Mahesh A Kori

Gearing is a very vital form of transmission of

mechanical power.

Power transmission is the movement of energy

from its place of generation to a location where

it is applied to performing useful work

A gear is a component within a transmission

device that transmits rotational force to

another gear or device

Universally adopted for almost all machine mechanisms.

TYPES OF GEAR TRANSMISSIONS:

Spur Gear Helical Gear

Herring bone Gear Bevel Gear

Rack and pinion Worm and worm wheel

Functions of Gears

Change the speed of rotation

Change the direction of rotation

Increase or reduce the magnitude of speed and torque.

Convert rotational movement into linear or vice versa (rack and pinion drive)

Change angular orientation (bevel gears).

Off set in the location of rotating movement (helical gears, and worm gear sets).

GEAR MANUFACTURING METHODS

Casting Pouring Injection Molding Powder metallurgy

Forming Cold Drawing Gear Rolling -> Forging

Metal Removal Process

Rotary Disc Wheel Milling (Disc Cutter) Form Grinding (Disc wheel) Thread Grinding

Linear Motion Tool Broaching Punching Shear Cutting

Rotary Threaded Wheel Hobbing Generation Grinding (Worm Type Grinding)

Reciprocating Tools Shaping (Pinion Type) Shaving (Rotary Type)

Gear Cutting & Finishing

TYPES OF GEARS

According to the position of axes of the shafts.

a. Parallel

Spur Gear

Helical Gear

Rack and Pinion

b. Intersecting

Bevel Gear

c. Non-intersecting and Non-parallel

worm and worm gears

MACHINING

in which the cutting tool (hob) forms the profiles of several teeth simultaneously during constant relative motion of the tool and blank.

Reproducing method, Generating method

in which the cutting tool is a

formed involve cutter, which

forms the gear teeth profiles

by reproducing the shape of

the cutter itself. In this

method, each tooth space is

cut independently of the other

tooth spaces. Generating processes Gear hobbing Gear shaping Rack planning

Form Cutting Milling machine Broaching machine Shaping machine

SPUR GEAR

Spur (straight cut) gears are widely used in parallel shaft applications, such as transmissions, due to their low cost and high efficiency. The design allows for the entire gear tooth to make contact with the tooth face at the same instant. As a result, this type of gearing tends to be subjected to high shock loading and uneven motion. Design limitations include excessive noise and a significant amount of backlash during high-speed operation.

APPLICATIONS

Spur gears are made in a great variety of sizes from less; than 25 mm to several cm in diameter. Spur gears are made with both 14.1 and 20 pressure angles. Spur gears are made of steel, bras, other metals and plastics.

Product Pump Gears

Application Oil pump for earth

moving equipments

Product Motorcycle Gear

Box Sets

Application 600cc, 800cc,

900cc, 1200cc

Motorcycles

Product Pump shafts

Application Small oil pump

for generators

Helical Gears can be used for transmitting motion between parallel shafts. Helical gears used for transmission at an angle are called SPIRAL GEARS.

Advantages of Helical Gears:

Noiseless motion even at higher speeds.

Smooth transmission of small gears with fewer teeth, at large transmission ratio ( up to 15:1)

HELICAL GEAR

These gears must be rigidly installed on the shaft. These gears result in an axial force in one direction depending upon the direction of rotation and

are used for transmitting small power.

Product Differential

Application Ford, GM, F350, F450, 7000cc,

7500cc, 8000cc, and 8500cc heavy

duty trucks

Product Forklift Gear

Box Sets

Application Forklift

Helical gears operate more quietly and smoothly than spur gears. This is due to the fact that teeth of helical gears slide one across the other rather than hitting each other as in spur gears.

Moreover, unlike spur gears, in helical gears several teeth of each gear are in contact at one time and thus the load is spread, resulting in greater strength than if only one tooth of each gear is in contact at a time as in spur gears, However, in helical gears, the friction, heat generated and wear is high because of the sliding action of one tooth on another Therefore helical gears should run in oil bath as in automobile transmission. Another disadvantage with helical gearing is that helical gears exert an end thrust which must be absorbed by a thrust bearing.

