What are Linear Bearings?
Linear bearings are a type of bearing that "bear" or support the load of the carriage during its single-axis linear movement and provide a low friction sliding surface for the guide rails. In a linear guide, the carriage is the component that travels in a straight line, back and forth, along the length of the guide rail. The guide rail is fitted and inserted into the linear bearing.
A linear bearing is a critical component of the linear guide assembly. Its applications are in cutting machinery, X-Y positioning tables, machine slides, industrial robots, and instrumentation systems. Either a motor-driven ball screw, lead screw, or manual force can be used to drive the motion. The single-axis motion is limited in the X-Y plane.
The types of linear bearings are divided into two main classifications: the rolling linear bearings and the plain linear bearings.
Linear Bearing Materials of Construction
The materials for constructing linear bearing components are the following:
Steel, an alloy primarily composed of carbon and iron, is the most popular material selection for linear bearings. Steel linear bearings are known to have excellent mechanical properties (i.e., high strength and rigidity); this can support heavy loads and provide smooth and precise motion. Carbon steel and stainless steel are common types of steel used in constructing steel linear bearings. Increasing the carbon content increases the hardness of steel; this affects the performance of the linear bearing.
Aluminum is a lightweight but high-strength metal. It is corrosion and chemical resistant. It is softer and less expensive than steel. However, aluminum linear bearings have lower load capacity than steel linear bearings. Aluminum linear bearings are also capable of providing smooth and precise motion.
Plastic linear bearings are softer, cheaper, and have a lower coefficient of friction than metallic bearings. Typical plastics used in linear bearings are nylon, polyethylene, and PVDF, and they are usually lined with a self-lubricating coating (e.g., PTFE). They are sometimes reinforced with fibers and fillers to enhance their weight-bearing capabilities. Plastic linear bearings can be used with softer shaft materials. However, they generally have lower load capacities and are limited to room temperatures.
Bronze is an alloy mainly composed of copper and zinc, with other additives such as manganese and phosphorus. It is a soft metal. Bronze linear bearings have a higher load capacity than plastic linear bearings. Because of the presence of metal-to-metal contact, however, they generate greater friction than plastic bearings; this necessitates maintenance of sufficient lubrication.
Ceramic linear bearings are typically fabricated from silicon nitride, aluminum oxide, zirconium oxide, and silicon carbide. They have high rigidity, which maintains travel accuracy and precision at high speeds. They have a high hardness, which increases their service life and abrasion resistance and reduces the generated particles from sliding of bearing components. They are also compatible with vacuum and ESD-sensitive devices and equipment.
In recirculating linear bearings, ceramic rolling elements are utilized for their higher speeds.
Composite bearings consist of a metal backing and a plastic sleeve or a PTFE liner. The polymeric component eliminates metal-to-metal contact; this lowers the coefficient of friction while maintaining the high load capacity of the bearing. The metal backing allows the bearing to dissipate its heat.
It is common practice to use different materials for the bearing and the guide rail; with the guide rail, it is usually harder to reduce friction. Material wear is concentrated in the contact surface of the linear bearing, which is the softer component. The guide rails, shafts, and bases (for plain linear bearings) are commonly constructed from hardened steel, ground steel, and anodized aluminum, which are all harder types.