The Indexing Table That Overshot Every Cycle
We used a pneumatic rotary index table to position parts between assembly stations. The table indexed 90° (4 stations). On the bench, it stopped accurately. On the floor, it overshot — the table kept rotating past the 90° stop. The reason: the table’s rotational inertia (the table plus the part) was too high for the pneumatic stop. The pneumatic brake couldn’t stop the table fast enough. It slammed into the stop and bounced back. We added a pneumatic cushion (a shock absorber at the stop position). The table decelerated into the stop and held. The mistake was not calculating the table’s kinetic energy and matching a shock absorber.
Pneumatic indexing table sizing is about the inertia and the stop. The table must start, rotate, and stop accurately. This article covers the design.
How a Pneumatic Index Table Works
A pneumatic index table (rotary indexer) rotates a fixture to multiple positions. Standard types:
- Pneumatic ( Geneva / cam-free): A pneumatic actuator drives the rotation. A mechanical stop (or a pneumatic brake) holds the position. Cheap, simple. But not as accurate as a cam indexer (article 31).
- Cam indexer (mechanical): A motor drives a cam that indexes precisely. More accurate, faster. But more expensive.
- Servo rotary table: A servo motor drives the rotation. Programmable position. Most flexible but most expensive.
For simple, low-speed, multi-position work, a pneumatic index table is standard.
Step 1: Number of Stations and Index Angle
The table has N stations. The index angle is 360°/N. For 4 stations, the index is 90°. For 6 stations, 60°.
The table’s design must match the index angle (the stop positions are at each station). Don’t pick a 4-station table and try to use it for 6 stations — the stops are wrong.
Step 2: Table Payload (Mass and Diameter)
The table carries the fixture and parts. The payload affects:
- Mass (kg): Heavier table = more inertia = harder to start and stop.
- Distance from center (radius): Mass at a larger radius creates more inertia than mass at the center. The table’s moment of inertia is I = m × r² (for a point mass).
For a table that’s 300 mm in diameter, carrying a 5 kg part at 100 mm radius: I = 5 × (0.1)² = 0.05 kg·m². For a 10 kg part at 150 mm radius: I = 10 × (0.15)² = 0.225 kg·m². The second case has 4.5× the inertia.
Step 3: Rotational Speed and Kinetic Energy
The table rotates at some speed (ω, rad/s). The kinetic energy at the stop is:
KE = ½ × I × ω²
For I = 0.1 kg·m² and ω = 2 rad/s (about 19 rpm): KE = 0.5 × 0.1 × 4 = 0.2 J. Small. But at ω = 5 rad/s: KE = 0.5 × 0.1 × 25 = 1.25 J. Larger. The stop (shock absorber) must absorb this energy.
The index time: how fast must the table rotate? If the table must index 90° (π/2 rad) in 1 second: ω = π/2 / 1 = 1.57 rad/s. At 0.5 seconds: ω = π rad/s = 3.14 rad/s. Faster index = more kinetic energy = bigger shock absorber.
Step 4: Shock Absorber (Stop)
The table slams into a mechanical stop at each position. A pneumatic shock absorber (or a hydraulic damper) cushions the stop. The shock absorber must absorb the kinetic energy without being overloaded.
E_shock ≥ KE = ½ × I × ω²
The shock absorber’s energy rating (J per cycle) must exceed the table’s KE. For KE = 1.25 J, use a shock absorber rated for 2 J (with margin).
The table that overshot had no shock absorber (just a hard mechanical stop). The table slammed in and bounced. Adding a shock absorber (sized for KE = 1.5 J) stopped it cleanly.
The index table rule: Calculate the table’s moment of inertia (I = m × r²), the rotational speed (ω), and the kinetic energy (KE = ½ I ω²). Size the shock absorber for KE with margin. The table that overshot had no cushion — a hard stop. Add a pneumatic shock absorber.
Step 5: Holding the Position
After the table stops, it must hold the position (against assembly forces). The table’s brake (or detent) locks the position.
- Detent (spring-loaded pin): A spring pin drops into a groove at each station. Holds against light forces.
- Pneumatic brake: A pneumatic disc brake clamps the table’s shaft. Holds against heavy forces (assembly, pressing).
If the work (assembly) applies force on the table, use a pneumatic brake (not just a detent). Otherwise the table creeps.
| Element | Sizing |
|---|---|
| Index angle | 360° / number of stations |
| Payload | Mass × radius² (moment of inertia) |
| Rotational speed | Index angle / index time |
| Kinetic energy | ½ × I × ω² |
| Shock absorber | Rated for KE with 1.5× margin |
| Holding | Detent (light) or pneumatic brake (heavy) |
Pneumatic vs Cam Indexer
For high-speed indexing (fast cycles, many parts), a cam indexer (article 31) is better. It stops accurately (no bounce) and handles high speed. But it’s more expensive.
For low-speed, simple, occasional indexing (a few cycles per minute), a pneumatic table is fine (with a shock absorber).
A Pneumatic Index Table Checklist
- How many stations? (Index angle = 360°/N.)
- What is the payload? (Mass and radius?)
- What is the moment of inertia? (I = m × r².)
- What is the index time? (ω = angle / time.)
- What is the kinetic energy? (KE = ½ I ω².)
- Is the shock absorber sized for KE? (With margin?)
- How is the position held? (Detent or pneumatic brake?)
- What assembly forces act on the table? (Brake sized?)
- Is the table’s rotation smooth? (No backlash?)
- Is there a home position sensor? (Confirms station 1?)
- Is the table lubricated? (Oil mist or greased?)
- Is this fast enough for the cycle time? (Or switch to cam?)
The Bottom Line
Pneumatic index table sizing is the kinetic energy at the stop. The table that overshot had no shock absorber — it slammed into a hard stop. Calculate the moment of inertia (I = m × r²), the rotational speed (ω = angle/time), and the kinetic energy (KE = ½ I ω²). Size a pneumatic shock absorber for that energy. Use a detent for light holding, or a pneumatic brake for heavy assembly forces. The table that stops cleanly every time wasn’t the biggest one — it had the right shock absorber.