HERRING-BONE GEAR

HERRING-BONE Gears are pairs of Helical Gears. These gears are used for large power transmission.

Advantages of Herring-bone Gears in addition to Helical Gears:

No Axial Force. It is balanced axially in both direction of rotation.

Large power transmission can be done at impact loads having frequent starts (inching).

These gears also must be rigidly installed on the shaft.

Herringbone gears are an improvement over the double helical gear design. Both right and left hand cuts are used on the same gear blank, which cancels out any thrust forces. Herringbone gears are capable of transmitting large amounts of horsepower and are frequently used in power transmission systems.

The differences in gear design create the need for significantly different lubrication designs. For instance, gears normally seen in automotive differentials are hypoid gears and require GL-5 concentration and

Bevel Gear transmission is employed for transmitting motion from one direction to the other at any angle. In general 90 transmission.

BEVEL GEAR

Bevel gears are available with straight, helical and spiral teeth. Where smooth running is required to avoid noise at high speeds, the straight bevel gears are inferior to the spiral bevel gears.

Bevel gears are more expensive to cut than spur gears. The most usual sizes of bevel gears are 30O mm diameter and under. Bevel gears are used 1:1 for angular drive.

A set of two bevel gears is used to transmit power from one shaft to another shaft which is not parallel to the first one. A pair of gears 1 and 2 for shafts whose centre lines meet at right angles.

Worm and worm wheel gives a large single stage speed reduction in perpendicular direction. Such transmission gives lower efficiency of transmission due to increased frictional losses. For a noiseless motion, worm 1 is usually made of medium carbon / alloy steel and the worm wheel 2 of bronze (or in some cast iron).

WORM & WORM WHEEL GEAR

used to connect shafts whose centre lines do not meet is the worm gear.

Worm gear sets employ a specially-machined worm that conforms to the arc of the driven gear. This type of design increases torque throughput, improves accuracy and extends operating life. Primarily used to transmit power through non-intersecting shafts, this style of gear is frequently found in gear reduction boxes as they offer quiet operation and high ratios (as high as 100:1). Downfalls with this type of gear set are its efficiency, high price per HP and low ratios (5:1 minimum).

Worms and worm gears are used when a large speed reduction ratio is wanted from one shaft to another.

Ratios as high as 70 to 1 can be made in with worm and worm gear, whereas the other types of gears cannot easily give more than about 6 to 1 in one pair. Nearly all worm gears work with shafts at 90, although they can be made for any shaft angle between about 80 and 100.

The teeth on the worm gear are helical and conform with the helix angle of the tooth on the worm. The helical tooth on the worm is a form of thread, similar to an acme thread. Worms may have single, double or triple threads. One revolution of a double thread worm revolves the worm gear aft amount equal to two teeth on the gear and so on.

RACK & PINION GEAR The rotary motion of pinion 1 (small gear) is transformed into linear motion of the rack 2. For transmitting large power worm & worm rack transmission is employed.

steering system on cars

SPUR GEAR NUMENCLATURE

SPUR GEAR NUMENCLATURE Nomenclature Symbol Formulae

Module

Diametral Pitch

Pitch

Number of teeth

Pitch Circle Diameter

Tooth height

Addendum

Dedendum

Root circle diameter

Outside diameter

Distance between the axis of the two mating gears t 1 & t2

Pressure angle

m

D.P.

P

t

D

h

h

h

D1

Do

A

m=p/ = D/t

t/D = /p = 1/m

P= .m = 3.14m

t = D/m

D = t.m

h = 2.2 m

h = m

h = 1.2 m

D1 = Do 4.4m

Do = m (t+2)

A = (t1 + t2) m/2

20(usually) or 14

DEFECTS OF THE GEARS

Wear of one, few or all the teeth.

One or few teeth broken or twisted.

Burrs on the bore or the key way.

Burrs on the internal splines of the gear bore or the tooth surface.

Crack or damage on the rim or the bore of the gear wheel.

