Robot 7th Axis Linear Track

Motionwell Automation designs and builds robot linear tracks in Singapore, the seventh axis an industrial arm stands on when the work is spread out along a line rather than concentrated in front of it. Our own ultra-low-profile track is 100 mm in overall height, repeats to plus or minus 0.05 mm on a ground-grade rack and pinion, and carries a slide platform load of 1.5 tonnes, with a maximum travel speed of 500 mm/s. The delivered configuration runs 2,850 mm of stroke, bolted directly to the factory floor with no pit. Drive is a servo motor through a planetary gearbox, guidance is dual linear ball bearing rails, and the track integrates with ABB IRC5 and OmniCore controllers as a coordinated external axis. Our heavy-duty robot track carries platform loads to 2,000 kg and strokes beyond 20 m for FANUC, ABB, KUKA and Yaskawa arms. Tracks are designed, assembled and tested at our Woodlands Link facility, and the company has delivered more than 150 special purpose machines since 2014 under ISO 9001:2015 and bizSAFE Level 3.

Where we stand, said plainly before you read further. We do not manufacture robot arms, controllers or drives; we buy them, and the track is one of the things we design around them. We do not build production welding cells, so if the reason for the track is a welding line, we are the wrong supplier for the process even where the mechanics sit inside our scope. We do not issue CE certificates and we are not a notified body. And where a catalogue travel unit from your robot vendor covers the stroke, the payload and the height you have available, that is the useful answer and it costs you a conversation rather than a commitment.

This page is about the track itself: what it buys you, what its height costs you, how it is driven over long travel, why it belongs on the robot controller rather than on the PLC, how cables survive the stroke, what accuracy figure to put in a specification, what the floor has to do, how the axis is protected, and when a track is the wrong answer. Overhead gantries covering a row of machines are a different machine with different economics, set out on the machine tending automation page; the wider cell scope around any arm is on our robot integration services page; the end-of-line version, where the track serves several pallet lanes, sits in the carton palletizing case study. If you already have a stroke, a robot model and a layout, skip ahead and talk to an engineer.

When Does a Robot Need a Linear Track Instead of a Bigger Robot?

Two situations, and it is worth being blunt about them because they are what reliably justifies the axis. The first is a part longer than the robot’s reach. The second is one robot that has to serve several stations which will not fit inside a single working envelope. Buying a bigger robot to cover a long part usually costs more than putting a normal robot on a track, and it does nothing for the multi-station case, because reach grows in a circle and stations sit in a line.

The distinction that decides it is spread against weight. A track answers spread: it moves the whole arm, so every point on the stroke is served by the same reach, the same payload curve and the same tooling. It does not add payload. An arm that cannot lift your part at full extension still cannot lift it after you have put it on a rail.

The third row deserves the weight it gets on our palletizing work. Where we put a column palletizer on a track, one robot on a 6 m track serves three pallet positions with 2 to 3 seconds of transit between them, absorbed into the natural pallet pattern cycle without reducing throughput. The trade is availability rather than speed. A track fault, or the robot itself, takes all three lanes down at once, while two robots on fixed bases fail independently. Where a stoppage on one product cannot be allowed to stop the other two, the second robot is the right purchase and we will say so.

The fourth row is a different machine rather than a variant of this one. If the machines already sit in a straight row and the parts drop vertically into them, the beam carries the distance overhead and leaves the floor clear, and the argument is made in full on the machine tending page rather than here.

What Does Track Height Cost You in Usable Working Height?

Everything the track is tall. The track sits between the floor and the robot base, so the whole working envelope moves up by the track height, and the height comes out of the process end rather than out of the ceiling.

A conventional robot travel unit takes 200 to 400 mm of vertical working height straight out of the arm’s usable envelope. That is why our low-profile track exists: at 100 mm overall it keeps the envelope where the process needs it, and it is low enough to avoid the floor pit that tracks in this payload class often require.

Where the lost height actually bites is not obvious from a datasheet, so check these three before accepting a track height. Reaching down: can the arm still get to the bottom of a tote, a machine table or the infeed conveyor, or has the base risen far enough that the wrist bottoms out? Reaching up: raising the base helps the top of a pallet stack and hurts the ceiling clearance, and on a low-bay unit the arm at full extension is what hits the sprinkler line. And reaching in: a machine tool door or a chamber opening has a fixed height, and a pedestal in that 200 to 400 mm band can turn a straight entry into an approach angle the wrist cannot hold.

