Using Lever Analogy Diagrams In Teaching Compound Planetary Gear Trains

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2006-1084: USING LEVER ANALOGY DIAGRAMS IN TEACHING COMPOUND
PLANETARY GEAR TRAINS
Gene Liao, Wayne State University
Dr. Liao received the B.S.M.E. from National Central University, Taiwan; M.S.M.E. from the
University of Texas; Mechanical Engineer from Columbia University; and the Doctor of
Engineering degree from the University of Michigan, Ann Arbor. He is currently an assistant
professor at the Wayne State University. He has over 15 years of industrial practices in the
automotive sector prior to becoming a faculty member.
© American Society for Engineering Education, 2006
Page 11.1393.1

Using Lever Analogy Diagrams in Teaching Compound Planetary
Gear Trains
Abstract
The planetary gear trains are widely used in many automotive, aerospace and marine
applications. The planetary gear trains are introduced to undergraduate mechanical engineering
students in the course of Kinematics and Dynamics of Machines. Traditional methods of
analyzing planetary gear trains, by means of torque and speed calculations, tend to be slow and
cumbersome. The complexity involved has, no doubt, kept many students from becoming
familiar with the capability of planetary gearing. It is our intent in this paper to describe the
lever analogy method of analysis and to present a miniature ‘cookbook’ of levers for various
planetary arrangements.
1. Introduction
The planetary gear trains, also known as epicyclic gear drives, are widely used in many
automotive, aerospace and marine applications. The planetary type mechanism is the most used
mechanical mechanism in conventional automatic transmissions for the majority of passenger
cars. As an application in automotive automatic transmission, the number of speed ratios is
determined by the kinematic structure and corresponding clutching sequence of its planetary gear
train.
The planetary gear trains are introduced to undergraduate mechanical engineering students in the
course of Kinematics and Dynamics of Machines. Traditional methods of analyzing planetary
gear trains, by means of torque and speed calculations, tend to be slow and cumbersome. The
complexity involved has, no doubt, kept many students from becoming familiar with the
capability of planetary gearing. It seems appropriate in the Kinematics and Dynamics of
Machines course to present a simpler method of analyzing and characterizing gear trains. The
method is called the Lever Analogy Diagrams which are commonly utilized in industry1-4.
The lever analogy diagram is very useful in analyzing gear train that has more than two
connected planetary gear sets. For a single planetary gear set, it is no need to add a level of
abstraction. The lever analogy is a translational-system representation of the rotating parts for
the planetary gear. In the lever analogy, an entire compound planetary gear train can usually be
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represented by a single vertical lever. The input, output and reaction torques are represented by
horizontal forces on the lever. The lever motion, relative to the reaction point, represents
rotational velocities. By using the lever, for an example, one can easily visualize the essential
functions of the transmission without addressing the complexities of planetary gear kinematics.
Most of the kinematics and dynamics textbooks utilize Train Value Formulation Method or
Tabular (Superposition) Method for analyzing of planetary gear trains. Using computer
simulation software, such as Working Model and Visual Nastran, to improve students
understanding of the mechanisms has rapidly increased in recent years 5, 6, 7, 8, 9. However, it
takes time and effort to construct demonstration models for instructional purpose. It is our intent
in this paper to describe the lever analogy method of analysis and to present a miniature
‘cookbook’ of levers for various planetary arrangements. It has been our instructional
experience that the use of this tool not only makes torque and speed calculations easy, but also
improves students’ ability to visualize the results and understand the effect of gear tooth ratios.
2. Modeling Procedure
The procedure of setting up a lever system analogous for planetary gear sets is: (1) replace each
gear set by a vertical lever; (2) rescale, interconnect, and/or combine levers according to the gear
sets’ interconnections; and (3) identify the connections to the lever(s), according to the gear sets’
connections. The lever is a basic building block of the analogy which replaces the planetary gear
set. The lever proportions are determined by the numbers of teeth on the sun and ring gears.
And then the next step is combining the levers and identifying the connections in the gear sets.
Figure 1 shows the free body diagrams of two meshing gears. The pitch diameter or teeth per
inch along the circumference is the same for two meshing gears, as illustrated in Equation (1).
The ratios of speeds and torques are shown in Equations (5) and (6).
Figure 1. Free body diagram of two meshing gears
2211 N/r2N/r2
ππ
= (1)
2211 r*wr*wV == (2)
gear 1
gear 2
T2, w2
T
1
, w
1
F
F
r2
r
1
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11 r*FT = (3)
22 r*FT = (4)
121221 N/Nr/rw/w ==∴ (5)
212121 N/Nr/rT/T == (6)
where V is tangential velocity at contact point; r1 and r2 are radii; w1 and w2 are angular velocity;
T1 and T2 are torques; N1 and N2 are number of teeth on gear 1 and 2 respectively.
