Robotic transporters are a key part of automating the welding of extremely large, heavy parts.

The maximum work radius of an extended-reach robot is about 3-meters (9.84 ft.)

If the parts you weld require more reach than that, ways have to be found to move the robot to the part to provide optimal torch access to the welds.

A variety of robot transporters are available, and each type lends itself to a particular type of work flow.

How do you decide whether your application is best-suited for a linear floor- or wall-mounted track, linear overhead gantry or radial transport beam solution?

Factors to Consider When Choosing a Robotic Transporter

  • Cost
    Cost is always one of the primary factors to consider when selecting a robot transporter configuration.

    A single robot costs less than a robot combined with a transporter, however a robot on a transporter can reduce the need for additional robots and can maximize the use of an investment in capital equipment. However, multiple robots can be used together on some types of transporters to improve productivity.

    Some types of transporters take up more floorspace, while others reduce floor space needs.

    Some have expensive installation requirements, while others have facility and structural limitations or require special accommodations for part handling.
  • Part Size and Configuration
    The physical dimensions — length, width and depth — of all of the parts that are to be welded robotically have to be considered.

    Do the parts need welds on all sides, on the outside only or on the inside as well?

    Also, are short, intermittent welds required or does the part call for continuous welds along its length? In the case of tubular parts, are the welds circumferential?
  • Part Flow
    Do the parts flow in an inline production configuration or do they enter the work cell from opposing sides?

    Are work stations parallel — next to each other — or opposing — on opposite sides of the robot?

    Are the parts loaded by an overhead crane, by forklift or via some other method?
  • Facility Structural Considerations
    Structural considerations include floor reinforcement and the thickness of the floor's concrete that are needed to support the robots and transporters, aisle width, ceiling height, support column spacing, overhead cranes, and such things as lighting and ductwork that could cause interference with the robots or transport structures.
  • Safeguarding Considerations
    Worker safety is important in any manufacturing plant.

    Safety equipment generally is designed to comply with the ANSI/RIA 15.06-1999 safety standard.

    Welding cells require perimeter guarding with arc flash protection curtains; personnel gates with safety interlocks; and light curtains at operator workstations and any pass-through openings to the welding cell.

    Transporters may require limiting devices to be used as interlocks for safeguarding the work space.
  • Production Requirements
    You will need to determine whether multiple robots will be needed to meet required production volumes.

    Calculations need to be based on the number, length and type of welds — that is, whether they are single or multi-pass — that are required per part.

    If these calculations show that multiple robots are required, you need to decide whether the robots need to share the same workspace, as in overlapping envelopes, or if separate, adjacent tracks will suffice. (See Photo 1.)

Types of Robot Transporters

  • Linear Floor Tracks

    Usually the least expensive robot transport option, linear floor tracks allow the robot to reach welds on very long parts.

    Tracks can be as long as 30 meters (98.4 ft.). Length is limited by the manipulator cables.

    One or more robots can be mounted on a carriage that either indexes between two or more fixed positions along the track length or is servo-controlled by the robot controller. Using a servo-controller allows continuous positioning of the robot anywhere along the length of the track.

    If the robot needs to move to more than a few positions along the track, a servo-controlled track typically is less expensive.

    Servo-controlled tracks also provide more programming flexibility. With a servo-controlled track, the robot can weld a continuous bead the length of the track. If coordinated motion control is included, the robot can be programmed to weld the part while it is turning.

    A robot on a linear track can only access the part from a single side, so the width and depth of parts that can be processed are limited to the reach of the robot arm. However, robots often are mounted on risers on the track carriage to increase their elevation and depth of reach.

    Floor tracks take up a significant amount of space but, with an opposing work station configuration, parts can be processed on both sides of the track.

    Work flow is flexible. The cell layout can be configured to allow parts to enter from both sides of the track or to move in and out in a straight line from multiple — parallel — work stations on one side of a longer-length track.

    Anti-collision software enables multiple carriages to travel on the same track without interference.

    Carriages also can be expanded to allow two robots to be mounted on a single platform.

    Separate carriages are advantageous when relatively few welds are spread out along the length of the part. The robots can operate independently and work on opposite ends of the part or can be used to simultaneously weld along the same seam.

    Two robots on a single carriage save the cost of an additional axis, and are suitable for applications in which multiple welds are grouped together. The carriage can be moved to a fixed position and the robots then can sequence the welds within their reach.

    For welding applications, the carriage should be large and heavy enough to support a bulk wire drum. That greatly reduces wire feed issues. Trying to feed wire through conduit in a cable tray is less reliable, and generally requires an assist feeder at the drum. That increases system complexity, cost, and maintenance issues.

    Safeguarding a linear track is not difficult if the manufacturer has provisions for mounting switches and cams to monitor the carriage position along the track. Such monitoring could include continuous position sensing or segmenting the travel into zones, depending on the risk assessment.

    Floor tracks require leveling when they are installed, but work with modest concrete floor requirements and have minimal height requirements.

  • Wall-mounted tracks

    Mounting a linear track in an elevated vertical position allows a robot to work from overhead. (See Photo 2).

    This configuration can give the robot a wider reach and allows them to reach down into part cavities. This design is different from floor tracks, and support columns for wall-mounting add expense.

    A robot on a wall-mounted track can only process parts directly in front of it, so this configuration lends itself to an inline part flow.

    The wall-mounted track creates a narrower workcell because parts are processed from one side and equipment can be located under the support structure. Care must be taken to allow for overhead clearance for the track and the rear of the robot on the carriage.

    Welding electrode delivery is more difficult with a wall-mounted track because it is harder to mount a wire spool on the carriage on an elevated rail. However, as with floor tracks, multiple carriages can be located on a single wall-mounted rail.

