Excavator(redirected from Mini-excavators)
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the principal type of machine designed to dig solid, soft rock or earth or fragmented hard rock and to load the material into hauling units, such as motor vehicles and railroad cars, or deposit it in spoil banks. In the USSR, excavators handle approximately 35 percent of the earthwork in construction and more than 80 percent of the open-pit mining excavations; the total annual volume of excavations in the USSR (1976) is 15 billion cum.
In the USSR excavators are divided into two groups according to the principle of operation: single-bucket excavators (power shovels) with discontinuous, or cyclic, action and multibucket excavators with continuous action. In the first group, all the major operations in a cycle are performed in a fixed sequence; in the second group they occur simultaneously.
Single-bucket excavators. The operating cycle of the widely used class of single-bucket excavators includes cutting or digging the ground as the bucket is simultaneously filled, moving the full bucket to the unloading point, unloading the bucket, and returning the bucket to the face. Depending on the power and the type of excavator and on the working conditions, the duration of the cycle ranges from 12 to 80 sec. The output of a single-bucket excavator with a bucket capacity of 1 cu m ranges from 100,000 to 350,000 cu m per year, or from 80 to 180 cu m/hr. Such machines are used for excavating any type of earth, including very hard and inhomogeneous earth containing large, solid inclusions. When used to work softer earth, they can be equipped with buckets of larger capacity. Hard rock and frozen earth are broken up (usually by blasting) before the excavators are used.
Single-bucket excavators are classified according to running gear as crawler-mounted, wheeled, walking, rail, floating, and combination rubber-tire-crawler types. They may be driven by a single engine or feature composite and/or individual drives for all operating equipment. The power unit may be an internal-combustion engine (diesel engines or sometimes carburetor or producer-gas engines) with electric, hydraulic, pneumatic, or combination-type motors. Single-bucket excavators may also be classified according to the type of control—manual, hydraulic, pneumatic, electrical, and composite types—and according to the type of operating equipment used—a bucket with a hinge and slide connection to the boom (forward dipper), a bucket with a hinge connection to the boom (backhoe), and a bucket with a flexible connection to the boom (dragline, clamshell, and other types). The most common type is the forward dipper (Figure 1, a); it may have any power rating and a bucket capacity up to 153 cu m (seePOWER SHOVEL). Forward dippers designed to load the excavated material into hauling units or deposit it in spoil dumps are best suited to working mineral deposits and digging foundation pits, mainly in hard rock and rocky earth as well as in solid soils of average hardness.
The backhoe (Figure l,b) is used to dig narrow trenches 0.7–1.5 m wide and up to 8 m deep, especially trenches with vertical walls (for laying sewer systems, oil pipelines, and foundation structures in hard and rocky earth) and small foundation pits.
The operating equipment of a dragline (Figure 1, c) comprises a boom and a bucket, which is suspended from the boom by means of hoisting and hauling (dragline) cables. The unit usually removes earth from a level below that on which it rests. Buckets of various capacities are used—more than 10 cu m for rocks that have been broken up beforehand. The duration of a cycle is usually 10 to 20 percent longer than that of a forward dipper. Approximately 40 percent of all excavators are operated with dragline equipment. Those that move in steps are called walking draglines.
Clamshell equipment (Figure 1, d) is used for loading operations, digging foundation pits with vertical walls, and working underwater. Plow units (Figure 1, e) are suitable for grading operations, removing pavements, and the like. Some excavator models designed mainly for construction work (with bucket capacities up to 5 cu m) are provided with various interchangeable equipment for earth removal, assembly, hoisting and conveying, loading and unloading, pile-driving and drilling operations, felling timber and uprooting stumps, and so on. They are usually called general-purpose excavators, universal excavators, or convertible shovels, because they are employed in many types of earth-moving and construction work. If the number of interchangeable equipment units is no more than three (for example, a shovel, dragline, and crane), they are known as semiuniversal excavators.
Excavators equipped with a hydraulic drive are widely used; they are suitable for use with all types of operating equipment. The equipment is hinged to a rotating platform and moved by means of hydraulic cylinders driven by high-pressure (10–40 me-gapascals) pumps. In this case hydraulic motors are usually used to transmit rotation (to turn and move). This type of single-bucket excavator is manufactured with buckets having small (up to 1.8 cu m), medium (2–3 cu m), and large (3.2–6 cu m) capacities; models with bucket capacities of 12–20 cu m are now being put into production in the USSR, primarily equipped with back-hoes. The specific power ranges from 133 to 213 kilowatts per cubic meter of bucket, depending on the operating conditions (soft or hard earth).
