SCRAPER PARTS AND DETAILS BASIC INFORMATION AND TUTORIALS

Scrapers ~ these machines consist of a scraper bowl which is lowered to cut and collect soil where site stripping and levelling operations are required involving large volume of earth.

When the scraper bowl is full the apron at the cutting edge is closed to retain the earth and the bowl is raised for travelling to the disposal area. On arrival the bowl is lowered, the apron opened and the spoil pushed out by the tailgate as the machine moves forwards.

Scrapers are available in three basic formats:

1. Towed Scrapers † these consist of a four wheeled scraper bowl which is towed behind a power unit such as a crawler tractor. They tend to be slower than other forms of scraper but are useful for small capacities with haul distances up to 300.

2. Two Axle Scrapers † these have a two wheeled scraper bowl with an attached two wheeled power unit. They are very manoeuvrable with a low rolling resistance and very good traction.

3. Three Axle Scrapers † these consist of a two wheeled scraper bowl which may have a rear engine to assist the four wheeled traction engine which makes up the complement.

Generally these machines have a greater capacity potential than their counterparts, are easier to control and have a faster cycle time.

To obtain maximum efficiency scrapers should operate downhill if possible, have smooth haul roads, hard surfaces broken up before scraping and be assisted over the last few metres by a pushing vehicle such as a bulldozer.

Typical Scraper Details

PARTS AND COMPONENTS OF ESCALATOR BASICS AND TUTORIALS

Escalators, or powered stairs, are used when it is necessary to move large numbers of people from floor to floor. They provide continuous movement of persons and can thus remedy traffic conditions that are not readily addressed by elevators.

Escalators should be viewed as preferred transportation systems whenever heavy traffic volumes are expected between relatively few floors. Escalators are used to connect airport terminals, parking garages, sports facilities, shopping malls, and numerous mixed-use facilities.

Although escalators generally are used in straight sections, spiral escalators also are available. Although expensive due to manufacturing complexities, they offer distinct advantages to both the designer and user because of their unique semicircular plan form.


An escalator resembles a powered ramp in construction. The major difference is that a powered ramp has a continuous treadway for carrying passengers, whereas the treadway of an escalator consists of a series of moving steps.

As for a powered ramp, the installation of powered stairs should conform with the requirements of the ‘‘American National Standard Safety Code for Elevators, Dumbwaiters, Escalators and Moving Walks,’’ ANSI A17.1.

(CLICK ON PHOTO TO ENLARGE)

An escalator consists of articulated, grooved treads and risers attached to a continuous chain moved by a driving machine and supported by a steel truss framework.

The installation also includes a handrail on each side of the steps that moves at the same speed as the steps; balustrades, or guards, that enclose the steps on each side and support the handrails; brakes; control devices; and threshold plates at the entrance to and the exit from the treadway.

The purpose of the threshold plates is to facilitate smooth passage of passengers between the treadway and landing.

The plates are equipped with a comb, or teeth, that mesh with and are set into grooves in the treadway in the direction of travel, so as to provide firm footing and to minimize the chance that items become trapped between treadway and the landing.

Each step is formed by a grooved tread portion connected to a curved and grooved riser. The tread and riser assembly is either a single die-cast piece or is assembled to a frame.

Both are suspended on resilient rollers whose axles are connected to the step chain that moves the steps. The step rollers ride on a set of tracks attached to the trussed framework. The tracks are shaped to allow the step tread to remain horizontal throughout its exposed travel.

TOWER CRANES BASIC AND CIVIL ENGINEERING TUTORIALS

TOWER CRANES BASIC INFORMATION
What Are Tower Cranes?


Much like a mobile crane, a tower-mounted crane moves loads by executing three motions: the hook is raised and lowered by means of a winch and fall, carried in a circular path by the swing gear, and carried in a radial motion by either luffing the jib or rolling a trolley carriage on its underside. The simplest of tower cranes have only these motions, but more complicated arrangements include mechanisms that allow the base to roll on a track, the crane to change elevation by climbing, or the jib to articulate on a hinge point.

In the twentieth century, tower crane and mobile-crane industries developed independently on opposite shores of the Atlantic Ocean in response to needs and cultural proclivities of their respective lands. Simply put, Europe was the realm of tower cranes, North America the realm of mobile cranes, and the rest of the world a patchwork of the two.

The globalized marketplace and rapid diffusion of knowledge in the present century have eroded differences in construction practices worldwide so that the selection of a crane type now is more likely to be determined by its suitability to the work than by the country where the work is taking place. At least that is the broad picture; contractors in some localities cling longer to traditional practices than others.

