Rotary index tables play a critical role in modern automated manufacturing across automotive, welding, assembly, aerospace, and heavy industrial applications. When integrated with industrial robots, these indexers can function as robot auxiliary axes, dramatically expanding reach, flexibility, and throughput.
Motion Index Drives manufactures a full line of high-precision rotary index tables and linear indexing systems engineered specifically to integrate with robot auxiliary axis motors, allowing rotary and linear motion to be controlled directly by the robot controller
SKY HOOK device using Motion Index Drives TMF and RT series indexers providing 2 additional axis to a Fanuc welding robot.
This article outlines the key engineering considerations you must understand before integrating a rotary indexer or linear indexing device as a robot auxiliary axis.
What Is a Robot Auxiliary Axis?
A robot auxiliary axis is an externally mounted rotary or linear axis that is driven and controlled by the robot’s controller rather than a standalone servo drive. Common examples include:
- Rotary index tables
- Trunnions and weld positioners
- Ferris wheel and Sky Hook welding systems
- Linear transfer and indexing conveyors
By integrating an auxiliary axis, the robot gains additional degrees of freedom, enabling:
- Better part access
- Reduced robot reach requirements
- Higher system flexibility
- Improved cycle times
Motion Index Drives dual trunnion system utilizing 3 FANUC auxiliary axis robot motors.
1. Auxiliary Axis Motors Must Be Properly Specified
Auxiliary axis motors are typically supplied by the robot manufacturer and must be purchased:
- At the time of robot order, or
- Matched to an existing robot using the robot’s serial number
Based on application data, including load, inertia, speed, and stop-time requirements, Motion Index Drives engineers recommend the correct auxiliary axis motor size and features to ensure compatibility with the indexing device
2. Robot Motion Profile Limitations Affect Indexer Size
Most robot controllers limit auxiliary axis motion to:
- Trapezoidal
- Triangular motion profiles
In both cases, acceleration and deceleration must be equal.
This limitation is significant because:
- Accelerating a load requires more torque than decelerating it
- Forced symmetry in accel/decel often increases required torque
- Indexer size may need to increase compared to a servo-driven system
In contrast, standalone servo-driven indexers can use asymmetrical motion profiles, allowing better optimization of acceleration and deceleration. When using robot auxiliary axes, this optimization is usually not available
3. Never Instantly Apply the Brake During E-Stops or Light Screen Events
One of the most common causes of premature indexer failure is instant brake application during:
- Light curtain violations
- Emergency stop conditions
All rotating mass systems must use programmed deceleration. Instant braking introduces extreme forces that can:
- Damage internal cams and followers
- Overload bearings
- Shorten motor and gearbox life
Most robot manufacturers offer special safety software packages that allow independent deceleration profiles for safety events. These packages are strongly recommended for auxiliary axis applications
4. Encoder Count Rollovers Must Be Addressed
If the auxiliary axis will rotate continuously in one direction, the control software must be capable of handling encoder count rollover.
Failure to manage rollover can result in:
- Position loss
- Axis faults
- Unexpected motion behavior
This is a software-level consideration that must be addressed early in system design.
5. Gear Ratios Must Be Entered Exactly
A common programming mistake is entering only the gearbox ratio into the robot controller.
For rotary index tables, the total reduction ratio must include:
- Gear reducer ratio (if present)
- Internal cam or transmission ratio of the indexer
Incorrect ratio input leads to:
- Positioning errors
- Poor accuracy
- Increased mechanical stress
Controllers that allow numerator/denominator input or exact ratio values should always be used for best results
6. Validation Is Critical for Long-Term Reliability
The #1 cause of auxiliary axis indexer issues is improper programming—not mechanical failure.
Motion Index Drives strongly recommends having a qualified technician:
- Validate motion profiles
- Review deceleration settings
- Confirm ratio and inertia calculations
Validation can be performed on-site or remotely via video, and it dramatically reduces the risk of premature failure
Zero-Backlash Cam Technology: A Major Advantage
Motion Index Drives auxiliary axis rotary index tables use cam-driven, zero-backlash technology, offering:
- True zero backlash
- Exceptional rigidity
- Multiple cam followers distributed around the dial OD
- High torque capacity in a compact footprint
This mechanical design is ideal for robotic applications where precision, repeatability, and vibration-free motion are critical
Mass Moment of Inertia: The Foundation of Proper Sizing
As with all rotary index table applications, mass moment of inertia is the most important sizing parameter.
In auxiliary axis systems, inertia calculations are used to:
- Determine achievable index speed
- Evaluate stopping forces
- Ensure acceptable inertia mismatch with the robot motor
Thanks to the rigidity of Motion Index Drives’ cam indexers, inertia mismatches of 10:1 or greater can still operate smoothly without vibration or motor overload when properly engineered
Small RT series indexer with explosion proof Motoman auxiliary axis motor
Stop Time Requirements Can Increase Indexer Size
Fast stop requirements—such as 100 ms emergency stops—can dramatically increase internal forces compared to longer stop times (e.g., 500 ms).
Using robot safety software to soften deceleration during safety events often allows:
- Smaller indexer size
- Lower system cost
- Longer component life
This tradeoff should be evaluated early in system design.
Beyond Rotary: Linear Indexing with Robot Auxiliary Axes
Motion Index Drives also offers LFA Precision Indexing Conveyors, fully programmable linear indexing systems that can be configured to accept robot auxiliary axis motors.
Key advantages:
- Linear layouts with rotary-level accuracy
- ±0.06 mm accuracy in X, Y, and Z
- Flexibility without sacrificing precision
Traditionally cam-driven, these systems are now increasingly requested in robot-driven configurations to meet modern flexibility demands
LFA precision indexing conveyors ready to accept robot auxiliary axis motors.
Final Thoughts: Engineer the Axis, Not Just the Motion
Successful robot auxiliary axis integration depends on:
- Accurate inertia data
- Proper motor selection
- Controlled acceleration and deceleration
- Correct ratio programming
- Professional validation
When engineered correctly, auxiliary axis indexers deliver years of reliable, high-precision performance in even the most demanding robotic applications.
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