Updating crane systems for optimal performance
This article offers several tips on how to update the crane bridge system by replacing components such as the hoist, controls, and electrification.
|Plant Engineering – December 2000
An overhead crane bridge can lead a long, productive life. For example, an electric crane built for the Cheyenne Railroad in 1884 is still in operation. Longevity of the overhead crane bridge is extremely important because it is often the most expensive part of the system.
This article offers several tips on how to update the crane bridge system by replacing components, such as the hoist, controls, and electrification.
When to update
Process machinery is becoming increasingly faster, while many end products are getting heavier. Perhaps the 3-ton steel coils produced in the past now weigh up to 5 tons because of production changes. Rather than replacing the whole crane system to handle the load increase, it may be possible to install a higher capacity hoist and upgrade the bridge.
Faster throughput and other changes in production flow also affect system requirements, such as speed and duty cycle. The hoist may now need to lift the prescribed load once every 5 min, rather than once every 60 min. Weight of the load, number of times the hoist works per hour, and number of hours or shifts per day all contribute to determining equipment needs.
Upgrading to a hoist with a higher service capacity and faster hoisting speed that can handle more cycles per hour effectively improves the whole overhead lifting system.
Consider a technology upgrade
While older magnetic contactor controls abruptly switched power to the hoist on and off buffered by resistors, newer products employ variable frequency drives that ease power to the hoist motor through programmed acceleration and deceleration ( see illustration ). There is no surge of electricity so the motor runs cooler, lasts longer, and experiences fewer problems. This kind of control also avoids shock loading to the drive train and gives better load control, because hoist speed is ramped up slowly and smoothly and loads are positioned more precisely with less swing.
Typically, older hoists are controlled by a pushbutton station that hangs from the hoist or bridge. Some crane systems employ cab controls, with the operator sitting in an overhead compartment that rides along with the crane. Obviously, these types of controls limit the operator’s mobility. Radio systems where the operator has the transmitter in hand and can control all crane motions from almost anywhere in the bay may be a better choice.
It might also be wise to modernize with a festoon cable system. Some old cranes use bare copper wires or steel angles to carry power to the hoist and trolley. (OSHA now prohibits exposed high-voltage systems like these on new equipment.) Festoon cables provide a safer, more consistent power supply.
Specifying a new hoist
Here is a checklist of questions the specifications should answer, along with some examples of how the answers might influence the type of hoist chosen. Giving each manufacturer the same data helps evaluate competitive bids.