When buying a roller conveyor for cartons or returnable totes, a request that states only width and length cannot show how the line will behave when a carton jams, turns, is damaged, or has to restart. Begin with the load’s underside, dimensional variation, transfer points, stop positions, permitted contact during accumulation, and access for maintenance. Then compare gravity rollers, continuously driven rollers, and motor-driven roller (MDR) zones against the work each process must perform. This guide is for production engineering, maintenance and procurement teams in Thailand. For a plant-wide route and layout study, read our conveyor system design guide. Here we concentrate on roller contact, drives, sensing, zones and test evidence.
The decision in brief: start with the load and the required stop
The first question is not which roller brand to purchase. Ask whether the unit load is stably supported, where an operator pushes it, where an automatic stop is needed, and whether cartons may touch when downstream equipment is blocked. A short, manually served transfer of rigid flat-bottom cartons may suit gravity rollers. A route that moves continuously without independent stops may suit a continuous drive. If each carton must be detected, held with a gap and released on downstream permission, evaluate zone control using MDR or another suitable drive. This is a screening framework; load, friction, speed, environment and safety still need to be checked. Dorner’s comparison similarly separates operator involvement from the need for precise powered movement.
| Process requirement | Option to compare | Verify before ordering |
|---|---|---|
| Manual loading and unloading over a short span | Gravity or push rollers | Slope or push force, runaway prevention, end stop |
| Continuous travel in one direction | Continuous drive | Load stability at start/stop; effect of stopping the whole line |
| Hold and release individual cartons | MDR or other zone drive | Carton and zone length, sensor location, receive signal |
| Soft, footed or irregular underside | Compare non-roller methods too | Loss of support, snagging and damage |
1. Inspect the underside of the actual load
Before reading a catalogue’s rated capacity, photograph and measure the underside. A corrugated seam, ribbed plastic tote, pallet feet, warped board and wet bag meet rollers differently. A foot may fall between rollers; a flexing bottom may add resistance; a corner may catch a guide. Maximum mass alone predicts none of these. Record minimum and maximum dimensions, offset centre of gravity, bottom stiffness, empty and loaded states, and protruding film or labels. If packaging changes by lot or supplier, include those variants in the samples.
Ask the designer to demonstrate stable support while the smallest article moves. Roller diameter and pitch affect continuity of support, resistance, noise, cleaning and bearing selection, but there is no universal pitch independent of the load bottom and tolerances. Run the smallest, most unstable and heaviest real samples instead of copying a standard dimension. Test the end transfer separately. A carton that travels smoothly along the straight bed can tip in an end gap; a pallet foot can catch as it enters the next machine.
Use a load sheet with product code, container type, mass range, underside photo, contact points, travel direction, required orientation, stacking rule, acceptable marks and cleaning conditions. Include damaged cartons and raised tape, not only a nominal drawing. Quality and operators should review this sheet with procurement because it defines product quality, not merely machine size.
2. Where gravity rollers fit, and where they do not
Gravity conveyor has no drive motor, but it is not a device that actively controls each carton. Distinguish a horizontal bed pushed by people from a sloped bed that moves by gravity. On a slope, speed depends on the load and contact conditions. Assess impact at the end, runaway and pinch points. On a horizontal bed, observe the repeated force and posture needed to move a loaded carton. Even the simple option needs a stop, side guide, ergonomic loading height, unloading path and maintenance access.
Gravity rollers can work as a short handoff or temporary holding area between manual tasks. When a process needs an exact stop, unattended input, frequent downstream blockage or separation of damage-sensitive products, a stop alone can transfer the control burden to operators. Compare gravity on whether that manual intervention is acceptable, not simply on purchase price. Dorner describes its gravity products as moving loads with human assistance or a powered conveyor. Verify that example against your own load in a real trial.
3. Continuous drives and MDR zones solve different problems
A continuously driven bed moves multiple rollers through a common drive and carries loads in one direction. This is straightforward when the route is clear. Independently holding one carton while controlling the next may require mechanical disengagement or additional controls. State where loads stop, whether rollers keep turning under a stopped load, and whether cartons push against one another. Two quotations that both say “powered roller conveyor” can describe different contact behaviour.
An MDR contains the motor in a roller and can drive neighbouring idlers through belts. Interroll’s RollerDrive page describes 24/48 V DC versions and their use in zero-pressure accumulation zones. It does not mean every MDR assembly has identical control. The board, sensor, PLC interface, power supply and failure behaviour depend on configuration. MDR names a drive architecture; load detection and zone logic produce controlled accumulation.
The Interroll RM 8310 example uses one RollerDrive in each zone to drive a defined set of idlers and uses a controller for zero-pressure accumulation. Do not transfer that model’s numerical ratings to another product. Put zone length, number of driven idlers, supply, sensors, control functions and replacement procedure on one comparison sheet for each bidder.
