Structure of guide rail 3

2. Rotate the guide rail
A guide rail that places some rotating bodies (balls, rollers, or needles) between the guide and moving parts, so that the two matching guide surfaces do not directly contact each other, is called a rotating guide rail.
The characteristics of rotating guide rails are low frictional resistance, lightweight and flexible movement; Low wear and tear, able to maintain accuracy for a long time; The difference in dynamic and static friction coefficients is small, and it is not easy to experience "crawling" phenomenon at low speeds, so the motion is uniform and smooth. Therefore, the rotating guide rail is becoming increasingly widely used in equipment that requires minimal movement and precise positioning. The disadvantages of the rotating guide rail are that the guide rail surface and the rotating body are in point contact or line contact, with poor vibration resistance and high contact stress. Therefore, high requirements are placed on the surface hardness of the guide rail, as well as on the external accuracy of the guide rail and the dimensional accuracy of the rotating body.
(1) Structural form
Ball guides - V-flat section ball guides, double V-shaped section ball guides, and circular section ball guides. Due to the point contact between the ball and the guide surface, the movement is light, but the stiffness is low and the load-bearing capacity is small. Commonly used in situations where the weight and load of moving parts are not large.
Roller (needle roller) guide rail - The rollers in the roller guide rail are in linear contact with the guide rail surface, so its load-bearing capacity and stiffness are greater than those of the ball guide rail, with better wear resistance and slightly lower flexibility. Rollers are sensitive to the unevenness of guide rails, easily causing lateral displacement and sliding, which increases the resistance of the guide rails, accelerates wear, and reduces accuracy. The larger the diameter of the roller, the more sensitive it is to the unevenness of the guide rail.
When the structural dimensions are limited, smaller diameter rollers can be used, which are called needle roller guides. Roller guide rail support is a standard component with advantages such as simple installation and lubrication, and easy adjustment and protection. Due to the rotation of the rollers within a closed raceway, they can be used on rails with large travel.
2) General questions for the design of rotating guide rails
1) Selection of structural form: The rotating guide rail is divided into two types based on its structural characteristics: open and closed. Open type rotating guide rails are used in situations where an external load is applied between two guide rails, and good contact between the guide rails can be maintained by the weight of the moving parts themselves. Closed guide rails are the opposite. Ball guides have the best flexibility, simple structure, and easy manufacturing, but they have low load-bearing capacity and stiffness. They are commonly used in situations where high precision, flexible movement, and light load are required. Roller (needle) guides have high stiffness and strong load-bearing capacity, but require high positional accuracy. The rotating guide rail adopts standard rotating bearings, with a simple structure, easy manufacturing, and convenient lubrication. Suitable for medium precision applications. To increase the load-bearing capacity of the rotating guide rail, preloading can be applied. At this point, the stiffness is high and there is no gap, resulting in a corresponding improvement in accuracy. However, the damping is greater than when there is no preload, making manufacturing complex and costly. Therefore, it is mostly used for precision guide rails.
2) Length selection: Generally, the length of the guide rail should be kept as short as possible while meeting the requirements of the guide rail's travel. To prevent the rotating body from falling off at the extreme position of the stroke, the length of the moving part should be=l+2a+Smax/2
In the formula, the length of the L-moving or guiding component is calculated by taking the shorter length (in millimeters) minus the spacing between the supporting points (in millimeters); A - Margin at extreme positions;
When using a cyclic rotating guide rail support, the stroke length of the moving parts is not limited. Rotating body size and number: The rotating body has a large diameter, high load-bearing capacity, and low frictional resistance. For ball guides, as the ball diameter increases, the stiffness also increases (the stiffness of roller guides is independent of the roller diameter). Therefore, if not limited by the structure, larger rotating bodies should be selected in limited quantities. The friction resistance of the needle roller guide is high, and the needle roller may slide. So try not to use needle roller guides as much as possible (especially when the needle roller diameter is less than 4 millimeters). As the number of rotating bodies increases, the bearing capacity and stiffness of the guide rail also increase. But the number of rotating bodies should not be too many, as too many will increase the uneven distribution of load on the rotating bodies, and the stiffness will actually decrease. If the number of rotating bodies is too small, manufacturing errors will significantly affect the guiding accuracy of the moving parts. Generally, on a rotating belt, the minimum number of rotating bodies is 12. Experience has shown that the weight of moving parts results in a load of 4 kilograms per centimeter per unit length on the roller; For ball guides, when the load on each ball is p23 (d) 12 kilograms (d is the diameter of the ball, in millimeters), the distribution of the load is relatively uniform. Increasing the length of the rollers in the roller guide can reduce contact stress and increase stiffness, but the unevenness of the load distribution also increases. For steel milling guides, the ratio of roller guide to diameter d<1.5, and for cast iron guides,//d can be increased (roller diameter is generally not less than 6 millimeters, and needle diameter is not less than 4 millimeters).
3) Stiffness and pre tightening method of rotating guide rail:
a) When the worktable moves back and forth, the pressure exerted by the worktable on the rotating bodies at both ends will change. The rotating body with greater force will deform more, while the one with less force will deform less. When the guide rail is in position 1, the forces on the rotating bodies at both ends are equal, and the worktable remains horizontal. However, when the guide rail moves to position 1 or I, the forces on the rotating bodies at both ends are not equal, and the deformations are inconsistent, causing the worktable to tilt at an angle of a, resulting in errors. In addition, the rotating body supports the worktable. If the stiffness of the worktable is different as shown in the figure below, elastic deformation will occur under the action of self weight and load, which will cause the worktable to be concave (sometimes even wavy), affecting the accuracy of the guide rail.
b) To reduce guide rail deformation and improve stiffness, in addition to selecting the shape, size, and number of rotating bodies reasonably and increasing the thickness of the worktable appropriately, preloading is commonly used.
Ways to improve the stiffness of the guide rail. Swallowtail shaped rotating guide rail, using a moving guide rail plate to obtain and control preload. Experimental results have shown that as the interference fit increases, the stiffness of the guide rail begins to increase sharply. After reaching a certain level, increasing the interference fit does not significantly improve the stiffness. The traction force increases with the increase of the interference fit, but does not change significantly within a certain limit. Once the interference fit exceeds a certain value, it increases sharply. Therefore, a fair interference fit should result in better rail stiffness and less traction.
4) Technical requirements: The quality of the guide rail depends on its manufacturing accuracy and installation accuracy. When designing, several technical conditions for rotating the guide rail should be formulated according to the usage requirements. The following items and data are available for reference:
The unevenness between the two guide rail surfaces is generally 3 microns;
The non straightness of the guide rail is generally 10-15 microns, with precision less than 10 microns. The diameter difference of the rotating body is not more than 2 microns for general guide rails, and the diameter difference of each group of rotating bodies is not more than 1 micron. For precision guide rails, the diameter difference of all rotating bodies is not more than 1 micron, and the diameter difference of each group of rotating bodies is not more than 0.5 microns. The taper of the rolling column is within the length range of the roller, and the diameter difference between the large and small ends is less than 0.5-1 micron. The surface smoothness should usually be scraped. Within 25x25 cm2, there are 20-25 contact spots, with the upper limit for precision and the lower limit for general precision.