Improvements big and small
Specialized shapes in belts, gears and other systems can address specific design issues, freeing systems to push through more torque and force. Take brakes, for example. On large magnetic brakes and clutches, coils can be level wound, with the copper wire arranged in perfect layers. "This way, the coil has the same turns in less space, for more torque," explains Jeff Pedu, Placid Industries, Lake Placid, NY.
Likewise, in gearboxes, replacing involute gearing with cycloidals and Novikovs lowers stresses and increases loadability for higher torque density. One caveat, according to Gerhard G. Antony, Neugard USA, Bethel Park, PA., is that these special tooth shapes are more difficult and expensive to machine, and some of them are unsuitable for high speeds.
Materials can also boost the torque output of a system. One of the most engineered materials available is case hardened steel. "Case hardened steel has greater load capacity than bronze and resist wear and backlash change," notes Russell Beach, Nissei Corp., Greenville, SC. Chromium, nickel and molybdenum in this alloy increase strength for increased load carrying.
Titanium, though frequently mentioned as the next super material, has about the same strength and durability as case hardened steel. "Titanium offers no advantages volume-wise," says Gerhard Antony, Neugart USA, "However, because it's about two times lighter than steel, it does offer more torque density per unit weight," he adds. Still, the high cost of titanium often limits its application to special designs.
Minimizing the interface between a motor and its application is one way to increase torque or force density. And the best way to accomplish this, according to John Calico, senior research engineer, Moog Components Group, Blacksburg, VA., is to remove the boundaries between the motor and application.
Instead of building a motor and attaching it to a system, build motor functions into the various elements of the machine, Calico suggests. For example, assemble the rotor directly onto the shaft it's supposed to drive. And build the stator mounting into surrounding machine parts. "The space you save by eliminating the housing can be filled with active motor lamination materials," says Calico.
The approach works best, Calico explains, if the motor is able to drive the load without intermediary components, such as belts, gears and pulleys. In addition to freeing up volume for more torque-producing materials, this eliminates inherent inefficiencies, making more torque available for the application.
Teeth with more bite
In gearboxes, increased tooth contact makes for more torque density. Not surprisingly, planetary sets � which engage multiple coplanar gears at once � are almost synonymous with torque density. Also called epicyclics, they're growing in use, largely because they're 40 to 60% more compact than traditional spur sets.
In standard gears, the answer is increasing tooth size. "Increasing torque capability per gearing stage equates to larger teeth and wider gear faces," says Mike Niemela, Bison Gear and Engineering Corp., St. Charles, IL. "But as the gear teeth grow in size, the number of teeth that will fit on a gear decreases, necessitating increased gear center distances to meet the gear ratio requirements," he adds. "We recently worked on an application where increased torque was needed; our normal gearmotors delivered 350lb-in. of torque from a specific envelope," says Niemela. "But increasing the center distance in the final output stage and changing from spur to helical gearing increased output power to 900lb-in. at 6 rpm."
Right angle sets � such as gears with angled or curved teeth that require mating pairs be perpendicular � rival planetaries in torque capabilities. "Right-angle gearing includes worm, bevel and hypoid types," explains Tom Provencher, Mijno Precision Gearing, Park Ridge, IL. "In double-enveloping gearing the hourglass shape of the worm and its throated gearwheel bring many teeth into contact for more load sharing and higher torque density."
High Torque � but for how long?
It's easy to crank a lot of torque out of a small system if the design only has to last a short time. “A gearbox can be rated for a very high torque if it need only last a couple of minutes, but will be rated much lower if it needs to last thousands of hours,” says
Gerhard Antony, Neugert USA. Likewise, compact systems can transmit exceptionally high force – if it is intermittent.
Target life expectancy is part of a fully defined torque or force rating. Volume and weight can be determined impartially; however, torque allows a number of interpretations. Catalogs often list acceleration torques, peak torques, 50% duty cycle torques, and so on. That's why designers should be certain torque values used to compare torque-density ratings are analogous.
"Torque density is an excellent measure allowing objective, impartial comparison, assuming variables are determined by objective criteria," says Antony, "For transparency reasons, the value used for torque density calculations should be the rated torque at continuous duty, for an exactly defined life."