Linear ball bearings – Selection, installation and maintenance

Linear ball bearings are important components in many mechanical engineering applications. The function of linear ball bearings is to enable precise and low friction movement along an axis. This article provides important guidance on the selection, installation and maintenance of linear ball bearings, so you can be sure to find the right bearing for your application and get the most out of it.

What is a linear ball bearing and how is it constructed?

Linear ball bearings are roller bearings with a special structure. They too operate with rolling elements (balls). In general, the term ball bearing is used to refer to a component that has an inner and outer ring separated by balls (rolling elements) that is designed to support a rotating element. However, a linear guide with a linear ball bearing is designed to guide a linear movement along a cylindrical shaft with as little friction as possible. For this purpose, a so-called recirculating ball mechanism is installed in a linear ball bearing. The recirculating ball mechanism allows the balls to roll linearly in relation to the shaft. At the end of the race, the balls are then returned by a recirculating track not in contact with the shaft. Linear ball bearings are designed for high-precision, strictly linear motions and cannot accommodate rotary motion.

Linear ball bearings are used in conjunction with linear axes. The linear movement mechanism usually uses steel roller balls for unlimited linear motion. The steel balls run continuously within the specified track of the ball cage while rolling on the surface of the linear shaft. Linear ball bearings perform linear motion with lower friction and high accuracy compared to plain bearings. Linear ball bearings are used in many applications, such as material handling equipment and semiconductor manufacturing lines.

In addition to linear ball bearings, there are also other forms of rolling bearings that allow linear movement.  This includes, for example, ball cage guides or needle guides. For ball cage guides, the balls are located in a cage that rolls and glides on the inner and outer surfaces. However, the cage also minimizes the path, which is why this form of rolling bearings is only suited for short linear motions. Needle guides require a guide shaft with flat surfaces for rolling the rolling elements. They are designed similar to the ball cage guide, but are able to absorb higher forces due to the needle-shaped rolling elements. The needle shape also prevents twisting. However, needle guides are only suited for short linear motions. Neither shape has a recirculation mechanism.

Linear ball bearings generally do not have an anti-rotation feature. Torque-transferring alternatives are ball grooved shafts, also called torque shafts, and needle guides. Since splined shafts can also be used as guide shafts, but are not roller-based shafts, these are not considered here.

Preliminary considerations for selecting linear ball bearings

A basic question for selecting linear ball bearings is: What is the shaft form? Linear ball bearings are designed for use with high-precision round shafts. Typically, two parallel linear shafts are used as a design-based approach for preventing possible radial rotation of the linear ball bearings. Alternatively, profile rails with carriages can also be used for higher load ratings or when space is restricted.

The next question to ask is: How is the linear ball bearing integrated? This has an impact on the shape. Linear ball bearings with flanges can for example be attached directly to the housing. The flange provides stable support and uniform force distribution. Linear bearings with housings offer greater environmental protection than linear bearings without housings, but are less flexible. Housings are easier to install and usually have a fixed stop that facilitates alignment.

MISUMI has three mounting options for linear ball bearings:

• Without flange: space-saving mounting, ideal for compact designs and restricted installation space
• with flange: round / square / compact flange (provides flexibility for positioning and fastening)
• with block shape / housing (provides stability and therefore high load capacity)

The following table provides an overview of available designs and shapes, such as flanged or open:

The following influencing variables should be considered in the last step:

• Environmental conditions
• Load rating and required speed
• Precision
• Chemical resistance
• Number of races
• Type of load

Depending on the type of load, one can for example choose between short or simple linear ball bearings or double linear ball bearings. The former are more suited for light loads, but not when a large moment load (offset load) is exerted. Double/multiple linear ball bearings should be used here. Generally speaking, linear ball bearings are not suited for rotary motion. A needle guide should for example be used for oscillating movements. For shock loads or in environments with increased dirt, plain bearings can also be a possible alternative to linear ball bearings. The choice of material for the roller elements also depends on the application. Steel balls have high hardness, are robust and can absorb high loads. Although they are cost-effective, they are also susceptible to corrosion. Linear ball bearings with stainless steel roller bodies are therefore the best choice in wet environments. The load capacity and resistance to external influences are consequently the key for the preliminary selection. Accuracy is another factor to consider. In high-precision applications, the bearing tolerance range must not be too large; the use of specialized, more expensive linear ball bearings is recommended. Ball cage guides can also be an alternative if the linear guide is intended to cover only a short travel distance.

In lower precision applications, the use of a cost-efficient item can often be a good choice. Evaluating the required accuracy is advisable. Costs can potentially be reduced further by using C-Value items. Cost-efficient linear bearings can for example be found in the MISUMI shop. Find these variants by entering the search term "cost-efficient" in the search field. You can also find more information on this in our article Plain Bearings - Reducing Costs with Cost-Efficient Products.

For example, the following cost-efficient linear bearings are available:

• Linear ball bearing with flange
• Linear ball bearing with double ring groove and seal
• Linear ball bearing with flange and double bushing

Linear Ball Bearing Installation Instructions

Precise installation of the linear ball bearings is essential for full function. The exact alignment of the bearing components is also important to ensure uniform running behavior. It is important to avoid forcibly inserting the axis at an angle into the bearing since the ball cage can otherwise be damaged.

Various mounting variants are presented below:

Tip for practitioners: Easy mounting with coupled clamping ring and cam levers

Using a clamping ring or cam lever makes mounting particularly easy. This can be very helpful, especially when aligning e.g. a positioning table for materials in manual installations. The clamping ring allows easy and secure positioning of the material table by manually clamping the linear ball bearings on the shaft without damaging the linear shaft.

Linear ball bearing maintenance

MISUMI linear ball bearings are treated with corrosion inhibitor oil. The corrosion inhibitor oil does not affect bearing greases. In addition, an MX lubrication unit can be used for continuous lubrication during operation. The following shows the effectiveness of the MX lubrication units and provides additional maintenance instructions.

The following test conditions were used: A linear ball bearing LMU12 with corrosion inhibitor oil and a linear ball bearing LMUX12 with lubricant unit MX were used as a sample. Other parameters:

• Effective load: 206 N (50% of dynamic load rating of 412 N)
• Average speed: 42 m/min (0.7 m/s)
• Lubricant: lubricating grease, only initial filling (only LMU-MX12)
• Shaft material: EN 13505 Equiv. 58HRC
• Operation: 24-hour continuous operation

The result of the endurance test: With an effective test load of 50% of the load capacity, the design with the MX lubrication unit achieved a continuous output of 2.5 times longer than without MX lubrication unit. These are reference values that are not considered a manufacturer’s warranty.

Mounting Error - Flange and countersunk bores or through hole

The following figure shows the correct and incorrect handling of linear ball bearings with flange and lubrication unit:

The MX lubrication units must never be used as a guide, as shown here in Figure (2). This can damage the housing. Instead, use a linear ball bearing with flange and guide as shown here as a mounting example in Figure (3) or switch to a design with a end-face flange, which is shown as a mounting example in Figure (1). Disassembling the MX lubrication unit and linear ball bearings will cause errors and should be avoided.

General Lubrication Instructions

Ball rows within linear ball bearings should be greased before and during use. This reduces friction and wear. Grease creates a layer between the balls and rolling surfaces, which simultaneously prevents seizing. Grease versions L, G and H are available for specific requirements. These are adapted to different operating conditions and loads.

An LTBC plating also extends the life of the linear ball bearings and linear shaft. Low Temperature Black Chrome (LTBC) coating protects against corrosion and minimizes reflection. Learn more about low reflection surfaces, LTBC and other options in this blog.