Crank Gear

Crank Gear

 

a mechanism for the transformation of one type of motion into another; it has a rotating element in the form of a crank or crankshaft attached to a shank and another element by rotating kinematic pairs (hinges). Crank gears usually have rotating and translatory kinematic pairs. A distinction is made between plane crank gears (with motion of all the elements in parallel planes) and three-dimensional crank gears, as well as between four-element and multielement types. Plane four-element crank gears are most common; they are divided into four-element-hinge, crank-slide, and crank-link gears.

There are two types of four-element-hinge crank gears—double-crank gears for transforming uniform rotation of one crank into nonuniform motion of the other (a specific example of which is the hinge parallelogram for transforming motion from one crank to another without a change in speed), and crank-rocker gears, which transform rotation of the crank into a rocking motion of the beam. Crank-slide mechanisms transform crank rotation into linear reciprocal motion of the slide, or vice versa, and are widely used in piston engines, pumps, compressors, and presses. Such crank mechanisms are divided into centered and displaced, on the basis of the position of the crank and slide. Crank-link mechanisms usually transform uniform rotation of the crank into nonuniform rotary motion, rocking, or reciprocal motion at the link. Crank gears with a rocking link are used in the driving mechanism for cutting in metal-cutting machines.

Most crank gears have dead positions, in which transfer of motion of the crank is impossible. Upon leaving these positions, the driven crank may change to the opposite direction of rotation. Piston engines usually pass through these positions because of inertia of the elements. When the drive crank is close to the dead positions, crank gears give a substantial power advantage, which is exploited in presses and other machines to obtain large forces on the slide. The complex motion of crank-gear connecting rods is sometimes used for the drive elements of such machines as dough kneaders and snow loaders.

In addition to plane four-element gears, plane multielement crank gears are used in a number of cases, such as the crank gears of drilling-head spindles, crank-bar and crank-bend mechanisms of forging presses, and crank-link gears for the drive of the primary motion of shaping machines. Three-dimensional four-element crank gears are used to produce rocking motion of a beam about an axis perpendicular to the axis of rotation of the crank.

N. IA. NIBERG

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