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Supplemental
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Read, follow, and understand schematics
◦ Electrical
◦ Hydraulic
◦ Pneumatic
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Interpret device and circuit operating from
drawings
Apply schematic readings in circuit
diagnostics
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Although it is common, not every
manufacturer follows the same
rules in schematics. This is a
general guide. Always check your
drawings against the machine.
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All drawings are drawn with the device off
and at rest
◦ Relays, switches, inputs, etc.
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Electrical Schematics
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Only those that are indicated to intersect
actually intersect
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USUALLY use industry standards
Marked with unique designators (R1, n1, etc)
Commonly labeled with part number, values
◦ When in doubt, get data sheet
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Commonly labeled, outlined
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Commonly marked TP
Allow for safe in-circuit
measurement
Matching point marked on board
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Labeled on schematic, matches on-board
designation
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Pins may be marked on board to aid pinpoint
diagnosis
Board layout drawings aid in component
location, board recognition
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Coil associated with
their outputs with
dashed lines
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Give tech overall theory of system
Help guide initial diagnosis
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Wire carrying signal commonly marked
◦ High/low states marked accordingly
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Actual conduction patch
Follow for possible unmarked test points
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Hydraulic and Pneumatic Schematics
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Many systems electric controlled
hydraulic/pneumatic
◦ Servos
◦ Pump motor controls
◦ Pressure/flow sensors
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May be the only one to service a device.
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Flow (Current)
◦ Movement of material
◦ Measured in volume/time (l/m, gal/hr)
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Pressure (Voltage)
◦ The exertion of force upon a surface by an object
◦ Measured in force/area (psi, n/m2)
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Restriction (Resistance)
◦ Opposition to flow
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Definition
◦ Measurement of force distributed over an area
◦ P = F/A
◦ Pascal’s law
 Pressure applied to an enclosed fluid is transmitted
undiminished to every portion of the fluid and
perpendicular to the walls of the containing vessel
◦ Can be varied by changing flow rate or area applied
Ex: 20 lbs applied to 1 square inch = 20 PSI
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Pressure varies inversely with volume
◦ Container must be sealed
 Decrease volume, pressure increases
 Increase volume, pressure decreases
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Gases are compressible
◦ Molecules can be moved closer
◦ Follow gas laws
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Liquids are not compressible
◦ Will transfer force throughout
◦ Will move to lower pressure area
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Create flow only
◦ Restrictions create pressure
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Displacement types
◦ Fixed - One output volume
◦ Variable – volume adjustable, based on demand
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Reversible will have two triangles
Fixed
Variable
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Converts hydraulic flow to
mechanical movement
Reversible will have two
triangles
Fixed
Variable
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The dashed line indicates
internal passages
Can have a spring, or can
be pressure operated
(sequence valve)
Pilot operated (relay)
◦ Activated by another
hydraulic pressure
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Can be single or double acting
◦ Single – one port, gravity or spring
reset
◦ Double – two port, hydraulic both
directions
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Direct fluid flow
Manually or externally actuated
◦ Hydraulic, pneumatic, or electrical
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Ex- Spool valve
◦ Multiple positions
◦ Allow for more control with
less components
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Hold unused reserve fluid
Sight glass may be used to indicate level
Can be several tank symbols in one drawing,
but only one common tank
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Indicate status of system
◦ Temperature
◦ Pressure
◦ Flow
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Can use a ball and seat or a spring behind the
ball
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Clean environment whenever possible
◦ Seals may have tight tolerances for pressure
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Air must be purged
◦ OEM procedure
◦ Cycle cylinders till air returned to tank
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Similar to hydraulics
◦ Use of check valves, control valves, cylinders, tanks,
and pumps
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Important differences
◦ Gases can be compressed
◦ Air usually vented to atmosphere
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Pressurized Storage Bottle
Regulator
3/2
Valves
Pneumatic Cylinders with
Speed Valves
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