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Success Stories:Engineered Systems

 

Crane Anti-Skew & Positioning Control

ApplicationOverhead cranes frequently skew due to poor railconditions, uneven wheel wear, wheel slippage or

unequal load conditions when the trolley is operatingat one end of the crane bridge. Crane skew is evenmore of a concern on long span bridge cranes andgantry cranes.

ProblemSkewing of the crane can cause excessive wheelwear and stress, especially to the wheel flanges. Itcan also produce horizontal forces at right angles tothe rail which can result in unusual stresses to thecrane runway beams and building structure.Polyurethane wheels on trackless

Long Span Crane & Gantry Crane System

gantry cranes tend to wear more rapidly than steel wheels. This often results in differing diameters of drivewheels, which subsequently cause the crane to skew. Not only does crane skew inhibit the smooth operationof a bridge crane, it also makes it more difficult to spot loads accurately in both manually operated andautomated applications.

SolutionMechanical methods used to reduce skew in the past, include:

increasing the crane end truck wheel base decreasing the clearance between the wheel flange and rail adding side guide rollers

An improved method for reducing crane skew is to use a combination of low slip motors and drives on eachend truck, with separate Electromotive IMPULSE® Adjustable Frequency Crane Controls, providingcontrolled speed and torque. This also helps prevent wheel and rail wear. For example, if the east end of thecrane bridge is detected to be traveling ahead of the west end, the east end motor will be driven slower toallow the west end to catch up.

The amount of allowable skew is dependent upon the application requirements. Using the above methodsindividually or combined improves crane alignment, thus reducing or eliminating crane skew, reducing wheelflange wear, and improving load positioning capabilities.

Technical DescriptionMechanical Crane Skew is shown on the following diagram.

View a larger version of the diagram. 

Several methods are available to control crane skew:

1. If the crane is equipped with independent motor drives on each end truck, the motors will 'slip'resulting in improved alignment keeping the crane in line. With crowned or special wheelconfigurations, the crane tends to be self-centering. Using separate low slip motors andElectromotive IMPULSE® Adjustable Frequency Crane Controls, helps reduce the results of craneskew. In these cases, the amount of skew is still dependent upon the mechanical clearance between

the wheel flange or guide roller and the rail. Although wheel and rail wear is reduced, spottingaccuracy is still affected.

2. On those crane systems, where spotting accuracy is critical, laser distance sensors combined with aPLC (programmable logic controller) and IMPULSE® Adjustable Frequency Drives can be used toprovide a more positive method of controlling skew. The distance sensors are connected to the PLCwhich in turn controls individual IMPULSE® Adjustable Frequency Crane Controls controllingindividual motors on each end truck. The PLC automatically adjusts the output signals to each AFDfor any distance deviations detected between sensors. This method of skew control often eliminatesskew during travel and at final positioning.

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Page 1 of 1Magnetek Material Handling - Success Stories - Crane Anti-Skew & Positioning Control

5/14/2010http://www.magnetekmh.com/solutions_engineeredsys-craneantiskew.htm