Single-bucket excavators may be classified according to purpose as construction, open-pit, combination construction and open-pit, and stripping excavators; there are also tunnel, floating, and other types. Construction excavators are the most common single-bucket type (approximately 60 percent worldwide); there are about 200 models produced by more than 80 domestic and foreign factories. They include general-purpose, light-duty excavators with bucket capacities of 0.05–1.8 cu m, crane lifting capacities of 0.7–45 tons, and weights of 1.5–65 tons; semiuniversal, medium-duty excavators with bucket capacities of 2–3 cu m, crane lifting capacities of 60–80 tons, and weights of 72–105 tons; and semiuniversal, heavy-duty excavators with bucket capacities of 3.25–6 cu m, crane lifting capacities of 100–150 tons, and weights of 115–210 tons. The specific weight per cubic meter of bucket capacity for working hard earth is 33–36 tons for such excavators. For the most efficient use, the excavators are equipped with buckets having a capacity of 25–60
percent greater for working soft earth and 20–25 percent smaller for very hard earth. Light-duty machines often have interchangeable crawler and rubber-tire running gear and a single-motor drive. Hydraulic excavators may have individual or composite drives.
Open-pit excavators are usually made with shovel equipment and only rarely with interchangeable dragline and crane equipment. They are notable for the high power of the working equipment, high operating forces and speeds, and a short operating cycle, which are responsible for high output (compared with other excavators of the same size) when used to load hauling units, particularly when working hard earth. Medium-duty excavators with bucket capacities of 2–4 cu m and heavy-duty excavators with capacities of 5–20 cu m and weights of 76–900 tons are designed for open-pit operations, especially with hard rock and earth that have already been blasted, and for loading large dump trucks. They are manufactured both as semiuniversal excavators and as special shovels with standard and extra-long operating eqiupment for overhead loading of hauling units located on the edge of a working face or excavation. They are equipped with crawler running gear and electric power equipment with individual drives for the operating equipment powered by an engine-driven generator. The specific power of the driving engine is 60 to 80 kilowatts per cubic meter of shovel capacity, the pressure exerted on the ground is 0.15–0.25 megapascals, and the speed is 0.6–1.5 km/hr. Hydraulic open-pit excavators have shovel capacities up to 8–9 cu m, and models with capacities of 12–20 cu m are under development.
Combination construction and open-pit excavators of medium and high power are designed mainly for the open-pit mining of nonmetallic building materials. In design they are midway between construction and open-pit excavators.
Stripping excavators are usually equipped with shovels for shifting the stripped earth from open pits to spoil dumps. Those with shovel capacities up to 15 cu m are also used to load hauling units located on the edge of the face. Bucket capacities range from 6 to 153 cu m, and the units weigh from 700 to 12,700 tons. The running gear of heavy-duty models consists of four paired crawler treads. The power equipment is the same as for open-pit excavators. The specific weight ranges from 45 to 90 tons per cubic meter of bucket capacity, and the speed is 0.3–0.5 km/hr. Besides power shovels, stripping operations may also call for the use of walking draglines with booms up to 100 m long, buckets with capacities of 5–168 cu m, and weights up to 14,000 tons. They are the principal equipment used for mining mineral deposits without haulage systems. In 1977 one of the world’s largest walking draglines was put into service in the USSR; it has a boom 100 m long and a bucket capacity of 100 cu m.
Tunnel and mine excavators are used underground in excavations having a large cross section to drive tunnels, construct the chambers of underground structures, and excavate minerals. Bucket capacities are from 0.75 to 1 cu m, and weights range from 16 to 30 tons.
Multibucket excavators. Multibucket excavators consist of operating equipment (buckets, an articulated bucket frame, and a boom), a metal superstructure to which the operating equipment is secured, drive mechanisms (power equipment), controls, a platform on which all the subassemblies and units are mounted, and running gear (the lower frame and engine). The running gear may be of the crawler (for any weight), rubber-tire (units up to 30–40 tons), rail, walking, or rail-walking types; the last two are suitable for any weight and for movement over soft earth. During operation, a multibucket excavator digs, scoops, transports, and unloads earth into an arterial conveyor or cars.
