Linear bearings are advantageous when it comes to handling heavy loads.
Simply put, a linear bearing provides guidance for movement while supporting the load of the carriage. Note that the contact area is important when determining the load capacity. For example, the basic ball bushing bearing design has a point of contact on the inner and outer race. This is beneficial because there is very little surface in contact. This results in a maximization of smoothness. This very feature is also the reason for the linear bearing’s biggest disadvantage.
However, the downside to having a single point contact is that all the force is concentrated on a single, small point. This creates a stress riser that limits this design to handling lower loads only. A slightly more advanced design consists of a ball-conforming groove on the outer plate. The purpose of this is to distribute the stress of the load across a large surface. Here we have a little more load capacity; however, it is still limited by the single point contact on the inner race of the shaft. This design typically has three times the load capacity as the ball bushing bearing, so it is used for medium-load capacity. A profile-load bearing with a ball conforming groove on the inner and outer race maximizes load capacity. For reference, these typically get ten times the load capacity as the precision steel type bearing.
A major factor to consider when selecting the correct bearing for a specific job is accuracy. Here, accuracy means the variation in the height over the travel of the bearing. Round rail bearings typically achieve accuracy of ±.001” (plus or minus one thousandth of an inch) of accuracy over the entire travel length of the bearing. Profile rails generally achieve one tenth of this accuracy. Round rail bearings have the added benefit of being able to be mounted on a non-qualified surface. This makes installation much easier compared to a profile rail bearing, which requires a milled or ground machine surface.
Another factor to consider is repeatability. Repeatability is dictated by pre-loading a bearing. There are three main ways to achieve this on a round rail bearing: oversize the shaft, oversize the bearing elements, or undersize the bore that the bearing is going into. The goal with any of these methods is to create an interference fit between each of the components.
Another factor that must be considered is configuration. Take rail configuration, for example. A popular configuration is two rails, each with two bearings per rail. When configuring bearings, two bearings per rail is highly advised. When determining which configuration will be used, always keep aspect ratio in mind. Aspect ratio is the distance between the rails versus the distance between the bearings. A 3:1 ratio is the maximum recommended. You will frequently see a 1:1 ratio in machine tools.
Other considerations to keep in mind are noise, speed, acceleration, temperature, and contamination. With all the versatility and design types available, you are certain to find linear bearings a great option for an assortment of practical needs.
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