LOADING, CARRYING, AND EXCAVATING
The proper packaging of material to assist in handling can significantly
minimize the handling cost and can also have a marked influence on the
type of handling equipment employed. For example, partial carload lots
of liquid or granular material may be shipped in rigid or nonrigid containers
equipped with proper lugs to facilitate in-transit handling.
Heavy-duty rubberized containers that are inert to most cargo are available
for repeated use in shipping partial carloads. The nonrigid container
reduces return shipping costs, since it can be collapsed to reduce
space. Disposable light-weight corrugated-cardboard shipping containers
for small and medium-sized packages both protect the cargo and
permit stacking to economize on space requirements. The type of container
to be used should be planned or considered when the handling
mechanism is selected.
SURFACE HANDLING
Lift Trucks and Palletized Loads
The basis of all efficient handling, storage, and movement of unitized
goods is the cube concept. Building a cube enables a large quantity of
unit goods to be handled and stored at one time. This provides greater
efficiency by increasing the volume of goods movement for a given
amount of work. Examples of cube-facilitating devices include pallets,
skids, slip sheets, bins, drums, and crates.
The most widely applied cube device is the pallet. A pallet is a low
platform, usually constructed of wood, incorporating openings for the
forks of a lift truck to enter. Such openings are designed to enable a lift
truck to pick up and transport the pallet containing the cubed goods.
Lift truck is a loose term for a family of pallet handling/transporting
machines. Such machines range from manually propelled low-lift devices
(Fig. 10.4.1) to internal combustion and electric powered ride-on
high-lift devices (Fig. 10.4.2). While some machines are substitutes in
terms of function, each serves its own niche when viewed in terms of
individual budgets and applications.
Pallet trucks are low-lift machines designed to raise loaded pallets
sufficiently off the ground to enable the truck to transport the pallet
horizontally. Pallet trucks are available as manually operated and propelled
models that incorporate a hydraulic pump and handle assembly
(Fig. 10.4.1). This pump and handle assembly enables the operator to
Fig. 10.4.1 Manually operated and propelled pallet truck.
raise the truck forks, and push/pull the load. Standard manual pallet
trucks are available in lifting capacities from 4,500 to 5,500 lb (2,045 to
2,500 kg), with customer manufactured models to 8,000 lb (3,636 kg).
While available in a variety of fork sizes, by far the most common is
27 in wide 3 48 in long (686 mm 3 1,219 mm). This size accommodates
the most common pallet sizes of 40 in 3 48 in (1,016 mm 3
Fig. 10.4.2 Lift truck powered by an internal-combustion engine.
1,219 mm) and 48 in 3 48 in (1,219 mm 3 1,219 mm). Pallet trucks
are also available in motorized versions, equipped with dc electric
motors to electrically raise and transport. The power supply for these
trucks is an on-board lead-acid traction battery that is rechargeable
when the truck is not in use. Control of these trucks is through a set of
lift, lower, speed, and direction controls fitted into the steering handle
assembly. Powered pallet trucks are available in walk and ride models.
Capacities range from 4,000 to 10,000 lb (1,818 to 4,545 kg), with forks
up to 96 in (2,438 mm) long. The longer fork models are designed to
allow the truck to transport two pallets, lined up end to end.

Stackers, as the name implies, are high lift machines designed to
raise and stack loaded pallets in addition to providing horizontal transportation.
Stackers are separated into two classes: straddle and counterbalanced.
Straddle stackers are equipped with legs which straddle
the pallet and provide load and truck stability. The use of the straddle
leg system results in a very compact chassis which requires minimal
aisle space for turning. This design, however, does have its trade-offs insomuch
as the straddles limit the truck’s usage to smooth level floors.
The limited leg underclearance inherent in these machines prohibits
their use on dock levelers for loading/unloading transport trucks. Straddle
stackers are available from 1,000 to 4,000 lb (455 to 1,818 kg)
capacity with lift heights to 16 ft (4,877 mm). Counterbalanced stackers
utilize a counterweight system in lieu of straddle legs for load and
vehicle stability (Fig. 10.4.3). The absence of straddle legs results in a
chassis with increased underclearance which can be used on ramps,
including dock levelers. The counterbalanced chassis, however, is
longer than its straddle counterpart, and this requires greater aisle space
for maneuvering. For materials handling operations that require one
machine to perform a multitude of tasks, and are flexible in floor layout
of storage areas, the counterbalanced stacker is the recommended machine.
Off-Highway Vehicles and Earthmoving
Equipment
by Darrold E. Roen, John Deere and Co.
The movement of large quantities of bulk materials, earth, gravel, and
broken rock in road building, mining, construction, quarrying, and land
clearing may be handled by off-highway vehicles. Such vehicles are
mounted on large pneumatic tires or on crawler tracks if heavy pulling
and pushing are required on poor or steep terrain. Width and weight of
the rubber-tired equipment often exceed highway legal limits, and use
of grouser tracks on highways is prohibited. A wide range of working
tool attachments, which can be added (and removed) without modification
to the basic machine, are available to enhance the efficiency and
versatility of the equipment.
Proper selection of size and type of equipment depends on the
amount, kind, and density of the material to be moved in a specified
time and on the distances, direction, and steepness of grades, footing
for traction, and altitude above sea level. Time cycles and pay loads
for production per hour can then be estimated from manufacturers’
performance data and job experience. This production per hour, together
with the corresponding owning, operating, and labor costs
per hour, enables selection by favorable cost per cubic yard, ton, or
other pay unit.
Current rapid progress in the development of off-highway equipment
will soon make any description of size, power, and productivity obsolete.
However, the following brief description of major off-highway
vehicles will serve as a guide to their applications.
Crawler Tractors These are track-type prime movers for use with
mounted bulldozers, rippers, winches, cranes, cable layers, and side
booms rated by net engine horsepower in sizes from 40 to over 500 hp;
maximum traveling speeds, 5 to 7 mi/h (8 to 11 km/h). Crawler tractors
develop drawbar pulls up to 90 percent or more of their weight with
mounted equipment.
Wheel Tractors Sizes range from rubber-tired industrial tractors for
small scoops, loaders, and backhoes to large, diesel-powered, two- and
four-wheel drive pneumatic-tired prime movers for propelling scrapers
and wagons. Large, four-wheel-drive, articulated-steering types also
power bulldozers.
