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Techno Linear Motion Catalog 19 Technical Information 5)    Load Bearing Mechanisms Load bearing mechanisms are the structural backbone of any linear/rotary motion system, and are a critical consideration.  This section will introduce most of the more common types of load bearing mechanisms found  in  linear  motion  machinery.    In  general,  bearings  allow  smooth,  low  friction  motion  between  two surfaces loaded against each other.  The motion can be either rotary (such as in a turning shaft) or linear (such as a machine part moving back and forth).  Some applications require that a bearing accommodate both types of motion simultaneously, which is referred to as a combination bearing.  In both cases there should be a strong attempt to provide enough lubrication to keep the bearing surfaces separated by a film of oil.  The absence of physical contact provides most bearings with long service lives. Bearings are evaluated on the basis of how much load they carry, at what speeds they can carry the load, and how long they will serve under those conditions.  Friction, start-up torque, shock- and impact-resistance, operating environment, rigidity, size, cost, complexity, and lubricating procedures are also important design considerations. The following diagram reveals the basic bearing families more widely found in linear motion systems.  The distinction between families is made by the type of motion and type of loading being considered.  Bearings accommodate rotational and/or translational motion.  Translational bearings, or linear bearings, are loaded perpendicular (radial) to the direction of motion.  Rotational bearings can be loaded either perpendicular to the axis of rotation (radial) or parallel to the axis of rotation (thrust). Most bearings can be classified into one of two general types.  The distinction between bearing types is made by the nature of the bearing mechanism.  The two types each have a number of different possible configurations, and some of the more common configurations will now be discussed.  Since many bearings are either specialized or of proprietary design, this discussion is by no means intended to be all exhaustive, but rather to serve as an introduction. Plain  Bearings,  or  Journal  Bearings,  are  the  most  basic  type  of bearing.  They have no moving parts, they support loads through sliding contact, and are usually the least costly to install. Some plain bearings are self-lubricating and maintenance-free while others are not.  In fact, some plain bearings are classified as to the lubricating method required. Typically, plain bearings will operate right up to the point of failure with little or no warning signs.  Since the motion involved is usually low level, the results of failure are typically noncatastrophic, and replacement or repair is fairly simple.  There are several ways to size plain bearings for an application, as outlined at the end of this section. Thrust TRANSLATION (OR LINEAR) JOURNAL OR RADIAL BEARING Bearing surface THRUST BEARING Radial Load ROTATION Motion Motion Load Load PLAIN BEARING Bearing Journal