Chapter 04

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MEng 2141Machine DrawingFikru Gebre (MSc.)Jimma UniversityLecture 6: GearsOutline Type of gearsTerminologies or nomenclatures

Introduction Gears are machine elements, which are used for power transmission between shafts, separated by small distance.Introduction

Types of Gears SpursHelicalBevelWormRack and pinion

Spur Gears

Are used in transmitting torque between parallel shafts6Helical Gears

Are used in transmitting torques between parallel or non parallel shafts, they are not as noisy as spur gears7Bevel GearsAre used to transmit rotary motion between intersecting shafts

Teeth are formed on conical surfaces, the teeth could be straight or spiral.8Worm Gears

Are used for transmitting motion between non parallel and non transmitting shafts, Depending on the number of teeth engaged called single or double. Worm gear mostly used when speed ratio is quiet high, 3 or more9Nomenclature

Smaller Gear is Pinion and Larger one is the gearIn most application the pinion is the driver, This reduces speed but it increases torque.10Internal Spur Gear System

11Nomenclature of spur-gear teeth.

Nomenclature of spur-gear teeth.

Nomenclature of spur-gear teeth.Pitch circle, theoretical circle upon which all calculation is based; its diameter is the pitch diameter. The pitch circles of a pair of mating gears are tangent to each other. A pinion is the smaller of two mating gears. The larger is often called the gear.The circular pitch p is the distance, measured on the pitch circle, from a point on one tooth to a corresponding point on an adjacent tooth.Thus the circular pitch is equal to the sum of the tooth thickness and the width of space.p=d/N

Nomenclature of spur-gear teeth.The module m is the ratio of the pitch diameter to the number of teeth.m, module=d/N pitch circle/number of teethp= mThe diametral pitch P is the ratio of the number of teeth on the gear to the pitch diameter.P, Diametral Pitch P=N/dpP=

Angle has the values of 20 or 25 degrees. Angle 14.5 have been also used. Gear profile is constructed from the base circle. Then additional clearance are given

16Drawing Bevel Gears

Drawing Bevel GearsStep 1:Calculate the following dimensions:

Diameter of the Pitch Circle for the GearAddendum (0.125)Dedendum (0.1446)

Diameter of the Pitch Circle for the PinionAddendum (0.125)Dedendum (0.1446)

Drawing Bevel GearsStep 2:Draw the pitch cones as shown in the diagram.

Draw the main center lines at right angles to each other to intersect at the cone apex (point O).

Lay off the pitch diameters of the gear (PA) and the pinion (PB).

Draw the lines OA, OP, and OB with form the pitch cone.

Drawing Bevel GearsStep 3:Draw the addendum and the dedendum as shown in the diagram.

Draw lines at A, P, and B perpendicular to OA, OP, and OB.

Lay off the addendum and dedendum distances and draw light lines to intersect at point O.

Drawing Bevel GearsStep 4:Lay off the face width for both the gear and the pinion, measuring along the pitch cone.

Drawing Bevel GearsStep 5:Calculate the values for the proportions for the remaining features for both the gear and the pinion. Complete the full section view as shown.

Drawing Bevel GearsStep 6:Normally a section view of the bevel gear is all that is required unless a second view is required to show such details as spokes. Establish the position for the center of the circular view. Lay off the main center lines. Project points 1, 2, 3, and 4 from the front view to the circular and draw light circles.Step 7:Lay off the radial center lines for each tooth. The tooth spacing equals 360 degrees / N. An arc whose radius is taken on the back cone is used as the pitch circle and a tooth is developed using standard spur gear formulas. Tooth sizes taken on the OD and pitch diameter are transferred to the front view, and the profiles for the teeth are drawn. Radial lines from these points are taken and the small end of the tooth is developed.Step 8:The teeth on the side or section view can new be drawn by projecting from the front view.Remaining chapters Bearing, spring, BearingsBearings are supports for shafts, providing stability, and free and smooth rotation.Rolling Contact Bearings load is transferred through rolling elements such as balls, straight and tapered cylinders and spherical rollers.Journal (sleeve) Bearings load is transferred through a thin film of lubricant (oil).

Rolling Contact Bearings

springA spring is a mechanical device designed to store energy when deflected and to return the equivalent amount o f energy when releasedspring