Precision Indexing Circular Conveyor

What is a Indexing Circular Conveyor System?

Author: ShanGong, Director of Engineering Department | Published: September 22, 2026 | Category: Circular Conveyor Systems

A precision indexing circular conveyor moves carriages around a closed loop in fixed steps and holds each one still while work is done on it. Two numbers decide whether such a line can be built at all: the station pitch and the loop length. They have to agree to a whole number of chain pitches, or the carriages will not return to the same stations lap after lap.

This page shows how that works on a real 800 mm elliptical line, and where the indexing accuracy actually comes from — which is not the chain.

Figure 1. The reference line: a 3,734 mm horizontal elliptical loop carrying ten carriages.

Reference Line: TMB01-T23-S800R180-Y762-10LK8-L2774-C20

Figure 2. Plan view: 800 mm arcs, 2,774 mm straight rails and 634 mm of clear space inside the loop.

The Station Pitch Has to Be a Whole Number of Chain Pitches

The carriages on this indexing conveyor system are fixed to the chain, so the chain sets the pitch. The line runs 12A chain, whose pitch is 19.05 mm. The 762 mm station pitch is exactly 40 of those pitches, and ten carriages close the loop at 400 pitches, or 7,620 mm of chain.

That is the whole design constraint in one line: 762 = 40 × 19.05, and 10 × 762 = 7,620. Nothing in the loop is rounded. Because 400 divides by both 10 and 8, the ten carriages and the eight positioned stations both land on chain pitches rather than between them.

Change one of those figures and the rest move with it. A different station pitch has to be a whole number of pitches; a different carriage count has to divide the loop; a longer straight track changes the loop length and therefore the pitch. This is why we ask for station pitch, carriage count and the internal space early — they are not independent preferences, and a precision indexing circular conveyor that is out by half a pitch cannot be corrected later by adjusting the tensioner.

Where the Indexing Accuracy Comes From

The chain does not hold the ±0.05 mm. A chain has clearance in every joint and elongates in service, so the position at the end of an index is approximately right, not precisely right. The accuracy comes from a mechanical positioning unit that engages the carriage after the move and pulls it onto a fixed datum.

On this line eight of the ten carriages are positioned. Four pneumatic cylinders do that work, not eight: each cylinder drives a long shaft carried in bearing seats, and each shaft turns two swing arms — one on each side of the loop. Each arm carries a CF8 follower that engages the positioning accessory on the carriage. Four cylinders, four shafts, eight arms, eight stations, and both sides of the loop are locked by the same stroke at the same moment.

Two consequences follow. The stations served by one shaft are paired, so their positions are set together when the line is commissioned, and any future adjustment moves both. And the positioning stroke is part of the cycle, so the time budget for an index is the move plus the lock, not the move alone.

Figure 3. Drive end: servo motor and 40:1 planetary gearbox mounted below the base plate.

Indexing Drive of this Precision Indexing Circular Conveyor

The drive is a Delta servo motor through a 40:1 planetary gearbox to a 76-tooth sprocket. One index advances the chain 40 pitches, which is 40/76 of a sprocket revolution, or about 21 motor revolutions. At the specified 400 mm/s chain speed, the sprocket turns at about 0.28 rev/s and the motor runs near 660 rpm, comfortably inside a servo’s rated speed.

Acceleration and deceleration should each be set to at least 0.2 s. On an indexing conveyor system carrying ten carriages, an abrupt start pulls the whole chain against its clearance and the carriages arrive with the loop still oscillating, which the positioning units then have to absorb.

For this build we recommended a 1.8 kW medium-inertia servo motor. Inertia matching matters more than rated power on a line like this: the reflected inertia of 580 kg of machine and ten carriages, seen through a 40:1 gearbox, is what the motor has to control during those 0.2 s ramps.

Guideway, Arcs and Why 800 mm

The loop is built from two T23 arc rails at 800 mm diameter and two LGT23-L2774 straight rails. T23 is the 23 mm machine-level guideway width, the smallest in the range that still suits a 3 kg carriage payload with a 100 × 85 mm plate; wider guideways (25, 28, 30, 44 and 50 mm) carry more load and more overhung moment, at the cost of a larger arc and a heavier loop.

Arc diameter is the second lever. A larger arc lengthens the machine but slows the angular rate at which a carriage swings through the turn, which keeps the load on the rollers and on the product lower at a given chain speed. At 800 mm and 400 mm/s a carriage takes about 3.1 s to travel the half-turn, so the arcs on this line are a transfer path rather than a working zone — which is exactly why two of the ten carriages are unpositioned at any moment.

Figure 4. Opposite isometric view of the same loop.

Carriage Plate and Fixture Mounting in Precision Indexing Circular Conveyor

Each carriage carries a 100 × 85 mm plate. The mounting pattern matters more than the size: four M6 tapped holes take the fixture screws, and two Ø5 H7 (+0.02/0) dowel holes, 8 mm deep, locate it. The screws hold the fixture; the dowels decide where it sits.

That distinction is what makes a precision indexing circular conveyor repeatable in service. A fixture removed for cleaning and refitted on its dowels returns to the same place, so the ±0.05 mm at the station is not lost the first time someone takes a fixture off. Design your fixture around the dowel pair, not around the screw holes, and keep the fixture centre of gravity over the carriage rather than cantilevered past its edge.

