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Different shaped rails fight assembly errors
Parallel-mounting surfaces for linear guide ways are critical to motion system performance
By John Lewis, Northeast Technical Editor -- Design News, December 20, 1999
Richmond, CA-Parallel-mounting surfaces for linear guide ways are critical to motion system performance. Excess friction from misaligned guide ways or rails makes operation noisy and stresses slider bearings to premature failure. Instead of wasting a lot of time machining the rails' mounting surfaces until they are perfectly parallel, Matrix Integrated Systems Inc. uses the Compact Rail High-Performance self-aligning linear system from Sparta, NJ-based Rollon Corp. for a robot that handles wafers during a process called ashing.
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| The U-rail’s flat raceways offer lateral freedom to the slider, eliminating parallel errors in axial assemblies when combined with a T-rail. |
The system achieves quiet operation; eases mounting and assembly by accommodating the parallelism, alignment, and mounting inaccuracies associated with unfinished mounting surfaces; and reduces system maintenance for end users. Matrix supplies ashing equipment for both 200-mm and 300-mm sized wafers to IC (integrated circuit) manufacturers. The ashers use a plasma source to essentially burn away extraneous photo-resist material from wafers in a dry, low-pressure process. The company claims that the 200-wph (wafer per hour) throughput on its Cheetah 200-mm and Jaguar 300-mm ashers is the world's fastest. "Rollon's rail system is quieter, easier to align, and offers lower maintenance than profile or round-rail designs," says Albert Wang, engineering manager at Matrix. Three different types of rails, T, U, and K, comprise Rollon's linear motion system. Each rail has a specific raceway shape that is designed to reduce friction and stress by accommodating axial or angular misalignment.
A Rollon K + U track and slider assembly supports a mounting plate that carries the 50- to 100-lb traverser/transport robot. A servo-driven belt drive provides 8 inch/sec velocity over a 16-inch stroke on the 200-mm asher, and greater than 11 inch/sec speeds over a 23-inch stroke on the 300-mm asher. "Before installing the Rollon system, we tried using a profile rail with ballscrew-driven slider assembly, as well as a round-rail system," explains Wang. "But at these speeds, noise was a problem. We chose the Rollon system primarily because it was quieter compared to the other designs."
 In contrast to common rail systems, Rollon’s internal raceway and rollers are sheltered from bumps and shocks. |
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 Instead of profile or round rails, Matrix chose Rollon’s K + U two-rail, self-aligning linear system because of quieter operation, easier assembly, and lower maintenance. |
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| Rail selection depends on the type of misalignment expected in the system. | ||
But parallelism, alignment, and mounting inaccuracies were also a problem. When the mounting surfaces for the linear guides are not perfectly parallel, it pushes the sliders out of alignment, and increases friction that adds stress and leads to slider failure. That's why setting up parallel linear-motion systems is critical, and most often a time-consuming step. Profile rails require the most mounting surface preparation during installation. Round rails require less mounting surface preparation, and are less de-pendent on alignment than are profile rails. But because Rollon's self-aligning linear system tolerates some mounting and structural errors, it may reduce assembly time in many applications. "The Compact Rail was the most cost-effective design," says Wang, "because it simplified installation, and reduced my labor cost."
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| The maximum acceptable difference in rail height, b, depends on the distance between the rails, a. |
The Compact Rail system's versatility stems from its original design. Unlike common rail systems, Rollon uses an internal raceway design that protects hardened and precision-ground raceway and roller surfaces from bumps and shocks. In addition, nickel plating and strong spring-loaded wipers protect the sliders "A wide range of rail shapes and sliders combine to offer precision, high load capacity, fast speeds, and smooth, quiet movement," says Rollon General Manager Andrew Cook. Three different rail styles, T, U, and K, are available in four outside dimensions: 18, 28, 43, and 63 mm. "The rails' specially shaped, internal raceways allow the system to compensate for unfinished surfaces such as aluminum or welded structures, and guarantee quality and precise movements," says Cook.
"The Compact Rail High Performance system offers the highest load capacity to size ratio among linear roller systems," says Cook. In fact, the system can handle up to 10,000-lb loads, can provide 60.003-inch linear precision, and can operate at speeds up to 9m/sec. Moreover, a range of rails and sliders allow engineers to achieve the precision, load capacity, and operating speeds to meet the requirements of a wide range of general engineering applications in automation, packaging, and robotics.
The type of misalignment expected in the linear-motion system determines the rail-system design, explains Cook. For example, the T + U system eliminates parallel errors in axial assemblies because U-series rails have flat raceways that offer lateral freedom to the slider.
When a T + U rail combination is used, the slider in the T-rail guides the movement and supports loads, while the slider in the U-rail compensates for structural or assembly parallelism errors while still sharing its part of the radial load.
For example, a pair of T + U rails let the slider work correctly even when the angle between the inner and outer lanes is not zero. The maximum value of the angle between the two lanes can be calculated using a known rail length.
While the T + U system solves axial alignment problems, the K + U system compensates for parallelism, alignment, and mounting errors in all directions. As the K rail guides the motion, carries the load, and accommodates structural and assembly errors, the U rail carries radial loads and allows the slider to overcome axial parallelism errors.
Additional details...Contact Andrew Cook, Rollon Corp. 532 Rte. 15, Sparta, NJ 07871; Tel: (973) 579-3400; Fax: (973) 579-3222
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