4. Zone length is more than carton length
Zone design defines how many units may occupy an area and when a unit can cross its boundary. If a long carton stops over two zones, a sensor can report an apparently empty zone, or only the adjacent drive may start and skew the load. Use the longest carton to set entry and exit sensing positions, stopping variation, desired separation and end gap. Test the shortest carton too: will it reliably break the photoeye and remain supported by rollers? Never size a mixed-product line from average carton length.
The Interroll module catalogue shows zones matched to maximum unit-load length, each with a RollerDrive, idlers, control card and photoeye/reflector. It illustrates the principle; it is not a universal allowance for every carton. Test actual samples at the intended stop speed and log stop scatter and sensing margin.
Evaluate sensors beyond their simple present/absent output. Dirt, transparent wrap, dark cartons, reflective tape, daylight and height changes can alter detection. Document whether the zone stops or moves after loss of detection, where an alarm appears and how an operator recovers. Show exactly which face of the carton a sensor observes on the drawing. Verify that location after installation. A short-circuit workaround for nuisance stops can defeat the intended control, so make diagnostic and adjustment routes usable.
5. Translate zero-pressure accumulation into acceptance behaviour
Zero-pressure accumulation holds units by zones when downstream is blocked rather than continuously pushing cartons together. Interroll ZoneControl provides an example in which a zone exchanges information with its neighbours. The label alone does not specify your process. Define whether all contact must be prevented or light contact is acceptable, and whether units restart one at a time or in another sequence.
At minimum, specify five timelines: normal inflow, blocked discharge, a sensor fault in the middle, emergency stop and release, and loss and return of power. For each, state what the sensors see, which rollers stop and the restart order. An undefined default on lost communication can leave a system that pushes cartons only during a fault. In a mixed-load line, test a long carton followed by a short one as well as the reverse order.
Do not treat “zero pressure” as a product-quality guarantee. Cartons may still slip slightly on rollers, touch guides or allow contents to move during a stop. Inspect carton surfaces and contents after transport. Make permitted scuffing, orientation and vibration explicit acceptance criteria. Contact between cartons and damage to products are different tests.
6. Define the sensor and PLC responsibility boundary
Even a self-contained zone controller has boundaries with upstream and downstream machines, safety circuits, a plant PLC and perhaps an MES. “PLC interface” alone leaves start permission, discharge request, receipt confirmation, full state, fault and reset without an owner. For each signal list its name, sender, receiver, normal state, timeout and action on communication loss. Decide whether equipment may restart automatically with cartons still on the bed after a power failure, in line with site safety rules.
Compare the maintenance burden of analogue interfaces, discrete I/O and fieldbus boards. They differ in wiring, diagnostics, spares and technician skills. Interroll’s ZoneControl describes configurations with or without PLC, but other suppliers need separate verification. Even when a vendor selects the board, the factory must approve supply capacity, network, existing-PLC changes and maintenance access.
Recovery deserves as much attention as normal running. After an operator removes a carton, do the sensor readings and internal zone states agree? Does switching from manual to automatic cause unintended motion? Can a technician identify the faulted zone from history? Put jam clearing into the design scenario rather than leaving it as a late sentence in the operating manual.
7. Compare the frame, environment and service access too
Include frame width, working height, legs, guides, end stops, roller replacement clearance and cleaning access. A frame may nominally fit the largest carton yet have insufficient clear width after guides are adjusted. Measure floor anchors, pipes, drains and forklift routes on site. If one roller cannot be removed without dismantling a neighbouring machine, count the maintenance interruption in the comparison.
Select material and finish for water, dust, corrosive cleaning chemicals, oil, temperature and static requirements at the actual station. In food or pharmaceutical packaging, a “washable” label is not a complete hygienic-design review. Inspect crevices, liquid traps, seals, wiring and post-cleaning restart. Interroll’s CleanLine roller curve is a water-cleaning-oriented example; its features do not apply to all roller conveyors. Put the site’s washing method and chemicals in the specification.
Assess nip points between rollers and frame, falling loads at ends and walkways crossed by people. Coordinate emergency stops and their stop scope with the whole line’s safety design. An interlock alone does not settle residual energy or restart procedure. Have a competent safety specialist confirm applicable standards for the machine and local rules.
8. Write an RFP with comparable inputs, outputs and exceptions
A request for proposal should describe process service before it lists metres of conveyor. Inputs include drawings and real samples, minimum and maximum mass, underside shapes, power, travel direction, adjoining heights and available floor area. Outputs include final position and orientation, damage limit, cycle requirement, acceptable stop time, alarms and logs. Exceptions include two-carton insertion, blocked photoeye without a carton, long units, reversed units, blocked downstream, power loss and manual removal.
