The Indexer That Vibrated at Every Index
We sized a cam indexer for a rotary dial station. The load was 25 kg on a 600 mm dial. We picked the smallest indexer that met the torque rating on the datasheet. On the floor, every index cycle, the dial rocked back and forth for half a second before settling. The cycle time target was 0.5 seconds per index. The settling alone took 0.8 seconds. The problem wasn’t the indexer’s torque — it was the inertia. The indexer’s allowed inertia was lower than the dial’s actual inertia, so the cam couldn’t control the motion smoothly. It slammed into position and the dial oscillated.
Cam indexer selection for automation isn’t just picking one that handles the torque. The inertia, the index angle, the dwell period, and the accompanying load all determine whether the dial stops cleanly or rocks. This article is how I select rotary indexers that settle fast and last.
How a Cam Indexer Works
A cam indexer (also called a rotary index table or Geneva-style indexer) converts continuous rotary input (from a motor) into intermittent output. The cam profile controls the acceleration and deceleration of the dial during the index motion. When the dial is dwelled (stationary), the cam holds it rigidly.
The key advantage over a servo-driven dial: during the dwell, the dial is mechanically locked. It doesn’t drift. A servo holding position has a small following error; a cam indexer dwell has zero motion. For precision assembly where the part must be perfectly still during the operation, this matters.
The Four Numbers That Size the Indexer
The datasheet gives you torque and index angle. But four numbers determine whether the indexer works in your application.
1. Index Angle and Dwell
The index angle is how far the dial rotates in one cycle. A 4-station dial indexes 90° per cycle. A 6-station dial indexes 60°. The dwell is the stationary period between indexes. The cam profile is designed for a specific index angle — you can’t use a 90° indexer for a 60° application.
Most indexers are available in standard index angles (30°, 45°, 60°, 90°, 120°, 180°). Pick the one that matches your station count. The number of stations = 360° / index angle.
2. Index Time
How long does the index take? A 90° index in 0.3 seconds is fast. The cam profile accelerates and decelerates within that time. The shorter the index time, the higher the acceleration, and the more torque and inertia the indexer sees.
Index time is set by the input RPM. A cam indexer with a 120° index angle running at 60 RPM has an index time of 0.33 seconds. If you need a faster index, you run the input faster — but the acceleration force increases.
3. Inertia Load (The One Everyone Misses)
The indexer’s datasheet lists an “allowed inertia” or “maximum moment of inertia.” This is the maximum load inertia (dial + tooling + parts) that the cam can accelerate and decelerate smoothly. Exceed it, and the cam can’t control the motion — the dial oscillates after indexing.
Calculate the dial inertia: J = (1/2) × m × r² for a solid disk. For a dial with tooling, approximate each major mass as a point at its radius and sum them up.
If the dial inertia exceeds the allowed inertia, you have two options: a larger indexer, or a shorter index time (which actually makes it worse — faster acceleration means more inertia force). The right fix is a bigger indexer.
4. Torque Load
The indexer sees torque from two sources: accelerating the dial (inertia torque) and external process forces (if a station pushes against the dial during index). The peak torque rating on the datasheet must exceed both. For most assembly stations, the process force during index is small (the station operates during dwell, not during index). The inertia torque dominates.
| Parameter | What to Check | Common Mistake |
|---|---|---|
| Index angle | Matches station count (360/stations) | Using wrong angle for station count |
| Index time | Meets cycle target | Too fast → oscillation |
| Inertia load | Below allowed inertia rating | Ignored → dial rocks after index |
| Torque load | Peak torque below rating | Only checking static torque |
Cam Profiles: Smooth Acceleration Matters
The cam profile determines how the dial accelerates and decelerates during the index. Different profiles give different motion characteristics.
Common Cam Profiles
- Modified trapezoid: Constant acceleration in the middle, smooth ramps at the ends. Standard. Good balance of speed and smoothness.
- Modified sine: Smoother acceleration transitions. Lower vibration. Slower than trapezoid.
- Modified cycloid: Smoothest motion, lowest residual vibration. Slowest. For high-precision, low-vibration applications.
For most assembly automation, modified trapezoid is the default. If the dial oscillates after indexing (the inertia problem), try a smoother profile — but you may need a larger indexer to handle the same load at the slower acceleration.
Mounting and Driving the Indexer
The indexer itself is half the system. The motor, coupling, and mounting determine whether it performs.
Drive Motor
The indexer runs at constant input RPM. A gearmotor or servo motor drives the input shaft. The motor doesn’t need to start/stop — it runs continuously. The cam handles the indexing.
Sizing: the motor needs enough torque to overcome the indexer’s friction and accelerate the dial. Most indexer datasheets specify the required input power. A 600 mm dial with 25 kg load typically needs a 0.4–0.75 kW gearmotor.
Coupling
The motor connects to the indexer input via a flexible coupling. The coupling compensates for small misalignment between the motor and indexer shafts. Use a bellows coupling (zero backlash) for precision. A jaw coupling works for general use but has slight play.
Mounting Orientation
Most indexers mount with the output shaft vertical (dial on top). They can also mount horizontally, but the internal cam lubrication is designed for vertical mounting. Check the manufacturer’s orientation requirements.
The settling test: After installing the indexer, run it at the production speed. Time how long it takes for the dial to settle after each index. If it settles in under 100 ms, you’re fine. If it rocks for 0.5 seconds or more, the inertia is too high for the indexer. Step up one size.
Indexer vs. Servo Dial: Which to Choose
Not every rotary station needs a cam indexer. A servo-driven dial (servo motor + gearbox + absolute encoder) is an alternative.
| Feature | Cam Indexer | Servo Dial |
|---|---|---|
| Dwell rigidity | Mechanically locked (zero motion) | Holding torque (small following error) |
| Index accuracy | ±15–30 arcsec (cam repeatability) | ±10–50 arcsec (encoder + gearbox) |
| Flexibility | Fixed index angle (change cam to change) | Any angle, programmable |
| Speed | Fast (0.2–0.5 s index) | Slower (servo acceleration limits) |
| Cost | Lower (for fixed cycles) | Higher (motor + drive + gearbox) |
| Maintenance | Oil change annually | Minimal |
Use a cam indexer when the station count is fixed, the cycle is high-volume, and you need rigid dwell. Use a servo dial when you need programmable angles, occasional changeover, or fewer stations where the cam cost isn’t justified.
A Cam Indexer Selection Checklist
- How many stations? (Determines index angle.)
- What is the dial mass and radius? (Calculate inertia.)
- What is the index time target? (Determines input RPM and acceleration.)
- Does the dial inertia exceed the indexer’s allowed inertia?
- What is the peak torque (inertia + process)?
- Does the cam profile suit the vibration requirement?
- Is the drive motor sized for the input power?
- Is the mounting orientation within the manufacturer’s spec?
- Will the dial settle within the required time?
- Is a cam indexer the right choice, or would a servo dial be more flexible?
The Bottom Line
Rotary indexer design for custom machines isn’t picking a size from a torque chart. It’s calculating the dial inertia, checking it against the allowed inertia, matching the index angle to the station count, and verifying the settling time. The indexer that rocks after every cycle wasn’t under-torqued — it was under-sized for the inertia. Size for inertia first, torque second, and the dial will stop where it should, every time.
