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Structure of linear guide rail
Guides in which rolling elements (balls, rollers, or needles) are placed between the guide member and the moving member so that the two mating guide surfaces do not directly contact are called rolling guides.
Publish Time:
2024-08-20
Source:
Guides in which rolling elements (balls, rollers, or needles) are placed between the guide member and the moving member so that the two mating guide surfaces do not directly contact are called rolling guides.
Rolling guides are characterized by low frictional resistance, smooth and flexible movement; low wear, and the ability to maintain accuracy for a long time; small difference between dynamic and static friction coefficients, and less prone to "creeping" at low speeds, thus ensuring uniform and stable movement. Therefore, rolling guides are increasingly used in equipment requiring micro-movement and precise positioning. The disadvantages of rolling guides are: the guide surface and rolling elements are point contact or line contact, poor vibration resistance, high contact stress, so high surface hardness is required for the guide; high requirements for the shape accuracy of the guide and the size accuracy of the rolling elements.
I. Structural Forms
Ball guides - V-flat rolling ball guides, double V-shaped rolling ball guides, and circular rolling ball guides. Because the balls and guide surfaces are point contacts, the movement is smooth, but the stiffness is low, and the load-carrying capacity is small. They are often used in occasions where the weight and load of the moving parts are small.
Roller guides - The rollers in the roller guides are in line contact with the guide surface, so their load-carrying capacity and stiffness are greater than those of ball guides, and they have better wear resistance and slightly less flexibility. Rollers are sensitive to the unevenness of the guide rail, easily causing lateral offset and sliding, which increases the resistance of the guide rail, 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 size is limited, rollers with smaller diameters can be used, and this type of guide is called a needle roller guide. Roller guide supports are standard components with advantages such as simple installation, lubrication, and easy adjustment and protection. Because the rollers rotate in a closed raceway, they can be used on guides with long strokes.
II. General Issues in Rolling Guide Design
1. Selection of structural form: Rolling guides are divided into open and closed types according to their structural characteristics. Open rolling guides are used when the external load acts between the two guide rails, and the weight of the moving part itself can maintain good contact with the guide rails. Closed guides are the opposite. Ball guides have better flexibility, simple structure, and easy manufacturing, but have small load-carrying capacity and low stiffness, and are often used in occasions with high precision requirements, flexible movement, and light loads. Roller (needle) guides have high stiffness and strong load-carrying capacity, but require high positional accuracy. Rolling guides using standard rolling bearings have a simple structure, easy manufacturing, and convenient lubrication. Suitable for medium-precision occasions. In order to increase the load-carrying capacity of the rolling guide, preload can be applied. At this time, the stiffness is high, and there is no gap, and the accuracy is correspondingly improved, but the damping ratio is larger than that without preload, the manufacturing is complex, and the cost is high. Therefore, it is mostly used for precision guides.
2. Selection of length: Generally, under the condition that the guide movement stroke is satisfied, the length of the guide should be as short as possible.
When using a circulating rolling guide support, the stroke length of the moving part is not limited. Size and number of rolling elements: The larger the diameter of the rolling element, the greater the load-carrying capacity and the smaller the frictional resistance. For ball guides, increasing the ball diameter increases the stiffness (the stiffness of the roller guide is independent of the roller diameter). Therefore, if the structure is not restricted, rolling elements with larger sizes should be selected. Needle roller guides have larger frictional resistance, and the needles may slide. Therefore, needle roller guides should be avoided as much as possible (especially when the needle diameter is less than 4 mm). When the number of rolling elements increases, the load-carrying capacity and stiffness of the guide also increase. However, the number of rolling elements should not be too large, as too many will increase the uneven distribution of the load on the rolling elements, and the stiffness will decrease. If the number of rolling elements is too small, manufacturing errors will significantly affect the guiding accuracy of the moving parts. Generally, there are at least 12 rolling elements on a rolling bearing.
III. Stiffness and Pre-tightening Methods of Rolling Guides:
1. When the worktable reciprocates, the pressure of the worktable on the rolling elements at both ends will change. The rolling elements with large force will deform more, and the rolling elements with small force will deform less. When the guide is at position 1, the forces on the rolling elements at both ends are equal, and the worktable remains horizontal: When the guide moves to position 1 or I, the forces on the rolling elements at both ends are unequal, and the deformations are inconsistent, causing the worktable to tilt at an angle α, thus causing errors.
2. In order to reduce guide deformation and improve stiffness, in addition to reasonably selecting the shape, size, and number of rolling elements and appropriately increasing the thickness of the worktable, the method of preloading is often used to improve the stiffness of the guide. For dovetail rolling guides, a movable guide plate is used to obtain and control the preload. Experiments have confirmed that with the increase of interference, the stiffness of the guide begins to increase sharply. After reaching a certain level, further increasing the interference will not significantly improve the stiffness. The traction force increases with the increase of interference, but the change is not large within a certain limit. When the interference exceeds a certain value, it increases sharply. Therefore, the reasonable interference should make the guide stiffness good and the traction force small.
IV. Technical Requirements
The quality of the guide depends on its manufacturing accuracy and installation accuracy. During design, several technical conditions for rolling guides should be formulated according to the usage requirements. The following items and data can be used for reference: The unevenness between the two guide surfaces is generally 3 μm;
The straightness of the guide is generally 10-15 μm, and the precision is less than 10 μm. The diameter difference of the rolling elements: For general guides, the diameter difference of all rolling elements is no more than 2 μm, and the diameter difference of each group of rolling elements is no more than 1 μm. For precision guides, the diameter difference of all rolling elements is no more than 1 μm, and the diameter difference of each group of rolling elements is no more than 0.5 μm: The taper of the roller is less than 0.5-1 μm within the roller length range: The surface finish should generally be scraped: Within 25x25 cm2, the number of contact spots is 20-25 points, the precision takes the upper limit, and the general takes the lower limit.
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