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Use your (gear) head for your next servo design

June 1, 2008
What gear systems are suitable for servo applications, and which don't make the grade? Well, gearboxes multiply torque and reduce speed; they're used

What gear systems are suitable for servo applications, and which don't make the grade? Well, gearboxes multiply torque and reduce speed; they're used with conventional electric motors. One example is an ac motor and its gearbox driving a conveyor at constant speed. Gear types typically employed in such applications include right angle, worm, and parallel-shaft spur (or helical) gearsets — either foot-mounted or mated with a NEMA C face.

On the other hand, gearheads are low-backlash planetary devices specifically designed for integration into servomotors. How so? Servomotors are used primarily in applications in which a load is repetitively accelerated and decelerated; servo-rated gearheads are designed to handle this repetitive action, as well as its high-acceleration torque requirements.


Low backlash is a major ingredient in getting a servo rating. Backlash, that free rotational movement that causes errors, is not an issue when gears turn only in one direction. However, in positioning applications, where a system turns clockwise and then returns counterclockwise, its final position may not match its original. This difference is the expression of error or backlash, also called gearshaft slop or play.

Reducing backlash requires the usual tradeoff between cost and performance; specifying low-backlash components does indeed minimize error. With their minimized backlash, servo gearheads can deliver the precise motion required for accurate servo positioning applications.

Factors that affect backlash include tooth shape and manufacturing quality; for the latter, irregularities can produce uneven, skewed tooth engagements.

Planetary advantages

Planetary gearheads are compact and highly efficient. For a given output torque, the power transmission capability per unit volume is unmatched. Efficiencies of 95% or better at rated loads per stage are typical.

In terms of gear ratios, other gear designs are generally limited. In contrast, planetary designs are available in lower ratios, as low as 3:1 and up to 175:1.

Too, their geometry allows a package in which the gearhead diameter approximates the servomotor diameter, so an ancillary benefit is that the output shaft is inline.

Available in both IEC and NEMA standard mounting configurations, gearheads are typically mounted to a servomotor via an adaptor plate and integrated self-locating input pinion clamps. Construction features such as large-diameter output shafts and augmented support for satellite gear shafts minimize torsional stress and deflection. Another feature provided by a few manufacturers is a backlash specification that lasts for the life of the product.

Gearhead application hints

When designing, first focus on motor selection, including the desired gear ratio. Determine continuous torque, accelerating torque, and speed. These values will be input to the gearhead.

Next, optimize the gearhead selection based on desired torque margin, backlash, and price. Specification sheets provide complete data, but pay particular attention to two torque ratings — maximum rated (continuous) torque, and maximum acceleration torque. If these input ratings are not exceeded, then the gearhead will operate within its design capability, and provide long life. Be sure that your design does not exceed the input torque rating of the gearhead.

In servo applications, often the motor can provide much more torque than is needed to drive the load. When this occurs, it is prudent to employ drive current-limiting functions. By limiting current, the amount of torque that can be input to a gearhead is also restricted — so in the event of a machine jam, the motor is prevented from supplying torque that could damage the gearhead. In some applications, the use of current limiting even enables specification of a gearhead smaller than that otherwise required to handle motor peak torque.

For backlash, specify what is needed in the application. For example, don't specify 3 arc-min. when 15 will perform adequately, because the lower the backlash, the more expensive the gearhead. On the same token, don't expect to save money by specifying higher backlash, such as 40 arc-min. Cost savings are not realized until about 90 arc-min., at the lower end of parallel shaft gears.

For gearhead specs, provide two input speeds — maximum speeds for both continuous duty and servo (or, in other words) acceleration/deceleration duty. Higher speeds lead to overheating and dynamic balancing issues.

One final note: Gearhead stops need to be controlled. When accelerating up to speed, electronic ramping is highly recommended. That's because a gearhead can be damaged if it hits a dead stop. Plug reversing an inertial load can also destroy a gearhead. Under such conditions, the gearhead can be wrenched and behaves like a spring that can break.

Extending life

Gearhead design life is typically indicated on data sheets. For example, a sheet may say a unit is rated for 15,000 operating hours. This means that at rated torque and speed, if temperature is kept within specifications, the gearhead will perform for the stated design life. Note that a 40-hour week is about 2,000 operating hours per year. A 24/7 operating schedule is about 8,700 hours per year.

After a gearhead has been operated for its rated life, to reduce the probability of irrevocable wear, it is prudent to refurbish the gearhead by replacing its bearings, lubricant, and seals, if present. Too, for longer life in an application, consider using a larger gearhead than normal sizing procedures require. Here, the gear teeth have to carry less force, so they lubricate better and generate less heat. Also consider operating at the lowest possible input speeds: Then the gear teeth and bearings see fewer revolutions and last longer. Finally, design the system so that the gearhead stays cool: Higher temperatures change the chemistry of the lubricants on gears and bearings, which shortens their lubricating powers exponentially. Excessive temperatures also thin lubricants, and shorten shaft seal life on oil-lubricated gearheads.

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To determine backlash, manufacturers lock the input pinion and clamp a balanced, symmetrical lever arm to the output shaft. Then, by applying a weight (to 5% of rated torque) first on one side of the arm, and then the other, a slight rotation is created. This rotation S is measured at a distance R and backlash is computed as Ø = S/R in radians.

Why test with that 5% weight? Loads heavier than that begin to induce torsional deflection, primarily in the output shaft. In servo-rated gearheads, relatively large diameter output shafts and other features minimize this potential error source.

Servo rating for super accuracy

A precise servo-rated gearhead mounted onto a brushless servomotor provides low backlash for accurate application positioning. The gearheads are mounted via an adaptor plate and integrated self-locating input pinion clamps. A variety of gearhead sizes, gear ratios, and backlash specs meets specific application needs.

Slow and strong

Planetary servo-rated gearheads multiply torque, provide inertia matching, and deliver precision in repeatable acceleration/deceleration applications. Typical ratios ranging from 3:1 to 175:1 are available in standard and stainless steel.

Right angle, consistent specs

Right-angle planetary gearheads provide high torque-to-size ratios and high torsional stiffness. Low-backlash units range from 5 to 15 arc-min. and can maintain those specifications over their full design life.

Key differences

Servo gearheads differ from conventional gear sets in that they're lighter, more accurate, and more responsive. They also carry more torque than similarly sized conventional motors.

For positioning applications

Servomotors for positioning applications come in many sizes and offer a variety of add-on items, including low backlash gearing for torque multiplication and inertia matching.

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