Bulldozer—Crawler Type (Fig. 10.4.4) This is a crawler tractor
with a front-mounted blade, which is lifted by hydraulic or cable power
control. There are four basic types of moldboards: straight, semi-U and
U (named by top-view shape), and angling. The angling type, often
called bullgrader or angledozer, can be set for side casting 25° to the right
or left of perpendicular to the tractor centerline, while the other blades
can be tipped forward or back through about 10° for different digging
conditions. All blades can be tilted for ditching, with hydraulic-power
tilt available for all blades.
APPLICATION. This is the best machine for pioneering access roads,
for boulder and tree removal, and for short-haul earthmoving in rough
terrain. It push-loads self-propelled scrapers and is often used with a
Fig. 10.4.4 Crawler tractor with dozer blade. (John Deere.)
rear-mounted ripper to loosen firm or hard materials, including rock, for
scraper loading. U blades drift 15 to 20 percent more loose material than
straight blades but have poor digging ability. Angling blades expedite
sidehill benching and backfilling of trenches. Loose-material capacity
of straight blades varies approximately as the blade height squared,
multiplied by length. Average capacity of digging blades is about 1 yd3
loose measure per 30 net hp rating of the crawler tractor. Payload is 60
to 90 percent of loose measure, depending on material swell variations.
Bulldozer—Wheel Type This is a four-wheel-drive, rubber-tired
tractor, generally of the hydraulic articulated-steering type, with frontmounted
blade that can be hydraulically raised, lowered, tipped, and
tilted. Its operating weights range to 150,000 lb, with up to 700 hp,
and its traveling speeds range from stall to about 20 mi/h for pushing
and mobility.
APPLICATION. It is excellent for push-loading self-propelled
scrapers, for grading the cut, spreading and compacting the fill, and for
drifting loose materials on firm or sandy ground for distances up to
500 ft. Useful tractive effort on firm earth surfaces is limited to about 60
percent of weight, as compared with 90 percent for crawler dozers.
Loader—Crawler Type (Fig. 10.4.5) This is a track-type prime
mover with front-mounted bucket that can be raised, dumped, lowered,
and tipped by power control. Capacities range from 0.7 to 5.0 yd3 (0.5
to 3.8 m3), SAE rated. It is also available with grapples for pulpwood,
logs, and lumber.
Fig. 10.4.5 Crawler tractor with loader bucket. (John Deere.)
APPLICATION. It is used for digging basements, pools, ponds, and
ditches; for loading trucks and hoppers; for placing, spreading, and
compacting earth over garbage in sanitary fills; for stripping sod; for
removing steel-mill slag; and for carrying and loading pulpwood and
logs.
Loader—Wheel Type (Fig. 10.4.6) This is a four-wheel, rubbertired,
articulated-steer machine equipped with a front-mounted, hydraulic-powered bucket and loader linkage that loads material into the
bucket through forward motion of the machine and which lifts, transports,
and discharges the material. The machine is commonly referred to
as a four-wheel-drive loader tractor. Bucket sizes range from 1⁄2 yd3
(0.4 m3) to more than 20 yd3 (15 m3), SAE rated capacity. The addition
Fig. 10.4.6 Four-wheel-drive loader. (John Deere.)
of a quick coupler to the loader linkage permits convenient interchange
of buckets and other working tool attachments, adding versatility to the
loader. Rigid-frame machines with variations and combinations of
front/rear/skid steer, front/rear drive, and front/rear engine are also used
in various applications.
APPLICATION. Four-wheel-drive loaders are used primarily in construction,
aggregate, and utility industries. Typical operations include
truck loading, filling hoppers, trenching and backfilling, land clearing,
and snow removal.
Backhoe Loader (Fig. 10.4.7) This is a self-propelled, highly mobile
machine with a main frame to support and accommodate both the
rear-mounted backhoe and front-mounted loader. The machine was designed
with the intention that the backhoe will normally remain in place
when the machine is being used as a loader and vice versa. The backhoe
digs, lifts, swings, and discharges material while the machine is stationary.
When used in the loader mode, the machine loads material into the
bucket through forward motion of the machine and lifts, transports, and
discharges the material. Backhoe loaders are categorized according to
digging depth of the backhoe. Backhoe loader types include variations
of front/rear/articulated and all-wheel steer and rear/four-wheel drive.
Fig. 10.4.7 Backhoe loader. (John Deere.)
APPLICATION. Backhoe loaders are used primarily for trenching and
backfilling operations in the construction and utility industries. Quick
couplers for the loader and backhoe are available which quickly interchange
the working tool attachments, thus expanding machine capabilities.
Backhoe loader mobility allows the unit to be driven to nearby job
sites, thus minimizing the need to load and haul the machine.
Scrapers (Fig. 10.4.8) This is a self-propelled machine, having a
cutting edge positioned between the front and rear axles which loads,
transports, discharges, and spreads material. Tractor scrapers include
open-bowl and self-loading types, with multiple steer and drive axle
variations. Scraper rear wheels may also be driven by a separate rearmounted
engine which minimizes the need for a push tractor. Scraper
ratings are provided in cubic yard struck/heaped capacities. Payload
capacities depend on loadability and swell of materials but approximate
the struck capacity. Crawler tractor-drawn, four-wheel rubber-tired
scrapers have traditionally been used in a similar manner—normally in
situations with shorter haul distances or under tractive and terrain conditions
that are unsuitable for faster, self-propelled scrapers.
Fig. 10.4.8 Two-axle articulated self-propelled elevating scraper. (John
Deere.)
APPLICATION. Scrapers are used for high-speed earth moving, primarily
in road building and other construction work where there is a
need to move larger volumes of material relatively short distances. The
convenient control of the cutting edge height allows for accurate control
of the grade in either a cut or fill mode. The loaded weight of the scraper
can contribute to compaction of fill material. All-wheel-drive units can
also load each other through a push-pull type of attachment. Two-axle,
four-wheel types have the best maneuverability; however, the three-axle
type is sometimes preferred for operator comfort on longer, higherspeed
hauls.
Motor Grader (Fig. 10.4.9) This is a six-wheel, articulated-frame
self-propelled machine characterized by a long wheelbase and midmounted
blade. The blade can be hydraulically positioned by rotation
about a vertical axis—pitching fore/aft, shifting laterally, and independently
raising each end—in the work process of cutting, moving, and
spreading material, to rough- or finish-grade requirements. Motor
graders range in size to 60,000 lb (27,000 kg) and 275 hp (205 kW)
with typical transport speeds in the 25-mi/h (40-km/h) range. Rigidframe
machines with various combinations of four/six wheels, two/
four-wheel drive, front/rear-wheel steer are used as dictated by the
operating requirements.
