Knowledge
Generally, the faster a crane component operates, the more important alignment is. Yet there’s more to this crucial subject than that, writes Richard Phillips, mechanical engineer at Casper, Phillips & Associates.
W n Richard Phillips.
orking for a company that offers a wide variety of services, including procurement, specification, design,
manufacturing review, modification and accident investigation, I hear a lot about machinery alignment. But it isn’t always widely understood in the crane industry.
Machinery alignment can refer to any mechanical component that is part of a system that involves motion. In very broad terms, alignment means that shaft centre lines are collinear or parallel; rails are straight, and wheels are all pointed in the same direction. As it pertains to overhead cranes and hoists, the two major areas for consideration are rotating machinery alignment and wheel/rail alignment on the bridge/trolley travel systems. The consequences of misalignment can be
disastrous. The worst-case scenario is a gearbox failure on a hoist when the crane is not equipped with an emergency brake on the drum. In the event of a gear failure, the motor brake cannot transmit
torque to the drum. This is essentially a single point of failure system, and the load will be dropped. The Crane Manufacturers Association of America (CMAA), a product group within Material Handling Industry (MHI), publishes alignment tolerances for crane rails. Most other design codes are clear on these requirements too, including the European Materials Handling Federation (FEM), which provides comprehensive guidance on alignment of various components, such as gearboxes and couplings. As CMAA, FEM, and other authorities would agree, machinery alignment is important in all cranes that are regularly used. Some alignment requirements are based on crane dimensions – the longer the span, the larger the alignment tolerance becomes, or a larger misalignment is allowed – but, as a rule of thumb, the faster a crane component operates, the more important alignment is. A shaft that spins quickly is going to have more strict alignment requirements than a hoist drum that spins as comparatively slow as 30rpm. Speed
n This was caused by poor rail alignment. The rail gauge increased too much at a certain point and the trolley wheels pinched the rail (wanted to pull the rail in).
f The trolley wheel flange started grinding against a rail and wearing. This can be a symptom of wheel misalignment and/or rail misalignment.
www.ochmagazine.com | Fall 2023 | 33
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