Types of Cranes and their Classification
Types of Cranes and their Classification
Cranes play an important role in the handling of materials by raising and moving loads, the mass of which is within their nominal capacity. There can, however, be wide variations in the duty, both for a single crane type, for example overhead travelling cranes, or between different crane types, for example between a builder’s tower crane and a heavy-lift dockside crane. The design of the crane has to take into account the conditions of service, in order to reach an appropriate level of safety and useful life.
Cranes are industrial machines which are mainly used for materials movements in construction sites, production shops, assembly lines, storage areas, power stations and similar places. Their design features vary widely according to their major operational specifications such as type of motion of the crane structure, weight and type of the load, location of the crane, geometric features, operating mechanisms, and environmental conditions.
A crane is having a hoist which consists of wire rope and hook. The crane is used both for lifting and lowering of materials, and to move them horizontally. Cranes uses one or more simple machines to create mechanical advantage and thus move heavy loads. Several factors are taken into consideration while selecting a crane. These factors include lifting capacity, crane use and application, and the number of work cycles which the crane is required to undergo.
A crane normally consists of three separate motions. These are (i) hoist which raises and lowers the material, (ii) cross travel which allows the hoist to be positioned directly above the material for placement, and (iii) long travel which allows the entire crane to move along the working area.
A crane is frequently a very large and complex piece of equipment. There are several individual components and design features which can be found on a typical crane. The components include brakes, wire ropes, electrical drive systems, automatic sensors, wheels, rails, buffers, cable reels, festoons, hoppers, over-load preventers, and anti-collision systems etc. There is now a choice of drives available such as electric or hydraulic. The components are required to be correctly sized. In addition, there are various design features and preferred solutions such as the effect of wind on cranes, design standards, welding methods, structural design and fatigue calculations, and finally the maintenance of the crane.
There are number important parameters which are to be taken into account to determine the suitability of a crane for a particular service. These parameters are (i) specifications, (ii) applicable codes and local regulations (iii) crane capacity, (iv) required span, (v) lift needed by the hoist, (vi) duty cycle (usage) of the crane, (vii) weight of the load and requirement of auxiliary hoist for lifting of the load, (viii) hook approach, (ix) desired length of runway system, (x) factors which are required to be considered in the design of runway and building structure, (xi) the operating environment (dust, paint fumes, outdoor or indoor etc.) of the crane, (xii) the required crane and trolley speeds, (xiii) the supply voltage, phases, and amperage, (xiv) the control system which is desired, (xv) existing cranes on the runway, (xvi) safety considerations which are needed to be followed, (xvii) maintenance requirements of the crane, and (xviii) the requirements of other accessories such as lights, warning horns, weigh scales, and limit switches, etc.
Basic components of a crane
The basic components of a crane are given below. Fig 1 shows major components of a top running double girder electric overhead travelling crane.
Bridge – It is the main travelling structure of the crane and spans the width of the building bay. It travels in a direction parallel to the runway. The bridge consists of two end trucks and one or two bridge girders depending on the equipment type. The bridge also supports the trolley and hoisting mechanism for the up and down lifting of the load.
End trucks – They are located on either side of the bridge. They house the wheels on which the entire crane travels. It is an assembly consisting of structural members, wheels, bearings, and axles etc., which supports the bridge girder(s) or the trolley cross member(s).
Bridge girder(s) – A bridge girder is the principal horizontal beam of the crane bridge which supports the trolley and is supported by the end trucks.
Runway – It consists of the rails, beams, brackets, and framework on which the crane operates.
Runway rail – It is the rail which is supported by the runway beams on which the crane travels.
Hoist – The hoist mechanism is a unit consisting of a motor drive, coupling, brakes, gearing, drum, ropes, and load block designed to raise, hold, and lower the maximum rated load. Hoist mechanism is mounted on the trolley.
There can be the requirement for the use of a second hoist on the bridge crane. This hoist can be used as an auxiliary hoist or be required in a process such as tilting / tipping. In case of handling of the long materials, like steel tubes and plates, the best solution are to have a crane with two hoists (and hooks) for better stability of the load ensuring safe lifting.
Trolley – It is the unit which carries the hoisting mechanism. It travels on the bridge rails in a direction at right angles to the crane runway. Trolley frame is the basic structure of the trolley on which are mounted the hoisting and traversing mechanisms.