Fig. 10.4.9 Six-wheel articulated-frame motor grader. (John Deere.)
APPLICATION. Motor graders are the machine of choice for building
paved and unpaved roads. The long wheelbase in conjunction with the
midmounted blade and precise hydraulic controls allows the unit to
finish-grade road beds within 0.25 in (6 mm) prior to paving. The
weight, power, and blade maneuverability enable the unit to perform all
the necessary work, including creating the initial road shape, cutting the
ditches, finishing the bank slopes, and spreading the gravel. The motor
grader is also a cost-effective and vital part of any road maintenance
fleet.

Excavator (Fig. 10.4.10) This is a mobile machine which is propelled
by either crawler track or rubber-tired undercarriage, with the
unique feature being an upper structure that is capable of continuous
rotation and a wide working range. The unit digs, elevates, swings, and
dumps material by action of the boom, arm, or telescoping boom and
bucket. Excavators include the hoe type (digging tool cuts toward the
machine and generally downward) and the shovel type (digging tool
cuts away from the machine and generally upward). Weight of the
machines ranges from 17,600 lb (8 tonnes) to 1,378,000 lb (626 tonnes)
with power ratings from 65 to 3644 hp (48.5 to 2719 kW).
Fig. 10.4.10 Excavator with tracked undercarriage. (John Deere.)
APPLICATION. The typical attachment for the unit is the bucket,
which is used for trenching in the placement of pipe and other underground
utilities, digging basements or water retention ponds, maintaining
slopes, and mass excavation. Other specialized attachments include
hydraulic hammers and compactors, thumbs, clamshells, grapples, and
long-reach front ends which expand the capabilities of the excavator.
Dumpers A dumper is a self-propelled machine, having an open
cargo body, which is designed to transport and dump or spread material.
Loading is accomplished by means external to the dumper. Types are
generally categorized into rear, side, or bottom dump with multiple
variations of front/articulated steer, two to five axles, and front/rear/
center/multiaxle drive.
APPLICATION. Dumpers are used for hauling and dumping blasted
rock, ore, earth, sand, gravel, coal, and other hard and abrasive materials
in road and dam construction and in quarries and mines. The units are
capable of 30 to 40 mi/h (50 to 65 km/h) when loaded (depending on
terrain/slopes), which makes the dumper an excellent choice for longer
haul distances.
Owning and Operating Costs These include depreciation; interest,
insurance, taxes; parts, labor, repairs, and tires; fuel, lubricant, filters,
hydraulic-system oil, and other operating supplies. This is reduced to
cost per hour over a service life of 4 to 6 years of 2,000 h each—
average 5 years, 10,000 h. Owning and operating costs of dieselpowered
bulldozers and scrapers, excluding operator’s wages, average
3 to 4 times the delivered price in 10,000 h.
ABOVE-SURFACE HANDLING
Monorails
Materials can be carried on light, rigid trackage, as described for overhead
conveyors (see below). Trolleys are supported by structural I
beams, H beams, or I-beam-like rails with special flat flanges to improve
rolling characteristics of the wheels. Size of wheels and smoothness
of tread are important in reducing rolling resistance. Figure 10.4.11
shows a typical rigid trolley for traversing short-radius track curves.
Typical dimensions for both types are given in Table 10.4.1. These
trolleys may be plain, with geared handwheel and hand chain, or motordrive.
For very low headroom, the trolley can be built into the hoist; this
is known as a trolley hoist.
Fig. 10.4.11 Monorail trolley. (CM Hoist Division, Columbus McKinnon.)
Overhead Traveling Cranes
by Alger Anderson, Lift-Tech International, Inc.
An overhead traveling crane is a vehicle for lifting, transporting, and
lowering loads. It consists of a bridge supporting a hoisting unit and is
equipped with wheels for operating on an elevated runway or track. The
hoisting unit may be fixed relative to the bridge but is usually supported
on wheels, permitting it to traverse the length of the bridge.
The motions of the crane—hoisting, trolley traversing, and bridging
— may be powered by hand, electricity, air, hydraulics, or a combination
of these. Hand-powered cranes are generally built in capacities
under 50 tons (45 tonnes) and are used for infrequent service where slow speeds are acceptable. Pneumatic cranes are used where electricity
would be hazardous or where advantage can be taken of existing air
supply. Electric cranes are the most common overhead type and can be
built to capacities of 500 tons (454 tonnes) or more and to spans of
150 ft (46 m) and over.
Single-Girder Cranes (Fig. 10.4.12) In its simplest form, this consists
of an I beam a supported by four wheels b. The trolley c traveling
on the lower flanges carries the chain hoist, forming the lifting unit. The
crane is moved by hand chain d turning sprocket wheel e, which is
keyed to shaft f. The pinions on shaft f mesh with gears g, keyed to the
axles of two wheels. An underslung construction may also be used, with
pairs of wheels at each corner which ride on the lower flange of I-beam
rails. Single-girder cranes may be hand-powered by pendant hand
chains or electric-powered as controlled by a pendant push-button station.
Fig. 10.4.12 Hand-powered crane.
Electric Traveling Crane, Double-Girder Type (Fig. 10.4.13) This
consists of two bridge girders a, on the top of which are rails on which
travels the self-contained hoisting unit b, called the trolley. The girders
are supported at the ends by trucks with two or four wheels, according to
the size of the crane. The crane is moved along the track by motor c,
through shaft d and gearing to the truck wheels. Suspended from the
girders on one side is the operator’s cab e, containing the controller, or
master switches, master hydraulic brake cylinder, warning device, etc.
The bridge girders for small cranes are of the I-beam type, but on the
Fig. 10.4.13 Electric traveling crane.
longer spans, box girders are used to give torsional and lateral stiffness.
The girders are rigidly attached to the truck end framing, which carries
the double-flanged wheels for supporting the bridge. The end frames
project over the rails so that in case of a broken wheel or axle, the frame
will rest on the rail, preventing the crane from dropping. One wheel axle
on each truck is fitted with gears for driving the crane or is coupled
directly to the shaft which transmits power from the gear reducer. On a
cab-operated bridge, a brake, usually hydraulic, is applied to the motor
shaft to stop the crane. Floor-operated cranes generally utilize spring
engaged, electrically released brakes.
