CMAA & AISC Crane Runway Rail Tolerance
It’s something that a lot of crane owners overlook, but it’s much more important than you may understand. A properly aligned crane is imperative to meet the performance requirements outlined by industry standards, and poor alignment can cause a chain reaction of issues, ultimately resulting in unnecessary wear-and-tear and unplanned downtime.
It’s the end user’s responsibility to ensure that crane runway tolerances are within the requirements outlined by CMAA. There are several factors that are essential to ensure your runway’s proper alignment: elevation levels, spans, straightness, and the overall condition of the runways.
WHY IS POOR ALIGNMENT SUCH A BIG ISSUE?
Poor runway alignment can cause crane racking, skewing, or binding. It can result in excessive stress to the runway beams and your building’s supporting structure. Most commonly, it causes extensive wheel wear and puts unnecessary strain on motor drives and other equipment.
Often times, companies will notice extensive wheel wear and change out their wheels rather than checking their runway alignment. It’s quite possible that if your trolley is experiencing extensive wheel wear, it’s due to a poorly aligned runway.
CMAA & AISC Requirements
The Crane Manufacturers Association of America’s (CMAA’s) specification #70 has eight different metrics for evaluating crane runways. These metrics are as follows:
1) Elevation of rails: ±3/8” above or below an assumed reference elevation
2) The maximum rate of elevation change: ¼” in 20’-0”
3) Rail to rail elevation: ±¼” within the nominal average span length
4) The maximum rate of change of rail to rail elevation: ¼” in 20’-0”
5) Straightness of rail: ± 3/8” on either side of an assumed rail centerline
6) The maximum rate of change of rail straightness: ¼” in 20’-0”
7) Variation in crane span: ±¼” difference relative to the nominal crane span
8) The maximum rate of change of variation in crane span: ¼” in 20’-0”
Additionally, AISC design guide #7, Industrial Buildings—Roofs to Anchor Rods, Chapter 19 indicates that the “Crane rails shall be centered on the centerline of the runway girders. The maximum eccentricity of the center to rail to centerline of girder shall be three-quarters of the girder web thickness”. While this quantity varies with girder size, the typical allowable value for the bulk of these runways is 3/8”.
If you are concerned with the condition of your runway and/or runway structure; give one of our experts at CRANE 1 a call and we will set up a consultation to discuss the specifics of a runway alignment survey and evaluation.