Bumper (buffer) – It is an energy absorbing device for reducing impact when a moving crane or trolley reaches the end of its permitted travel, or when two moving cranes or trolleys come into contact. This device can be attached to the bridge, trolley, or runway stop.
Fig 1 Major components of a top running double girder electric overhead travelling crane
Types of cranes
There are several types of cranes. Some of them are given below.
Overhead travelling crane – An overhead travelling crane is also known as a bridge crane. It is a type of crane where the hook and the line mechanism runs along a horizontal beam that itself runs along two widely separated rails normally in a long factory building and runs along rails along the two long walls of the building. The crane includes a hoist to lift the items, the bridge, which spans the area covered by the crane, and a trolley to move along the bridge.
The bridge girders of the overhead travelling crane can be built using typical steel beams or a more complex box girder type. The advantage of the box girder type configuration results in a system which has a lower deadweight yet a stronger overall system integrity. The configuration of overhead travelling crane can be either under running or top running.
Under running crane is known as under slung crane. These cranes are distinguished by the fact that they are supported from the roof structure and run on the bottom flange of runway girders. Under running cranes are typically available in standard capacities upto 10 tons (special configurations upto 25 tons and over 27 metres spans). Under running cranes offer excellent side approaches, close head-room and can be supported on runways hung from existing building members if adequate. The advantages of the under running cranes are (i) very small trolley approach dimensions meaning maximum utilization of the width and the height of the building, and (ii) the possibility of using the existing ceiling girder for securing the crane track. The disadvantages of the under running crane are (i) reduced hook height because of the location of the runway beams, (ii) the load being applied to the roof is higher than that of a top running crane, and (iii) loading of runway beams needs careful sizing of the flanges otherwise, the flanges can ‘peel’ off the beam.
In the top running cranes, the crane bridge travels on top of rails mounted on a runway beam supported by either the building columns or columns specifically engineered for the crane. Top running cranes are the most common form of crane design where the crane loads are transmitted to the building columns or free standing structure. These cranes have an advantage of minimum headroom / maximum height of lift. Fig 2 shows important parameter of overhead crane.
Fig 2 Important parameters of overhead crane
There are different types of overhead travelling cranes with many being highly specialized, but the great majority of the cranes fall into one of the three categories namely (i) single girder bridge crane, (ii) double girder bridge crane, and (iii) monorail.
Single girder bridge crane consists of a single bridge girder supported on two end trucks. It has a trolley hoist mechanism which runs on the bottom flange of the bridge girder. Single girder cranes cost less since (i) only one girder is needed, (ii) the trolley is simpler, (iii) freight expenses are reduced, (iv) installation is faster, and (v) runway beams are lighter. However, not all cranes are to be designed with a single girder. Normally, if the crane has to handle more than 15 ton or the span is more than 25 meters, a double girder crane is a preferred option. Double girder cranes are also highly suitable where the crane needs to be fitted with walkways, cabins, magnet cable reels or other special equipment.
Double girder bridge crane consists of two bridge girders supported on two end trucks. The trolley runs on rails on the top of the bridge girders. Double girder bridge cranes are more typical when needing heavier capacity systems from 10 tons and above.
Monorail is used for several applications such as production assembly line or service line, where only a trolley hoist is needed. The hoisting mechanism is similar to a single girder crane with a difference that the crane does not have a movable bridge and the hoisting trolley runs on a fixed girder. Monorail beams are normally ‘I’ beams.
Gantry cranes – These cranes are essentially the same as the regular overhead travelling cranes except that the bridge for carrying the trolley or trolleys is rigidly supported on two or more legs running on fixed rails or other runway. These ‘legs’ eliminate the supporting runway and column system and connect to end trucks which run on a rail either embedded in, or laid on top of, the floor.
The crane frame is supported on a gantry system with equalized beams and wheels that run on the gantry rail, normally perpendicular to the trolley travel direction. This crane comes in all sizes, and some can move very heavy loads.
Mobile crane – Mobile crane is mounted on a carrier normally a truck which provides the mobility for the crane. This crane has two parts namely (i) a carrier which is frequently referred to as the ‘lower’ and (ii) a lifting component which includes the boom also referred to as the ‘upper’. These are mated together through a turntable which allows the upper to swing from side to side. The present day hydraulic truck cranes are normally single engine machines, with the same engine powering the under-carriage and the crane. The upper is normally powered through hydraulics run through the turntable from the pump mounted on the lower. Earlier the hydraulic truck cranes had two engines. One in the lower is used for the crane to travel on the road and ran a hydraulic pump for the outriggers and jacks. The second in the upper ran the upper through a hydraulic pump of its own.
