Tower n hydraulic Crane

8/7/2019 Tower n hydraulic Crane

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Parts of a Tower CraneAll tower cranes consist of the same basic parts:

•

The base is bolted to a large concrete pad that supports the crane.• The base connects to the mast (or tower ), which gives the tower crane its height.• Attached to the top of the mast is the slewing unit -- the gear and motor -- that allows the

crane to rotate:

On top of the slewing unit are three parts:

• The long horizontal jib (or working arm ), which is the portion of the crane that carriesthe load. A trolley runs along the jib to move the load in and out from the crane's center:





The machinery arm contains the motor that lifts the load, along with the control electronics thatdrive it and the cable drum, as shown here:

The motors that drive the slewing unit are located above the unit's large gear :

http://www.howstuffworks.com/motor.htm



http://www.howstuffworks.com/gear.htm






Now let's find out how much weight this equipment can handle.

How Much Weight Can They Lift?A typical tower crane has the following specifications:

• Maximum unsupported height - 265 feet (80 meters)The crane can have a total height much greater than 265 feet if it is tied into the buildingas the building rises around the crane.

• Maximum reach - 230 feet (70 meters)• Maximum lifting power - 19.8 tons (18 metric tons), 300 tonne-meters (metric ton =

tonne)• Counterweights - 20 tons (16.3 metric tons)

The maximum load that the crane can lift is 18 metric tons (39,690 pounds), but the crane cannotlift that much weight if the load is positioned at the end of the jib. The closer the load is

positioned to the mast, the more weight the crane can lift safely. The 300 tonne-meter ratingtells you the relationship. For example, if the operator positions the load 30 meters (100 feet)from the mast, the crane can lift a maximum of 10.1 tonnes.

The crane uses two limit switches to make sure that the operator does not overload the crane:

• The maximum load switch monitors the pull on the cable and makes sure that the loaddoes not exceed 18 tonnes.

• The load moment switch makes sure that the operator does not exceed the tonne-meter rating of the crane as the load moves out on the jib. A cat head assembly in the slewingunit can measure the amount of collapse in the jib and sense when an overload conditionoccurs.



Now, it would be a pretty big problem if one of these things fell over on a job site. Let's find outwhat keeps these massive structures standing upright.

Why Don't They Fall Over?When you look at a tall tower crane, the whole thing seems outrageous -- why don't thesestructures fall over, especially since they have no support wires of any kind?

The first element of the tower crane's stability is a large concrete pad that the constructioncompany pours several weeks before the crane arrives. This pad typically measures 30 feet by 30feet by 4 feet (10 x 10 x 1.3 meters) and weighs 400,000 pounds (182,000 kg) -- these are the

pad measurements for the crane shown here. Large anchor bolts embedded deep into this padsupport the base of the crane:

So these cranes are essentially bolted to the ground to ensure their stability. In the next section,you'll learn how tower cranes "grow."

How Do They Grow?Tower cranes arrive at the construction site on 10 to 12 tractor-trailer rigs. The crew uses amobile crane to assemble the jib and the machinery section, and places these horizontalmembers on a 40-foot (12-m) mast that consists of two mast sections. The mobile crane thenadds the counterweights .



The mast rises from this firm foundation. The mast is a large, triangulated lattice structure,typically 10 feet (3.2 meters) square. The triangulated structure gives the mast the strength toremain upright.



To rise to its maximum height, the crane grows itself one mast section at a time! The crew uses atop climber or climbing frame that fits between the slewing unit and the top of the mast. Here'sthe process:

1. The crew hangs a weight on the jib to balance the counterweight.2. The crew detaches the slewing unit from the top of the mast. Large hydraulic rams in the

top climber push the slewing unit up 20 feet (6 m).3. The crane operator uses the crane to lift another 20-foot mast section into the gap opened

by the climbing frame. Once bolted in place, the crane is 20 feet taller!

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Once the building is finished and it is time for the crane to come down, the process is reversed --the crane disassembles its own mast and then smaller cranes disassemble the rest.

To maneuver the load, the boom has to be able to move right and left, as well as up and down.Underneath the operator's cab is a Rotex gear on a turntable bearing that turns at 2 revolutions

per minute (rpm). It is driven by a bidirectional, hydraulic motor mounted on the cab and housedin a metal cover to prevent injuries. The rotation is controlled by a foot-operated, hydraulic pedalin the cab.



The large gear under the cab is the Rotex gear, which allowsthe cab to swivel and move the boom from side to side.

Hydraulic truck cranes are used to lift heavy loads to tall heights, and it's important that the truck be completely stable during the lifting operation. The tires don't offer the stability needed, so the

truck employs outriggers that act as balances to keep the crane from leaning too much to oneside or the other. The outriggers use hydraulics to lift the entire truck, tires and all, off theground. The outriggers are comprised of the beam, which is the leg of the outrigger, and the pad,which is the foot. Sometimes, "floats" are placed under the pad to dissipate the force of the craneand the load over concrete or pavement. Floats are usually wood planks that are lined up tocreate a base that is larger than the pad itself.

The outriggers keep the crane balanced during a lift. The insetshows all outriggers extended.

The outriggers are only one mechanism used to balance the crane during lifting operations. Thereare also detachable counterweights that can be placed on the back of the crane on the undersideof the cab. These counterweights prevent the crane from tipping forward during operation. The

http://www.howstuffworks.com/tire.htm

http://www.howstuffworks.com/tire.htm



amount of counterweight needed for a particular lift is determined by the weight of the load, theradius of the boom and the boom's angle during operation. The 70-ton Link-Belt truck crane hascounterweights that come in 4,000-pound (1,814-kg) sections. Counterweights are only usedduring lifts; they have to be removed before the truck can be driven.

Prior to any lift, the operator enters data into a computer inside the cab, including the weight of the object to be lifted and the height to which it is to be lifted. This computer serves as theoperator's backup, warning the operator if the crane is being pushed beyond its capability. Usinga binder of charts in the cab, the operator also determines the angle of lift and the radius of the

boom. Once all of this is entered, the computer can track the progress of the lift and warn thedriver if the crane is nearing its limitations. If the boom is lifted too high for the load amount, aseries of lights just above the inside of the front window will begin to light up. These lights arethe warning lights for the Load Moment Indicator (LMI).

If the operator tries to lift a load too high, the Load MomentIndicator will light up.

There are at least two other people needed to perform a lift properly, including the oiler and thesignalman . The oiler is responsible for making sure that all of the crane's parts are in place andsecured prior to any lift. He or she also acts as a spotter during a lift to ensure that the lift is

being performed properly. The signalman, as the name suggests, gives hand signals to theoperator during the lift to make sure the load is being maneuvered correctly. Click here to seesome of the hand signals used during a lift.

Hydraulic truck cranes provide brute strength to move objects, machines and even large animalsthat would otherwise be very difficult to budge. Using a very simple principle of hydraulics,these machines move thousands of pounds with relative ease, making them an essentialcomponent of most construction projects and a great example of the power of basic physics.

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Tower n hydraulic Crane