Tower cranes are the lifting machines of choice worldwide for most mid- and high-rise building construction. They are used also on expansive sites where the broad hook sweep and the relative ease of coordinating multiple tower cranes is an advantage.

There are niche markets for these as well; cable stay and suspension bridges, offshore oil platforms, and power plants are some examples. In most of the world outside U.S., small tower cranes are used for modest-size residential and commercial projects.

Many of these rigs are self-erecting machines that are pulled to the site by truck. In North America, similar work would utilize a small telescopic crane.

Gradually, however, self-erecting tower cranes are penetrating the U.S. market. There is some doubt  whether these machines should be classified as tower cranes; though their operating motions fit thepattern, their deployment—setup time, operation, inspection, maintenance, and demobilization—is more like that of mobile cranes.

Freestanding hammerhead tower cranes range up to about 300 feet (91 m) in height; for luffing tower cranes the limitation is less. Though most tower cranes free-stand and remain at a fixed height, various self-climbing arrangements permit a tower crane to attach to a building under construction and rise with it.

With such supplemental means of support, a tower crane can ascend to any building height. Very high line speeds up to 1000 ft/min (5 m/s) available with some models yield good production rates even at extraordinary heights.

Some machines can operate in winds up to 45 mi/h (70 km/h), which is far above mobile-crane wind limits. Lift capability of tower cranes is gauged by a moment rating expressed as tonne-meters. The tonne-meter rating is obtained by multiplying rated capacity in metric tons by the working radius in meters.

This is done according to a method that averages a range of boom lengths and working radii. The smallest machines used for light construction have ratings of about 20 meter-tons and the very largest in production exceed this by a factor of about a hundred. Most used for heavy construction are in the range of 150 to 650 meter-tons.

The cost associated with installing and removing a tower crane is small for the self-erecting type but can be considerable for most others. At minimum, those costs would include trucking, hiring a rigging crew and an assist crane, construction of a foundation, electrical hookup, and the services of a trained technician. More complicated installations such as those which climb have considerable additional expenses.

The high costs of installation, as well as the considerable investment of time and planning, make all but the smallest tower cranes a tool for longer-term projects where these expenditures can be amortized.

Excluding self-erectors, tower crane designs follow the “erector set” concept; that is, they are composed of multiple components of common design. Components connected together by pins or bolts are often interchangeable among an array of models.

As a tower is a cantilever subjected to high-bending moments that shift and reverse, each tower leg must be designed to resist alternating compressive and tensile forces. The usual stress levels and the expected number of lifetime loading cycles are not exceptional; thus leg design does not pose a difficult fatigue problem for designers.

The connections between tower sections are another matter, being a perpetually nettlesome design and maintenance concern. Some designs use bolts and others use pins to join the legs of one section to the next. Bolts must be preloaded to carefully controlled levels.

Failure to establish and maintain the preload can result in bolt fatigue failure. The alternative pin-type connection requires tight fabrication tolerance; imperfect towers can be either too sloppy or difficult to fit together. They also must be carefully designed to avoid the pitfall of premature fatigue failure.

With the exception of a few diesel-powered machines with hydrostatic drives, tower cranes are powered by electricity and motors driving all the machinery are almost universally electric, too. The hoist motors on older machines have high- and low-speed ranges with stepped increments in each range.

More recent models have variable-frequency drive or other forms of continuously adjustable speed motors with friction or eddy-current brakes and creep speed. Automatic acceleration for all motions is typical on many cranes. Remote controls are sometimes offered.

A tower crane is said to be top-slewing—slewing being the preferred tower crane term for swing motion—if the swing circle is mounted near the tower top and it’s said to be bottom-slewing if the swing circle is near the. Bottom-slewing machines might have become largely obsolete had the concept not been revived by recent developments with self-erecting machines.

CRANE SUPPORTED (HANGING) LEADERS USED IN PILING BASICS AND TUTORIALS

CRANE SUPPORTED (HANGING) LEADERS USED IN PILING BASIC INFORMATION
What Are Crane Supported Leaders Used In Piling?


Although the complete piling rig with its base frame and leaders supported by a stayed mast provides the best means of ensuring stability and control of the alignment of the pile, there are many conditions which favour the use of leaders suspended from a standard crawler crane.

Rigs of this type have largely supplanted the frame-mounted leaders for driving long piles on land in the UK and USA. The usual practice is to link the leaders by the head of the crane jib and to control their verticality or backward or forward rake by means of adjustable stays near the foot of the leaders.

The latter bear on the ground through an enlarged foot which can be levelled by a screw jack. BSP International Foundations Ltd. TL series leaders (Figure 3.6) have heights of 19.0m and 21.9m and carry hammers of up to 3 tonne mass.