The trolley consists of a frame which carries the hoisting machinery
and is supported on wheels for movement along the bridge rails. The
wheels are coupled to the trolley traverse motor through suitable gear
reduction. Trolleys are frequently equipped with a second set of hoisting
machinery to provide dual lifting means or an auxiliary of smaller
capacity. The hoisting machinery consists of motor, motor brake, load
brake, gear reduction, and rope drum. Wire rope winding in helical
grooves on the drum is reeved over sheaves in the upper block and
lower hook block for additional mechanical advantage. Limit switches
are provided to stop the motors when limits of travel are reached.
Current is brought to the crane by sliding or rolling collectors in contact
with conductors attached to or parallel with the runway and preferably
located at the cab end of the crane. Current from the runway conductors
and cab is carried to the trolley in a like manner from conductors
mounted parallel to the bridge girder. Festooned multiconductor cables
are also used to supply current to crane or trolley.
Electric cranes are built for either alternating or direct current, with
the former predominating. The motors for both kinds of current are
designed particularly for crane service. Direct-current motors are usually
series-wound, and ac motors are generally of the wound-rotor or
two-speed squirrel-cage type. The usual ac voltages are 230 and 460,
the most common being 460. Variable-frequency drives (VFDs) are
used with ac squirrel cage motors to provide precise control of the load
over a wide range of speeds. Cranes and hoists equipped with VFDs are
capable of delicate positioning and swift acceleration of loads to the
maximum speed. Standard, inexpensive squirrel cage motors may be
used with VFDs to provide high-performance control of all crane motions.
The capacities and other dimensions for standard electric cranes
are given in Table 10.4.2.
Gantry Cranes
Gantry cranes are modifications of traveling cranes and are generally
used outdoors where it is not convenient to erect on overhead runway.
The bridge (Fig. 10.4.14) is carried at the ends by the legs a, supported
by trucks with wheels so that the crane can travel. As with the traveling
crane, the bridge carries a hoisting unit; a cover to protect the machinery
from the weather is often used. The crane is driven by motor b through a
gear reduction to shaft c, which drives the vertical shafts d through
bevel gears. Bevel- and spur-gear reductions connect the axles of the
wheels with shafts d. Many gantry cranes are built without the cross
shaft, employing separate motors, brakes, and gear reducers at each end
of the crane. Gantry cranes are made in the same sizes as standard
traveling cranes.
Special-Purpose Overhead Traveling Cranes
A wide variety can be built to meet special conditions or handling
requirements; examples are stacker cranes to move material into and out
of racks, wall cranes using a runway on only one side of a building,
circular running or pivoting cranes, and semi-gantries. Load-weighing
arrangements can be incorporated, as well as special load-handling devices
such as lifting beams, grapples, buckets, forks, and vacuum grips.
Rotary Cranes and Derricks
Rotary cranes are used for lifting material and moving it to points covered
by a boom pivoted to a fixed or movable structure. Derricks are
used outdoors (e.g., in quarries and for construction work), being built
so that they can be easily moved. Pillar cranes are always fixed and are
used for light, infrequent service. Jib cranes are used in manufacturing
plants. Locomotive cranes mounted on car wheels are used to handle
loads by hook or bulk material by means of tubs, grab buckets, or
magnets. Wrecking cranes are of the same general type as locomotive
cranes and are used for handling heavy loads on railroads the guyed or stiff-leg type, and are either hand-slewed or power-swung
with a bull wheel. Figure 10.4.15 shows a guyed wooden derrick of the
bull-wheel type. The mast a is carried at the foot by pivot k and at the
Fig. 10.4.14 Gantry crane.
top by pivot m, held by rope guys n. The boom b is pivoted at the lower
end of the mast. The rope c, passing over sheaves at the top of the mast
and at the end of the boom and through the pivot k, is made fast to drum
d and varies the angle of the boom. The hoisting rope e, from which the
load is suspended, is made fast to drum f. The bull wheel g is attached to
the mast and swings the derrick by a rope made fast to the bull wheel
Fig. 10.4.15 Guyed wooden derrick.
and passing around the reversible drum h. In derricks of the self-slewing
type, the engine is mounted on a platform attached to the mast and the
derrick is swung by a pinion meshing with a gear attached to the foundation.
Either the bull-wheel or the self-slewing type may be made of
steel or wood construction and may be of the guyed or stiff-leg type.
Figure 10.4.16 shows a column jib crane, consisting of pivoted post a and
carrying boom b, on which travels either an electric or a hand hoist c.
The post a is attached to building column d so that it can swing through
approximately 270°. Cranes of this type are rapidly being replaced by
such other methods of handling materials as the mobile lift truck or the
automotive-type crane. Column jib cranes are built with radii up to 20 ft
(6 m) and for loads up to 5 tons (4.5 tonnes). Yard jib cranes are generally
designed to meet special conditions.
Fig. 10.4.16 Column jib crane.
Locomotive Cranes
The locomotive crane (Fig. 10.4.17) is self-propelled and provided with
trucks, brakes, automatic couplers, fittings, and clearances which will
permit it to be used or hauled in a train; it can function as a complete
unit on any railroad. Locomotive cranes are of the rotating-deck type,
consisting of a hinged boom attached to the machinery deck, which is
turntable-mounted and operated either by mechanical rotating clutches
Fig. 10.4.17 Locomotive crane. (American Hoist and Derrick.)
or by a separate electric or hydraulic swing motor. The boom is operated
by powered topping line, with a direct-geared hoisting mechanism to
raise and lower it. Power to operate the machinery is deck-mounted, and
the machinery deck is completely housed. The crane may be powered
by internal-combustion engine or electric motor. The combination of
internal-combustion engine, generator, and electric motor makes up the
power arrangement for the diesel-electric locomotive crane. Another
power arrangement is made up of internal-combustion engines driving
hydraulic pumps for hydraulically powered swing and travel mechanisms.
The car body and machinery deck are ballasted, thereby adding
stability to the crane when it is rotated under load. The basic boom is
generally 50 ft (15 m) in length; however, booms range to 130 ft (40 m)
in length. Locomotive cranes are so designed that power-shovel, piledriver,
hook, bucket, or magnet attachments can be installed and the
crane used in such service. Locomotive cranes are used most extensively
in railroad work, steel mills, and scrap yards. The cranes usually
have sufficient propelling power not only for the crane itself, but also
for switching service and hauling cars.
Truck Cranes
The advent of the truck crane has changed significantly the methods of
lifting and placing heavy items such as concrete buckets, logs, pipe, and
bridge or building members. Truck cranes can, without assistance, be rapidly equipped with accessory booms to reach to 260 ft (79 m)
vertically—or 180 ft (55 m) vertically with 170 ft of horizontal reach.
