|Selecting and Applying Linear Motion Devices
Rolling element linear motion bearings are widely used to guide, support, locate, and accurately move machinery components and products in industries in which automation exists. Designers have a wide range of alternatives for providing accurate linear motion. The primary criteria for selecting linear motion devices include the footprint of the mass being moved and the envelope available, load and its orientation, accuracy, travel, operating environment and duty cycle required of the device. Designers have the choice of ball or crossed roller, straight line design or recirculating type, linear bushing and shaft assemblies and positioning stages. Each type of linear motion device has its own set of advantages and disadvantages. Let's look at the basic characteristics and selection criteria of each of these devices.
Type of Rolling Elements
Ball bearing or crossed roller rolling elements are the most common choices. The ball bearing type typically uses a set of four hardened and ground shafts that surround the balls at four points.(fig1) This design offers the advantage of being self-cleaning because contaminants are not able to get between the balls and shafts due to the point contact and tend to be pushed away as the bearing operates. The arched or v-grooved rail configuration is also used and offers somewhat greater stability in overhanging load conditions.
Crossed rollers (fig 2), alternately crisscrossed with each other, are also used with the arched or v-grooved rail system or, move between a set of four, partially flat, parallel smooth rods surrounding the rollers. The additional grinding process that flattens the shafts allows the rollers to move on a flat surface, increasing greatly their load capacity relative to the ball type. The rollers are specially configured to have a diameter greater than their length, allowing them to lie at 45 degrees to each other without interfering and offering the same load capacity in any direction or orientation. The rollers have line contact with the way surface as compared to the point contact of balls. This bearing design allows crossed roller slides to carry larger loads and absorb greater impacts.
Straight-line or non-recirculating linear bearings (fig3), have rolling elements that move on a straight track and are separated by a retainer. The bearing reaches the end of its travel when the retainer or rolling element contacts a limiting component, typically either a screw head or end cap. This travel distance is determined by the relationship of the retainer length to the carrier length. Standard units usually have a travel equal to one-third of the carriage length. Maximum total travel can be as much as 1X the carriage length.
Straight-line design allows the manipulation of preload an important capability that is increasingly needed in modern industrial equipment design. A tool steel gib is used internally on the side of the bearing to uniformly load all the rolling elements in the bearing and reduce looseness or play. A row of staggered set screws accessible from the side of the assembly allows the preload to be set and adjusted. The amount of preload has the opposite effect upon the axial play, and the friction characteristics. Increased preload reduces play and increases friction (fig4). Preload may be quantified using a gram force gauge allowing customization to the application. Straight-line design offers the lowest coefficient of friction because the rolling elements are separated from each other and they are not required to turn corners or to describe an oval path as they do in recirculating designs.
Recirculating Linear Bearings
Recirculating designs (fig 5) offer travel that is not limited by carriage length. In this design the rolling elements revolve within an oval track inside the carriage. Increased friction and stiction characteristics are caused by the need for rolling elements to travel through corners. The relatively small carriages are often not sufficient to support the size of the part being moved or guided and therefore double or even triple carriages are necessary, increasing the size and complexity of the bearing. Long travel is possible within a reduced envelope since the carriage need only be long enough to allow recirculation of the rolling elements. High accuracy is also possible because the base and carriage can be machined, ground and matched with each other.
Linear bushing and shaft assemblies
Linear bushing and shaft assemblies (fig 6), typically require an assembly consisting of two shafts, with supports or hangers, four bearings and a housing. An important advantage of this type of device is that travel can be as long as the shaft itself, often up to 12 feet. A disadvantage is that complicated mounting and precise alignment of the shafts and supports is necessary for proper operation. Linear bushings typically provide relatively high load carrying capacity but, on the other hand, are only about half as accurate as the other main types, can be noisy and exhibit undesirable friction and stiction characteristics.
