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Indexing Conveyors for Delicate, High-Value Assemblies

In this article, an indexing conveyor is a unit-handling conveyor that moves an assembly a fixed step, stops it at a station for work, and moves it again; when the assembly is a flight unit, an electronics module, or a precision subassembly, the stop and the move between stops are design inputs alongside the throughput. UTEC Industrial designs, engineers, machines, fabricates, and installs custom material handling systems for aerospace and heavy industry from its Spokane Valley, WA facility, integrating Allen-Bradley PLC and motion control with in-house CNC machining, heat treating, and stress relief. The article covers how precision chain and servo axes hold an index position, what the Logix motion instructions offer and warn about for index moves, where a mechanical cam still fits, and the guarding, safety functions, sensing, and acceptance steps that protect the product and the people at each station. The conveyor sits inside the build chain, design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring, and as engineering reasoning, its index repeatability depends on the frame and machined interfaces upstream as well as on the drive and controls downstream.

Where does an indexing conveyor fit among unit-handling conveyors?​

ASME B20.1-2024, the Safety Standard for Conveyors and Related Equipment, has a publisher scope stating that it applies to the design, construction, installation, maintenance, inspection, and operation of conveyors and conveying systems in relation to hazards. It says the conveyors may be of the bulk material, package, or unit handling types, where the installation is designed for permanent, temporary, or portable operation, and that the Standard applies, with the exceptions it notes, to all conveyor installations. It excludes any conveyor designed for, installed for, or used primarily for the movement of human beings, but it does apply to certain conveying devices that incorporate within their supporting structure work stations or operator's stations specifically designed for authorized operating personnel. As this article reads that scope, an indexing conveyor that carries assemblies falls within the unit-handling types it names. This article states no B20.1 clause requirements; those are read from the standard itself.

CEMA publishes the vocabulary and the application reference for this class:

  • Terms. ANSI/CEMA 102-2022, Conveyor Terms & Definitions, is described on its store page as listing and defining over 1,500 terms, including 100 illustrations, with many of the more than 150 types of conveyors illustrated.
  • Application. The CEMA Application Guide for Unit Handling Conveyors, 2nd edition (2016, 680 pages), is described on its store page as intended to help designers, engineers, building contractors, operations managers, and procurement managers make informed decisions about designing, developing, or purchasing conveyor systems that will optimally integrate into customer-defined systems.

The difference between unit-load and bulk handling, and which CEMA standards fall on each side, is set out in the unit-load vs. bulk-solids article and is not repeated here (ASME B20.1-2024; ANSI/CEMA 102-2022; CEMA Application Guide for Unit Handling Conveyors, 2nd ed., 2016).

What makes an assembly delicate from the conveyor's point of view?​

For a conveyor, "delicate" means the assembly can be damaged by the handling itself. Three sources from space-hardware and electronics practice bear on it:

  • Mechanical shock and ESD in spacecraft handling. ECSS-Q-ST-20-08C covers the storage, handling, and transportation of spacecraft hardware, and applies to ground support equipment where its clauses say so. Its §6.3.1 calls for a dedicated procedure, with risk analysis and mitigation in every critical or hazardous step, and its NOTE 1 gives mechanical shock and ESD as examples. Its §6.3.4 calls for non-interruptible sequences to be identified.
  • Grounding of moving equipment. NASA-HDBK-8739.21 states that carts, wagons, trams, or other mobile equipment must be grounded while in use in an ESD protected area (§7.11.3). Its Table 7-1 verification is less than 1 ohm tied directly to equipment ground, or less than 10⁹ ohm to common point ground through conductive flooring.
  • ESD program. ANSI/ESD S20.20-2021 covers the administrative and technical requirements for establishing, implementing, and maintaining an ESD control program. It is cited here at standard level only.

An indexing conveyor is fixed equipment, not a cart, so the handbook's cart clause does not apply to it by its own terms. As engineering reasoning, a pallet that rides on plastic guides or rollers can still be insulated from the grounded frame, which raises the question that clause answers. Material selection, cleanliness, and ESD grounding for equipment near flight hardware are covered in the cleanroom and ESD requirements article. This article stays with the conveyor's own contribution: the acceleration and jerk of each index, the datum the part is located to, and what happens when a move is interrupted (ECSS-Q-ST-20-08C, §1, §6.3.1, and §6.3.4; NASA-HDBK-8739.21, §7.11.3 and Table 7-1; ANSI/ESD S20.20-2021).