These cranes are normally able to travel on highways, eliminating the need for special equipment to transport the crane. These cranes can have special features for rough terrain movement and all terrain movements. When working on the job site, outriggers are extended horizontally from the chassis then vertically to level and stabilize the crane while stationary and hoisting. Many truck cranes have slow travelling capability (a few kilometers per hour) while suspending a load. Great care is to be taken not to swing the load sideways from the direction of travel, as most anti-tipping stability then lies in the stiffness of the chassis suspension.
Most cranes of this type also have moving counter-weights for stabilization beyond that provided by the outriggers. Loads suspended directly are the most stable, since most of the weight of the crane acts as a counterweight. Manufacturer calculated charts (or electronic safeguards) are used by crane operators to determine the maximum safe loads for stationary (outriggered) work as well as (on-rubber) loads and travelling speeds. Mobiles cranes can have a range in lifting capacity from around 12 tons to around 1,200 tons.
Telescopic crane – A telescopic crane has a boom which consists of a number of tubes fitted one inside the other. A hydraulic or other powered mechanism extends or retracts the tubes to increase or decrease the total length of the boom. These types of booms are frequently being used for short term construction projects, rescue jobs, lifting boats in and out of the water, etc. The relative compactness of telescopic booms makes them adaptable for many mobile applications. However, it is to be noted that while telescopic cranes are not automatically mobile cranes, many of them are. These are frequently truck-mounted.
Tower crane – The tower crane is a modern form of balance crane. It is normally fixed to the ground on a concrete and sometimes attached to the sides of structures as well. Tower cranes frequently give the best combination of height and lifting capacity and are used in the construction of tall buildings and tall industrial structures. This crane frequently gives the best combination of height and lifting capacity. The base of the crane is attached to a mast which gives the crane its height. Further the mast is attached to the slewing unit (gear and motor) which allows the crane to rotate. On top of the slewing unit there are three main parts which are namely (i) the long horizontal jib (working arm), (ii) shorter counter jib, and (iii) the operator’s cabin.
The jib (colloquially being called the ‘boom’) and counter-jib are mounted to the turntable, where the slewing bearing and slewing machinery are located. The counter-jib carries a counterweight, normally of concrete blocks, while the jib suspends the load from the trolley. The hoist motor and transmissions are located on the mechanical deck on the counter-jib, while the trolley motor is located on the jib. The crane operator either sits in a cabin at the top of the tower or controls the crane by radio remote control from the ground. In the first case the cabin of the operator is normally located at the top of the tower attached to the turntable, but can be mounted on the jib, or partway down the tower.
The lifting hook is operated by using electric motors to manipulate wire rope cables through a system of sheaves. In order to hook and unhook the loads, the operator normally works in conjunction with a signaller (known as a ‘rigger’). They are most often in radio contact, and always use hand signals. The rigger directs the schedule of lifts for the crane, and is responsible for the safety of the rigging and loads.
A tower crane is normally assembled by a telescopic jib (mobile) crane of greater reach. In the case of tower crane which has risen while constructing very tall skyscrapers, a smaller crane (or derrick) is frequently be lifted to the roof of the completed tower to dismantle the tower crane afterwards.
Tower crane can also be self erecting type. Self erecting type tower crane is also called self-assembling or ‘kangaroo’ crane. It lifts itself off the ground using jacks, allowing the next section of the tower to be inserted at ground level or lifted into place by the partially erected crane itself. The crane can thus be assembled without outside help, or can grow together with the building or structure it is erecting.
Crawler crane – A crawler is a crane mounted on an undercarriage with a set of tracks which is also called crawlers. These crawlers provide stability and mobility to the crane. Crawler cranes have both advantages and disadvantages depending on their use. Their main advantage is that they can move around on site and perform each lift with little set up, since the crane is stable on its tracks with no outriggers. A crawler crane is also capable of travelling with a load. The main disadvantage is that these cranes are very heavy, and cannot easily be moved from one job site to another without significant expense. Typically a large crawler is to be disassembled and moved by trucks, rail cars or ships to its next location. Crawler cranes range in lifting capacity from around 35 tons to 3,000 tons.
