Understanding Overhead Crane Deflection and Criteria

Feb 22, 2024

Overall, deflection is literally defined as “deviation from an allotted course”. It can mean that something is deviated by turning aside or by coming off course. For overhead cranes, this definition translates to the relative vertical or horizontal displacement of a member or part of an overhead crane under load. So how does that impact the design of the crane, and how is the vertical and horizontal deflection calculated? And, how do organizational specifications differ from crane to crane and from association to association? Let us explain.

Vertical Deflection Criteria

Vertical Deflection Criteria is the (vertical) deflection ratio allowed for a lifting device. Vertical deflection differs from horizontal deflection, but both are taken into account for enclosed track bridge cranes. Vertical deflection refers to the change in position of a crane’s bridge, track, or boom along the vertical axis.

Most systems are manufactured to an approximate deflection, because manufacturers have no control over installation, foundation rigidity, or the standard variation in thickness tolerances for the piping, tubing, steel plate, and sheet metal. That means some variation above or below deflections defined by manufacturers should be considered normal. That being said, when overhead cranes are installed according to the standard installation manual and maintained according to the manufacturers installation maintenance manual, you can be assured of the safety of lift products and their ability to handle chosen rated capacities and performance standards.

When measuring deflection for comparison to the manufacturer or industry standard, the deflection is measured at 100 percent capacity, rather than 125 percent capacity. For each type of crane, the deflection value differs depending on the crane’s overall length, span, or reach.

For workstation (enclosed track) bridge cranes, the vertical deflection value is less than for heavier bridge cranes. Enclosed track workstation bridge cranes have an approximate deflection of L/450 where “L” the length, or span, of the crane. In other words, to determine the deflection of your workstation bridge crane, you must first know its span or length. This equation is typically measured in inches or centimeters because the total deflection should be very minor. If your deflection is significantly higher than a value measured in inches, you’re in big trouble. To measure your deflection, divide the span of your specific system (the length in inches or centimeters) by the deflection rating defined for that type of crane.

For example, the deflection for a 34-foot bridge on a workstation bridge crane can be determined by first changing the unit of measurement from feet to inches. A 34-foot bridge is 408 inches long (feet x 12= inches). Divide 408 inches by the specified deflection rating for enclosed bridge cranes (L/450). That will give you an approximate deflection of one inch (.9 inches).

Deflection is determined for other overhead crane systems using the same method. The criterion is the same, but the terminology and deflection limits differ slightly from system to system. For instance, when determining the deflection for a workstation (enclosed track) bridge crane, we would use an approximate deflection of L/450, as specified above. However, when determining the deflection for a gantry crane, the deflection rating is L/600 for steel gantries, and L/450 for aluminum gantries. Steel will almost always have a slightly lower deflection due to its rigidity compared to that of aluminum. To determine the deflection rating of an all-steel gantry with a span (beam length) of 25 feet, you would follow the same equation we used to determine the deflection for workstation bridge cranes, only using the deflection rating for steel gantry cranes. In order to do so, we would first have to change the unit of measurement to inches. A span of 25 feet (multiplied by 12) is 300 inches. Next, we would determine the deflection by dividing the span of the gantry in inches by the deflection rating specified by the manufacturer. For Spanco, this number is, as specified above, L/600. A gantry crane spanning 300 inches has an approximate deflection of a half an inch (.5 inches).

To determine the deflection for a jib crane, the equation remains the same, but the terminology can also differ from system to system, and the deflection ratings differ once again. Some manufacturers will refer to the length or span of the jib as the “reach”. That’s why you might see a deflection rating for jib cranes referred to using the letter “R” rather than the letter “L”. It depends on your manufacturer. But, for the purpose of determining your deflection, keep in mind that “R” simply stands for “Reach” and “L” stands for “Length”. Whether it’s referred to as the reach, the span, or the length, the deflection equation still remains the same. When determining the deflection for jib cranes, the type of jib in question is also an important factor.

At Spanco, we have five different jib crane series, and each series has several installation types that impact the deflection rating. For instance, our 100 Series Freestanding Jib Crane has a deflection rating of L/150 (or R/150). That number is the same for our 200 Series Mast-Style Jib Cranes and our 300 Series Wall-Mounted Jib Cranes. However, our 400 Series Articulating Jib Cranes have a deflection rating of L/200 (or R/200), and our 500 Series Workstation Jib Cranes have a deflection rating of L/150 OR L/225, depending on the way the crane is mounted. For Freestanding Workstation Jib Cranes, there’s a higher deflection rating. But for our 501 Series Wall-Cantilever Workstation Jib Cranes, we use a lower deflection rating of L/225 due to its wall-mount. For a 501 Series Wall-Cantilever Workstation Jib Crane, the deflection for a crane with a 12-foot span can be calculated similarly to the abovementioned systems. First, we would need to change our unit of measurement to inches. In this case, a 12-foot span (or reach) is equivalent to 144 inches. If we divide 144 inches by the deflection rating of 225, we get a deflection of a little more than a half an inch (.64 inches).

Horizontal Deflection Criteria

Horizontal Deflection Criteria is the deflection ratio allowed for a bridge crane or runway. Horizontal deflection, unlike vertical deflection, impacts parts of the crane that run horizontally. This is taken into account for enclosed track systems, including workstation bridge cranes and workstation jib booms.

Most manufacturers design for a lateral deflection for runways and cranes of L/400.  The “L” in this case refers to the span of the bridge crane from the runway support center. That number is divided by the deflection rating of 400. For example, to determine the deflection for a ceiling-mounted workstation bridge crane with bridge length of 40 feet, we must first translate the unit of measurement to inches. A 40-foot span is equivalent to 480 inches. If you divide 480 inches by the specified deflection rating of 400, the horizontal deflection for that particular crane is approximately 1.2 inches.

Rigidity Requirements

It’s crucial for manufacturers to conform to theoretical considerations and to subject their systems to a variety of checks like stress analyses and horizontal and vertical deflection analyses of bridges, beams, masts, columns, and other parts. These systems should conform to theory and their static structural response must preserve the response of the original crane structure in order to pass these tests.

According to rigidity requirements laid out by CMAAS (Crane Manufacturers Association of America), the following are the values for the deflection ratings of the crane girder:

  • Vertical deflection is defined as the permissible deflection ratio allowed for a lifting device. For bridge cranes this value is usually L/700. For a Workstation Bridge Crane, the value is more (L/450) because the enclosed track is lighter.
  • Horizontal deflection is a deflection ratio allowed for a bridge crane or runway. For regular bridge cranes, this value is usually L/600. For a Workstation Bridge Crane, the value is greater (L/400).

The rigidity requirement for horizontal deflection is essential to prevent oblique traveling of the crane.

Testing Requirements

According to OSHA and ASME, crane static strength tests are typically specified at 125 percent of the crane’s rated capacity. Neither standard, however, specifies an acceptable tolerance over or under the 125 percent figure. ASME B30.2 does in fact reference a figure in its interpretation of load testing, which suggests a tolerance of 0%/-4% on the weight of the test load. In effect, this suggested a test load weighing between 120 percent and 125 percent of the rated crane capacity (i.e.: 125% -125% x 0.04 = 120%).

Furthermore, any overhead crane that has been significantly modified, or that was installed after January 1999, must be static strength tested before being put into service.

If you have any questions about deflection or the deflection rating values outlined in this blog, please feel free to comment below. To ensure your deflection values are correctly calculated, it’s important to reach out to your local overhead crane distributor or a qualified engineer for more information.