Cranes – Design, Practice, and Maintenance phần 3 potx

Themore complicated WL drive has great advantages compared to driveswith slipring motors or DC motors with resistance control.. Number of sheaves: dependson rope system in trolley 3.3 Ca

Fig 3.1.2 Fluid coupling

fluid coupling is an excellent type of drive as it gives smooth tion of the complete belt system

accelera-The slipring motor

The slipring motor is a drive which is now little used but it is still worthmentioning The alternating current slipring motor is speed-controlled

by resistances These resistance-steps can be switched on or off by thecontroller If torque is required: the more resistance, the lower thespeed ‘No resistance’ gives the speed curve of the normal squirrel cagemotor The brushes of the motor need regular maintenance; the resist-ances can burn out and rust Therefore resistances made of stainlesssteel have preference

Fig 3.1.3 Slipring motor: resistance controlled

The Ward–Leonard drive

The Ward–Leonard (WL) drive can be considered as a ‘better DC ve’ (The DC drive with resistance control is not further described.) Themore complicated WL drive has great advantages compared to driveswith slipring motors or DC motors with resistance control

dri-The main motor, which is a squirrel cage motor, runs at a constantspeed during the workshift on the crane It drives a Ward–Leonardgenerator for each mechanism The generator is directly coupled to themain motor and gives a regulated voltage and current to the respectivemotor which forms the drive-element of the crane mechanism Thespeed control of this drive-element can be stepless

With a three-field generator like the Ward–Leonard–Kra¨mer themaximum torque can be fixed exactly at the desired level This givesexcellent drives for the hoisting mechanisms of grabbing cranes whichdredge under water and for the drives of cutter-dredgers and similardevices Cosphi compensation is not necessary The Ward–Leonard–

Fig 3.1.4 Ward–Leonard–Kra¨mer (hoist motion)

must be carefully monitored, is the average accelerating torque edge of how to design and manufacture these powerful Ward–Leonarddrives has unfortunately been largely lost.

Knowl-Direct current full-thyristor systems

In the last twenty years the direct current full-thyristor drive has becomethe successor to the resistance-controlled AC drives and DC drives andthe Ward–Leonard drives

The stepless controlled full-thyristor direct current motor is availablefor all mechanisms and all capacities It can be regarded as fool proof.Regular maintenance is needed to attend to the brushes, and collectors

in the motors Dust caused by wear and tear of the brushes has to beremoved from time-to-time and the brushes have to be adjusted,checked, and replaced to prevent breakdown and loss of efficiency.These motors can be totally enclosed or drip-watertight, self-ventilated

or ventilated by an external, continously running ventilator lated) Field weakening can occur, normally to a level of approximately

(force-venti-1500 to 2000 rev兾min depending on the power range and field sation The normal voltage is 400 V or 500 V Cosphi compensation isneeded to achieve a cosphi of approximately 0,9

compen-Alternating current drives with frequency control

To reduce maintenance on the motors as much as possible, the facturers of electrical systems have developed and now use AC motorswith frequency control Since 1995 a good working system has beenachieved AC frequency control is also available for hoisting mechan-isms using large amounts of power

manu-The motors are of a simple design However these are special squirrelcage motors The electrical control is somewhat more complicated thanthat of the full-thyristor systems, and forced ventilation is not normallyrequired Control of these motors is always stepless Field weakening,

up to 2000 to 2200 rev兾min – based on a four-pole motor, is possible byincreasing the frequency Torque–speed curves can be adjusted within alimited range

It is safe to assume that the research and development of the design

of motors will continue and that further advances will be made ever, this drive offers the most appropriate and suitable answer for thenext ten years Cosphi compensation may be necessary to achieve acosphi level of approximately 0,9 depending on the type of the drive

