Thursday, December 1, 2011
The regulations set forth by OSHA clearly says that you are not to pick up more than the rated capacity of the crane. OSHA 1910.179 N states that The crane shall not be loaded beyond its rated load except for test purposes as provided in paragraph (k) of this section. Not only can you not exceed the rated capacity of the crane, but you also have to factor in the lowest rated capacity of the device your using to lift with. If you have a below the hook device you have to subtract the weight of the device from the rated capacity of the crane. Then if the rated capacity of the device is smaller than the rated capacity of the bridge or hoist then you cannot exceed the capacity of the device.
You should never pick more than the rated capacity unless it is for a load test which will be conducted at or near 125% of the rated load. So many factors go into the design of a crane that you would have to look at the safety factor of all your components. You would have to look at the wire rope, the motors, bearings, sheaves, drums, wheels, rails, hoisting speeds, and beam and steel sizes.
Now when you try to figure out what is your cranes safety factor, you have to be careful as to whether your crane manufacturer built the crane based on CMAA standards or FEM standards. So it is important to know what the difference is between the two standards. CMAA is the Crane Manufacturers Association of America and FEM is Federal European De La Manutention. This is the standards for design and manufacturing internationally of overhead traveling cranes. FEM specifications are written based on the input from the World’s largest crane companies. Then of the CMAA specifications are written based on the input from the largest American crane companies. FEM will separate each main part of the crane (hoist, trolley, and bridge). However CMAA groups them together as one structure and machinery. I think the problem you can have with this is that with FEM you can get an under and over designing of certain components because it does not always take into account the entire crane system as CMAA does. CMAA safety factor for the wire rope is 5:1 for standard cranes, and 8:1 for molted metal cranes. However, FEM safety factor for the wire rope of 3.6:1 and up to 9:1 depending on the crane group the crane is classified in. There are 6 classes of cranes for FEM and for CMAA. These different classifications were designed to allow for the most economical crane to be specified.
FEM 1Bm 1Am 2m 3m 4m 5m
CMAA A B C D E F
These classifications are based on the usage of the crane. They are similar to each other but different enough that it can put your cranes in different classifications.
FEM Life in Hours
CMAA Life in Hours
Typically with the FEM standards you are getting a heavier duty hoist than with the CMAA standards. FEM also factors their calculations using the long ton while CMAA goes by the short ton. The difference is 220lbs per ton. This does not seem like a lot but when you are comparing a heavier capacity hoist it is a drastic difference. For example if your crane is a 30 ton hoist, CMAA standards will build the crane for 30 tons. Likewise that same hoist by FEM standards will be built for an additional 6,138lbs.
You also have to consider the deflection of your bridge beam when factoring in what your crane can pick up. When a load test is conducted on your crane deflections are measured. CMAA has an allowable deflection of live load tolerance. However FEM has no such rule. If we go by CMAA standards the standard allowable deflection is .1125% of the span of the bridge. A good rule to go by that I always used as a technician to find an acceptable tolerance is, Jib cranes tolerance is the span/150 or 450 depending upon the span. A single girder crane has an allowable tolerance of the span/603 and double girder cranes have a tolerance of the span/888. These tolerances are based on 100% of the rated capacity.
In the end the crane operator is the one who is responsible for the lift. It is the operator’s responsibility to know what the capacity of the crane and any lifting devices. If the operator has any doubts to the safety of the lift then they should not lift the load. Anytime that the lift is going to be at near capacity it is a good practice to test the brakes. The operator tests the brakes by raising the load only a few inches off of the ground and applying the brakes. There should be no drift in the hoist motion when the operator is not raising or lowering the load. This is an excellent safety measurement and should be used.
Many factors are taken into account when considering the safety factor for your crane. There are so many different parameters that get you to your true safety factor that you would have to re-engineer the entire crane to figure out what it truly is. Once true stress is calculated at its maximum on all load bearing components and the usage of the crane is calculated then the true safety factor can be calculated. The standards are guidelines to help with the design of the crane. However, they also give limits and requirements for most vital crane components. Although cranes may be built according to CMAA standards, all electric hoists are designed and rated according to FEM standards. These standards are in place to make the operator and those working around the crane safe. Be safe and never exceed the capacity of the crane. Ensure your crane operator has been properly trained and your equipment is being inspected as well as maintained with a through preventive maintenance program.