Introduction and Technical Analysis of Reducer Types Used in Industrial Robot Arms

At the heart of industrial automation, robot arms owe their critical features such as precise motion capability, high repeatability, and heavy load-carrying capacity to essential components: reducers. This “Field Guide and Technical Article” aims to provide a comprehensive overview for automation engineers, technicians, and industry professionals by delving into the types of reducers used in industrial robot arms, their working principles, technical specifications, and field applications. Due to the dynamic nature of robotic systems, each joint (axis) of a robot arm must reduce the high speed of the motor to the desired low speed while simultaneously multiplying and transmitting the torque generated by the motor. This is where specially designed high-performance reducers, which go beyond traditional gearboxes, come into play. These reducers not only reduce speed and increase torque but also directly affect critical parameters such as precision, repeatability, rigidity, and backlash in the robot’s motion. An incorrectly selected or poorly maintained reducer can significantly degrade robot performance, lead to production errors, and even shorten the system’s lifespan. Therefore, a deep understanding of industrial robot reducers is vital for the efficiency and sustainability of modern manufacturing facilities. The most common reducer types used in robotic applications are Harmonic Reducers (Strain Wave Gearing), Cycloidal Reducers, and RV Reducers (Robotic/Precision Cycloidal Reducers), specifically developed for robotic applications. Each type has its unique advantages, disadvantages, and application areas. This article will explain the fundamental engineering principles of these reducers, shed light on their practical applications in the field, and provide the necessary technical information to make the most appropriate reducer selection. Additionally, reducer maintenance, common problems, and suggested solutions will be covered comprehensively.

Working Principle and Technical Data of Reducer Types Used in Industrial Robot Arms

The precise and powerful motion capability of industrial robot arms largely depends on the specialized reducers they house. These reducers convert the high-speed, low-torque motion from servo motors into the low-speed, high-torque, and extremely precise motion required for robot joints. Key features sought in robotic applications include high torque density, low backlash, high rigidity, compact design, and long lifespan. Let’s now examine these reducer types in detail:

Harmonic Reducers (Strain Wave Gearing)

Harmonic reducers are widely used in small and medium-sized robot arms, collaborative robots (cobots), and applications requiring precise positioning due to their compact size, lightweight construction, and nearly zero backlash characteristics. Their working principle is based on the continuous contact of a flexible gear (Flexspline) with a fixed gear (Circular Spline) as it is deformed by a wave generator (Wave Generator).

• Wave Generator: Typically consists of an elliptical cam and a bearing around it. It is connected to the servo motor shaft and forces the Flexspline into an elliptical shape, ensuring the teeth make contact with the circular spline.
• Flexspline: A thin-walled, flexible metal cup (usually steel) with teeth on its outer surface. When deformed by the wave generator, a portion of its teeth engages with the circular spline.
• Circular Spline: A rigid, circular ring with teeth on its inner surface, usually two more teeth than the Flexspline. It is fixed to the chassis or the stationary part of the robot.

Working Principle: As the wave generator rotates, it deforms the Flexspline into an elliptical shape. Along the major axis of this ellipse, the Flexspline teeth fully engage with the Circular Spline teeth. Along the minor axis, the teeth are disengaged. For every full rotation of the wave generator, the Flexspline teeth are shifted by two teeth relative to the Circular Spline teeth. This differential movement provides a high reduction ratio (typically from 50:1 to 320:1). The robot joint connected to the output of the Flexspline receives this low-speed, high-torque motion.

Advantages: High reduction ratios in a single stage, extremely low backlash, compact size, lightweight, high positioning accuracy, high torque density.

Disadvantages: Limited shock load capacity, potential for fatigue and wear in the Flexspline, relatively high cost, may be insufficient for heavy-duty applications requiring high rigidity.

Cycloidal Reducers

Cycloidal reducers operate on a principle similar to planetary gear systems but use cycloidal discs and pins instead of traditional gears. This structure offers high shock load capacity, high rigidity, and a long lifespan. They are preferred in heavy-duty industrial robots and applications requiring high torque.

• Input Shaft and Eccentric Bearing: The input shaft, connected to the servo motor, carries one or two eccentric bearings.
• Cycloidal Discs: Mounted on the eccentric bearings of the input shaft, these discs have specially curved cycloidal teeth. Typically, two discs are placed with a 180-degree phase difference.
• Pin Ring (Output Pins): Consists of a series of pins fixed to the reducer housing, which engage with the teeth of the cycloidal discs.
• Output Shaft: A shaft connected to pinions or directly to the discs, transmitting the rotational motion of the cycloidal discs to the robot joint.