Rail road crane – This crane has flanged wheels for use on railroads. The simplest form is a crane mounted on a flat car. Most of these cranes are normally purpose built and have high lifting capacities. The design differs according to the type of work but the basic configuration is similar in all cases. The configuration normally consists of a rotating crane body which is mounted on a sturdy chassis fitted with flanged wheels. The body supports the boom and provides all the lifting and operating mechanisms. On larger cranes, a cabin for an operator is normally provided. The chassis is fitted with buffing and coupling gear to allow the crane to be moved by a locomotive, although several rail road cranes are also self-propelled to allow limited movement around a work site.
Floating crane – These cranes are used mainly in the constructions of bridges and ports. They are also used for occasional loading and unloading of especially heavy or awkward loads on and off the ships. Some floating cranes are mounted on a pontoon; others are specialized crane barges with a lifting capacity sometimes exceeding 9,000 tons. These cranes are used to transport entire bridge sections. Floating cranes are also used to salvage sunken ships.
Aerial crane – This crane is sometimes called sky crane. It is helicopter designed to lift large loads. Helicopter is able to travel to and lift in areas which are difficult to reach by conventional crane. Aerial crane is normally used to lift units / loads onto shopping centres and high rise structures. This crane can lift anything within their lifting capacity. It is being used for the erection of transmission towers in difficult terrains. It also performs disaster relief after natural disasters for clean up, and during wild-fires it is able to carry huge buckets of water to extinguish fires.
Jib crane – It is a type of crane where a horizontal member (jib or boom), supporting a movable hoist, is fixed to a wall or to a floor mounted pillar. Jib cranes are used in industrial premises and on vehicles. The jib can swing through an arc, to give additional lateral movement, or be fixed.
Further to above cranes there are other types of cranes which are bulk handling crane, loader crane, stacker crane, deck crane, level luffing crane, and hammer head crane etc.
Essential parameters for specifying electric overhead cranes
There are certain parameters which are essential for specifying electric overhead cranes. These parameters are described below.
Crane capacity – It is the rated load, which is required to be lifted. Rated load means the maximum load for which a crane or individual hoist is designed and built by the manufacturer and shown on the equipment identification plate.
The rated capacity of crane is the live load which can be lifted by the crane. The rated load is defined as the maximum working load suspended under the load hook. Load block and ropes are not included in the rated load.
The design load for the crane system is based on the rated capacity plus 15 % for the weight of the hoist and trolley (rated capacity x 1.15) and an additional 25 % for impact (rated capacity x 1.25) for a total design capacity (rated capacity x 1.4). 25 % impact factor is good for hoists speeds upto 15 metres per minute.
The capacity of crane is the maximum rated load (in tons) which a crane is designed to carry. The net load includes the weight of possible load attachment. For example, a 10 ton crane allow to pick up a 10 ton load provided the hoist weighs 1.5 ton or less and the hoist speed is less than 15 metres per minute. Under no conditions, the crane is to be loaded beyond its rated capacity.
Lift height – The rated lift height means the distance between the upper and lower elevations of travel of the load block and arithmetically it is normally the distance between the beam and the floor, minus the height of the hoist. This dimension is critical in most applications as it determines the height of the runway from the floor and is dependent on the clear inside height of the building. It is to be remembered that any slings or below the hook devices which influence this value are to be included.
Runway height – It is the distance between the grade level and the top of the rail.
Clearance – It is the vertical distance between the grade level and the bottom of the crane girder.
Clear span – It is the distance between columns across the width of the building. Building width is defined as the distance from outside of the eave strut of one sidewall to outside of the eave strut of the opposite sidewall. Crane span is the horizontal centre distance between the rails of the runway on which the crane is to travel. Typically distance is around 500 mm less than the width of the building. The requirement of the span of the crane depends on the crane coverage width dictated by the application.
The crane steel structure is selected to be either a single or double girder crane construction according to the span and the maximum load handling capacity.
Building height – Building height is the eave height which normally is the distance from the bottom of the main frame column base plate to the top outer point of the eave strut. Eave height is the distance from the finished floor to the top outer point of the eave strut. There is to be a safety distance between the top edge of the crane runway rail and the first obstacle edge in the building (e.g. roof beams, lights, and pipes).
Runway length – It is the longitudinal run of the runway rail parallel to the length of the building.
