When the shaft of an electric motor turns, there’s still work to do.
The rotation of the shaft has to be transformed into enough force to move a conveyor belt, run a machine, etc. This is almost always done with an arrangement of gears attached to the shaft. The nature of those gears, and how they’re integrated with the motor, varies considerably, depending on the application and the preferences of equipment manufacturers and end users.
Perhaps the most basic distinction is between keeping the motor and gearing separate or integrating them into one unit. The latter are usually known as gearmotors. Separate gearing units attached to motors are called gearboxes, or sometimes speed reducers, due to how they transform the high rotational speed of the motor’s shaft into a slower gear rotation with higher torque.
Gearmotors have the advantage of being an all-in-one package, which is compact, cost-efficient and convenient – at least initially. However, most of that benefit goes to the equipment manufacturer, says Kyle Verploegen, business development manager for gearing at the Dodge division of ABB. The problem comes for the end user when the motor needs maintenance or replacement.
“When a motor fails, the end user wants to know that they can get a motor readily available on the shelf,” Verploegen says. But when a gearmotor fails, a replacement isn’t so available: “It’s not sitting on a shelf and it may take a week, maybe two, maybe longer, to get that replacement motor” from the manufacturer through an authorized agent or distributor, as it needs to have the correct pinion gear installed first, he says.
Some gearbox manufacturers tout ease of replacement for their motors. “We’ve created our gearmotor to be different than its common definition in the market,” says Lauren Forseth, an applications engineering manager at Rexnord. “Since the gearbox typically lasts longer than the motor, our design allows for the motor to be easily replaced in application. Also, because we have an integral motor bushing and no alignment requirement, the motor can be mounted in only a few minutes.”
Advantages of gearmotors include compactness, ease of installation and general ease of maintenance. The latter is mostly because some gearmotors are sealed and permanently lubricated with grease that never needs changing.
Brother International Corp. supplies such motors, says applications engineer Tom Colacino: “Brother gearmotors use grease as a lubricant that never needs to be replaced, so long as the gearmotor is used within its rated range.” Maintenance issues that do need attention, Colacino says, include dirt and dust accumulation on the motor casing or fins, corrosion of the gearmotor coating and uncoated components, and loosening of fasteners due to vibrations generated while operating.
Gearmotors come in specialized forms, especially when used for material handling. Two of these are shaped to fit in as integral parts of a conveyor line: drum motors and powered rollers.
Drum motors are used at the end of a conveyor belt, replacing sprockets and chain drives; in a sense, they’re powered sprockets. They have the advantages of compactness, with no chains or other externally moving parts to present hazards. They’re also more energy-efficient than most drive-chain setups, easier to install and easier to maintain.
Yet for all that, drum motors have only about a 7% market share of all conveyor drives in North America, according to leading manufacturer Van der Graaf (VDG). Problems include overheating with subsequent lubrication failure, and difficulty in changing belt speeds, which an increasing number of applications now require.
But VDG took care of those problems, says Rick Zander, business development manager. “By stopping and evaluating the product, we initiated a design team that looked at root causes.” They brought down the running temperature by lowering internal pressure, improving the gear sets and modifying the laminations on the motor’s wire windings. Varying speeds were achieved by adding a variable-frequency drive to alter the current being fed to the AC motor.
On a roll
Roller motors are a type of gearmotor for roller conveyors, which are generally used for heavy, flat-bottomed loads like cases and pallets. They’re the same size and shape as ordinary rollers, but inside is a 24-volt DC motor and gearbox.
Roller motors usually are mechanically linked to several adjacent ordinary rollers. Often these linkages are about the same width as the cases or whatever they’re propelling, making it easy to slow down or otherwise maneuver individual units. This kind of control can be facilitated by specialized digital controllers.
Pulseroller, for example, features the ConveyLinx controller, which runs two rollers simultaneously and can connect to programmable logic controllers or other outlets elsewhere on the line for overall control. ConveyLinx “can also be transformed into a fully functional PLC and be used as a standalone program driving multiple motors or one motor and devices,” says Dave Sellers, vice president of operations and marketing.
Sellers notes that powered rollers save energy in two ways: through consuming less energy than the AC motors that would typically be used in motor/gearbox combos, and through intermittent operation.
“Powered rollers have the advantage of typically only running when needed, which can save up to 70% in energy cost in some conveying applications,” he says.
Gearbox/motor combinations are more common than enclosed gearmotors, simply because they’re more familiar and easier to replace and maintain. This is particularly true in material handling, because plants often maintain the motors and power trains for conveyors in-house; motors in machinery are more likely to be left to the manufacturer or another outside party.
When using standalone gearboxes, the basic decisions are the type and arrangement of the gears and shafts. That depends in turn on the load to be carried, how much energy efficiency is desired, safety issues and other considerations.
One of the most common drive configurations is the worm gear, a threaded cylinder that runs on an axis perpendicular to that of a larger gear, increasing the gearbox’s torque. Worm gears are inexpensive and reliable, but take up a relatively large space and need regular lubrication and other maintenance.
Bevel gears are an alternative. They also feature gears whose axes are at right angles, but instead of riding atop a larger gear, a gear that is “beveled” or cut at an angle meshes sideways with the slanted teeth of a larger gear. Hypoid gears combine the cylindrical shape of a worm gear with the sideways meshing of a bevel gear.
Gearboxes with bevel and hypoid gearing are perceived to be more expensive than worm-gear ones, but their big advantage is efficiency in transforming the motor’s shaft rotation into torque, which saves energy.
“The main and most important difference is gearing efficiency, which reduces energy usage and total cost of ownership,” says Laurie Andres, marketing manager for Nord Gear Corp. “Worm products can range from 40% to low 90s for efficiency, while bevel products achieve up to 97%.”
Another consideration for conveyors is how the gearbox will be mounted. There are generally two options: Putting it beneath the conveyor and connecting it to sprockets that lead to the conveyor’s drive pulley; or connecting it directly to the conveyor’s driveshaft.
The first option, sometimes known as “foot-mounting,” is the most common in unit conveyance. Combining the gearbox with an arrangement of sprockets allows the gearbox and motor to be tucked under the conveyor, and allows speed adjustment by changing sprockets. These moving parts, however, are more of a challenge to maintain.
Connecting the gearbox output directly to the driveshaft is a simpler arrangement with fewer moving parts, generally used for lighter conveying applications. However, it places the gearbox and motor outside the conveyor – a potential hazard for anyone working nearby.
There’s a universe of gears and gear-motor combinations available to help electric motors do their jobs. Matching them to the needs of an individual application will go a long way toward having conveyors and other equipment that’s efficient and trouble-free.