Motion Control - The need for speed

July 10, 2003
Hybrid rail guides lead to lighter, faster systems.

The need for speed

Wayne M. Greer
     SKF Motion Technologies
     Bethlehem, Pa.

Motion-component manufacturers like SKF are experimenting with hybrid material rail guides to decrease weight and increase attainable speeds.

Today's motion-control applications often demand higher speeds to boost productivity and throughput. When you get right down to it, the system's mass largely determines maximum speed. And that has led manufacturers to reexamine the materials they use to build components.

One trend is the increasing interest in hybrid materials for motion-system components such as rail guides. Hybrid products offer the potential for lighter weight and higher performance. As an example in linear-guidance systems, profile rail guides have begun to evolve in this direction with emerging hybrid designs.

Conventional steel linear-profile rails guides usually consist of a profile rail with four precision-ground raceways and a slide unit with four ball-circulation paths. This design offers a number of benefits. Specifically, square configuration raceways result in a guidance system with good rigidity, capable of withstanding moment loads in all directions, with equal load-carrying capacity in all four directions. Also, ready-to-mount units can minimize installation and adjustment requirements and the design of these systems can compensate for inaccuracies in adjacent components. Hybrid guide technology can potentially deliver other advantages by reducing drive power requirements and a machine's overall manufacturing and operating costs.

An inherent drawback of traditional linear-rail guides is that the weight of the steel rails and slide units naturally limits a machine's operating speeds: inertia increases linearly with mass and acceleration. As a result, the feasibility of lightweight designs is gaining interest to help promote more dynamic production, transport, and handling systems. For instance, reducing weight to 80% of previous levels means significantly less system inertia, and speeds to 10 m/sec.

The density of steel historically used for guides (approximately 8 gm/cm3) is almost three times the density of aluminum (less than 3 gm/cm3), making aluminum a viable material for hybrid guides. Hybrid bearings can incorporate aluminum, engineered plastics, and hardened steel inserts for roller contacts. The composite designs lower rail-guide weight without significantly reducing load-carrying capacity.

Interest in lighter profile rail guides has been generated, in part, by an increasing number of smaller and lighter products to be transported or positioned in high-tech industries, leading to a quest for smaller and lighter machines. Today, size and weight have earned virtually equal attention with requirements for load-carrying capacity and stiffness when designing linear systems.

Another reason for the interest is the systems themselves. Multiaxis handling systems, in particular, stand to benefit from lightweight hybrid guides. In these systems, the base axis must support two, three, or even four other axes. Weight reductions of the supported axes could add up considerably to offer previously unattainable increases in the dynamics of the base axis and machine performance.

The first hybrid rail-guide designs consisted of a lightweight material to provide the required structural stiffness and a special steel to promote running accuracy between the rails and rolling elements, as well as the load-carrying capacity of the system. Prototype linear-profile rail guides replace the conventional heavyweight steel rails with special lightweight materials. Only special steel raceways are incorporated into the rail body. Hardened raceways accommodate the high Hertzian stress typically associated with rolling-element bearings.

Further prototype development and evaluation have shown aluminum base bodies can be up to 65% lighter, and as much as 80% lighter for lightweight material/metal systems, without sacrificing system stiffness or running accuracy and repeatability.

Full production of standard off-the-shelf hybrid linear-profile rail guides awaits further testing and refinements with traditional production methods. In the meantime, an accelerated search is currently underway for suitable materials, whether aluminum, ceramic, or other appropriate alternatives to lower weight, increase speeds, and improve corrosion resistance. However, depending on the type of materials used, there could be some reductions in operating temperature limits.

Regardless of material choices, using hybrid rail guides calls for close cooperation between machine designers and linear-guide specialists in the early stages of machine design: In particular, bringing performance characteristics such as stiffness and running accuracy in line with material capabilities.

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