How Do I Size a Rotary Indexing Table Drive Correctly?

At 5 r/min, the table reaches 0.524 rad/s. The number that matters during acceleration is angular inertia multiplied by angular acceleration; during constant speed, it is friction and transmission loss. Positioning error is then set by feedback resolution, backlash, compliance, sensor repeatability, counter bandwidth, and the distance between the measurement point and the table.

Motion quantities and design limits

The design data changed during development, so freeze the mechanical basis before selecting a motor. The initial concept used a 2,300 mm table, 10 stations, 5 r/min, and 4,000 kg at full load. A later correction referred to 400 kg, while the updated six-station design used a 1,500 mm table, six 55 kg workpieces at a 570 mm radius, 5 r/min, and 2-second acceleration and deceleration.

Resolve whether the stated 400 kg includes the six workpieces. That single definition changes the calculated acceleration torque. Also record bearing friction, external process forces, total reduction, gear efficiency, backlash, permissible settling time, and required holding torque. Read these values from the bearing, gearbox, drive, and coupling data rather than inserting assumed efficiencies.

Feedback and locating approaches

Encoder count alone is only one part of positioning accuracy. A high-resolution device can report motion on the motor shaft while the table remains displaced by gearbox backlash or torsional windup. Compare architectures at the load, where the tolerance applies.

For a production indexer, the preferred architecture is load-side feedback or repeatable one-direction approach combined with a mechanical locating feature at each station. Use the drive to enter the capture region, then use the locator to establish final position. If mechanical indexing is unavailable, mount feedback as close to the table as practical and measure the complete backlash band before accepting the design.

Recommended control architecture

Use the Siemens S7-1200 for sequencing, a hardware high-speed counter for incremental feedback, a repeatable home sensor, and a drive capable of controlled acceleration and deceleration. Keep command generation and position feedback separate: the command requests motion, while the feedback proves where the table actually stopped.

The table ultimately indexes in one direction from station 1 through station 10 in the original operating concept. That materially improves repeatability because every target can be approached on the same gear flank. On startup, find home from the prescribed direction, establish zero, and retain the same rotational direction for normal indexes. If reverse motion is required for recovery, move past a known point and re-approach it in the normal direction before accepting position.

An inductive sensor and metal target can define home, but the sensor is not a precision encoder. Its switching distance, target geometry, mounting rigidity, and approach speed determine repeatability. Capture a consistent switching edge at low speed, apply any measured home offset, and verify the final station mechanically.

Load inertia and acceleration torque

This is heat and mechanics, not logic. A table can need little torque at steady speed yet demand substantial torque during a short acceleration. For a rigid axis:

ω = 2πn/60

α = Δω/Δt

Tacc = Jtotal × α

At 5 r/min, ω = 0.524 rad/s. With a 2-second acceleration from rest, α = 0.262 rad/s².

If 400 kg already includes the workpieces, construct the actual inertia from the table structure and each load rather than adding both rows. Loads at the rim contribute through the square of radius, so total mass alone cannot size this axis.

Add bearing friction, seals, gear friction, any unbalanced load, and the inertia of rotating transmission components. Reflect table torque to the motor with:

Tmotor = Ttable/(itotal × ηtotal)

Reflect load inertia with:

Jreflected = Jload/itotal²

The reported 3.2 N·m motor requirement cannot be checked from a reduction of i = 40 alone because an additional crown-and-pinion reduction, total efficiency, and friction torque were not specified. Record every reduction stage and its efficiency in the calculation. Check both peak acceleration torque and continuous thermal torque against the drive’s speed-torque data.

Encoder resolution at the table rim

A 500-pulse/revolution encoder gives 50 pulses per 36° station when mounted directly on a ten-station table and decoded at one count per stated pulse. That arithmetic does not provide acceptable peripheral resolution on a 2,300 mm diameter table:

C = πD = π × 2300 mm = 7226 mm

Peripheral increment = C/N = 7226/500 = 14.45 mm per count

Station 3 is nominally count 150, but commanding count 150 does not make the motor stop instantaneously on that edge. The controller must begin deceleration before the target and close the remaining position error at low speed. Mechanical coast, loop delay, backlash, and feedback location determine final error.

Confirm whether the encoder rating means pulses, cycles, or decoded counts per revolution. A quadrature receiver may count one, two, or four edges per cycle. Use the effective counter resolution in both position and frequency calculations.

A proposed 40 mm pinion driving a rack around a 2,000 mm table produces a geometric ratio of 50:1. With a 1,000-count encoder on the pinion shaft, one count advances the pinion circumference by approximately π × 40/1000 = 0.126 mm. That is also approximately 0.126 mm of rack and table-rim travel; dividing it by 50 again would apply the ratio twice.

