13. HIGH-VACUUM PUMPS operate with
the rotating plunger action of liquid pumps.
Sealing oil lubricates the three moving parts.
Parts are accessible without disturbing con-
nected piping. These pumps are used to
rough pump a vacuum before connecting a
diffusion pump; to evacute light bulbs and
electronic tubes, and to vacuum dry and dis-
till. Single pumps draw vacuums from 2 to 5
microns; in series to 0.5 micron, and com-
pound pumps draw to 0.1 micron. They can
be run in reverse for transferring liquids.
Diagonal cored slots, closed by a slide pin,
form the passageway and inlet valve. Popper
or feature outlet valves are used.
14. A COMPACT MULTI-PLUNGER INTENSIFIER, this hydraulic booster is
designed to convert low pressure to high pressure in any oil-hydraulic circuit. No
additional pumps are required. Because of its six plungers, the pressure flow from
the booster is both smooth and uninterrupted. High-pressure pumps are not
required, and no operating valves are needed to control the high-pressure system.
Small cylinder and ram assemblies can be used on operating equipment because the
pressure is high. Operating costs can be low because of the efficient use of con-
nected horsepower. The inertia effects of the small operating rams are low, so high
speed operations can be attained. These boosters were built in two standard sizes,
each of which was available in two pressure ranges: 2 to 1 and 3 to 1. Volumetric
output is in inverse proportion to the pressure ratio. All units have a maximum
7,500 psi discharge pressure. Pistons are double-acting, and the central valve
admits oil to pistons in sequence and is always hydraulically balanced.
373
15. THIS SELF-PRIMING PUMP gives a
rapid and smooth transition from priming
cycle to centrifugal pumping. The pump
starts with its priming chamber full. Liquid is
recirculated through the impeller until the
pump is primed. As priming liquid circu-
lates, air is drawn through impeller and
expelled through the discharge. When all air
is evacuated, discharge velocity closes the
priming valve completely. These pumps can
have open or closed impellers. Solids up to 1
in. can be passed through a 3 in. size pump
with an open impeller.
16. INTERNAL SCREW PUMPS can easily transfer high-viscosity petroleum
products. They can be used as boiler fuel pumps because they deliver a pulseless
flow of oil. For marine or stationary systems, the characteristic low vibration of
screw pumps has allowed them to be mounted on light foundations. The absence
of vibration and pulsing flow reduces strain on pipes, hose and fittings. The
pumping screws are mounted on shafts and take in liquid at both ends of the pump
body and move it to the center for discharge. This balanced pumping action
makes it unnecessary to use thrust bearings except in installations where the
pump is mounted at a high angle. The pumps can be used at any angle up to verti-
cal. Where thrust bearings are needed, antifriction bearings capable of supporting
the load of the shaft and screws are used. The intermeshing pumping screws are
timed by a pair of precision-cut herringbone gears. These are self-centering, and
do not allow the side wear of the screws while they are pumping. The pump is
most efficient when driven less than 1,200 rpm by an electric motor and 1,300
rpm by a steam turbine.
Sclater Chapter 11 5/3/01 1:15 PM Page 373
374
ROTARY-PUMP MECHANISMS
Fig. 1 (A) A Ramelli pump with spring-loaded vanes to ensure con-
tact with the wall; vane ends are rounded for line contact. (B) Two
vanes pivot in the housing and are driven by an eccentrically
mounted disk; vanes slide in glands and are always radial to the
housing, thus providing surface contact. (C) A housing with a cardioid
curve allows the use of a single vane because opposing points on the
housing in line with the disk center are equidistant.
Fig. 2 Flexible vanes on an eccentric rub-
ber rotor displace liquid as in sliding-vane
pumps. Instead of the vanes sliding in and
out, they bend against the casing to per-
form pumping.
Fig. 3 A disk mounted eccentrically on
the drive shaft displaces liquid in continu-
ous flow. A spring-loaded gland separates
the inlet from the outlet except when the
disk is at the top of stroke.
Fig. 4 A rotary compressor pump has a
link separating its suction and compression
sides. The link is hinged to a ring which
oscillates while it is driven by the disk.
Oscillating action pumps the liquid in a con-
tinuous flow.
Fig. 5 A gear pump transports liquid
between the tooth spaces and the housing
wall. A circular tooth shape ha sonly one tooth
making contact, and it is more efficient than an
involute shape which might enclose a pocket
between two adjoining teeth, recirculating part
of the liquid. The pump has helical teeth.
Fig. 6 A Roots compressor has two
identical impellers with specially shaped
teeth. The shafts are connected by external
gearing to ensure constant contact between
the impellers.
Fig. 7 A three-screw pump drives
liquid between the screw threads along
the axis of the screws. The idle rotors
are driven by fluid pressure, not by
metallic contact with the power rotor.
Sclater Chapter 11 5/3/01 1:15 PM Page 374
375
Fig. 8 The housing of the Hele-
Shaw-Beachum pump rotates the
round-cranked shaft. Connecting
rods attached to the crank ring
cause the pistons to oscillate as the
housing rotates. No valves are nec-
essary because the fixed hollow
shaft, divided by a wall, has suction
and compression sides that are
always in correct register with the
inlet and outlet ports.
