Excavator Criteria

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54 Maximum number of cycles per hour, 60 min -;- 0.883-min cycle - 68 Maximum volume per hour (68 cycle x 33 cu ft) I 27 = 83 lcy If the unit operates at a 45-min-hour efficiency, the probable production will be 83 lcy ' 4 5 62 lcy I hr 60 (3. IE) HYDRAULIC EXCAVATORS General Information Machines which make use of hydraulic pressure to develop bucket penetration into mate1i.als are classified by the digging motion of the bucket. l11e hydraulically controlled boom and stick, to which the bucket is attached, may be mounted on either a crawler or a \Vhc:el tractor base. A downward arc unit is classified as a "hoe." It develops excavation breakout force by pulling the bucket toward the machine and curling the bucket inward. An upward motion unit is known as a "front shovel. 11 A shovel develops break out force by crowding mate1ial mvay from the machine. The doVv'Jlward swing of a hoe dictates usage for excavating below the nmning. gear. l11e boom of a shovel swings upward to load; therefore, the machine requires a material face above the ru1111mg gear to work against. Front Shovels Front shovels are used predominately for hard digging above track level, and loading haul units. The loading of shot rock would be a typical application. A shovel is capable of developing a high breakout force. The material being excavated should be such that it will stand with a faiTly ve1tical face. Crawler-mounted shovels have very slow travel speeds, less than 3 mph. Figure 8-1 illustrates a crawler-mounted shovel. :··i- , .. ''·'. FIGURE 3.11 A hydraulic-operated front shovel loading material into a truck. (Caterpillar Inc.)

description

Plant Management

Transcript of Excavator Criteria

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54

Maximum number of cycles per hour, 60 min -;- 0.883-min cycle - 68

Maximum volume per hour (68 cycle x 33 cu ft) I 27 = 83 lcy

If the unit operates at a 45-min-hour efficiency, the probable production will be

83 lcy ' 45 62 lcy I hr 60

(3. IE) HYDRAULIC EXCAVATORS

General Information

Machines which make use of hydraulic pressure to develop bucket penetration into

mate1i.als are classified by the digging motion of the bucket. l11e hydraulically controlled boom

and stick, to which the bucket is attached, may be mounted on either a crawler or a \Vhc:el

tractor base. A downward arc unit is classified as a "hoe." It develops excavation breakout force

by pulling the bucket toward the machine and curling the bucket inward. An upward motion

unit is known as a "front shovel. 11 A shovel develops break out force by crowding mate1ial mvay

from the machine. The doVv'Jlward swing of a hoe dictates usage for excavating below the nmning.

gear. l11e boom of a shovel swings upward to load; therefore, the machine requires a material

face above the ru1111mg gear to work against.

Front Shovels

Front shovels are used predominately for hard digging above track level, and loading

haul units. The loading of shot rock would be a typical application. A shovel is capable of

developing a high breakout force. The material being excavated should be such that it will stand

with a faiTly ve1tical face. Crawler-mounted shovels have very slow travel speeds, less than 3

mph. Figure 8-1 illustrates a crawler-mounted shovel.

:··i- , .. -~·~·?·!·',~·~·:> ''·'.

FIGURE 3.11 A hydraulic-operated front shovel loading material into a truck. (Caterpillar Inc.)

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(3.13) Size of a Front Shovel

The size of a shovel is indicated by the size of the bucket. e\pressed in cubic yards

1l1ere are three different bucket rating standards, PCSA Standard No. 3. Society of Auto1110-

tive Engineers (SAE) Standard J-296, and the Committee on European Construction Equipment

(CECE) method. All the methods are based only on the physical dime11sions of the bucket and

do not address tbe "bucket loading motion" of a specific machine. For buckets greater tll;111 a

3-cu-yd capacity. ratings are on 1/4 cu-yd inte1vals and they are on 1/8 cu-yd intervals for btH.:kL~ts

less than 3 cu yd in size.

Strnck capacity. The volume actually enclosed by the bucket Vvith no allowance for bucket kcth

is the stmck capacity.

