Fractional horsepower gearmotors are many-faceted drive systems. But you must look at them from all angles to make them sparkle in your applications. Here’s what they are and tips on applying them
A gearmotor is an integrated assembly in which a gearbox and a drive motor combine in one unit. Gearmotors are the best solution for many motion control applications including those that require frequent starts and stops. In short, gearmotors serve as transducers that convert electrical energy to mechanical work. They provide speed reduction and torque multiplication in a more compact package than separate motors and gearboxes, especially suiting them for applications where space is at a premium.
Typical applications for fhp gearmotors include conveyors, ice dispensers, commercial restaurant equipment, chemical mixers, packaging machinery, medical and exercise equipment, and battery-powered vehicles, including handicapped mobility applications.
Gearmotors are also time-saving devices for designers, because they can greatly simplify or eliminate the need to match motor and gearing. The right choice between gearmotor or separate motor and gearbox hinges on application requirements, budget, and gearmotor limitations with respect to speed, torque, and load.
In analyzing relative cost, compare not only the original dollar outlay for the integrated assembly vs. separate components, but also expected life span and replacement considerations if there is a significant discrepancy in the stress on either the motor or the gearbox. In other words, are motor and gearbox well-matched?
A good manufacturer engineers products for optimal torque throughput by considering the balance between durability and strength ratings of the gearing system, and by designing bearing and lubrication systems for long life and good overload capacity. On gearmotors, motor and gearing are matched to provide balance. Reliability and life-testing prove the designs and full-load tests at final assembly assure quality. The supplier’s application staff can offer help, including laboratory testing, to match the gearmotor with equipment requirements.
Custom design may sometimes be indicated. Usually, a manufacturer likes to see eventual production of at least several thousand units per year to justify the investment in product development. In return for tooling and engineering charges, the original equipment manufacturer winds up with a proprietary design that often produces a performance edge. For one major gearmotor manufacturer, about 30% of annual production is in custom proprietary designs.
At present, there are no uniformly accepted standards for fhp gearmotors. However, four principal standards-making bodies influence gearmotor design:
• The American Gear Manufacturers Association (AGMA).
• The National Electrical Manufacturers Association (NEMA).
• German national standards (Deutsches Institut für Normalisation — DIN).
• The International Organization for Standardization (ISO).
ISO standards are years from worldwide acceptance as national standards are merged into ISO documents, due to standards-review process complexities.
Over the years, competitive forces among manufacturers have brought development of products which, while dissimilar in design, offer similar mounting configurations, gear ratios, torque, and speed properties. These gearmotor designs have become a sort of quasi industry standard that gives users product choices.
There are two major gearmotor categories:
• Parallel shaft.
• Right angle.
A parallel-shaft gearmotor has an output shaft axis in-line or offset but parallel to the motor shaft, Figure 1. It typically uses spur, helical, or planetary gearing. Parallel-shaft gearmotors have high torque transmission efficiency (and, thus, power efficiency), but are generally not so quiet as right-angle gearmotors.
Right-angle gearmotors serve well in tight spaces, particularly where the designer needs to have torque “turn a corner.” The motor shaft connects to a worm shaft in the gear drive. The worm powers a worm gear on the output shaft. Thus, the drive output shaft axis is perpendicular to the motor-shaft axis, Figure 2. Right-angle gearmotors have lower efficiency than parallel-shaft gearmotors due to the worm drive, but they tend to produce less gear noise.
You can get gearmotors with ac or dc motors. Table 1 gives some recommendations. AC gearmotors serve mostly in constant or nearly constant- speed applications. DC gearmotors typically serve on adjustable-speed applications, and where high starting torque or reversibility is needed. When selecting brush-commutated dc motors, take care in sizing for brush life.
Brushes may be replaceable. You can use “Bullock’s Rule for Brush Life”: Life is inversely proportional to the change in speed or the square of current through the brush.For example, a gearmotor is capable of 3,000-hr brush life at 2,000-rpm input motor speed, drawing 2-A brush current. At 4,000 rpm, you could expect brush life to be 1,500 hr at the same 2-A current. At the same 2,000-rpm speed, but at 1-A current, expect four times the brush life, that is, 12,000 hr.
Output shaft options
You can get many custom shaft options including keyways, flats, through holes, threads, or splines. Other common shaft options include shoulders, special lengths, shaft materials for corrosion resistance or food-handling applications, and surface plating.
Another shaft option frequently available is of shaft position. For offset parallel- shaft gearmotors, there are position choices of the output shaft relative to the face of the gearhead at 12 (typical), 3, 6, and 9 o’clock. Rightangle gearmotor shaft position options include right or left-hand shaft and dual output shafts.
Speed and torque ratings
A gearmotor’s output speed equals motor shaft speed delivered to the gearhead, divided by gear ratio. With an adjustablespeed motor, there is a range of output speeds. For example, if the gear ratio is 20:1 and motor operating speed range is 100 to 3,000 rpm, then gearmotor output speed range is 5 to 150 rpm.