Rack and Pinion, Belt or Ballscrew on a Long Axis?

Three drives get proposed for a travelling robot base, and the length of travel settles the choice long before precision does. Our tracks use a servo motor through a planetary gearbox onto a ground-grade rack and pinion, with dual linear ball bearing rails carrying the load.

The reason rack and pinion wins on a robot track rather than on a positioning stage is what is riding on it. A robot on a carriage is a large mass with a moving centre of gravity, and it puts reaction torque into the axis every time the arm accelerates. That load is a stiffness problem, not a resolution problem. A drive that is compliant, or whose compliance changes as the carriage moves, gives you an axis that settles differently at each station, and that settling time is a cost you only see once the cell is running.

Two consequences belong in the specification. Because the rack splices, a stroke can be extended later by adding rack, rail and frame rather than replacing the axis. And because the pinion mesh carries the load, backlash is dealt with mechanically at build time; it cannot be tuned out in software afterwards.

Why Does the Track Have to Be a Coordinated External Axis?

Because a track running as a separate machine turns every station change into a handshake, and those handshakes cost more cycle time than people expect.

ABB IRC5 and OmniCore controllers treat the track as a coordinated external axis. The track servo is driven by the robot controller, the track appears in the same motion planner and the same coordinate frame as the arm, and track motion is planned with the arm rather than sequenced after it. The arm can begin its approach while the carriage is still moving, and the controller resolves the combined path rather than executing two independent moves in series.

None of this makes the PLC route wrong in every case. A track that parks at three fixed stations with long dwell at each one works perfectly well as an indexed axis. The moment transit has to hide inside the arm’s own motion, the coordinated axis is what makes it possible at all.

Either way, verify reach and cycle time in offline simulation before anything is built. A pose that cannot be reached is a mechanical problem rather than a software one, and on a track it is usually at one end of the stroke where nobody checked.

How Do Cables and Services Survive the Full Stroke?

Every service the robot needs has to travel with it: motor power and encoder feedback, safety circuits, fieldbus, compressed air for the tooling, and whatever the process adds. Our tracks carry them in an energy chain running along the travel with power, signal and air routed inside one managed run, under a protective cover.

Four things decide whether that installation lasts.

Cable specification. Continuous-flex cable rated for the travel, not standard cable tied into a chain. On cleanroom and long-travel builds we standardise on igus chain and flex-rated cable so the jacket does not craze after a few million cycles. This is the same discipline that applies to the dress pack on the arm itself, covered on the end of arm tooling page.

Bend radius. It has to be respected at design time. A jacket cracked at a tight bend is a failure that reappears every time it is patched, and on a track it fails in the middle of the stroke where it is least accessible.

Fill and separation. Chain fill governs whether cables move freely or grind against each other, and power and signal want separating for noise as much as for wear. Adding a service after the chain is sized is where a well-built axis starts failing intermittently.

Where the chain lies. On a machine tending duty the chain sits in the swarf zone unless somebody routes it out of one. A carrier filling with hot chips becomes an abrasion problem long before it becomes a jam, so route it away from the chip conveyor discharge and give the chain a cover.

The track also needs travel protection independent of software: end-of-travel limit switches on the axis, so an overrun is caught by hardware rather than by a parameter someone can edit.

Repeatability or Accuracy: Which Number Belongs in Your Specification?

They are different measurements, and specifying the wrong one is how a track ends up either over-bought or unable to do the job.

Repeatability describes how tightly the axis returns to a position it has already been taught. Our low-profile track repeats to plus or minus 0.05 mm through the rack and pinion drive with encoder feedback, and the heavy-duty series is specified at plus or minus 0.1 mm. Accuracy is a different claim: it describes how close the axis gets to a position it has been told to go to in a coordinate system, anywhere along the stroke. On the column palletizing track we specify plus or minus 0.5 mm positioning along the track using an absolute linear encoder, which is a positioning figure rather than a repeat figure. Know which one your specification is asking for before you compare two suppliers’ numbers.