Mechanical Models
TOWER CRANE. (Fig. 10.4.18a and b). Has vertical and horizontal
members together with a boom and jib. Permits location close to building
with horizontal reach. Without jib, capacity to 27 tons (24.5 tonnes).
With jib, reach to 180 ft (55 m). With jib, vertically to 190 ft (58 m).
Lifting capacity based upon using outriggers.
CONVENTIONAL CRANE. (Fig. 10.4.18c). With boom and with or
without jib. Boom plus jib to 260 ft (79 m) and 125 tons (113 tonnes)
capacity. Maximum working weight 230,000 lb (104,000 kg).
Fig. 10.4.18 Mechanical tower crane with vertical and horizontal extensions.
(a) Tower working heights; (b) normal crane position; (c) crane with conventional
boom. (FMC Corp.)
Table 10.4.3 Conventional Crane* Capacity and Limits of Operation
Boom On outriggers
Length, Radius, Angle, Point
ft ft deg height, ft Rear lb Side lb
30 11 81.0 33.5 250,000 250,000
30 25 46.3 24.5 123,300 123,300
60 16 80.7 63.1 145,100 145,100
60 50 39.7 41.0 52,200 50,100
90 20 81.3 92.9 131,600 131,600
90 80 31.5 49.8 30,500 26,000
180 40 79.2 180.6 46,700 46,700
180 170 21.6 68.9 8,300 6,200
230 50 79.0 229.6 21,000 21,000
230 220 18.9 77.5 2,900 1,800
* Multiply ft by 0.30 for m, lb by 0.45 for kg.
SOURCE: FMC Corporation.
GENERAL CHARACTERISTICS. (Table 10.4.3). 8 3 4 drive wheels with
air brakes on all eight wheels. Power hydraulic steering. Removablepin-
connected counterweights front or rear removable for roadability.
Hydraulic Models
SELF-PROPELLED. (Fig. 10.4.19a). Short wheelbase, two axle, single
cab; 181⁄2 tons (16.8 tonnes) capacity with two telescoping sections to
64 ft. Addition of a jib to 104 ft (32 m) reach.
Fig. 10.4.19 Hydraulic crane with (a) self-propelled and (b) truck-type bases.
(FMC Corp.)
HYDRAULIC TRUCK. (Fig. 10.4.19b). Three or four axle, two cabs,
crane functions from upper cab; 45 tons (41 tonnes) capacity with three
telescoping sections to 96 ft (29 m). Addition of a jib and boom extension
to 142 ft (43 m).
GENERAL CHARACTERISTICS. Hydraulic extensions save setup time
and provide job-to-job mobility. Equipment such as this comes under
Commercial Standard specification CS90-58, ‘‘Power Cranes and
Shovels.’’ Similar equipment, called utility cranes, without the highwaytruck-
type cabs, is also available.
Cableways
Cableways are aerial hoisting and conveying devices using suspended steel
cable for their tracks, the loads being suspended from carriages and
moved by gravity or power. The most common uses are transporting
material from open pits and quarries to the surface; handling construction
material in the building of dams, docks, and other structures where
the construction of tracks across rivers or valleys would be uneconomical;
and loading logs on cars. The maximum clear span is 2,000 to
3,000 ft (610 to 914 m); the usual spans, 300 to 1,500 ft (91 to 457 m).
The gravity type is limited to conditions where a grade of at least 20
percent is obtainable on the track cable. Transporting cableways move
the load from one point to another. Hoisting transporting cableways
hoist the load as well as transport it.
A transporting cableway may have one or two fixed track cables,
inclined or horizontal, on which the carriage operates by gravity or
power. The gravity transporting type (Fig. 10.4.20-I) will either raise or
lower material. It consists of one track cable a on which travels the
wheeled carriage b carrying the bucket. The traction rope c attached to
the carriage is made fast to power drum d. The inclination must be
sufficient for the carriage to coast down and pull the traction rope after
it. The carriage is hauled up by traction rope c. Drum d is provided with
a brake to control the lowering speed, and material may be either raised
or lowered. When it is not possible to obtain sufficient fall to operate the
load by gravity, traction rope c (Fig. 10.4.20-II) is made endless so that
carriage b is drawn in either direction by power drum d. Another type of
inclined cableway, shown in Fig. 10.4.20-III, consists of two track
cables aa, with an endless traction rope c, driven and controlled by drum
d.When material is being lowered, the loaded bucket b raises the empty
carriage bb, the speed being controlled by the brake on the drum. When
material is being raised, the drum is driven by power, the descending
empty carriage assisting the engine in raising the loaded carriage. This
type has twice the capacity of that shown in Fig. 10.4.20-I.
A hoisting and conveying cableway (Fig. 10.4.20-IV) hoists the material
at any point under the track cable and transports it to any other point. It
consists of a track cable a and carriage b, moved by the endless traction
rope c and by power drum d. The hoisting of the load is accomplished by power drum e through fall rope f, which raises the fall block g
suspended from the carriage. The fall-rope carriers h support the fall
rope; otherwise, the weight of this sagging rope would prevent fall
block g from lowering when without load. Where it is possible to obtain
a minimum inclination of 20° on the track cable, the traction-rope drum
d is provided with a brake and is not power-driven. The carriage then
Fig. 10.4.20 Cableways.
descends by gravity, pulling the fall and traction ropes to the desired
point. Brakes are applied to drum d, stopping the carrier. The fall block
is lowered, loaded, and raised. If the load is to be carried up the incline,
the carriage is hauled up by the fall rope. With this type, the friction of
the carriage must be greater than that of the fall block or the load will
run down. A novel development is the use of self-filling grab buckets
operated from the carriages of cableways, which are lowered, automatically
filled, hoisted, carried to dumping position, and discharged.
The carriage speed is 300 to 1,400 ft/min (1.5 to 7.1 m/s) [in special
cases, up to 1,800 ft/min (9.1 m/s)]; average hoisting speed is 100 to 700
ft/min. The average loads for coal and earth are 1 to 5 tons (0.9 to 4.5
tonnes); for rock from quarries, 5 to 20 tons; for concrete, to 12 yd3
(9.1 m3) at 50 tons.
The deflection of track cables with their maximum gross loads at midspan
is usually taken as 51⁄2 to 6 percent of the span. Let S 5 span
between supports, ft; L 5 one-half the span, ft; w 5 weight of rope,
lb/ft; P 5 total concentrated load on rope, lb; h 5 deflection, ft; H 5
horizontal tension in rope, lb. Then h 5 (wL 1 P)L/2H; P 5 (2h 2
wL2)/L 5 (8hH 2 wS2)/2S.
