Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PowerPoint to accompany Krar Gill Smid Technology of Machine Tools 6 th Edition Gears Unit 69

69-2 Objectives Identify and state the purposes of six types of gears used in industry Apply various formulas for calculating gear-tooth dimensions

69-3 Transmitting Rotary Motion from One Shaft to Another Shafts parallel but distant: flat belt, large pulleys –Speed controlled by size of pulley Shafts parallel but close: V-belt –Reduces excessive slippage of flat belt –Speed controlled by variable speed pulleys Shafts parallel but close: two rollers –Slippage is main problem, speed not constant Gears used to eliminate slippage and produce positive drive

69-4 Gears and Gearing Used to transmit power positively from one shaft to another by successively engaging teeth Used in place of belt and other friction drives May be used to increase or decrease speed of driven shaft –Decreasing or increasing torque of driven member

69-5 Spur Gears Generally used to transmit power between two parallel shafts Teeth straight and parallel to shafts Used where slow-to moderate-speed drives required Gear Pinion Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

69-6 Internal Gears Used where shafts parallel and centers must be closer that achieved with spur or helical gearing Provides for stronger drive Provides speed reductions with minimum space requirement Used where mush torque is required Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

69-7 Helical Gears Used to connect parallel shafts or shafts at an angle Progressive action of teeth Run smoother and quieter than spur gears Stronger than spur gears since more than one tooth engaged Thrust bearings often required to overcome end thrust Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

69-8 Herringbone Gears Used where necessary to overcome end thrust Two helical gears placed side by side, with one-half having left-hand helix and other half right-hand helix Give smooth, continuous action and eliminate need for thrust bearings Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

69-9 Bevel Gears Two shafts located at an angle with axial lines intersecting at 90º Miter gears Same size gears Angular bevel gears Shafts' angle other than 90º Hypoid gears Shaft at 90º, but not in same plane Do Not Intersect! Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

69-10 Worm and Worm Gear Used when considerable reduction in speed is required Shafts are at right angles Worm may be single- or multiple-start thread –Double-start thread will revolve worm gear twice as fast as worm with single-start thread Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

69-11 Rack and Pinion Used to convert rotary motion to linear motion Straight or flat gear may have straight teeth to mesh with spur gear, or angular teeth to mesh with helical gear Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

69-12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gear Terminology

69-13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Addendum Radial distance between pitch circle and outside diameter or height of the tooth above the pitch circle Formula: A = CP x.3183 Knowing Circular Pitch

69-14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Clearance (CL) The radial distance between the top of one tooth and the bottom of the mating tooth space Formula: Knowing Diametral Pitch

69-15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dedendum (D) The radial distance from the pitch circle to the bottom of the tooth space. The dedendum is equal to the addendum plus the clearance. Formula: Knowing Diametral Pitch

69-16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Diametral Pitch (DP) The ratio of the number of teeth for each inch of pitch diameter of gear. Formula: Knowing Pitch Diameter

69-17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Number of Teeth (N) Formula: Knowing Diametral Pitch and Outside Diameter

69-18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Outside Diameter (OD) Formula: Knowing Diametral Pitch and Number of Teeth

69-19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pitch Diameter (PD) The diameter of the pitch circle that is equal to the outside diameter minus two addendums. Formula: Knowing Diametral Pitch and Number of Teeth

69-20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Whole Depth (WD) The full depth of the tooth or the distance equal to the addendum plus the dedendum. Formula: Knowing Diametral Pitch

69-21 More Gear Terminology Center distance –Shortest distance between axes of two mating gears or distance equal to one-half the sum of the pitch diameters Chordal addendum –Radial distance measured from top of tooth to point where chordal thickness and pitch circle intersect edge of tooth Chordal thickness –Thickness of tooth measured at pitch circle or length of chord that subtends arc of pitch circle

69-22 More Gear Terminology Circular pitch –Distance from point on one tooth to corresponding point on next tooth measured on pitch circle Circular thickness –Tooth thickness measured on pitch circle –Also known as arc thickness Involute –Curved line produced by point of stretched string when unwrapped from given cylinder

69-23 More Gear Terminology Linear pitch –Distance from point on one tooth to similar point on next tooth of gear rack Module (metric gears) –Pitch diameter of gear divided by number of teeth Pitch Circle –Circle that has radius of half the pitch diameter with center at axis of gear

69-24 More Gear Terminology Pitch circumference –Circumference of pitch circle Pressure angle –Angle formed by line through point of contact of two mating teeth and tangent to the two base circles and line at right angles to centerline of gears Root circle –Circle formed by bottoms of tooth spaces

69-25 More Gear Terminology Root diameter –Diameter of root circle Tooth thickness –Thickness of tooth measured on the pitch circle Working depth –Distance gear tooth extends into tooth space of mating gear (equal to two addendums)

69-26 More Formulas for Spur Gears See Table 69.1 in text!