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Draglines and Clamshells

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1 Draglines and Clamshells
Chapter 18 Draglines and Clamshells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Drag buckets and clamshells are attachments hung from a lattice-boom crane.

A dragline excavator is especially useful when there is need for extended reach in excavating or when material must be excavated from underwater. Clamshell excavators provide the means to excavate vertically to considerable depths.

4 DRAGLINES The Monighan Co. patented a walking mechanism for draglines in These machines were used extensively in building levees on the Mississippi River.


6 DRAGLINES A dragline works, as the name implies, by dragging a dragline-type bucket toward the machine.

7 DRAGLINE BUCKET A dragline bucket consists of three parts: the basket, the arch, and the cutting edge. Buckets are generally available in three types: (1) light duty, (2) medium duty, and (3) heavy duty.

8 DRAGLINES The greatest advantage of a dragline over other machines is its long reach for digging and dumping. A dragline does not have the positive digging force of a hydraulic shovel or hoe.

9 DRAGLINES Draglines are used to excavate material and load it into hauling units or to deposit it in levees and spoil piles near the pits from which it is excavated.

10 DRAGLINES Dragline components consist of a drag bucket and a fairlead assembly. Wire ropes are used for the boom suspension, drag, bucket hoist, and dump lines.

11 DRAGLINE PRODUCTION The output of a dragline should be expressed in bank measure cubic yards (bcy) per hour. This quantity is best obtained from field measurements. Table 18.2 gives approximate dragline digging and loading cycles for various angles of swing.

12 OPTIMUM DEPTH OF CUT A dragline will produce its greatest output if the job is planned to permit excavation at the optimum depth of cut. Based on using short-boom draglines, Table 18.3 gives the optimum depth of cut for various sizes of buckets and classes of materials.

Table 18.3 presents ideal dragline production capability based on digging at optimum depths with a swing angle of 90°. The table also assumes maximum efficiency, for example, a 60-min hour.

For any other depth or swing angle, the ideal output of the machine must be adjusted by an appropriate depth-swing factor. The effect of the depth of cut and swing angle on dragline production is given in Table 18.4.

15 EXAMPLE A 2½-cy short-boom dragline is to be used to excavate wet, sticky clay. The depth of cut will be 14.8 ft, and the swing angle will be 60°. Determine the probable production of the dragline. There are 65,000 bcy of material to be excavated. How long will the project require?

16 Step 1. Ideal Production Determine the ideal production from Table 18.3: Based on a 2½-cy bucket size and wet, sticky clay material: Ideal production = 175 bcy.

17 Step 2. Optimum Depth of Cut
Determine the percent of optimum depth of cut, Eq Optimum depth of cut Table 18.3: ft.

18 Step 2. Optimum Depth of Cut
Table 18.3: ft. Percentage of optimum depth of cut Eq. 18.1 = 120%

19 Step 3. Cut/Swing Angle Correction Factor
Step 3. Determine the depth of cut/swing angle correction factor from Table 18.4: Percentage of opt. depth = 120% Swing angle = 60 Depth of cut/swing angle correction factor = 1.09

20 Step 4. Efficiency Factor
Determine an overall efficiency factor based on the expected job conditions. Draglines seldom work at better than a 45-min hr: Efficiency factor =

21 Step 5. Production 175  1.09  0.75 = 143 bcy/hr
Determine production rate. Multiply the ideal production by the depth/swing correction factor and the efficiency factor: 175  1.09  0.75 = 143 bcy/hr

22 Step 6. Production Determine soil conversion, if needed (Table 4.3). Not necessary in this example.

23 Step 7. Time Determine total hours, Eq : Total hr = hr

24 CLAMSHELLS The clamshell is a vertically operated bucket capable of working at, above, and below ground level.

25 CLAMSHELLS Clamshells are used primarily to remove materials from vertical excavations such as cofferdams, pier foundations, and sheet-lined trenches.

26 CLAMSHELLS There are hydraulic clamshell buckets that are mounted on the stick of hydraulic hoes.

27 CLAMSHELL BUCKETS The two jaws of a clamshell bucket clamp together when the bucket is lifted by the closing line.

28 CLAMSHELL BUCKETS Clamshell used to cleanout 8 ft diameter steel piles.

Because of the variable factors that affect the operations of a clamshell, it is difficult to give average production rates. The critical variable factors include the difficulty of loading the bucket, the size load obtainable, the height of lift, the angle of swing, and the method of disposing of the load.

30 SAFETY Keep personnel away from the swing area of dragline and clamshell cranes. These machines must be operated so as not to expose persons in the area to a hazard. The crane operator must not swing the boom and bucket, whether loaded or empty, over the top of trucks and drivers.

31 SAFETY Another common accident, with cranes equipped as draglines or with clamshell buckets, is where a machine falls into the excavation it is digging. These accidents often happen when ground conditions deteriorate after a rain. Management must always be cognizant of changing ground conditions.

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