Gear box

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Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Module 2 - GEARS
Lecture 17 – DESIGN OF GEARBOX
Contents
17.1 Commercial gearboxes
17.2 Gearbox design.
17.1 COMMERCIAL GEARBOXES
Various commercial gearbox designs are depicted in Fig. 17.1 to 17.10. These include
single to multistage ranging from spur, helical, bevel to worm gears.

Fig.17.1 Commercial Gearbox Design

Fig.17.2 Two stage helical gearbox

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Fig.17.3 A single stage bevel gearbox

Fig.17.4 Worm gearbox

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Fig.17.5 Worm gearbox, sectional front and side views

Fig.17.6 Worm gearbox, withoutcooling fins, sectional front and side views

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Fig.17.7 Helical gear lubrication with idler gear

Fig.17.8 Spur gear lubrication
with stream by nozzles

Fig.17.9 A double reduction spur gear box

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.MayuramFig.17.10 Bevel gear – Spur gear transmission
17.2 HELICAL GEARBOX DESIGN - PROBLEM 1
In a turbine drive 300 kW power is transmitted using a pair of double helical gear. The
pinion speed is 2950 rpm and that of the gear is about 816.5 rpm. There are no space
constraints on the gear drive. Selecting suitable materials, design the pinion and the
gear to last for 108 cycles. Design thegearbox completely.
Data: W = 300kW; n1 = 2950rpm; n2 ≈ 816.5 rpm; Desired Life 108 cycles.
Solution:
1. Angular speed of the input shaft

ω1 

2πn1 2π x 2950

 308.77 rad / s
60
60

1000W 1000x300

 971.6Nm
ω
308.77

2. Torque:T1 

3. The details of the gear design carried out are given in Table 1 and 2.

Indian Institute of Technology Madras

Machine Design II

Prof.K.Gopinath & Prof. M.M.Mayuram

The final specifications of the pinion and gear are:
20o pressure angle involute teeth with helix angle of 35o, ha =1mn, hf =1.25mn.
i= Z2 / Z1 =105/29= 3.62
Table 17.1 Gear dimensions
Element Z

mn
mm

d mm

da mm db mm

dr mm

mt mm

Pinion

29

5

177.01

187.01

161.76

164.51

6.104

Gear

105

5

640.92

650.92585.69

628.42

6.104

Table 17.2a Gear specifications
Element

Φn

φt

B

Pinion

20o

23.96o

Gear

20o

23.96o

mm

pt mm

pa

mm

70

19.165

27.37

70

19.165

27.37

Table 17.2b Gear specifications
Element

CRt

CRa

CR

FS sb

FS sH

Pinion

1.3044

1.2787

2.583

1.99

1.73

Gear

1.3044

1.2787

2.583

1.891.53

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Fig.17.11 (a) Pinion

Fig.17.11 (b) Gear

All dimensions are in mm and not to scale

Fig.17.12 A commercial double helical gearbox

Indian Institute of Technology Madras

Machine Design II

Fig.17.13 Gear box outer
dimensions (tentative)

Prof. K.Gopinath & Prof.M.M.Mayuram

Fig.17.14 Pinion shaft layout diagram

All dimensions are in mm
4. Shaft design is based on the ASME equation:
Tangential load on the shaft: Ft = T/r = 971.6/0.088.5 = 11kN
Fr =Ft tan Ø =11tan23.96o= 4.89kN
F = (Ft2 + Fr2)0.5
= (112 + 4.892)0.5=15.42kN
Bending moment at C
M = Fl /4 =15.42x0.15/4=0.58 kNm
Fig.17.15 Shaft Loading
By ASME code equation for shaft design we have,

d16
(Kb M)2  (K t T)2
 (1  k)[  ]

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

k = 0.2 i.e, 20% reduction in strength due to keyway is assumed. From Table 17.3, for
rotating shaft with minor shock loads, Kb = 1.5 and Kt = 1.0.
Taking C45 steel for the shaft, σyp = 360 MPa
τyp = σyp /2 = 360/2 = 180 MPa and taking factor of...
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