Counter frequency and PLC capture

At 225 r/min, a 1,000-pulse/revolution encoder produces:

f = 225/60 × 1000 = 3750 Hz

That is below the cited 5 kHz counter limit only when the limit and the encoder calculation use the same edge convention. Four-edge quadrature decoding would produce 15,000 counts/s. Read the S7-1200 CPU and signal-board documentation for the selected input mode, maximum count frequency, supported voltage level, wiring topology, and input filtering.

Update position with a hardware high-speed counter. A normal cyclic PLC rung samples too slowly and with too much timing variation to count a multi-kilohertz encoder reliably. The PLC program should read the captured hardware count, extend or normalize it as required, compare it with the motion target, and supervise motion plausibility.

Check the complete signal chain: encoder output type, receiver compatibility, cable shielding, reference potential, maximum cable length from the applicable manuals, and the maximum count rate at the highest possible overspeed. The design frequency must include commissioning jogs and fault-driven overspeed, not only the nominal 5 r/min table speed.

Motor and drive selection

Low table speed does not by itself favor a stepper. The reduction can place the motor near 500–600 r/min, where available torque depends on winding, bus voltage, drive current, and acceleration. Select from the manufacturer’s speed-torque curve and include gearbox losses. A motor rated by 0.3 N·m holding torque cannot be approved by comparing that number with a calculated running torque.

A vector-controlled inverter application may require an inverter-duty, separately ventilated induction motor, particularly when continuous torque is needed at low shaft speed. A six- or eight-pole motor can reduce the required gear ratio and associated losses, but select pole count only after calculating motor speed, transmission efficiency, and cooling.

A pulse-train PLC output can perform the positioner role for a compatible step-and-direction drive. Confirm pulse frequency, electrical interface, direction timing, acceleration generation, and how the drive reports ready, in-position, following error, and fault. Adding an encoder to a stepper does not automatically make a PLC function intended for another motor type into a closed-loop stepper controller; the selected drive or PLC motion function must explicitly close that loop.

Homing, backlash, and accumulated error

Incremental counting does not inherently accumulate one count of error at every station. If no counts are lost and the mechanical relationship remains fixed, targets should be absolute multiples from home: 0, 50, 100, 150, and so on for the 500-count, ten-station example. Error accumulates when pulses are missed, the measuring wheel slips, the coupling moves, the counter wraps incorrectly, or backlash changes the relationship between encoder and table.

Run a home search at every machine power-up because an incremental encoder cannot prove that the table remained stationary while control power was absent. Use a reference sequence that approaches the same sensor edge from the same direction and at the same final speed. Home again after encoder faults, count plausibility faults, mechanical work, coupling disturbance, unexpected reverse motion, or any event that makes the count-to-table relationship uncertain.

For one-direction production indexing, keep the gear train loaded on one flank. If recovery requires reverse travel, perform backlash take-up and a new reference approach before automatic operation. Measure repeatability by approaching the same station repeatedly from the production direction, then repeat the test after a reverse move; the difference exposes lost motion.

Commissioning procedure

1. Freeze the design mass, load locations, diameter, station count, acceleration time, settling time, and tolerance at the actual pickup radius.
2. Calculate table inertia from each component. Add acceleration torque, measured or conservatively specified friction torque, process torque, and transmission losses.
3. Select the total reduction and calculate motor speed, peak torque, continuous torque, and reflected inertia. Check the drive and motor curves rather than holding torque or nominal power alone.
4. Select the feedback location. Prefer table-side measurement or a mechanical locator when backlash exceeds the position budget.
5. Calculate counts per station, peripheral increment per count, and maximum counter frequency using the actual decoding mode.
6. Configure the S7-1200 hardware counter and motion output for compatible electrical signals. Add drive-ready, fault, in-position, home, and motion-permission interlocks.
7. Home from the defined direction at reduced speed. Set the machine coordinate only on the qualified sensor edge and apply the measured home offset.
8. Index through every station at reduced acceleration. Record commanded count, actual count, physical station error, settling time, motor current, and drive status.
9. Repeat each station from the normal direction, then introduce a reverse move and repeat the test to quantify backlash.
10. Run at production acceleration and full load. Confirm peak current during ramps, continuous current at steady cycling, gearbox temperature, stopping repeatability, and locator engagement.

Resolution passes only when the measured physical error remains inside tolerance across all stations, loads, temperatures, and approach histories. Encoder counts alone are not the acceptance measurement; use a suitable external dimensional or angular reference at the pickup point.

Power-loss recovery and sequence verification

A power interruption can leave a transfer arm between the table and CNC with a workpiece still gripped. Position retention cannot reveal whether the interrupted transaction was loading or unloading, nor whether a part has been released. Store sequence state, but validate it against physical sensors after power returns.

Automatic restart must remain inhibited after an interrupted cycle. Present a supervised recovery mode that lets the operator move individual actuators under interlocks, determine workpiece occupancy, and return the mechanism to a defined recovery state. Homing a loaded or partially engaged mechanism can create a collision, so permit reference motion only when arm position, grip state, vertical clearance, table state, and machine-side occupancy satisfy the recovery conditions.

Frequently asked questions