Heaped capacity. Both PCSA and SAE use a I: I angle of repose for evaluating heaped capiil·it\

CECE specifies a 2: l angle of repose.

Fill factors. Rated heaped capacities represent a net section bucket volume: therefore. they m11s1

be conec,ted to average bucket payload based on the charac,te1istics of the mate1ial being handled.

Manufacturers usually suggest factors, commonly named "fill factors," for making such correc­

tions. Fill factors are percentages which, when multiplied by a rated heaped capacity. adjust tlie

volume by accounting for how the specific material will load into the bucket

lt is always best to conduct field tests based on the weight of rnate1ial per bucket lucid

to validate fill factors.

TABLE 3.7

Fill factors for front shovel buckets

Material

B<lllk clay; eanh

Rock-earth mixture

Rock-poorly blasted

Rock-well blasted

Shale. s8ndstone-standing bank

Fill factor (%)

100-110

105-115

85-100

100-110

85-100

3.14) BASIC PARTS AND OPERATION OF A FRONT SHOVEL

The basic pa11s of a front shovel include the mounting, cab, boom, stick. and bucket.

With a shovel in the con-ect position, near the face of the mate1ial to be excavated, the

bucket is lowered to the floor of the pit, with the teeth pointing into the face. A crowding force

is applied by hydraulic pressure to the stick cylinder at the same time the bucket cylinder rotall's

the bucket through the face. lf the height of the face is just right, conside1ing the type of material

and the size of the bucket, the bucket will be filled when it reaches the top of the face If

the height of the face, refoned to as the "height of cut", is too shallow. it will not be possible

to fill the bucket completely without making a second pass at the face If the height of the face

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is grea1er than required to fill the bucket, it will be 11ecessa1y to reduce the depth of' penet 1 at in11

of the bucket into the fuce if the full fuce is to be excavated in one pass or to stai1 the excavation

above the floor of the pit. Material left near the floor of the pit will be excavated after the

upper po1tion of the face is removed.

FIGURE 3.12

Basic parts of a hydraulic front shovel

:ii.15) Selecting a Front Shovel

ln selecting a shovel, the user should consider the probable co11centnitio11 of Hork 10

be perfo1med. Two fundamental factors which should be taken into account are the cost per

cubic yard of mate1ial excavated and the job conditions under which the shovel will operate

In estimating the cost per cubic yard, one should consider the following factors

I. The size of the job. as a larger job may justify the hjgher cost of a lart>:e shm el

2. The cost of transpo1ting the machine (a large shovel will involve more cost than a s111all

one).

3. The combined cost of drilling, blasting, and excavating. For a large shovel these co-;ts

may be less than for a small shovel as a large machine will handle more massive rocks

than a smalJ one. Large shovels may permit savings in d1illing and blasting costs.

4 The direct w1it cost per cubic yard for the excavation (less for a large shovel than J()r

a small one).

The following job conditions should be considered in selecting the size of a shovel:

I. If the mate1ial to be excavated is hard and tough, the bucket of the large shovel.

which exe1ts higher digging pressures, will handle the material more easily

2 If blasted rock is to be excavated, the large-size bucket will handle bigger indi\ idual

pieces.

3. lf the time allotted for the completion of a project requires a high hourly production.

either multiple small shovels or a large shovel must be used.

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4. ·n1e size of available hauling w1its should be considered in select in,?- the size of' a slim d.

If small hauling units must be used, the size of the shovel should be small_ whereas if

large hauling units are available, a large shovel should be used.

Manufacrw-ers' specifications should always be consulted for the exact values or nrncliine

clearances. The maximum bucket dumping height is especially irnponant when the shovel is loading

haul units.

· f3 .16) Shovel Production

There are four elements in the production cycle of a shovel:

I. Load bucket

2. Swing with load

3. Dump load

4. Return swing

lt should be noted that a shovel does not travel during the digging and loading cycle

Travel is li.rnjted to moving into or along the face as the excavation prngresses

Typical cycle element times under average conditions, for 3-to "- cu yd SI/.e sli1)\ el-;.

will be:

I.