The 610mm and 835mm square section lattice leaders have a height to the cathead of 22.5 and 38m respectively, and can carry combined pile and hammer loads of 13 tonne and 21 tonne respectively.

Backward and forward rakes of up to 1:3 are possible depending on the stability of the crawler crane. There is a practical limit to the length of pile which can be driven by a given type of rig and this can sometimes cause problems when operating the rig in the conventional manner without the assistance of a separate crane to lift and pitch the pile.

The conventional method consists of first dragging the pile in a horizontal position close to the piling rig. The hammer is already attached to the leader and drawn up to the cathead. The pile is then lifted into the leaders using a line from the cathead and secured by toggle bolts.

The helmet, dolly and packing are then placed on the pile head and the assembly is drawn up to the underside of the hammer. The carriage of the piling rig is then slewed round to bring the pile over to the intended position and the stay and angle of the crane jib are adjusted to correct for vertically or to bring the pile to the intended rake.  The problem is concerned with the available height beneath the hammer when it is initially drawn up to the cathead.

Taking the example of leaders with a usable height of 20.5m in conjunction with a hammer with an overall length of 6.4m, after allowing a clearance of 1m between the lifting lug on the hammer to the cathead and about 0.4m for the pile helmet, the maximum length of pile which can be lifted into the leaders is about 12.7m.

A somewhat longer pile could be handled if the leaders were of a type which allows vertical adjustment. Occasionally it may be advantageous to use leaders independent of any base machine. Thus if only two or three piles are to be driven, say as test piles before the main contract, the leaders can be guyed to ground anchors and operated in conjunction with a separate petrol or diesel winch.

Guyed leaders are slow to erect and move, and they are thus not used where many piles are to be driven, except perhaps in the confines of a narrow trench bottom where a normal rig could not operate.

CHICAGO BOOM DERRICK BASICS AND TUTORIALS

CHICAGO BOOM DERRICK BASIC INFORMATION
What Is A Chicago Boom Derrick?

Chicago Boom Derrick

A Chicago boom can be mounted on a building frame during or after construction, on a tower, or on any frame. Indeed, Figure 1.19 shows one installed on a power plant stack.

When a boom or strut is assembled in the form of a Chicago boom, it can range from as little as 10 ft (3 m) to as much as 125 ft (38 m) in length, and capacities can range from a low of, say, ¼ ton (225 kg) to a practical upper limit of perhaps 35 tons (32 t).

In the not too distant past, booms were made of wooden poles. Short lightweight booms are easily and inexpensively made of single steel pipes fitted with the necessary attachments, but most booms are trussed, or latticed, structures of angle irons or tubing or a combination of the two. Aluminum and synthetic composite booms are plausible, too, where site conditions favors such unconventional materials.

The topping lift usually employs ordinary hoisting blocks; one is fitted at the boom tip held off with steel straps while the opposite end is mounted at the pivot fitting on the support structure also with straps.

The upper load block may consist of sheaves built into the boom head, or it may be a common block suspended on straps.  The purpose of straps is to allow clearance between the rope suspension system and adjoining boom elements through the full range of luffing motion.

A Chicago boom is able to hoist materials to a height above the boom foot, with the horizontal reach limited by the length of the boom and the swing arc by the host structure. Swing guys often are fitted to the boom tip and are run laterally on each side to a point of anchorage.

Wind, friction at the pivots, and the resistance of the opposing guy must be overcome when pulling on the line to swing. With a manual arrangement, the guys are fiber ropes arranged with several parts of line. Hand pulling through several parts can take several minutes to swing the boom through 90°. Where production economics justifies the expense to attain greater speed, mechanical swing systems are used.

In the typical installation, a two-drum winch is used to power the hoisting and topping motions, but when the work involves only lifting and swinging, the topping motion will not be needed. A fixedrope guy line can then be installed, or to make adjustment easier and to provide flexibility.

Visual control can be established when the winch is located at the floor level of the boom foot, particularly when the loads are to be hoisted to this floor, but the winch can be located at any level. When the winch is too large or too heavy to be lifted in the job-site material hoist or in the elevator of an existing building, it may be necessary to use a small temporary winch to operate the derrick in order to hoist the working winch.

Alternatively the winch can be positioned on the ground. When the winch is on the ground, the operator has direct communication with the ground crew and can have the boom and load in view at all times.

When the winch is on the same floor as the boom foot and the loads are to be hoisted to that floor, the operator has direct communication with the swing and load-landing crew and has the boom but not the load in view at all times.

THE BASIC HOISTING MECHANISM BASICS AND TUTORIALS

THE BASICS OF HOISTING MECHANISM
What Is Hoisting? How Hoisting Works?

