By Sean M. FeyrerThe miniaturization of linear-motion systems is much more involved than simply reducing the dimensions of guideways, slide units, ball bearings, or ball and roller screws. In addition to saving space, smaller components must be designed, specified, and manufactured for exacting precision and full integration to achieve optimum performance.
In electronics applications the trend is toward finer structures and smaller components with increasingly higher degrees of integration. The same picture has emerged in mechatronics and the assembly of PC boards, as well as in the medical-equipment field. Other linear motion applications adopting smaller components range from semiconductor packaging equipment to systems for aligning and splicing fiber-optic cables. Countless others abound.
The smaller world of linear motion components includes many noteworthy technologies. For instances, miniature profile rail guides offer compact stainless solutions in applications demanding maximum component density in minimum installation space. Typical applications include equipment in the semiconductor, precision mechanics, mechanical engineering, micro-assembly, electronics, and medical industries. They are designed to deliver precise linear positioning, smooth operation, high load-carrying capacity, high stiffness, and unlimited stroke.
The numbers tell the story. The smallest rail width is 7 mm, which corresponds with a proportional reduction in the size of the carriages or blocks. Whereas slide units (for rail size 15 mm) are 71 mm long and 47 mm wide, their smaller counterparts now measure just 23.5 mm long and 17 mm wide and offer a weight reduction factor of approximately 20 times their counterparts.
Different carriage designs meet particular load and stiffness demands (long carriages promote higher load-carrying capacity and wide carriages promote higher moment stiffness.) High-speed linear motion up to 3 m/sec can be achieved.
SKF’s miniature linear ball bearings are suitable for pick-and-place applications where space is at a premium. |
Miniature linear ball bearings are well-suited for small pneumatic systems in pick-and-place and handling applications where available space for components is minimal. These compact bearing solutions apply the reliable technology of recirculating balls.The design of the “size-friendly” linear ball bearings consists of a plastic cage carrying hardened steel raceway segments and guiding the ball sets. The raceway segments are engineered to maximize the length of the load-carrying zone, and the design of the plastic cage allows for all the balls in the bearing to run on the same pitch circle diameter. Ball recirculation occurs smoothly with no resulting cage loads.
Cages can accommodate maximized ball diameters to achieve higher load capacity, enhanced running quality, and extended service life. Longer raceway segments further contribute to load-bearing capacity.
These types of bearings can additionally be sealed to protect against contaminants and enhance lubricant retention. Depending on application, they can be fitted with integral double-lip seals consisting of an inner lip that prevents lubricant from escaping within the bearing and an external lip that wipes the shaft surface as the bearing moves.
Where corrosion resistance is a factor, the bearings can be specified with raceway segments and balls manufactured from high-alloy stainless steel. Combined with stainless shafts, all-stainless designs can be realized.
In precision-positioning system applications where rotary motion must be transferred to linear motion, miniature ball screws can come into play. They, too, have kept pace with the ever-shrinking world, especially in laboratory instrument applications including syringe pumps and automated pipettes.
Diameters for these assemblies can be as small as 6 mm and leads as small as 2 mm. Despite their dimensions (compared with larger versions), they can accommodate relatively high dynamic loads. And, smaller ball screws can facilitate smaller overall instrument designs.
Components such as this miniature slide from SKF USA Inc. can help provide ultra-smooth motion in fiber-optic applications. |
In addition to these several “downsized” components, significant strides have been made in creating fully integrated systems requiring less space and fewer assembly steps. Two examples of such system innovations are miniature slides and miniature linear stages.
Miniature slides are compact and complete assemblies that serve to provide precision guidance and ultra-smooth motion for delicate operations in the fiber-optics (aligning/splicing cables), medical, pneumatics, semiconductor, micro-assembly, and electronics industries. Depending on the application, running accuracies of 2 mm over a stroke of 100 mm can be achieved.
Rail units can range from 7 to 15 mm with widths from 17 to 32 mm and lengths from 26 to 165 mm, depending on specific type. They are made of stainless steel to resist corrosion and have been hardened to optimize endurance and performance.
In addition to small size and big performance, miniature slides offer plug-and-play advantages. They can be installed and/or replaced easily and reliably, because the units are pre-assembled and ready to mount. Unlike cross-roller systems with four rails and cages to be assembled, miniature slides bolt into place without any adjustment for preload.
Miniature linear stages are fully integrated units that also allow for plug-and-play operation to provide controlled linear movement in high-precision pick-and-place positioning applications. Demand has grown for these assemblies in the laboratory, semiconductor, high-tech, packaging, and automation industries.
Composite slides, as part of stage assemblies, have been developed in widths as small as 60 mm. Travel speeds up to 500 mm/sec can be realized.
The stages typically incorporate drive and guidance mechanisms using standard components, such as precision ground ball screws and miniature profile rail guides, for optimized application versatility and quick turnaround.
Whenever and wherever miniature linear-motion system components may be used, potential performance challenges can arise unless they are recognized and prevented before problems occur. For example, the ball size in miniature profile rail guides ordinarily would be expected to affect a system’s smooth-running characteristics. This potential risk can be eliminated by grinding the raceways of the rails such that each ball has a four-point contact. This serves to maintain smooth-running performance without jeopardizing system stiffness. And, as with their larger counterparts, smaller components will be susceptible to all the usual external forces, including vibration and contamination. System and component designs should compensate appropriately to minimize possible risks.In general, potential problems can be averted at the outset by taking advantage of the design and engineering expertise offered by experienced linear motion system manufacturers. Such professional support can prove invaluable in designing customized integrated solutions to fit (small or otherwise) application requirements.
Sean M. Feyrer is linear guidance business manager for SKF USA Inc., 1530 Valley Center Pkwy, Bethlehem, PA 18017. More information is available at www.linearmotion.skf.com or by calling 800-541-3624.ADVERTISEMENT