Multibucket excavators are divided into two basic categories depending on whether the operating equipment is of the chain or wheel type. In chain excavators with from 12 to 40 buckets, the buckets are attached to an endless chain that moves along rollers in guides on the bucket frame. Most models can provide an accurate final configuration for the banks of excavations and are therefore used in profiling operations (Figures 2,a and 2,b). In wheel excavators the operating member is a wheel having from six to 12 (sometimes up to 24) buckets (Figure 2,d). Earth is emptied out of the wheel buckets directly onto a conveyor located on a rotating boom or through a feeder; it is then transferred to an unloading conveyor. The total distance the earth is transported may be as much as 150 m, the digging height may reach 50 m, and the digging depth may range to 25 m. The wheel is almost completely freed from the task of transporting the earth, so that the excavation rate can be increased to 3–4.4 m/sec instead of the 0.4–1.2 m/sec of chain machines. Light-duty models with buckets having capacities ranging from 25 to 150 liters (l) can be used in open pits for excavating nonmetallic building materials; those of medium power with buckets of 200–450 l capacity are used for large construction projects; and heavy-duty models with buckets of 500–4,500 l capacity are used in large open pits. Wheel excavators are also used for the stockpiling and unloading of large volumes of materials at rates up to 12,500 cu m/hr or more. The excavator weight per liter of bucket capacity ranges from 0.9 to 1.5 tons; the average productivity ranges from 1.8 to 2.5 cu m/hr per liter of bucket capacity.
A distinction is made between multibucket excavators having buckets that move transversely with respect to the movement of the excavator, those with buckets that move longitudinally, and revolving excavators. Transverse excavators (Figure 2) are mainly chain types that move while operating in a direction perpendicular to the plane of motion of the buckets. They are able to excavate overhead and underneath, and they are mounted on rail or crawler running gear. Longitudinal excavators may be chain or wheel types that move while operating in a direction parallel to or coincident with the plane of motion of the buckets; they thus serve as trenchers. Such machines are usually of low power, with weights up to 75 tons, and are mounted on crawler or rubber-tire running gear. They are designed for digging trenches 0.35–2 m (rarely 3 m) wide with depths from 1.2–2 m (for wheel excavators) to 8 m (for chain excavators). They are also manufactured as units to be mounted on tractors, rubber-tire prime movers, and trucks. Revolving multibucket excavators are of the chain type, with a bucket frame mounted on a rotating platform that turns on a supporting circle; they usually have crawler running gear (Figure 2,c). Provision for 360° rotation increases the weight of an excavator by 30–40 percent. The bucket capacity may reach 3,600 l. Such excavators are used to work dummy roads and to excavate two buckets alternately from one horizon by an upper and a lower face.
Multibucket excavators are most productive for concentrated open-pit operations and earthwork in soft, medium, and uniformly hard earth, especially when the transverse contours of the faces are constant or vary little and slopes of the excavation are regular. Light-duty excavators with bucket capacities up to 150 l can operate with any type of hauling unit; medium-duty and heavy-duty machines operate with railroad and conveyor haulage.
In addition to stripping operations in open pits, multibucket excavators are used for land reclamation and spoil dumping. In land reclamation, chain machines are usually used to excavate transversely; they are notable for the low specific pressure exerted on the ground (approximately 3 × 104 pascals) and are designed to dig banked trenches and small channels (Figure 2,e). They can excavate to a depth of 2.5 m and dig channels up to 12 m wide. The bucket capacity ranges from 25 to 75 l, and the excavators weigh from 12 to 20 tons. The specific power is approximately one kilowatt per liter of bucket capacity, and the productivity ranges from 1.5 to 3 cu m/hr per ton of weight (the higher value applies to the small models). Machines that excavate longitudinally are occasionally used with special equipment. Stripping shovels (Figure 2,f) with wheel or chain operating equipment and a cantilever dumping conveyor are used to handle large-volume spoil-dumping operations when the overburden in open-pit mines is excavated with multibucket excavators. They pile the loose earth of the overburden onto upper spoil banks to a height of 25 m or onto lower spoil banks to a height of 50 m.
