Application Note on Crane Duty

What is an overhead crane?

An overhead crane is a type of crane where the hook-and-line mechanism runs

along a horizontal beam that runs along two widely separated rails. Often it is in a

factory building and runs along its walls. Overhead cranes are commonly known as

Electrical Overhead travel or EOT cranes.

What are the types of Overhead cranes?

There are three main types of EOT cranes:

1) Jib Crane: A jib crane has one fixed leg, a cantilevered boom that rotates around the fixed leg, and a hoist/trolley unit that traverses the boom and lifts loads for placement somewhere else. A jib crane is helpful in a high-repetition lift process where an overhead crane would be otherwise tied up. Jib cranes are typically one to two tons, but sometimes reach five ton capacity.

Fig 1: Jib Crane

2) Overhead Bridge Crane: An overhead bridge crane operates on a raised and fixed track. Typically the height under and overhead bridge crane is 20’ or more. On an overhead crane, end trucks are equipped with wheels to provide for moving the entire crane. A bridge girder (single or double) spans the rails, and provides support for the hoist/trolley unit. Finally, the trolley traverses the bridge girders and usually has a hoist mounted on it to lift the load with either chain or wire rope.

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Fig 2: Overhead Bridge Crane

3) Gantry Crane: A gantry crane is similar to a bridge crane, but typically has legs at either end of the bridge girders, which are then mounted with end trucks at the bottom of the legs to move the gantry along ground-level tracks. Gantry cranes are more frequently found outdoors where no raised runway is present. Also, small non-powered gantry cranes are used in light duty applications such as small machine shops or automobile garages. These gantry cranes typically do not have a fixed path, but rather have rubber tires

Fig 3: Gantry Crane

Basics of Contactor switching duties/Definitions as per IEC 60947-4-1

Conventional thermal Current: The conventional free air thermal current Ith for motor starters in enclosures is defined as an eight hour current. It is the maximum current which can be conducted for this period of time during which no switching operations are carried out, without the need for intervention and without the

temperature limit being exceeded.

AC1 duty: It involves the switching of non inductive or slightly inductive loads eg: resistive furnaces. While running on AC1 duty, the switching device must be capable of making and breaking one times the rated operational current.

AC2 duty: It involves the switching of slip ring induction motors. AC-2 utilization category is of particular importance in the selection of contactors for use with slip ring motors in cranes. While running on AC 2 duty, the switching device must be capable of making and breaking 2.5 times the rated operational current. This in not always a realistic value as in many cases the actual dynamic loading of the contactor may be substantially lower.

Types of Movements of an Overhead Crane:

There are three main movements in an Overhead crane: Up travel, Cross travel and Long travel.

1) Up travel and the associated interlocks: This includes the Vertical motion of the hoist inorder to pickup and drop off the load. The motion of the hoist is controlled by a motor Depending on the direction of motion and whether the hoist is loaded or not, the motor will have to perform forward/reverse motoring or Forward/Reverse breaking operation.