The main purpose of linear motion products is to move something in a straight line from one place to another in one direction. Usually, this is achieved using bearings with rolling elements. Traditionally, this type of rolling element was a ball or roller, but now some linear motion bearings do not include rolling elements, so they are called "sliding" bearings. These sliding bearings are only plastic or ceramic bushings, sometimes lined with PTFE or oil immersed plastic to help reduce sliding friction. Linear motion can be achieved without bearings, but moving the load without bearings requires more effort. The use of bearings can significantly reduce friction. For example, the friction coefficient of a circulating ball bushing can be as low as 0.001, while the friction coefficient of steel to steel (with lubrication) is 0.10. Linear bearings can move in multiple ways. They can be driven by screws, belts, chains, racks, and small gears, or simply pushed by hand. Linear motion products can be combined to form a multi-directional system with 2 or 3 axes. For example, XY workbench (2-axis) or CNC machine tool (3-axis).

 

 

 

For simplicity, various linear motion products are discussed based on their types.

 

Linear bearings with ball bearings provide the lowest friction. The two main products are circulating ball bearings and motion guidance systems.

 

1. Circular ball bushings are traditional linear bearings used in conjunction with hardened and ground shafts. Ball bearings have a very small point contact area and can achieve very low rolling friction. However, the small contact area means that they exert a significant force on the shaft, so they can only be used for hardening the shaft (such as steel shaft Cf53 or stainless steel shaft X90CrMoV18).

2. The motion guidance system (referred to as ball guides and linear guides) also uses circular balls, but they operate on irregular guides. The ball grooves in these guides are typically Gothic arched to maximize load-bearing capacity. Once again, the guide rail must be hardened to prevent deformation caused by high load balls. These systems are usually more capable of withstanding torque than traditional circular linear bearings.

The basic load rating of roller bearings is higher than that of ball bearings of the same size. However, their tolerance to lateral forces is poor. Roller bearings are mainly used as rollers but can also be used for cross-roller guides.

 

1. Track bearings are bearings that roll along the corresponding track on their outer diameter. Usually, they contain rollers as rolling elements, but sometimes they may contain balls (usually when radial load-bearing capacity is required). Track bearings are widely used and come in various styles. The simplest is a heavy-duty fully loaded roller bearing (such as NUTR), while others have a shaped outer ring (V-shaped or concave) to correspond to a formed (sometimes hardened) guide rail. They can be connected using sockets, studs, or weldable hubs. Other designs include an auxiliary axial roller to adapt to lateral loads (such as composite roller bearings)

2. Roller guides are similar to ball guides but provide greater load capacity compared to ball versions of the same size. Due to their compactness and accuracy, they are often used in medical applications.

3. The cross roller slider provides high accuracy and load capacity. However, their design properties make them susceptible to contamination, so they are mainly used in clean rooms or vacuum applications.

In the past 20 years, sliding bearings (or bushings) have developed rapidly with the development of low friction plastics. Sliding bearings are usually made of plastic or ceramic. Although they provide reduced friction, the friction coefficient is usually much higher than that of rolling element bearings.

 

2. Ceramic linear bearings provide properties similar to plastic linear bearings. They are quiet and smooth, usually providing the same size as traditional circular linear bearings, and can be lubricated free. In addition, some allow a combination of rotational motion and linear motion.

 

When choosing linear motion products, many factors need to be considered. The range of available products is wide, with each product having multiple shapes and sizes. Obviously, price is always a factor, but each system has its own advantages and disadvantages, which should be considered in design.

 

Factors to consider include:

 

Load capacity (does it involve any time?)

 

L · Precision (micrometer precision from micro ball guides to lower precision rollers)

 

Support arrangement (full support, end support, cantilever or telescopic)

 

Environment (dirty/dusty, clean room, vacuum, outdoor...)

 

Corrosion resistance (medical, pharmaceutical, food industry)

 

Size and scale (compact, heavy-duty, or in between)

 

Friction (about the drive system)

 

Product Selection Table (Comparison of Advantages and Disadvantages - English Version)

*Friction varies depending on load, surface finish, and alignment. The value is for comparison purposes only.

 

 

When specifying linear motion products, it is important to consider all the advantages and disadvantages of each product. Special attention should be paid to friction, as the sliding friction coefficient (sliding liner) is significantly higher than the rolling friction coefficient (ball and roller). The minimum friction value in the above table is 0.001, while the maximum value is 0.4, which is a coefficient of 400 times. Obviously, the force required for ordinary bushings (especially plastic bushings) to drive systems is much higher, but that means they can still play a role in lightweight applications.

 

For high-precision applications, recirculating ball bushings and ball guides remain industry leaders. Ball bushings have a wide range of applications and can be used for full support (open type) or end support (closed type). The linear ball and roller guides must be fully supported from above or below. Although roller linear guides provide higher load capacity, recent product development of ball guides has narrowed the load capacity gap between them. Ball guides are more popular than roller linear guides, so their prices are often lower.

 

If dirt is a problem, track bearings are clearly the best solution. They provide low friction coefficients and have different systems to adapt to various loads (such as cam followers, V-shaped bearings, and combined roller bearings).

 

The micro market is often satisfied by circulating ball bushings and micro ball guides. Due to their extremely precise properties, they are expensive and should only be specified when absolutely necessary.

 

Stainless steel products and professional coatings can achieve corrosion resistance. The product range is limited, and the cost may be significantly higher than standard products. Similarly, only specify corrosion resistance when necessary (such as in food, pharmaceutical, and offshore industries).

 

The correct selection of linear motion products is crucial for designing their lifespan. Before selecting the type of rolling element (ball, roller, or flat surface) and specifying the final product, it is important to consider all of the above aspects. Although cost always has a significant impact, product suitability is crucial.