chapter 14 spur and helical gears - ksu · equation (14-1) is known as lewis equation and y is...
TRANSCRIPT
1
CHAPTER 14
Spur and Helical Gears
In this chapter we are going to analyze and design Spur and
Helical gears to:
Resist bending failure of the teeth
Pitting failure of tooth surfaces.
Failure by bending will occur when the significant tooth
stress equals or exceeds either the yield strength or the
bending endurance strength.
A surface failure occurs when the significant contact stress
equals or exceeds the surface endurance strength.
2
In the first two sections a little of the history of the analyses
from which current methodology developed is going to be
covered.
The American Gear Manufacturers Association (AGMA)
has for many years been the responsible authority for the
distribution of knowledge pertaining to the design and
analysis of gearing. The methods this organization presents
are in general use when strength and wear are primary
considerations.
3
The general AGMA approach requires a great many charts and graphs.
Many of these charts and graphs can be omitted by choosing a single pressure angle and by using only full-depth teeth.
This simplification reduces the complexity and makes possible a better development of the fundamentals and hence should constitute an ideal introduction to the use of the general AGMA method.
4
14.1 The Lewis Bending Equation
Static Effect
An equation for estimating the bending stress in gear teeth in which the tooth form entered into the formulation was introduced by Wilfred Lewis.
To derive the basic Lewis equation (See Fig. 14-la) which shows a cantilever of cross-sectional dimensions F and t, having a length l and a load Wt, uniformly distributed across the face width F.
5
The section modulus I/c is Ft2/6, and therefore the bending
stress is:
(a)2
6
/ Ft
lW
cI
M t
6
The maximum stress in a gear tooth occurs at point a as shown
in figure 14-1a.
Using the similarity of triangles, we can write:
(B)
=l
x
t / 2
l
tx
l
t
t
x
4
2/
2/
2
7
From equation (a): we can rewrite it as the following:
(C)
Substitute (b) into (c) and multiply the numerator and
denominator by the circular pitch p, we find
(d)
Letting y = 2x/3p, we have
(14-1)
6/4
1
4/
1
6/
16222 ltF
W
ltF
W
Ft
lW ttt
xpF
pW t
3/2
Fpy
W t
8
Equation (14-1) is known as Lewis equation and y is called
the Lewis form factor.
Most engineers prefer to employ the diametral pitch in
determining the stresses. This is done by substituting P=π / p
and Y= π y in equation (14-1). This gives
(14-2)
Where
(14-3)
Equation (14-2) considers only the bending of the tooth. And
the effect of the radial load Wr is neglected.
FY
PW t
3
2xPY
9
The value of Y can be found
in table 14-2. (These values
are for a normal pressure
angle of 20o, Full-Depth
Teeth, and a Diametral Pitch
of Unity in the Plane of
Rotation)
Number of
TeethY
Number of
TeethY
12 0.245 28 0.353
13 0.261 30 0.359
14 0.277 34 0.371
15 0.290 38 0.384
16 0.296 43 0.397
17 0.303 50 0.409
18 0.309 60 0.422
19 0.314 75 0.435
20 0.322 100 0.447
21 0.328 150 0.460
22 0.331 300 0.472
24 0.337 400 0.480
26 0.346 Rack 0.485
10
Eq. (14-3) also implies that
the teeth do not share the load
the greatest force is exerted at the tip of the tooth.
the contact ratio should be somewhat greater than unity, say
about 1.5, to achieve a quality gearset. If, in fact, the gears are
cut with sufficient accuracy, the tip-load condition is not the
worst, because another pair of teeth will be in contact when
this condition occurs.
11
Examination of run-in teeth will show that the heaviest loads
occur near the middle of the tooth. Therefore the maximum
stress probably occurs while a single pair of teeth is carrying
the full load, at a point where another pair of teeth is just on
the verge of coming into contact.