Hook approaches – Maximum hook approach is the distance from the wall to the nearest possible position of the hook. The smaller the distance is, the better can the floor area be utilized. It is necessary to check the optimum hook approaches of the crane so that when combined with the true lift of the hoist, the most of the available floor space can be utilized. This is also termed as side hook approach.
End approach – End approach describes the minimum horizontal distance, parallel to the runway, between the outermost extremities of the crane and the centre line of the hook.
Bridge, trolley and lift speeds – The rate at which the bridge or trolley travels or at which the hoist lifts is normally specified in metres per minute. The crane operating speeds are selected to allow safe operation while using the pendant. Dual operating speeds, normally a fast and slow speed with a ratio of 4:1 are normally used but for optimum control a variable speed control system is the strongly recommended control system.
Electrical and control requirements – There are two circuits in most hoist electrification systems, power and control. The first is the power circuit. The power circuit provides the energy to lift loads, and run other motors that perform work. Since bridges, trolleys, and hoists move during operation they are to be powered by appropriate means. The second is the control circuit. It is the secondary low voltage electrical circuit to supply power for the control functions.
The circuit voltage is not to exceed 600 volts for alternating current (AC) or direct current (DC). The runway power is normally by conductor bar and hoisting trolley by festoon cable. The control circuit voltage at pendant pushbuttons is not to exceed 150 volts for AC and 300 volts for DC. Other control options include radio control, free-floating pendant (festooned) or hoist-mounted pendant.
The crane or hoist is normally operated by some type of push button arrangement held in the hand of the operator. The benefit of reducing shock hazard by reducing the voltage and current are obvious.
Other than the above parameters, there can be some specific conditions applicable to a particular application. Further, there are some other conditions which are to be specified for the design of the crane. These include (i) the existing operating environment (dust, paint fumes, outdoor, etc.), (ii) existing cranes on the runway so that the use of a collision avoidance or collision warning system can be considered, (iii) requirement of a catwalk on the crane for maintenance access, and (iv) accessories which are needed such as lights, warning sirens, weigh scales, and limit switches etc.
Classification of cranes
Classification of cranes defined in ISO 4301 part 1, considers only the operating conditions which are independent of the type of crane and the way it is driven.
Two cranes with the same rated capacity and span can differ in their average load intensity and / or expected loading cycles. There are different standards which classifies cranes based on the service class. The Crane Manufacturer Association of America (CMAA) classifies bridge cranes according to average load intensities and number of cycles. On the other hand, the classification of hoists by the International Organization for standardization (ISO), European Federation Standard FEM (Federation Europeene de la Manutention) and Hoist manufacturer Institute (HMI) is according to more rigorous requirements, which include number of starts and maximum running time per hour.
CMAA crane classification and its comparison with other classification are given below. There are six different classifications of cranes by CMAA based on the duty cycle of crane.
Class A (standby or infrequent service) – This crane is the lightest crane as far as duty cycle is concerned. This service class covers cranes where precise handling of equipment at slow speeds with long idle periods between lifts. Capacity loads can be handled for initial installation of equipment and for infrequent maintenance. Typical examples are cranes used in power houses, public utilities, turbine rooms, motor rooms, and transformer stations. This is the lightest crane as far as duty cycle is concerned.
Class B (light service) – This service class covers cranes where service requirements are light and speeds are slow. Loads can vary from no load to occasional full capacity. Lifts per hour can range from 2 to 5, and average 3 meters per lift. Examples of class B cranes include service buildings, light assembly operations, repair and maintenance shops, and light ware housing etc.
Class C (moderate service) – Class C cranes are those cranes whose service requirements are deemed to be moderate. These cranes handle loads which average 50 % of the rated capacity with 5 to 10 lifts per hour, averaging around 5 meters per lift, with not over 50 % of the lifts at rated capacity. In terms of numbers, most of the cranes are built to meets class C requirements. Examples of class C cranes are the cranes normally used in mill machine rooms and machine shops etc.
Class D (heavy service) – In class D service crane, loads approaching 50 % of the rated capacity is handled constantly during the work period. High speeds are desirable for this type of service with 10 to 20 lifts per hour averaging around 5 meters per lift with not more than 65 % of the lifts at the rated capacity. Typical examples of cranes with heavy service are steel pant production shops, steel warehouses, foundries, fabricating shops, heavy machine shops, container yards, and lumber mills etc. Cranes can be with standard duty buckets or magnets operations where heavy duty production is needed.