How does precision conveyor chain hold an index position?​

Tsubaki's 1997 Complete Guide to Chain names wear and elongation as the major obstacles to using chain in accurate indexing drives, and describes precision conveyor chains, developed in the 1980s for this application, which it says do not wear or elongate. It gives examples from its own product line:

  • Construction. Needle bearings are installed between the pins and bushings, which adds rolling elements and eliminates the sliding friction between them.
  • Elongation. Immediately after installation the chain stretches a little, less than 0.03 percent, to fit the contacting surfaces, and after that it does not stretch.
  • First example. In automatic installation of electronic parts with 30 stations, a conveying speed of 10 m/min, and a 0.6 s index, the positioning accuracy is ±0.2 mm, using a positioning pin.
  • Second example. A more accurate chain is described for an assembling machine with 46 stations, 10 m/min, a 1.0 s index, and a stopping accuracy of ±0.15 mm; each part is measured, and the installing accuracy is ±0.1 mm.

The same section gives the handling limits. The contact surfaces between pins, needles, and bushings are small, and a load larger than the needle bearing's allowable static load causes permanent deformation and the chain will not operate correctly; the guide says the chain tension, including inertia, should be lower than the rated allowable load. Because of the chain's low bending resistance, it will vibrate on the low-tension return side, and guide rails or guide rollers help. The stainless version has a limited allowable load because of the low hardness of its contacting parts, and it elongates gradually. For engineering-class precision chain, made-to-order machined-tooth sprockets are used instead of standard flame-cut sprockets.

These are one maker's product examples from 1997, not general performance figures. The ±0.2 mm figure is stated with a positioning pin, so as engineering reasoning, the final location in that example comes from the pin engaging the pallet or attachment, with the chain delivering the part close enough for the pin to engage (U.S. Tsubaki 1997, The Complete Guide to Chain, Applications §3.1 and §3.2, pp. 149–153).

Why does the shape of the index move matter for a delicate load?​

On a servo-driven indexer, the move between stations is a programmed motion profile. In Rockwell Automation's Logix5000 Controllers Motion Instructions reference manual, the Motion Axis Move (MAM) instruction is used to move an axis to a specified position. Its move types include Absolute, which moves the axis to an absolute position, and Incremental, which moves it a specified distance from where it is now. Its Profile operand selects Trapezoidal (0) or S-Curve (1). In the operand table for the Motion Axis Jog (MAJ) instruction, the manual says a value must always be entered for the accel and decel jerk operands, that the instruction uses them only if the profile is configured as S-curve, and it gives 100 (% of Time) as values to get started.

Two academic sources describe why the profile choice matters, at book and abstract level only:

  • Vibration and load effects. The publisher's description of Biagiotti and Melchiorri's Trajectory Planning for Automatic Machines and Robots says suitable trajectories are planned to avoid undesired effects such as vibrations or even damage to the mechanical structure, and that the choice of trajectory has implications for actuator and reduction-gear dimensioning, the vibrations and efforts generated on the machine and on the load, and tracking errors.
  • Higher-order trajectories. Lambrechts, Boerlage, and Steinbuch's abstract, on trajectory planning and feedforward design for single-axis motion control, reports an algorithm for higher-order trajectories with bounds on all considered derivatives for point-to-point moves, shows these are time-optimal in the most relevant cases, and reports that simulations and hardware-in-the-loop experiments showed superior effectiveness of fourth-order feedforward compared with lower-order feedforward.

None of these sources studies product damage on an indexing conveyor. The link to the product is this article's engineering reasoning: a trapezoidal profile changes acceleration abruptly at the start and end of each ramp, an S-curve limits the rate of that change, and a loosely fixtured or top-heavy assembly on a pallet is the part of the load least able to follow an abrupt change. On the same reasoning, the failure modes to test for are a part shifting on its pallet and a fixture striking its locator at the end of the index (Rockwell Automation MOTION-RM002O-EN-P, pp. 118, 127, 130–131, and 142; Biagiotti and Melchiorri 2008, publisher description; Lambrechts et al. 2005, abstract).

