Precise movements are made with precision slides. Reciprocating linear bearing accuracy, the possible runout in any plane, is typically as low as 0.005 in./in. Because linear bearings are only as accurate as their riding surfaces, modern machining and grinding capabilities make submicron tolerances possible. Standardizing issues — runout, parallelism, and repeatability — are also less problematic than ever, as overall precision continues to increase — though often at an increased price. To avoid costly, unnecessarily tight tolerancing, many designs and accuracies are available to satisfy specific application criteria. For the best design, footprint, weight, environment, and orientation of masses surfing on bearings determine which designs are usable for an application; required accuracy, travel, and duty cycle determine which is most appropriate.
Non-recirculating linear bearings consist of either ball or cylinder rolling elements, spaced by a honeycomblike cage, that move on a straight track. The bearing reaches the end of its travel when either the retaining cage, or a 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; it is twice the difference of carriage and retainer lengths. On a standard unit, travel usually equals about one-third the carriage length; however, travel length sometimes equals carriage length.
Straight-line ball slide assemblies use ball bearings for rolling elements. They are held in position by a cage made of Delrin, Teflon, copper, stainless steel, aluminum, and other plastics. Typically, the balls roll on a set of four hardened and ground shafts that surround them. Arched, v-grooved rails are also used, because they offer greater stability in overhanging load conditions. This design is selfcleaning because balls push contaminants away as they roll; pollutants rarely get caught at the ball/race point contact.
Straight-line alternately crossed rollers are small cylinders held in a crisscrossed pattern by a cage. Like ball rollers, crossed rollers either travel on arched, v-grooved rails, or four parallel rods. However, when rods are used, they are ground partially flat. The rollers travel on the resulting flat surface for line contacts with the rod. Compared to the load capacity allowed by point contact of balls, crossed rollers carry very heavy loads. They also absorb greater impacts. The rollers are specially configured to have a diameter greater than their length, allowing them to lie at 45° to each other without bumping one another; this also makes for equal load capacities for all directions and orientations.
Crossed roller slides provide the accuracy needed for precise positioning — for example, on tooling packages. The straight-line accuracy of crossed roller slides is about 0.0001 in./in. of travel; this is measured by aligning the line of travel with a master straight edge, then observing position with a gauge or indicator mounted on the slide. Positional repeatability can approach 0.0001 in., with coefficients of friction as low as 0.003.
Recirculating linear bearings
Recirculating designs offer travel not limited by carriage length. They have either cylindrically shaped or spherical rolling elements. In this design the rolling elements loop through an oval track inside the carriage. However, increased friction and stiction are caused when rolling elements travel through the oval’s two tight bends. The relatively small carriages are often not sufficient to support the size of moved or guided parts, so double or even triple carriages are necessary. This increases bearing size and complexity. The main benefit of circulating designs is the long travel possible within a reduced envelope, since the carriage need only be long enough to allow rolling element recirculation. High accuracy is also possible because the base and carriage can be machined, ground, and matched.
Linear bushing and shaft assemblies
Bushing assemblies typically require two shafts with supports or hangers, four bearings, and a housing. The recirculating steel balls are located in four ball retainers spaced evenly throughout the bushing. The bearings’ balls roll on the shafts, allowing axial travel. This gives the advantage of a travel length as long as the shaft itself, often up to 12 ft.
The bushing’s load bearing plates push on the steel balls to increase load capabilities. Because the contact areas of the balls and shaft are less than that of the linear guide, frictional resistance is greatly reduced. Linear bushings also provide relatively high load-carrying capacities. Drawbacks include a complicated mounting and the need for precise shaft and support alignment. They are only about half as accurate as the other main types; they can also be noisy and exhibit undesirable friction and stiction.
The life of a linear bearing is actually expressed as a length — the distance a bearing can travel under specified conditions. Because exact life cannot be predicted, a rating is often used to approximate when a bearing might start showing signs of old age. In industry, L10 is the most common rating; if Bearing Model X has that rating, then 90% of apparently identical Model X bearings will successfully travel the calculated L10 length before failing. Three fudge factors go into the rating; they account for the effects of speed, temperature, and load.
Often the most important criterion, slide load capacities are rated according to orientation. Mass loads on horizontal slides are most common, but normal forces are applied to mounting surfaces in many orientations. Regardless of positioning, rated loads are best centered and evenly distributed over the slide. Further, bases should be fully supported on flat mounts to prevent concentrated or distributed bending forces on the slide. Protruding support arms reduce accuracy; because the supports act as levers, load capacity also drops. Moment load ratings and allowable force formulas are available and should be consulted when these forces occur.
The speed coefficient fs can be calculated:
A load type factor fw accounts for vibrations, impacts, and inertial forces applied to the bearing. Smooth operation is assigned an fw value of 1 to 1.5; varied loading, 2 to 3. Linear bearing footprints and envelopes should be sized for maximum physical support of manipulated objects. Overhanging loads and forces can be accommodated, but should be kept at a minimum; this should be noted especially when using straight-line slides, because of the longer travels to accommodate carriage and stop interference.
For a rated life of Ln,
When speeds are slower than 30 ipm, they will not degrade life and fs can be assumed to be one.
Similarly, temperature factor ft quantifies the effects of heat on slide life. For temperatures below 200°F, ft is unity. Bearings of materials other than hardened steel can have ft values of 0.75 above 400°F; this approaches unacceptably low values. For these higher temperature environments, though hardened steel bearing components operate, they do so with unavoidably reduced life. Excessive heat softens the contacting surfaces of slides, in turn reducing load rating. Heat also stresses elements associated with slides, such as retainers and lubricants; they must be constructed of durable materials.