HANGERS.

A hanger is used when a shaft bearing is to be suspended The figure on page 203 shows a form of hanger made by a leading manufacturing company.

from the ceiling.

The frame of the hanger is divided and the parts are con nected by bolts. With such a form, the shaft may be more easily removed than when the hanger frame is a solid piece.

The units for determining the leading dimensions of a shaft hanger are the diameter d of the shaft and the drop D of the hanger.

The following proportions are suitable for shafts ranging in. in diameter:

from 14 in. to 4

A = 6 d

+ .45 D;

2 d + .03 D;

4 d +.35 D;

2 d + .3 D;

.5d+.01 D;

1.25 d;

= 2 d + .25 D;

1.5 d + .05 D;

e

= .125 d + .01 D;

=

=

2 d;

I

=

.4d;

J

K

=

=

=

=

.375 d;

.25 d + .125";
.625 d;

.15 d+.375'';
2.4d+.3125";

.08 d;

.125d+.0625";

.2d;

.4d;

.2 d;

.375 d + 1";

.09 d +.25";

.75 d;

1.3125d+.125";
1.25 d +.1875";

.1d;

.25 d +.25'';

.125 d +.0625";
2.2d;

= 4d;

m = 1.4 d +.375";

n =

0 =

01 =

5220

Ρι

=

=

=

d;

.25 d;
.0625 d;

d;

.0625 d;

.4d;

NOTE. To find R1, draw the arc J; also, draw the arc Q tangent to P; then, draw a straight line tangent to these arcs, and R will be the distance along the center line determined by B included between this tangent and the upper face of the hanger. Having found R1, make R equal to it.

The radius Ti is made equal to three-eighths of the thickness at the middle.

The steps of the ball-and-socket bearings are of cast iron, and are bored to fit the journal without using either lining or brasses. The ball and the recesses in the ends of the plugs, into which the ball is fitted, should be faced. The screw threads on the plugs may be cast on the plugs or turned, the latter being preferable. It is customary to use 2 threads per inch for all sizes of plugs.

BELT PULLEYS.

The accompanying table gives the dimensions of a set of cast-iron belt pulleys ranging from 6 in. to 72 in. in diameter, as

D

made by a well-known manufacturing company. These pulleys are so designed that the number of patterns may be kept within reasonable limits, and at the same time have the dimensions correspond as nearly as possible with well-established rules.

The letters over the columns of dimensions given in the table correspond to the letters in the figure.

In all cases the num

ber of arms is 6, and the arms increase in size toward the hub, the taper being in. per ft.

In order to prevent heavy stresses in shafts and bearings, pulleys that are to run at high speeds must be carefully

balanced. Perfect balance involves two conditions: (a) the center of gravity of the pulley must lie in the center line of the shaft, (b) the straight line joining the centers of gravity of any pair of opposite halves of the pulley must be perpendicular to the center line of the shaft.

The usual method of balancing a pulley is to rivet a weight to the light side and test the balance by putting the pulley on a mandrel that is placed on two carefully leveled ways on which it can roll with very little friction. If the center of gravity of the pulley lies in the center of the shaft, the pulley will stay in position when stopped with any point of its circumference over the mandrel; if, however, one side of the pulley is heavier, the mandrel will roll until the heavy side is at the lowest possible point.

While the above method does not determine whether or not the second condition of perfect balance is fulfilled, it is generally sufficient for pulleys running at ordinary limits of speed and reasonably well made.

In some cases, however, a failure to meet the requirements of the second condition of perfect balance may result in unsatisfactory running and severe stresses in the shaft and its bearings. Consider a pulley in which the center of gravity of one half is at the right of a line perpendicular to the center line of the shaft while the center of gravity of the opposite half is on the left of the perpendicular. This condition will not affect the balance of the pulley when tested by the mandrel rolling on the ways; when, however, the pulley revolves around the center line of the shaft, the centrifugal forces of the two halves act in opposite directions and along different lines. These forces thus form a couple that tends to bend the shaft. Since the centrifugal force is proportional to the square of the number of revolutions, it is apparent that, at high speeds, the bending effect may be considerable, even though the lack of symmetry is not very great.

It is usually considered unsafe to run a cast-iron pulley, gear-wheel, or flywheel at a higher rim speed than 100 ft. per sec. Since the centrifugal force increases in direct proportion to the cross-section of the rim, it is evident that it is useless to try to provide against it by putting more material in the rim.