Lecture - 2 Design of a Helical-Bevel Gear...

Lecture - 2Design of a Helical-Bevel Gear Boxg

Part-II

(March 20 2017)(March 20, 2017)

Design of Helical & Bevel Gearsand Layout of Gear Box

1

Design of an Industrial General Purpose Reduction Gear Unit :

Data: The TWO- stage (1st stage Bevel and

Tasks (contd….) :Data: The TWO stage (1 stage Bevel and 2nd. Stage helical) reduction gear box has the following specifications.

(20 to 22 different problems). Z1

Z3 Z4

GROUP POWER(kW)

INPUT RPM

OUTPUTRPM

DUTY OVERHAULTIME LUBRICATIONSub

GroupDescription

IA, B, C, D,

12 1500 170 A,E,I,M,Q, U

Precision, Intermittent,

No shock2 years

Forced

II 10 1800 200 B,F,J,N,R General, Oil Sump

Z2

E, F,G,H Continuous,Medium shock

IIII, J, K, L

09 1450 125 C,G,K,O,S, V

General, Intermittent,Heavy Shock

Oil Sump

IVM,N,O,P

07 1200 125 D,H,L,P,T Precision, Continuous,

Medium Shock

Forced

V 05 1500 140

Assembled plan view(Not of the same one as below)

t i h ld d

VQ,R,S,T

05 1500 140

VIU, V

06 950 100 Horizontal input and vertical output (Forced Lubrication)

1st. stage ratio should not exceed 3.In general non co-axial horizontal input and output (except otherwise mentioned).

2

Gear Tooth Terminology

3

Gear Tooth Terminology (Contd.)

[N t W ki l b diff t f t d d

Pressure Angle ( ) : Also known as ‘angle of obliquity’, is the inclination of the“line of action” of the contact force between a pair of meshing teeth withrespect to a line drawn tangent to pitch circles at pitch point.

[Note- Working pressure angle may be different from standardpressure angle].

Direction of ‘line of action’ depends on driver & driven gears and their directions of rotation.

4


Base Circle: It is an auxiliarycircle used in involute gearingcircle used in involute gearingto generate tooth profiles.

Line of actions a commontangent, which passes throughth it h i t b th b

[Note: For a particular generated

the pitch point, both basecircles.

involute gear the base circleremains fixed and can beconsidered as reference. ]

Pinion & Gear: In a pair of gears in mesh smaller oneis called ‘Pinion’ and the bigger one is called ‘Gear’is called Pinion and the bigger one is called Gearirrespective of any of them is driver or driven.

5


Module (m) : Pitch circle diameter / Number of teeth.

It is standardized and expressed in mm.

[Note: In case of helical teeth ‘normal module’ (i em[Note: In case of helical teeth normal module (i.e.,

module in normal direction is standard one].

It is followed in Metric and SI unit system.

nm

Standard Module: 1, (1.25), 1.5, 2, 2.5, 3, 3.5, 4, 4.55 6 7 8 10 12 15 17 20 255 6 7 8 10 12 15 17 20 25

Diametral Pitch (DP) : Average number of teeth

per unit length (inch) of pitch circle diameter.

To compare with module system 1”/DP i.e., 25.4/DP gives

l l t t d d d la value close to a standard module.

Standard 1, 2, 3, 4, ………………….. 25 6

Involute Toothed Gear : Fundamental Relations :

Pitch diameter

Referring to straight tooth spur Gear:

mZrd pp 2Pitch diameter

mZrp pc /2Circular pitch (arc)

p

Base circle radius

cospb rr

7

Involute Toothed Gear : Fundamental Relations (Contd.):

Referring to straight tooth spur Gear:

Tip or Addendum circle radius

maZr

( ‘-‘ for internal toothed gear)

mar fa

2

Root or Dedendum circle radius

mdZr fd

2

( ‘+’ for internal toothed gear)

8

Involute Toothed Gear : Fundamental Relations (Contd.): Recapitulation :Referring to straight tooth spur Gear:

Tooth thickness ( ) and space ( )

at standard pitch circle of

tt

st

at standard pitch circle of

uncorrected (standard) gear

they are equal (arc). ttst

tt st

c= p / 2 = πm / 2

9

Minimum Number of Teeth in a Gear, Interference and Undercut

2sin

2 fc

aZ

10

Gear DesignGear Design

Straight Tooth Spur Gear

11

Strength of gear teeth-Lewis St e gt o gea teet e sequation

12


Stress at root

22

66bthF

btM t

Stress at root

btbt

Where, b is the width of gear.

Ymht

6

2

Y is called the Lewis form factor

13


Y Lewis form factorY Lewis form factor

Y = 0.484 - 3.28 / ZFormative number of teeth.Z

bYmFt o at e u be o teetZ

bYSF

Introducing Allowable Strength And velocity factor

bYmcSF vot

14


Helical Tooth Spur Gear

15

Design of Helical Gear

Pitch Diameter of Helical Gear

3cos

ZZFormative number of teeth is expressed as:Zcos

16


Design of first stage gear set:

Module on the basis of bending strength:Module on the basis of bending strength:

The Lewiss Formula for module calculation.

2Tcosβm =n3 d

m = S ψYZc cv wc c

17


Centre Distance of Mating Helical Gear Pair

cosnpcp

p

pZpZD

cosngcg

g

pZpZD

2/)( gp DDA gpn ZZmA

2gp gpcos2

18


Straight Tooth Bevel Gear

19

Bevel Gear Nomenclature & Terminology :

20Straight Tooth Bevel Gear.

Different Type of Bevel Gears:

Straight Bevel

Spiral Bevel

21

Hypoid Bevel

Herringbone BevelWorm & Worm Wheel

Straight Bevel Gear Design:

Module (m, in meter) can be estimated as:b

b2

2Tm

For straight tooth bevel gear:

mean r

2

3 (1 )bevel

do

v w

m S ZYc c

mean r

Mean PCD (Straight Bevel)

2 mean r Z m 2 bevelmean r Z m

90o

Other relations.

90p g

sin / sin /p g p gZ Z And,

22Straight Tooth Bevel Gear.

p g p g

Design of a Bevel- Helical Two Stage Gear Box:

IMPORTANT:

Complete the Gear Design Partand

fDraw the layout of gears on 27 March, 2017

Z1

Z2

Z3 Z4

IMPORTANT:Al S b it f h d k t h f

Assembled plan view

Also Submit a freehand sketch of plan view (one copy per group).

23of 2-stage (Bevel-Helical) gear box.(Top cover open)

(Not of the same one as below)

Lecture - 2 Design of a Helical-Bevel Gear...

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