The free body diagram of a planetary gear is shown Figure 2, where N, r, T, and w represent the
teeth number, radius, torque, and angular velocity of gear respectively. The subscripts p, pc, s
and r are respectively for planet, planet carrier, sun, and ring gears. The sum of torques at pinion
is shown in Equation (9). Equation 10 illustrates the torque ratio between sun and ring gears.
FFF,0rFrF 21p2p1 ==∴=∗−∗ (7)
rsrsrrss N/NT/T,r*FT,r*FT =∴== (8)
0TTT pcrs =−+ (9)
Therefore,
sr
r
pcr
sr
s
pcs NN
N
*TT,
NN
N
*TT +
=
+
= (10)
The tangential velocities at ring and planet gears are V, and
rpcpppr r*wr*wValso,r*wV +== (11)
r
r
p
p
N
r**2
N
r*2
π
π
=
∗
(12)
rpcpprr N*wN*wN*w += (13)
rs
Tr,wr
F
1
F
1
F2
Tpc,
wpc
Ts,
ws
rp
Tp, wp
rr
F
2
planet
gear
ring
gear
sun gear
carrier or
arm
Figure 2. Free body diagram of planetary gear
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Using common tangential velocities at sun and planet gears gives:
spcppss N*wN*wN*w +−= (14)
Sum of the Equations (13) and (14), it gives:
)NN(*wN*wN*w srpcssrr +=+ (15)
2.1 Single Planetary Gear Set
The stick diagram for a single planetary gear set is illustrated in Figure 3(a) and the lever
replacement is shown in Figure 3(b). Using lever diagram, the torque equations are derived from
Equations (16) to (18), and angular speeds in Equations (19) and (20). The justification for these
substitutions may not be obvious, but it can show that the horizontal force and velocity
relationship of the lever are identical to the torque and rotational velocity relationship of the gear
set. For an example shown in Figure 4, when the carrier of a simple gear set is grounded, the
ring and sun rotate in opposite directions at relatively speeds inversely proportional to their
numbers of teeth; and the corresponding points on the analogous lever behave the same.
Figure 3. Lever representations of single planetary gear
sr
r
pcr NN
N
*TT +
= (16)
srpc TTT += (17)
sr
s
pcs NN
N
*TT +
= (18)
r
spc
rs
sr
N
ww
NN
ww −
=
+
− (19)
sun gear shaft
ring gear
planet gear/carrier
Ts
Tpc
carrier
T
r
ring
sun
w
r
wpc
w
s
N
S
Nr
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s
rs
s
r
rs
r
pc w
NN
N
w
NN
N
w+
+
+
=∴ (20)
Figure 4. Example of lever representations of single planetary gear
2.2 Interconnections Between Planetary Gear Sets
The interconnections between gear sets are replaced by horizontal links connected to the
appropriate places on the levers. Whenever two gear sets have a pair of interconnections, the
relative scale constants and placement of their analogous levers must be such that the
interconnecting links are horizontal. Levers connected by a pair of horizontal links remains
parallel, and therefore can be replaced functionally by a single lever having the same vertical
dimension between points. This is illustrated using an example shown in Figure 5 along with the
levers representing the two simple planetary gear sets. Let teeth numbers at planetary gear 1 are
65 and 33 teeth for ring and sun gear respectively; and at planetary gear 2 are 55 and 21 teeth for
ring and sun gear respectively. Figure 5(a) is a stick diagram of two planetary gear sets, 5(b) is
two levers representations, and 5(c) is the process of combining two levers.
(a) A stick diagram of two connected planetary gear sets
sun
carrier
ring
Ts ,ws
Tr ,wr
ring 1
(65 teeth)
carrier 1
planetary
gear 1
planetary
gear 2
output shaft
input shaft
carrier 2
ring 2
(55 teeth)
sun 1
(33 teeth)
sun 2
(21 teeth)
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(b) Lever representation of each gear set
(c) Combining of two lever representations
Figure 5. Two connected gear sets and lever representations
3. Application of Lever Analogy in Hybrid Powertrain
To bring the lever analogy into a more interest stage, an Electric Variable Transmission (EVT)
used in hybrid powertrain is selected as an example. Figure 6 shows an EVT consisting of three
planetary gear sets, three clutches, and three power sources (engine and two electric motors).