12
Dynamic Effect
Dynamic effect occurs when a pair of gears is driven at
moderate or high speed and noise is generated
Barth first expressed the velocity factor in terms of AGMA
standards and can be represented as (SI units):
(14-6a)
Where V is the pitch-line velocity in m/s
profile)cast iron,(cast 05.3
05.3 VK
13
Barth equation is modified for cut or milled profile (see
equation 14-6b)
(14-6b)
Later AGMA added two velocity factors for shaped and
ground profiles and can be written as:
(14-6c)
(14-6d)
profile) milledor (cut 10.6
10.6 VK
profile) shapedor (hobbed 56.3
56.3 VK
profile) groundor (shaved 56.5
56.5 VK
14
V is in ft/min
15
By introducing the velocity factor into (14-2) gives
(14-7)
The metric version of this equation is
(14-8)
Where
FY
PWK t
FmY
WK t
(N) load theofcomponent tangential
(mm) modile them
(mm) width face the
tW
F
16
As a general rule, spur gears should have a face width F from
3 to 5 times the circular pitch p.
Equations (14-7) and (14-8) are important because they form
the basis for AGMA approach to the bending strength of gear
teeth. They are in general use for estimating the capacity of
gear drives when life and reliability are not important
considerations.
The equations can be useful in obtaining a preliminary
estimate of gear sizes needed for various applications.
17
Example 14-1
A stock spur gear is available having a diametral pitch of 8
teeth/in, a 1.50 in face, 16 teeth, and a pressure angle of 20°
with full-depth teeth. The material is AISI 1020 steel in as-
rolled condition. Use a design factor of nd = 3 to rate the
horsepower output of the gear corresponding to a speed of
1200 rev/m and moderate applications.
18
Solution
The term moderate applications seems to imply that the gear
can be rated by using the yield strength as a criterion of failure.
From Table A-20, we find:
Sut = 55 kpsi=379MPa and Sy = 30kpsi=207MPa
A design factor of 3 means that the allowable bending stress is
207/3 = 69 MPa
The pitch diameter is N/P = 16/8 = 2 in, so the pitch-line
velocity is:
smftdn
V /2028.3)628(0051.0min/62812
1200)2(
12
19
Using milled profile (eq. 14-6b) for the velocity factor
From table 14-2, for N =16 teeth Y = 0.296, thus
525.11.6
3028.9
1.6
1.6
VK
6.93hphp)746/1.5171(1.5171)10(60
16201200)4.252(
)10(60
35,-13equation From
1620525.1
)69)(296.0)(4.25125.0)(1.38(
33
WdnW
H
NK
FmYW
FmY
WK
t
allt
t
20
Example 14-2
Estimate the horsepower rating of the gear in the previous
example (14-1) based on obtaining an infinite life in bending.
21
Solution
The rotating-beam endurance limit is estimated from Eq. (7-8)
To obtain the surface finish Marin factor ka, for machine surface
MPa191)379(504.0504.0' ute SS
934.0)191(70.2
265.0,7.2
265.0
b
uta aSk
ba
22
For the size factor kb
948.03.0
495.025.0
0555.0)8(2
)296.0(3
2
3
281.08
25.1
8
125.11
,)/4(
5.1
)(808.0)(808.0
0107.
5.0
5.05.0
e
b
e
e
dk
indint
inP
Yx
inPP
l
xlt
inF
Fthbd
23
for load factor kc =1, for bending
Stress concentration factor Kf
For a 20o full-depth tooth the radius of the root fillet is
denoted rf, where rf = 0.3/P=0.3/8=0.0375 in
From Figure A-15-6
MPa12.169)191)(1)(1)(1)(1)(948.0)(934.0(
1
'
efedcbas
fed
SkkkkkkS
kkk
15.025.0
0375.0
t
r
d
r f
43.1)168.1(63.01)1(1
63.0 7.20, figure From
1.68 ,3 can take we,/ Since
tf
t
KqK
q
KD/ddD
24
These results should be accepted only as preliminary estimates
to alert you to the nature of bending in gear teeth.
hp96.3)746/2952(2952)10(60
58.925525.1
)42.39)(296.0)(4.25125.0(1.38
MPa42.39/
MPa373.563
12.169
3
WdnW
H
NK
FmYW
K
n
S
t
v
allt
foallall
d
e
oall
25
Dolan and Broghamer stress
concentration formula
(14-9)
Where
ML
fl
t
r
tHK
diameterpitch the dedendum, the radius,fillet angle, pressure
2/
4583662.0290.0
4583662.0316.0
4583662.034.0
2
dbr
rbd
rbr
M
L
H
f
f
f
27
14.2 Surface Durability
In this section, the failure of the surfaces of gear teeth, which
is generally called wear is studied. To obtain an expression for
the surface-contact stress, the Hertz theory is employed.
