Motors Part 14
H H are catches which can be unhooked from the stems of the valves A by the governor rods J J.
The vertical links K, K are connected at their lower ends with the pistons of dash pots, and have their upper ends attached to the valve spindles, and act to close the valves A A when the catches H are released by the governor rods J by means of the weights of the pistons in the dash pots.
[Ill.u.s.tration: _Fig. 76. Corliss Valve-operating Mechanism._]
The dash pots L L act in such a manner as to cushion the descent of the links K and thus prevent undue shock. M is a wrist plate pin by which the valve rod C can be released from the wrist plate.
The whole purpose of the mechanism is to provide a means for closing the valves which are at the steam inlet ports, by a sudden action. The exhaust valves, on the other hand, are not so tripped but are connected directly with the wrist plate which drives all four of the valves.
The wrist plate or spider has a rocking motion, being driven by an eccentric rod from the engine-shaft. The mechanism thus described gives a variable admission as the load varies, but a constant release of the exhaust and a constant compression to act as a cushion.
[Ill.u.s.tration: _Fig. 77. Angle Valve._]
It gives a high initial pressure in the cylinder, and a sharp cut off, hence it is found to be very efficient.
Angle Valve.--One of the most useful is the angle valve, which is designed to take the place of an angle bend or knee in the line of the piping. The mechanism is the same as in the well known globe valve construction, the bonnet A being on a line with one of the right-angled limbs of the body.
The pressure of the fluid should always be on the lower side of the valve C, coming from the direction of the arrow B, for the reason that should the steam pressure be constant on the other side, it would be difficult to repack the gland D without cutting off the steam from the pipe line.
[Ill.u.s.tration: _Fig. 78. Rotary Valve._]
[Ill.u.s.tration: _Fig. 79. Two-way Rotary._]
Referring back to the ill.u.s.tration of the globe valve, it will be noticed that the same thing, so far as it pertains to the direction of the steam, applies in that construction, and a common mistake is to permit the pressure of the steam to be exerted so that it is constantly acting against the packing of the spindle.
Rotary Valves.--Two forms of rotary valves are shown, one as ill.u.s.trated in Fig. 78, where the rotating part, or plug, A has one straight-way opening B, which coincides with two oppositely-projecting ports C, D.
The other form, Fig. 79, has an L-shaped opening E through the rotating plug F, and the casing, in which the plug is mounted has three ports, one, G, being the inlet, and the other two H, I, at right angles for the discharge of the fluid.
[Ill.u.s.tration: Fig. 80. Rotary Type.]
[Ill.u.s.tration: _Fig. 81. Two-way Rotary Type._]
Rotable Engine Valves.--So many different forms of the rotable valve have been made, that it is impossible to give more than a type of each.
For engine purposes the plugs are usually rotated in unison with the engine shaft, and a single delivery valve of this kind is shown in Fig.
80.
This has three ports in the casing, namely the inlet port A, and two outlet ports C, D. The plug has a curved cut out channel E, and this extends around the plug a distance equal to nearly one-half of the circ.u.mference, so that the steam will be diverted into, say, B, for a period equal to one-quarter turn of the plug, and then into port C, for the same length of time.
Fig. 81 shows a valve which has a double action. The plug G has two oppositely-disposed curved channels, H, I, and the casing has a single inlet port J, and two oppositely-disposed outlet ports K, L.
[Ill.u.s.tration: _Fig. 82. b.u.t.terfly Throttle._]
[Ill.u.s.tration: _Fig. 83. Angle Throttle._]
When the plug turns the port L serves to convey the live steam to the engine, while the other port K at the same time acts as the exhaust, and this condition is alternately reversed so that L acts as the discharge port.
Throttle Valves.--The throttle valves here ill.u.s.trated are those used in connection with gasoline engines. The best known is the _b.u.t.terfly_ valve, shown in Fig. 82, and this is also used as a damper, for regulating the draft in furnaces and stoves.
This type is made in two forms, one in which the two wings of the valve are made to swing up or down in unison, and the other, as ill.u.s.trated, where the disk A is in one piece, and turns with the spindle B to which it is fixed.
[Ill.u.s.tration: _Fig. 84. Slide Throttle._]
[Ill.u.s.tration: _Fig. 85. Two-slide Throttle._]
In Fig. 83 the wing C is curved, so that by swinging it around the circle, the opening of the discharge pipe D is opened or closed.
Another design of throttle is represented in Fig. 84. One side of the pipe A has a lateral extension B, which is double, so as to receive therein a sliding plate C, which is easily controllable from the outside.
Fig. 85 shows a form of double sliding plate, where the double lateral extensions project out in opposite directions, as at D, D, and within these extensions are sliding plates which are secured together in such a way that as one is pushed in the other also moves in, and thus acts in unison to close or to open the s.p.a.ce between them. It is the most perfect form of throttle valve, as it causes the gases to open directly into the center of the outgoing pipe.
Blow-off Valves.--The ill.u.s.tration shows a type of valve which is used on steamboats and very largely on farm boilers throughout the country.
The pipe A from the boiler has cast therewith, or otherwise attached, a collar B, which has a standard C projecting upwardly at one side, to the upper end of which is hinged a horizontal lever D, which has a weight at its other end.
[Ill.u.s.tration: _Fig. 86. Blow-off Valve._]
The upper end of the pipe has a conically-ground seat, to receive a conical valve E, the stem of which is hinged, as at F, to the level. The weight may be adjusted to the pressure desired before blowing out and the only feature in this type of valve is the character of the valve seat, which is liable, through rust, and other causes, to leak.
Pop, or Safety Valve.--As it has been found more desirable and practical to use a form of valve which is not liable to deterioration, and also to so arrange it that it may be manually opened, the _Safety Pop_ valve was devised.
[Ill.u.s.tration: _Fig. 87. Safety Pop Valve._]
This is shown in Fig. 87, in which the valve seat base A, which is attached to the top of the boiler, has a cup-shaped outlet B, that is screwed to it, and this carries a lever C, by means of which the valve may be manually opened.
A vertical sh.e.l.l D is attached to the cup-shaped portion, and this has a removable cap E. The valve F is seated within a socket in the base, and has a disk head, to receive the lower end of a coiled spring G.
The spring is supported in position by a stem H which extends down from the head, and an adjusting nut I serves to regulate the pressure desired before the steam in the boiler can act.
CHAPTER XI
CAMS AND ECCENTRICS
More or less confusion arises from the terms _cams_ and _eccentrics_. A cam is a wheel which may be either regular in shape, like a _heart-wheel_, or irregular, like a _wiper-wheel_.
The object in all forms of cams is to change motion from a regular into an irregular, or reversely, and the motion may be accelerated or r.e.t.a.r.ded at certain points, or inverted into an intermittent or reciprocating movement, dependent on the shape of the cam.
A cam may be in the shape of a slotted or grooved plate, like the needle bar of a sewing machine, where a crank pin works in the slot, and this transmits an irregular vertical movement to the needle.
A cam may have its edge provided with teeth, which engage with the teeth of the engaging wheel, and thus impart, not only an irregular motion but also a turning movement, such forms being largely used to give a quickly rising or falling motion.
What are called _wiper-wheels_ are designed to give an abrupt motion and such types are used in trip hammers, and to operate stamp mills. In harvesters, printing presses, sewing machines, and mechanism of that type, the cam is used in a variety of forms, some of them very ingenious and complicated.
[Ill.u.s.tration: _Fig. 88. Heart-shaped._]
[Ill.u.s.tration: _Fig. 89. Elliptic._]