7.5 Conditions of Nonundercutting of Planar Internal Noncircular Gears 133 The limiting condition of undercutting corresponds to positions wherein the tra-
jectory of point Mreaches point K, which belongs to the addendum curve of the internal noncircular gear tooth profile. The trajectory of point Mdepends on the method of generation of the internal noncircular gear by the shaper. Here, axial generation is considered, which means that center distance between the shaper and the internal noncircular gear is modified according to expressions ofx(Of 2)andy(Of 2) (see Eqs. (7.4.1) and (7.4.2)).
Two approaches are considered for determination of the limiting value of the pitch radius ρs∗ of the shaper for avoidance of undercutting. In consequence, the limiting value of shaper teeth,Ns∗, is obtained as
Ns∗ =2ρs∗
m (7.5.1)
wheremis the module of the gear drive.
7.5.1 Approach A
The limiting condition of undercutting can be represented numerically by a set of nonlinear equations. Solution of such a set of nonlinear equations implies determi- nation of the limiting value of the pitch radiusρs∗. The proposed approach is based on the following algorithm:
(1) Point Mbelongs to the addendum circle of the shaper, and this condition is represented by
f1(θsM, ρs∗)= |r(M)s (θsM, ρs∗)| −(ρ∗s +1.2m)=0 (7.5.2) Here,|r(M)s (θsM, ρs∗)| =
(rsxM)2+(rsyM)2, and vectorr(M)s (θsM, ρs∗) is represented by Eq. (7.4.5) whereinρs=ρs∗is considered as a variable.
(2) PointKbelongs to the profile2of the noncircular gear tooth, and this condi- tion implies that equation of meshing Eq. (7.4.14) is satisfied at pointK,
f2(θsK, θ2K, ρs∗)=0 (7.5.3) (3) PointKbelongs to the addendum curve of the noncircular gear, and this condi-
tion is represented by
f3(θsK, θ2K, ρs∗, θσK2)= |r(K)2 (θsK, θ2K, ρs∗)| −(r2(θσK2)−m)=0 (7.5.4) Here,
(i) |r(K)2 (θsK, θ2K, ρ∗s)| =
(r2xK)2+(r2yK)2 where vector r(K)2 (θsK, θ2K, ρ∗s) is ob- tained by matrix transformation as
r(K)2 (θsK, θ2K, ρs∗)=M2s(θ2K)rs(θsK, ρs∗) (7.5.5) (ii) r2(θσK2) is the amplitude of polar vector of the centrodeσ2corresponding to
pointKandθσK2 is its polar angle.
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134 Design of Internal Noncircular Gears
(4) Polar angleθσK2satisfies
f4(θsK, θ2K, θσK2, ρ∗s)=tanθσK2−r2xK(θsK, θ2K, ρs∗)
r2yK(θsK, θ2K, ρs∗) =0 (7.5.6) (5) The trajectory of pointMin coordinate systemS2may be obtained by applica-
tion of matrix transformation
r(M)2 (θsM, ρs∗, θ2)=M2s(θ2)rs(θsM, ρs∗) (7.5.7) whereinθ2is the parameter of the curve that represents the trajectory ofM.
(6) Intersection of curve represented by Eq. (7.5.7) and profile2at pointKrepre- sented by Eq. (7.5.5) provides a vectorial equation
r(M)2 (θsM, ρs∗, θ2)−r(K)2 (θsK, θ2K, ρs∗)=0 (7.5.8) that yields two additional scalar equations
f5(θsM, θsK, θ2K, ρs∗, θ2)=r2xM−r2xK =0 (7.5.9) f6(θsM, θsK, θ2K, ρs∗, θ2)=r2yM−r2yK =0 (7.5.10) (7) A system of six nonlinear equations f1, f2, f3, f4,f5, f6is obtained as
f1(θsM, ρs∗)=0 (7.5.11) f2(θsK, θ2K, ρs∗)=0 (7.5.12) f3(θsK, θ2K, ρs∗, θσK2)=0 (7.5.13) f4(θsK, θ2K, θσK2, ρs∗)=0 (7.5.14) f5(θsM, θsK, θ2K, ρs∗, θ2)=0 (7.5.15) f6(θsM, θsK, θ2K, ρs∗, θ2)=0 (7.5.16) and may be solved numerically for determination of unknowns (θsM, ρs∗, θsK, θ2K, θσK2, θ2).
7.5.2 Approach B
Approach B is based on the results provided in Table7.5.1, developed by Litvinet al.
(Litvinet al., 1994), to determine the maximal number of teeth for various pressure angles in the case of internal circular involute gears (see Chapter 11 of Litvin &
Fuentes, 2004).
Figure 7.5.2shows the substitution of centrode σ2 of the internal noncircular gear by a new centrodeσ2c of a circular gear with pitch radius equal to the mini- mal radius of the centrodeσ2. Centrodeσ2cis rigidly connected to systemS2c. The trajectory of pointMin systemS2cis an extended hypocycloid.