2.

Load bucket

Svving with load

Dump load

7-9 sec

4-6 sec

2-4 sec

4. Return swing 4-5 sec

The actual production of a shovel is affected by numerous factors, including the.

1. Class of material

2. Height of cut

3. Angle of swing

4. Size of hauJjng units

5. Operator skill

6. Physical condition of the shovel

·111e production of a shovel should be expressed in cubic yards per hour based on ha11k­

measure volume (bey per hour). 'll1e capacity of a bucket is based 011 its heaped volu111t: 111

order to obtain the bank-measure volume of a bucket of material, the average loose \ olu111e

should be divided by I plus the swell, expressed as a fraction. For example, if a 2-cu-yd bucket,

excavating mate1ial whose swell is 25%, vvill handle an average loose volume of 2.25 cu yd,

the bank-measure volume will be 2.25/I .25 = 1.8 cu yd. If this shovel can make 2.5 cvl'les

per min, which includes no allowance for lost time, the output will be 2.5 , 1.8 = 4.5 bey

per min, or 270 bey per hour. TI1is is an ideal production, which will seldom_ if e\'er. be

ex'}Je1ienced on a project. Ideal production is based on digging at optimum height with a LJ0°

swing and no delays.

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The actual production of a shovel vviJI be significantly lower than the ideal because

operators do not operate continuously at peak efficiency, nor do they operate a full ()0 minutes

every hour. Machines require minor repairs, haul units tend to bunch together and other 111inor

delays always affect production.

Ideal production assumes no intenuption of the loading cycle. Transp01tation Research

Board (TRB) studies have sh0Vv11 that actual production times for slim els used in higll\\ ay

construction excavation operations are 50-75% of the available \>..orki11g tillle Tlie1efo1 e. p1 o­

duction efficiency is only 30-45 min per hour. The factors causing tlii:-. loss of productio11 ;i1 e

I. Shon moves to position for digging

2. Handling oversize rnate1ial

3. Cleanup of the loading area

4. Haul-unit exchange

5. Lack of haul unit to load

6.. Operator breaks

In the field the machine vvill require time to move into the digging. position as the !i1ce

is advanced. One study found this type of motion to be required after about 20 bucket lnads.

and on the average it required 36 sec. When handling shot rock. carefltlly evaluate the amount

of oversize mate1ial to be moved .. A machine vvith a bucket whose bite width and pocket arc

satisfactory for the average size pieces may spend too much time handling individual O\ersize

pieces A larger bucket_ or a larger machine, or changing the blasting pattern should be con­

sidered when there is a large percentage of oversize material

The use of auxiliary equipment in the loading area, such as a dozer. can reduce clea11up

delays. Control of haul units and operator break are within the comrol of field management.

0 17) The Effect of the Height of Cut on Shovel Production

If the height of the face from which a shovel is excavating. lll<lle1ial is too shallu\\. it

will be difficult or impossible to fill the bucket in one pass up the foce. rhe operator will l1ave

a choice of making more than one pass to fill the bucket, which will increase the time per cycle.

or with each cycle be may cany a pa11ly filled bucket to the hauling unit. In either case the

effect will be to reduce the production of the shovel.

If the height of the face is greater than the minimum required tu Jill the bucket with

favorable crowding and hoisting forces, the operator may do one of three things. He may reduce

the depth of the bucket penetration into the face in order to fill the bucket in one full stroke

ll1is vviJl increase the time for a cycle. He may start digging above the base of the face_ and

then remove the lower po1tion of the face later. Or he may mn the bucket up the full height

of the face and let the excess eaith spill down to the bottom of the face, to be picked up

later. ll1e choice of any one of the procedures will result in lost time, based on the time rcqttiiccl

to fill the bucket when it is digging at optimum height.

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·n1e PCSA bas published findings on the optimum height of cut based on data from studies

of cable-operated shovels (see Table 3-8). In the table the percent of optimum height of' cut

is obtained by dividing the actual height of cut by the optimum height for the given mate1ial and

bucket then multiplying the result by !00. Thus, if the actual height of cut is 6 ft and the opti1nurn

height is 10 ft. the percent of optimum height of cut is ( 6/10) · I 00 --, 60%.