Gearhead output torque equals gear ratio times input torque from the motor, less an amount attributed to losses due to seals, bearings, and gearing inefficiencies. The weakest link determines the output torque rating; torque is limited by either geartrain or motor. In intermittent- duty applications, a duty-cycle analysis can often provide a smaller, more economical selection. To avoid overheating and premature mechanical failure, take care not to exceed gearmotor ratings continuously.
Motion controllers, interfaces
You can use gearmotors with a variety of controllers and interface devices. The manufacturer can advise on the best gearmotor match to your own system, or sometimes the appropriate motion-control and interface device.
In selecting a gearmotor, probably the most important consideration is desired life. It relates directly to design and application. Several factors contribute to gearmotor life, including:
• Duty cycle.
• Load characteristics.
• Ambient temperature.
• Gear material, design, and accuracy.
Cyclic loads exceeding the geartrain’s durability rating result in premature failure. Exceeding the geartrain’s strength rating results in catastrophic tooth failure. Most gear designers try for a strength rating twice the durability rating. That should give you a good feel for the geartrain’s ability to absorb occasional peak load beyond the durability or normal torque rating.
However, gearmotor manufacturers differ in their definitions of normal operating conditions and design life. You need to determine that the gearmotor rating is suitable for the load demands and duty cycle of your application. Try to fill in all blanks in the supplier’s application data sheet and test a prototype, if feasible.
The traditional characterization, Table 2, has been to slot operation as “continuous,” “intermittent,” or “occasional,” with continuous being 8 to 10 hr/day at rated load; intermittent, several min/hr; and occasional, 15 to 30 min/day with individual operating cycles of 2 min or less. However, given the computing power most gearmotor makers now have, an accurate definition of duty cycle can translate into an optimum recommendation of gearmotor. Some makers offer calibrated drives with recording devices to empirically determine load characteristics.
A load may be primarily frictional, inertial, or a combination. A frictional load is due to resistive frictional forces that must be overcome. It tends to be constant and requires a small torque spike to start motion and a smaller, relatively constant torque to maintain motion. Inertial load relates to the mass and concentration of the load relative to the driveshaft. It typically requires high starting torque but very low running torque.
Radial and axial load on the gearmotor output shaft are also important. Radial or overhung load is the net force perpendicular to the driveshaft. It results from a mass supported by the shaft or is created by devices such as sprockets or pulleys. Axial load is parallel to the shaft axis. It must be opposed by some component of the gearmotor. Most manufacturers provide overhung loads ratings. They can provide axial ratings if you ask.
Reversing, braking, overrunning, back-driving
Generally, gearmotors are not designed to handle rapid stopping or plug reversing of inertial loads (changing direction without stopping motion first). General good practice says disengage or bring the load to a stop before reversing gearmotor direction.
A large inertial load may require a clutch and brake to keep it from driving the gearmotor forward after the motor is de-energized, or backward due to system backlash. These two conditions, called overrunning and back-driving loads respectively, can harm both a gearmotor and its driven system. Advances in control technology can provide electronic compensation in some such applications.
There are many gearmotor mounting options, including foot, flange, and base mounting. Orientation can play critical roles in proper gear lubrication and in seals, placement of oil fill plugs, and venting when used. Moreover, orientation can introduce axial load components. Check the manufacturer’s recommendations.
Excessive temperature can harm both motor and gearbox. Most gearmotors are designed for maximum ambient temperature of 40 C (104 F). Designs generally take into account some heat sinking at the mounting. Gearmotor selection must consider environments that add thermal insulation such as lint, some chemicals, or mud. Also, frequent stops and starts can cause overheating. Permanent split capacitor ac motors are generally recommended for this situation.
Proper lubrication is a key factor in life of gearmotor gears, bearings, and seals. Either oil or grease is used, with oil preferred in most applications due to its greater ability to move and coat the gear teeth, leading to longer life. Grease is sometimes used in low power applications where shaft seals are eliminated for cost or efficiency reasons. It is also used occasionally to avoid product contamination from oil leakage. Synthetic lubricants offer extended temperature ranges and can serve well in high or low ambient temperatures. Oil-lubricated gearboxes generally run quieter than grease-lubricated ones. Grease can entrain air, producing a popping sound as it is squeezed through the gear mesh.
Noise and vibration
Some applications, such as office machines and medical devices, require low noise and vibration. Excitation at the gear mesh is a primary source of noise and vibration. It can result from gearhandling damage such as nicks or burrs, meshing errors, shaft deflection, or dynamic bearing loads. Resonance with gear housings and driven machinery can amplify it. Sheet-metal housings enclosing overall equipment are notorious culprits. If your application requires special consideration in this area, most gearmotor manufacturers can make recommendations to minimize noise and vibration.
A look ahead
Research is progressing on materials, manufacturing processes, heat treatment, and electromagnetics that will help cut costs and enhance performance of motors and gearing and thus, of gearmotors. New motor designs combined with advanced gear technology should let gearmotors shrink while increasing torque throughput. Smart gearmotors, equipped with feedback devices and integrated with advanced controls, are becoming realities. It is an exciting time for gearmotor technology and application.
Ronald Bullock is President, Bison Gear & Engineering Corp., Downers Grove, Ill.