Building on the Basics

Engineer Jagvir Goyal tells us about essential tools and tackles that enable the smooth operation of sophisticated machinery and technologies on site. Recent years have witnessed the arrival of versatile construction equipment that can handle arduous tasks and make the construction of incredibly complex looking structures fast and easy. Yet, most equipment, however sophisticated, requires essential accessory equipment, tools and tackles to do the job optimally. Chain pulley blocks Chain pulley blocks are often used at sites to lift small loads vertically and manually. As the name suggests, these consist of a sheaved pulley provided with a hook and a chain passing over the pulley. The hook is inserted over a firm rod or pipe at a higher level. There is another moveable pulley of smaller diameter at a lower level with a hook attached to it in the reverse direction. This pulley and hook are used to hook the load to be lifted up. The chain is now pulled manually and the load starts moving up. This is one of the oldest and cheapest equipment that finds much use at construction sites because of simplicity in operation. Quite inexpensive, this tackle should always be kept available at site. Producers have perfected the art of production of these blocks by using high grade steel alloy for the chains and cutting the gears from special steel. The hooks are made of IS-certified steel and each chain pulley block is tested for 50 per cent overload before it makes it to the market. IS 3832 specifies the test. The load carrying capacity of chain and pulley blocks now produced goes from a mere 0.5 tonne to 10 tonne. Hand winches Hand winches are used at site either to lift a load through ropes or cables or control the movement of large components suspended in the air during their erection. For lifting small loads, as per winch capacity, the wire rope or cable is wound on the winch drum and the rope or cable is released or pulled back manually through the winch handle. The other end of the rope or cable is tied to the load to be lifted after passing it over a pulley fixed at a higher level. Hand winches prove very useful in guiding a large and heavy component being raised to higher levels through powered erection equipment. Take the example of a structural steel platform being erected at, say, 150 m height in a tall RCC chimney. The steel platform will have two or four structural steel plate girders of large span, in the range of 13 m to 20 m. Erection of such girders, fabricated at site, is a gigantic task. When these girders are being raised to the required level, hand winches can play a major role in safe erection. If jute ropes are used over the winches and two hand winches are used with jute ropes from them tied to each end of the girder, the girder can be prevented from swaying too much in the air and going out of control. As the girder is in suspended form, there will be no load from it on the hand winches but the sway of the girder can be controlled by the men on hand winches by pulling or releasing the jute ropes to keep the girder in stable position till it finally rests at the desired location and is then aligned as per laid design. Hand winches are produced for different load carrying capacities, up to 20 tonne, by varying the gear sizes. More advantageous than chain and pulley blocks, hand winches demand no consumables and require minimum maintenance. This inexpensive equipment proves very useful at construction sites. These can be floor or wall-mounted, but generally floor-mounted winches are used at construction sites. Mechanical engineers often modify them temporarily at site to convert them into power winches. Motorised hoists In addition to chain and pulley blocks and hand winches, another useful tackle is a motorised hoist that runs on electric power, diesel or compressed air. It has a hoisting gearbox that can be mounted directly with a rope drum; the motor is mounted with a geared brake drum. Compressors need space for installation; therefore, these hoists are generally electrically or diesel-operated. Motorised hoists operate along guides provided for them. The hook of the hoist picks up the load and the hoist steers it up vertically. Failsafe brakes are preferred in such hoists. The lifting capacity goes up to 50 tonne or so. Overhead cranes Often provided in workshops, station buildings and power houses, overhead cranes come in handy to lift loads without occupying any space on the floor to accommodate them. These have a box-like structure, designed and strengthened to withstand loads and connected with bridge trolley operated by motors. Wire ropes and hooks are tested for overload. The capacity of single girder cranes generally goes up to 15 tonne. The span and capacity are the two main factors that determine the overhead crane to be chosen from a wide range available from more than 100 manufacturers in India. Jacks Almost all construction equipment use jacks for one purpose or other. They are used either for a lifting or pushing action. Jacks are either manual or hydraulic type. While manual jacks mostly use gears to draw the action required from them, hydraulic jacks use hydraulic oil to exert pressure on a surface that further transmits it to lift a weight or push a piston. These are produced in high and medium pressure categories; provided with high pressure seals to prevent leakages, mountings and smooth lowering mechanism on release of the load; and tested to sustain the designed pressure. These are single or double acting, spring return or load return type, provided with heat-treated alloy steel body and selected by mechanical engineers with respect to the purpose to be served. Extraction jacks serve the special purpose of extracting a rod or pipe embedded in a structure. When the oil exerts pressure in them, they grip the bar to be extracted and pull it out. Such jacks are often useful in vertical slip form operations where expensive jack rods embedded in the structure need to be retrieved for reuse or to save money. Hoses Like hydraulic jacks, hydraulic hoses too play an important role in successful production and use of construction equipment. Hydraulic hoses are reinforced tubes that can withstand the pressure of hydraulic oil flowing inside them. Hose reinforcement may be braid or spiral type. Braid type reinforcement, often used, is provided in a criss-cross pattern. Spiral type reinforcement is provided in layers. The inner surface of the hoses is lined with a liner that does not react with the hydraulic oil passing through it. Normally, this liner is made of rubber or fabric that may withstand the pressure and doesn’t leak under