TABLE 3-8

Factors for height of cut and angle of swing effett on shovel produrtion

Percent of

optimum depth 45 60

Angle of swing

(degrees)

75 90 I 20 150 180

40 0 CJ3 0.89 0.85 0.80 0 72 0.65 0 5q

60 1.10 1.03 0.96 0.91 0.81 0.73 0.66 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

80 1.22 1.12 1.04 0.98 0.86 077 06lJ

100 1.26 1.16 1.07 l.00 088 07CJ 071

120

l.+0

160

I 20

I 12

l.03

l 11

10-i

1.03

0 97

0 96 0.90

0 q7 () 8(l 0 77 () 7()

0.91 081 073

0.85 0.75 0 67

Much of the PCSA work concerned smaller size machines. less than 3 cu vd. In 11ll1st

cases those small shovels have been replaced by other t)1)eS of e....:ca\ at ors. track or rubber

tire loaders Hut some general guidelines can still be g.kaned from the siudies.

The optimum height of cut ranges from 30 to 50~o of the ma\'.irnum digging height. \\ ith

the lower percentage being representative of easy to load mate1ials. such as loam. sand, or grm el.

Hard to load materials, sticky clay or blasted rock, necessitate a greater optimum height, iu the

range of 50% of the maximum digging height value. Common ea1th would require-slightly less

than 40% of the maximum digging height.

( 3 L 18) The Effect of the Angle of Swing on Shovel Production

·rue angle of swing of a shovel is the horizontal angle, expressed in degrees, between

the position of the bucket when it is excavating and the position when it is discharging the load.

ll1e total time in a cycle includes digging, swing to the dumping position. dumping, and returning

to the digging position. lf the angle of swing is increased, the time for a cycle will be increased.

whereas if the angle of swing is decreased, the time for cycle will be decreased. The ideal

production of a shovel is based on operating at a 90° swing and the optimum height of cut.

The efiect of the angle of swing on the production of a shovel is illustrated in Table 3.8. The

ideal production should be multiplied by the proper conversion factor in order to conect the

production for any given height and swing angle. For example, if a shovel which is digging at

optimum depth has the angle of swing reduced from 90° to 60°, the production will be increased

by 16%.

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Ex:11nple 3.5 : A 5-cu yd shovel having a maximum digging height of 34 ft is being u~ed to

load poorly blasted rock. The face being worked is 12 ft high and the haul units can be

positioned so that the swing angle is only 60°. What is the adjusted ideal production if the ideal

cycle time is 2 I sec.

Optimum height for this machine and material (poorly blasted rock):

0.50 x 34 ft (max. height)= 17 ft

Fill factor Table 3.7: 85-100%, use 90%

Ideal production per 60 min hour.

(60 min hr + (21 sec/60 sec-min) x 5 cu yd x 0.9

Percent optimum height, 12 ft = 0.71 17 ft

CoITecting for height and swing, from Table 3. 8, by interpolating, 1. 08

The adjusted ideal production of the shovel is

771 x l .08 = 833 Icy per 60-min lu

77 l Icy/hr

Although the infonnation given in Tables 3. 7 and 3.8 is based on extensive field studies.

the reader is cautioned against using it too literally without adjusting for conditions which will

probably exist on a pa11icular project.

( 3. N) Production Efficiency Factor

As eve1y constructor knows, no two projects are alike. ll1ere are ceitain conditions at

every job over which the constrnctor has no control. These conditions must be considered 111

estimating the probable production of any piece of equipment including shovels.

A shovel may operate in a large, open quany situation with a fom well-drained floor.

where trucks can be spotted on either side of the machine to eliminate lost time waiting for

haul units. The tenain of the natural ground may be unifonnly level so that the height of cut

will always be close to optimum. The haul road is not affected by climatic conditions, such as

rains. These would be excellent workillg conditions.

