The lifting crab of a EOT crane is an independent two motion machine and consists of the hoist machinery built on to a frame, which is mounted on runner wheels, and driven by a motor through suitable gearing. The crab is also known as trolley.
Various designs of crabs, arranged with C.I. side frames, mild steel side frames are in use. But now a days, preference is given to the built up side frames on which the machinery is mounted.
The function of the crab is to provide cross travel motion to the hoisting machinery along the length of the main bridge of the crane. The entire hoisting machinery is mounted on the crab which moves on runner wheel over rails fitted on the main bridge of the crane.
The motion of the crab is derived from an electric motor. A speed reduction gear box and a brake are the essential components of a crab gearing. The motion of the motor is transmitted through the reduction gear box via the brake. The HP of motor is calculated by the formula:
HP=W*S*C / 4500
where,
W=total load of the crane including the dead load any pay load in tons;
S=Speed of the longitudinal motion in meter per minute;
C=Tractive effort in Kg per ton.
The reduction gear box is designed to obtain a speed ratio which can reduce the motor RPM to the appropriate RPM of the crab runner wheel to produce the desired speed of the crab. The reduction ratio per stage of the gearbox is fixed at 4 to 5 and is the same for all stages.
The maximum load in tons is the customer’s requirement and must be specified clearly. Design of almost all the components of the crane are influenced by this parameter. In some cases it is necessary to arrange for a second hoisting machinery called the auxiliary hoist, to lift a much lighter load. If the load at which the crane is operated in general is considerably less than the maximum load, an auxiliary hoist may be used. Cranes used for power plant maintenance is an example of such cases where auxiliary hoist may be fitted. For an entirely different reason auxiliary hoist may be fitted with cranes used for foundries, steel casting shop handling ladles containing molten metal. In these industries the main hoist of the crane is used for hoisting the ladle containing the molten metal, while the auxiliary hoist is used to lift the ladle very slowly to pour the liquid in the mould.
An auxiliary hoist machinery consists of a completely separate hoist mechanism built on a second crab which may run either above or below the crab of the main hoist. The auxiliary hoist is generally of much less capacity than the main hoist.
Span of the Crane:
The distance between the centre to centre of the gantry rails over which the wheels of the bridge run. The section of the gantry rails should also be specified for the design of the wheels of the bridge.
To arrange for the correct height of lift this parameter should be specified. If the crane is required to lift out of pits or from below the floor level, particulars should be specified.
Class of Work of the crane:
For the purpose of design of their frames, every crane and hoist is classified with respect to the frequency of application, variation of magnitude of the load and the effect of impact.
Cranes are divided into four classes.
Light duty cranes come under class I type.
Heaviest duty crane are class 4 type.
Light duty and Heavy duty cranes are discerned by the working period, effective load and dynamic effect. It should be remembered that this classification is applicable to the entire crane and the structure.
The working period is short if the crane is operated for less than 500 hours per year. The effective load is high if the crane is used to lift loads greater than two third of its safe working load on more than one thousand occasions per year.
The dynamic effect may be considered low if the speed of travelling of both crab and bridge or hoist are each less than 100 m per minute. If the speed of operation is higher than this the dynamic effect is high.
Headroom:
The clear height available from the gantry rail level to the lowest overhead obstruction is called the head room. This should be specified by the customer. The height of the hoisting machinery over the crab should not foul with the structural obstruction.
End Clearance:
The distance from the centre of gantry rail horizontally to the nearest obstruction on either side is called the end clearance.
Operating speeds under Loaded Condition:
The operating speeds in m/min for the main hoist, auxiliary hoist, cross traverse and long travel should usually be specified by the customer. When there are no special requirements, the speeds will follow the usual standard practice for the size of crane under consideration.
Apart from the above mentioned important parameters the customer should also specify the following particulars:
Electrical details like the type (AC or DC),
Voltage,
Number of phase,
Frequency,
Electrical control,
Whether the control is to be in operator’s cabin for hand operation, or whether from floor by means of pendant.