It was shown that the contact stress between two cylinders
may be computed from the equation:
(a)
Wherebl
Fp
2max
cylinders oflength
togethercylinders two thepressing force
pressure surfacelargest max
l
F
p
28
(14-10)
2/1
21
2
2
21
2
1
/1/1
/1/12width-half
dd
EE
l
Fb
E1, ν1
E2, ν2
29
To adapt these relations to the notation used in gearing, we
replace F by Wt/cos(ϕ), d by 2r, and l by the face width F.
With these changes, we can substitute the value of b as given
by equation (14-10) in equation (a). Replacing pmax by σC, the
surface compressive stress (Hertzian stress) is found from the
equation:
(14-11)
Where r1 and r2 are the instantaneous values of the radii of
curvature on the pinion- and gear-tooth profiles, respectively,
at the point of contact.
2
2
21
2
1
212
C/1/1
/1/1
cos EE
rr
F
W t
30
By accounting for load sharing in the value of Wt used,
equation (14-11) can be solved for the Hertzian stress for any
or all points from the beginning to the end of tooth contact
Noted that the first evidence of wear occurs near the pitch line.
The radii of curvature of the tooth profiles at the pitch point
are:
(14-12)
where ϕ is the pressure angle and dP and dG are the pitch
diameters of the pinion and the gear, respectively.
In equation (14-11), that the denominator of the second group
of terms contains four elastic constants, two for the pinion and
two for the gear.
2
sin
2
sin21
GP dr
dr
31
As a simple means of combining and tabulating the results for
various combinations of pinion and gear materials, AGMA
defines an elastic coefficient CP by the equation
(14-13)
With this simplification, and the addition of a velocity factor
Kν, equation (14-11) can be written as:
(14-14)
Where the sign is negative because σC is a compressive stress.
2/1
22 11
1
G
G
P
P
P
EE
C
2/1
21
11
cos
rrF
WKC
t
PC
32
Example 14-3
The pinion of Example 14-1 and 14-2 is to be mated with a 50
tooth gear manufactured of ASTM No. 50 cost iron. Using the
tangential load of 382 lbf, estimate the factor of safety of the
drive based on the possibility of a surface fatigue failure.
33
Solution From Table A-5:
From example 14-1, the pinion pitch diameter is dP = 2in, and
for the gear dG = 50/8 =6.25in
Thus from equation (14-12):
1817
105.14
02111
1030
02921
1
211.0 Mpsi,5.14 ,292.0 Mpsi,30
2/1
6
2
6
2
P
GGPP
C
EE
in069.12
20sin25.6 in,342.0
2
20sin221
rr
34
The face width is given as F = 1.5 in and Kν =1.525 from
example 14-1. Substituting all these values in equation (14-14)
with pressure angle of 20o, thus the contact stress is
psi72400
069.1
1
342.0
1
20cos5.1
380525.11817
11
cos
2/1
2/1
21
rrF
WKC
t
PC
35
The surface endurance strength of cast iron can be estimated
from
For table A-24, HB = 262 for ASTM No. 50 cast iron, Thus
SC = 0.32(262) = 83.8 kpsi
Thus the factor of safety can be found as:
cycles, 10for kpsi, 32.0 8
BC HS
35.14.72
8.83
load imposed
loadfunction of loss22
n
36
14.3 AGMA Stress Equations
Two fundamental stress equations are used in the AGMA
methodology, these are:
The bending stress
The pitting resistance (contact stress).
In AGMA terminology, these are called stress numbers and
are designated by a lowercase letter s instead of the Greek
lower case σ. The fundamental equations are:
(14-15)
J
BH
t
so
t
Bmdso
t
Y
KK
bmKKKW
J
KK
F
PKKKW
1
units) (SI
units)customary (U.S.