    Limiting devices can be mounted to the robot arm to mute safeguards while the robot is overhead and become active when it moves down into a workstation. As with floor tracks, safeguarding switches also can be added to monitor the carriage position.

  • Linear Gantries

    Gantries extend the envelope of the robot beyond the reach of a wall-mounted track by suspending the robot overhead and adding additional axes. (See Photo 3.)

    The Y-axis of travel allows a wider part to be accessed by providing motion perpendicular to the length of travel. A vertical Z-axis allows a robot to reach deeper into part cavities. With the addition of gantry axes, a shorter-reach robot can be used to reduce the weight requirement for the gantry drives. Gantries with shorter axes can be supported from one side on a single rail. Larger span gantries utilize a bridge structure with supports on each side. Driving a bridge gantry requires special robot controller software to synchronize the motion of two independent motors to position each leg of the gantry.

    Gantries can add a great deal of expense to a robot system, but they are one of the few ways to process very large parts.

    A part positioner normally is combined with a gantry to turn the part for robot access. Most robots feature software to allow the coordination of all axes — the robot, the base, and the positioner — to achieve long continuous welds or circumferential welds in a single pass.

    A sufficient Z-axis stroke can allow the robot to weld the inside truck bodies, containers or other box-like parts while a longer Y-axis could allow the robot to work on the outside surfaces. Of course, the longer the spans, the larger the required supporting structure and the higher the cost.

    Long cable length and cable management become an issue the longer the stroke of the robot transporter.

    For gantry systems, it is not uncommon to mount the robot controller and process equipment to the gantry structure so that only primary power is supplied the length of the track, and so that the robot and process cables are limited to the Y- and Z-axis stokes.

    The other concern for a gantry system is overhead clearance, particularly for systems that use the Z-axis stroke.

    Large parts normally require overhead cranes to load and unload. Users can add interlocks between the robot gantry and overhead crane to keep the crane out of the gantry area while the robot is operating or while the Z-stroke extends up in the air.

  • Multiple floor track configurations

    Using two floor tracks on either side of a large part may be less expensive than a using a single robot on an overhead gantry, and the multiple arcs provided by the dual track configuration can enhance productivity. (See Photo 4.)

  • Radial Gantries

    Radial gantries provide one of the most economical overhead robot transporter solutions. (See Photo 5.)



A rotary base is less expensive than a linear axis, but it has a fixed boom length.

However, a single robot mounted on a two-meter to three-meter boom can expand its working volume by nearly 10 times.

Robots can be wall-mounted or ceiling-mounted on the end of the gantry boom. Wall-mounted robots provide a deeper envelope, allowing robots to reach into part cavities or access vertical surfaces, such as the outside of a box.

Ceiling-mounted robots provide a wide, flat envelope that is suited to processing flat panels such as mower decks or truck sides. Care needs to be taken with ceiling-mounted robots that wire feeders or cables do not hang down below the arm so they do not contact or snag the part or fixturing.

As with linear gantries, part positioners usually are combined with radial gantries to improve their flexibility. However, the orientation of the positioner in the workcell affects the robot's access to the part.

Locating a headstock/tailstock positioner perpendicular to the boom allows the robot to process longer parts using the length of the boom to move the robot closer to the headstock or tailstock.

The width of the part is limited to the reach of the robot — nearly double the reach for ceiling-mounted robots — and part depth that can be accessed is limited. If the headstock/tailstock positioner is located so it is in line with the gantry boom, then the boom can be pivoted to either side of the positioner to allow the robot to access the part from each side, providing more depth.

The boom length is fixed, so the robot can only access parts limited in length to approximately twice the robot's reach. Booms can easily be moved out of the way to improve overhead crane access for loading and unloading large parts.

It is not possible to use multiple carriages with radial gantries, but it is possible to combine multiple gantries in the same work cell. (See Photo 6)

Combining multiple gantries enlarges the part envelope that can be reached, and adds multiple arcs.

Similarly, radial gantries do not lend themselves to in-line processing.

However, two work stations can be are located on either side of the base, 180 degrees apart to expand the work envelope, and limiting devices can be integrated into the boom axis to interlock the booms, to provide safeguarding for either work station and to prevent overhead crane intrusion into the active workstation. Also, smaller robots can be mounted to a radial gantry to enhance their reach at a reduced cost.

Radial gantries also can be scaled up to include large extended-reach robots.

Because a radial gantry supports the robot as a cantilevered load, it must be rigid and have a solid foundation.

Special foundation requirements, ballast or base plates may be needed to counteract the forces of a moving robot at the end of the boom.

Utilities normally are routed through the interior of the boom, and an assist feeder for welding wire can be mounted to the wire drum at the base of the structure. Process equipment, such as nozzle cleaners, may be mounted on the boom in proximity to the robot.

Radial gantries often are used in applications in which short, intermittent welds or processing is done across the part, while linear tracks often are applied to applications that require long, continuous weld seams.

While coordinated motion between the robot, boom, and positioner is possible to achieve continuous welds, it is more common to “park” the boom in a stationary position and move the robot to weld joints within its reach.

Summary

In summary, the dimensions of the part usually dictate the method of transporting the robot.

In some cases, two radial gantries or floor tracks might be more economical than a single multi-axis gantry solution.

Using extended-reach robots on a single-axis transporter is less expensive than multi-axis transporter solutions.

If multiple robots are used in the same workspace, then programming and safeguarding for them must be addressed. It is most cost-effective to perform as much welding as possible on subassemblies in smaller cells, and limit the welding time of bigger components in large workcells to maximize their throughput.

If you would like to contact Chris Anderson, call him at 937.847.3216 or e-mail chris.anderson@motoman.com.