Ball bushings, which are also known as “linear ball bearings“, are an important product in linear motion. This article deals with the different variants of ball bushings and their properties but first provides you with basic information regarding their history, characteristics and design.
History
Ball bushings are considered to be the oldest product with rolling element recirculation in the field of linear motion and therefore the first to be categorised under the term “linear guide”. The beginnings of ball bushings can be traced back to the First World War. At that time, they were developed to move the flaps of the tail units of US military aircraft, which ran on wire ropes. However, the first ball bushings as we know them today were not developed until a few decades later, in the 1940s. Nowadays, ball bushings are standardised in accordance with DIN ISO 10285 and DIN ISO 14728.
Features and structure
There are two basic types of ball bushings: Standard and super ball bushings.
Standard ball bushings consist of a metal outer cylinder, balls as rolling elements, which are guided by a cage, and optional seals.
The design of the super ball bushings is generally similar to that of the standard version; their rolling elements are also balls and seals can also be used here if necessary. Unlike standard ball bushings, however, they have a plastic body into which the raceways, which are made of steel, are inserted.
Load capacity
Compared to linear guides and ball splines, ball bushings have a significantly lower load-capacity. The rolling elements run on a shaft and have minimal point contact with the shaft, which is why it is not possible for ball bushings to support relatively heavy loads. More information on the topic of load ratings can be found here at linearwizard.com.
The load-capacity of ball bushings varies depending on the arrangement of the raceways and the number of circuits. The following applies: the more circuits and the larger the diameter of the ball bushing, the higher the load capacity. With regard to closed and adjustable ball bushings, the term “directional” or “non-directional installation” is used, depending on how the force is applied to the row of rolling elements. If a ball bushing is installed in a directional manner, the ball circuits are aligned in such a way that the force is optimally distributed over the circuits so that the maximum load capacity can be achieved; at the same time, the nominal service life is increased. The load ratings from the data tables increase by a factor that is specified in the catalogue depending on the number of ball circuits. In the case of non-directional installation, the position of the ball circuits is not considered during assembly. The load ratings specified in the data tables are the minimum values that apply to the most unfavourable position of the ball circuits for non-directional installation.
The catalogue value refers to non-directional installation, the maximum load rating Cmax refers to directional installation.
Directional or non-directional installation is not possible with open ball bushings. A distinction is made here between the load in radial and in reverse radial. If the balls are loaded in the tensile direction, the load rating is ultimately lower, as the ball bushings tend to bend upwards.
Lubricant requirement
Due to the point contact, ball bushings require little lubricant. Although they do lose grease during operation, this is not a decisive factor in practice as it is only a small amount and ball bushings are not used in highly dynamic applications. Due to this low lubricant requirement, ball bushings are maintenance-free in many applications after initial greasing. It is the exception rather than the rule that ball bushings have a lubrication hole in the outer body for relubrication. In addition, the lubricant requirement naturally depends on the individual application.
Friction coefficient
The minimal contact surface due to the point contact also results in an even lower friction coefficient between the rolling elements and the raceway compared to ball screw drives and linear guides. Ball bushing guides are considered to run very smoothly.
Deflection maps
There are two different deflection types for ball bushings: tangential and radial deflection. The type most commonly used today is the tangential deflection: The loaded balls are guided into the cage before they are subsequently returned unloaded and without contact to the shaft. The tangential deflection is the most compact design.
An alternative is radial recirculation, in which there are a large number of ball circuits, which – as with a rail guide – are returned in an end cap into a return bore in the nut body above the raceways. Although higher load ratings can be achieved with this type of recirculation than with tangential recirculation, this radial recirculation is no longer used in today’s ball bushings due to the high costs involved. Especially since the development of the relatively inexpensive linear guides, their production is no longer profitable, which is why the radial deflection is hardly found on the market anymore.
The tangential deflection (left) is used today in both standard and super ball bushings, while the radial deflection (right) is no longer found in new products.
Areas of application
Ball bushings are suitable for applications where little load is moved and high precision is not required. Typical areas of application include office and packaging machines, but they are also commonly used in medical technology.
SNR brand ball bushings
As the products of individual manufacturers differ from one another, the following section will focus on a few products from the SNR brand.
Standard ball bushings in ISO dimensions are designed as single or tandem versions in closed, adjustable and open designs. Super ball bushings are only available in single versions in open and closed designs. In addition, standard cylindrical single ball bushings are available in JIS dimensions and closed design. The same applies to ball sleeves, another type of ball bush. Ball bushings can also be integrated into linear units, which are available in single, tandem and quattro designs and can be combined with both standard and super ball bushings. These are available in open, adjustable and closed form as well as in several other designs. Various accessories are also available for ball bushings, including shafts and clamping elements. More detailed information on SNR brand products can be found here.
If you are wondering what calculations are required for ball bushings or how they are installed, you can take a look at the other articles on linearwizard.com.
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