Bumper manufacturers are seeing an alarming trend in which many overhead and gantry cranes are being designed and built according to CMAA/OSHA standards or other regulations rather than to the more conservative AIST Technical Report No. 6, also known as TR#6. AIST Technical Report No. 6 is a consensus standard developed by leading industry experts for the specification of overhead crane design. Conceived primarily for steel mill cranes, AIST Technical Report No. 6 is being more widely used as crane owners realize the full level of protection that bumpers designed to this standard can provide. When a crane is installed, it is load tested to be capable of making a full capacity lift, and it is typically able to travel at a full rated speed. However, cranes delivered with bumpers that only meet CMAA standards do not absorb or dissipate the total energy of a full speed/full load impact.
Building designers and crane builders rely on knowing the actual forces imparted as a result of a crane stop. CMAA and other standards only account for a portion of these forces. What happens to the remainder of the energy of impact? If energy is not absorbed or dissipated by the correct bumper, it is absorbed by the crane or building structure.
Buying a CMAA-rated bumper is often seen as a way to save money, but the cost to properly apply, install, and maintain adequate bumpers is only a fraction of the potential cost that could result from a destructive or fatal crane stop or collision. A CMAA/OSHA-rated bumper will likely be destroyed by a full speed impact.
The ideal bumper system will bring a moving object to rest without damage to itself or the moving object, and it will keep load swing and shock to a minimum. A bumper that is built to AIST Technical Report No. 6 is like an insurance policy for crane owners—despite the higher initial cost, owners should look at it as an investment vs. the chance of failure with a cheaper component.
Examining the differences
What are some of the technical differences between AIST and CMAA/OSHA crane bumpers? AIST-compliant bridge bumpers have a maximum deceleration rate that shall not exceed 16 fps² at 50 percent speed and must be rated to handle 100 percent of the energy of impact at full rated speed. By contrast, bumpers built to CMAA/OSHA standards have a maximum deceleration rate that shall not exceed an average 3 fps² at 20 percent speed. They shall have energy absorbing or dissipating capacity to stop the crane when traveling with power off at a speed of at least 40 percent of the rated load speed. Note there is no mention of the percentage of energy that a CMAA/OSHA-rated bumper must absorb or dissipate. Physically, CMAA/OSHA-rated bumpers can be less than half the size of AIST bumpers for the same crane.
The difference between AIST and CMAA deceleration requirements is more significant due to the higher operating speeds of steel mill cranes and the fact that many mill cranes are still cab operated. The shock of deceleration is either lessened by the bumper, or it is absorbed by the building and the crane and felt by the operator.
Note that collision avoidance systems, limit switches, and other electrical or mechanical means may not be fail safe. The bumper is the last line of defense to protect people, equipment, and the product.
When selecting bumpers, building designers or trolley builders need to know the exact end force on the building or trolley end stops resulting from a crane, trolley or transfer car impact in order to properly design the structure. In an existing building, be aware of limitations in the actual structure.
Finding the right bumper
Using AIST Technical Report No. 6 to size and apply crane bumpers is the best way to ensure a strong defense against impact damage. The crane does not care which standard you use to calculate the energy, but you should.
When selecting a bumper, provide the bumper designer and manufacturer the necessary criteria to make a proper choice. This includes the:
Other factors that you should consider supplying to the bumper designer and manufacturer include:
When installing bumpers, consider mounting them at the end stops rather than on the crane. This will make it easier to inspect, maintain, or repair because the crane may not need to be taken out of service. End-stop mounting also allows flexibility in mounting arrangements, and any leaks can be found easily in one location.
The First Law of Thermodynamics explains that energy cannot be created or destroyed. It is important for crane owners to know that the energy of impact is either absorbed or dissipated by the correct bumper, or it is absorbed by the crane or building structure. For this reason, bridges, trolleys, transfer cars, and other machines that depend on bumpers should use AIST Technical Report No. 6 to size the bumpers based on the application details—and not on the initial cost.
Lift & Access is part of the Catalyst Communications Network publication family.
