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Understanding
Connections
What Other Items Do I
Need to operate My Cylinder?
Additional items needed to complete your job
will depend entirely on your pump selection and its relationship to the
cylinder(s) or tools that you are using. Items such as hoses, fittings, gages,
and in-line valves are commonly referred to as accessories or hydraulic system
components. In order to begin the selection process for these auxiliary
components, let’s look at some typical setups that we can have.
In Figure 1 the most basic arrangement is shown. A
pump is supplying pressure to a single-acting cylinder. All that is needed is a
hose to convey the pressurized hydraulic fluid and possibly some threaded or
quick-connect fittings, sometimes referred to as couplers, to connect the hose
to the pump and cylinder.
However, with the addition of a gage, we can monitor the amount of hydraulic
fluid pressure the pump is producing. If the pump that you select has a
built-in discharge pressure gage, then all the better.
Adding a directional control valve provides the next level of system
enhancement. Control valves can be purchased separately and installed in the
hydraulic circuit (Figure 3). This is referred to as a remote mounted valve.
Control valves are often supplied as part of the pump assembly and are mounted
directly on the pump’s reservoir (Figure 4):
If the pump you select does not have a built-in pressure release knob (valve),
then a load-lowering valve must be installed in the hydraulic circuit, usually
near the cylinder:
When one pump is used but more than one cylinder is involved, a means of
distributing the hydraulic fluid to the various users is needed. This is
handled through tee fittings and/or a component called a manifold. These items
are shown in Figure 6. The manifold can optionally be equipped with manual
handles. This allows the manifold to act as a compound shut-off valve that can
regulate the flow of hydraulic fluid to or from the cylinders or a sequencing
valve which provides control of primary and secondary circuits. Tee gauge
adapters provide a means of installing pressure or tonnage gauges anywhere
along the hydraulic circuit.
What Cylinder Mounted
Accessories are Available?
Interchangeable plunger head inserts and
fixture attachments for both plungers and cylinder bodies expand cylinder jack
capabilities. Here is a list of just some of the items that are available:
·
Serrated saddles for
adhering to the load;
·
Plunger clevis eye
attachments for fastening load hooks;
·
Swivel or tilt caps to
adjust to uneven loads or unlevel floors;
·
Special automotive
repair attachments for spreading and pulling.
How Do I Pick the
Right Hose?
Before any hose selection there are a few
important safety points that must be considered. The hose picked must be rated
for a minimum of 10,000 psi internal fluid pressure. While the rated burst
pressure of the hose may far exceed this value, never operate your system
greater than 10,000 psi. Careful consideration should be given to the hose
installation arrangement with regards to bends, turns, and pinch-points. Hoses
should not be handled when pressurized. Adhere to the maximum temperature
rating of the hose and make sure the type of hydraulic fluid being used is
compatible with the core tube hose material.
Based on an understanding of the purpose and
relationship of the hose in the hydraulic circuit, picking the correct hose for
your job involves determining the right size, material, and length.
Size
Hose intended for hydraulic cylinder jack use
is commercially offered in ¼ inch and ⅜ inch nominal diameter. The smaller size
is logically more economical. The larger size should be used where higher
hydraulic fluid flow rates are involved or long hose lengths are required.
Material
Hoses are available in a variety of materials
each with its special purpose. For general purpose applications either
thermoplastic (nylon or polyurethane) or rubber hose is acceptable.
Thermoplastic hose consists of a core that is reinforced with either polyester
or steel wire braids, covered with a polyurethane jacket. This jacket provides
excellent abrasion resistance but should not be used where excessive heat or
weld splatter could be encountered. Rubber hose generally consists of a two
layer core, reinforced with two plys of steel wire braids, all covered with a
rubber jacket. It is the choice for superior flexibility and for those jobs
where long hose lengths are needed. Special materials are used to manufacture hoses
that are classified as non-conductive. This material should be selected where
electrical isolation is required.
Length
The job will often dictate the length of hose
needed. Aside from this fact, hose length can have a very meaningful effect on
system performance. For a given hose size, the longer the length the higher the
resisting (back) pressure the pump sees and the slower the cylinder jack
retraction speed. Length (and size) have a direct bearing on the volume of
hydraulic fluid needed to complete the task. This is best illustrated by an
example:
Example:
Question: Not counting the cylinder’s
capacity, what amount of hydraulic fluid will be required to fill four (4) ⅜
inch diameter hoses, 20 feet long? Answer:
Volume = Number of hoses x length (feet) x
size factor for ⅜
= 4 x 20 x 0.75
= 60 fluid ounces (≈ ½ gallon)
When the nominal hose size is reduced to ¼
inch diameter?
Volume = Number of hoses x length (feet) x
size factor for ¼
= 4 x 20 x 0.33
= 26 fluid ounces (≈ 1½ pints)
It is advisable to fill long hoses with
hydraulic fluid before filling the pump’s reservoir. This will insure that
sufficient fluid volume is available to perform the job at hand. Hoses can be
ordered with standard lengths from 2 to 100 feet.
What Hose-End Fittings
are Available?
The hydraulic power industry has somewhat
standardized on a nominal size of ⅜ inch for hose-end fittings, regardless of
the hose diameter. Threaded fittings almost universally use the 18 NPTF style
thread. This designation means 18 threads per inch of the National Pipe Thread
Fine series.
Two general categories of fitting styles
exist: field-attachable and permanent. Field attachable fittings (or couplers)
use numerous means to accomplish the connection to include screw-together and
clamp type. The permanent type means that the coupling mechanism is permanently
attached to the hose end via cold-forming powered machinery. This is the type
most often offered on rubber and thermoplastic hose. They are used primarily by
heavy equipment manufacturers and maintenance facilities.
How Do I Pick the
Right Manifold?
A manifold is similar to a hydraulic control
valve in that it is a device that directs the flow of hydraulic fluid in a
cylinder hydraulic circuit. It can be remotely situated in the circuit or it
can be mounted directly on the hydraulic pump. When pump-mounted it generally
forms the base of the directional control valve. The manifold consists of a
body with several internal flow passages. Fluid is channeled through these
passages, often times by optional valves that consist of a plug, stem, and
handle. Selection of the number of ports or openings is entirely dependent on
the job application. Manifolds bodies can be round, hexagonal, or block
(rectangular) in geometry. The figure below is a typical block manifold with
valves.