Another shovel may be used to excavate mate1ial for a high way cut through a hill. TI1e

height of cut may vaiy from zero to considerably more than the optimum height. The sides of

the cut must be carefully sloped. The cut may be so nanow that a loaded trnck must be moved

out before an empty truck can be backed into the loading position. As the truck must be spotted

behind the shoveL the angle off swing will approximate 180°. The floor of the cut may be muddy,

which will delay the movement of the trucks. On a project of this type the shovel will have

a severely reduced productivity.

Besides project-specific factors, there are constructor or management factors which \\ill

suppress or enhance production efficiency. The advance planning and foresight of the construc­

tor in controlling the work and orgacizing the job will affect production. Factors that should

always be considered and addressed are :

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I. Maintenance of equipment

2. Availability of repair paits

3. Project housekeeping

4. Haul-road condition

5. Loading area layout

6. Haul-unit sizing and number

7. Competency of field management

The estimator must consider all these factors and decide upon an efficiency factor with

which to adjust peak production. To select the precisely coITect efficiency factor eve1y time is

very difficult. Expe1ience and good judgment are essential to selecting the approp1iate efficiency

factor. Sometimes it will be found that actual job production is below the estimated production:

in such cases the selected factor should be reconsidered to guide titture estimating, but the.: job

conditions should also be carefully examined.

Example 3.6 : A 3 cu yd shoveL having a maximum digging height of 30 ft, will be used on

a highway project to excavate well-blasted rock. The average face height is expected 10 he.:

22 ft. Most of the cut will require a 140° swing of the shovel in order 10 load the haul units

Detennine the estimated production in cubic yards bank measure.

Fill factor, From table 3. 7, well-blasted rock 100-110%, use 100%

Load 9 sec (because of mate1ial)

Swing loaded 4 sec

Dump 4 sec (into haul units)

Swing empty 4 sec

21 sec or ,35 lTI1ll

Ideal production:

(60 min-hour/0.35 min) x 3 cu yd x l 0 = 5 I 4 lcy/hr

Optimum height, 50% of max: 0.5 x 30 ft = 15 ft

Percent optimum height 22 ft = 14 7% 15 ft

Height ClUd swing factor:

Adjusted production:

Percent swell, Table 1. l:

from table 3.8, for 147% and 140°, by interpolation, 0.73

514 Icy x 0. 73 = 375 Icy/hr

Well-blasted rock = 60%

375 Icy/hr 7 16 = 234 bey/hr

lf the TRB information is used, the efficiency would be 30-45 working minutes per hour

234 bey/Jn ' 45 = 175 bey/hr - 60

The best estimating method is to develop specific histo1ical data by type of machine and

project factors. But infonnation such as the TRB study, which presents data from thousands of

shovel cycles, provides a good bench mark for selecting an efficiency factor.

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FIGURE 3.13 Crawler-mounted hydraulic hoe loading a truck.

FIGURE 3.14 Crawler-mounted hydraulic hoe handling 72-in reinforced concrete pipe

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(3.?0)Hoes

Hoes are used p1inmily to excavate below the irntural sudace of the grouud 011 \\hicl1

the machine rests. A hoe is sometimes refened to by other names. such as backhoe or hack

shovel. Hoes are adapted to excavating trenches and pits for basements, and the smaller machines

can handle general grading work. Because of their positive control, they are supe1ior to dra­

glines in operating on close--range work and loading into haul units (see Fig 3.13). In stonn drain

and utility work the hoe can pe1fonn the trench excavation and handle the pipe, eliminating. a

second machine.

The Basic Parts and Operation of a Hoe

The basic pai1s and operating ranges of a hoe are illustrated in Fig 3. 15.

Table 3. 9 gives representative dimensions and clearances for hydraulic crawler mounted

hoes. Buckets are available in va1ying widths to suit the job requirements.

Wheel-mounted hydraulic hoes are available with buckets up to I Y2 cu yd. The maxi­

mum digging depth for the larger machines is about 25 ft. With all four out1iggers dow11, the

large machines can handle 10,000 lb loads at a 20 ft radius. These are not production exca­

vation machines. They are designed for mobility and general puq)Ose work.