Then the harder point: neither number is placement accuracy on your part. The axis says nothing about gripper slip, part tolerance, fixture wear or thermal drift, which is usually what you end up measuring on the finished work. A long axis adds two of its own. Thermal growth accumulates with length, so a long rail in a hall whose temperature moves between the night shift and the afternoon does not hold the same absolute geometry all day. And the floor the track is bolted to is not a straight line, so the axis inherits the floor’s shape unless the levelling work removes it.

The practical answer is to stop asking the axis to be accurate and teach the process instead. Teach positions at each station rather than deriving them from a nominal track coordinate. Where a part has to be found rather than assumed, put a camera on it and correct the move, the approach set out on our machine vision inspection page. And match the tolerance to the part rather than to the datasheet: on carton palletizing we accepted plus or minus 1 mm placement because the cartons themselves hold plus or minus 2 to 3 mm, and chasing tighter placement than the part holds is wasted money.

What Does the Floor Have to Do, and Do You Need a Pit?

For our low-profile track, no pit. It bolts directly to the factory floor and levels on adjustable feet, six of them on our low-profile design, and that is the whole civil scope. Avoiding the pit is most of the argument for the 100 mm height: a pit means civil works, a permanent hole in the floor plan, drainage and a cleaning problem in any clean or food environment, and it is not something you undo when the line is rearranged.

What the floor still has to do is real work. The track is a straightedge bolted onto a surface that is not straight, so the levelling and shimming is what converts a good axis into a good installation. Three things to settle before the frame is drawn.

Flatness and level along the whole run. Measure it, do not assume it. A long track laid on a floor with a fall in it either gets shimmed to level or is built to follow the floor, and those are different machines. Levelling is done across the whole run at once rather than foot by foot, because correcting one foot moves its neighbours.

Anchoring against reaction loads. The anchors do not only carry the static weight of the robot and the carriage. They take the reaction every time the arm accelerates and every time the carriage decelerates with a robot, tooling and part on top of it. On the low-profile track that travelling mass is up to a 1.5 tonne platform load, and on the heavy-duty series up to 2,000 kg. Those are horizontal loads into the slab, not just vertical ones.

Floor loading and the route in. The distributed load along the track and the point loads at the feet both need checking against the slab, particularly on an upper floor. And the track arrives in sections that have to reach the installation position through the doors and aisles that exist, which decides how the frame is split.

How Do You Protect the Track from Chips, Dust and Washdown?

Start from the fact that decides the answer: a rack and pinion axis is open by design. The pinion has to engage the rack and the bearing blocks have to engage the rails, so unlike a ballscrew inside a tube the working surfaces are not fully sealed. Protection is about keeping contamination off the engagement and getting it off again when it lands.

Be precise about what a washdown rating covers. IP65, the rating on our food-grade electrical enclosures, covers dust ingress and low-pressure water jets, so hose-down and wipe-down sanitation is fine. It is not a high-pressure or steam rating, so caustic foam followed by a pressure rinse belongs on the specification before the frame is drawn rather than at the first deep clean. Washdown changes the guarding hardware too: a standard dry-environment interlock switch fails early and takes the line down with it.

The honest boundary is that environment is a whole-axis design decision, not a cover ordered at the end. A track specified for a dry assembly hall and later moved into a wet zone is a rebuild.

What Does the Safety Case Look Like When the Robot Base Moves?

It grows with the stroke, and that is a cost which is easy to under-estimate. A fixed robot has one safeguarded space. A robot on a 20 m track has a safeguarded space 20 m long, plus the crushing and shearing hazards of the carriage and the track itself, which exist whether or not the arm is moving.

The method is unchanged. Start from an ISO 12100 risk assessment that determines the limits of the machine and covers every operating mode, including setup, teaching, cleaning and fault clearing, which is when guards get opened and hands go where the designer never pictured them. The robot and the integrated system are then covered by ISO 10218-1:2025 and ISO 10218-2, with each safety function rated for performance level under ISO 13849-1:2023. ISO 10218-1 was published in February 2025 as the third edition and the first substantive revision since 2011, so check which edition your contract names.

On the tracks we build, the typical safeguarding is a perimeter guard along the travel with a safety laser scanner on the load side, and reduced speed in teach mode. Four track-specific points follow from that.

