外文翻译 凸轮和齿轮 Cams_and_Gears.doc

河北工程大学毕业设计Cams and GearsA cam is a convenient device for transforming one motion into another.The machine element has a curved or grooved surface which mates with a follower and imparts motion to it. The motion of the cam (usually rotation) is transformed into follower oscillation,translation or both. Because of the various cam geometries and the large number of can and follower combinations,the cam is an extremely versatile mechanical element.Although a cam and follower may be designed for motion,path,or function generation,the majority of applications utilize the cam and follower for function generation.The most common can types according to cam shapes are: disk or plate translating (two-dimensional or planar) , and cylindrical (three-dimaensional or spatial) cams. Follower can be classified in several ways: according to follower motion, such as tranlation or oscillation; according to whether the translational (straight-line) follower motion is radial or offset from the center of the cam shaft; and according to the shape of the follower contact suface (e.g., flat-face, roller, point (knife-edge), spherical, planar curved, or spatial-curved surface).In the case of a disk cam with a radial (in-line) translating riller follower the smallest circle. that can be drawn tangent to the cam suface an concentric with the camshaft is the base circle. The tracer point is a point at the cent of the roller that generates the pitch curve. The pressure angle is the angle between the direction of the path of the roller center and normal to the pitch curve through the center of the roller and is the complement of the transmission angle. Neglecting friction, this normal isis collinear with the contact force between the cam and follower. As in a linkage,the pressure angle varies during the cycle and is a measure of the ability of the cam to transfer movtive effort to the follower, which, in the pressence of friction, would tend to bind the follower in the guide.Numerous applications in automatic machinery require intermittent motion. A typical example will call for a rise-dwell-return and perhaps another dwell period of a specified number of degrees each, together with a required follower displacement measured in centimeters or degrees. The designers job is to lay out the cam acordingly. The first decision to be made is to choose the cam follower type . The specified application may dictate the combination of the cam and follower. Some factors that should enter into the decision are: geometric considerations, dynamic considerations, environmental considerations and economic matters.Once a type of cam and follower pair has been selected, the follower motion must bu chosen.Therefore, the velocity, acceration, and in some cases further derivatives of the displacement of the follower are of great importance.Gears machine elements that transmit motion by means of succssively engaging teeth. Gears transmit motion from one rotating shaft to another, or to a rack that translates. Numerous applications exist in which a constant angular velocity ratio (or constant troque ratio) must be transmitted between shafts. Based on the variety of gear types available,there is no restriction that the input and the output shafts need be either in-line or parallel. Nonlinear angular velocity ratios are olso available by using noncircular gears .In order to maintain a constant angular velocity,the individual tooth profile must obey the fundamental law of gearing; for a pair of gears o transmit a constant angular velocity ratio,the shape of their contacting profiles must bu such that the coommon normal passes through a fixed point on the line of the centers.There are several standard gear types. For applications with parallel shafts, straight spur gear, parllel helical, or herringbone gears are usually used. In the case of intersecting shafts, straight bevel or spiral bevel gears are employed. For nonintersecting and nonparallel shafts, crossed helical, worm, face, skew bevel or hypoidgears would be acceptable chices. For spur gears, the pitch circles of mating gears are tangent to each other. They roll on one another without sliding. The addendum is the height by which a toth projects beyond the pitch cirle (also the radial distance between the pitch circle and the addendum circle). The clearance is the amount by which the dedendum (tooth height below the pitch circle) in a given gear exceeds the addendum of its mating gear. The tooth thickness is the distance across the tooth along the arc of the pitch circle while the tooth space is the distance beween adjacent teeth along the arc of the pitch circle. The backlash is the amount by which the width of the tooth space exceeds the thickness of the engaging tooth at the pitch circle.In the force analysis of spur gears,the forces are assumed to act in a single plane. Helical gears are used to transmit motion between parallel shafts. The helix angle is the same on each gear,but one gear must have a right-hand helix and the other a left-hand helix.The shape of the tooth is an involute helioid. If a piece of paper cut in the shpe of a parallelogram is wrapped around a cylinder, the angular edge of the paper becomes a helix.If we unwild this paper, each point on the angular edge generates an involute curve. The surface obtained when every point on the edge generates an involute is called an involute helicoid.The initial contact of spur-gear teeth ia a line extending all the way across the face of the tooth. The initial contact of helical gear teeth is a point, which changes into aline as the teeth come into more engagement. In spur gears the line of contact is parallel to the axis of the rotation; in helical gears, the line is diagonal across the face of the tooth.It is this gradual engegement of the teeth and the smooth transfer of load from one to another, which give helical gears the ability to transmit heavy loads at high speeds.Helical gears subject the shaft bearings to both radial and thrust loads. When the thrust loads become high or are objectionable for other reasons, it may be deirable to use double helical gears. A double helical gears (herrongbone) is equivalent to two helical gears of opposite hand, mounted side by side on the same shaft. They develop opposite thrust reactions and cancel out the thrust load. When two or more single helical gears are mounted on the same shaft, the hand of the gears should be selected so as to produce the minimum thrust load.Crossed-heliical,or spiral,gears are those in which the shaft centerlines are neither parallel nor intersecting. The teeth of rossed-helical gears have point contact with each other, which changes to line contact as the gears wear in. For this reason they will carry out very small loads and are mainly for instrumental applications, and are definitely not recommended for use in the transmission of power. There is no difference between a crossed helical gear and a helical gear until they are mounted in mesh with each other. A pair of meshed crossed helical geears usually have the same hand; that is, a right-hand driver goes with a right-hand driven. In the design of crossed-helical gears,the minimum sliding velocity is obtained when helix angle are equal. However, when the helix angles are not equal, the gear with the larger helix angle should be used as the drixer if both gears have the same hand.Worm gears are similar to crossed helical gears.The pinion or worm has a small number of teeth, usually one to four, and since they completely wrpe around the pitch cylinder they are called threads. Its mating gear is called a worm gear, which is not a true heical gear. A worm and wrom gear are usually at right angle.The wrom gear is not ahelical gear because its face is made concave to fit the curvature of the wrom in order to provide line contact instead of point contact. However, a disadvantage of wrom gearing is the high sliding velocities across the teeth, the same as with crossed helical gears. The wrom and wrom gear of a set have the same hand of helix as for crossed helical gears, but the helix angles are usually quite different. The helix angle on the wrom is generally quite large, and that on the gear very small. Because of this, it is usual to specify the lead angle on the wrom, which is the conplement of the wrom helix angle, and the helix angle on the gear;the two angles are equal for a 90-deg. Shaft angle.When gears are to be used to transmint motion between intersecting shafts, some from of bevel gear is required. Although bevel gears are uaually made for a shaft angle of 90-deg., they may be produced for almost any shaft angle. The teeth may bu cast, milled, or generated. Only the generated teeth may be classed as accurate. In a typical bevel gear mounting, one of the gear is often mounted outboard of the bearing. This means that shaft deflection can more pronouced and have a greater effect on the co