In case of double girder crane, the trolley housing hoisting and cross traversing machinery move on the top of two bridge girders. The bridge girders are of box construction made from rolled plates. However for smaller capacities and spans, the bridge girders are also made from standard rolled sections.
For standard workshop duty crane the hoisting unit is comprised of compact wire rope hoist unit. While in case of heavy duty or higher capacity cranes, the hoisting unit is made from fabricated and machined rope drum housing left hand and right hand grooves to achieve true vertical lift. The rope drum is connected to total enclosed helical gearbox through flexible drum coupling. The gearbox is driven by electric motor either sq. cage induction type or slip ring type. Fail safe hydraulically operated thrustor brake or DC brake is provided on input shaft of gearbox to prevent accidental lowering of load in the event of power failure.
For special steel mill ladle handling application, the wire rope arrangement can be supplied with four rope independent suspension to prevent falling of load in case of failure of one wire rope. The hoisting gearbox can be supplied with integral planetary arrangement and two hoisting motors for achieve hoisting motion at 50% of full speed in case of failure of one hoisting motor. Planetary gearbox can also be supplied for operating 4 rope grab bucket to achieve hoisting as well as grab opening/ closing operations.
Special features for hoisting arrangement include motorised hook rotation, two hook blocks suspended from hoist drum supporting lifting beam, special anti sway rope arrangement for rotating lifting beams, magnets suspended from hook block/ lifting beam, tong suspended from rope system, grab bucket suspended from wire rope system or hook type grab, hot metal handling crane with motorised hook rotation, fixed mast crane to prevent load sway during acceleration and deceleration.
In addition to Main Hoist (MH) an Auxiliary Hoist (AH) can also be provided on the same trolley. On customer’s request, two or three trolleys can be operated on the same pair of bridge girders for special application with a facility for load summation.
The cross travel machinery comprises of motor gearbox and brake unit coupled to wheel assemblies with live axles. The spherical roller bearings housed in L type of bearing housing are provided for heavy duty application.
The bridge girders are made of plate box construction of ample strength and rigidity. Full depth vertical diaphragms are provided inside the box for stability. Short diaphragms are provided in between full depth diaphragms to support the rail section that support the trolley. Horizontal diaphragms are provided as needed to prevent buckling of web plates.
For special applications wide box girder design is provided as per customer’s requirement. In this case the electrical panels are housed inside the box girder to protect the electrical and electronic components from metallic dust as well as heat. As an alternative to wide box girder design, a completely enclosed electrical house supported on crane platform can be provided.
Full length walkway platforms are provided along the LT drive side girders for heavy and extra heavy duty cranes. Platform/ Walkways are provided on all other cranes as per customer’s specifications.
Power is supplied to trolley through flexible festoon cable system. For lower capacity and light duty cranes, compact festoon cable system or drag chain system is used for trolley power supply. For higher capacity/ heavy duty cranes beam rider trolleys moving on heavy duty I beams supply power to machinery installed on the trolley.
The girders are supported at ends by rigid end carriages that house long travel wheels. The arrangement of end carriage is decided by application as well as number of long travel wheels that support the load. Crane capacity, runway rail size, duty classification of crane, LT speed are some of the factors that affect the sizing of LT wheels.
Depending on design requirement cranes with 4, 8, 12, 16 and longer travel wheels are provided for crane LT drive. The drive unit comprises of motor, gearbox and brake unit. Hollow output shaft gearbox that is installed directly on wheel axle is provided for light and medium duty crane. For heavy and extra heavy duty cranes the long travel gearbox, motor unit is mounted on a separate LT machinery platform and the gearbox is connected to Long travel wheel through a pair of half geared coupling and floating shaft.
In case of heavy duty cranes, the hoist, CT and LT gearboxes are made from MS fabricated steel duty stress relieved before machining. The helical gears are machined hob cut and are housed in precision machined housings. Gears are dipped in oil bath and are splash lubricated.
The motor speed and torque can be controlled through variable frequency variable voltage drives commonly known as VVVF or VFD drives as per customer’s requirement. Slip ring motors with resistance control is also employed as per the buyer’s choice.