Construction World - Indian Edition | February 2008

Special report - Equipment

Electrifying Excavators

Engineer JAGVIR GOYAL tells us about the amazing versatility of the excavators available today.

The first activity in the construction of any new structure is excavation for its foundations. Normally, this occurs with much fanfare. However, the real tempo is built and the pace is set only if excavation work moves at a fast speed. The movement of machines at the site is an electrifying scene and work finds its rhythm. In contrast, if excavation work moves at a slow speed, the spirit of producing a new structure is lost and the project dawdles.

Thus, excavation work is an activity that decides the future pace of construction of any structure.

Excavation basics
Excavation work needs to be planned well by drawing an excavation plan of the area. All precautions are to be taken against caving of sides by maintaining proper slope in them or providing shoring and supports wherever required. Vertical cuts in excavation should be avoided at any cost as these prove risky and costly ultimately. All excavated earth is always stacked away from the cutting edge to avoid its falling over the workers. A major percentage of fatal accidents at construction sites occur during excavation activity owing to caving in of the sides. The Jasola Delhi accident is a recent example.

Excavation equipment
Highly versatile excavators being produced today give unbelievable output and can handle any kind of job. These can be operated mechanically, hydraulically or by cable. Further, they can be track or tyre-mounted. Track-mounted excavators are preferred as they can work in waterlogged areas or areas of low bearing capacity. They can also move on steeper slopes than tyre-mounted excavators. However, the speed of tyre-mounted excavators is higher. Therefore, the choice is made according to site requirements. Excavation equipment includes backhoes, shovels, drag-lines, clam shells and hydraulic excavators.

In backhoes, buckets move towards the operating machine, while in shovels, the bucket moves away from the body of the machine. Hoes excavate earth at levels below their own position while shovels excavate earth above their positioning level. Backhoe loaders have an arm and bucket on the back and a loader on the front. Draglines have buckets suspended from the booms by means of wire ropes run from the cabin by powerful motors. There are separate hoist ropes and drag ropes in a drag line. The drag rope drags the bucket on the area to be excavated and also keeps it balanced. Draglines are suitable for excavation below their own level and have their own limitations as hoist and drag ropes can't be extended too long. These are more suitable for mining work. Clamshells have their buckets in two halves with teeth on each half, opened and closed hydraulically and supported by another hydraulic circuit to drag the bucket.

Hydraulic excavators
Hydraulically operated excavators have taken over the excavation scene now. These are able to rotate in any direction, work at a fast pace and are highly reliable. The best part of this equipment is the forearm-like movement of the bucket with teeth. It can be set to any angle to rip off a large chunk of soil from the earth's surface.

The basic components of a hydraulic excavator are the track or tyre undercarriage, cabin built over a pivot, hydraulic system, boom, ram and bucket. The sharp teeth of the bucket cut through the soil and the bucket scoops it up. A hydraulic jack moves the arm. Two levers provided on the right side of the operator help him to virtually control all the movements of the excavator. These levers require no extra effort and can be operated with ease. The upper body of a hydraulic excavator can swivel through 360° by rotating on the slewing ring. Though the excavator has been designed by taking a cue from man's movements, it can swivel more than a man's torso.

Basically, a diesel engine at the back of the excavator provides all the power to drive the hydraulic equipment. Further, a hydraulic motor at the front drives the machine forward. All hydraulic power is produced by exerting great pressure on hydraulic oil, which further drives the motors, pushes the pistons of jacks, and so on. Thus, the oil-carrying pipes and their joints have to be very strong.

Essential features
While buying an excavator, parameters that need to be studied include the capacity of the bucket of the excavator; the power of the excavator; cycle time and hourly output; reach; maintenance requirements; POL consumption; cost; and travelling speed.
Excavators being produced today have a bucket capacity as high as 5 cu m. The higher the bucket capacity of an excavator, the more its weight. A 5-cu-m bucket capacity excavator weighs around 90 tonne. A useful feature of excavators is that their track width is adjustable according to site requirements. Almost all excavators are now equipped with a dozer blade as pushing the excavated earth and mounting it for collection and dumping into dumpers are crucial to achieve optimum efficiency.

An excavator can achieve the rated output only if it is backed by earth-carrying dumpers with earth-disposal capacity matching the excavation capacity of the excavator. A dumper always has to be ready and in position to receive excavated earth when the boom of the excavator swings back with a filled bucket for unloading. Thus, the fleet of dumpers also has to be reviewed while assessing an excavator's performance.

