Rolling guides www.mapeng.net 马棚网机械资讯 Rolling linear guides and guide-ways are widely used in practice, alongside plain linear guides. The following advantages are obtained when compared with plain guides: light running forces due to rolling friction, no stick-slip, trouble-free installation and immediate availability due to standardization of the rolling elements. www.mapeng.net 马棚网机械资讯 The main disadvantage of this type of guide when compared with hydrostatic and hydrodynamic guides lies in the low dam effects in the direction normal to the movement. www.mapeng.net 马棚网机械资讯 Figure 1 shows in its upper section a rolling guide using a roller chain. Generally, rolling elements travel half the distance moved by the slide. When roller chains are employed, their length must at least equal the sum of half the slide travel plus its own length. Hence, for long movements rolling guide ways in general employ recirculating roller elements, as shown in the lower half of Fig. 1. In this case the rollers run in an endless track so that the movement is limited only by the length of the running surface. www.mapeng.net 马棚网机械资讯 For accurate operation of the complete rolling guide way, consideration must not only be given to the quality of both guiding surfaces but also to the dimensional and geometric accuracy of the rolling elements and their control in the cage. When cylindrical rollers are used, inaccurate cages as well as non-parallel guide way surfaces can lead to angular motion and side thrust of the rollers on the cage, which in turn will result in damage to the guiding surfaces and the cage itself, due to the friction generated. This problem is not encountered when balls are used as rolling elements, but such guides have the disadvantage of even lower stiffness and load-carrying capacity when compared with roller linear guides (point contact instead of line contact). www.mapeng.net 马棚网机械资讯 Rolling-contact bearings. Ball bearings. Figure 2(top) Shows two views of www.mapeng.net 马棚网机械资讯 one quarter of the most widely used type of modem bail bearing. It consists of four parts: an inner race b, an outer race c, the balls d, and a separator or retainer e, which is also called a cage. www.mapeng.net 马棚网机械资讯 www.mapeng.net 马棚网机械资讯 Fig. 1 Principles of construction for linear roller guideswww.mapeng.net 马棚网机械资讯 The races in which the balls roll have grooves with radii of curvature slightly greater than the radius of a ball. Some bearings are assembled by moving the inner race radially, inserting the balls, centering the inner race, spacing the bails uniformly, and applying the retainer that holds the balls loosely in an equally spaced position. Other bearings have a notch in one of the races that permits more balls to be inserted. With more balls, the bearing can carry more radial load R (Figure 2[top]), but less thrust load T on account of the notch. www.mapeng.net 马棚网机械资讯 Although the bearing in Figure 2 (top) is classified as a radial ball bearing, ft has considerable thrust or axial-load capacity. The thrust capacity T can be increased by extending the races to sections ff so that the groove in the outer race is deeper on the left side and the groove in the inner race is deeper on the right side; some angular-contact bearings are made like this. If all of the load on a bearing is axial, an axial-thrust bearing can be used. This is simply two grooved washers with balls between them. www.mapeng.net 马棚网机械资讯 Roller bearings. These bearings have rollers instead of balls between the races. The rollers may be cylindrical, tapered, or spherical. Cylindrical roller bearings have rollers with a length to diameter ratio of about one and run in cylindrical races, one of which may have retaining shoulders; they cannot carry a thrust load. Needle bearings have long, small-diameter rollers and may be installed with or without retainers and with or without races. www.mapeng.net 马棚网机械资讯 Needle bearings cannot carry thrust loads, but they have the highest radial load capacity.www.mapeng.net 马棚网机械资讯 www.mapeng.net 马棚网机械资讯 Fig2 Types of bearings www.mapeng.net 马棚网机械资讯 Tapered roller bearings (Figure 2) have a high load capacity and when used in opposed pairs can carry thrust loads in either direction. Spherical roller bearings have either convex or concave rollers in both single and double rows. www.mapeng.net 马棚网机械资讯 All these bearings have a sell-aligning ability obtained by grinding either the outer or inner race spherical. The most commonly used type of spherical roller bearing is the double-row bearing with convex rollers. www.mapeng.net 马棚网机械资讯 Self-Aligning Ball Bearingswww.mapeng.net 马棚网机械资讯 Self-aligning ball bearings normally have two rows of balls that roil in a common spherical race in the outer ring, as shown in Figure. Because of this design, the inner ring, with the ball complement, can align itself freely around the axis of the shaft. When the shaft bends under load, the bearing will follow the deflection of the shaft without resistance. Self-alignment also contributes to smooth running by neutralizing the effect of the balls wobbling in the grooves. This bearing is therefore particularly useful in applications in which it is difficult to obtain exact parallelism between the shaft and housing bores. Thrust Ball Bearings the simplest form of thrust ball bearings is shown in Figure. In this type of bearing, a single row of balls set in a separator runs in two similar grooves formed in the stationary and revolving rings. The revolving ring is fixed to a shaft. These grooves are usually shallower than the groove in a deep-groove radial ball bearing. Thrust ball bearings are designed to carry pure thrust load and if any radial load is present, separate radial bearings must be used. From the viewpoint of both economics and simplicity of design, it is wise to seek the use of an angular-contact bearing where both radial and thrust loads are present.
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