Excavators

Mansoor Azam Qureshi

NUST

Islamabad

Excavator

An excavator has been defined as a power-driven digging machine, usually mounted on crawler tracks.

The major types of excavators used in construction include hydraulically powered excavators and the members of the crane-shovel family.

However, dozers and scrapers are also capable of excavating soil and rock.

Type of Excavators

Hydraulic power is the key to the versatility of many excavators.

Hydraulic front shovels are used predominantly for hard digging above track level and for loading haul units.

Hydraulic hoe-type excavators are used primarily to excavate below the natural surface of the ground on which the machine rests.

The loader is a versatile piece of equipment designed to excavate at or above wheel/track level. Unlike a shovel or hoe, a loader must maneuver and travel to position the. bucket to load or dump.

Additionally, there are a variety of excavators available for specialty applications.

Main components of Excavator

Main components of Excavator

Advantage of Hydraulic controls

The hydraulic control of machine components provides

– Faster cycle times.– Positive control of attachments.– Precise control of attachments.– High overall efficiency.– Smoothness and ease of operation.

Bucket capacity

Bucket payload can be measured by volume or weight

Volume is typically stated as struck measure of loose volume meaning that the material excess is scraped off flush with the top of the bucket.

Heaped at a specific angle of repose

Bucket payload An excavator’s bucket payload (actual amount of

material in the bucket on each digging cycle) is dependent on bucket size, shape, and certain soil characteristics, i.e., the fill factor for that soil. Fill factors for several types of material are listed below.

Average Bucket Payload = (Heaped Bucket Capacity) x (Bucket Fill Factor)

Excavator operating Ranges

A Maximum loading height of bucket with teeth

B Maximum reach at ground level C Maximum digging depth D Maximum vertical wall E Maximum depth of cut for level

bottom (straight clean up) F Maximum bucket hinge pin

height G Maximum height, to bucket

teeth at highest arc

Operating Ranges for different Booms and Sticks

Excavator load capacity for different boom and buckets

*Liftingcapacity @ 20 ft.t Lifting capacity @ 25 ft.

Selecting excavator

Maximum excavation depth required Maximum working radius required for digging

and dumping Maximum dumping height required Hoisting capability required (where

applicable, i.e., handling pipe and trench boxes

Excavator Production Estimate

Depend upon – Bucket heap capacity Q in LCY– Fill factor F– Cycle time t in sec– Efficiency in min/hr E– Swell factor

Production in BCY= x x

Cycle Time for typical excavator

Example

A crawler hoe having a 3~-cy bucket is being considered for use on a project to excavate very hard clay from a borrow pit. The clay will be loaded into trucks having a loading height of 9 ft 9 in. Soil-boring information indicates that below 8 ft. the material changes to an unacceptable 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?

Size of bucket, 3 1/2 cy

Bucket fill factor (Table), hard clay 80 to 90%; use average 85%

Typical cycle element times. Optimum depth of cut is 30 to 60% of maximum digging depth

8/23 = 0.34 8/27 = 0.3 within limits– Load bucket 7 sec – Swing with load 6 sec– Dump Load 4 sec– Return swing 5 sec– Cycle Time 22 sec

Efficiency factor = 50 min/hr Swell factor = 35% Production= (3600x3.5x0.85/22) + (50/60) + (1/

[1+0.35]) = 300 BCY/hr

Check maximum height for loading= 21 to 22 ft available is ok for dump truck height

Type of Buckets Attatchments

Helpful Hints Digging should be planned so that dipper teeth cut as near as

possible to the line of the digging cable. Length and depth of cut should be judged to produce a full

dipper at every pass. Full loads on every pass produce more pay dirt than a faster cycle with partly filled dipper. Full loads should be the first objective, followed by speed increases for fast cycles.

A hoe will dig fairly hard materials. Where possible, blasting will often be less expensive than bulling through hardpan and rack strata with the hoe dipper.

Using the dipper teeth as a pick axe by extending handle to maximum reach, then dropping Front end to break ledge rock is very bad practice, the result being serious Front end damage.

Once the trench is open, ledge rack can be broken by pulling dipper up under the layers. Top layers are pulled first with one or two layers lifted at a time