pressure. The liner material shouldn’t be dissolvable in the oil being run through hoses. If it happens, this can badly affect construction equipment and damage the expensive motor. Thus, there should be no compromise on the quality of hoses. Leakage or bursting of hydraulic hoses during equipment operation releases all the pressure built by hydraulic oil and defeats the purpose for which the equipment has been deployed. Therefore, hydraulic hoses and their end fittings must be very strong to take and absorb the exerted oil pressure. Covering layer of hoses should therefore be strong enough to protect the inner liner and reinforcement inside it. In addition to the pressure exerted by the oil, the hose material should withstand high temperatures too. Care should always be taken to have spare hydraulic hoses of each diameter at the site for immediate replacement of a leaking, damaged or burst hose. Care should also be taken to avoid subjecting a hose to unnecessarily high pressure and temperature. The quality of hydraulic oil should also be checked as inferior quality oils react with hose material and damage it, leading to damage of pumps or motors. The maximum bending radius of hoses should also be checked while selecting them and later during their use. Unnecessarily high or acute bending of hoses may shorten their lives or may distort them permanently. Hydraulic power packs Hydraulic power packs are self-contained units that make the use and working of entire hydraulic equipment possible by supplying hydraulic oil to it and regulating its supply. They consist of a hydraulic oil reservoir that is kept filled with oil, supply pressure manifold, supply pressure regulator, fluid return valve, the motor and its starter, gauges for supply pressure, supply relief, valve adjustments and drain plugs. The power pack is located as close to the valves to be operated as possible. The platform erected for the location of power pack is kept perfectly level and stable and at level. The power pack is anchored to its platform with bolts to minimise vibrations. It is always kept protected from direct sun or rain for best use and control. The operation is simple. The hydraulic fluid is pumped to create pressure on the control valves and action begins. However, the manufacturer’s instructions regarding maximum pressure on the control valves and level of the hydraulic fluid in the reservoir need to be followed carefully. For the operation of vertically moving upward slip forms, the hydraulic power pack has to be erected on a special platform near the working deck to supply oil under pressure to all jacks erected along the circumference of chimney to lift the yokes. The power pack is to be taken up right to the top of the chimney along with all other equipment. Similarly, during the construction of cooling towers, hydraulic power packs operate the jacks of hydraulic jump forms. Here, the job is more strenuous as the cooling tower circumference is much larger than that of a chimney and requires more number of jacks. Indian companies like Delta, Powerjac, Fluid Power, Romac, Techno, Precise and others market hydraulic power packs of different capacities ranging from 15 litre to as high as 1,000 litre, capable of supplying oil under a pressure range of 700 kg per sq cm or more and power supply of 10 hp. Earth compactors This small equipment pays dividends during compaction of earth in a restricted area. Small in size, easy to operate and run by a single operator, it can be driven to fine pockets of the area under compaction. Often light in weight, earth compactors provide high frequency and low amplitude and are fitted with one-directional impulse vibrators for ramming and forward movements. These are best suited for sandy soils and may be fully non-cohesive or semi-cohesive. For small jobs like driveways of buildings, earth compactors prove useful and handy. Sometimes these prove to be the most suitable equipment even for large projects with space constraints. Here is an example. During the construction of large natural draught cooling towers at a power project site, the base design provided by German consultants was so evolved that a 2 m wide and 4 m RCC trench of 96 m outer diameter and 92 m inner diameter had to be constructed and filled with non-cohesive soil, compacted to maximum density. The area was large and related to a very important structure but restricted for movement of vibratory rollers or compactors. The area was ultimately planned to be filled in layers of 200 mm thickness, each layer compacted to maximum density by deploying a number of vibratory earth compactors. Ultimately, all the layers were filled and compacted to desired density to achieve finishing level, only with the help of small but useful vibratory earth compactors. Earth compactors are produced by many companies like Aquarius, Jaypee, Sona, Sany, Cosmos, Gubbi, Surya and others. The plate used is 6 to 15 mm thick with motor capacity of 3 to 5 hp. Plate size is generally around 2 ft by 3 ft. The compactor is either diesel, petrol or electricity-driven with depth effect reaching between 6 and 15 inches depending upon power rating and vibrating frequency varying between 2,500 to 4,000 vibrations per minute. Earth compactors weigh between 100 kg and 400 kg. Clamps and D shackles Clamps are items that are required in plenty at a construction site. They are used to connect, fasten, grip, secure or support a component of any equipment. Rigid and swivel clamps are pipe clamps that form part of the staging and formwork erected for a building. These are required in thousands in number at a site, depending upon the magnitude and height of the structure. In addition, there are clamps classified as per their shape. While C clamps are used to grip an object, U clamps are an essential item used at construction sites to connect wire ropes to different spares and tackles. These are in steel or stainless steel, with both ends threaded and connected with a strip and nuts. D shackles are also wire rope fittings, sort of large sized U clamps, and very useful in connections. The two ends of the D have eyes through which screw pins or round pins are passed and tightened. These can withstand loads up to 150 tonne. At present, over 100 firms manufacture quality D shackles in India. Quick Bytes • Hand winches prove very useful in guiding a heavy component being raised to higher levels through powered erection equipment.• While manual jacks mostly use gears to draw the action required from them, hydraulic jacks use hydraulic oil to exert pressure on a surface.• Often light in weight, earth compactors provide high frequency and low amplitude.