Sound hydraulic system
The success of an excavator depends on the soundness of its hydraulic system. This should always be kept in mind while buying an excavator. Frequent breakdowns owing to the failure of this system completely ruin momentum of work and the excavator becomes a burden. Many hydraulic cylinder actions with variable strokes are involved in the excavation process. Any pressure drop owing to leakage of oil or non-operation of a valve can steal the major advantage of the excavator.

Among the important components of the excavator hydraulic system are its filters, pipes, hoses, seals, valves, cylinders and oil. Filters of hydraulic systems must work efficiently for smooth working of the hydraulic pump, to avoid cavitations and the long life of the system. The latest range of excavators promises almost double life of hydraulic filters. Availability of spare pipes, hoses and seals is important as, sometimes, one of these hoses or pipes may burst under pressure. Connections of hoses to machine parts are the most vulnerable points. Therefore, excavators with least hoses and more tubes should be preferred. Filters need to take out every minor particle of the hydraulic oil to avoid any cavitations effect or erosion of pumps. Regular filter cleaning is a very important aspect of maintenance.

Excavator accessories
Hydraulic excavators can have a number of attachments and accessories used to handle particular jobs. Some of these attachments are:

Excavator rippers: These are useful in breaking up hard soil and particularly helpful in trenching and pipeline work.
Rock buckets: These are suitable for handling hard rocks and resist highly abrasive forces. Made of high-strength steel, these buckets have the thickest wear plates and can achieve better penetration than general purpose buckets.
Concrete cutter jaws: This attachment helps in demolition and cutting of concrete structures.

Crusher jaws: They are particularly helpful in demolition of RCC structures.
Primary pulverizer jaws: These demolish concrete and separate the reinforcement from it in a single operation. The time of secondary processing is thus saved.

Pin grabbers and couplers: These help excavators in picking up and using any work tool.
Grapples: These can handle loose materials as well as irregularly shaped loads.
Rotation shears: These can rotate at a full 360° and demolish even steel structures.
They can be mounted on the boom or stick of the excavator.
While the primary function of a hydraulic excavator is to excavate soil to a larger reach and depth, it acts as the most effective demolition tool for any type of structure, steel or RCC.

Indian requirements
With India in the grip of an infrastructure development boom, the need of the hour is to make available excavators of high capacity, thus completely eliminating the need of importing excavation equipment and saving on high cost of spare parts. The main manufacturers of hydraulic excavators are Caterpillar, Hitachi, L&T, JCB, BEML, Mitsubishi, Poclain, Volvo and Liebherr. In India, L&T, BEML and JCB rule the market while Poclain excavators are available in plenty. In fact, excavators are known as JCB or Poclain at sites of work! JCB, which stands for Joseph Cyril Banford, is in fact the founder of the UK-based company. And in February 2007, a robust excavator, the L&T Komatsu PC 130-7, was launched in India by L&T. JCB's JS 210 L and JS 330 L with maximum engine power of 128 HP and 239 HP are also available in India.

Caterpillar produces the larg-est variety of hydraulic excavators to cater to all small and big jobs. Known as CAT, the machines are divided into five categories: Mini, Small, Medium, Large and Extra Large. Further, a number of machines are produced under each category. A buyer can configure his machine and get a price estimate. For example, the CAT 345C L excavator is an amazing machine with 345 hp net power, a travel speed of 4.4 kmph and a digging depth reach of 9 m. Its maximum bucket capacity is 3.8-cu-m, the bucket itself weighs about 1.76 tonne. The machine has a reach of 13-m at ground level.
CAT unveiled its D series excavator 325D at Excon 2007 held in Bangalore in November 2007. Its 330D and 320D models are already in operation in India. CAT's D series excavators particularly suit Indian projects owing to their comparatively lower buying and running costs, ability to take higher hydraulic pressure and lower oil consumption. The filter replacement period is almost double that of C series machines. In fact, the 325D model now introduced fills the large gap between 320D and 330D models.
Manufacturers often project specifications and data showing bucket-fill capacity, operating weight, engine output, fuel tank capacity, rated RPM, maximum travelling speed, length of boom, maximum dumping height, maximum horizontal reach of excavator, maximum digging depth, tyre size and wheel base (for wheeled excavators) or ground pressure (for track based excavators). These should be examined with respect to specific needs at the work site.



 

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