As a follow up to one of our older posts Understanding Overhead Crane Deflection and Criteria posted on December 23, 2014, I want to talk about why deflection is so important in the jib crane selection process. To recap, crane deflection is the amount of vertical or horizontal displacement of a member or part of an overhead crane. To understand deflection, picture a diving board with a person at the end of it. As the person gets closer to the end, the board deflects more. Cranes also deflect when there is a load on them.
The first question you might ask yourself is how do you measure deflection? Generally speaking, manufacturers design their cranes to deflect a certain amount depending on the type of crane and standard they are designed to meet. Spanco’s deflection guidelines for jib cranes are L/150 for all series except the Wall-Mounted Tie-Rod Supported (301 Series), which is designed to L/600 at midspan, and the Wall-Mounted Workstation (501 Series), which is designed to L/225. L stands for the length in inches of the crane member that is deflecting and is divided by the manufacturer’s specified allowable rate. If you have a jib crane with an 8’ boom and the manufacturer allows L/150, your measured maximum deflection at 100% of the crane’s rated capacity should be approximately 0.64” ([8 x 12]/150 = 0.64). Likewise, a crane designed to L/100 would have approximately 0.96” of maximum deflection. That would mean about 50% more deflection, and unlike the example of the diving board, steel booms are only meant to deflect a certain amount over time before there could be a catastrophic failure. Crane deflection calculation does not take the existing building or mounting structure into consideration.
Now deflection gets tricky with jib cranes because there is no published national standard for allowable deflection. Manufacturers can design to whatever deflection guidelines they want based on their own level of risk tolerance. This fact is extremely important to understand because manufacturer X and manufacturer Y can both make what appears to be the same type jib crane, but they may be designed based on completely different allowable deflections. Deflection is one of the most critical, yet overlooked, characteristics a buyer should consider when purchasing a jib crane.
As an end-user, purchaser, or crane dealer, it is important to make sure that you are comparing capacity and deflection when selecting a jib crane. If brand X’s jib crane deflects more than brand Y’s jib crane, brand X’s jib crane will be harder to operate. From an ergonomic stand point, higher deflection means that the user must push the load uphill more, leading to a risk of premature fatigue. From a safety stand point, if the deflection is higher, it could make the load roll on its own from one end of the boom to the other and possibly strike something or, even worse, someone. In fact, fatalities due to crushing or being struck are a big OSHA concern. The final point to review is price. One jib crane may appear to be cheaper, but it will probably have a higher deflection rate. Some may find the decrease in expense to be enticing; however, the cheaper jib crane may be harder to operate. Once installed, typically, a motorized trolley is then needed to push the load uphill, which is an added expense likely not needed on a higher quality jib crane with less deflection.
Each application is different. I always urge purchasers and end users to call manufacturers and ask them for all technical information, especially deflection, to get the best system for their project. Sometimes what appears to be less expensive can end up costing you and your organization in terms of efficiency and safety.
About the Author
Tim Bambrick holds a degree in Mechanical Engineering from Thaddeus Stevens College and has spent 15 years in the material handling, fall protection, and safety industries. Tim has extensive industry experience ranging from working as an Engineer to working in a product development role for multiple companies. Tim is currently the Regional Sales Manager for the Mid-Atlantic and Eastern Canada territory for both Spanco and Rigid Lifelines.