The planetary gear system in the EVT plays an important role in distributing the power among
the engine and two electric machines. In planetary gear set 1, the planet carrier shaft is
connected to the engine output shaft through a clutch (C1). This EVT may perform in a hybrid
or purely electrical manner depending on the on/off status of the C1. The ring gear shaft (in
planetary gear set 1) and the sun gear shaft (in planetary gear set 3) are connected to the
transmission output shaft through clutch 2 (C2). Similarly, the planet carrier shaft in planetary
gear set 2 is connected to the transmission output shaft through clutch 3 (C3). The lever diagram
is shown in Figure 7.
When C1 and C3 are engaged (with C2 is off), the EVT runs in the first mode. In this mode, the
engine drives the output shaft through planetary gear sets 1 and 2. Electric machine B, serving
as a motor, also gives additional power to the output shaft through planetary gear set 2 and C3.
Simultaneously, through the ring gear in planetary gear set 3, part of the engine power drives
electric machine A which serves as a generator. Unit A converts the mechanical energy into
electrical energy which is stored in the battery and drives unit B. The speed diagram for the
first mode is illustrated in Figure 8.
sun 1
carrier 1
ring 1
33
65
sun 2
carrier 2
ring 2
21
55
sun 1
carrier 1
ring 1
sun 2
carrier 2
ring 2
33
65
21
55
sun 1
carrier 1
ring 1
33
65
21
55
sun 2
carrier 2
ring 2
33
18
5521
21
65 =
+
∗
47
5521
55
65 =
+
∗
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Figure 6. Schematics of two-mode EVT (from US Patent 5,558,588)10
(a) Combining planetary gears 1 and 2 (b) Combining planetary gears 1, 2 and 3
C3
R1
S1
P1
planetary
gear 3
C2
output
shaft
input
shaft
Engine
Electric
machine A
Electric
machine B
R2
S2
P2
R3
S3
P3
planetary
gear 1
planetary
gear 2
C1
C Clutch
S Sun gear
P Planet carrier
R Ring gear
S1, S
2
P1
R1
N
S1
N
R1
R2
P2
N
S2
N
R2
R3
P3
S3
N
S3
N
R3
S1
P1
R1
N
S1
N
R1
R2
P2
S2
N
S2
N
R2
S1, S2
P1
R1
N
S1
N
R1
R2
P2
N
S2
N
R2
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Figure 7. Lever diagram of EVT
In the second mode shown in Figure 9, the engine drives the output shaft through planetary gear
sets 1 and 3. Electric machine A, serving as a motor, also gives additional power to the output
shaft through planetary gear set 3 and C2. Simultaneously, through the ring gear in planetary
gear set 2, part of the engine power drives electric machine B that serves as a generator, or the
power of unit B will drive the output shaft as B serves as a motor.
Figure 8. Speed diagram of first mode
S1, S2
P1
R1
N
S1
N
R1
R2
P2
N
S2
N
R2
R3
P3
S3
N
S3
N
R3
Motor B
Engine
Motor A
Output
C2
C3
N
S1
N
R1
N
S2
N
R2
S1,
S2
P1
R1
R2
P2
R3
P3
S3
N
S3
N
R3
Motor B,
W
B
Engine, WEngine
Motor A, WA
Output, WOutput
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Figure 9. Speed diagram of second mode
Based on speed diagram, the output shaft speed in the first mode is derived as:
Engine
SRR
S
B
SRR
RR
output W
NNN
N
W
NNN
NN
W∗
++
+∗
++
+
=
221
2
221
21 (21)
The speed equations in the second mode is illustrated as:
Output
SSRR
SRR
B
SSRR
S
Engine W
NNNN
NNN
W
NNNN
N
W∗
+++
++
+∗
+++
=
1221
221
1221
1 (22)
4. Summary
Using lever analogy diagrams, the increasing complex mechanical system, such as automotive
transmission, is much easier to understand. The lever analogy allows easy analysis of
mechanical transmissions of angular velocity and torque. It is our intent in this paper to describe
the lever analogy method of analysis and to present a miniature ‘cookbook’ of levers for various
planetary arrangements. The following steps should be followed:
(1) Replace planetary gear sets with their equivalent levers
(2) Rescale the levers such that their interconnections are horizontal
(3) Combine levers if possible
(4) Identify inputs, outputs, and reaction for each gear
(5) Solve lever system for angular speeds and torques respectively
N
S1
N
R1
N
S2
N
R2
S1, S2
P1
R1
R2
P2
R3
P3
S3
N
S3
N
R3
Motor B,
W
B
Engine, WEngine
Motor A, WA
Output, WOutput
Output, WOutput
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01-0129, 2003.
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