COMPONENTS OF BRIDGE CRANES
The function of a crane is to move the lifted load horizontally and longitudinally in the building. The lifted load is usually supported with a hook which is cabled to a hoist. The hoist is supported by a trolley which moves horizontally along the crane bridge. The crane bridge is connected to a number of crane trucks at each end depending on the capacity and span. The crane trucks can have 2, 4 or 8 wheels based on the capacity of the crane. The wheels ride along a crane rail which is supported by runway beams. The figure below illustrates the basic crane components;
1. Bridge - The Bridge is the principal structural component of an overhead crane. It spans the width of the building and comprises one or more load bearing beams or girders. These may be fabricated steel box-girders or rolled-steel joists. The bridge carries the hoist trolley, which travels along the length of the girders during operation.
2. Runway - The track and support system on which the crane operates. The runway girders are usually considered a part of the building structure and are designed accordingly.
3. Runway Rail - The rail supported by the runway beams on which the crane travels.
4. End trucks - Located on either side of the bridge, the end trucks house the wheels on which the entire crane travels. It is an assembly consisting of structural members, wheels, bearings, axles, etc., which supports the bridge girder(s) or the trolley cross member(s). Electric drive motors typically two-speed or variable-speed units power the wheels and move the crane into the required position. Brakes are mounted on the drive motors and are essential to prevent uncontrolled loads becoming dangerous, and are often electrically operated. Electrical limit switches cut power to the drive motors and prevent the crane from colliding with the building structure at the end of the travel range.
5. Electric rope 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. The hoist mechanism is mounted to the trolley.
6. Trolley or Crab - The ‘crab’ is the ‘cross travel unit’ from which the hook is lowered and raised. A top-running trolley on a double girder crane runs on rails fitted to the top of the crane bridge. An underhung trolley on a single-girder crane runs on the bottom flange of the crane beam, with drive units connected directly to the trolley. The trolley carries the electric wire rope hoist that supports the load block and hook through a system of pulleys. A variable-speed AC motor on the hoist drives the load up or down. Limit switches prevent the load block from colliding with the trolley.
7. Bumper (Buffer) - An energy absorbing device intended 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 may be attached to the bridge, trolley or runway stop.
8. Controls - Controls for an EOT crane are usually mounted in an operator pendant or remote console and comprise various push buttons and switches that operate relays and contactors mounted on the crane. Drive motors and the hoist motor draw substantial currents during operation and require appropriately rated contactors to switch them on and off. Variable frequency inverters provide speed control for motors where accurate positioning is essential. A master contactor is triggered by a main switch and cuts off all power to the crane if a dangerous situation occurs.