How to improve the problem of poor rigidity of linear bearings on pneumatic press machines

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Linear bearings are widely used as a motion unit that can achieve linear motion of the shaft, also known as linear motion systems, and are fully used in conjunction with cylindrical shafts for linear travel.

From the perspective of motion mode, the shaft and bearing can perform linear reciprocating motion in the axial direction, or relative rotation. If you have used our linear bearings, you can know that the actual point contact between the shaft and the bearing is a steel ball, and the actual motion is rolling friction. It has the advantages of small moving drag force, no crawling, and high motion accuracy, but also has disadvantages such as poor impact resistance and rigidity.

Let's take a case study where a linear bearing guide structure was used on a certain pneumatic press. The basic parameters are as follows: using a single standard closed linear bearing as the guide, the bearing has an inner diameter of 50mm, an outer diameter of 80mm, and a length of 100mm.

The shaft moves axially inside the bearing, and at a distance of 160mm from the midpoint of the bearing, the shaft is subjected to a radial load of 1kN, and the load ZUI ultimately acts on the linear bearing. This structure requires sufficient rigidity to ensure the guiding accuracy of the shaft during motion. In commonly used mechanical design manuals, there is generally no detailed information on how to choose the fit between linear bearings and shafts.

When designing this structure for the first time, we can refer to the relevant information of linear guide column products from linear bearing manufacturers. For example, based on experience, the manufacturer provides a 50mm diameter linear guide column with a size tolerance of 50 (g6). The fit between the linear bearing and the bearing seat adopts a small interference fit, which can ensure that the bearing seat has sufficient stiffness.

From a professional perspective, the factors that affect the structural rigidity of linear bearings include the rigidity of the shaft, the rigidity of the bearing seat, the rigidity of the bearing, and the clearance between the bearing and the shaft.

Due to the different tolerances on the inner diameter of linear bearings provided by different manufacturers, it is recommended to first confirm the inner diameter tolerance of the linear bearings with the bearing manufacturer when designing the shaft, and then design the outer diameter size of the shaft to ensure that there is interference fit.

After the assembly of the redesigned shaft and linear bearing, due to the presence of interference, the shaft cannot move freely within the bearing and needs to apply a thrust of 10-30N to make its relative motion, which is a relatively moderate interference fit state. The general experience is that the larger the bearing, the greater the interference.

After changing the fit form between the linear bearing and the shaft from clearance fit to interference fit, the clearance between the shaft, ball, and raceway is eliminated, which is equivalent to applying a certain amount of preload between the three, and has a significant effect on improving the rigidity of the linear bearing.

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