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. 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 maximum (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 impacts any part of the crane that stands vertically, including the mast, columns, wall, etc.
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 safety standards, the deflection is measured at 100 percent capacity, rather than 125 percent capacity. According to ANSI Standards (ANSI/ASME B30.2) for Overhead Travelling Cranes Operational and Running Tests: “Standard deflection must be measured with a load of 100 percent of the rated capacity and must not exceed the allowable deflection specified by the applicable design standard.” 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 a deflection limit of L/450. The letter “L” literally stands for the length of the crane, or span. In other words, to determine the deflection of your workstation bridge crane, you must first know its span or length. This equation is always measured in inches. That’s because if your deflection is higher than a value measured in inches, you’re in big trouble. Deflection should be very minor. To measure your deflection, use the deflection limit defined for that type of crane and divide that number by the length (or span) of your specific system. If your bridge length is 34 feet, you will divide the deflection limit defined by your manufacturer (L/450 for workstation bridge cranes).
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 limit for enclosed bridge cranes (L/450). That will give you a deflection of less than 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 a deflection limit of L/450, as specified above. However, when determining the deflection for a gantry crane, the deflection limit is L/600 for steel gantries, and L/450 for aluminum gantries. Steel will almost always have a slightly higher deflection limit due to its rigidity compared to that of aluminum. To determine the deflection 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 limit 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 limit specified by the manufacturer. For Spanco, this number is, as specified above, L/600. A gantry crane spanning 300 inches has a 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 limits 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 limit 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 is irrelevant in this case. 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 limit. For instance, our 100 Series Freestanding Jib Crane has a deflection limit of L/150 (also known as 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 limit of L/200 (or R/200), and our 500 Series Workstation Jib Cranes have a deflection limit of L/150 OR L/225, depending on the way the crane is mounted. For Freestanding Workstation Jib Cranes, there’s a lower deflection limit. But for our 501 Series Wall-Cantilever Workstation Jib Cranes, we use a higher deflection limit 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 limit 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 maximum 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 maximum 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 limit 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 limit of 400, the horizontal deflection for that particular crane is 1.2 inches.
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 prove to 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 OSHA and ANSI the following maximum values for the deflection of the crane girder must normally not be exceeded in order to avoid undesirable dynamic effects and to secure the function of the crane:
- Vertical deflection is defined as the maximum 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 less (L/450) because the enclosed track is lighter.
- Horizontal deflection is a maximum 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 less (L/400).
In the absence of more detailed calculations, it is acceptable to assume that the top flange resists the whole horizontal force. The rigidity requirement for horizontal deflection is essential to prevent oblique traveling of the crane. The vertical deflection is normally limited to a value not greater than 25 mm to prevent excessive vibrations caused by the crane operation and crane travel.
According to OSHA and ASME, crane load 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 load tested before being put into service. The deflection test defined by OSHA and ASME suggests that structural deflections must be measured with loads of 100 percent of the rated capacity and must not exceed the allowable deflections specified by the applicable design standard (deflection limits, as listed above). OSHA and ASME also specify that the load must travel over the full length of the bridge and trolley runways during these load tests, and only runway parts that have been successfully load tested may be placed into service.
If you have any questions about deflection or the deflection limit 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.