For a CNC to operate correctly, each axis on the machine must travel in a very straight, tight line. This critical function is dependent on linear bearings and rails that are precisely matched to system load requirements and feature a minimum deflection, vibration and friction. Often overlooked and underestimated, linear bearings and rails play a key role in determining system accuracy, repeatability and quality of motion. Additionally, linear bearings and rails exert a direct and substantial influence on the performance of other key drive train components, such as: rack & pinion, ball screw and lead screw.

Linear bearing and rail assemblies are available in both sliding (plain) and rolling contact configurations. Examples of sliding configurations include, boxways, dovetails and twin rails. Whereas, rolling configurations are commonly based on either v-wheel or recirculating ball bearings. Because rolling contact bearings feature 10-20 times less friction and significantly less wear than sliding types, they have become the standard for the high speed, high acceleration demands of CNC routers. And, based on better load distribution and superior rigidity, recirculating ball bearing configurations have rapidly replaced v-wheel type rolling bearings.

Linear Bearing Dependencies

The primary dependencies for linear bearing performance are alignment, rigidity and precise pre-load. To prevent binding and premature wear it's critical that linear bearings feature precise running parallelism, straightness, flatness and mounting surface rigidity. Effective linear bearing damping and bearing to rail rigidity are dependent on precise bearing to rail pre-load. It warrants stressing that a linear bearing assembly acts like a very stiff spring, displacements caused by mounting misalignments and thermal growth caused by high levels of friction generate harmful forces that negatively impact the accuracy and service life of a linear bearing assembly.

In significant measure the machine component onto which linear bearings are mounted will influence bearing performance. Irregularities (roughness / waviness) and lack of rigidity in a mounting surface will be promptly passed to a linear bearing assembly, creating significant stresses which can cause accelerated bearing wear and systemic operational problems in the form of heat build-up, excessive vibration, diminished accuracy and rough motion.

All Linear Bearing Are Not Equal

It is worth noting that sub-standard bearing assemblies offer an array of harmful by-products. Poor quality steel leads to premature sub-surface failure. Poor quality heat treating which doesn't achieve sufficient hardness results in a lack of resistance to deflection and component abrasion. Poor surface finish quality causes a high coefficient of friction within the bearing assembly, which carries the penalty of reduced system performance and pre-mature failure. While poor quality machining and grinding procedures cause diminished accuracy and rough motion.