Transcript Document

Unit 15 Fluid Conductors and
Connectors
Connectors
Up to this point we have discussed components of fluid power
systems without attention to their connections. Conductors
carry fluid from the power source to the system components and
eventually the point of operation. To function properly, as well
as safely, conductors must be able to withstand the highest
possible pressure of the system, deliver the fluid at required
volumes, withstand high operating temperatures, and remain
leak free during operation.
Conductors are classified as either rigid, semi-rigid, or flexible.
Conductors are referred to b type such as pipe, tubing, and hose.
Pipe is considered rigid. Tubing is considered semi-rigid, and
hose is considered to be flexible. The first conductor we will
discuss is pipe.
Pipe and Materials and Sealing
Pipe is a rigid conductor which means it is not meant to be bent.
Consequently, pipe must be connected using fittings and, as a rule,
everywhere there is a connection in a fluid power system there exists
the possibility of a leak.
Materials
There are many materials that could be used for pipe
but only seamless steel is recommended for hydraulic
systems. Galvanized pipe is not recommended because
of chips.
Sealing
Pipe threads have to be tapered and usually use NPTF
type thread.
ID, OD, and Wall Thickness and
Pipe Size
Wall thickness determines the pressure rating of the pipe.
Schedule 40, 80, and 160 are most common and are designated as
low pressure, high pressure, and very high pressure respectively.
Pipe Threads
To form a leak proof seal, pipe threads are tapered on their outside
diameter about 1/16th per inch. This taper causes an interference fit
which is suppose to create a seal. In reality pipe threads don’t seal
very well without the help of pipe tape or joint compounds.
Pipe Threads
NPT
NPTF
NPTF type pipe thread is recommended for hydraulic systems. The
major difference NPT and NPTF type thread is in the clearance left
between the crest and root of the thread. With the NPTF type, there
is no clearance which aids in sealing.
Pipe Fittings
Since pipe is heavy walled and not designed for bending into
shape, fittings must be used to make pipe connections. Fittings
are also used on tubing to connect sections.
Pipe Installation Considerations
Since metal is removed when threading, it is critical that metal
shavings not be allowed to get in the system. When applying
sealing compounds, make sure not to allow any of the compound to
overlap the end of the pipe. Never start sealant tape any closer to
the edge than two threads. When tightening pipe threads, be
aware that the wedging action generates radial force on the outer
pipe and that over tightening pipe threads can result in a rupture.
Tubing
The main advantage of tubing is that it can be bent into shape
and thus requires fewer fittings. Fewer connections generally
means there will be less possibility of leaks. Tubing is also
known for its ability to absorb vibration and has a smooth
inside finish which is good for fluid movement.
Tubing Materials
Steel
Copper
Brass
Aluminum Stainless Steel Plastic
Although tubing may be made of many different
materials, fluid power systems only use one or more
of them. Generally, carbon steel tubing is the choice
of hydraulic systems and either steel or plastic
tubing for pneumatic systems. Aluminum, copper,
and brass have the tendency to fracture. Stainless
steel is expensive.
Tubing Size
Tubing size is determined by outside diameter. As tubing wall
thickness increases, its internal diameter decreases. As wall
thickness increases, the burst pressure of the tubing also increases.
Tubing Size and Pressure Rating
Three very important terms
associated with tubing are: burst
pressure, working pressure, and
safety factor.
Burst pressure is the point at
which a tube will rupture at a
given pressure value. Working
pressure is the value considered
safe to operate the system at
under normal circumstances.
Safety factor is the ratio
between working pressure and
burst pressure. As a general
rule, a good safety factor is at
least 4 to 1.
Tube Fittings
Tube fittings generally fall into one of two categories: flared and
flare-less. A flared tube fitting, shown above at left, consists of a
body, sleeve, and a nut. A seal is generated between the tube
flare and the body when the nut is tightened against the sleeve
and tube. The main reason of the sleeve is to keep the nut from
twisting the tube flared end when the nut is tightened. It is
important to note that some flare assemblies, such as low
pressure 45 degree flares, do not use a sleeve. Flare-less tube
fittings use basically the same components except that the seal is
created by a donut shaped piece that is sometimes referred to as
a “ferrule” rather than a sleeve.