Class E (severe service) – Cranes with class E service are capable of handling loads approaching the rated capacity throughout its life with 20 or more lifts per hour at or near the rated capacity. Application of canes with class E include magnet, bucket, magnet / bucket combination cranes or fertilizer plants, steel plants, cement plants, scrap yards, lumber mills and container handling etc.
Class F (continuous severe service) – Cranes with class F service are to be capable of handling loads approaching rated capacity continuously under severe service conditions throughout its life. Typical examples of such cranes include custom designed specialty cranes essential for performing the critical work tasks affecting the total production facilities. This type of crane is to provide the highest reliability with special attention to ease of maintenance features.
HMI hoist duty ratings are given in Tab 1. The table provides an idea of the relative significance of the duty cycle ratings for the different electric hoists. It is to be noted that the duty cycle determination for a particular application involves obtaining a significant amount of additional information and expertly applying it to the intended use.
|Tab 1 Duty cycle ratings of various electrical hoists|
|HMI class||Operation based on 65 % capacity||Details|
|Maximum on time||Maximum starts||Maximum on time||Maximum starts|
|minutes per hour||per hour||From cold start|
|H1||7.5 (12.5 %)||75||15||100||Power-house and utilities, infrequent handling, hoists used primarily to install and service heavy equipment, loads frequently approach capacity and hoist idle for long periods between use.|
|H2||7.5 (12.5 %)||75||15||100||Light machine shop fabricating, service and maintenance; loads and utilization randomly distributed; rated loads infrequently handled. Total running time not over 12.5 % of the work period.|
|H3||15 (25 %)||150||30||200||General machine shop fabricating, assembly, storage, and warehousing; loads and utilization randomly distributed. Total running time not over 25 % of work period.|
|H4||30 (50 %)||300||30||300||High volume handling of heavy loads, frequently near rated load in steel warehousing, machine and fabricating shops, mills, and foundries, with total running time not over 50 % of the work period. Manual or automatic cycling operations of lighter loads with rated loads infrequently handled such as in heat treating or plating operations, with total running time frequently 50 % of the work period.|
|H5||60 (100 %)||500||Not applicable (Note 1)||Not applicable (Note 1)||Bulk handling of material in combination with buckets, magnets, or other heavy attachments. Equipment frequently cabin operated. Duty cycles approaching continuous operation are frequently necessary. User must specify exact details of operation, including weight of attachments.|
|NOTE 1 : Not applicable since there are no infrequent work periods in Class H5 service.|
AISE (American Iron and Steel Engineers) also provides for different service classes for cranes covered under AISE Technical Report No. 6, ‘Specifications for Electric Overhead Travelling Cranes for Steel Mill Service’. Like CMAA, AISE also provides a numerical method for determining crane class based on the expected load spectrum. Without getting into the specifics of this method, AISE does generally describe the different service classes (load cycles) as (i) Service Class 1 (N1) with less than 100,000 cycles, (ii) Service Class 2 (N2) with 100,000 to 500,000 cycles, (iii) Service Class 3 (N3) with 500,000 to 2,000,000 cycles, and (iv) Service Class 4 (N4) with over 2,000,000 cycles.
Further AISE describe the different load classes as (i) L1 which includes cranes which hoist the rated load exceptionally, and normally hoist very light loads, (ii) L2 which includes cranes which rarely hoist the rated load, and normally hoist loads about 1/3 the rated capacity, (iii) L3 which includes cranes which hoist the rated load fairly frequently, and normally hoist loads between 1/2 and 2/3 or the rated capacity, and (iv) L4 which includes cranes which are regularly loaded close to the rated capacity. Based on the load classes and load cycles, the CMAA chart (Tab 2) below helps determine the class of the crane.
|Tab 2 Chart for determining class of crane|
|Load Classes||Load cycles|
|20,000 to 100,000 cycles||100,000 -500,000 cycles||500,000 – 2,000,000 cycles||Over 2,000,000 cycles|
For the determination of the crane duty group according to FEM, the factors which are needed are (i) load spectrum which indicates the frequency of maximum and smaller loadings during examined time period, and (ii) class of utilization which is determined according to number of hoisting cycles during lifetime of crane. By combining of these factors duty group of the crane is to be selected. Tab 3 gives comparison of different standards for crane classification.
|Tab 3 Comparison of various standards|
|* Based on 63 % of mean effective load|
|** Based on 65 % of mean effective load|