37
Where for U.S. customary units (SI units),
)factor ion concentrat-stressfillet root includes(which
strengh bendingfor factor geometry the
factor thickness-rim the
factoron distributi-load the
(mm)in member,narrower theof width face the
pitch diameteral e transversthe
factor size the
factor dynamic the
factor overload the
(N) Ibf load, ed transmittl tangentiathe
f
J
B
Hm
d
s
o
t
K
YJ
K
KK
bF
P
K
K
K
W
mt: transverse metric module
38
Before you try to digest the meaning of all these terms in
equation (14-15), view them as advice from AGMA
concerning items the designer should consider whether you
follows the voluntary standard or not. These items include
issues such as:
Transmitted load magnitude
Overload
Dynamic augmentation of transmitted load
Size
Geometry: pitch and face width
Distribution of load across the teeth
Rim support of the tooth
Lewis form factor and root fillet stress concentration
39
The fundamental equation for pitting resistance (contact stress)
is
(14-16)
Where
The evaluation of all these factors is explained in the sections
that follow.
I
R
w
Hso
t
E
f
P
m
so
t
P
C
Z
Z
bd
KKKKWZ
I
C
Fd
KKKKWC
1
units) (SI
units)customary (U.S.
resistance pittingfor factor geometry the)(
(mm)in pinion, theofdiameter pitch the)(
factorcondition surface the)(
)N/mm(Ibf/in t,coefficien elastican )(
1
22
I
WP
Rf
Ep
ZI
dd
ZC
ZC
40
14.4 AGMA Strength Equations
41
Following this convention, values for AGMA bending strength, designated here as St, are to be found in Figs. 14-2, 14-3, and 14-4, and in Tables 14-3 and 14-4.
Since AGMA strengths are not identified with other strengths such as Sut, Se, or Sy as used elsewhere in this book, their use should be restricted to gear problems.
42
Figure 14-2: Allowable bending stress number for through-hardened steels. The SI equations are St , σFP =0.533HB + 88.3 MPa, grade 1, and σFP = 0.703HB+ 113 MPa, for grade 2.
43
Figure 14-3: Allowable bending stress number for nitrided
through-hardened steel gears (i.e., AISI4140, 4340, St. The SI
equations are σFP =0.568HB + 83.8 MPa, grade 1, and σFP =
0.749HB+ 110 MPa, for grade 2.
44
Figure 14-4: Allowable bending stress number for nitriding through-
hardened steel gears St. The SI equations are σFP =0.594HB + 87.76
MPa Nitralloy grade 1, σFP =0.784HB + 114.81 MPa Nitralloy grade
2, σFP =0.7255HB + 63.89 MPa 2.5% chrome, grade 1, σFP
=0.7255HB + 153.63 MPa 2.5% chrome, grade 2, and σFP =
0.7255HB+ 201.91 MPa 2.5% chrome, grade 3.
45
46
In the AGMA approach the strengths are modified by various
factors that produce limiting values of the bending stress and
the contact stress. Using similar notation to that in equation (1-
6) we shall term the resulting modifications the allowable
bending stress σall and the allowable contact stress σC, all . The
equation for the allowable bending stress is:
(14-17)
Where for the U.S. customary units (SI units),
Z
N
F
FP
RT
N
F
t
all
YY
Y
S
KK
Y
S
S
units) (SI
units)customary (U.S.
47
ratio stress a safety, offactor AGMA the
factorsy reliabilit the
factors re temperatuthe
stress bendingfor factor cycle stress the
)(N/mmlbf/in stress, bending allowable 22
F
ZR
T
N
FPt
S
YK
YK
Y
S
48
The equation for the allowable contact stress σC,all is
(14-18)
Where the upper equation is in U.S customary units and the
lower equation is in SI units, Also
Z
WN
H
HP
RT
HN
H
C
allC
YY
ZZ
S
KK
CZ
S
S
,
units) (SI
units)customary (U.S.
49
The values for the AGMA allowable stress numbers
(strengths) for bending and contact stress are for
Unidirectional loading
10 million stress cycles
99 percent reliability
ratio stress a safety, offactor AGMA the
factorsy reliabilit the
factors re temperatuthe
resistance pittingfor factors ratio hardness the
factor life cycle stress the
)(N/mmlbf/in stress,contact allowable 22
H
ZR
T
H
N
HPC
S
YK
YK
C
Z
S
50
Figure 14-5: Contact-fatigue strength SC at 107 cycles and
0.99 reliablity for through-hardened steel gears. The SI
equations are σHP =2.22HB + 200 MPa, grade 1, and σHP
=2.41HB + 237 MPa, grade 2
51