Geared Up

Converting input provided by a primary mover into an output of lower RPM and correspondingly higher torque.

Gearheads are important speed reducers that come in various sizes, capacities and speed ratios. They are mainly used for applications where there is either a need for increasing the resolution of the motor, or the need for a large amount of torque for a small package.

When specifying for gearheads, there is a variety of factors to consider. According to Miriam Metcalfe, Marketing at Wittenstein, footprint, sizing and application are among the most important:

Footprint: What are your space requirements? Is an inline (coaxial) gearbox or a right-angle gearbox appropriate?
Sizing: Because of the overall high efficiency of a servo gearbox, engineers need to appropriately size based on the torque and speed requirements.
Application: For continuous running applications requiring precision, servo gearboxes specifically designed to handle this should be specified.

Doug Rightmier, application manager, Oriental Motor U.S.A.; and the engineering team at Myostat Motion Control, believe it is all about the backlash of the mechanism and gearbox design.

In applications that utilize gearheads, “precise positioning [is required] and the gearheads need to adhere to this requirement,” says Rightmier. “Backlash is the amount of “play” in the gears when the direction of the motor input is reversed and it can have an effect on positioning accuracy in bi-directional operations.”

Spur vs. Helical

The main difference between spur and helical gearboxes is how their gear teeth are cut by certain angles.

“The spur gear is a cylindrical gear on which the gear teeth are straight and cut parallel to the shaft,” says Rightmier.

“Helical gears offer a refinement over spur gears [because they] can be meshed in a parallel or crossed orientations,” says the engineering team from Myostat Motion Control. “The angled teeth engage more gradually than do spur teeth, causing them to run more smoothly and quietly.

While helical gears have the advantages of lower noise and higher strength, they are more sensitive to axial loading; therefore, careful consideration has to be made in the design.

Robotic Applications

According to Myostat, a cycloidal drive or cycloidal speed reducer is a mechanism for reducing the speed of an input shaft by a certain ratio. They are capable of high ratios in compact sizes.

“They are best known for being used in clocks and watches,” says Rightmier. “Of late, they have found their way into robotics applications [and] offer a good advantage of high gear reduction in a small package.”

However, Rightmier points out, that they have poor efficiency due to sliding contact and the cycloid motion causes torque and speed ripple on the output shaft.

Low Heat, Long Duty

According to Miriam Metcalfe, Wittenstein, energy efficiency was not a buzzword when the company developed the first servo gearbox in 1983.

“We now know that the more heat that is given, the less energy efficient a gearbox is, [so Wittenstein’s] most recent development is the SP+ -L version [that features] long duty and exceptionally low heat.”

Rightmier states, “The primary concern [when selecting gearheads based on energy efficiency,] is getting as much mechanical power out of the system based on the electrical power being put into the system.”

As long as the gear ratio will deliver the needed torque at the required speed, the customer will see an increase in energy efficiency when compared to using a larger motor that would consume more power.

Myostat claims power losses in gear systems are associated primarily with tooth friction and lubrication churning losses. Churning losses are relatively independent of the nature of the gears and the gear ratios – they are primarily related to the peripheral speed of the gears passing through the fluid.

Myostat continues to explain that the efficiency of a gearbox can be reached by minimizing the backlash of the gearbox. It depends on the application and size of the gearbox, because size and manufacturing process will often affect the efficiency/backlash/tolerance of the gear design.