For track cables, a factor of safety of at least 4 is advised, though this
may be as low as 3 for locked smooth-coil strands that use outer wires of
high ultimate strength. For traction and fall ropes, the sum of the load
and bending stress should be well within the elastic limit of the rope or,
for general hoisting, about two-thirds the elastic limit (which is taken at
65 percent of the breaking strength). Let P 5 load on the rope, lb; A 5
area of metal in rope section, in2; E 5 29,500,000; R 5 radius of
curvature of hoisting drum or sheave, whichever is smaller, in; d 5
diameter of individual wires in rope, in (for six-strand 19-wire rope, d5
1⁄15 rope diam; for six-strand 7-wire rope, d 5 1⁄9 rope diam). Then load
stress per in2 5 T1 5 P/A, and bending stress per in2 5 Tb 5 Ed/2R. The
radius of curvature of saddles, sheaves, and driving drums is thus important
to fatigue life of the cable. In determining the horsepower required,
the load on the traction ropes or on the fall ropes will govern,
depending upon the degree of inclination.
Cable Tramways
Cable tramways are aerial conveying devices using suspended cables,
carriages, and buckets for transporting material over level or mountainous
country or across rivers, valleys, or hills (they transport but do not
hoist). They are used for handling small quantities over long distances,
and their construction cost is insignificant compared with the construction
costs of railroads and bridges. Five types are in use:
Monocable, or Single-Rope, Saddle-Clip Tramway Operates on
grades to 50 percent gravity grip or on higher grades with spring grip
and has capacity of 250 tons/h (63 kg/s) in each direction and speeds to
500 ft/min. Single section lengths to 16 miles without intermediate
stations or tension points. Can operate in multiple sections without
transshipment to any desired length [monocables to 170 miles (274 km)
over jungle terrain are practical]. Loads automatically leave the carrying
moving rope and travel by overhead rail at angle stations and transfer
points between sections with no detaching or attaching device required.
Main rope constantly passes through stations for inspection and oiling.
Cars are light and safe for passenger transportation.
Single-Rope Fixed-Clip Tramway Endless rope traveling at low
speed, having buckets or carriers fixed to the rope at intervals. Rope
passes around horizontal sheaves at each terminal and is provided with a
driving gear and constant tension device.
Bicable, or Double-Rope, Tramway Standing track cable and a
moving endless hauling or traction rope traveling up to 500 ft/min (2.5
m/s). Used on excessively steep grades. A detacher and attacher is required
to open and close the car grip on the traction rope at stations.
Track cable is usually in sections of 6,000 to 7,000 ft (1,830 to 2,130 m)
and counterweighted because of friction of stiff cable over tower saddles.
Jigback, or Two-Bucket, Reversing Tramway Usually applied to
hillside operations for mine workings so that on steep slopes loaded
bucket will pull unloaded one up as loaded one descends under control
of a brake. Loads to 10 tons (9 tonnes) are carried using a pair of track
cables and an endless traction rope fixed to the buckets.
To-and-Fro, or Single-Bucket, Reversing Tramway A single track
rope and a single traction rope operated on a winding or hoist drum.
Suitable for light loads to 3 tons (2.7 tonnes) for intermittent working on
a hillside, similar to a hoisting and conveying cableway without the
hoisting feature.
The monocable tramway (Fig. 10.4.21) consists of an endless cable a
passing over horizontal sheaves d and e at the ends and supported at
intervals by towers. This cable is moved continuously, and it both supports
and propels carriages b and c. The carriages either are attached
permanently to the cable (as in the single-rope fixed clip tramway), in
Fig. 10.4.21 Single-rope cable tramway.
which case they must be loaded and dumped while in motion, or are
attached by friction grips so that they may be connected automatically
or by hand at the loading and dumping points. When the tramway is
lowering material from a higher to a lower level, the grade is frequently
sufficient for the loaded buckets b to raise the empty buckets c, operating
the tramway by gravity, the speed being controlled by a brake on
grip wheel d.
Fig. 10.4.22 Double-rope cable tramway.
The bicable tramway (Fig. 10.4.22) consists of two stationary track
cables a, on which the wheeled carriages c and d travel. The endless
traction rope b propels the carriages, being attached by friction grips.
Figure 10.4.23 shows the arrangement of the overhead type. The track
cable a is supported at intervals by towers b, which carry the saddles c in
which the track cable rests. Each tower also carries the sheave d for
supporting traction rope e. The self-dumping bucket f is suspended from
carriage g. The grip h, which attaches the carriage to traction rope e, is controlled by lever k. In the underhung type, shown in Fig. 10.4.24, track
cable a is carried above traction rope e. Saddle c on top of the tower
supports the track cable, and sheave d supports the traction cable. The
sheave is provided with a rope guard m. The lever h, with a roller on the
end, automatically attaches and detaches the grip by coming in contact
Fig. 10.4.23 Overhead-type double-rope cable tramway.
with guides at the loading and dumping points. The carriages move in
only one direction on each track. On steep downgrades, special hydraulic
speed controllers are used to fix the speed of the carriages.
The track cables are of the special locked-joint smooth-coil, or tramway,
type. Nearly all wire rope is made of plow steel, with the old
cast-steel type no longer being in use. The track cable is usually pro-
Fig. 10.4.24 Underhung-type double-rope cable tramway.
vided with a smooth outer surface of Z-shaped wires for full lock type or
with a surface with half the wires H-shaped and the rest round. Special
tramway couplers are attached in the shops with zinc or are attached in
the field by driving little wedges into the strand end after inserting the
end into the coupler. The second type of coupling is known as a dry
socket and, though convenient for field installation, is not held in as high
regard for developing full cable strength. The usual spans for level
ground are 200 to 300 ft (61 to 91 m). One end of the track cable is
anchored; the other end is counterweighted to one-quarter the breaking
strength of the rope so that the horizontal tension is a known quantity.
The traction ropes are made six-strand 7-wire or six-strand 19-wire, of
cast or plow steel on hemp core. The maximum diameter is 1 in, which
limits the length of the sections. The traction rope is endless and is
driven by a drum at one end, passing over a counterweighted sheave at
the other end.