Fig. 9 A disk drives the oscillating arm which
acts as piston. The velocity of the arm varies
because of its quick-return mechanism. Liquid is
slowly drawn in and expelled during the clockwise
rotation of the arm; the return stroke transfers the
liquid rapidly.
Fig. 10 A rotating cylinder block is mounted concentrically
in a housing. Connecting-rod ends slide around an eccentric
guide as the cylinders rotate and cause the pistons to recipro-
cate. The housing is divided into suction and compression
compartments.
Fig. 11 A rotary-reciprocating pump that is normally operated
manually to pump high-viscosity liquids such as oil.
OFFSET PLANETARY GEARS INDUCE ROTARY-PUMP ACTION
Two planetary gears are driven by an offset sun gear to provide the pumping action in this positive-displacement pump. A successively
increasing/decreasing (suction/compression) is formed on either side of the sun and planet gears.
Sclater Chapter 11 5/3/01 1:15 PM Page 375
376
MECHANISMS ACTUATED BY PNEUMATIC OR
HYDRAULIC CYLINDERS
Fig. 1 A cylinder can be used with a first-
class lever.
Fig. 2 A cylinder can be used with a sec-
ond-class lever.
Fig. 3 A cylinder can be used with a
third-class lever.
Fig. 4 A cylinder can be linked up directly
to the load.
Fig. 5 A spring reduces the thrust at the
end of the stroke.
Fig. 6 The point of application of force
follows the direction of thrust.
Fig. 7 A cylinder can be used with a bent
lever.
Fig. 8 A cylinder can be used with a
trammel plate.
Fig. 9 Two pistons with fixed strokes
position the load in any of four stations.
Fig. 10 A toggle can be actuated by the
cylinder.
Fig. 11 The cam supports the load after
the completion of the stroke.
Fig. 12 Simultaneous thrusts in two dif-
ferent directions are obtained.
Sclater Chapter 11 5/3/01 1:16 PM Page 376
377
(Note: In place of cylinders, electrically powered thrust units or
solenoids can be used.)
Fig. 13 A force is transmitted by a cable.
Fig. 14 A force can be modified by a sys-
tem of pulleys.
Fig. 15 A force can be modified by
wedges.
Fig. 16 A gear sector moves the rack
perpendicular to the piston stroke.
Fig. 17 A rack turns the gear sector.
Fig. 18 The motion of a movable rack is
twice that of the piston.
Fig. 19 A torque applied to the shaft can
be transmitted to a distant point.
Fig. 20 A torque can also be applied to a
shaft by a belt and pulley.
Fig. 21 A motion is transmitted to a dis-
tant point in the plane of motion.
Fig. 22 A steep screw nut produces a
rotation of the shaft.
Fig. 23 A single-sprocket wheel pro-
duces rotation in the plane of motion.
Fig. 24 A double-sprocket wheel makes
the rotation more nearly continuous.
Sclater Chapter 11 5/3/01 1:16 PM Page 377
378
FOOT-CONTROLLED BRAKING SYSTEM
This crane braking system (see figure) operates when the main
line switch closes. The full depression of the master-cylinder
foot-pedal compresses the brake-setting spring mounted on the
hydraulic releasing cylinder. After the setting spring is fully com-
pressed, the hydraulic pressure switch closes, completing the
electric circuit and energizing the magnetic check valve. The set-
ting spring remains compressed as long as the magnetic check
valve is energized because the check valve traps the fluid in the
hydraulic-releasing cylinder. Upon release of the foot peal, the
brake lever arm is pulled down by the brake releasing spring,
thus releasing the brake shoes.
LINKAGES ACTUATE STEERING IN A TRACTOR
Hydraulic power for operating the brakes,
clutch, and steering of a 300 hp diesel-powered
tractor is supplied by an engine-driven pump
delivering 55 gpm at 1200 psi. The system is
designed to give a 15-gpm preference to the
steering system. The steering drive to each
wheel is mechanical for synchronization, with
mechanical selection of the front-wheel, four-
wheel or crab-steering hookup; hydraulic
power amplifies the manual steering effort.
Sclater Chapter 11 5/3/01 1:16 PM Page 378
379
FIFTEEN JOBS FOR PNEUMATIC POWER
Suction can feed, hold, position, and lift parts, form plastic
sheets, sample gases, test for leaks, convey solids, and
de-aerate liquids. Compressed air can convey materials, atomize
and agitate liquids, speed heat transfer, support combustion,
and protect cable.
Sclater Chapter 11 5/3/01 1:16 PM Page 379
15 Jobs for Pneumatic Power (continued )
TEN WAYS TO USE METAL DIAPHRAGMS AND
CAPSULES
380
A metal diaphragm is usually corrugated (Fig. 1) or formed to some irregular profile. It can be
used as a flexible seal for an actuating rod. The capsule (Fig. 2) is an assembly of two
diaphragms sealed together at their outer edges, usually by soldering, brazing, or welding. Two
or more capsules assembled together are known as a capsular element (Fig. 3). End fittings
for the capsules vary according to their function; the “fixed end” is fixed to the equipment. The
“free end” moves the related components and linkages. The nested capsule (Fig. 4) requires
less space and can be designed to withstand large external overpressures without damage.