Structure and Mass of this Precision Indexing Circular Conveyor

The base plate is 3,624 × 850 × 20 mm in 45# steel, and the finished machine weighs about 580 kg. Steel rather than aluminium was chosen because the loop is nearly 3.8 m long and has to stay flat under ten carriages that stop and start continuously; the plate is the datum that the arcs, straight rails and chain guides all register to.

Mass has practical consequences worth planning for. The line needs lifting equipment on arrival, the supporting floor or frame has to carry it without deflecting, and the route from the loading bay to the installation point needs checking against the 3,734 mm length before despatch.

Turning the Index Into Throughput

Cycle time on an indexing conveyor system is the move, plus the positioning stroke, plus the dwell your process needs. The move is fixed by the geometry: 762 mm at 400 mm/s is about 1.9 s, and the two 0.2 s ramps add roughly another 0.2 s. Add the lock and release of the positioning units, then add the longest station dwell in the loop.

Indexes per hour is then simply 3,600 divided by that cycle, and every index advances all ten carriages one station at once — so the line produces at the rate of its slowest station, not its average one. If one station needs three seconds and the rest need one, the answer is not a faster chain; it is either splitting that operation across two stations or giving it two carriages. Our cycle time calculation guide on circular-motion.com works through the arithmetic.

Working Space Inside the Loop of Precision Indexing Circular Conveyor

This closed loop conveyor keeps 634 mm of clear space inside the ellipse. Customers often mount their own mechanism there — a press, a dispenser, a camera column — so the inner dimension is part of the requirement rather than an outcome. Give us the internal envelope you need and we will set the straight track length around it, then re-derive the station pitch so the loop still closes on whole chain pitches.

Figure 5. Straight section: carriages at 762 mm pitch running on the T23 guideway.

Installation and Alignment of Precision Indexing Circular Conveyor

The loop is only as accurate as the surface under it. The customer’s mounting frame should be flat to within ±0.15 mm over any 3 m and ±0.3 mm over any 10 m, measured after the frame is levelled in place, because a twisted frame pulls the base plate and opens the rail joints.

At the joints themselves the step and gap between rail sections is held to 0.03 mm, and on a twin-rail loop the joints on the two rails are staggered so a carriage never crosses both at the same instant. We assemble, align and run the loop in the factory before despatch, so on site the work is levelling, bolting down and re-checking the joints rather than building the geometry from parts.

Maintenance and Lubrication

Lubricate the guideway and rollers with one of three approved lubricants: a spray-type penetrating lubricant (WD-40 class) every 3–5 days, a liquid guideway oil (ISO VG 46 class) every 2 weeks, or a solid lubricating grease for long-interval, effectively maintenance-free running. The intervals apply whether the lubricant is applied to the rail directly or through a fitted felt device.

Beyond lubrication, three checks keep an indexing line in tolerance: chain tension at the tensioner, the swing arms and followers at the positioning stations, and the fasteners at the carriage and fixture interfaces. Chain elongation shows up first as a carriage arriving slightly short before the positioning unit pulls it in — worth logging, because it tells you when the chain is due rather than waiting for a missed index.

Options in Precision Indexing Circular Conveyor

Three options are worth considering at the enquiry stage rather than later. A bellows cover protects the guideway and drive where liquid or dust is present. Stainless steel carriers and a 316 stainless guideway suit washdown and hygienic processes — see our stainless steel circular conveyor page. And the servo interface (pulse, EtherCAT or PROFINET) should match your controller; this line was specified as pulse type.

Specifying a Line Like This

Six figures let us size a precision indexing circular conveyor without a second round of questions: carriage count, station pitch or required throughput, payload per carriage, how many stations must be positioned and where, the internal space you need inside the loop, and the servo interface your controller uses. Environment matters too — this line was specified for a general industrial environment, so it carries no covers.

FAQ about Precision Indexing Circular Conveyor

How accurate is the indexing?

This line is built to ±0.05 mm repeatability at the positioned stations. Carriages that are not positioned stop within the accuracy the chain alone can hold, which is a different order of magnitude, so tell us which stations need the tight figure.

Can every carriage be positioned?

Yes. Here eight of ten are positioned because two carriages pass through the arcs while the others are working. Positioning every carriage means more cylinders, arms and followers, and a slightly longer lock time in each cycle.

What sets the cycle time?

The move, the positioning stroke and the dwell your process needs. The move itself is fixed by the station pitch and the chain speed: 762 mm at 400 mm/s takes under two seconds, plus the ramps.

Can the loop be made longer later?

Not by simply adding track. The loop has to remain a whole number of chain pitches, so extending a line means a new chain length, a new station pitch or a different carriage count. Plan the final layout before the first build.

How do I mount a fixture on the carriage?

Through the four M6 holes, located on the two Ø5 H7 dowel holes in the plate. Keep the load over the carriage and tell us the fixture mass and height, because both change the moment the guideway sees at 400 mm/s.

What happens if the chain stretches?

The tensioner takes up the slack, and the positioning units continue to set the final position at each station. Elongation shows first as a larger correction at the lock; when that correction grows, the chain is approaching replacement.

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