1. Overview
The parts that support and guide the movement of moving components along a certain trajectory are called guide rail pairs, also commonly referred to as guide rails. The motion trajectory of moving parts can be straight, circular, or curved. Rolling circular guides can be classified as rolling thrust bearings, while curved guides are rarely used in machinery.
Guideway is a very important component in machines, especially in machine tools. The machining accuracy of machine tools is directly related to the precision of guide rails. For precision machine tools produced in small batches, the machining workload of guide rails accounts for about 40% of the total machining workload of the machine tool. Moreover, once the guide rail is damaged, maintenance is extremely difficult.
According to the principles of kinematics, a guide rail is a device that constrains moving components to only one degree of freedom. The guide rail surface located on the supporting component in the guide rail pair is the bearing surface, called the static guide rail, which is relatively long; The other guide rail surface is located on the moving component, called the moving guide rail, which is relatively short. The moving components with moving guide rails are often referred to as workbenches, sliding tables, and commonly used guide rail surfaces include flat and curved surfaces. The circular arc guide rail surface forms a cylindrical guide rail; The combination of different flat guide surfaces forms a sliding friction between rectangular guide surfaces, which is called a sliding guide. A rolling element is placed between the guide surfaces to convert friction into rolling friction, which is called a rolling guide.
There are two types of guide rails: closed and open. Closed guide rails can withstand overturning torque, while open guide rails cannot.
2. Basic requirements for guide rail design
2.1 Basic requirements for guide rail design
(1) Guidance accuracy
Guidance accuracy refers to the accuracy of the motion trajectory of the moving component when it moves along the guide surface of the guide rail. The main factors affecting guidance accuracy include the geometric accuracy of the guide rail bearing surface, the structural type of the guide rail, the contact accuracy of the guide rail pair, surface roughness, the stiffness of the guide rail and supporting components, the oil film thickness and stiffness of the guide rail pair, as well as the thermal deformation of the guide rail and supporting components.
The geometric accuracy of a linear motion guide generally includes: straightness in the vertical and horizontal planes; The parallelism between two guide rail surfaces. The geometric accuracy of the guide rail can be expressed as the error in the total length of the guide rail or the error per unit length.
(2) Accuracy retention
Accuracy retention refers to the ability of a guide rail to maintain its original geometric accuracy during operation. The accuracy retention of guide rails mainly depends on their wear resistance and dimensional stability. The wear resistance is related to factors such as the material matching, stress, machining accuracy, lubrication method, and performance of protective devices of the guide rail pair. In addition, residual stress inside the guide rail and its supporting components can also affect the accuracy retention of the guide rail.
(3) Motion sensitivity and positioning accuracy
Motion sensitivity refers to the Z-small stroke that a moving component can achieve; Positioning accuracy refers to the ability of moving components to stop at a specified position as required. The sensitivity and positioning accuracy of motion are related to factors such as the type of guide rail, friction characteristics, motion speed, transmission stiffness, and quality of moving components.
(4) Motion stability
The smoothness of guide rail movement refers to the performance of the guide rail without crawling during low-speed or slight movement. Stability is related to factors such as the structure of the guide rail, the matching of guide rail materials, lubrication conditions, lubricant properties, and the stiffness of the transmission system for guide rail movement.
(5) Vibration resistance and stability
Vibration resistance refers to the ability of the guide rail to withstand forced vibration and impact, while stability refers to the performance of not experiencing self-excited vibration under given operating conditions.
(6) Stiffness
The ability of guide rails to resist force deformation. Deformation will affect the relative position and guiding accuracy between components, which is particularly important for precision machinery and instruments. The deformation of the guide rail includes the deformation of the guide rail body and the contact deformation of the guide rail pair, both of which should be considered.
(7) Structural craftsmanship
Structural craftsmanship refers to the degree of difficulty in processing the guide rail pair (including the components where the guide rail pair is located). On the premise of meeting design requirements, manufacturing and maintenance should be as convenient and cost-effective as possible.
2.2 The main contents of guide rail design include:
① Select the structural type, cross-sectional shape, and combination form of the guide rail based on its working conditions and load-bearing characteristics.
② Perform mechanical calculations on the guide rail to determine the structural dimensions.
③ Determine the clearance, tolerance, and machining accuracy of the guide rail pair.
④ Select guide rail materials, match friction surface hardness, and use surface finishing and heat treatment methods.
⑤ Select the preload of the guide rail, design the loading method and device for the preload.
⑥ Choose the compensation method and adjustment device for the worn guide rail surface.
⑦ Choose the lubrication method for the guide rail, design the lubrication system and protective device.
2.3 Design principles of precision guide rails
For precision guides with high requirements for geometric accuracy, motion accuracy, and positioning accuracy (such as guides for CNC machine tools and measuring machines), the following principles should be followed in design:
(1) The principle of error compensation meets the following three requirements to enable the guide rail system to achieve mutual error compensation:
① Intermediate elastic links, such as rolling plastic strips or fluid films, must be installed between the guide rails.
② There should be sufficient pre tension between the guide rails to compensate for contact errors.
③ The manufacturing error of the guide rail should be less than the deformation of the intermediate elastic body (component).
(2) Principle of non-interference in accuracy
Only when the various accuracies of the guide rail are not affected by each other during manufacturing and use can high accuracy be easily achieved. For example, the straightness of the rectangular guide rail and the straightness of the side guide rail do not affect each other during manufacturing; The changes in the lateral dimensions of the flat V guide rail combination guide rail will not affect the working accuracy of the guide rail.
(3) Principle of approaching dynamic and static friction coefficients
When designing guide rail pairs, the dynamic and static friction coefficients of the contact surface of the guide rail should be as close as possible, in order to achieve high repeated positioning accuracy and low-speed stability. Rolling guide rails, ordinary sliding guides with embedded plastic plates or plastic sheets, have small friction coefficients and similar static and dynamic friction coefficients.
(4) Principle of automatic fitting of guide rail pairs
To achieve high precision of the guide rail, it is necessary to have the automatic fitting feature of the guide rail pair. Horizontal guide rails can be fitted by the weight of moving components; Other guide rails must be fitted with additional spring force or roller pressure.
(5) The principle of full contact
The length of the fixed guide rail must ensure that the moving guide rail is in contact with the entire length of the fixed guide rail at the two extreme positions of the Z-axis travel (not exceeding the fixed guide rail), to ensure that the guide rail pair always makes full contact during the contact process.
(6) Compensation force deformation and thermal deformation principles
The guide rail and its supporting components will deform when subjected to force or temperature changes. When designing the guide rail and its supporting components, efforts should be made to make them deform to the required shape. Like the crossbeam guide rail of a gantry machine tool, it is made into a convex shape to compensate for the bending deformation caused by the weight of the spindle box (or tool holder).
2024 March 4th Week VAFEM Product Recommendation:
VAFEM single bearings, manufactured to original equipment quality, are assembled in Langenbach, along with all the necessary components. There are four different models: grooved and angular roller bearings,tapered roller bearings,integrated wheel bearing units (hub units with built-in electronic sensors), and the bearings used for ABS, TCS, CPWM and navigation systems. In VAFEM catalogue you will also find images of each individual item. Our range of wheel hubs has also been considerably extended.
2024-03-22