An Introduction to Machine Drawing and Design Part 8

XV. STUFFING-BOXES.

[Ill.u.s.tration: FIG. 50.]

In fig. 50 is shown a gland and stuffing-box for the piston rod of a vertical engine. A B is the piston rod, C D a portion of the cylinder cover, and E F the _stuffing-box_. Fitting into the bottom of the stuffing-box is a bra.s.s bush H. The s.p.a.ce K around the rod A B is filled with _packing_, of which there is a variety of kinds, the simplest being greased hempen rope. The packing is compressed by s.c.r.e.w.i.n.g down the cast-iron gland L M, which is lined with a bra.s.s bush N. In this case the gland is screwed down by means of three stud-bolts P, which are screwed into a f.l.a.n.g.e cast on the stuffing-box. Surrounding the rod on the top of the gland there is a recess R for holding the lubricant.

[Ill.u.s.tration: FIG. 51.]

[Ill.u.s.tration: FIG. 52.]

The object of the gland and stuffing-box is to allow the piston rod to move backwards and forwards freely without any leakage of steam.

Fig. 51 shows a gland and stuffing-box for a horizontal rod. The essential difference between this example and the last is in the mode of lubrication. The gland f.l.a.n.g.e has cast within it an oil-box which is covered by a lid; this lid is kept shut or open by the action of a small spring as shown. A piece of cotton wick (not shown in the figure) has one end trailing in the oil in the oil-box, while the other is carried over and pa.s.sed down the hole A B. The wick acts as a siphon, and drops the oil gradually on to the rod. In this example only two bolts are used for s.c.r.e.w.i.n.g in the gland; and the f.l.a.n.g.es of the gland and stuffing-box are not circular, but oval-shaped.

In the case of small rods the gland is made entirely of bra.s.s, and no liner is then necessary. Fig. 52 shows a form of gland and stuffing-box sometimes used for small rods. The stuffing-box is screwed externally, and carries a nut A B which moves the gland.

EXERCISE 51: _Gland and Stuffing-box for a Vertical Rod._--Draw the views shown in fig. 50 to the dimensions given. Scale 6 inches to a foot.

EXERCISE 52: _Gland and Stuffing-box for a Horizontal Rod._--Fig.

51 shows a plan, half in section, and an elevation half of which is a section through the gland f.l.a.n.g.e. Draw these to a scale of 6 inches to a foot, using the dimensions marked in the figure.

EXERCISE 53: _Screwed Gland and Stuffing-box._--Draw, full size, the views shown in fig. 52 to the given dimensions.

A more elaborate form of gland and stuffing-box is shown in fig. 53.

This is for a large marine engine with inverted cylinders, such as is used on board large ocean steamers. The stuffing-box is cast separate from the cylinder cover to which it is afterwards bolted. The lubricant is first introduced to the oil-boxes marked A, from which it pa.s.ses to the recess B, where it comes in contact with the piston rod. To prevent the lubricant from being wasted by running down the rod, the main gland is provided with a shallow gland and stuffing-box which is filled with soft cotton packing, which soaks up the lubricant.

The main gland is screwed up by means of six bolts, and to prevent the gland from locking itself in the stuffing-box, it is necessary that the nuts should be turned together. This is done in a simple and ingenious manner. One-half of each nut is provided with teeth, and these gear with a toothed wheel which has a rim only; this rim is held up by a ring C.

When one nut is turned, all the rest follow in the same direction.

[Ill.u.s.tration: FIG. 53.]

EXERCISE 54: _Gland and Stuffing-box for Piston Rod of Large Inverted Cylinder Engine._--The lower view in fig. 53 is a half plan looking upwards, and a half section of the gland looking downwards. The upper view is a vertical section. Complete all these views and add an elevation. Scale 3 inches to a foot.

_Note._--The large nuts, the wheel, the supporting ring, and small gland are made of bra.s.s.

_Dimensions of Stuffing-boxes and Glands._

_d_ = diameter of rod. _t__{1} = thickness of _d__{1} = diameter of box (inside). stuffing-box f.l.a.n.g.e.

_l_ = length of stuffing-box _t__{2} = thickness of gland bush. f.l.a.n.g.e.

_l__{1} = length of packing s.p.a.ce. _t__{3} = thickness of bushes in _l__{2} = length of gland. box and gland.