Fig. 10.4.25 shows a loading terminal. The track cables a are anchored
at b. The carriage runs off the cable to the fixed track c, which makes a
180° bend at d. The empty buckets are loaded by chute e from the
loading bin, continue around track c, are automatically gripped to traction
cable f, and pass on the track cable a. Traction cable f passes around
Fig. 10.4.25 Cable-tramway loading terminal.
and is driven by drum g.When the carriages are permanently attached to
the traction cable, they are loaded by a moving hopper, which is automatically
picked up by the carriage and carried with it a short distance
while the bucket is being filled. Figure 10.4.26 shows a discharge terminal.
The carriage rolls off from the track cable a to the fixed track c,
being automatically ungripped. It is pushed around the 180° bend of
track c, discharging into the bin underneath and continuing on track c
Fig. 10.4.26 Cable-tramway discharge terminal.
until it is automatically gripped to traction cable f. The counterweights h
are attached to track cables a, and the counterweight k is attached to the
carriage of the traction-rope sheave m. The supporting towers are A
frames of steel or wood. At abrupt vertical angles the supports are
placed close together and steel tracks installed in place of the cable.
Spacing of towers will depend upon the capacity of the track cables and
sheaves and upon the terrain as well as the bucket spacing.
Stress In Ropes (Roebling) The deflection for track cables of tramways
is taken as one-fortieth to one-fiftieth of the span to reduce the
grade at the towers. Let S 5 span between supports, ft; h 5 deflection,
ft; P 5 gross weight of buckets and carriages, lb; Z 5 distance between
buckets, ft; W1 5 total load per ft of rope, lb; H 5 horizontal tension of
rope, lb. The formulas given for cableways then apply. When several
buckets come in the span at the same time, special treatment is required
for each span. For large capacities, the buckets are spaced close together,
the load may be assumed to be uniformly distributed, and the
live load per linear foot of span 5 P/Z. Then H 5 W1S2/8h, where W1 5
(weight of rope per ft) 1 (P/Z). When the buckets are not spaced
closely, the equilibrium curve can be plotted with known horizontal
tension and vertical reactions at points of support.
For figuring the traction rope, t0 5 tension on counterweight rope, lb;
t1 , t2 , t3 , t4 5 tensions, lb, at points shown in Fig. 10.4.27; n 5 number
of carriers in motion; a 5 angle subtended between the line connecting
the tower supports and the horizontal; W1 5 weight of each loaded
carrier, lb; W2 5 weight of each empty carrier, lb; w 5 weight of
Fig. 10.4.27 Diagram showing traction rope tensions.
traction rope, lb/ft; L 5 length of tramway of each grade a, ft; D 5
diameter of end sheave, ft; d 5 diameter of shaft of sheave, ft; f1 5
0.015 5 coefficient of friction of shaft; f2 5 0.025 5 rolling friction of
carriage wheels. Then, if the loads descend, the maximum stress on the
loaded side of the traction rope is
t2 5 t1 1 o(Lw sin a 1 1⁄2nW1 sin a)
2 f2o(Lw cos a 1 1⁄2nW1 cos a)
where t1 5 1⁄2t0[1 2 f1(d/D)]. If the load ascends, there are two cases: (1)
driving power located at the lower terminal, (2) driving power at the
upper terminal. If the line has no reverse grades, it will operate by gravity at a 10 percent incline to 10 tons/h capacity and at a 4 percent
grade for 80 tons/h. The preceding formula will determine whether it
will operate by gravity.
The power required or developed by tramways is as follows: Let V 5
velocity of traction rope, ft/min; P 5 gross weight of loaded carriage,
lb; p 5 weight of empty carriage, lb; N 5 number of carriages on one
track cable; P/50 5 friction of loaded carriage; p/50 5 friction of empty
carriage; W 5 weight of moving parts, lb; E 5 length of tramway
divided by difference in levels between terminals, ft. Then, power required
is
hp 5
NV
33,000 SP 2 p
E
6
P 1 p
50 D6 0.0000001 WV
Where power is developed by tramways, use 80 instead of 50 under
P 1 p.
BELOW-SURFACE HANDLING (EXCAVATION)
Power Shovels
Power shovels stand upon the bottom of the pit being dug and dig above
this level. Small machines are used for road grading, basement excavation,
clay mining, and trench digging; larger sizes are used in quarries,
mines, and heavy construction; and the largest are used for removing
overburden in opencut mining of coal and ore. The uses for these machines
may be divided into two groups: (1) loading, where sturdy machines
with comparatively short working ranges are used to excavate
material and load it for transportation; (2) stripping, where a machine of
very great dumping and digging reaches is used to both excavate the
material and transport it to the dump or wastepile. The full-revolving
shovel, which is the only type built at the present (having entirely displaced
the old railroad shovel), is usually composed of a crawlermounted
truck frame with a center pintle and roller track upon which the
revolving frame can rotate. The revolving frame carries the swing and
hoisting machinery and supports, by means of a socket at the lower end
and cable guys at the upper end, a boom carrying guides for the dipper
handles and machinery to thrust the dipper into the material being dug.
Figure 10.4.28 shows a full-revolving shovel. The dipper a, of cast or
plate steel, is provided with special wear-resisting teeth. It is pulled
through the material by a steel cable b wrapped on a main drum c.
Gasoline engines are used almost exclusively in the small sizes, and
diesel, diesel-electric, or electric power units, with Ward Leonard control,
in the large machines. The commonly used sizes are from 1⁄2 to
5 yd3 (0.4 to 3.8 m3) capacity, but special machines for coal-mine stripping
are built with buckets holding up to 33 yd3 (25 m3) or even more.
The very large machines are not suited for quarry or heavy rock work.
Sizes up to 5 yd3 (3.8 m3) are known as quarry machines. Stripping
shovels are crawler-mounted, with double-tread crawlers under each of
the four corners and with power means for keeping the turntable level
when traveling over uneven ground. The crowd motion consists of a
chain which, through the rack-and-pinion mechanism, forces the dipper
into the material as the dipper is hoisted and withdraws it on its downward
swing. On the larger sizes, a separate engine or motor is mounted
on the boom for crowding. A separate engine working through a pinion
and horizontal gear g swings the entire frame and machinery to bring the
Fig. 10.4.28 Revolving power shovel.
dipper into position for dumping and to return it to a new digging
position. Dumping is accomplished by releasing the hinged dipper bottom,
which drops upon the pulling of a latch. With gasoline-engine or
diesel-engine drives, there is only one prime mover, the power for all
operations being taken off by means of clutches.
