Planetary Gear Heads Basics

Planetary gear heads are used on high precision motion control applications that require high torque to volume ratio (ranging from a factor of two to a factor of 100, depending on the ratio), high torsional stiffness and low backlash the specifics of which will vary by application. Planetary gear heads increase the torque by the factor of the gear head ratio. A planetary gear head will also reduce the motor speed by the gear head ratio, making it possible to run the motor at a higher, more efficient rpm. The inertia reflected back to the motor is reduced for increased stability. Finally, using a planetary gear head allows machine builders to use a smaller, less expensive motion control package.

Helical gearing improves the performance of a planetary gear head by increasing the contact ratio. The contact ratio is defined as the number of teeth in the mesh at any given time. The typical contact ratio for spur gearing is 1.5 while the contact ratio for helical gearing is more than doubled to 3.3. The increased contact ratio provided by helical gearing provides:

  • 30% to 50% more torque capacity than spur type planetary gearing·
  • Increased load sharing which results in longer life
  • Smoother and quieter operation·
  • A reduction in backlash of 2 arc-min

The helix angle of the gear head also has a significant impact on performance, as the higher the helix angle, the more teeth in the mesh at any one time. So increasing the helix angle from the typical 12o to 15o results in a 17% to 20% increase in torque capacity over conventional helical gearing, and a 40% increase compared to straight-cut spur gears. A 15° helix angle also provides a smoother and quieter operation.

The helical cut of the gear teeth causes an axial load on the motor shaft. This needs to be compensated for in the bearing design of the gear head. Many helical gear heads use ball bearings with little or no axial load capabilities. This can contribute to gearing or motor bearing failure. A better approach is the use of tapered roller bearings, such as are used in all Micron Helical gear heads, to completely compensate for this axial load.

Single-stage planetary gear heads provide possible gear ratios ranging from 3:1 to 10:1. Gear ratios are part of choosing a high-performance geared motor. The gear ratio can be no higher than 10:1 because the pinion gear can be made only so small. Gear ratios of less than 3:1 are not possible with a planetary design since the pinion and outer ring gear would need to be nearly the same size, which would not leave room for the planet gears. A gear ratio range between 4:1 and 8:1 provides optimal pinion and planetary gear size, higher performance and longer life. Gear ratios of greater than 10:1 can be achieved with an additional planetary stage, although this will normally add to the length.

Crowning is a modification of the gear tooth profile that optimizes gear mesh alignment in order to increase torque capacity and reduce noise. It also enhances load distribution on the tooth flank, thereby reducing high-stress regions that can result in surface pitting.

A certain amount of clearance is needed for a planetary gear head to function effectively. Clearance helps avoid excessive heat and wear in the gearing and ensures proper lubrication. But the small gap between the gear teeth leads to lost motion. Real-world gear heads also cannot have infinite torsional stiffness, so additional lost motion is generated by windup in the gear head. It shows the rotation of the input and output shafts of a planetary gear. The vertical distance between the two lines is lost motion.

There is no strict standard regulating how gear head backlash should be measured. This leads to some confusion and misconceptions. Some manufacturers may take an average of four or more points on the output shaft to provide a backlash specification. For example, using this method a unit with backlash measurements of 4, 6, 10 and 12 arc-min. would have a rating of 8 arc-min. This author takes the position that backlash should be rated at the highest point on the output shaft so the measurements stated above should yield a 12 arc-min measurement. Additionally, some manufacturers measure backlash by applying less than 2% of the rated torque, which produces a lower backlash measurement and doesn’t provide true backlash ratings over the entire life of the product. Other manufacturers measure backlash by using 2% of the rated torque as standard to provide backlash ratings that are true for the life of the product.

The backlash will increase over time. A planetary gear head might have a backlash of 8 arc-min out of the box but increase to 15 arc-min over six months of use, for example. So the accuracy of a planetary gear head over its lifetime is an important specification for most users.

Gear head sizing and selection, troubleshooting

Planetary gear heads also have some common problems. Factors of troubleshooting that contribute to gear head noise include input speed, gear head ratio, output torque, radial and axial loads and mounting errors.

Proper gear head sizing and selection is critical to achieving long and reliable life. The following approximate method can be used as a starting point.

Application Torque (Tr) — Continuous Motor Torque (Tm) x Ratio x Efficiency (e)

Determining the exact gear head sizing requirements requires consideration of the complete motion profile including speed, torque, acceleration, deceleration, and cycles. A de-rating factor can be used to reduce the rating to take high cycling into consideration.

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