The penetration force into the material being excavated is achieved by the stick cylinder

and the bucket cylinder. ll1e maximum crowd force is developed when the ~1ick cylinder operates

perpendicular to the stick (see Fig 3.16). As in the case of the dragline (see Fig 3.7), the best

digging is therefore at the bottom of the arc because the geomet1y of the boom, stick, bucket,

and the Liydraulic cylinders at that point exe11s the maximum force drawing the stick in and cmling

the bucket.

TABLE 3.9 Representative dimensions, loading clearance, and lifting capacity for hydraulic crawler hoes

I I

Sit<· :--1irh '

buC~\'I lt'll;'.ill

(CU ~di I lf!J i

; I )- I

'

I 11 I/ -

I ~ - I '

I ~ f,.. I·'

2 - .. J.j

~~ 7-111

3 10-1 I

>! S ·I 2

4 I II

' ' ,,

Liftin~ 1..ip;u .. ·11~ ·· 2r· 1:

'Lif11n~t..Jp.11.11;. 1 1 ~.;'.1

I \laximum ! reach \la\imum i

'II ground digging I lnel depth I (fl I (ft)

19-22 12-15

24··27 U>-1 S

26-33 16-2.<

27-35 17-21

29-38 J~-27

32-40 20-29

38-42 25-30

.\6- .«J 23-·27

44 29 -111--1(, ~h- ~2

Lifting capacity (o 15 fl ·----·

\la\imum Shor! Mick Long stirk loading height Fron I Side Front Side

(ft) (lb) (lb) (lb) (lh)

14-16 2.900 2,600 2.900 2,600

17-19 7. JOO 5.:lOO 7.200 .~. \()()

17-25 12.800 9.000 9.300 'J.:'00

18-23 17.100 10.100 17.700 I I. I 00

19-24 21.400 14.500 21.600 14.200

20-26 32,600 21,400 31.500 24.400

24-26 32.900' 24.600' 30.700' 26.200

21-22 :n.200· 21.900' 32.400' 22.000'

27 47.900' 33.500'

:5-2() 34.100 27.)(){I' .11.Wll 27.li(J()

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FIGURE 3.15

Basic parts and operating ranges of a hydraulic hoe. A, dumping height B, diggrng reach~ C, max. digging

depth.

1.i z·1.;_i.:r

I l I I:'\ I Ji' I~

.. ~

FIGURE 3.16

Arrangement of a hoe's hydraulic cylinders to develop digging forces.

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(3 . .27)Bucket Rating for Hydraulic Hoes

Hoe buckets are rated like shovel buckets by PCSA ru1d SAE standards using 1: 1 angle

of repose for evaluating heaped capacity. Buckets should be selected based on the material being

excavated. The hoe CaJ1 develop high penetration forces. By matching bucket width and bucket

tip radius to .the resistance of the material, one can take full advantage of the hoe's potential.

For easily excavated mate1ials wide buckets should be utilized. When excavating rocky mate1ial

or blasted rock, a nanow bucket with a short tip radius is best. In utility work the width of

the required trench may be the critical consideration. Fill factors hydraulic hoe buckets are

presented in Table 3.10.

Hoe Operating Efficiency and Production

The same elements which affect shovel production are applicable to hoe excavation

operations. 111e optimum depth of cut will depend on the type of material being excavated and

bucket selection. As a rule, the optimum depth of cut for a hoe is usually in the range of 30-

60% of the machine's maximum digging depth. Table 3.11 presents cycle times for hydraulic track

hoes based on bucket size and average conditions.

Example 3.7 : A 311 cu yd crawler hoe is being considered for use on a project to excavate

very hard clay from a bonow pit. 111e clay will be loaded into trucks having a loading height

of 9 ft 9 in. Soil-b01ing information indicates that below 8 ft the mate1ial changes to an un­

acceptable silt material. What is the estimated production of the hoe in cubic yards bank measure,

if the efficiency factor is equal to a 50 min hour ?