MESHING OF GEARS

The spur and helical gears must be assembled in such a way that the axes of the meshing gears are parallel and the distance between their axes is correctly established.

The correct meshing of the spur and helical gears is decided by applying blue paste on the teeth of the driving gear, giving it several rotations and seeing to the blue marks on the teeth faces of the meshing driven gear.

(a) Correct meshing covering 70 80% tooth surface contact.

(b) Axis not parallel.

(c) Axis not parallel and distance between them is too small.

(d) Axis not parallel and distance between them is excess.

(e) Axis are parallel and distance between them is excess.

(f) Axis are parallel and distance between them is too small.

LUBRICATION

Proper Lubrication with timely addition / replacement plays vital role in maintaining the gear boxes with efficiency and increasing its working life.

Oil level should be checked in all the gear boxes oil indicators / dip sticks.

Oil level should be in between the minimum and maximum limits of the dip stick (or indicator).

Testing of the lubricating oils used to be carried out periodically. The following tests are to be done

1. Dirt contamination 2. Moisture 3. Volatile materials (Benzene, kerosene, spirit etc,) 4. Viscosity 5. Acidity 6. Alkalinity due to soda (Na2 Co3) etc.,

Base circle : It is a circle from which involute form is generated. Only the base circle on a gear is fixed and unalterable.

Base pitch (pb): It is the distance from one face of a tooth to the corresponding face of an adjacent tooth on the same gear, measured along the base circle. Sometimes called the 'normal pitch'.

BASE CIRCLE

Pitch is the distance between a point on

one tooth and the corresponding point

on an adjacent tooth.

Pitch circle. A circle, centered on and

perpendicular to the axis, and passing

through the pitch point. Sometimes also

called the 'pitch line', although it is a

circle.

Pitch diameter (D). Diameter of a

pitch circle. The nominal gear size is

usually the pitch diameter.

PITCH CIRCLE

Circular pitch (p). The distance from one face of a tooth to the corresponding face of an adjacent tooth on the same gear, measured along the pitch circle.

Diametral pitch (Pd). The ratio of the number of teeth to the pitch diameter.

Module (m). The module of a gear is equal to the pitch diameter divided by the number of teeth.

ADDENDUM AND

DEDENDUM Addendum : The radial distance from the pitch surface to

the outermost point of the tooth.

Dedendum : The radial distance from the depth of the tooth

to the pitch surface.

A point of contact is any point at which two tooth profiles touch each

other.

The pitch point is the point of tangency of two pitch circles and is on the

line of centers.

POINT OF CONTACT

The line of action is also

called path of action.

It is the straight line

passing through the pitch

point and tangent to both

base circles.

LINE OF ACTION

LANDS

Bottom land is the

surface at the bottom of a

gear tooth space adjoining

the fillet.

Top land is the

(sometimes flat) surface of

the top of a gear tooth.

ANGLES ASSOCIATED WITH GEARS Helix angle (). The angle between a tangent to the helix and the gear

axis. It is zero in case of a spur gear.

Lead angle (). The angle between a tangent to the helix and a plane

perpendicular to the axis. It is the complement of the helix angle

which is usually given for helical gears.

Pressure angle (). The complement of the angle between the

direction, that the teeth exert force on each other, and the line joining

the centers of the two gears. The pressure angle is a constant for a

given gear

Gear Milling

Advantages

General purpose equipment and machines are used.

Comparatively simple and easier setup is needed.

Simple and cheap cutting tools are used.

It is suitable for piece and small size production.

Drawbacks

Inaccurate process due to profile deviations and indexing errors.

Low production capacity due to the idle time loss in indexing, approaching, and withdrawal of the tool. However, the productivity can be enhanced by multi-workpiece setup.

Gear Broaching

Rotating Broach

Progressive Broaching

Gear Shaping

Progressive Shaping Head

Gear Planing

Gear Hobbing

Gear Shaping with Pinion Cutter

Gear shaping with Rack Cutter

Gear Shaving

Gear Grinding

Gear Lapping