Stopping distance is longer. The stopping performance that sets scanner and light curtain distances under ISO 13855 now includes a travelling mass of up to 1.5 tonnes on the low-profile track and up to 2,000 kg on the heavy-duty series. Measure it on the built machine rather than calculating it from the arm alone.

Safe axis limiting is worth specifying. Ask whether the arm’s safe speed, safe standstill and safe axis range limiting functions cover the track running as a coordinated external axis or only the arm. Where they do, the carriage can be restricted to part of the stroke while a person works at the far end, which changes where access points can be without removing the guarding.

Teaching happens along the whole length. Somebody will be inside the guarded space, at reduced speed, at every station. The access strategy has to work at all of them rather than at the one nearest the door.

Trapping points travel. The gap between the moving carriage and anything fixed beside the track is a shearing hazard that moves along the run. It is designed out with clearance and covers at concept stage, or it is guarded forever afterwards.

Singapore does not require CE marking, but the Workplace Safety and Health Act requires dangerous parts of machinery to be securely fenced and places duties on the occupier and on anyone supplying machinery for use at work. Motionwell delivers the physical scope that follows: guard fencing and interlocked access doors, safety laser scanners, safety-rated stop circuits, and LVD and CE testing. The full method is on our machine safety and CE marking page.

When Is a Linear Track the Wrong Answer?

Six situations where we would talk you out of one.

The work is heavy rather than spread. A track moves the arm; it does not raise the payload curve. If the arm is marginal on your part at reach, it is still marginal at every point along the stroke, and the answer is a larger arm or a different architecture rather than an axis under it. The selection logic sits on our cobot versus industrial robot comparison and on the robot integration services page.

Every cycle needs a traverse. A track pays for itself when transit hides inside a wait that already exists, the way 2 to 3 seconds of pallet-to-pallet transit disappears into a pattern cycle. If the robot has to traverse the full stroke on every single cycle, you have added transit time to every cycle and bought a slower cell.

Downtime at one station cannot take out the others. One track, one carriage, one robot: a fault anywhere stops every station on the rail. Two arms on fixed bases fail independently, and where a stoppage on one product cannot be allowed to stop the others, the second arm is the right purchase.

The floor cannot be given up. The track occupies its footprint along the entire stroke, permanently, and the guarded area grows with it. Where the floor is the binding constraint, an overhead gantry keeps the aisle clear underneath, which is the comparison run on the machine tending automation page.

The layout is not a line. Stations on both sides of an aisle, or scattered around a cell, are the case a linear axis cannot serve. A track is one dimension of freedom, and no amount of stroke turns it into two.

The process is production welding. We decline production welding cells outright, whatever the mechanics look like.

One case is not a reason against a track but a reason to design it differently: two robots sharing one rail. Two robots can share one rail on independent carriages, and that configuration needs collision avoidance designed in from the start rather than added afterwards as a program interlock.

Frequently Asked Questions

Is a robot on a linear track cheaper than buying a bigger robot?

Usually, when the problem is distance rather than weight. Buying a bigger arm to cover a long part normally costs more than putting a normal arm on a track, and it does nothing at all for the multi-station case, because reach grows in a circle while stations sit in a line. A track also carries a cost the bigger arm does not: it occupies floor along its whole length, the guarded area grows with the stroke, and one track fault stops every station it serves. Where a stoppage at one station cannot be allowed to stop the others, buy the second robot instead.

How much vertical working height does a seventh axis cost me?

Whatever the track is tall, because the track lifts the robot base by its own height and the arm's envelope moves up with it. A conventional robot travel unit takes 200 to 400 mm of vertical working height before the robot has done anything, which is why tracks in this payload class are often set into a floor pit to get it back. Our low-profile track is 100 mm overall and bolts directly to the floor, so the arm keeps more of its usable envelope and the civil work disappears. Check the top of your reach, not just the bottom, before you accept a track height.

Does the linear track need its own controller and PLC program?

It should not. ABB IRC5 and OmniCore controllers treat the track as a coordinated external axis, so the track servo is driven by the robot controller and the track motion is planned together with the arm rather than sequenced after it. The practical difference is that the arm can start moving while the carriage is still travelling, positions are taught in one frame, and there is no move-wait-move handshake between two machines at every station. Running the track as an independent PLC axis is possible, and it costs cycle time at every transit and adds an interface someone has to own.