Motors Part 15
[Ill.u.s.tration: Fig. 90. Double Elliptic.]
Cams are also used for cutting machines, or in tracing apparatus where it would be impossible to use ordinary mechanism. All such forms are special, requiring care and study to make their movements co-relate with the other parts of the mechanism that they are connected up with.
Simple Cams.--Fig. 88 shows a form of the most simple character, used, with some modifications, to a larger extent than any other. It is called the _heart-shaped_ cam, and is the regular type.
Fig. 89 is an elliptical cam, which is also regular. What is meant by _regular_ is a form that is the same in each half portion of its rotation.
Fig. 90 is a double elliptic, which gives a regular movement double the number of times of that produced by the preceding figure, and the differences between the measurements across the major and minor axes may vary, relatively, to any extent.
[Ill.u.s.tration: _Fig. 91. Single Wiper._]
[Ill.u.s.tration: _Fig. 92. Double Wiper._]
[Ill.u.s.tration: _Fig. 93. Tilting Cam._]
Wiper Wheels.--Wiper wheels are cams which give a quick motion to mechanism, the most common form being the single wiper, as shown in Fig.
91.
The double wiper cam, Fig. 92, has, in some mechanism, a p.r.o.nounced difference between the lengths of the two fingers which form the wipers.
The form of cam shown in Fig. 93 is one much used in iron works for setting in motion the tilt hammer. Only three fingers are shown, and by enlarging the cam at least a dozen of these projecting points may be employed.
Cam Sectors.--Fig. 94 shows a type of cam which is designed for rock shafts. The object of this form of cam is to impart a gradually increasing motion to a shaft. a.s.suming that A is the driving shaft, and B the driven shaft, the cam C, with its short end D, in contact with the long end E of the sector F, causes the shaft B to travel at a more accelerated speed as the other edges G, H, approach each other.
[Ill.u.s.tration: _Fig. 94. Cam Sector._]
[Ill.u.s.tration: _Fig. 95. Grooved Cam._]
[Ill.u.s.tration: _Fig. 96. Reciprocating Motion._]
Cylinder Cam.--Fig. 95 shows one form of cylinder A with a groove B in it, which serves as a means for moving a bar C back and forth. The bar has a projecting pin D, which travels in the groove.
This form of movement may be modified in many ways, as for instance in Fig. 96, where the drum E has a sinuous groove F to reciprocate a bar G to and fro, the groove being either regular, so as to give a continuous back and forth movement of the bar; or adapted to give an irregular motion to the bar.
[Ill.u.s.tration: _Fig. 97. Pivoted Follower for Cam._]
Double Cam Motion.--Cams may also be so arranged that a single one will produce motions in different directions successively, as ill.u.s.trated in Fig. 97. The horizontal bar A, hinged at B to the upper end of a link C, has its free end resting on the cam D.
The arm A has also a right-angled arm E extending downwardly, and is kept in contact with the cam by means of a spring F. Connecting rods G, H, may be hinged to the arm E and bar A, respectively, so as to give motion to them in opposite directions as the cam revolves.
Eccentrics.--An eccentric is one in which the cam or wheel itself is circular in form, but is mounted on a shaft out of its true center. An eccentric may be a cam, but a cam is not always eccentric in its shape.
The term is one in direct contrast with the word _eccentric_.
[Ill.u.s.tration: _Fig. 98. Eccentric._]
[Ill.u.s.tration: _Fig. 99. Eccentric Cam._]
Fig. 98 shows the wheel, or the cam, which is regular in outline, that is circular in form, but is mounted on the shaft out of its true center.
In this case it is properly called an eccentric cam but in enginery parlance it is known as the eccentric, as represented in Fig. 99.
Triangularly-Formed Eccentric.--Fig. 100 ill.u.s.trates a form of cam which has been used on engines. The yoke A being integral with the bar B, gives a reciprocating motion to the latter, and the triangular form of the cam C, which is mounted on the shaft D, makes a stop motion at each half-revolution, then produces a quick motion, and a slight stop only, at the half turn, and the return is then as sudden as the motion in the other direction.
[Ill.u.s.tration: Fig. 100. Triangularly-formed Eccentric.]
CHAPTER XII
GEARS AND GEARING
For the purpose of showing how motion may be converted from a straight line or from a circular movement into any other form or direction, and how such change may be varied in speed, or made regular or irregular, the following examples are given, which may be an aid in determining other mechanical devices which can be specially arranged to do particular work.
While cams and eccentrics may be relied on to a certain extent, there are numerous places where the motion must be made positive and continued. This can be done only by using gearing in some form, or such devices as require teeth to transmit the motion from one element to the other.
The following ill.u.s.trations do not by any means show all the forms which have been constructed and used in different machines, but they have been selected as types merely, in order to give the suggestions for other forms.
Racks and Pinions.--The rack and pinion is the most universal piece of mechanism for changing motion. Fig. 101 ill.u.s.trates it in its most simple form. When constructed in the manner shown in this figure it is necessary that the shaft which carries the pinion shall have a rocking motion, or the rack itself must reciprocate in order to impart a rocking motion to the shaft.
[Ill.u.s.tration: Fig. 101. Rack and Pinion.]
[Ill.u.s.tration: Fig. 102. Rack Motion.]
This is the case also in the device shown in Fig. 102, where two rack bars are employed. A study of the cams and eccentrics will show that the transference of motion is limited, the distances being generally very small; so that the rack and pinions add considerably to the scope of the movement.
The Mangle Rack.--The device called the _mangle rack_ is resorted to where a back and forth, or a reciprocating movement is to be imparted to an element by a continuous rotary motion.
[Ill.u.s.tration: Fig. 103. Plain Mangle Rack.]
[Ill.u.s.tration: Fig. 104. Mangle Rack Motion.]
[Ill.u.s.tration: Fig. 105. Alternate Circular Motion.]
The plain mangle racks are shown in Figs. 103 and 104, the former of which has teeth on the inside of the opposite parallel limbs, and the latter, Fig. 104, having teeth not only on the parallel sides, but also around the circular parts at the ends.
This form of rack may be modified so that an alternate circular motion will be produced during the movement of the rack in either direction.
Fig. 105 is such an instance. A pinion within such a rack will turn first in one direction, and then in the next in the other direction, and so on.
If the rack is drawn back and forth the motion imparted to the pinion will be such as to give a continuous rocking motion to the pinion.
Controlling the Pinion.--Many devices have been resorted to for the purpose of keeping the pinion in engagement with the teeth of the mangle rack. One such method is shown in Fig. 106.
[Ill.u.s.tration: Fig. 106. Controlling Pinion for Mangle Rack.]
The rack A has at one side a plate B, within which is a groove C, to receive the end of the shaft D, which carries the pinion E. As the mangle rack moves to such a position that it reaches the end of the teeth F on one limb, the groove C diverts the pinion over to the other set of teeth G.
All these mangle forms are subst.i.tutes for cranks, with the advantage that the mangle gives a uniform motion to a bar, whereas the to and fro motion of the crank is not the same at all points of its travel.