Figure 1.1 shows a lifting device with a load attached to the lower block and the block in turn supported by two ropes, or parts of line, suspended from the upper block. Each rope must therefore carry half the weight of the load; this gives the system a mechanical advantage of 2. Had the load been supported by five ropes, the mechanical advantage would have been 5. Mechanical advantage is governed by the number of ropes actually supporting the load.

As parts of line are added, the force needed to raise or lower the load decreases, and load movement speed decreases as well. The blocks contain pulleys, or sheaves, so that the rope is in one continuous piece from the end attached to the upper block to the winding drum.

This makes the force in all parts of the rope uniform in a static system. The value of the rope load is found by dividing the weight of the lifted load by the mechanical advantage; in Figure 1.1 the lifted load would include the lower block, sometimes called the hook block.

When the distance between the upper and lower blocks is great, it is necessary to include the weight of the parts of line as well. The load in the rope is also equivalent to the force that must be generated at the winding drum in order to hold the load.

The effects of friction come into play as soon as the system is set into motion. Friction losses occur at the sheave shaft bearings and in the wire rope itself, where rope losses result when the individual wires rub together during passage over the sheave.

These losses induce small differences in load between each rope segment (i.e., each section of rope from sheave to sheave). The loss coefficient can vary from a high of about 4½% of rope load for a sheave mounted on bronze bushings to a low of as little as 0.9% for a sheave on precision ball or roller bearings. An arbitrary value of 2% is a reasonable approximation for sheaves on common ball or roller bearings when the rope makes a turn of 180°.

The tension in the rope at the winding drum is different when the load is raised and when it is lowered. Friction losses are responsible for this difference. When load-weighing devices that operate by reading the tension in the line to the drum are used, the variation is readily observed.

When an unloaded hook must be lowered, lowering will be resisted by friction, by the weight of the rope between the upper block and the deflector sheave, and by the inertia of the winding-drum mass. Mechanical advantage works in reverse in this case, as a mechanical disadvantage so to speak, so that the weight at the hook must exceed the rope weight multiplied by the mechanical advantage plus an allowance to overcome friction and inertia.

EXCAVATING AND EARTH PLACING MACHINERY BASICS AND TUTORIALS

EXCAVATION AND EARTH MOVING EQUIPMENT AND MACHINES
What Are The Different Excavation and Machine Moving Equipment?


Bulldozers (‘dozers’) are used for cutting and grading work, for pushing scrapers to assist in their loading, stripping borrowpits, and for spreading and compacting fill. The larger sizes are powerful but are costly to run and maintain, so it is not economic for the contractor to keep one on site for the occasional job.

Its principal full-time use is for cutting, or for spreading fill for earthworks in the specified layer thickness and compacting and bonding it to the previously compacted layer. It is the weight and vibration of the dozer that achieves compaction, so that a Caterpillar ‘D8’ 115 h.p. weighing about 15 t, or its equivalent, is the machine required; not a ‘D6’ weighing 7.5 t which is not half as effective in compaction. The dozer cannot shift material very far, it can only spread it locally.

A dozer with gripped tracks can climb a 1 in 2 slope, and may also climb a slope as steep as 1 in 1.5 provided the material of the slope gives adequate grip and is not composed of loose rounded cobbles. On such slopes of 1 in 1.5 or 1 in 2 the dozer must not turn, but must go straight up or down the slope, turning on flatter ground at the top and bottom. It is dangerous to work a dozer (and any kind of tractor) on sidelong ground, particularly if the ground is soft.

Dozers cannot traverse metalled roads because of the damage this would cause, and they should not be permitted on finished formation surfaces. Sometimes a flat tracked dozer (i.e. with no grips to the tracks) can be used on a formation if the ground is suitable.

Motorized scrapers are the principal bulk excavation and earth-placing machines, used extensively on road construction or earth dam construction. Their movement needs to be planned so that they pick up material on a downgrade, their weight assisting in loading; if this cannot be managed or the ground is tough, they may need a dozer acting as a pusher when loading.

This not only avoids the need for a more expensive higher powered scraper, but reduces the wear on its large balloon tyres which are expensive. The motorized scraper gives the lowest cost of excavation per cubic metre of any machine, but it needs a wide area to excavate or fill and only gentle gradients on its haul road. It cannot excavate hard bands or rock, or cut near-vertical sided excavations.

The face shovel, or ‘digger’ can give high outputs in most types of materials, including broken rock. It comes in all sizes from small to ‘giant’; but for typical major excavation jobs (such as quarrying for fill) it would have a relatively large bucket of 2–5m3 capacity. The size adopted depends on what rate of excavation must be achieved, the capacity of dump trucks it feeds to cart away material, and the haul distance to tip or earthworks to be constructed.