CRANES 101: OVERHEAD CRANE BASICS

 




In industrial applications, you’ll find a wide variety of cranes, each for a specific use. Where there is a need to lift and transport a load that is beyond human capability, you may find a crane. From small to large, there are single girder & double girder, gantry cranes, jib cranes, mobile cranes, there are even floating cranes for building oil rigs in the sea. Most people are familiar with tower cranes, the machines erected on the tops of buildings used in construction, a common sight in most urban environments, but what are overhead cranes

 

An overhead crane, also known as a bridge crane, is typically found in industrial environments, and is used for manufacturing or maintenance applications to aid in the lifting, maneuvering, and unloading of materials. Steel mills, paper mills, warehouses, manufacturing companies, mining companies, transportation companies, oil and gas companies and utility companies all use overhead cranes. Because these cranes are mounted close to the ceiling and travel through the overhead space, they have been dubbed “overhead” cranes. Unlike forklifts, overhead cranes offer the advantage of not having to occupy valuable floor space that might otherwise be needed for production or storage, and are reliable where efficiency or downtime are critical factors. 




Manual crane from the late 19th century used for unloading
small loads from ships at the Port of Barcelona, Spain.
photo by Biser Todorov, Creative Commons Licence


 

What are the Components of an Overhead Crane?


Once you have invested in an overhead crane to improve worker safety and efficiency, you’ll want to familiarize yourself with the components. The electrical, mechanical, and power systems, may need to be repaired or replaced over time. A regular preventative maintenance program that monitors the wear that comes with normal use can keep you ahead of expensive repairs, or emergency breakdowns. Knowing your basic components will help you understand where your problems are, should they arise.



 


Working from the load outward:


Hoist –the mechanism that is responsible for lifting the load. Hoists are powered by electricity  or compressed air, and can be connected to the load by wire rope or chain. The load can be picked up directly using a large, metal hook, or attached to the hoist via chain, synthetic slings, wire slings or other lifting devices.

 

Trolley – the mechanism that moves the hoist along the bridge girder, in order to position the lifting hook over the load and to transport it across a distance.

 

End Trucks – the carriers for the crane bridge. End trucks are comprised of encapsulated wheels, and travel along the runway by use of motors or hand-pulled by an operator.   

 

Bridge —the main structure consisting of one or two load-bearing girders, that glides horizontally on the runway.

 

Crane Electrification – electrical circuitry used to give functionality to the crane. There are two circuits in most crane electrification systems: power and control. The power circuit provides the energy to lift loads and run motors and the controls operate the functionality of the crane, such as through hand-held push button devices directing the hoist’s movement. Controls can include radios, limit switches, overload devices, contactors, and other components.   

 

Runway Conductors – the method by which power transfers from the building to the crane. There are two main ways that cranes are powered: festoon and conductor bar. Festoon provides electrification from your building to your crane through a coated wiring that stretches and stacks as the bridge moves along the runway. The conductor bar provides electrification from your building to your crane through rigid strips of conductors and a corresponding collector attached to the bridge—similar to a city tram.

 

Runway Structure – the combination of beams, rail, and structural framework on which the crane is supported and operates.


Some distinguishing features of overhead cranes include whether it is top running or under running and the number of girders used on the bridge. These configurations indicate an adaptation to certain conditions or duty classes required by the lifting application.

 

A top-running crane has a bridge which travels on rails attached to the top of the runway beams. The primary advantage of the top-running crane is that it’s located higher in the building, avoiding potential obstructions, and allowing for a greater lifting height.


An under-running crane has a bridge which travels on the bottom flange of the runway beams. The primary advantage of the under-running crane is that it’s located slightly lower in the building, avoiding potential interferences caused by limited space above the crane.

 

Single girder cranes can be configured as both top or under-running, operating with just a single bridge beam attached to the end trucks. Properly applied these cranes can give years of service. Double girder cranes have the strongest construction, utilized for heavy duty lifting applications, or high duty class. The double girder is mainly a top running crane. The hoist is supported on the top between the two girders and the lifting rope or chain of the hoist falls in between the two girders to give more support for heavy lifts and travels.


Buyer Tips for Bridge Cranes 


When you are ready to make a purchase of a crane system for your business, make sure you select the proper equipment for your unique application. If you choose the lowest price crane bid without understanding the variances in equipment quoted, it may result in additional long term maintenance costs for you. Prior to compiling specifications for RFQ submittal, here are some basic points to consider:

 

  1. What type of service will the overhead crane be used for?
  2. What is the frequency (or how often will the bridge crane be used)?
  3. Will the speed of the bridge, trolley and hoist match your production requirements?
  4. Does your budget match your desired specifications?

 

Long-term planning considerations:


1.     What are the long-term maintenance costs associated with less expensive crane systems and components, should you choose them?

2.     Are replacement motors and other parts readily available for your equipment from multiple suppliers or just one?

3.     Does your crane provider meet your company’s minimum liability insurance requirements?

4.     Does your crane provider have engineering and manufacturing capabilities or do they outsource these services?

5.     Are the replacement components readily available, to eliminate potential long lead times and increased costs?

6.     Does your crane provider have their own installation team or do they outsource installation?

7.     Not having a qualified crane company install the runway? Make sure the runway is installed to meet the CMAA runway tolerance specifications. An improperly installed runway will either delay installation of the bridge crane or create future problems and add additional costs.

8.     Are you considering whether or not you will be able to increase the capacity of the crane or add a 2ndbridge in the future? If so, verify if the runway beam size and rail quoted is adequate for future upgrades.


Resources for novice overhead crane buyers:


Government agencies like OSHA, the Occupational Safety and Health Administration, US Department of Labor and industry associations like ASMEthe American Society of Mechanical Engineers and the CMAA, Crane Manufacturers Association of America all provide overhead crane safety and design guidance. Look to OSHA 1910.179 for regulations for overhead cranes and ASME B30 for safety standards as they apply to material handling equipment, cranes, hoists, rigging equipment and below-the-hook lifting devices. 


The CMAA is a trusted resource for information on overhead cranes. They deliver technical specifications and resources that promote safety in the design, operation, and maintenance of overhead cranes. Ace CEO, Josh Arwood is currently serving as the President of the CMAA and several Ace employees are involved in helping create new crane specifications and safety standards within the organization.


Does your crane provider adhere to all CMAA manufacturing guidelines and specifications? The CMMA offers several helpful buyer’s guides to get you started. 


Feel free to contact an Ace representative to walk you through any of these questions and considerations at (800) 733-2231. For more detailed information on bridge cranes, crane specifications and duty classes, see our bridge crane page on aceindustries.com




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