_t_ = thickness of metal in _d__{2} = diameter of gland bolts.

stuffing-box. _n_ = number of bolts.

+----------------------------------------------------------+ _d_ _d__{1} _l_ _l__{1} _l__{2} _t_ _t__{1} +-----+---------+-----+---------+---------+------+---------+ 1 1-3/4 3/4 2 1-1/2 7/16 1/2 1-1/2 2-1/2 1-1/4 2-5/8 2 9/16 11/16 2 3-1/2 1-3/4 3-1/4 2-1/2 11/16 7/8 2-1/2 4-1/8 2-1/4 3-7/8 2-7/8 13/16 1-1/16 3 4-3/4 2-3/4 4-1/2 3-1/4 15/16 1-1/4 3-1/2 5-1/4 3 5-1/8 3-5/8 1 1-3/8 4 5-7/8 3-1/4 5-3/4 4 1 1-3/8 4-1/2 6-3/8 3-1/2 6-3/8 4-3/8 1-1/16 1-9/16 5 7 3-3/4 7 4-5/8 1-1/16 1-9/16 6 8 4-1/4 8-1/4 5 1-1/8 1-11/16 +----------------------------------------------------------+

+-------------------------------------------------+ _d_ _t__{2} _t__{3} _d__{2} _n_ +-----+-----------------+---------+---------+-----+ 1 _t__{2}=_t_ 3/16 7/16 2 1-1/2 when gland 1/4 5/8 2 2 f.l.a.n.g.e is 5/16 3/4 2 2-1/2 made of cast 5/16 7/8 2 3 iron and 3/8 1 2 3-1/2 _t__{2}=_t__{1} 3/8 1 2 4 when gland 7/16 1 2 4-1/2 f.l.a.n.g.e is 7/16 7/8 4 5 made of 7/16 1 4 6 bra.s.s. 1/2 1-1/4 4 +-------------------------------------------------+

The proportions of glands and stuffing-boxes vary considerably but the above table represents average practice.

EXERCISE 55:--Make the necessary working drawings for a gland and stuffing-box for a locomotive engine piston rod 2-1/2 inches in diameter, to the dimensions given in the table.

XVI. VALVES.

Professor Unwin divides valves, according to their construction into three cla.s.ses as follows:--(1) flap valves, which bond or turn upon a hinge; (2) lift valves, which rise perpendicularly to the seat; (3) sliding valves, which move parallel to the seat.

Examples of flap valves are shown in figs. 54 and 55; two forms of lift valves are shown in figs. 56 and 57, and in figs. 58 and 59 are shown two forms of slide valve. The slide valve shown in fig. 58 moves in a straight line, while that shown in fig. 59 (called a c.o.c.k) moves in circle.

_India-rubber Valves._--In india-rubber valves there is a grating covered by a piece of india-rubber, which may be rectangular, but is generally circular, and which is held down along one edge if rectangular, or at the centre if circular. Water or other fluid can pa.s.s freely upwards through the grating, but when it attempts to return the elasticity of the india-rubber, and the pressure of the water upon it, cause it to lie close on the grating, and thus prevent the return of the water. The india-rubber is prevented from rising too high by a perforated guard. In fig. 54 is shown an example of an india-rubber disc valve. A is the grating, B the india-rubber, C the guard secured to the grating or seat by the stud D and nut E. The grating is held in position by bolts and nuts F. The grating and guard are generally of bra.s.s.

India-rubber disc valves are also shown on the air-pump bucket, fig. 47.

EXERCISE 56: _India-rubber Disc Valve._--Fig. 54 shows a vertical section and a plan of an india-rubber disc valve. In the plan one-half of the guard and india-rubber are supposed to be removed so as to show the grating or seat. Draw these views, and also an elevation. A detail drawing of the central stud is shown in fig.

16, page 18. In fig. 54 the elevation of the guard is drawn as it is usually drawn in practice, but if the student has a sufficient knowledge of descriptive geometry he should draw the elevation completely showing the perforations. Scale 6 inches to a foot.

[Ill.u.s.tration: FIG. 54.]

[Ill.u.s.tration: FIG. 55.]

_Kinghorn's Metallic Valve._--The action of this valve is the same as that of an india-rubber valve, but a thin sheet of metal (phosphor bronze) takes the place of the india-rubber.