What goes wrong when an index move is changed while it is running?​

Rockwell's manual carries a programming-guidelines warning on the risk of velocity and end-position overshoot. It applies if move parameters are changed dynamically by any method, either by changing move dynamics with the Motion Change Dynamics (MCD) or Motion Coordinated Change Dynamics (MCCD) instruction, or by starting a new instruction before the last one has completed. Under that warning:

  • Trapezoidal. A trapezoidal velocity profile can overshoot if maximum deceleration is decreased while the move is decelerating or is close to the deceleration point.
  • S-curve. An S-curve profile can overshoot if maximum deceleration is decreased while the move is decelerating or close to the deceleration point, or if maximum acceleration jerk is decreased while the axis is accelerating; the manual notes that jerk can be changed indirectly if it is specified in % of time.

A separate guideline for the Motion Axis Stop (MAS) instruction, on the next page, says that when an MAS is executed, the axis uses the new deceleration and jerk rates for the motion already in process, which can cause it to overshoot its speed, overshoot its end position, or reverse direction, and that S-curve profiles are more sensitive to parameter changes.

As engineering reasoning, end-position overshoot on an indexing conveyor means the pallet passes its station datum, into a locating pin, a process tool, or the next pallet. That argues for treating index dynamics as fixed recipe values that are changed only at standstill, for stopping with the same profile the move was started with, and for testing overshoot at acceptance with the heaviest and lightest loads the conveyor will carry (Rockwell Automation MOTION-RM002O-EN-P, Programming Guidelines, pp. 86–87 and 120).

What traps does a rotary index table set when it is programmed as a rotary axis?​

A dial or turntable indexer can be programmed as a Rotary-mode axis, and the MAM instruction's rotary move types carry conditions that the manual marks as important:

  • Rotary Shortest Path. Use it only if the axis Positioning Mode is Rotary. It is a special type of absolute move that takes the axis to the specified position in the shortest direction, regardless of its current position, and through 0° if needed. It can be started while the axis is moving or standing still, but it cannot move the axis more than one revolution with a single move.
  • Rotary Positive. Use it only while the axis is standing still and not moving; otherwise, the manual says, the axis could move in the wrong direction.

For a table carrying fixtured assemblies past operators and tooling, a move in an unexpected direction is the named failure mode. As engineering reasoning, three rules follow. Station positions are commanded as absolute targets, so an aborted index recovers to a known station rather than adding a partial step to the next one. Rotary Positive moves are interlocked to a confirmed standstill. The one-revolution limit on Rotary Shortest Path is checked in the logic that computes each target. Whether the direction of a table may reverse at all belongs in the machine's risk assessment, and drive-integrated safe-direction monitoring is described in the transfer-car positioning and interlocks article (Rockwell Automation MOTION-RM002O-EN-P, MAM move types, pp. 130–131 and 156–157).

When does an electronic cam replace a mechanical cam indexer?​

As this article describes it, a mechanical cam indexer turns a constant-speed input into an index-and-dwell output through the shape of a machined cam. Norton's Cam Design and Manufacturing Handbook is a reference for that approach; its contents include chapters on double-dwell cam curves, single-dwell cam curves, dynamics of cam systems covering force, torque, and vibration, residual vibrations in cam-follower systems, and failure of cam systems by stress, wear, and corrosion. This article cites it at contents level only and gives no motion-law coefficients from it.

The electronic alternative is a servo axis following a cam profile in the controller:

  • Publisher's view. The publisher's description of Biagiotti and Melchiorri's book says the concept of "electronic cams" has replaced, in the design of automatic machines, the classical approach based on "mechanical cams". That is the publisher's claim, not a survey result.
  • Controller instruction. In the Logix motion manual, the Motion Axis Position Cam (MAPC) instruction provides electronic camming between any two axes according to the specified cam profile.

