SPUR GEAR-1

8/8/2019 SPUR GEAR-1

1/26

SPUR GEARS

INTRODUCTION

GEARS ARE MACHINE ELEMENTS USED TO

TRANSMIT ROTARY MOTION BETWEEN TWO

SHAFTS, NORMALLY WITH A CONSTANT

RATIO. THE PINION IS THE SMALLEST GEAR ANDTHE LARGER GEAR IS CALLED THE GEAR

WHEEL. A RACK IS A RECTANGULAR PRISM WITH

GEAR TEETH MACHINED ALONG ONE SIDE- IT IS IN

EFFECT A GEAR WHEEL WITH AN INFINITE PITCHCIRCLE DIAMETER. IN PRACTICE THE ACTION OF

GEARS IN TRANSMITTING MOTION IS A CAM

ACTION EACH PAIR OF MATING TEETH ACTING AS

CAMS.


2/26


3/26


4/26


5/26


6/26

GEAR DESIGN HAS EVOLVED TO SUCH A LEVEL

THAT THROUGHOUT THE MOTION OF EACH

CONTACTING PAIR OF TEETH THE VELOCITY RATIOOF THE GEARS IS MAINTAINED FIXED AND THE

VELOCITY RATIO IS STILL FIXED AS EACH

SUBSEQUENT PAIR OF TEETH COME INTO

CONTACT. WHEN THE TEETH ACTION IS SUCHTHAT THE DRIVING TOOTH MOVING AT CONSTANT

ANGULAR VELOCITY PRODUCES A PROPORTIONAL

CONSTANT VELOCITY OF THE DRIVEN TOOTH THE

ACTION IS TERMED A CONJUGATE ACTION. THE

TEETH SHAPE UNIVERSALLY SELECTED FOR THE

GEAR TEETH IS THE INVOLUTE PROFILE.


7/26

CONSIDER ONE END OF A PIECE OF STRING IS

FASTENED TO THE OD OF ONE CYLINDER AND THE

OTHER END OF THE STRING IS FASTENED TO THE

OD OF ANOTHER CYLINDER PARALLEL TO THE

FIRST AND BOTH CYLINDERS ARE ROTATED IN THE

OPPOSITE DIRECTIONS TO TENSION THE

STRING(SEE FIGURE BELOW). THE POINT ON THE

STRING MIDWAY BETWEEN THE CYLINDER P IS

MARKED. AS THE LEFT HAND CYLINDER ROTATES

CCW THE POINT MOVES TOWARDS THIS CYLINDER

AS IT WRAPS ON . THE POINT MOVES AWAY FROM

THE RIGHT HAND CYLINDER AS THE STRING

UNWRAPS. THE POINT TRACES THE INVOLUTE

FORM OF THE GEAR TEETH.


8/26


9/26

THE LINES NORMAL TO THE POINT OF CONTACT

OF THE GEARS ALWAYS INTERSECTS THE CENTRE

LINE JOINING THE GEAR CENTRES AT ONE POINT

CALLED THE PITCH POINT. FOR EACH GEAR THE

CIRCLE PASSING THROUGH THE PITCH POINT IS

CALLED THE PITCH CIRCLE. THE GEAR RATIO IS

PROPORTIONAL TO THE DIAMETERS OF THE TWO

PITCH CIRCLES. FOR METRIC GEARS (AS

ADOPTED BY MOST OF THE WORLDS NATIONS)

THE GEAR PROPORTIONS ARE BASED ON THE

MODULE, m.

m = (PITCH CIRCLE DIAMETER (mm)) / (NUMBER OF

TEETH ON GEAR).

IN THE USA THE MODULE IS NOT USED AND

INSTEAD THE DIAMETRIC PITCH DP IS USED


10/26

Profile of a standard 1mm module gear teeth for a gear

with Infinite radius (Rack ).Other module teeth profiles are directly proportion . e.g.

2mm module teeth are 2 x this profile


11/26

TERMINOLOGY - SPUR GEARS


12/26


13/26


14/26

Spur Gear Design

The spur gear is simplest type of gear manufactured

and is generally used for transmission of rotary motion

between parallel shafts. The spur gear is the first

choice option for gears except when high speeds,

loads, and ratios direct towards other options. Other

gear types may also be preferred to provide more silent

low-vibration operation. A single spur gear is generally

selected to have a ratio range of between 1:1 and 1:6

with a pitch line velocity up to 25 m/s. The spur gear

has an operating efficiency of 98-99%. The pinion ismade from a harder material than the wheel. A gear

pair should be selected to have the highest number of

teeth consistent with a suitable safety margin in

strength and wear.


15/26

The minimum number of teeth on a gear with a normal

pressure angle of 20 degrees is 18.

The preferred number of teeth are as follows

12 13 14 15 16 18 20 22 24 25 28 30 32 34 38 40 45 50

54 60 64 70 72 75 80 84 90 96 100 120 140 150 180

200 220 250


16/26

Materials used for gears

Mild steel is a poor material for gears as as it has poorresistance to surface loading. The carbon content for

unhardened gears is generally 0.4%(min) with

0.55%(min) carbon for the pinions. Dissimilar materials

should be used for the meshing gears - this particularlyapplies to alloy steels. Alloy steels have superior fatigue

properties compared to carbon steels for comparable

strengths. For extremely high gear loading case

hardened steels are used the surface hardening methodemployed should be such to provide sufficient case

depth for the final grinding process used.


17/26


18/26


19/26


20/26

Equations for basic gear relationships

It is acceptable to marginally modify these relationshipse.g to modify the addendum /dedendum to allow Centre

Distance adjustments. Any changes modifications will

affect the gear performance in good and bad ways...


21/26


22/26


23/26

Module (m)

The module is the ratio of the pitch diameter to thenumber of teeth. The unit of the module is milli-

metres.Below is a diagram showing the relative size of

teeth machined in a rack with module ranging from

module values of 0,5 mm to 6 mm


24/26

Normal Pressure angle

An important variable affecting the geometry of the

gear teeth is the normal pressure angle. This is

generally standardized at 20o. Other pressure anglesshould be used only for special reasons and using

considered judgment. The following changes result

from increasing the pressure angle:

Reduction in the danger of undercutting andinterference

Reduction of slipping speeds

Increased loading capacity in contact, seizure and

wear

Increased rigidity of the toothing

Increased noise and radial forces

Gears required to have low noise levels have

pressure angles 15o to17.5o


25/26

Contact Ratio

The gear design is such that when in mesh the rotating

gears have more than one gear in contact and

transferring the torque for some of the time. This

property is called the contact ratio. This is a ratio of the

length of the line-of-action to the base pitch. The higher

the contact ratio the more the load is shared between

teeth. It is good practice to maintain a contact ratio of

1.2 or greater. Under no circumstances should the ratiodrop below 1.1.


26/26

A contact ratio between 1 and 2 means that part of thetime two pairs of teeth are in contact and during the

remaining time one pair is in contact. A ratio between 2

and 3 means 2 or 3 pairs of teeth are always in

contact. Such as high contact ratio generally is notobtained with external spur gears, but can be developed

in the meshing of an internal and external spur gear pair

or specially designed non-standard external spur gears.

SPUR GEAR-1

Documents

Transcript of SPUR GEAR-1