Machines that employ linear axes, as in Gantry architecture, rely on precision linear guides to obtain the required positioning accuracy. The challenge of controlling motion straightness becomes tougher the longer the travel, the heavier the load and the tighter the accuracy specifications are.
As the length of the guides exceeds the dimensions practically available in a casting or a granite slab, the use of RHS beam to support the guideway becomes the obvious choice.
INA Linear Technology (a division of the Schaeffler Group of Germany) and Industrial Techno Logic (ITS Group) teamed up to supply the machinery industry with precision long travel systems, comprised of ITS’ heavy duty RHS steel beams and INA’s linear bearings. Machine designer can now implement positioning systems up to 8 meter in length, achieving motion precision, high stiffness, long-term stability, structural simplicity and cost effectiveness. Such systems have been successfully implemented in demanding applications as extra-wide format industrial digital printers, where gantry axes float 200Kg print head over a 5.1 -meter wide billboard print at speeds of 2.2 m/sec and accelerating at over 1 g, while maintaining total motion accuracy of better than 70 µm yaw, pitch and roll.
ITS Group has invested over two years of relentless efforts in developing its support beams. Working closely with its raw material, forming, machining and tool suppliers, ITS was able to fine tune a unique beam fabrication process that consistently delivers the required dimensional accuracy and stability. This process consists of several steps – formation, stress relaxation and machining, in several cycles. Finally the linear bearings and drive are installed and fine-tuned:
It starts with the raw material. High grade construction steel plates are heat-treated, folded into a rectangular hollow section (RHS) and welded along the seam. The resulting profile has the advantageous toughness and the uniform thickness of the rolled steel plate, plus the strength and stiffness contributed by the high moment of inertia of the RHS.
Several cycles of heating and gradually cooling the beam between controlled temperatures help relieve residual stresses. Vibrating the beam mechanically completes the treatment to achieve a stress-free structure that will not deform during machining and will retain its straightness throughout its service life. Aging on the shelf for a few months (“shelf relaxation”) stabilizes the beam and prepares it for the following step.
All linear bearing guideways are only as straight as the surface they are attached to. To ensure that ITS beams meet their precision tolerances, engineers from ITS and Iscar Metalworking, the world-renowned cutting tool manufacturer, carefully defined the exact tools and milling parameters for each step of the entire machining process. Multiple steps in a colossal machining center gradually planarize the surface intended to support the guideway. Periods of shelf relaxation in-between milling passes relieve stresses and allow the beam crystallographic structure to recover. In the last pass, which takes about 4 hours, a single tool tip cuts the last few hundredths of a millimeter, while the beam lies unfastened on the table, supported only by its own weight. The beams are mounted in the machining center in the same orientation and support points as they are intended to be installed in the equipment. This way it achieves its accuracy at its natural position, under the same deflection forces.
Guideway Installation and Fine Tuning
Most beam-supported linear positioning systems are custom-designed for a specific customer application. The beam dimensions, bearing type, drive mechanism and precision grade are designed to match the travel, payload, velocity and accuracy requirements of the equipment they are built for.
The beam, machined to tolerance, arrives at ITS’ assembly floor. Skilled technicians install the linear bearing guideway and the drive mechanism – a direct drive linear motor, lead screw or belt drive.
The beam is supported the same way it will be mounted in the equipment. The guideways are installed and a load similar to the designed payload is mounted on the carriage. The beam is ready for its final test – motion linearity. Two digital instruments measure the three types of linearity deviations – yaw, pitch and roll – in micro-radians precision.
One instrument measures pitch and roll; the other pitch and yaw. Having two independent instrument measure the same entity cross-verifies the readings. Both instruments are computerized, and the readings are automatically analyzed and reported.
The straightness of the machined beam is sufficient for most applications. In more demanding cases, ITS’ computerized instruments guide the technician in fine-tuning the guideway into tighter tolerances.
In critical applications, where a performance of a linear encoder depends on the exact parallelism of the reading head to the scale, ITS installs a special router onto the carriage, and utilize the guideway itself to make a final machining pass to bring the encoder seat into perfect parallelism.
The result of this meticulous installation and fine tuning procedure is a precise, heavy-duty axis, built to carry the burden of repetitious motions at high accelerations, many times in hostile environment, while maintaining its accuracy over years of service.