The trade-off below is engineering reasoning, not a finding of these sources. A mechanical cam fixes the motion law, dwell angle, and index angle in hardware, so it cannot be mis-set in the field, but changing the product pitch or index time means a new cam. An electronic cam puts the same choices in a profile that can be changed for each product, and that flexibility brings the parameter-change overshoot risk described above, so the profile needs the same change control as any other recipe value (Norton 2009, Cam Design and Manufacturing Handbook, contents; Biagiotti and Melchiorri 2008, publisher description; Rockwell Automation MOTION-RM002O-EN-P, p. 196).

Which guarding rules and safety standards apply at the stations?​

An indexing conveyor has pinch points at every station where a moving pallet meets a locator, stop, or fixed frame, and power-transmission parts at its drive. In the US, two OSHA general-industry sections bear on it:

  • General machine guarding. 29 CFR 1910.212(a)(1) requires one or more methods of machine guarding to protect the operator and other employees in the machine area from hazards such as those created by point of operation, ingoing nip points, rotating parts, flying chips, and sparks. Under (a)(2), guards are affixed to the machine where possible, or secured elsewhere if for any reason attachment to the machine is not possible, and a guard must not offer an accident hazard in itself.
  • Drive sprockets and chains. 29 CFR 1910.219(f)(3) states that all sprocket wheels and chains shall be enclosed unless they are more than seven (7) feet above the floor or platform. Where the drive extends over other machine or working areas, protection against falling shall be provided, and the paragraph does not apply to manually operated sprockets. The section covers mechanical power-transmission apparatus; it does not say whether a chain that carries the pallets is a power-transmission chain.

The safety design itself comes from the machine's risk assessment under ISO 12100:2010, General principles for design — Risk assessment and risk reduction. Four further standards, cited at standard level, frame it:

  • ISO 14118:2017 specifies requirements for designed-in means to prevent unexpected machine start-up, to allow safe human interventions in danger zones, for all energy sources; it does not specify performance levels, safety integrity levels, or machine-specific means.
  • ISO 14119:2024 specifies principles for the design and selection of interlocking devices associated with guards.
  • ISO 13855:2024 specifies requirements for the positioning and dimensioning of safeguards with respect to the approach of the human body or its parts; approaches such as running, jumping, or falling are not considered, and it applies to safeguards used on machinery for the protection of persons 14 years and older.
  • ISO 13849-1:2023 specifies a methodology and provides related requirements, recommendations and guidance for the design and integration of safety-related parts of control systems that perform safety functions, for high demand and continuous modes of operation only; it does not apply to low demand mode of operation.

Drive-integrated functions for a stopped position, such as safe operating stop and safely limited position, are described in the transfer-car positioning and interlocks article and are not repeated here (OSHA 29 CFR 1910.212-1974; OSHA 29 CFR 1910.219; ISO 12100:2010; ISO 14118:2017; ISO 14119:2024; ISO 13855:2024; ISO 13849-1:2023).

What sensing and controls confirm each index?​

A servo indexer knows where its motor is; the station needs to know where the part is. The intelligence layer is built from four pieces:

  • Motor feedback. The Kinetix 5700 user manual's drive-system overview table describes the ERS3 and ERS4 inverters as supporting DSL and Hiperface encoder feedback (p. 16). Its Appendix D gives the current regulator loop settings (p. 340) and refers to position-loop, velocity-loop, and torque-loop axis configurations (p. 356).
  • Controller tasks. In the Logix 5000 design considerations manual, controller tasks can be configured as continuous, periodic, or event (Ch. 5, p. 39), and a periodic task performs a function at a specific time interval (p. 41). Event-task triggers are tabulated on p. 43, and the trigger list differs by controller family (Ch. 4, p. 30).
  • Torque removal. The Kinetix manual states that 2198-xxxx-ERS3 and ERS4 drives are equipped for hardwired and integrated safe torque-off (p. 241). It also states that disabling the power transistor output does not provide physical isolation of the electrical output that is required for some applications (p. 250). As engineering reasoning, it is therefore not a substitute for electrical isolation during maintenance.
  • Station confirmation. The index is confirmed at the station by a sensor independent of the motor encoder, such as a pin-engaged switch or a pallet-present sensor, before the station's process is allowed to start.