How-Fig 3.1.5 DC full thyristor

In low speed crane-travelling mechanisms, the option of using onedrive for all the motors under the two sill-beams of the cranes is poss-ible Because all the motors will receive the same frequency, synchroniz-ation between the motors is not absolutely necessary providing that thewheel loads and the wind loads on each sill-beam of the crane do notdiffer significantly However, it is preferable to use one drive for eachsill-beam and also to make ‘cross-over’ connections between the motors

on the two sill-beams This ensures exact synchronization

Warning Especially with AC frequency control, but often also with

DC-Full-Thyristor Control the Electromagnetic ability (EMC ) due to the Higher Harmonics plays animportant role

Compat-To prevent disturbances by this Electro Magnetic ference (EMI) special double-shielded cables must be used.These screens or shields consist of a copper foil wrappingand optimized copper wire braiding

Inter-On both ends of the cable special EMC glands must beused These must be well-earthed and connected to steelboxes

In the bigger motors insulated bearings should also be used

Fig 3.1.6 AC frequency control: torque–speed diagram for hoisting/lowering

Fig 3.1.7 2B800 kW Holec AC frequency control motors in the hoisting winch

of a grab-unloader

Hydraulic drives

We now concentrate on the Ha¨gglunds hydraulic drive, which consists

of a control system; an electric motor; an oil tank; a pump; and ahydraulic motor The pump is driven by an electric motor, which runswith a fixed speed The oil flow from the pump is controlled by either

a Squashplate or a tilting cylinder block, the angle of which can bechanged by a signal from the control system The motor pumps the oilwhich flows into the motor cylinders and presses the pistons radiallyout towards the camring The speed of the motor is stepless variable.This system has a low moment of inertia and a high starting torque(200 to 300 percent of a nominal rated torque) A brake system canalso be provided on these drives

Fig 3.1.8 Winches with Ha¨gglunds hydraulic drives

As already mentioned in Wire Rope Reeving Systems (Section 2.1),there are quite a number of reeving systems for hoisting mechanisms.The main types are considered in Figs 3.2.1(a) to 3.2.1(e).

Fig 3.2.1(a) Container cranes with machinery trolley (Hoisting winch on the

trolley.) Number of rope sheaves: minimum

Fig 3.2.1(b) Container cranes with rope trolley (Hoisting winch fixed on the

bridge.) Number of rope sheaves: depending on wire rope layout

Fig 3.2.1(c) Grab unloader with main and auxiliary trolley Number of rope

sheaves: see Fig 2.1.2

Fig 3.2.1(d) Level luffing crane Number of rope sheaves: see sketch

Fig 3.2.1(e) Stacking crane with ‘rope tower’ Number of sheaves: depends

on rope system in trolley

3.3 Calculating the requisite power of the hoisting motors

For calculating the requisite motor power the following items must beconsidered:

(a) the resistance due to normal (nominal ) hoisting;

(b) the resistance due to acceleration of the rotating masses;(c) the resistance due to acceleration of the linear moving masses;(d) for the hoisting mechanism shown in Fig 3.2.1(e) the influence

of the anglesα have to be taken into account, as the forces andthe motor power are multiplied in this wire rope system

with f G 1

cosα

whereα is then half of the biggest angle between the wire ropeswhen the load is in the highest position

gearings and rope

Torque (Nm) kiloWatts (kW)Addition:

restric-For example: faG160 percent G1,6

The motor(s) must be able to deliver

G924,831,6

G578 kW

(which is lower than N1:

so use N1G733 kW)Take motor(s): N G733 kW (2 · 366 kW)

n G783 rev兾min

S3– 60 percent rating (see under Section 3.7)

faG160 percent

Field weakening

Let us assume that it is also necessary to hoist (and lower) the load of

200 kN with a speed of û G120 m兾min (ûG2 m兾sec) and an acceleration

time of 4 sec The motor(s) then run at n G2 · 783 G1566 rev兾min

Torque (Nm) kiloWatts (kW)Addition:

Fig 3.3.1 DC FT torque–speed diagram

For grabbing winches

Follow the same calculation method

Example 2

Assume the weight of the loaded grab is 36 t; the nominal speed

120 m兾min

Unloader type as Figs 2.1.2 and 3.2.1(c)

With empty grab the speed is 150 m兾min

Torque (Nm) kiloWatts (kW)Addition:

