A guide to gearhead selection

Choosing the right gearhead is key to "torquing up" servomotor performance, according to Rob Pearson, Product Manager for ZF Industrial.

When faced with the task of selecting a gear reducer for servomotor applications, a design engineer often makes the selection based on performance considerations, such as torque capacity, degree of backlash and, torsional stiffness.

In contrast, the manufacturing engineer, who must integrate the reducer/motor combination and other components into a final assembly, has a different set of requirements.

These include a simple and reliable method of mounting the reducer to the motor, smooth and quiet operation and low maintenance.

Usually, the design engineer's task is simplified, because the most common reason - by far - for turning to gear reducers is the requirement for more torque; or more precisely, torque output above that which is economically available from a servomotor on its own.

The gear reducer provides a compact and efficient solution to this requirement.

There is one drawback, however, top-end speed (typically 3000-5000rev/min) is compromised, but the higher available torque more than compensates for this.

Employing a gear reducer also solves the common problem of settling time delays, experienced with many servo-controlled systems.

These often result if load inertia is high compared with motor-armature inertia.

Gear reducers overcome this common problem by reducing the reflected inertia (ie the load inertia seen by the control system) by the square of the gear reduction ratio.

For example, a 5:1 reduction ratio provides a 25:1 reduction in the reflected load-inertia ratio, ensuring stable system operation and optimal machine performance.

When high torque performance is required from a gear reducer, the best option is to specify a planetary type unit, such as ZF's PG series.

These reducers provide the highest power densities available in an electrical-actuator system, together with all the traditional advantages of a planetary gearbox design: low weight, compactness and low noise, plus a combination of high efficiency and dynamic operation.

Planetary gear heads have high torque ratings because several gears uniformly share the load.

The mixing motion of the planet gears with the outer ring gear keeps all teeth well lubricated.

A single drop of oil on one tooth evenly redistributes over the entire gear set, which is not the case for nonplanetary gear reducers.

Planetary reducers also offer low backlash, which can be important for positioning accuracy and servo system stability.

Crucial to the consistent and reliable performance of planetary gearheads is a specification that includes precision ground gears and high quality roller bearings.

Taking the example of the gears first: on ZF's PG range, precision grinding (in pairs) of the planetary gears realises extremely low backlash performance throughout the range, down to less than or equal to 3arc-min.

This operation also provides benefits in terms of quiet operation, with noise levels of the order of less than or equal to 65dB.

As regards the use of roller bearings, the value of these is that they allow higher loads on the output shaft.

Their endurance, in combination with the high duty gearing, allows repeated stopping of the gearmotor without detracting from its precision performance.

In addition to physical design considerations, another major factor in gearhead selection is how easily and quickly the unit can be attached to servomotors and integrated into machines.

A flexible design in this respect can make a large contribution to cost savings for the machine builder.

Units such as the PG range, that are designed with OEMs in mind, achieve integration flexibility by offering the user a choice of attachment options, typically a shrunk fit connection with clamping element on a flat shaft, or a machined shaft with key way and feather key.

In operating terms, the planetary gearhead user should look for a nominal service life of 15,000 hours.

If the unit has fully enclosed design, with IP65 ingress protection, and is grease sealed, then the gearhead operation should be essentially maintenance free during its service life span - even in adverse environments.

Employing the alternative of oil lubrication can be very messy and may not be possible in hygiene conscious environments, such as food.

In addition, whereas grease filled units offer the flexibility to be mounted in any orientation; this is not usually the case when using oil as the lubrication medium.

ZF planetary gearheads are available in both single and twin stage versions, in a range of ratios between 4:1 and 100:1.

The gearheads are rated for motor input speeds from 3000 to 10,000rev/min as standard, and are suitable for use with a wide range of motor frame sizes.

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