Ask bidders to answer each numbered requirement as compliant, compliant with conditions, or excluded. Conditions must identify extra parts, cost, schedule and separate tests. Request a drawing of roller pitch, minimum support, zone length, stop tolerance, sensor type, control signals, power and replacement method. Separate conveyor, sensors, control cabinet, upstream integration, installation, FAT, SAT, training and spares in the quotation. This makes a low equipment price with omitted commissioning visible.
Our broader material-handling equipment selection guide explains RFP, FAT/SAT and whole-method selection. This article applies that discipline specifically to roller contact, stopping and sensing. Route-wide merges and branches need separate layout engineering.
9. FAT needs more evidence than a smooth-running video
At factory acceptance testing (FAT), use load samples and control scenarios to verify the purchase specification at the builder’s facility. If actual cartons cannot be supplied, record differences from substitutes and explicitly transfer open tests to SAT. Agree test numbers, conditions, pass criteria, records and witnesses before testing. The Rockwell Automation functional acceptance manual calls for test cases and acceptance criteria that cover requirements, interfaces, faults and exceptions. It is a test-planning reference, not a source of roller-conveyor performance limits.
Observe support for smallest and largest cartons; stop positions of empty and loaded cartons; continuous inflow; gap when discharge is blocked; photoeye masking and cable faults; state recovery after manual removal; emergency stop and reset; and power recovery. Keep videos and logs with sample code, underside condition, mass and direction. A bare “pass” is not enough when a packaging change later needs investigation. Retest a fault under the same conditions after correction.
10. SAT proves the installed boundaries and maintenance method
Site acceptance testing (SAT) uses the real floor, adjoining machines, power, network and operators after installation. FAT cannot expose every height mismatch, site-lighting effect on sensors, PLC timing difference or inaccessible roller. A Rockwell Automation procurement specification calls for renewed I/O checks and control verification after installation. It concerns a DCS, yet the principle of checking the installed interface after factory testing applies here.
At SAT, repeat stop and restart cycles with real cartons and actual release intervals, not just a long uninterrupted run. Check transfer tilt, detection of small cartons in mixed flow, fault location and safe jam removal by local maintenance staff. Include final wiring drawings, PLC and controller backups, spares, training records, and owners and deadlines for open items in acceptance. Decide in advance which safety and quality issues cannot be carried into production as a temporary exception.
11. Before-order checklist and post-start review
Build a trial matrix of difficult load combinations
One representative carton can hide production faults. List plausible combinations: smallest and lightest, smallest and fully loaded, largest and heavy, or a warped bottom at the higher operating speed. Every combination need not receive identical repetitions, but prioritize weak support, poor sensor visibility and high stopping momentum. If clear shrink film or a glossy label is used, test the orientation that puts it in the sensor path. Straightening every carton by hand before a trial does not prove robustness to the real process.
Record more than “moved / did not move.” Note leading-edge stop position and scatter, guide contact, sliding on rollers, film snagging, alarm display, recovery action and product appearance. Film the underside and transfer end from useful angles and tie footage to sample ID and settings. After tuning, rerun the same carton and release interval. This distinguishes a lucky single pass from repeatable behaviour.
Measure both sides of the existing-line interface
For a retrofit, measure actual adjoining heights, floor slope, anchor positions and direction rather than relying only on drawings. If the existing machine discharges a carton at an angle, the entry condition changes. Trial the smallest or footed load across any end gap. A guide that solves the handoff may make cleaning or manual removal difficult, so review access at the same time.
Pair the existing machine’s permission to discharge with the new bed’s permission to receive. If both wait indefinitely for the other, the line stops without a fault. If both move unconditionally, cartons may overlap at the interface. Record state transitions and timeouts in one signal table reviewed by the PLC owner and machine builder. Matching signal names alone does not establish sequence.
Compare spares and site recovery time
Choose spare rollers, belts, sensors and boards according to local availability, tools and acceptable downtime as well as failure likelihood. A spare roller is of little help if changing it means dismantling a neighbouring machine. Ask the bidder to demonstrate replacement of one roller and one sensor, then feed the required clearance back into the layout. Can maintenance identify the failed zone without a supplier-only device?
If cleaning is daily, identify where debris is removed between rollers and the safe stop procedure. If washing can shift a photoeye, define readjustment and a short post-cleaning trial. The ability to restore the machine to its known state is a selection criterion alongside nominal running performance.
- Do you have underside photos and samples of the smallest, largest and least stable load?
- Are empty, full and deformed cartons included in trials?
- For gravity rollers, have push force, slope and end stopping been observed?