Fill factor: From table 3. l 0, hard clay 80-90%, use 80%

Cycle time: From table 3.11, 3 1/2 cu yd machine, 22 sec or 0.37 mm

Ideal production:

(60 min-hr/0.37 mm cycle) x 3Y2 cu yd x 0.8 454 Icy/hr

From table I. I.

Percent swell dry clay = 0.35

Adjusted production:

454 cy/hr x 50 nun hr

1. 3 5 60 mm hr = 280 bey/hr

Check : optimum depth, 30-60% maximum digging depth from Table 3. 9, 23-27 ft

8/23 34% > 30% okay

8/27 30% 30% okay

Maximum loading height, from Table 3.9, 21-22 ft

9 ft 9 ~ < 21 okay

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Table 3.10

Fill factors for hydraulic hoe buckets

Mate1ial

Moist loam/sandy clay

Sand and gravel

Rod\.-poorly blasted

Rock-well blasted

Hard. tough clay

Table 3.11

Fill factor (%)

100-110

95-110

40-50

60-75

80-90

Excavation cycle times for hydraulic crawler hoes under average conditions

Bucket Load Swing Dump Swing Total size bucket loaded bucket empty cycle

(cu yd) (sec) (sec) (sec) (sec) (sec)

< I 5 4 2 ,.,

14 .)

J · J I 12 6 4 2 ,.,

15 .)

2 21 2 6 4 ,.,

4 17 .)

~ 7 5 4 4 20 .)

31:: 7 6 4 5 22

4 7 6 4 5 22 .:; 7 7 4 6 24

66

Hoe cycle times are usually of greater duration than those for shovels, because, alter

making the cut, the hoe bucket must be raised above the g:roWld level in order to load a haul

unit or to get above a spoil pile. lf in Example 3.7 the trncks can be spotted 011 the floor of

the pit the bucket will be above the truck when the cut is completed. Then it would not be

necessary to raise the bucket any higher before swinging and dumping. 1l1e spotting of haul units

below the level of the hoe will increase production.

Factors to be carefully considered in selecting a hoe and planning its utilization are:

I Maximum digging depth required

2. Minimum digging depth available

3. Working radius for digging and dumping

4. Dumping height required (trench operations)

5. Digging with required (trench operations)

6. Clearance for canier, superstrncture, and boom

7. Hoisting capability

In trenching operations the prirruuy concern is to match the hoe's ability to excavate linear

feet of trench per unit of time with the pipe laying production. Usually, the volume of mate1ial

moved is not the question.

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( '.::/ 2" \Gradalls ../. (. J

n1e gradall is a utility machine which combines the operating features or the hoe. dra­

gline, and motor grader. TI1e full revolving superstructure of the unit can be mounted 011 either

crawler tracks or wheels. The unit is designed as a versatile machine for both excavation and

cation.

FIGURE 3.17

Grad;:dl shaping the side slopes of a ditch.

The bucket of a gradall can be rotated (that is, the gradall's arm can rotate) 90° or

more, allowing it to be effective in reaching restricted working areas and where special shaping

of slopes is required. Note its use in Fig 3.17. The three-pait telescoping boom can be hy­

draulically extended or retracted to vary digging or shaping reach. It can exert breakout force

both above and below ground level

When used in a hope application to excavate below the mnning gear, its production rate

\Viii be less than a hoe equipped with an equal size bucket. Similarly, it can perfonn dragline­

type tasks, but it does have limited reach compared to a dragline. Because the macillne pro- ·

vides the operator with positive hydraulic control of the bucket it can be used as a finishing

tool frn fine-grading slopes and confined areas, tasks which would normally be motor grader

work if there were no space constraints.

3.?3) Loaders

General Information

Loaders are used e\.1:ensively in construction work to handle and transport bulk rnate1ial

such as ea1th and rock, to load trucks, to excavate earth, and to charge aggregate bins at asphalt

and concrete plants.

Page 15: Excavator Criteria

FIGl;RE 3.18

Tr::ick-tvpe loader

FIGURE 3.19 Wheel-tractor loader

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