BALL BEARINGS. BALL bearings find many applications in an automobile as thrust bearings and shaft bearings. When well proportioned for their load and the speed of the shaft, they give great freedom from friction with a minimum of lubrication and adjustment. Ball bearings are more suited to slow than high speeds, especially when heavy pressures are in question, and are more useful as thrust bearings than in any other capacity, such as behind bevelgear wheels or worm gears, where the load is steady. Where shocks are to be encountered, ball bearings have no place, owing to the liability of the balls to split. Thrust bearings are usually of the four-point type, an illustration of which is seen in Fig. 44. To design a bearing of this kind, the number and diameter of the balls must first be decided upon from consideration of the load and speed. The radius of the pitch-circle of the balls can then be determined from where R the radius of the pitch-circle, r = the radius of one ball, and the angle subtended by the ball, as in Fig. 44. The angle can be obtained from where N the number of balls it is proposed to use. Table No. 46, p. 170, will be of service in this connection. The points A and B may be located anywhere on the vertical line CD. The circle shown passing through the centres of the balls is the pitch-circle. The bearing surfaces of the ball races are described by drawing lines from the points A and B tangent to the circles representing the two balls. If, instead of drawing the tangents from A and B, the points C and D at the intersections of the pitch-circle with the vertical line are used, the angles a, a will each be 45°, To assist in determining the size of ball to use, the following table will be useful: In designing a ball bearing, whether for taking a thrust or carrying a shaft, a certain amount of clearance between the balls is necessary to allow them freedom of movement, and this should be about 0.005 inch between every two balls, but the total amount allowed in any bearing should not exceed one-third the diameter of the ball used. From formula 61 the table No. 17, below, has been calculated, and will be found to save time when setting out a bearing. TABLE 17. DIAMETER OF BALL PITCH-CIRCLES, CALCULAted for BallS OF 1-INCH DIAMETER. Number of balls. Diameter of pitch-circle. Number of balls. Diameter of pitch-circle. It may be assumed that the friction of a ball bearing is independent of the speed and the number of balls, but the ball used should be as large as possible, as the friction varies, roughly speaking, as the square of the diameter of the ball in inverse ratio. To provide a good factor of safety, and to allow for unequal distribution of the load, it is usual to assume that the whole load is carried by one ball, and to design accordingly. The safe loads given in the above table are calculated for a speed of 150 revolutions per minute of the bearing. As the speed is increased the value of the safe load will be decreased, it being safe to assume that, should the speed be doubled, the load should be decreased by one-third. To determine the pitch-circle diameter for balls of any other diameter than 1 inch, multiply the tabular number in Table 17 by the diameter of the ball selected. CARRIAGE SPRINGS. THE available data on the design of carriage springs is very limited, the method most often followed being a process of trial and error. For single springs, known as grasshopper" springs, the writer uses the formula given by D. K. Clarke, as follows in which B = width of plates in inches. Tthickness of plates in inch. S C constant = 11.3. To determine the deflection in inches per ton of load, the most reliable formula the writer is acquainted with is that given in the Practical Engineer pocket-book, and repeated here where D = deflection in inches per ton of load. L= span of spring in inches. C = constant 40,000 for single and 20,000 for = double springs. B = width of plates in inches. |