Sclater Chapter 11 5/3/01 1:16 PM Page 380
381
A differential pressure gage (Fig. 5) with opposing cap-
sules can have either single or multicapsular elements.
The multicapsular type gives greater movement to the
indicator. Capsules give improved linearity over bellows
for such applications as pressure-measuring devices. The
force exerted by any capsule is equal to the total effective
area of the diaphragms (about 40% actual area) multiplied
by the pressure exerted on it. Safe pressure is the maxi-
mum pressure that can be applied to a diaphragm before
hysteresis or set become objectionable.
A pressure gage (Fig. 6) has a capsular
element linked to a dial indicator by a three-
bar linkage. Such a gage measures pres-
sure or vacuum relative to prevailing atmos-
pheric pressure. If greater angular motion of
the indicator is required than can be
obtained from the three-bar linkage, a quad-
rant and gear can be substituted.
An absolute pressure gage (Fig. 7)
has an evacuated capsular element
inside an enclosure that is connected
to the pressure source only. The
diaphragm allows the linkage move-
ment from the capsule to pass
through a sealed chamber. This
arrangement can also be used as a
differential pressure gage by making
a second pressure connection to the
interior of the element.
An expansion compensator (Fig. 8) for oil-filled systems
takes up less space when the capsules are nested. In this
application, one end of the capsule is open and connected
to oil in the system; the other end is sealed. Capsule
expansion prevents the internal oil pressure from increas-
ing dangerously from thermal expansion. The capsule is
protected by its end cover.
A capsule pressure-seal (Fig. 9) works like a
thermometer system except that the bulb is
replaced by a pressure-sensitive capsule. The
capsule system is filled with a liquid such as sili-
cone oil and is self-compensating for ambient and
operating temperatures. When subjected to exter-
nal pressure changes, the capsule expands or
contracts until the internal system pressure just
balances the external pressure surrounding the
capsule.
A force-balanced seal (Fig. 10) solves the problem, as in the seal of Fig. 9, for
keeping corrosive, viscous or solids-bearing fluids out of the pressure gage. The
air pressure on one side of a diaphragm is controlled so as to balance the other
side of the diaphragm exactly. The pressure gage is connected to measure this
balancing air pressure. The gage, therefore, reads an air pressure that is always
exactly equal to the process pressure.
Sclater Chapter 11 5/3/01 1:16 PM Page 381
382
DIFFERENTIAL TRANSFORMER SENSING DEVICES
Gage pressure bellows transmitter. The bellows is connected to
a cantilever beam with a needle bearing. The beam adopts a different
position for every pressure; the transformer output varies with beam
position. The bellows are available for ranges from 0–10 in. to 0–200
in. of water for pressure indication or control. Differential diaphragm pressure transmitter. Differential pres-
sures P
1
and P
2
act on the opposite sides of a sensitive diaphragm
and move the diaphragm against the spring load. The diaphragm dis-
placement, spring extension, and transformer core movement are
proportional to the difference in pressure. The device can measure
differentials as low as 0.005 in. of water. It can be installed as the pri-
mary element in a differential pressure flowmeter, or in a boiler wind-
box for a furnace-draft regulator.
Absolute pressure bellows transmitter. This transmitter is similar
to the differential diaphragm transmitter except for addition of a refer-
ence bellows which is evacuated and sealed. It can measure nega-
tive gage pressures with ranges from 0–50 mm to 0–30 in. of mer-
cury. The reference bellows compensates for variations in
atmospheric pressure.
Cantilever load cell. The deflection of a cantilever beam and the
displacement of a differential transformer core are proportional to the
applied load. The stop prevents damage to the beam in the event of
overload. Beams are available for ranges from 0–5 to 0–500 lb. And
they can provide precise measurement of either tension or compres-
sion forces.
Absolute pressure Bourdon-tube transmitter. This device can
indicate or control absolute pressures from 15 to 10,000 psi, depend-
ing on tube rating. The reference tube is evacuated and sealed, and
compensates for variations in atmospheric pressure by changing the
output of the reference differential transformer. The signal output con-
sists of the algebraic sum of the outputs of both the primary and ref-
erence differential transformers.
Proving ring. The core of the transmitting transformer, T
1
, is fas-
tened to the top of the proving ring, while the windings are stationary.
The proving ring and transformer core deflect in proportion to the
applied load. The signal output of the balancing transformer, T
2
,
opposes the output of T
1
, so that at the balance point, the null point
indicator reads zero. The core of the balancing transformer is actuated
by a calibrated micrometer that indicates the proving ring deflection
when the differential transformer outputs are equal and balanced.
Sclater Chapter 11 5/3/01 1:16 PM Page 382
Không có nhận xét nào:
Đăng nhận xét