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The main disadvantage of this type of guide when compared with hydrostatic and hydrodynamic guides lies in the low damping effects in the direction normal to the movement.
Gener¬ally, 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 recirculation roller elements,. In this case the rollers run in an endless track, so that the movement is limited only by the length of the running surface.
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 cylin¬drical 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 con¬tact).
Rolling-contact bearings.
Ball bearings. 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.
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 balls 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 in¬serted. With more balls, the bearing can carry more radial load R but less thrust load Ton account of the notch.
Although the hearing is classified as a radial ball bearing, it 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 the entire load on a bearing is axial, an axial-thrust bearing can be used. This is simply two grooved washers with balls between them.
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 re¬tainers and with or without races.
Needle bearings cannot carry thrust loads, but they have the highest radial load capacity.
Tapered roller bearings have a high load capacity and when used in opposed pairs can carry thrust loads in either direction. Spherical roller bearings have convex or concave rollers in both single and double rows.
All these bearings have a self-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.
Clutches
A clutch is a device for quickly and easily connecting or disconnecting a rotatable shaft and a rotating coaxial shaft.
Friction clutches.
Friction clutches have pairs of mating conical, disk, or ring-shaped surfaces and means for pressing the surfaces together. The pressure may be created by a spring or by a series of levers locked in position by the wedging action of a conical spool. On a spring-loaded clutch the operator, by controlling the rate, at which the spring pressure is applied to the clutch, can regulate the speed of clutch engagement and the torque applied to the driven shaft. There is always some slippage, however, and the efficiency of a friction clutch can never exceed 50 percent; i.e., during a clutching operation at least one-half of the input energy is lost by friction in the clutch and produces heat.