Note: To keep the grab well-closed during hoisting you are strongly

advised to give the closing motor more ‘pull ’ than the holding motor –

55 percent or 60 percent of the total ‘pull ’

Wet sand or soil can produce suction when the grab is drawn out of

the material, therefore famust be sufficient to counteract this force

Remark: The acceleration of linear moving masses can also be

trans-ferred into the acceleration of rotating masses:

Acceleration of linear moving masses Transfer

Fig 3.3.2 4000 ton floating crane ‘Asian Hercules’

Acceleration of linear moving masses Transfer

In general, we have 3 types of different systems and calculations

A Direct driven trolleys or motor trolleys

B Trolleys, which are pulled by wire ropes

C Rope driven trolleys for grab-unloaders with a main- and anauxiliary trolley.

For the direct driven or motor trolleys account must be taken of thepossibility of slip between the direct driven wheel and the rail, underbad weather conditions

Factors to be considered are:

1 The resistance due to nominal travelling

2 The resistance due to the current supply- or festoon system

3 The resistance due to the influence of the wind on the trolley andthe load

4 The resistance due to the acceleration of the rotating masses

5 The resistance due to the acceleration of the linear movingmasses

For systems A and B, the motor trolleys and rope driven trolleys,

we arrive at the following calculation; after checking the maincharacteristics

Main characteristics

Trolley travelling speed m兾min 150 m兾min 210 m兾min

m兾sec ûG2,5 m兾sec û G3,5 m兾sec

(Full-motor (Full-rope

Weight of the total load t W2G55 t W2G55 t

Wheel resistance of the

G0,05 kN兾tEfficiency of gearings

(and rope sheaves) ηt

Full rope trolley or

Fig 3.4.1

Full motor trolley of a

crane with the hoisting

winch on the trolley

Motor speed nm rev兾min nmG1500 rev兾min

trolley is ropedriven

Example A Example BReduction between

3.4.1(A) Direct driven trolleys or motor trolleys;

wheel slip control

Addition: Drive forces on Needed motor

Total, during acceleration: ΣFaG81,66 kN ΣNaG251,44 kW

(for control of theslip between railand wheel )

The needed motor power must now be greater thanΣN G76,35 kW and

ΣNaG251,44兾fa fais the maximum torque of the motors, which should

not be greater than faG2 (MmaxG200 percent of Mnom) So:

ΣN must be greater thanΣN G76,35 kW and ΣN must be

as an unloaded motor driven trolley:

Loaded trolley Unloaded trolley

(container)

WtG90 t

Average wheel load P G130兾4G32,5 t P G90兾4G22,5 t

3.4.2(B ) Trolleys pulled by wire ropes or rope driven trolleys

(We continue with the trolley with the main characteristics mentionedunder Section 3.4.2(A) We repeat these for clarity.)

m兾sec û G3,5 m兾sec

(full rope trolley)

Efficiency of gearings and

Influence of the wind:

Resistance due to festoon system kN F2G1,5 kN

hoist-wire ropes it can

be necessary to

calculate

approxi-mately twice as much

for F1and N1)

2 Resistance due to the

Drive forces

1 Nominal travelling F1G5,4 kN N1G22,2 kW

3 Wind q G150 N兾m2

F3G12 kN N3G49,4 kWAcceler rot masses,

Acceler linear

5 masses, taG4 sec F5G78,75 kN N5G324,3 kWTotal during

The needed motor power must be greater than

ΣN G77,8 kW and ΣN G424 兾 fa

fais again the maximum torque factor of the motors, which should – in

this case – not be greater than faG2,2 (MmaxG220 percent of Mnom.)So:

ΣN must be greater thanΣN G77,8 kW and ΣN must be greater than

This type of high throughput unloader always has high-speed rope leys with acceleration and deceleration times of only approximately

trol-3 sec

Therefore it is necessary to take the extra rope-pull of the acceleratingand swinging load into account when calculating the requisite motorpower (See Fig 2.1.2 for a diagram of this assembly.)