Applications requiring anti-friction devices for intermittent motion can be satisfied by the use of manually operated positioning stages (fig 7). Available with either ball or crossed roller bearing elements, and configured with one, two or three axes of motion, positioning stages are generally spring loaded against a linear actuation device, most commonly a micrometer head. The devices will smoothly translate to a position which can be measured and then locked into position as the operation commences. Changes in part size or length can be easily accomplished by manipulating the measuring head.
Footprint and envelope
The linear bearing should be sized so that it gives as much physical support as possible to the object being manipulated. Overhanging loads and forces can be accommodated but should be minimized if possible. Straight line types require a longer space to operate than recirculating because the relationship between the moving carriage and travel is fixed.
Load and its orientation
The load is often one of the most critical selection factors in selecting linear motion devices. The rated load capacity of a slide may be a mass load on a horizontal slide or a force load normal to the mounting surface in any position. The rated load must be centered and distributed over the slide and the base must be fully supported on a flat mounting surface so the slide is not subject to concentrated or distributed bending forces. Loads supported by protruding arms reduce accuracy and load capacity by acting as levers or ratio arms. Moment load ratings and formulas to calculate allowable forces are available and should be consulted in applications in which these forces occur (fig 8).
For many industrial applications requiring reciprocating linear bearings accuracy of 0.005” per inch is common. Much more precise runout, parallelism, and repeatability are available. Modern machining and grinding capabilities allow sub-micron tolerances to be achieved. A linear bearing will only be as accurate as the surfaces with which it is interacting. Accuracy comes at a price. Many different levels of accuracy are available to match the specific application criteria (fig9).
Hardened steel bearing components may be used in high temperature environments but life is reduced at temperatures above 212F at which point the hardness begins to degrade (fig10). Care should be taken when applying linear bearings in these applications that the associated elements of the bearing are constructed of materials that can withstand extreme temps. High temperature materials should be specified for the retainers and special lubrication may be needed.
Large amounts of dirt and grit or airborne contaminants can affect the operation of linear bearings While ball slides using cylindrical shaft ways are considered self cleaning due to their design it is possible to find industrial environments in which their life may reduced. When using crossed roller bearings, care should be taken to isolate the units or provide a bellows type cover to protect the internal parts if the environment warrants it.
Requirements common in the semiconductor and electronics industries are satisfied through the use of electroless nickel, an alloy of nickel and phosphorous produced by autocatalytic chemical reduction with hypophosphite. The plating allows static charges to dissipate, helping the slides to meet requirements for automated equipment. Standard anodized coating, used on conventional slides, is an insulator that can hold a static charge because of the insulating effect of the coating material. Electroless nickel plating, on the other hand, is a preferred treatment in the semiconductor industry for non-stainless steel surfaces because it provides a conductive path for dissipating static charges. The best way to eliminate static on machine components is to make sure the surface of the component has a direct path to ground. Proper grounding dissipates any discharges away from the product.
Out-gassing of slide components can be reduced or eliminated for vacuum applications by eliminating anodized and oxided finishes, labels, lubricants, nonmetallic retainers and venting blind holes.
Linear motion bearings are designed to reciprocate and operate at high speeds. The duty cycle (fig 10) affects the life of the bearings and should be considered when selecting slides for constant duty environments Formulas are available to calculate the predicted life of bearings in differing situations.
Del-Tron Precision began operations in 1974 supplying original equipment manufacturers with the world’s first commercially available subminiature ball slide. Since then, thousands of Del-Tron ball slides have been incorporated into medical analyzing and testing machines, semiconductor processing equipment, computer peripherals, assembly systems, scientific instruments and many other machines. Del-Tron’s modern corporate campus boasts highly automated computer controlled equipment and final inspection of 100% of all products has been Del-Tron’s policy since its inception.
By Edward Keane
Del-Tron Precision, Inc.
For more information contact Del-Tron Precision, Inc, 5 Trowbridge Drive, Bethel, CT 06801. Phone: 203-778-2727 Fax: 203-778-2721 Internet: www.deltron.com
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