The fourth item is this article's engineering practice, not a source requirement. As engineering reasoning, when the servo reports in-position and the station sensor disagrees, the logic faults and holds rather than retrying the index, because a retry with a misplaced part is how a delicate assembly gets struck twice. UTEC Industrial, a Rockwell Automation Recognized System Integrator, integrates Allen-Bradley ControlLogix and CompactLogix controllers with VFD and servo drives for this kind of motion and interlock logic (Rockwell Automation 2198-UM002E-EN-P, 2018, pp. 16, 241, 250, and App. D; Rockwell Automation 1756-RM094N-EN-P-2025, Ch. 4 p. 30 and Ch. 5 pp. 39, 41, and 43).

How is an indexing conveyor tuned and proven before it carries real hardware?​

Tuning depends on the real load. The Kinetix 5700 manual states that actual bandwidth values (Hz) depend on the application and can require adjustment once motor and load are connected (p. 211). For permanent-magnet motors, it says that using the load observer with a default set of gains can yield high performance out of the box, and that most of the time there is no need to perform an auto-tune procedure (p. 207). As engineering reasoning, the index is therefore tuned and its overshoot measured with a pallet and a mass simulator that match the real assembly's mass and center of gravity, before any real hardware rides on it.

As engineering reasoning, not a statement in ECSS, this article applies ECSS-Q-ST-20-08C's MGSE requirements to a conveyor that serves as handling equipment for space hardware. Those requirements extend past commissioning:

  • Ready-for-use criteria. The MGSE user manual states the criteria for use (§6.2.5).
  • Logbook. Repairs, maintenance, nonconformance reports, and modifications are recorded (§6.2.6).
  • Reuse. Before generic MGSE is used, or other project-specific MGSE is reused, the MGSE design authority documents a suitability analysis covering interfaces, performances, safety factors, history log, design lifetime, as-designed and as-built documentation, and the user manual (§6.2.3).
  • Handover. Proof of validation and certification is transmitted with the MGSE in case of handover (§6.2.8 b).

Load testing and acceptance documentation for aerospace handling equipment are covered in the MGSE article. UTEC Industrial performs factory acceptance testing (FAT) and on-site commissioning, which is where index repeatability and overshoot can first be measured on the built conveyor (Rockwell Automation 2198-UM002E-EN-P, 2018, pp. 207 and 211; ECSS-Q-ST-20-08C, §6.2.3, §6.2.5, §6.2.6, and §6.2.8).

Where does an indexing conveyor sit in the design-to-monitoring chain?​

Index accuracy is decided along the whole chain, not at the drive alone:

  • Design and engineering. Station pitch, index time, the datum each assembly is located to, and whether a locating pin or the drive sets final position are fixed before the frame is drawn.
  • Parts machining. Locator bores, rail seats, and sprocket teeth are machined interfaces. Tsubaki's guide says made-to-order machined-tooth sprockets are used, instead of standard flame-cut sprockets, for its engineering-class precision chain.
  • Fabrication, weld fatigue, and stress relief. A welded frame that moves after machining moves its stations with it. Stress relief ahead of final machining is covered in the stress-relief article for machine bases and frames.
  • Drives, controls, and tuning. Profile, jerk, and servo tuning are set against the real load. The Kinetix manual's p. 211 note says actual bandwidth values depend on the application and can require adjustment once motor and load are connected.
  • Monitoring. Following error, in-position time, and station-sensor agreement logged for each index show drift before it becomes a strike.

Apart from the Tsubaki sprocket statement and the Kinetix p. 211 note, the steps and the monitoring list above are engineering reasoning. UTEC Industrial machines to tolerances as tight as ±0.001 in and performs automated vibratory stress relief and CMM inspection on the frames it builds, which is where the locator and rail interfaces behind an index position are made and checked (U.S. Tsubaki 1997, The Complete Guide to Chain, Applications §3.1; Rockwell Automation 2198-UM002E-EN-P, 2018, p. 211).