7.5 Conditions of Nonundercutting of Planar Internal Noncircular Gears 135
Table 7.5.1. Maximal number of shaper teeth for internal circular involute gears.
Pressure angle Generation method Gear teeth Shaper teeth Axial 25≤N2≤31 Nc≤0.82N2−3.20 αc=20◦ Axial 32≤N2≤200 Nc≤1.004N2−9.162
Two-parameter 36≤N2≤200 Nc≤N2−17.6 Axial 17≤N2≤31 Nc≤0.97N2−5.40 αc=25◦ Axial 32≤N2≤200 Nc≤N2−6.00
Two-parameter 23≤N2≤200 Nc≤N2−11.86 αc=30◦ Axial 15≤N2≤200 Nc≤N2−4.42
Two-parameter 17≤N2≤200 Nc≤N2−8.78
Figure 7.5.2. Illustration of the trajectory described by the tip of the shaper tooth during the relative motion between the shaper and the equivalent internal circular gear.
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136 Design of Internal Noncircular Gears
Table 7.5.2. Gear data.
Number of teeth of the elliptical pinion,N1 31 Number of teeth of the internal noncircular gear,N2 62 Number of revolutions of the elliptical pinion,n 2
Module,m 4.0 mm
Pressure angle,α 20.0◦
Eccentricity of elliptical centrodeσ1,e 0.2
Approach B is based on the following algorithm:
(1) The minimal curvature radius of centrodeσ2 of the internal noncircular gear, ρ2,mi n, is obtained by consideration of (see Eq. (2.8.8))
ρ2(θ2)=
r2(θ2)2+ dr2
dθ2
23/2
r2(θ2)2+2 dr2
dθ2
2
−r2(θ2)d2r2
dθ22
(7.5.17)
dρ2
dθ2
=0 (7.5.18)
Expressionsr2(θ2),dr2(θ2) dθ2
, andd2r2(θ2)
dθ22 have been represented in Section7.4.
(2) The number of teeth for the equivalent internal circular gear is obtained as N2∗= 2ρ2,mi n
m (7.5.19)
(3) The limiting value of the number of shaper teeth can be obtained by application of Table7.5.1(see Chapter 11 ofLitvin & Fuentes, 2004).
7.5.3 Numerical Example
An internal noncircular gear drive based on a conventional elliptical pinion is con- sidered. The gear data are shown in Table7.5.2.
Parameteraof the elliptical centrodeσ1is obtained as (see Eq. (4.3.26)) a= N1mπ
4π/2
0
1−e2sin2θ1dθ1
=62.631089 mm
Table 7.5.3. Solution for Approach A.
Profile parameter of pointM,θsM 0.517701 rad Limiting value of the pitch radius of the shaper,ρs∗ 82.541688 mm Profile parameter of pointK,θsK 0.172244 rad Generalized parameter of motion for pointK,θ2K −0.139680 rad Polar angle for polar vector of pointK,θσK2 −0.031313 rad Parameter of the trajectory of pointM,θ2 0.620299 rad
7.5 Conditions of Nonundercutting of Planar Internal Noncircular Gears 137
Figure 7.5.3. Observation of undercutting whenNs =42 and nonundercutting whenNs=41.
Center distanceEof the gear drive is obtained as (see Eq. (7.3.6)) E=a[−1+
1+(n2−1)(1−e2)]=60.737850 mm
APPLICATION OF APPROACH A. The solution of the set of the nonlinear equations corresponding to the Approach A provides the values of magnitudes (θsM, ρs∗, θsK, θ2K, θσK2, θ2) shown in Table7.5.3.
The limiting value of the number of shaper teeth is obtained as Ns∗= 2ρ∗s
m =41.2708
Figure7.5.3shows the trajectories of point Min two cases: for Ns =41 (ρs = 82 mm) and forNs =42 (ρs =84 mm). The detail A of Fig.7.5.3shows that under- cutting occurs forNs =42, whereas undercutting does not occur forNs =41.
APPLICATION OF APPROACH B. The minimal radius of curvature of centrode σ2 is obtained atθ2=0 (see Fig.7.3.1(a)). In this case,
ρ2,mi n= 1 1
E+a(1+e)+ e a(1−e2)
=93.589402 mm
The number of teeth of the equivalent internal circular gear,N2∗, is obtained as N2∗ =2ρ2,mi n
m =46.794701
Applications of results shown in Table 7.5.1(see Chapter 11 ofLitvin & Fuentes, 2004) provides the following limiting value of the number of shaper teeth:
Ns∗=1.004N2∗−9.162=37.8199
Approach B provides a more conservative result than the exact solution pro- vided by Approach A.
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8 Application for Design of Planetary Gear Train with Noncircular and Circular Gears