Precision Gearboxes | Low Backlash, High Torque

Precision gearboxes are one of the main components of robotic joints and reduce motor speed while increasing motor torque, allowing for smooth, controlled movement. Precision servo gearboxes are critical to high-performance robotics because of the stability they provide and the positional accuracy making them an indispensable method of creating movement. In high precision gearbox, low or zero-backlash is extremely important for eliminating slack in the mechanical system that may cause unwanted movement or impact reproducibility and repeatability.

Types of precision gearboxes include:

• Precision planetary gearboxes - Small and high torque density.
• Harmonic drives (strain wave gears) - Provide excellent zero backlash with high-quality precision and a small package size.
• Cycloidal reducers - Very durable and able to withstand shock-loads.

Harmonic drives are a favorite gear design in applications such as: robotic arms or manipulator arms where a high degree of positional accuracy and smooth motion is required in a small package size.

In a robotic joint, a precision reduction gearbox serves the purpose of reducing the motor's speed while increasing the output torque, which allows for precise and controlled movement of the robot’s joints. A precision reduction gearbox is a critical component of advanced automation and robotic systems.

The major advantages and functions of a precision gearbox in robotics:

• Increased Torque: A robotic gearbox delivers a torque increase at the robot's joint. This allows a robotic joint to move heavy loads or resistive force with higher precision and control.
• Speed Reduction: A robotic speed reducer allows for high speed motor rotation to be slowed down to produce controlled motion. This level of control is needed for the pick-and-place movement or for welding modes of functions or surgical robotics.
• Zero Backlash: High performance designs such as harmonic drive gearboxes, or strain wave gearboxes, have eliminated mechanical play (backlash) of joints. This is foundational to precision gearboxes as mechanical backlash was problematic to  originally designed servo motors in industrial robots and CNC machines, and robotic arm systems.
• Space Saving and Lightweight: using advanced compact reduction gearboxes, such as cycloidal reducers, or harmonic gear systems, can produce high torque in a small package, making them ideal solutions for robotic systems with limited space or weight burdens.
• Reliable Motion Control: A robotic joint mechanism supports the delivery of repeatability of position for the system that contributes to improved operational accuracy and increased longevity of the robotic or surgical system.

In high-precision robotics applications, having a "low or zero backlash" mechanism is essential. Mechanical backlash directly impacts a robot's positional accuracy, precision, and responsiveness which are all crucial to achieving high-quality robotic movements.

Here are the reasons why backlash is important:

1. Positional Accuracy:

Backlash (or unwanted mechanical play) between the gear teeth will cause lag or overshoot in the motion of the motor. For precision applications such as robotic assembly, surgical robotics, or micro-manipulation even a minimal gap may lead to a positional error.

2. Repeatability:

In industrial settings, the robot needs to go back to the same position over and over again. A low or zero-backlash gearbox, such as a harmonic drive gearbox, or a strain wave gearbox, can repeat motions with precision meaning there is no drift or slippage.

3. Stable Motion Control:

Backlash does not yield smooth movements during rapid changes of direction as it leads to vibration or instabilities. A zero-backlash gearbox will provide a smooth transition to a new motion and is especially important in applications where trajectory control is paramount.

4. Critical to high precision tooling:

Applications such as CNC milling, laser cutting applications, and robotic arms used in electronics require tool movements which are exact, and the absence of backlash means that there will be no lag or deviations.

5. Supports closed-loop control systems:

Feedback-based control such as servo motors with encoders are impacted by any play (or back-off system) due mechanical constraints resulting in reduced fidelity of the control motion. Closed-loop controlled systems will utilize precision gearboxes for low backlash.

• High precision: Made to provide low backlash and optimal torque transfer.
  
• Variety of options to scale: Payoff from micro to industrial size, designed to maximize torque/speed ratios.
  
• Enduring construction: Guild with premium alloys and surface treatments for reliability and longevity.
  
• Custom precision gearboxes: Configurable design for load, space and duty cycles.
  
• Centrally produced and assembled: Total quality, fit and performance control from CNC machining to assembly.
  
• Designed for integration: Can be integrated with a variety of robotic arms, actuators and motion platforms.

Every kind of precision gearbox reducer (planetary, harmonic (strain wave), and cycloidal) has its own pros and cons and design differences. This article will help you understand how each gearbox reducer compares in robotics and automation:

1) Planetary Gearbox

• The design consists of a central sun gear with planet gears mounted to a ring. It has a high torque density in a small footprint.
• The gearbox provides good efficiency and moderate backlash that can be further minimized in precision designs.
• It fits many applications, including industrial automation, CNC machines, and general robotics applications.
• It is generally a lower cost solution than the other types of high-precision gear.

2) Harmonic Drive (Strain Wave Gearbox)

• A harmonic drive gearbox uses a flexible spline and wave generator, which produces smooth, flexible motion with a high degree of gear reduction.
• This gearbox has near-zero backlash, and is well-suited for precision applications such as surgical robots, space robotics, and gimbals.
• Harmonic drives are typically compact and lightweight but have a high torque capability. They are generally more expensive (not always!) and more fragile than the other types.

3) Cycloidal Reducer

• Cycloidal reducers function based on rolling motion achieved through cycloidal discs and needle bearings resulting in reduced wear and superior shock capability.
• They are well-suited for the heavy-duty robots and collaborative arms which have the loads in the weighty class.
• Cycloidal reducers provide low backlash and smooth motion resulting in repeatable, accurate tasks.

The harmonic drive (also known as a strain wave gear) stands out because of its compact size and precision.

Harmonic Drives: Compact and Precise

• Compact Size:

Harmonic drives allow for high reduction ratios, within a very small, lightweight housing. This is due to its unique strain wave mechanism, which does not require large and stacked gear stages as do most traditional reducers. This makes them perfect for applications where space is approximately zero, such as surgical tools, humanoid robot joints, and space/aero systems.

• Precision and near zero backlash:

A major benefit of harmonic drives is their ability to provide low to no backlash and in practical use near to no gap or lag as the direction of motion changes. This benefits operations that need accuracy such as robotic surgery and optical alignment.

• High torque-to-size ratio:

Harmonic drives permit the transmission of relatively high torque in size and weight which allows for robotic systems with both good strength and good agility. The efficiency of the harmonic drive is necessary in specific types of robots, such as miniature systems.

• Smooth and repeatable motion:

The flex spline and wave generator of a harmonic drive allow for continuous engagement of teeth, benefiting smooth rotation and repeatable motion which is important for robotic arms and actuators  with high expectations of precision and motion.