Working Principle: As the input shaft rotates, the eccentric bearings move the center of the cycloidal discs in a circular orbit. The cycloidal discs, in contact with the pins in the pin ring, slowly rotate in the opposite direction by one tooth (or pin) per revolution. The use of two cycloidal discs reduces vibration and improves load distribution. This slow rotational motion is transmitted to the output shaft, achieving a high reduction ratio (typically from 30:1 to 300:1) and high torque.

Advantages: High shock load capacity, high rigidity, long lifespan, low backlash, high torque capacity, compact design.

Dezavantajları: May be slightly larger than harmonic reducers, more complex assembly.

RV Reducers (Robotic/Precision Cycloidal Reducers)

Field Considerations for Reducer Types Used in Industrial Robot Arms

• Correct Selection and Sizing:
  Reducer selection must be made according to the robot’s application requirements. Load capacity, speed profile, acceleration/deceleration times, required positioning accuracy, and environmental conditions (temperature, humidity, dust) must be considered. The nominal torque capacity, peak torque capacity, and backlash value of the reducer directly affect the robot’s dynamic performance. Oversizing leads to unnecessary cost and weight increase, while undersizing results in premature failures and low performance. Safety factors must be considered, especially for dynamic loads and shock loads.
• Precise Mounting and Alignment:
  The mounting of the reducer to the robot joint and servo motor must be extremely precise. Misalignment, axial or radial loads, put unnecessary stress on the reducer’s bearings and gears, shortening its lifespan, causing overheating, and abnormal noises. Mounting bolts must be tightened to the manufacturer’s specified torque values and checked regularly. Correct selection and mounting of couplings are also critical for absorbing vibrations and compensating for alignment errors.
• Proper Lubrication and Periodic Maintenance:
  For reducers to operate efficiently and have a long lifespan, the correct type and amount of lubricant must be used. The manufacturer’s recommended lubricant type (grease or oil), viscosity, and change intervals must be strictly adhered to. Incorrect lubrication increases friction, leading to overheating, wear, and loss of efficiency. Oil levels should be checked regularly, signs of contamination should be looked for, and oil changes should be performed at specified intervals. The thermal stability of the lubricant is particularly important for robots operating at high temperatures.
• Thermal Management and Overheating Control:
  Reducers generate heat due to friction during operation. Overheating can lead to lubricant degradation, seal leaks, and expansion of internal components, causing increased backlash or seizing. The reducer must be kept within its thermal limits, considering the robot’s operating environment temperature, cycle time, and load profile. If necessary, additional cooling solutions (e.g., oil coolers) should be considered. Temperature monitoring with thermal sensors is beneficial for early detection of potential problems.
• Backlash Control and Monitoring:
  In robotic applications, backlash is a critical parameter for positioning accuracy and repeatability. Reducer backlash values can increase over time due to wear. Periodic backlash measurements and monitoring indicate the health of the reducer. Backlash exceeding a certain tolerance reduces the robot’s accuracy and can cause vibrations. While backlash adjustment is possible in some reducers, in most cases, the reducer may need to be replaced once the wear level is reached.
• Vibration and Noise Analysis:
  Abnormal noises (humming, clicking, grinding) or increased vibration levels from the reducer can indicate a malfunction or wear in internal components (gears, bearings). Regular vibration analysis and acoustic measurements are important for early detection of potential failures and prevention of unplanned downtime. When such abnormalities are observed, the robot should be inspected immediately, and necessary interventions should be made.

Common Problems and Solutions for Reducer Types Used in Industrial Robot Arms

Industrial robot reducers, being complex and high-precision components, can occasionally encounter various problems. Accurate diagnosis and rapid resolution of these issues are critical to maintaining production efficiency.

1. Excessive Backlash Increase:

• Problem: Looseness in robot movements, positioning errors, vibrations, and loss of repeatability.
• Causes: Natural wear in gears or cycloidal discs, bearing play, overloading of the reducer, insufficient or incorrect lubrication.
• Solutions:
  • Inspection and Adjustment: Some reducer models may have backlash adjustment mechanisms (though rare in robotic reducers and requiring expert intervention).
  • Lubrication Check: Ensure the correct lubricant is used and lubrication intervals are followed. Contaminated or degraded lubricant accelerates wear.
  • Load Profile Analysis: Check if the robot is continuously operating above its nominal load. Overloading accelerates wear.
  • Replacement: If wear is advanced and backlash is out of tolerance, replacing the reducer or worn internal components (gear set, bearings) is usually the only solution.

