Materials in Motion

Tractor Drives: Overhead Crane Motorization

Power drives—often referred to as tractor drives—provide motorized movement of overhead bridge cranes. They are used to motorize the travel of the hoist, trolley, or bridge. They are often used when loads regularly exceed 1000 pounds. These drives are also extremely useful when transporting material into hard-to-reach locations where operator movement is limited due to machinery or other obstacles. This often occurs when the operator can’t stand close enough to the crane to move it manually. Motorizing the crane allows the operator to use the system from a distance. They are also helpful during applications with high-cycle times that require frequent lifting of heavy, awkward, and bulky loads.

Historically, cranes have been powered using several methods of motorization: mechanic, hydraulic, pneumatic, or electric.

Mechanically powered systems provide motion using the force applied at the handle of the crane multiplied by a system of gears and pulleys. Mechanical power has been available for centuries, making it an economical and easy method for motorizing certain lift systems. Oftentimes, these systems are used for extremely light loads that require minimal lifting over a long period of time.  They are generally only suitable for cranes with moderate lift power and for work that doesn’t require fast lift and transfer of heavy loads.

Hydraulic power is not used often for facility cranes, although steam is often used for locomotive cranes or cranes traveling on rails. The advantage is that it is entirely self-contained, and it can travel using its own motive power without the use of electricity, gears, or air driven motors. Even so, pneumatic power has largely superseded the use of hydraulic power in manufacturing.

Today, most overhead cranes are motorized using either air power or electricity. Pneumatic (air-powered) systems carry power using compressed gas—usually air—as a fluid for transmitting energy. An air-operated hoist, for example, is fairly common for moderate-sized systems. There are clear advantages for using pneumatic powered systems, but, since the advent of electricity as a motive power for cranes, the use of electric power has far surpassed any other type of motorization available.

Pneumatic systems provide infinite availability of their own energy source, whereas outside sources of energy are required to produce electricity. Since air is the main component of pneumatic power, it is unlimited, easily accessible and channeled at any location. What’s more, air can be used flexibly at various temperatures—even in extreme conditions, and can be safer than electric power. Air power is easy to use, easy to install, and provides quick and safe transfer of power at multiple speeds.

Of course, there are also distinct disadvantages to using air-powered systems. Electric power is by far the most common source of motorizing overhead cranes in practically every industry. That’s because pneumatic systems require the installation of specific air-producing equipment, and compressed air must be well prepared in order to meet most safety requirements. It also requires the addition of necessary lubricants and other expensive equipment such as compressors, filters, dryers, and regulators.

Electric power is commonly used to motorize the travel of the hoist trolley or bridge crane. Overall, electricity is readily available, cleaner, easier to use, and even safer—in some respects. While electrocution and fire hazards are always a concern when using electric power, pneumatic and hydraulic power both pose other safety risks. Using electricity to power your crane eliminates the risk of compressed air or fluid leaks, broken pipes, and excess exhaust.

Electricity also travels over very long distances quite easily, making it available throughout your entire facility without the addition and installation of costly equipment.  Electric motorization is silent, transformable, convertible, easily stored, and moves nearly at the speed of light.

The convenience of using an electric motor to power an overhead crane is often reason enough to avoid pneumatic motorization altogether. Electric motorization also lends itself to multiple options and features, making it highly adaptable to almost any application. Enclosed track electric tractor drives provide the highest level of speed, thereby improving cycle time and overall production. Furthermore, most electric tractor drives feature standard 208 to 575V, 60Hz, 3-phase power, worm-gear-reducer drive designs, explosion proof controls, variable frequency drives, and weather-resistant upgrades. Although there may be certain instances when pneumatic power is necessary for special applications, the overall accessibility and cost-efficiency of electric power, combined with its ease of use and overall speed make it the most popular method of overhead crane motorization to date.

Kristina Harman

Technical Writer | Spanco.com

Kristina Harman is the senior technical writer and content manager for Spanco, Inc. Kristina has twelve years of experience in content development, technical communications, and copyediting. She holds a Bachelor of Science Degree in English from Towson University and a Master of Education Certification in English from Johns Hopkins University. She is a member of the Society for Technical Communication and the American Medical Writers Association.

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