Gear is a toothed machine part that meshes with another toothed part to transmit motion or change speed or direction.Classification of Gears: Spur gears coupling two parallel shafts Gear types
Herringbone gear Spiral bevel gear Bevel gear Worm gearSpeed Ratio of Pair of Gears The speed ratio of a pair of gears may be defined as the ratio of the angular speed of the driving gear to the angular speed of the driven gear and is equal to the number of teeth on the driven divided by the number of teeth on the driver.Addendum and Dedendum Circles Addendum circle is the circle passing through the outer ends of the teeth of a gear. Dedendum or root circle is the circle passing through the bottom of the spaces.
Addendum Distance and Dedendum Distance. Tooth Depth Addendum distance or, more commonly, the addendum is the radius of the addendum circle minus the radius of the pitch circle. Rooth distance or dedendum is the radius of the pitch circle minus the radius of the root circle. Total tooth depth is the sum of dedendum and addendum. Working depth is equal to the sum of the addenda of mating gears.Face and Flank of Tooth. Acting Flank Face of the tooth or tooth face is that portion of the tooth curve which is outside the pitch circle. Flank of the tooth is the part of the tooth curve inside the pitch circle. Acting flank is that part of the flank which comes in contact with the face of the tooth of the other gear.Face Width of Gear Face width of the gear is the length of the gear tooth measured along an element of the pitch surface.Clearance Clearance is the distance measured on the line of centers between the addendum circle of one gear and the root circle of the other, when they are in mesh.Tooth Thickness. Space Width. Backlash Tooth thickness, T is the width of the tooth (arc distance) measured on the pitch circle. Space width or tooth space, S is the arc distance between two adjacent teeth measured on the pitch circle. Backlash, B is the difference between the space width and tooth thickness.Pitch Circles and Pitch Point
The circle D, drawn through P with center S, is the pitch circle of the gear A, and the circle D1 is the pitch circle of gear B. Note that the pitch point P is the point of tangency on the center line of the pitch circles, which is equivalent to cylinders in pure rolling contact.Tooth Thickness. Space Width. Backlash Tooth thickness, T is the width of the tooth (arc distance) measured on the pitch circle. Space width, S or tooth space is the arc distance between two adjacent teeth measured on the pitch circle. Backlash, B is the difference between the space width and tooth thickness.Circular Pitch The distance from the center of one tooth to the center of the next tooth, measured on the pitch circle. Pc = circular pitch, inches; D = pitch circle diameter, inches; T = number of teeth. Two gears that mesh together must have the same circular pitch.Diameteral Pitch Diamteral pitch, sometimes called as pitch number, is the term ordinarily used to designate the tooth size; it is equal to the number of teeth divided by the diameter of the pitch circle. Pd = diameteral pitch or pitch number.Relation between Circular pitch and Diameteral Pitch From and , we have: That is, the product of the circular pitch and the diameteral pitch is equal to π.
Arc and Angle of Action Arc of approach is the arc of the pitch circle where the tooth profile cuts the pitch circle when a pair of teeth first comes in contact until they are in contact the pitch point. cP is the arc of approach on gear 4 and dP the arc of approach on gear 2. The lengths of the arcs of approach are equal. Arc of recess is the arc of the pitch circle from contact at the pitch point until where the tooth profile cuts the pitch circle when the pair of teeth comes out of contact. Pe is the arc of recess on gear 4 and Pf the arc of recess on gear 2. The lengths of the arcs of recess are equal. Arc of action consists of the arcs of approach and recess. cPe is the arc action of gear 4 and dPf is the arc of action of gear 2. The lengths of the arcs of action are equal. Angle of approach, α subtends the arc of approach. Angle of recess β, subtends the arc of recess. Angle of action, Φ subtends the angle arc of action and is equal to the angle of approach plus the angle of recess.The Path of Contact Path of contact is the line drawn through all the points at which the teeth touch each other (in this case the line aPb).Pressure Angle or Obliquity of Action Pressure angle, θ is the angle between the line drawn through the pitch point perpendicular to the line of centers and the line drawn from the pitch point to the point where a pair of teeth is in contact.