Gears

Spur gear: Gears whose axes are parallel and whose teeth are parallel to the center line of the gear are called spur gears. They are used to transmit power form one shafts or element to another in cases where those shafts have their axes parallel.

Spur gearing is used over a wide range of articles-form small watches, precision measuring instruments, machine tools to gear boxes fitted in motor cars and aero engines, etc.

Spur gear elements and toothed parts: Common and usual types of wheel teeth form is show and the terms in general use in connection with spur gearing are given below:

Addendum: The radial heights form the pitch circle to the tip of the tooth.

Dedendum: The radial depth forms the pitch circle to the bottom of the tooth space in the mating gear.

Clearance: The radial distance from the top of tooth to the bottom of the tooth space in the mating gear.

Face of tooth: The surface of a tooth b/w the pitch line and the top of the tooth.

Flank of tooth: The surface b/w the pitch line and the bottom of the tooth.

Tooth surface: Includes both face and flank.

Thickness: The thickness of the tooth on the pitch circle.

Working depth: The greatest depth to which a tooth of one gear extends into the tooth space of a mating gear, equals addendum plus Dedendum minus the clearance.

Outside circle: The circle that contains the top of the teeth. The diameter of this circle is called outside diameter.

Pitch circle: An imaginary circle through the pitch point having its center at the axis of the gear. The diameter of pitch circle is called pitch diameter (d’).

Root circle: The circle that contains the bottom of the teeth. The diameter of this circle is called root diameter.

Pitch point: The points of contact of pitch circles.

Pitch line: The line of contact of two pitch surfaces.

Circular pitch (p): The distance measured on the circumference of the pitch circle form a point of one tooth to corresponding points on the next tooth.

Module (m): The pith diameter in millimeter divided by the number of teeth.

Pressure angle or angle of obliquity: The angle which the common normal to the two teeth at the point of contact makes with the common tangent to the two pitch circles at the pitch points.

Forms of wheel teeth: The standard form of tooth is involution in form except that a slight easing of the point is permissible. It has many advantages, the chief that involutes gears will work will work well together when the center distances are slightly varied. The standard pressure angles for teeth are 14½° and 20°.

Helical gear: Helical gears are gears in which the teeth are cut in the form of helix around the gear. Helical gearing is used to connect parallel shafts as well as non-parallel, non-intersecting shafts.

The pitch surfaces are cylindrical as in spur gearing, but the teeth, instead of being parallel to the axes, wind around the cylinders helical like screw threads.

Helical gear for parallel shaft: The conception of a helical gear is simplified by considering it as a spur gear with the teeth twisted. The teeth of helical gear with parallel axis have line connect, just as do spur gear. This provides gradual engagement and continuous connect of the engaging teeth. The efficiency of transmission with helical form of teeth is high and less friction loss.

So they can be used to transmit large powers, and larger velocity ratio can be obtained in one step than is possible with ordinary spur gears. One disadvantage in the use of helical gears is that the teeth impart axial trust to each other. Double helical gears are called herringbone gears, which give the smooth-running advantages of helical gear.

Helical gear for non-parallel shafts: Helical gear for non-parallel shafts, non-intersecting shafts may be designed for any angle b/w the shafts but the shafts are usually at right angles. The tooth action of this gear is quite different from that of the helical gears for parallel shaft.

The former have merely point contact, while the latter have line contact. In the former case, also there is a larger amount of sliding in the direction of the common tangent to the tooth elements, which is entirely absent in latter case. Helical gear for non-parallel shafts, non-intersecting shafts may consequently be used for comparatively light serves.

Spiral gear: is used to connect non-parallel, non-intersecting shafts. The pitch surfaces are cylindrical and the teeth have point contact. These gears are, therefore suitable only for transmitting small power. The center distance for a pair of spiral gears is the shafts making any angle b/w them.

Bevel gear: When two shafts, the axes of which intersect, are to be connected by gearing, the wheels are known as bevel gears the teeth are cut on a conical surface, such as bevel gears. In a bevel gear the teeth are cut on a conical surface, such as would be represented by a truncated cone. In the great majority of bevel gear drivers the shafts are at right angles, but the angles b/w the shafts may be either greater or less angle than 90°. In such cases the gears are called angular bevel gears. When the angle b/w the shafts is 90° and two gears of a pair are equal, the gear are called mitre gears. When the pitch angle of a bevel gear is 90°, it is called crown gear.

Bevel gear may be divided into four classes: - Straight-tooth bevel gears, Spiral bevel gears, Skew bevel gears, Hypoid.

Worm gear: worm gearing is essentially a special form of helical gearing in which the teeth have line connect and the axes of the driving and driven shaft are usually at right angles and do not intersect. The distinction b/w helical gearing may be explained as follows: if the number of threads or teeth is such that no one threads makes a complete turn, the result is a gear. If on the other hand, a thread makes a complete turn, the result is a worm and the mating gear is called worm wheel. The action of this gearing is like the action of a screw and nut and this class of gearing is, therefore, sometimes called screw gearing. Worms are designated like screw threads as right-hand or left-hand single-thread worm, right-hand double-hand worm, etc. Worm gearing is commonly employed to obtain higher velocity ratios can conveniently be obtained forms other forms of gearing.

Rack and pinion: The function of a rack and pinion is to transform circular motion to rectilinear motion. Small gear are called pinion and rack are a series of teeth on a straight line. They may be considered as spur gear of infinite radii. Lathe, drill, planer, etc. are fitted with rack and pinion to convert rotary motion to-line motion.