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Gearbox Motors vs Integrated Gearmotors

Wednesday - 16/04/2008 08:44
The benefits of planetary gearboxes, and when to choose between a gearbox motor combination and an integrated gearmotor.
Gearbox Motors vs Integrated Gearmotors
Only about one-third of motor-driven motion systems use gearing, even though gearheads benefit size-constrained applications and those running at 1,000 rpm or less. Other advantages of using a gearhead with a servomotor abound.
 
Torque Multiplication
 
Gearheads provide a mechanical advantage when mounted to the motor output shaft. The number of gears, and the number of teeth on each, create a mechanical advantage defined by a ratio. If a motor generates 200 lb-in. of torque, attaching a 5:1 gearhead generates output torque approaching 1000 lb-in. depending on gearhead efficiency.
 
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Speed Reduction
 
Gearheads are often called gear reducers because most increase output torque while decreasing output speed. A motor running at 3,000 rpm fitted with a 10:1 ratio gearhead outputs 300 rpm. This speed reduction improves system performance because many motors do not operate efficiently at low rpm.
 
Consider a stone-grinding mechanism that must run at 15rpm. At this speed, motor cogging makes grinding wheel rotation inconsistent.
 
The variable resistance of the stone being ground also renders wheel rotation unpredictable. In contrast, fitting a 100:1 gearhead to the motor lets the latter run at 1,500 rpm. The motor-gear-head combination provides more-consistent output force and smooth wheel rotation despite friction and load variations.
 
Inertia Matching
 
Over the past 20 years, servomotor manufacturers have introduced lightweight materials, dense copper windings, and high-energy magnets for motors that generate more torque for a given frame size. While beneficial, this trend has increased the risk of inertial mismatches between servomotors and the loads they control.
 
Should load inertia significantly exceed that of the motor, it causes excessive overshoot or increased settling times — both of which decrease production line throughput. On the other hand, a motor too large for an application has a high initial cost and consumes more power to accelerate its own inertia.
 
Here, a gearhead can help match motor inertia to load inertia, resulting in a more-responsive system. The motor experiences reflected inertia equal to the load inertia divided by the square of the gearhead ratio. Rapid start and stop applications benefit most from well matched motor and load inertias.
 
Cost Savings
 
Using a gearhead to multiply torque, reduce speed, and match inertia also helps cut system cost by permitting smaller motors and drives. Consider an application requiring 200lb-in. at 300rpm. Driving this load with a servomotor alone (assuming industry-standard performance characteristics) necessitates a 142-mm frame size and a drive supplying 30A continuous.
 
Such a system costs about $6,000. In contrast, including a gearhead cuts motor and gearhead frame size to 90mm — and the resulting mechanical advantage permits a smaller drive as well. Such a system costs about $3,300.
 
Configuration Options
 
If a gearhead makes sense, an engineer must determine whether in-line or right-angle gearing is appropriate for the design’s footprint. Next, one must decide between an integrated gearmotor or a more-common gearhead-motor pair.
 
In-line gearheads are commonly used for motion-control applications because they have higher torque output and lower backlash than right-angle gearheads. They also cost less than right-angle types. As the name implies, in-line gearheads have an output shaft that is in-line (centered with) the motor shaft.
 
On the other hand, a right-angle gearhead is often used when the servomotor needs to fit into a tight space. Here, the output shaft of the right-angle gearhead is at 90° to the driving motor shaft. Gearing such as worm sets have an inherent right-angle design because the worm’s drive axis (screw) is 90° to the worm-gear axis.
 
Another consideration is whether to use a separate gearhead assembly or an assembly that is built into the motor. Most motion designs incorporating gearing use separate assemblies. This lets engineers specify the best motor and gearhead for an application, even if they’re from different manufacturers.
 
Typical gearheads can be