What should an indexing conveyor specification define?​

A request for an indexing conveyor should state enough for the builder to design the motion, the locating, and the protection together. UTEC Industrial has built heavy-duty conveyor systems for Lockheed Martin for aerospace manufacturing. A complete request for an indexing conveyor states:

  • The assembly. Mass, center of gravity, fixturing, allowable acceleration or shock if the program has one, ESD sensitivity, and cleanliness class.
  • The cycle. Number of stations, station pitch, index time, dwell time, and the stopping or positioning accuracy required, with how it will be measured.
  • The locating method. Whether a positioning pin, a hard stop, or the drive alone sets final position.
  • The motion. Trapezoidal or S-curve profile, whether dynamics may ever change while a move is running, and, for a rotary table, which rotary move types are allowed.
  • The drive and chain. Servo or cam indexer, and for chain, the elongation and tension limits the chain maker states, including inertia.
  • Protection. The risk assessment basis under ISO 12100:2010, guarding at stations and drive, and the stop and restart behavior after a fault.
  • Proof. FAT tests for repeatability and overshoot at minimum and maximum load, and for space hardware, the ECSS-Q-ST-20-08C user manual and logbook deliverables, as this article applies them above.

The protection items trace to ISO 12100:2010, the space-hardware deliverables to ECSS-Q-ST-20-08C, and the chain limits to the chain maker's own data, not to a general standard (ISO 12100:2010; ECSS-Q-ST-20-08C, §6.2.5 and §6.2.6; U.S. Tsubaki 1997, The Complete Guide to Chain, Applications §3.1).

Related Articles

References​

  • ASME B20.1-2024: Safety Standard for Conveyors and Related Equipment. ASME, 2024.
  • ANSI/CEMA 102-2022: Conveyor Terms & Definitions. Conveyor Equipment Manufacturers Association, 2022.
  • Conveyor Equipment Manufacturers Association. CEMA Application Guide for Unit Handling Conveyors, 2nd ed. CEMA, 2016.
  • ECSS-Q-ST-20-08C: Space product assurance — Storage, handling and transportation of spacecraft hardware. ECSS Secretariat, ESA-ESTEC, 2014.
  • NASA-HDBK-8739.21: Workmanship Manual for Electrostatic Discharge Control (Excluding Electrically Initiated Explosive Devices). National Aeronautics and Space Administration, 2010.
  • ANSI/ESD S20.20-2021: ESD Association Standard for the Development of an Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies, and Equipment (Excluding Electrically Initiated Explosive Devices). EOS/ESD Association, Inc., 2021.
  • Otoshi K (supervising ed.), Kanehira M (ed.). The Complete Guide to Chain, 1st English ed. U.S. Tsubaki, 1997. ISBN 0-9658932-0-0.
  • Rockwell Automation MOTION-RM002O-EN-P: Logix5000 Controllers Motion Instructions (Reference Manual). Rockwell Automation, 2025.
  • Biagiotti L, Melchiorri C. Trajectory Planning for Automatic Machines and Robots, 1st ed. Springer, 2008. ISBN 978-3-540-85628-3.
  • Lambrechts P, Boerlage M, Steinbuch M (2005). "Trajectory planning and feedforward design for electromechanical motion systems." Control Engineering Practice, 13(2), 145-157.
  • Norton RL. Cam Design and Manufacturing Handbook, 2nd ed. Industrial Press, 2009. ISBN 978-0-8311-3367-2.
  • OSHA 29 CFR 1910.212-1974: General Requirements for All Machines. U.S. Department of Labor, 1974.
  • OSHA 29 CFR 1910.219: Mechanical Power-Transmission Apparatus. U.S. Department of Labor, 1974 (as amended through 2004).
  • ISO 12100:2010: Safety of machinery — General principles for design — Risk assessment and risk reduction. ISO, 2010.
  • ISO 14118:2017: Safety of machinery — Prevention of unexpected start-up. International Organization for Standardization, 2017.
  • ISO 14119:2024: Safety of machinery — Interlocking devices associated with guards — Principles for design and selection. International Organization for Standardization, 2024.
  • ISO 13855:2024: Safety of machinery — Positioning of safeguards with respect to the approach of the human body. ISO, 2024.
  • ISO 13849-1:2023: Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design. International Organization for Standardization, 2023.
  • Rockwell Automation 2198-UM002E-EN-P (2018): Kinetix 5700 Servo Drives User Manual. Rockwell Automation, 2018.
  • Rockwell Automation 1756-RM094N-EN-P-2025: Logix 5000 Controllers Design Considerations. Rockwell Automation, 2025.

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