Look for custom cranes
August 1st 2010

Overhead cranes are an investment that lasts. It is vitally important, therefore, to have the right crane for the job says Andrew Pimblett, md of Street Crane Company

Not all overhead cranes are the same. There are standard cranes, but very few standard applications. Overhead cranes should be customised for the demands placed on them and considered as an integral part of the logistics process if they are to be effective at the outset and reliable in the long run.

Look carefully at the warehouse throughput – both now and into the future. This will help in assessing the speeds required in long travel, cross travel and hoisting. Consider also the maximum number of daily crane movements and proportion of lifts and transfers at the upper end of the crane's safe working load (SWL).

This analysis allows the mechanisms in the crane are appropriately rated. An over rated crane will cost more at the outset, but an under rated crane may prove troublesome and more costly in repairs and downtime.

There are a number of industries, steel and glass, for example, where the distribution chain has a significant warehousing requirement between the producer and the eventual consumers.

These intermediates ensure local availability, break bulk and perform basic operations such as converting material to the sizes that end users require.

In the glass industry major manufacturers like Pilkington and StGobain have warehouses for the bulk storage of product from their continuous processes. Bulk glass storage is generally in a pack where several large sheets are stored in a frame (see picture above right). Handling devices allow these packs to be safely transported and stored in the warehouse.

Further down the supply chain glass is frequently handled in sheets of various sizes. In these applications vacuum lifting devices are more commonly used for secure lifting and transportation and to avoid harsh handling of the product.

Throughout the glass supply chain, cranes are specified with close control and slow speed final placement of the product. Typically cranes in the glass industry have medium to high duty ratings.

Steel may leave the mill as plate, coil or rolled section. In each case there are dedicated devices that safely and efficiently handle the load. Plate may be handled by vacuum lift or more usually by a magnetic device. Grabs are available for handling coil while structural sections are most commonly handled using convention hooks with strategically placed slings for load stability. In this aluminium warehouse (see picture below left) the cranes have been engineered to place loads precisely without damaging the foil. Typically cranes in the steel industry will have high to medium duty ratings.

Safety considerations Safety is a vital consideration in specifying any crane. Even though cranes are engineered with a margin of safety, it is important not to exceed this. Capacity restrictors (overload cut out devices) are mandatory in EU countries.

Load stability is another consideration. Is the load small and compact or large and cumbersome? Small compact loads can be easily handled by a single hoist, but it may be worthwhile with more cumbersome loads to consider twin hoist lifting for additional stability.

Crane speed is another factor. If a crane with load accelerates rapidly the load will initially create drag, but if the crane is then stopped, the load may swing dangerously. In both cases this could lead to slippage and additional dynamic loads are placed on the crane structure. These effects can be largely eliminated by the use of appropriate control technology to control acceleration and deceleration such that drag and over swing are virtually eliminated.

There are also a whole range of safety accessories that can be installed. Lights and audible alarms warn of the crane approach.

Floodlights are useful on yard cranes and in sparsely lit warehouses. Collision prevention systems are used where two or more cranes share the same track.

Modern cranes are designed to provide long term service with reliability. However all mechanical devices require periodic servicing. This can be more rationally planned where hours of use meters are in use to indicate the optimum time for preventative maintenance.We recommend that maintenance is conducted on a planned basis by a reputable company.

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