History. The idea of creating an earth-moving machine is attributed to Leonardo da Vinci, who proposed a design for a dragline excavator in the early 1500’s. The first steam dredger was built at the Izhora Factory under the direction of A. A. Béthenc-ourt in the 1810’s, and a single-bucket, nonrevolving excavator was patented in 1835 by W. S. Otis (USA). Single-bucket excavators came into general use in the late 1800’s with the expansion of railroad construction. The first electric excavators appeared in 1910, and full-revolving, crawler-mounted excavators with internal-combustion engines appeared in 1912 (with diesel engines in 1916). Full-revolving, medium-power excavators with large operating equipment, mounted on railroad chassis, were first used for open-pit mining in the 1920’s. In Russia the production of single-bucket excavators began in 1901 at the Putilov Factory (now the Leningrad Kirov Factory). Excavator production was organized in the USSR in 1931. In 1947 the Urals Heavy Machine-building Plant became the first in the world to begin series production of open-pit excavators with bucket capacities of 3–5 cu m; in 1958 the plant manufactured a walking dragline excavator with a bucket capacity of 25 cu m and a boom 100 m long. In 1965 the Novokramatorskii Machine-building Plant manufactured a power shovel with a 35-cu m bucket for working hard earth. A walking dragline with a bucket capacity of 100 cu m and a boom length of 100 m was fabricated at the Urals Heavy Machine-building Plant in 1975. In 1976 the same plant also readied production for a heavy-duty open-pit power shovel with a bucket capacity of 20 cu m, designed to work hard earth.
The manufacture of excavators abroad is concentrated in the USA (Marion Power Shovel Company, Inc., Bucyrus-Erie Company, and Harnischfeger Corporation), Great Britain, the Federal Republic of Germany, the German Democratic Republic, France, Japan, the Polish People’s Republic, the Czechoslovak Socialist Republic, and other countries. In 1968 the Bucyrus-Erie Company fabricated a walking dragline with a bucket capacity of 168 cu m for working soft earth, and the Marion Power Shovel Company manufactured a power shovel with a bucket capacity of 153 cu m.
The multibucket excavator designed for use on land was invented in France by M. Couvreux in 1860 and was used in the construction of the Suez Canal. By the end of the 1800’s multi-bucket excavators were widely used in Germany for the open-pit mining of brown coal. In Russia the production of multibucket excavators was initiated at the Putilov Factory in 1901. In 1921 excavators appeared in Germany with bucket capacities of 525 l; in 1933, with capacities of 1,200 l; in 1939, with capacities of 1,500 l; and in 1941, with capacities of 2,000 l. In 1958 a wheel excavator with a bucket capacity of 3,600 l was put into operation in the Federal Republic of Germany. In 1967 the Novokramatorskii Machine-building Plant began delivery of high-power wheel excavators with a production capacity of 3,000 cu m/hr, and in 1973, models with a production capacity of 5,000 cu m/hr.
Future development. The principal development trend in the manufacture of excavators is to increase productivity through the use of machines with greater unit power. The makeup of the excavator fleet is changing with the introduction of hydraulic types. The proportion of alloy steels used is being increased to 35–40 percent of the total excavator weight in light-duty excavator designs, 40–50 percent in medium-duty designs, and 70–80 percent in heavy-duty designs. The duration of the operating cycle is being reduced by increasing the speed of the individual operating movements. Rotary excavators are being designed for working rock with a specific excavating resistance up to 16 × 106 pascals. New trenchers with a productivity of 2,500–3,000 cu m/hr are designed to work earth with a specific excavating resistance of 0.8 × 107–1.0 × 107 pascals; they can dig trenches 2–2.5 m wide and as much as 3 m deep at a rate of 400–500 m/hr, which is 10–20 times faster and 5–6 times more economical than existing excavators of the same class are capable of. So-called “baby” excavators weighing 300–800 kg are now widely used. An automatic control system is being developed for continuous-action excavators; it regulates all the parameters that determine the progress of the operating cycle. Systems are being designed on this basis with self-optimizing algorithms for working the foot of banks and entire faces. Automatic control systems are being created to handle all the machines of a continuously operating complex—spoil dumpers, conveyors, and gantries.
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