Gearing is used in only one-third of the motion control systems currently in service.
Gearless system owners take note: using a gearhead with a servo motor or using an integrated gearmotor can enable the use of a smaller motor, thereby reducing the system size and cost.
Primary advantages of gears
Torque multiplication. The gears and number of teeth on each gear create a ratio. If a motor can generate 100 in-lbs of torque and a 5:1 ratio gear head is attached to its output, the resulting torque will be close to 500 in-lbs.
RPM reduction. When a motor is running at 1,000 rpm and a 5:1 ratio gearhead is attached to it, the speed at the output will be 200 rpm. This speed reduction can improve system performance because many motors do not operate efficiently at very low rpm. For example, we recently helped to develop a stone-grinding mechanism requiring the motor to run at 15 rpm. This slow speed makes turning the grinding wheel difficult because the motor tended to cog. The variable resistance of the stone being ground also hindered ease of turning. With a 100:1 gearhead, letting the motor run at 1,500 rpm, the motor and gear head provided smooth rotation, while the gearhead output provided a more constant force with its output rotating at 15 rpm.
Inertia matching. Servo motors are generating more torque relative to frame size thanks to lightweight materials, dense copper windings, and high-energy magnets. The result is greater inertial mismatches between servo motors and the loads they are trying to control. Using a gearhead to better match the inertia of the motor to the inertia of the load enables use of a smaller motor and results in a more responsive system that is easier to tune.
System cost savings. All of the above can result in smaller motors and drives, lowering system cost.
Choosing the gearhead
Gearhead characteristics determine their appropriate application. Here are some rules of thumb:
Spur gears have teeth that run perpendicular to the face of the gear. They are compact, cost-effective, and capable of high gear ratios. Disadvantages include being noisy and prone to wear.
Worm-gear drives are typically lower-priced. Their inherent design creates a right-angle output. The simplicity of the driven worm gear allows for various configurations such as hollow shaft or double shaft outputs. Advantages: high precision, low noise, and low maintenance. Disadvantages: relatively low efficiency and being non-reversible.
Planetary-gear drives (such as Parker’s Stealth helical planetary gear head) resemble the arrangement of the solar system. A central (sun) gear drives the planetary gears surrounding it. The planetary gears ride within a ring gear and rotate the output shaft of the gearhead. Advantages: compact size, high efficiency, low backlash, and a high torque-to-weight ratio. Disadvantages: complex design (and possibly higher costs).
Harmonic gear drives contain a wave generator, flexspline, and circular spine. Advantages: low weight, compact design, no backlash, high gear ratios, high torques and coaxial input and output. Disadvantages include lower torsional stiffness and greater mechanical wear as a result of the preload caused by the flexspline.
Cycloidal drives have an input shaft that drives an eccentric bearing, which then drives a cycloidal disk. The drive’s output shaft has rollers that fit through holes in the face of the cycloidal disk. Cycloidal speed reducers are capable of high ratios but are small in size. Disadvantages include increased vibration, caused by the cycloidal motion, which can cause wear on the teeth of the cycloidal disk.
- Select the gearing that best suits your application.
- Discover the power that gears add to your motion control system.
Article contributed by Jeff Nazzaro, gearhead and motor product manager for Electromechanical Division, Parker Hannifin Corporation. Get more information on electromechanical gearheads on www.parker.com.
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