Factors to be considered are:

1 The resistance due to nominal travelling of the main-trolley plusloaded grab

2 The resistance due to nominal travelling of the auxiliary trolley(at half the speed of the main trolley)

3 The resistance due to the influence of the wind on the trolley andthe load

4 The resistance due to the acceleration of the rotating parts

5 The resistance due to the acceleration of the main trolley

6 The resistance due to the acceleration of the auxiliary trolley

7 The resistance through the sharp acceleration of the loaded grabwhich swings to its extremes

Main characteristics: example

m兾sec û G4,33 m兾sec

Weight of the auxiliary trolley mt W3G13 t

Wheel resistance of the trolley

JtGΣmomof inertia of the rot parts kg m2

1 Resistance due to

nominal travelling

of the main trolley

plus loaded grab:

Calculation: ropes (kN) power (kW)

Drive forces

Taking this great swing into account is necessary

The motor must be able to develop a great torque!)

(bulk unloader) ropes (kN) power (kW)

1 Nom trav of main

2 Nom trav aux trolley F2G0,65 kN N2G1,65 kW

3 Wind, q G250 N兾m2

F3G6,3 kN N3G32,1 kW

4 Acceler rot masses,

5 Acceler main trolley F5G23,1 kN N5G117,7 kW

6 Acceler aux trolley F6G9,4 kN N6G23,94 kW

7 Acceler of grab

(2α1G16,6 degrees) F7G128,25 kN N7G653,3 kW

8 Total during acceleration ΣF G170,75 kN ΣN G1000,2 kW

If we take faG2,5 as the maximum torque factor, the motor powershould be:

Fig 3.4.3 The swinging grab of an unloader

Fig 3.4.4 Grab of a floating unloader

Make a schematic diagram (as Fig 3.5.1) and estimate the weight ofthe boom and it’s centre of gravity.

Fig 3.5.1

For an example we use the following:

– Centre of rotation Gthe hinge point

of the boom GO

– Distance from O to the centre of

– Distance from hinge point to centre

– The asked for time for

– Creeping time for the boom when

starting and ending the hoist

movement, including latching: t2G1 min

– Time for really hoisting the boom: t3Gt1At2G5,5A1

G4,5 min

– Average speed: ûGPQAPR

t3

GL1AL2

t3

ûG50A174,5

G7,33 m兾min– Total efficiency of the wirerope

sheaves, the drum and the gearbox: ηG0,86

– Force in the boom-hoist-tackle, when

starting to hoist the boom: F1GG · B

(During the hoisting of the boom the force F1 becomes lower as the

distance A becomes greater.)

(The influence of the wind from the rear becomes greater when theboom is hoisted.)

Take the motor power: N G280 kW

n G1500 rev兾min

25 percent; eventual 40 percent rating

faG160 percent

The tackle of the boom hoist mechanism is 2 times Z 2 · Z G2 · 9 G18.

This means that the force F1 is taken by a bundle of 2 · 9 G18 wire

ropes

The efficiency of the sheaves before the drum is:ηsG0,91

The force in each of the 2 wire ropes, which are connected to the boomhoist drum is then:

F2G F1

2 · Z ·ηs

F2G 201 000

2 · 9 · 0,91G12 271 kg

forestays is taken into account, it will be found that these underpartsgive some assistance in lifting the first few degrees from the horizontal.This means that the force in the boom-hoist tackle is reduced over thefirst 6 degrees by some 7 percent, thus using less motor power Thisphenomenon can help in difficult cases.

Normally the wire ropes of the boom-hoist mechanisms will offer

a safety factor of about six, against rupture However, a boom-hoistmechanism is a system that can give rise to difficulties if one of the twowire ropes should fail Cases of a boom or even a whole crane collapsingwhen a wire rope breakdown occurs are well known in the annals ofcrane history If one wire rope should fail, the safety factor againstrupture of the remaining wire rope would only be three which is nothigh enough for safe operation

The safest manner for the reeving-in of the boom-hoist mechanism isshown in Fig 3.5.2

Fig 3.5.2 Ideal reeving system for boom-hoist mechanism