the drops, and in thickness must not be less than 1”, owing to difficulties of casting.

S

S

B

Fig. 161.

(122) Overhead Bracket. A bracket for bolting to an overhead beam is shown in Fig. 162, the pedestal for a 2" diameter shaft being a separate fitting bolted and wedged to the horizontal projecting arm. The square bosses B B are for the holding down bolts. The size of the top flange evidently depends upon the size of the beam, or the space available for supporting the bracket.

The calculation of the necessary amount of metal for such brackets as these is somewhat too difficult for a work of this character. But the student should notice that there are both bending and shearing forces acting upon the projecting arms, and that the T section of the arms, as shown at A, Fig. 161,

offers the most economical form for strength and stiffness, the latter being very necessary. On the fixing bolts of Fig. 161 there is a shearing stress acting on both bolts B and C, together with a tensile stress on the top bolt B, caused by the tendency of the bracket to turn downwards about the bottom edge of the flange

IL

Fig. 162.

as a hinge. On the bolts of Fig. 162 there are tensile stresses only, hence other things being equal, the bolts for a bracket as in Fig. 161 should be larger than for a bracket as in Fig. 162. It is doubtful if the makers of these brackets are able to calculate their necessary section, they are guided rather by previous experience, and the brackets are probably abnormally strong.

EXAMPLES.

Make drawings to a scale of 6" 1', showing side and end elevations and plan of the following brackets for shaft bearings:EX. 6.-Wall bracket for a journal 13" diameter, 3" long, as in Fig. 161, distance from centre to wall 14". Pedestal part fitted with simple bottom brass with side flanges, width of cap and part through which bolts pass 2", projection on both sides 23" diameter to make up length of journal. Bolts for cap " diameter, 5" centres, oil cup for glass needle lubricator, cast with cap. Savealls 4" wide, 14" deep, each 3" from centre to outside in direction of shaft length. Wall flange 16" long, 6" wide, " thick, inside web 1" wide, " thick; top flange " thick, web 11" wide; bottom flange" thick, web 1" wide. Top and bottom flanges taper as shown from 6" to 2". Length of wall

flange above top flange 3". Bosses for bolts in order to keep them close into top and bottom flange.

EX. 7.-Bracket for overhead beam as in Fig. 162 for 2' shaft pedestal. Show pedestal in position, its design to be as in Fig. 154.

SECTION XXIV.

STUFFING BOXES.

THERE are a number of rods used in engines, and engine fittings of different kinds which require in working to move to and fro through the covers or casings of cylinders, or other vessels containing steam, gas, or air, without allowing any of the internal fluid to escape.

To prevent this leakage these rods require to pass through "stuffing boxes," ," which consist essentially of circular boxes containing rings of "packing" tightly surrounding the rods in such a way as to prevent the passage of fluid between it and the rod. The packing is kept in position and forced out against the rods by a loose bush called a "gland" held in place by studs and nuts (see Fig. 163a).

For example, piston and slide valve rods of steam cylinders, pumps and rams for water pressure, spindles of cocks and valves, all require to pass through stuffing boxes, and as it is highly important to prevent leakage it is necessary for the student to understand the construction of stuffing boxes, and to be able to

design them in connection with any of th numerous parts to which they are fitted.

The number of patent stuffing boxes and different kinds of packing (metallic, hemp, asbestos fibre, &c.) at present in the market is far too large for description here. The student would do well to notice their chief features in the advertisements of the different engineering journals, but only general principles will be dealt with in the following remarks:

In making drawings of stuffing boxes it is not usual to show the packing, or to state what kind it is to be.

(123) Common Form of Stuffing Box.-Fig. 1636 shows a

stuffing box as fitted for the air pump plunger of a horizontal condenser (Fig. 197), which will serve as a good example of the most common form of construction.

The piece marked G is the "gland" which compresses the rings of packing in the space P P against the plunger R, when screwed in by the studs and nuts. For rods up to 24" or 3" diameter this. is generally made wholly of brass, but for larger sizes is of cast iron with a brass bush as shown in the figure. The most usual shape of the flange of the gland, when only two studs are used, is shown in the end view. Large boxes are fitted with three or four studs and circular flanges, and have frequently an arrangement of toothed gearing (a toothed ring on the gland piece and small pinions which form the nuts gearing into it), so that when one nut is turned all shall turn, and thus screw the gland in all round. equally

At the back of the box is fitted a brass bush, B, to keep the rod from rubbing on the hard metal of the cover or casing.

(124) Lubrication of Stuffing Boxes.--It is necessary to arrange for the rod and gland to be well lubricated. A convenient method when the box is horizontal is to extend one part of the gland flange and leave it hollow to form an oil cup, the oil being siphoned down the small supply pipe to the rod by a piece of cotton wick; a piece of thin brass hinged at one end forming a cover, this is shown in the figure, where () C is the oil cup, p the pipe, and L the lid.

With vertical boxes, lubrication is best effected by attaching a separate lubricator leading to a groove cut in the gland.

(125) Proportions of Stuffing Boxes.--These are generally decided by very practical considerations, the following being the most important :—

The length of a stuffing box for high pressures should be greater than for low pressures, whether steam or water, therefore the boxes of H.P. cylinders usually contain more packing than those of L.P. cylinders for the same diameter of rod.

Whenever possible it is always better to fix long stuffing boxes than short ones, they give far less trouble in working and require but little attention. Special limitations of space as in marine engines may sometimes handicap the designer, but generally speaking, a long box should be aimed at.

These same remarks apply also to the length of gland and of the back bush. They are subject to much wear, and, therefore, last longer when of good length. Frequently, as in the case of slide valve rods, they act as guides, and are made of extra length, especially when the rods do not extend through the back of the cylinder or casing, forming what are called "tail rods" (see Fig. 181).

The stuffing boxes of valves and cocks are of smaller size compared with the spindle diameter than those of cylinders and slide valve chests, as the spindle is only subject to occasional rotary movement, and the packing may be screwed up much harder against the rod.

The following proportions are taken from stuffing boxes on a number of engines and fittings, made by different makers :—

(126) Size of Packing.-Packing is made in sizes increasing by", but it is always sufficiently pliable to be compressed into slightly smaller spaces. The following sizes are measured between the rod and box, and, therefore, give all that is necessary to determine the diameter of the box, as, for example, a 1" rod using packing would require a box of diameter equal to 11⁄2 + (2 × 23".

=