How significant are capacity ratings on bridge cranes? The capacity rating is part of every inspector’s checklist, but are we taking these for granted?
The Standards and Regulations state that bridge girders must have a capacity sign on each side of the bridge girder and in the case of dual trolley hoists each load block must include the capacity of each trolley/hoist.
Sounds simple, however it wasn’t long ago we were asked to provide pricing on proof testing a number of cranes. The customer provided us with a listing of cranes. One of the listed cranes had a capacity of 40 tons. Having visited the customer’s site a few years back we couldn’t remember a 40 ton bridge crane being at this site. We decided to revisit the site. What the customer described as a 40 ton crane was actual labeled 20/20 ton which had two 20 ton top riding trolley/hoists. Each load block was labeled 20 ton. We questioned the customer about the 40 ton rating. It didn’t look like a 40 ton crane. The
customer was very adamant about the capacity and said this unit has always been proof tested at 40 ton by others.
We still had doubts and upon leaving we recorded the manufacturer’s serial number. You guessed it, the manufacturer confirmed our suspicions. The bridge
capacity rating turned out to be 20 tons. We asked why they would label the bridge 20/20 ton. According to the manufacturer the customer wanted the capability of lifting a single pick on one hoist of 20 ton or spread a large load out of 20 ton between both hoists. Of course we asked the manufacturer why they didn’t just label the bridge girders at 20 ton. They claim “this is the way they have always done it and it is accepted practice”. When we
told them that this crane had been proof tested in the past at better than 200% of rating, their answer was “when the crane was purchased everyone was aware of the correct capacity”. That was 30 years ago.
Another problem in the industry is the fact that when a manufacturer designs a crane with two trolley hoists the design may take into account the wheel loads of eight wheels. When they do this the capacity load on the girder is spread out, this allows the manufacturer to design the crane using lighter girders. The problem with this is if the girder uses dual or single capacity signs, doesn’t mean that the girder will handle the capacity load from a four
wheel trolley.
The biggest fear we have is a customer removing twin trolley hoists and adding one trolley hoist at a total capacity of the two removed. We all know that years ago steel pricing was a lower percentage of over all cost in the manufacturing process. However steel prices today have become a very significant cost. Overhead cranes designed today seem to have become much lighter in regards to steel used.
In summary owners/users of overhead cranes must maintain manuals that include design specifications on bridge girder loadings and be available for review when inspections and proof testing is performed. Hopefully someday this would be incorporated into the Standards and Regulations.
Richard is a friend and colleague at the Crane Certification Association of America and General Manager of Advanced Overhead Crane.
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