Frictional linear bearings are useful in many applications. Apart from their simplicity, they tend to have high load capacities as concentric or parallel surfaces mate over a large contact area. They also provide good stiffness between the journal and bearing elements.
Despite these advantages, there are drawbacks and difficulties, among which is stick-slip. Stick-slip, which often results in jerky, inaccurate operation, is caused by large differences in static and kinetic friction. Bearing surfaces resist sliding up to a point. Beyond that, however, a much lower kinetic frictional coefficient comes into play, and the force initially needed to overcome static friction is suddenly out of proportion to what s required to maintain movement.
Experience shows that stick-slip is usually limited to lower velocities, and is particularly troublesome when approaching a predetermined position in numerically controlled servo systems. It can also lead to vibration and chatter that interferes with precision processes such as machining.
Hydrostatic bearings and air bearings, in which bearing surfaces are completely separated by a layer of pressurized fluid, provide high load capacities without the problems associated with sliding friction. But these bearings can be costly, and maintaining fluid pressure requires an additional power supply.
Sometimes its preferable, then, to use rolling element linear bearings in positioning systems. But while rolling elements do the job nicely in many precision machines, their design and selection can be tricky.
Rolling bearings operate with a very small contact area, and localized stresses can take their toll on bearing elements. Therefore roller-to-race conformity, surface finish, and hardness are crucial parameters. Heavy loads can permanently deform rollers and raceways if the geometry is wrong or component quality is sub-par.
Point or elliptical contact, as occurs with ball bearings, can be adjusted by tailoring the conformity between the race and ball radius. The conformity is the radius of the race divided by the ball diameter. The closer the geometric conformity, the greater the contact area and load distribution and the lower the stress. (Note also that the contact area becomes more elliptical than round.) Lower stress also means less strain, so the system will be stiffer. A ball rolling over a flat or convex raceway, on the other hand, is an example of poor conformity. Such arrangements will require substantially larger ball diameters to reasonably distribute stress and maintain stiffness.
Preloading is another way to stiffen rolling-element linear slides — it increases the pressure area and can make the slide up to three times stiffer. Dont overdo it though, excessive preloading isn t good. The increased ball deformation, even while it remains in the elastic range, causes the ball to scuff, rather than roll, along most of the contact ellipses major axis. True rolling occurs only at the center of contact.
Scuffing is directly related to contact area, so the ball-to-track conformity is also an influence. As percent conformity goes down surfaces are closer in shape and the amount of scuffing gets worse. The limit, 50% conformity, means the ball and race are in full-face contact. Although load capacity and stiffness are high, scuffing will be excessive.
Ball contact angle also affects stress levels and wear. Some configurations have gothic race shapes that allow the ball to contact at more than one ellipse, offset from the race centerline. This distributes the load without requiring excessive conformity (and a drawnout ellipse) that can promote scuffing and frictional wear.
Apart from concerns of ball and race contact, there are various ways the overall guide assembly can be arranged. Linear roller bearings can have circulating or non-circulating rollers. With circulating systems, travel is limited not by the bearing but by the length of the rail or guide. Non-circulating linear bearings, though, are limited by the length of the bearing outer sleeve or housing, since the displacement of the balls relative to the races is only counteracted by reversing the motion.
Bearings and guide ways can be set together in various configurations. Bearing circuit assemblies, or bearing blocks, can be double-sided, allowing ball-to-race contact along both edges of the block; two separate rails bound the block on either side. Similarly, the guide can be double-sided, with the opposite edges having races to receive a bearing block. In this way two bearing blocks can be connected across the guide to move as one. Using doublesided components is a compact, efficient way to keep the slide aligned and capable of multidirectional loads.
Besides supporting both downward and sideways pressure, having multiple linked ball and race systems provides a spline action. With diametrically opposed balls and races the slide will resist torque while allowing free axial movement.
Denny Z. Dvorak is President of Precision Motions Co., West Hartford, Conn.