This valve is now largely used in the pumps of marine engines, and is shown in fig. 55 as applied to an air-pump bucket. Three valves like the one shown are arranged round the bucket.

EXERCISE 57: _Kinghorn's Metallic Valve._--Fig. 55 shows an elevation and plan of one form of this valve. In the plan one-half of the guard and metal sheet are supposed to be removed, so as to show the grating, which in this case is part of an air-pump bucket.

Draw the views shown, and also a vertical section of the guard through the centres of the bolts. All the parts are of bra.s.s except the valve proper, which is of phosphor bronze. Scale 6 inches to a foot.

_Conical Disc Valves._--A very common form of valve is that shown in figs. 56 and 57. This form of valve consists of a disc, the edge of which (called the face) is conical. The conical edge of this disc fits accurately on a corresponding seat. The angle which the valve face makes with its axis is generally 45. If the disc is raised, either by the action of the fluid as in the india-rubber valve, or by other means, an opening is formed around the disc through which the fluid can pa.s.s. The valve is guided in rising and falling either by three feathers underneath it, as in fig. 56, or by a central spindle which moves freely through a hole in the centre of a bridge which stretches across the seat, as in fig. 57. The lift of the valve is limited by a stop above it, which forms part of the casing containing the valve. The lift should in no case exceed one-fourth of the diameter of the valve, and it is generally much less than this. The guiding feathers (fig. 56) are notched immediately under the disc for the purpose of making available the full circ.u.mferential opening of the valve for the pa.s.sage of the fluid. These notches also prevent the feathers from interfering with the turning or sc.r.a.ping of the valve face.

Conical disc valves and their seats are nearly always made of bra.s.s.

EXERCISE 58: _Conical Disc Valves._--Draw, half size, the plans and elevations shown in figs. 56 and 57. In fig. 57 the valve is shown open in the elevation, and in the plan it is removed altogether in order to show the seat with its guide bridge.

[Ill.u.s.tration: Plan of Valve. FIG. 56.]

[Ill.u.s.tration: Plan of Seat. FIG. 57.]

_Simple Slide Valve._--The form of valve shown in fig. 58, often called the _locomotive slide valve_, is very largely used in all cla.s.ses of steam-engines for distributing the steam in the steam cylinders. The valve is shown separately at (_d_), (_e_), and (_f_), while at (_a_), (_b_), and (_c_) is shown its connection with the steam cylinder.

It will be observed that the valve itself is in the shape of a box with one side open, the edges of the open side being f.l.a.n.g.ed. When the valve is in its middle position, as shown at (_a_), two of these f.l.a.n.g.ed edges completely cover two rectangular openings S_{1} and S_{2}, called _steam ports_, while the hollow part of the valve is opposite to a third port E, called the _exhaust port_. As shown at (_a_) the piston P would be moving upwards and the valve downwards. By the time the piston has reached the top of its stroke the valve will have moved so far down as to partly uncover the steam port S_{1}, and admit steam from the valve casing C through S_{1} and the pa.s.sage P_{1} to the top of the piston.

The pressure of this steam on the top of the piston will force the latter down. While the above action has been going on, the port S_{2} will have become uncovered, and the hollow part of the valve will be opposite both the steam port S_{2} and the exhaust port E, so that the steam from the under side of the piston, and which forced the piston up, can now escape by the pa.s.sage P_{2}, the steam port S_{2}, and the exhaust port E to the exhaust outlet O, and thence into the atmosphere, if it is a non-condensing engine, or into the condenser if it is a condensing engine, or into another cylinder if it is a compound engine.

After the piston has performed, a certain part of its downward stroke, the valve, which has been moving downwards, will commence to move upwards, and when it has reached a certain point it will cover the port S_{1}, and shut off the supply of steam to the top of the piston. It is generally arranged that the steam shall be cut off before the piston reaches the end of the stroke. When the piston reaches the bottom of its stroke the valve has moved far enough up to uncover the port S_{2} and admit steam to the bottom of the piston, and to uncover the port S_{1} and allow the steam to escape from the top of the piston through the pa.s.sage P_{1}, the port S_{1}, the port E, and outlet O. In this way the piston is moved up and down in the cylinder.

The valve is attached to a valve spindle S by nuts as shown, the hole in the valve through which the spindle pa.s.ses being oval-shaped to permit of the valve adjusting itself so as to always press on its seat.