Cam Mechanism
cam mechanism
[′kam ‚mek·ə‚niz·əm]Cam Mechanism
a mechanism that usually consists of two moving parts (a cam and tappet) and a fixed part (the support). Cam mechanisms produce almost any type of motion of the driven element (the tappet) with continuous motion of the drive element (the cam). To reproduce a complex path of motion of the working units, such as in knitting machines, two cams transmit motion to the tappet.
Cam mechanisms are compact and may be easily introduced into the general scheme of a machine. Some of their drawbacks are heavy wear in the double-action kinematic pair and a tendency toward disconnection at high speeds. To reduce wear, the elements of a cam mechanism are made of high-quality steel and then are tempered and carefully worked. Loss of contact between the cam and the tappet is prevented by forcible connection of the pair, using springs that constantly press the tappet to the cam, or geometrical fastening, in which the cam or tappet is given a shape that makes impossible separation of the tappet from the cam (for example, the cam is constructed with a groove into which the tappet roller fits).
Cam mechanisms are used in various areas of machine building, such as internal-combustion engines, metal-cutting machines, and machines of the food industry, in which the cam mechanism performs a programmed operation, as well as in automated machines, in which cam mechanisms perform control functions, connecting and disconnecting working parts at the proper moment.
REFERENCES
Mashinostroenie: Entsiklopedicheskii spravochnik, vol. 9. Moscow, 1949. Pages 102–10.Levitskii, N. I. Kulachkovye mekhanizmy. Moscow, 1964.
Popov, N. N. Raschet i proektirovanie kulachkovykh mekhanizmov. Moscow, 1965.
I. G. GERTSKIS
Cam mechanism
A mechanical linkage whose purpose is to produce, by means of a contoured cam surface, a prescribed motion of the output link of the linkage, called the follower. Cam and follower are a higher pair. See Linkage (mechanism)
A familiar application of a cam mechanism is in the opening and closing of valves in an automotive engine. The cam rotates with the cam shaft, usually at constant angular velocity, while the follower moves up and down as controlled by the cam surface. A cam is sometimes made in the form of a translating cam. Other cam mechanisms, employed in elementary mechanical analog computers, are simple memory devices, in which the position of the cam (input) determines the position of the follower (output or readout).
Although many requisite motions in machinery are accomplished by use of pin-jointed mechanisms, such as four-bar linkages, a cam mechanism frequently is the only practical solution to the problem of converting the available input, usually rotating or reciprocating, to a desired output, which may be an exceedingly complex motion. No other mechanism is as versatile and as straightforward in design. However, a cam may be difficult and costly to manufacture, and it is often noisy and susceptible to wear, fatigue, and vibration.
Cams are used in many machines. They are numerous in automatic packaging, shoemaking, typesetting machines, and the like, but are often found as well in machine tools, reciprocating engines, and compressors. They are occasionally used in rotating machinery.
Cams are classified as translating, disk, plate, cylindrical, or drum (see illustration). The link having the contoured surface that prescribes the motion of the follower is called the cam. Cams are usually made of steel, often hardened to resist wear and, for high-speed application, precisely ground.
The output link, which is maintained in contact with the cam surface, is the follower. Followers are classified by their shape as roller, flat face, and spherical face. Followers are also described by the nature of their constraints, for example, radial, in which motion is reciprocating along a radius from the cam's axis of rotation; offset, in which motion is reciprocating along a line that does not intersect the axis of rotation (illus. b); and oscillating, or pivoted (illus. a). Three-dimensional cam-and-follower systems are coming into more frequent use, where the follower may travel over a lumpy surface.