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Crane Anti-Skew & Positioning Control


Application

Overhead cranes frequently skew due to poor rail conditions, uneven wheel wear, wheel slippage or unequal load conditions when the trolley is operating at one end of the crane bridge. Crane skew is even more of a concern on long span bridge cranes and gantry cranes.

Problem
Skewing of the crane can cause excessive wheel wear and stress, especially to the wheel flanges. It can also produce horizontal forces at right angles to the rail which can result in unusual stresses to the crane runway beams and building structure. Polyurethane wheels on trackless gantry cranes tend to wear more rapidly than steel wheels. This often results in differing diameters of drive wheels, which subsequently cause the crane to skew. Not only does crane skew inhibit the smooth operation of a bridge crane, it also makes it more difficult to spot loads accurately in both manually operated and automated applications.

Solution
Mechanical 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 each end truck, with separate Electromotive IMPULSE® Adjustable Frequency Crane Controls, providing controlled speed and torque. This also helps prevent wheel and rail wear. For example, if the east end of the crane bridge is detected to be traveling ahead of the west end, the east end motor will be driven slower to allow the west end to catch up.

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

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 wheel configurations, the crane tends to be self-centering. Using separate low slip motors and Electromotive IMPULSE® Adjustable Frequency Crane Controls, helps reduce the results of crane skew. 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, spotting accuracy is still affected.
  2. On those crane systems, where spotting accuracy is critical, laser distance sensors combined with a PLC (programmable logic controller) and IMPULSE® Adjustable Frequency Drives can be used to provide a more positive method of controlling skew. The distance sensors are connected to the PLC which in turn controls individual IMPULSE® Adjustable Frequency Crane Controls controlling individual motors on each end truck. The PLC automatically adjusts the output signals to each AFD for any distance deviations detected between sensors. This method of skew control often eliminates skew during travel and at final positioning. 
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