Flared Tube Fitting
37 degree Flare Fitting SAE “B”
Ferrule Compression Fitting
JIC 37 degree flares are recommended for high pressures above
1000PSI. SAE 45 degree flares are primarily used in pneumatic
systems and are for pressures below 1000PSI. The flare-less tube
fitting can be used in both hydraulic and pneumatic systems and has
pressure ranges up to 3000PSI. Flare-less tube fittings should be
used on medium and heavy wall tubing. The most critical step in
making a single flare tube fitting is forming the flare. The tube must
be cut square, cleaned free of burrs, measured exactly, and flared
without galling, over-thinning, or splitting the end of the tube.
Flare Less Tube Fitting
Sleeve Type Compression Fitting
O-Ring Type Compression Fitting
The types above are used when tubing cannot be flared
or to avoid flaring. In both cases the sleeve holds the
tube in place.
Tubing Installation
As a rule, tubing runs should be kept as short as possible with
as few bends or turns as possible but not completely straight.
Bends do help absorb vibration and allow for expansion and
contraction.
Tube Bending Radius
Hydraulic tubing is bent with a special tool. Each tube has a
standard bend radius to which the tube may be bent without
flattening, kinking, or wrinkling. The rule of thumb for
calculating the bend radius is given by the following formula:
Bend Radius + 3 X Outside Diameter of Tube. The bend
radius is measured from the inside center of the radius to the
centerline of the tube, not the outside as done with a hose.
Hose
Reinforcement
Inner Tube
Outer Cover
The standard components of a three piece hose assembly are the
inner tube, reinforcement layer, and the outer cover. The inner
tube comes into direct contact with the fluid and must be
compatible with it. The reinforcement layers provide strength
for the hose and must be able to withstand the system pressure.
The outer cover protects the other two by providing a contact
surface for the hose. The outer cover should not be allowed to
rub against anything during normal operation.
Hose Construction
Although basic hose construction includes three pieces, hoses
can be constructed with multiple reinforcement layers made
from many materials including steel that give the hose three
possible ratings: suction, medium pressure, high pressure, and
very high pressure.
Hose Size
Hose size is given by a “dash” number which relates to
the inside diameter of the hose. The dash numbers
graduate in 1/16th inch increments. A hose with a –16
imprinted on the side would have an inside diameter
of 1 inch. A hose could have a very large outside
diameter but a very small inside diameter so trying to
judge inside diameter by examining the outside
diameter is not practical.
Hose Pressure Classifications
The pressure rating of a hose is determined by its construction. The
more reinforcement layers a hose has, the more pressure it can
stand. In addition to layers, the type of reinforcement also pays a
role in determining the pressure rating of a hose.
Hose Fittings
Hose fittings can be classified as either permanent or reusable.
Permanent hose fittings are installed on the hose by “crimping”
and cannot be disassembled. Reusable hose fittings are screwed
or clamped on the hose end. The big difference between skive
and no-skive hose is in the thickness of the outer cover. The
thicker cover requires a different fitting shell.
Quick Couplings
Quick couplings are used for convenience because they can
installed and removed by hand and in situations where there will
be repeated connecting and disconnecting of lines. There are
three kinds of couplings: single shutoff, double shutoff, and
straight through. Single shutoffs are used with compressed air.
Double shutoffs are used with liquids, and straight through are
used where flow must be unrestricted.
Hose Installation Considerations
Avoid twisting hose. Small twists in a hydraulic hose can
have devastating results. Do not install hose tight as in the
illustration on the right. Hose needs room to expand and
contract.
Hose Installation Considerations
Hose movement needs to be considered on equipment
using pivoting actuators. The cover should not be
allowed to come into contact with any other machine
part as the machine operates.
Hose Installation Considerations
Hydraulic hose will expand with pressure and at the same
time shorten in length. Secure all hoses firmly being
careful to allow for some movement.
Hose Bend Radius
Two conditions that contribute to shortened service life are installing
the hose twisted or over bent. A small twist in a hose, as little as 10
degrees, can shorten the life of the hose by 90 percent. Examine the
“lay line” during installation to ensure there are no twists in the
hose. The minimum recommended bend radius of a hose is
determined by manufacturers who specify limits on charts.