2. Overheating:

• Problem: Abnormally high temperature in the reducer housing, lubricant degradation, hardening of seals and leaks, loss of efficiency.
• Causes: Overloading, insufficient or incorrect type of lubrication, contaminated lubricant, lack of ventilation, excessively high ambient temperature, increased internal friction (bearing or gear damage).
• Solutions:
  • Load Control: Review the robot’s load profile and ensure it operates within its nominal capacity.
  • Lubrication Check: Ensure the correct viscosity and amount of lubricant recommended by the manufacturer are used. Check the contamination status of the lubricant and replace if necessary.
  • Environmental Conditions: Check the operating environment temperature and ensure the reducer stays within its thermal limits. Apply additional cooling (air circulation, cooling fan) if necessary.
  • Internal Damage Check: If the above steps do not resolve the issue, a detailed inspection for internal bearing or gear damage may be required.

3. Abnormal Noises and Vibrations:

• Problem: Humming, grinding, clicking, or friction noises during operation, abnormal vibrations in the robot arm.
• Causes: Worn or damaged gears/cycloidal discs, bearing failure, incorrect assembly (alignment issues), loose fasteners, lack of lubrication.
• Solutions:
  • Listening and Observation: Try to identify the source and type of the sound. Observe how the sound changes with different robot motion profiles (slow, fast, loaded, unloaded).
  • Lubrication Check: Insufficient lubrication can cause friction noises. Check the lubricant level and quality.
  • Mounting Check: Check the tightness, alignment, and coupling connections of the reducer and motor mounting bolts.
  • Internal Damage Detection: If noises and vibrations persist, the reducer should be disassembled to check for wear, cracks, or damage in internal components (gears, bearings). Damaged components should be replaced.

4. Oil Leaks:

• Problem: Oil leakage from the reducer housing or seals.
• Causes: Wear, aging, or damage to oil seals (gaskets), excessive internal pressure (if ventilation is blocked), cracks in the housing, incorrect assembly.
• Solutions:
  • Seal Replacement: Identify the source of the leak and replace damaged oil seals or gaskets. Ensure original spare parts are used.
  • Ventilation Check: Check if the reducer’s ventilation hole is blocked. Blocked ventilation can increase internal pressure, leading to leaks.
  • Housing Check: Although very rare, cracks can occur in the reducer housing. In this case, the entire reducer usually needs to be replaced.
  • Oil Level: Overfilling with oil can also cause leaks. Ensure the oil is at the correct level.

5. Insufficient Torque Transmission or Slippage:

• Problem: The robot being unable to perform the desired movement under certain loads, feeling weak, the motor being excessively strained.
• Causes: Overloading of the reducer, severe wear/damage in internal gears or cycloidal discs, slippage in the connection (coupling) between the servo motor and the reducer.
• Solutions:
  • Load Analysis: Re-evaluate the robot’s operating load and torque requirements. Check if the reducer has sufficient torque capacity.
  • Coupling Check: Check the tightness and integrity of the coupling connection between the servo motor and the reducer. If there is slippage, tighten or replace the coupling.
  • Internal Damage Inspection: Check the internal structure of the reducer for broken teeth, worn tooth profiles, or other mechanical damage. In such cases, damaged components must be replaced, or the reducer must be renewed.

In case of any malfunction, the most appropriate approach is to first consult the technical manuals of the robot manufacturer or reducer supplier and seek support from authorized service personnel. Since reducers are one of the most critical and complex parts of a robot, unauthorized interventions can lead to greater damage.

Conclusion and Expert Advice on Reducer Types Used in Industrial Robot Arms

Reducers used in industrial robot arms are the unseen heroes of modern automation. These specially designed components, such as Harmonic, Cycloidal, and RV reducers, form the backbone of production lines by meeting the high speed, torque, precision, and repeatability requirements of robots. Each reducer type has its unique advantages and disadvantages, and making the right choice significantly impacts the robot’s performance, lifespan, and overall efficiency. For expert advice or to request a quote, contact Mermak CNC on WhatsApp today!