Overhead Crane

[edit] Introduction

An overhead crane, also known as a bridge crane or overhead travelling crane, is a lifting device used for handling materials in factories, workshops, warehouses, power stations and material yards. It takes its name from its bridge-like structure, which spans the working area and travels along elevated runway rails, typically mounted on runway beams supported by the building columns or by a separate independent structure. Overhead cranes are widely used in manufacturing, steel production and metallurgy, power generation, mining, ports, railways and construction, and are an essential part of many industrial buildings.

[edit] Components and operation

An overhead crane mainly consists of a bridge made up of one or two main girders, end carriages containing the travel wheels, a trolley (or crab) carrying the hoisting mechanism, long travel and cross travel drives, and an electrical power supply and control system. Power is usually supplied to the crane through conductor bars or festoon cables.

The crane handles loads in three dimensions: the bridge moves longitudinally along the runway (long travel), the trolley moves laterally along the bridge (cross travel), and the hoist raises and lowers the load vertically. This allows a load to be lifted and placed anywhere within the rectangular area covered by the crane.

[edit] Types

The main types of overhead crane include:

• Single girder cranes: with the hoist usually running on the bottom flange of a single girder, used for lighter loads and shorter spans.
• Double girder cranes: with the trolley running on rails on top of two girders, used for heavier loads, longer spans and greater hook heights.
• Top-running cranes: in which the end carriages run on rails mounted on top of the runway beams.
• Underslung (underhung) cranes: in which the end carriages run on the bottom flanges of the runway beams, often suspended from the roof structure, which avoids the need for runway columns.

Gantry and semi-gantry cranes are related types in which one or both ends of the bridge are supported on legs running on rails at ground level, rather than on elevated runways.

[edit] Selection and classification

The selection of an overhead crane depends on its rated capacity, span, height of lift, duty classification, speeds, method of control and operating environment. Capacities range from less than 1 tonne to several hundred tonnes. The span is determined by the width of the building or bay, and the height of lift by the clear height available beneath the crane and the size of the loads to be handled.

Cranes are classified according to how frequently they are used and the proportion of their rated capacity that they typically lift. Under ISO 4301-1 and the European crane standards, overall crane classification ranges from A1 (light, infrequent use) to A8 (heavy, continuous use), with separate classes for individual mechanisms. The classification determines the design life and fatigue performance of the structure and mechanisms. In Europe, the design of bridge and gantry cranes is covered by BS EN 15011, together with the BS EN 13001 series.

There are three main methods of control. Pendant or floor control, in which the operator walks with the load using a pendant control station, is suitable for lower capacities and less frequent use. Cab control, with the operator in a cab attached to the crane, is suited to heavy-duty, high-frequency operation and to situations where a clear view of the load from the floor is not possible. Radio remote control allows the operator to choose a safe position with a good view of the load and is often used in hazardous or harsh environments. Some cranes are also automated or semi-automated.

[edit] Building design considerations

Overhead cranes have a significant effect on the design of the building in which they are installed. Crane loads, including vertical wheel loads, horizontal surge forces from trolley and bridge movement, and longitudinal braking forces, must be allowed for in the design of the runway beams, columns, bracing and foundations, and fatigue must be considered for frequently used cranes. Runway beams must meet strict deflection and alignment tolerances to allow the crane to travel smoothly and to avoid excessive wear on the wheels and rails. In steel portal frame buildings, crane runways are often supported on brackets attached to the columns, or on separate crane columns for heavier cranes.

Adequate clearances must be provided between the crane and the building structure, services and lighting, and safe access must be provided for inspection and maintenance. End stops and buffers are required at the ends of the runway.

[edit] Safety, inspection and testing

In the UK, overhead cranes are subject to the Lifting Operations and Lifting Equipment Regulations 1998 (LOLER) and the Provision and Use of Work Equipment Regulations 1998 (PUWER), and guidance on safe use is provided by the BS 7121 series. Lifting operations must be planned by a competent person, appropriately supervised and carried out safely. Cranes must be thoroughly examined by a competent person before first use, after installation at a new location, and at regular intervals, normally at least every 12 months, or every 6 months where they are used to lift people, and lifting accessories at least every 6 months, unless a written examination scheme specifies otherwise.

Before each period of use, the operator should carry out pre-use checks, including the operation of controls, limit switches and brakes, and the condition of wire ropes, chains, hooks and safety catches. Wire ropes should be discarded when broken wires, wear, corrosion or deformation reach the limits set by the manufacturer or relevant standards. During operation, the crane should be started, moved and stopped smoothly to avoid load swing, and the rated capacity must never be exceeded. Regular planned maintenance, including lubrication, adjustment and replacement of worn components, is essential for safe and reliable long-term operation.

Installation and dismantling must be carried out by competent personnel in accordance with the manufacturer's instructions and relevant safety requirements. Before being put into service, the crane should be commissioned and tested, typically including no-load tests of all motions and limit switches, a static overload test (commonly at 125% of rated capacity) and a dynamic overload test (commonly at 110% of rated capacity), to confirm that it operates correctly and meets its design requirements.

[edit] Related articles on Designing Buildings

• Design considerations for steel-framed warehouses
• Factory
• Gantry
• Hoist
• Industrial building
• Lifting Operations and Lifting Equipment Regulations (LOLER)
• Portal frame
• Provision and Use of Work Equipment Regulations PUWER
• Steel frame
• Types of crane