Practically all power shovels are readily converted for operation as
dragline excavators, or cranes. The changes necessary are very simple in
the case of the small machines; in the case of the larger machines, the
installation of extra drums, shafts, and gears is required, in addition to
the boom and bucket change.
The telescoping boom, hydraulically operated excavator shown in Fig.
10.4.29 is a versatile machine that can be quickly converted from the
rotating-boom power shovel shown in Fig. 10.4.29a to one with a crane
boom (Fig. 10.4.29b) or backhoe shovel boom (Fig. 10.4.29c). It can dig
ditches reaching to 22 ft (6.7 m) horizontally and 9 ft 6 in (2.9 m)
below grade; it can cut slopes, rip, scrape, dig to a depth of 12 ft 6 in
(3.8 m), and load to a height of 11 ft 2 in (3.4 m). It is completely
hydraulic in all powerized functions.
Dredges
Placer dredges are used for the mining of gold, platinum, and tin from
placer deposits. The usual maximum digging depth of most existing
dredges is 65 to 70 ft (20 to 21 m), but one dredge is digging to 125 ft
(38 m). The dredge usually works with a bank above the water of 8 to
20 ft (2.4 to 6 m). Sometimes hydraulic jets are employed to break
down these banks ahead of the dredge. The excavated material is deposited
astern, and as the dredge advances, the pond in which the dredge
floats is carried along with it.
The digging element consists of a chain of closely connected buckets
passing over an idler tumbler and an upper or driving tumbler. The
chain is mounted on a structural-steel ladder which carries a series of
Fig. 10.4.29

rollers to provide a bearing track for the chain of buckets. The upper
tumbler is placed 10 to 40 ft (3 to 13 m) above the deck, depending
upon the size of the dredge. Its fore-and-aft location is about 65 percent
of the length of the ladder from the bow of the dredge. The ladder
operates through a well in the hull, which extends from the bow practically
to the upper-tumbler center. The material excavated by the buckets
is dumped by the inversion of the buckets at the upper tumbler into a
hopper, which feeds it to a revolving screen.
Placer dredges are made with buckets ranging in capacity from 2 to
20 ft3 (0.06 to 0.6 m3). The usual speed of operation is 15 to 30 buckets
per min, in the inverse order of size.
The digging reaction is taken by stern spuds, which act as pivots upon
which the dredge, while digging, is swung from side to side of the cut by
swinging lines which lead off the dredge near the bow and are anchored
ashore or pass over shore sheaves and are dead-ended near the lower
tumbler on the digging ladder. By using each spud alternately as a pivot,
the dredge is fed forward into the bank.
Elevator dredges, of which dredges are a special classification, are
used principally for the excavation of sand and gravel beds from rivers,
lakes, or ocean deposits. Since this type of dredge is not as a rule
required to cut its own flotation, the bow corners of the hull may be
made square and the digging ladder need not extend beyond the bow.
The bucket chain may be of the close-connected placer-dredge type or
of the open-connected type with one or more links between the buckets.
The dredge is more of an elevator than a digging type, and for this
reason the buckets may be flatter across the front and much lighter than
the placer-dredge bucket.
The excavated material is usually fed to one or more revolving
screens for classification and grading to the various commercial sizes of
sand and gravel. Sometimes it is delivered to sumps or settling tanks in
the hull, where the silt or mud is washed off by an overflow. Secondary
elevators raise the material to a sufficient height to spout it by gravity or
to load it by belt conveyors to the scows.
Hydraulic dredges are used most extensively in river and harbor work,
where extremely heavy digging is not encountered and spoil areas are
available within a reasonable radius of the dredge. The radius may vary
from a few hundred feet to a mile or more, and with the aid of booster
pumps in the pipeline, hydraulic dredges have pumped material through
distances in excess of 2 mi (3.2 km), at the same time elevating it more
than 100 ft (30 m). This type of dredge is also used for sand-andgravel-
plant operations and for land-reclamation work. Levees and
dams can be built with hydraulic dredges. The usual maximum digging
depth is about 50 ft (16 m). Hydraulic dredges are reclaiming copper
stamp-mill tailings from a depth of 115 ft (35 m) below the water, and a
depth of 165 ft (50 m) has been reached in a land-reclamation job.
The usual type of hydraulic dredge has a digging ladder suspended
from the bow at an angle of 45° for the maximum digging depth. This
ladder carries the suction pipe and cutter, with its driving machinery,
and the swinging-line sheaves. The cutter head may have applied to it
25 to 1,000 hp (3.7 to 746 kW). The 20-in (0.5-m) dredge, which is the
standard, general-purpose machine, has a cutter drive of about 300 hp.
The usual operating speed of the cutter is 5 to 20 r/min.
The material excavated by the cutter enters the mouth of the suction
pipe, which is located within and at the lower side of the cutter head.
The material is sucked up by a centrifugal pump, which discharges it to
the dump through a pipeline. The shore discharge pipe is usually of the
telescopic type, made of No. 10 to 3⁄10-in (3- to 7.5-mm) plates in lengths
of 16 ft (5 m) so that it can be readily handled by the shore crew.
Floating pipelines are usually made of plates from 1⁄4 to 1⁄2 in (6 to
13 mm) thick and in lengths of 40 to 100 ft (12 to 30 m), which are
floated on pontoons and connected together through rubber sleeves or,
preferably, ball joints. The floating discharge line is flexibly connected
to the hull in order to permit the dredge to swing back and forth across
the cut while working without disturbing the pipeline.
Pump efficiency is usually sacrificed to make an economical unit for
the handling of material, which may run from 2 to 25 percent of the total
volume of the mixture pumped. Most designs have generous clearances
and will permit the passage of stone which is 70 percent of the pipeline
diameter. The pump efficiencies vary widely but in general may run
from 50 to 70 percent.
Commercial dredges vary in size and discharge-pipe diameters from
12 to 30 in (0.3 to 0.8 m). Smaller or larger dredges are usually specialpurpose
machines. A number of 36-in (0.9-m) dredges are used to
maintain the channel of the Mississippi River. The power applied to
pumps varies from 100 to 3,000 hp (75 to 2,200 kW). The modern 20-in
(0.5-m) commercial dredge has about 1,350 bhp (1,007 kW) applied to
the pump.
Diesel dredges are built for direct-connected or electric drives, and
modern steam dredges have direct-turbine or turboelectric drives. The
steam turbine and the dc electric motor have the advantage that they are
capable of developing full rating at reduced speeds.
Within its scope, the hydraulic dredge can work more economically
than any other excavating machine or combination of machines.

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