- For driven rollers, is behaviour under a stationary carton stated?
- For MDR, are zone length, idler connections, supply and sensors drawn?
- Are downstream blockage, simultaneous insertion, sensor fault and power return specified?
- Are PLC and adjoining-machine signals and timeouts assigned?
- Are end gaps, guides, stops, floor clashes and roller replacement space measured?
- Are real samples, pass criteria and records agreed separately for FAT and SAT?
- Can local technicians locate a fault, clean and replace a part safely?
Decide how improvement will be measured before acceptance. Record human interventions, carton dwell time, damage, and jam recovery time using the same definitions before and after installation. Separate stops caused by the roller, sensor, downstream machine and loading method. A higher catalogue speed is not improvement if the line jams more. Track awkward lifting and manual clearing as site-level outcomes too.
Plan for product changes: document carton-size limits, trials for new SKUs, roller and sensor part numbers, and ownership of controller backups. A configuration only the vendor can change may add lifecycle cost. Conversely, unrestricted on-site changes can undermine detection and separation. Establish approval, backups and retest after settings change.
Frequently asked questions
12. Work through one process before selecting a product
Consider a returnable tote travelling from an inspection machine to a packing bench. Today an operator loads a cart and waits for packing to have space. A mechanisation request should not begin with conveyor length. Observe the inspection release interval, how long packing cannot receive, tote-size variation, ribs on the underside and where an operator must remove a tote. If contents are impact-sensitive, transfer posture and stopping shock become requirements too.
If packing is nearly always ready and an operator can comfortably handle a short level section, trial gravity rollers. If packing periodically closes, observe the physical queue at a gravity end stop. If totes cannot contact one another, independent zones may be necessary. Before naming MDR, check how many zones fit the longest tote, sensor positions and the existing inspection machine’s discharge-permission signal. Align the sending and receiving contract first.
If tote feet tilt between rollers, other options remain: closer pitch, another support element, or a belt conveyor. Do not quietly repair every tote by hand to make a chosen conveyor appear suitable. If the inspection discharge height varies, put adjustment range and end gap on the installation drawing. Measured process conditions let bidders answer the same problem.
The RFP for this example could require safe holding while packing is unavailable and ordered release on reopening; acceptable tilt and damage for both smallest and fully loaded totes; and a stop plus fault location if sensing fails. At FAT, simulate the inspection signals. At SAT, test real signal timing and manual tote removal. Assign test owners and acceptance authority in advance. This is a reasoning example, not a claim that a particular product or benefit has been achieved.
After production starts, do not put all stoppages in one “conveyor fault” bucket. Record load supply, detection, drive and adjoining-machine permission separately, together with time and tote code. Then the team can tell whether a certain underside fails, or whether sensors move after cleaning, and choose the right corrective action from evidence.
Before handover, observe an ordinary site operator using only the procedure to locate a stop, remove the tote safely and restart. A successful demonstration under that condition connects factory testing to stable everyday operation.
Will MDR automatically stop cartons from touching?
No. MDR describes the drive inside the roller. Separation depends on sensors, zone length, logic and stopping accuracy. Even with “zero-pressure accumulation” in a quotation, test downstream blockage using the smallest and largest cartons and record the allowed contact.
Does gravity conveyor need no electrical or control specification?
It has no motor, yet adjoining equipment height, end stop, human intervention and runaway protection still matter. At an automatic feed into a gravity section, permission or full detection may be needed. Define the whole-machine boundary.
Is a longer zone always safer?
A longer zone uses more space and can reduce holding density for short cartons; a short zone may be bridged by a long load. Size against the longest load, stop variation, sensing positions and transfer gap, then test short loads too.
What is often missing from a low-price quotation?
Control boards and sensors, existing-PLC modification, end transfer hardware, safety measures, on-site tuning, FAT/SAT, spare rollers and replacement labour. Map every quote onto one numbered RFP and make omissions visible.
Summary
Roller-conveyor selection turns on how a load meets the rollers, how the carton to be stopped is detected, and what happens when discharge is blocked. Gravity, continuous drive and MDR zones differ in their required human work and scope of control. Start the RFP with real undersides, prove load-and-control combinations at FAT, and verify the installed handoff and maintenance routine at SAT. Turning uncertain behaviour into specific tests before purchase reduces start-up jams and quality issues.
Bring underside photos, minimum and maximum mass, a short process video, records of jams or damage, and existing signal lists. That allows gravity, continuous drive and MDR zones to be compared under the same conditions. TOMAS TECH supports Thai manufacturing sites with load and process definition, RFPs, control-interface review and FAT/SAT criteria. Contact our team with the section you are considering and the stop or quality problem you need to solve.
