INDUSTRIAL ROBOT COMPONENTS

Seventh Axis Tracks and Robot Transfer Units Explained

Published: 

Seventh Axis Tracks and Robot Transfer Units Explained

A seventh axis track, also called a robot transfer unit or RTU, is a floor rail with a moving carriage that carries a robot along its length. It lets one robot serve several machines or work a part longer than its reach. The controller runs the track as an extra axis, so the arm and carriage move in one program.

How a Track Works

The robot bolts to a carriage plate. The carriage rides on linear rails and is often driven by a servo motor through a rack and pinion. Because the robot controller drives that motor, the track becomes part of the robot. When you teach a point, the carriage position is stored with it, so the arm and track move together. Our post on robot transfer units goes over the parts of a typical RTU.

That extra axis needs its own motor, its own amplifier, and the controller set up to run it. Our write up on understanding servo amplifiers explains what the amplifier does. The robot's cables also have to travel with the carriage, which is why tracks use a cable carrier. See our post on robot cable management for how that is laid out.

When a Track Makes Sense

  • A row of machines. One robot on a track can load and unload several lathes or mills in a line. Our post on machine tending robots covers that setup.
  • Long weldments. Frames, rails and long fabrications can run past the reach of any single arm. A track lets a robotic MIG welding cell follow the seam.
  • Several pallets or lines. Palletizing robots on a track can build loads at more than one station.
  • Moving parts down a cell. On long material handling runs, the robot carries the part instead of a conveyor.

A track is not always the answer. If the robot only needs to reach a little higher or a little further, a robot riser or a longer arm can do it with less to maintain. The long arm L models in our FANUC R-2000 series post are worth a look before you add an axis.

Track, Gantry or Positioner

A floor track is one way to add an axis. An overhead gantry is another. A gantry carries the robot on a beam above the work, which keeps the floor open for operators and forklifts. The gantry cell we have uses an R-2000iB/220U, which FANUC lists at 220 kg and 2,470 mm. There is also an R-2000iB/220US for rail or ceiling mounting.

The third option moves the part instead of the robot. A weld positioner turns the part so the torch always has a good angle. Our Genesis dual trunnion weld cells and the Pema 5000 CR robotic weld positioner work this way. One of our FANUC Arc Mate 120iD robots comes with two aux axis kits.

What to Check on a Used Track

  • Travel. Measure the usable stroke from stop to stop, not just the length of the rail.
  • Carriage load. Add up the robot, the tooling and the part. The robot's mass is on its data sheet. An M-710iC/50, for example, weighs about 560 kg before you put anything on it.
  • Bolt pattern. The carriage plate has to match your robot's base, or you need an adapter plate.
  • Rails and rack. Look for chipped teeth, dents, rust and dry rails. Run the carriage end to end by hand if you can and feel for rough spots.
  • Motor and drive. Find out whether the servo motor and gearbox come with it. Spare FANUC aiS 8/4000 servo motors and FANUC servo amplifiers are in stock, listed by exact part number. Match the part number to what your controller needs.
  • Cable carrier. Check for broken links and worn cables inside it.
  • Floor. The whole length has to be flat and anchored. A robot floor plate under the ends helps, but the slab still has to be sound.
  • Freight. Tracks are long and heavy. Everything ships FOB Imlay City, Michigan, and the buyer arranges freight.

Tracks, Gantries and Robots in Stock

Stock changes all the time. Click through for current units.

For the full robot list, see our used FANUC robots.

Related Guides

Taking a Track Out of Service?

We buy used robots, tracks and gantries, from a single RTU to a full plant. Go to our sell your used machinery page or ask about robot appraisals.

For questions on a track or gantry, call 586-752-0090, extension 1 for sales, or get in touch with sales.