In automation design, the choice between a Stepper Motor and a Servo Motor is often decided by budget. But looking at the price tag alone is a mistake that leads to machine failure. Steppers are excellent for holding loads stationary (high holding torque). Servos are kings of high-speed motion. If you choose a stepper for a high-speed application, it will lose torque and "miss steps." If you choose a servo for a simple low-speed application, you have wasted $500. This guide explains the physics behind the choice. Table of Contents 1. Open Loop vs. Closed Loop (The Risk) 2. The Torque Curve: Speed Kills Steppers 3. Inertia Mismatch 4. Selection Summary Advertisement 1. Open Loop vs. Closed Loop (The Risk) The biggest difference is not the motor itself, but how it is controlled. Figure 1: Steppers run "blind" (Open Loop). Servos use an encoder to verify position (Closed Loop). ...
If you are looking for a robust indexing box for your dial application, the Sankyo AD Alpha Series is a top contender. Apart from its product specifications, we have expanded this guide to include useful technical context on why these drives are chosen over other technologies.
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Design and Core Technology
Figure 1: The low-profile AD Alpha Series unit features a ground pilot flange for precise mounting.
Sankyo's AD/Alpha Series features a low-profile cast iron housing with a ground pilot flange for mounting dial plates, hubless gears, or weldment fixtures.
The heart of the system is the Globoidal Cam mechanism. Unlike barrel cams, a globoidal cam is tapered to match the radius of the needle-bearing cam followers. This optimizes the contact area, maximizing torque transfer and significantly extending service life.
Above the indexing dial, a stationary hollow center allows engineers to route air lines, electrical wiring, or sensor cables directly through the center of the table—a critical feature for modern automation cells.
Why Choose Cam Indexing Over Direct Drive?
In an era of servo motors and direct drives, why do engineers still specify cam indexers?
- Zero Backlash Rigidity: The pre-loaded cam followers ensure that once the table is in the "dwell" (stopped) position, it is mechanically locked. This provides superior rigidity against machining forces (like drilling or pressing) compared to a servo motor holding position electronically.
- Simplicity: No complex tuning or feedback loops are required. The motion profile (acceleration/deceleration) is permanently machined into the hardened steel cam.
- Cost-Effectiveness: For fixed-station applications (e.g., always stopping at 4, 6, or 8 positions), a cam indexer is often significantly cheaper than a high-torque direct-drive motor.
Construction and Maintenance
Figure 2: Cutaway view showing the robust internal cam and follower assembly.
The AD Series is designed to be maintenance-free. The housing is sealed and filled with synthetic gear oil before shipping, meaning no routine oil changes are typically required for the life of the unit. This makes it ideal for clean-room environments or hard-to-access machinery.
Most units are driven with geared motors mounted within the profile height of the housing, eliminating the need for spacers or clearance holes in your base plate.
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Common Applications
- Automated Assembly: High-speed assembly of small consumer electronics or medical devices.
- Filling and Capping: Bottle filling lines where smooth acceleration/deceleration (S-curve motion) is required to prevent spillage.
- Welding Stations: The high rigidity allows for precise positioning of heavy weld fixtures.
- Packaging: Carton erecting and inspection stations.
Key Features
- Flexible Stops: 2 to 32 incremental output stops.
- Large Capacity: Dial diameters range from 280mm to 3600mm (1 to 12 feet).
- Speed: Maximum 200 cycles per minute, bi-directional indexing.
- Accuracy: ±30 arcsec indexing accuracy (average ±9 arcsec).
- Drive Flexibility: Index mounts in any position with right-angle gearmotor mounting in 90° increments.
- Control: Variable frequency drive (VFD) controllers utilize electronic braking, eliminating brake dust and wear.
Stationary Output Flange Riser Option
Figure 3: The center riser option provides a non-rotating platform for cameras or sensors.
This option is used if part handlers or measurement equipment are required to be mounted directly above the rotary dial. The riser acts as a rigid, stationary post in the center of the rotating table.
Each riser incorporates a tolerance male centering pilot for precise fixture mounting. It is bolted to the index output flange and includes extended grease fittings for easy access.
Find more details about Sankyo AD Alpha Series - Dial Index at Sankyo Automation Official Site.
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