2-2 Preload Design and Function
The miniature assembly consists of three parts: the guide pillar, the guide sleeve and the steel ball bushing as the sliding part, as shown in Fig. 2-2-1. The steel ball is precisely riveted into the brass retainer, so that it is not easy to affect the moving performance. The guide pillar, steel ball and guide sleeve are hardened and finely machined, and their material structure is stabilized by careful heat treatment.
The ball slides freely between the pre-pressurized guide bushings and the guide posts, allowing linear, rotary, or compound linear rotary motion. During linear motion, the path travel of the steel ball bushing is always 1/2 the distance between the bushing and the guide post, and the maximum length of linear motion is determined by the total length of the steel ball bushing and the guide post.
Preload
Accurate preload is a prerequisite for optimal motion, and HEADWAY offers a customized preload selection service that is determined by machining tolerances. Preload is the difference between two opposing balls in contact with the shaft and the inside diameter of the guide sleeve.
As shown in Fig. 2-2-2, the preload ensures that the miniature components are absolutely free of backlash. For applications with special conditions, the required preload can be specified at the time of ordering. The amount of preload is highly dependent on the size of the product.
2-2-2
The meaning of Preload
A small preload allows the microcomponent to run very smoothly, but the stiffness is limited. Load capacity and stiffness increase with increasing preload. A high preload will result in more rolling friction. Excessive preload will lead to interference and heavy running. Microcomponents may be overloaded due to high material stresses on the surface. This can be avoided by adjusting the fit tolerances of the micro-components. Therefore, the guide bush must be handled gently to ensure that the diameter is not deformed during assembly.
Preload Value
The preloads shown in Table 2-2-3 are based on theoretical knowledge and practical experience. With these preload values, the microcomponents achieve a high level of stiffness and very smooth motion.
When ordering micro-components consisting of micro-guide rods, micro-ball bushings and micro-guide sleeves, these components are already matched at the factory.
This ensures that the preload is precisely adjusted at optimum conditions.
| Dia Post (mm) |
Preload (μm) |
|---|---|
| 2,5/3 | 0,5-2 |
| 4/5 | 1-3 |
| 6/8 | 2-4 |
| 10/12 | 3-5 |
| Dia Post (mm) |
Preload (μm) |
|---|---|
| 14/25 | 4-7 |
| 30/42 | 4-8 |
| 50-63 | 6-10 |
| 80/100 | 8-12 |
The relationship between the Dia guide pillar and the steel ball:
Ball diameter has an effect on the amount of friction; larger balls roll more easily than smaller ones, and on the other hand a large number of smaller balls will produce better vibration damping than a few larger ones. For this reason, and because installation space is often limited, smaller balls are preferred in engineering design. By selecting guides and ball bushings from a miniature series with smaller balls, the required mounting space for the miniature assembly is reduced. HEADWAY has determined the shaft and ball diameters of the miniature assembly as well as the distribution of the number of balls through a detailed study.
Coefficient of friction µ:
Miniature components run without lubrication and the right side friction coefficient applies to radial loads.
High µ = 0.001-0.002
Average µ = 0.003-0.004
Low µ = 0.005-0.008
The rolling friction of a micro-component is determined by the effect of internal loads due to preload and external radial forces. If the radial load is low, the proportion of friction caused by the preload and retainer is dominant. Therefore, the coefficient of friction μ increases when the radial load decreases. Due to the low radial load and the need for a very smooth motion, a low preload must be used.
Impact Variables
1. The surface finish of the steel balls
2. Values of preload and load
3. Number of balls
4. Cage friction
5. Lubrication
