellent corrosion resistance.
Steel vs.
Aluminum
Steel is tougher against fatigue. It is employed under toughest
working conditions. Aluminum is more lightweight and its non-corrosive body
holds up better to weather. It is not for use under constant motion (for
example in production) and under constant use wears sooner.
Lock Nut (also called "Locking Collar")
These hydraulic cylinders are used for lifting & holding
load over extended period of time. The plunger has a locking collar (nut) for
mechanical securing of load when extended periods of non-hydraulic support are
required meaning the plunger is threaded and has a nut. Once you lift the
plunger out of the cylinder, you can screw down the "locking" nut
which will permanently hold the load without creeping.
Hollow (also called
"Center Hole")
In addition to general purpose lifting or horizontal pushing,
the hollow cylinder is used for pulling. The jack has a center hole that runs
from the top of the ram through the bottom which can be used to pull a chain or
rope through the center hole to tug, pull and create tension. This is used, for
example, to tighten the wire ropes on suspended bridges.
Understanding the Terms and Specifications
Ram
= Piston = Plunger
The plunger is the part of the jack that rises out of the
housing (cylinder body). The plunger is also referred to as a
"Piston" or "Ram".
Stroke
The "stroke" means how high the jack is able to lift
from the retracted position. The "stroke" it is the height difference
between a retracted and extended jack.
Oil
Flow = Flow Rate
The oil flow also called "flow rate" is the amount of
oil, the pump delivers to the cylinder (... for hand pumps with one hand
stroke, ... for electric and air pumps per minute). The larger the flow rate,
the faster the cylinder will rise.
Some pumps are equipped with double speed: With the cylinder
unloaded, the pump delivers a high volume of oil for fast cylinder speed. Once
the load is engaged, the pump automatically switches to the slower high
pressure lifting stage.
How many hand strokes are required for a hand pump to raise a
cylinder?
Hand strokes = Cylinder oil capacity / Oil volume per stroke
Dimensions
The "collapsed height" is the height of the cylinder
when fully retracted. The "extended height" is the height of the
cylinder at full stroke.
The width of any cylinder is given as the "outside
diameter". Flat cylinders have more of an oval shape. The "outside
diameter" is measured at the widest outside-to-outside point. The hollow
cylinder has a center hole. The "inside diameter" gives the width of
the hole.
The cylinder effective area is the piston's surface area that
comes in contact with the load.
Oil
Capacity
The "usable oil capacity" in the pump must always be
larger than the oil capacity in the cylinder or the cylinder will not lift to
the maximum extended height. Also take into account that the hoses and fittings
will have to be filled with oil when you pressurize the system. Consequently
make sure you have plenty more oil in the pump than the cylinder requires,
especially if you are operating long hoses and possibly multiple hoses and
accessories. As a rule of thumb please allow 20 percent additional usable oil
volume in the pump than the cylinder(s) requires.
Note that the pump has a larger oil tank than its usable oil
capacity. Often the usable oil capacity is only 75 percent of the tanks volume
so you may find a 1 gallon tank on a pump with only 0.75 gallon usable oil. The
reason is that the oil requires some breathing room especially when it is
flowing back into the pump under high pressure.
Low-cycle
vs. High-cycle
General purpose cylinders are limited to the number of raise and
retract cycles they can sustain before maintenance considerations become an
important factor.
Low-cycle: Infrequent use of cylinder. Mainly used in
construction and maintenance.
High-cycle: Continuous up and down movement of plunger/piston.
Used in production. 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. High-cycle cylinders are often used
as components in manufacturing machines.
Aluminum jacks cannot be used for high cycle applications. As a
rule of thumb, pumps require 20% more oil volume than jack(s) for low-cycle
applications and double the oil volume of jack(s) for high-cycle applications.
Cylinders are always considered "Low-cycle" cylinders
unless otherwise labelled as "High-cycle" cylinders.
Single
vs. Double Acting
Cylinders are always considered "Single-acting"
cylinders unless otherwise labelled as "Double-acting" cylinders.
Single
Acting - Oil is pumped one
way into jack to extend/raise piston out of cylinder – Either
steel spring in jack OR weight of lowering load retracts/pulls down piston and
pushes oil back into pump. Jacks are ideal for lifting, holding, pushing &
pulling.
Double
Acting - Oil is pumped two ways: (1) into jack to extend/raise
piston out of cylinder and (2) out of jack to retract/push down piston and
return oil back into pump. Jacks are ideal for lifting and controlled descent.
Single
vs. Double Speed
Single
Speed - The pump operates the jack at one and the same
consistent speed.
Double
Speed - With the cylinder unloaded, pump delivers a high volume
of oil for fast cylinder speed. Once the load is engaged, the pump
automatically switches to the slower high-pressure lifting stage.
SCFM
SCFM stands for Standard Cubic Feet per Minute. It denotes how
much air volume (measured in cubic feet) is needed per minute from an air
supply (compressor /shop air) to operate an air over hydraulic pump.
NPTF
NPTF stands for National Pipe Thread Fine. It denotes the inside
diameter of a fitting or hydraulic hose. The standard sizes are 3/8 inch NPTF
(also referred to as fast flow) or 1/4 inch NPTF.