Journal of Chongqing University-Eng. Ed.
Vol. 2 No. 1
June 2003
An overview on research developments of toroidal
continuously variable transmissions*
Nabil Abdulla Attia1,2, QIN Datong1, LI Huaying3
1
The State Key Laboratory for Mechanical Transmission, Chongqing University, Chongqing 400044, P.R. China
2
Faculty of Engineering, Helwan University, Mataria, Egypt
3
College of Agricultural Engineering, Southwest Agricultural University, Chongqing 400716, P.R. China
Received 6 January 2003; revised 15 April 2003
Abstract: As environmental protection agencies enact new regulations for automotive fuel economy and emission, the toroidal
continuously variable transmissions (CVTs) keep on contribute to the advent of system technologies for better fuel consumption
of automobiles with internal combustion engines (ICE). Toroidal CVTs use infinitely adjustable drive ratios instead of stepped
gears to achieve optimal performance. Toroidal CVTs are one of the earliest patents to the automotive world but their torque
capacities and reliability have limitations in the past. New developments and implementations in the control strategies, and
several key technologies have led to development of more robust toroidal CVTs, which enables more extensive automotive
application of toroidal CTVs. This paper concerns with the current development, upcoming and progress set in the context of the
past development and the traditional problems associated with toroidal CVTs.
Key words: continuous variable transmission (CVT); toroidal; history; development; transmission; technologies
1. Introduction
Over the past century, the advancements of research
and development in fuel economy and emissions have
been retarded, in spite of implementation by depleting
a lot of money. Thus, the ideal interim solution is to
further optimize the overall efficiency of internal
combustion engine (ICE) vehicles. One potential
solution to treat the fuel economy dilemma is the
toroidal traction drive CVT, an old concept that has
only recently become the hope to automotive makers.
Toroidal CVTs can potentially reduce the fuel
consumption because the engine can run under most
efficiently attributed to its smooth power delivery
without any shift shock and robust acceleration as the
power is delivered continuously with little loss of
driving force during its ratio changes compared with
traditional transmissions.
The fundamental theory behind toroidal CVTs has
inevitable potential to achieve the fuel regulations and
booming success in commercialization. The Nissan
Motors’ Cedric and Gloria models put in the Japanese
market in November 1999 have encouraged
manufactures to spend a lot in further developments in
this field. In spite of CVTs usage in automobiles for
decades, limited torque capabilities and questionable
reliability have inhibited their growth. Today, ongoing
toroidal CVTs research has led to more powerful
transmissions and wider automotive application, which
has increased the demand for further development and
ultimately given a rigid foundation for toroidal CVTs
in the world of automotive infrastructure [1].
2. Configurations of the toroidal CVTs
Toroidal CVTs consists of input and output toroidal
disks with tilting rollers inside the toroidal cavity.
Speed ratio can be accomplished by steering the power
roller on the disks. Such drives can be configured based
on its geometry into full toroidal (on-centre type) and
half toroidal (off–center type), as shown in Fig. 1 [2-3].
(a)
(b)
Output disk
Input disk
Power roller
Input disk
Power roller
O′
Output disk
θ
O′
O
θ
Axis of
rotation I
O
E
Fig. 1. Schematic geometry of toroidal CVT: (a) full toroidal
CVT; and (b) half toroidal CVT
2.1 Full toroidal CVTs (on-centre type)
Nabil Abdulla Attia: Male; Born in 1968 in Egypt; PhD candidate; Research field: automotive transmission.
*
Funded by the Ford-NSFC Foundation of China (No. 50122151).
Vol. 2 No. 1
Journal of Chongqing University-Eng. Ed.
2.1.1 Structure
As shown in Fig. 1 (a), in the geometry of a full
toroidal CVT, the straight line OO′ passes between the
contact points of the power roller while input and
output disks pass through the center of the cavity circle
that forms the input and output disks (on-centre type).
As a result, the reaction force to the contact force
generated for power transmission between the power
roller and both input and output disks does not apply to
sustaining the bearing of the power roller. In addition,
each of the tangent lines to the contact points O and
O′ are parallel and has no intersection point.
2.1.2 Characteristics
The full toroidal CVT is characterized by a contact
angle of 2θ 0 =180o and that the tilting rotation center of
the power rollers coincides with the center of the cavity.
Spin occurs significantly as much as seven times more
than that in a half toroidal CVT, but a large capacity
thrust bearing is not required since no thrust force is
generated. Thus, the power roller supporting part can
be made of a simple structure. Consequently, it is much
easier to arrange several power rollers in the space
between the set of input and output disks of a full
toroidal. Also, the full toroidal uses a hydraulic piston
to generate the loading force.
83
bearings are required to support the thrust force of the
power roller. Also, the half toroidal uses a loading cam
to generate the loading force.
3. Background and history of toroidal CVTs.
The toroidal type CVTs have a long historical
background, which can be classified into the following
development periods
3.1 Early developments
One of the earliest known examples of the principle
of the toroidal CVT was patented by Charles W. Hunt
in Richmond. As shown in Fig. 2, an oscillating wheel
(E) is placed between disks B and D and by varying the
C.W.HUNT.
angle ofFig.
this2. wheel,
is toroidal
changed
onbythe
spherical
The
firstspeed
patent
of
CVT
Hunt
Counter
Shaft For
Driving Machinery
No.19,472.
surfaces of disks
B and D [4]. Patented Nov.27,1877
2.2 Half toroidal CVTs (off-center type)
2.2.1 Structure
As shown in Fig.1 (b), in a half toroidal CVT, the
straight line OO′ does not pass through the center of
the cavity circle that forms the input and output disks
(off-center type), and the tangent lines to the contact
points O and O′ have an intersection point E . When
this point is on the rotation axis, the spin of the contact
points becomes zero. In general, the spin in the half
toroidal CVT is smaller than the full toroidal CVT
because the locus of a intersection point is nearby the
rotation axis I in the entire range of the transmission.
2.2.2 Characteristics
A half toroidal CVT is characterized by a contact
cone angle with a range of 2θ 0 = 100° to 140° at the
power roller transmitting power between the input and
output disks and that the tilting rotation center of the
power rollers for transmitting power between the input
and output disks is off the center of cavity. The cone
angle is given to the half toroidal CVT to minimize
spin loss in the traction power transmission zone, but
this leads to the generation of a thrust force, so that
Fig. 2. The first patent of toroidal CVT by Hunt
Because of their simplicity, full toroidal CVTs was
first used in automobiles during the early 1900s, and
automobiles equipped with a full toroidal CVT were
called “ friction drive cars” which featured with metalto-metal contact resulting in low durability. It was out
of use after 1915 or around because it did not meet
much commercial success [2].
Richard Erban in Austria installed a drive for a
traction drive with a lubricant between the rolling
elements in automobile 40 HP in 1924 and received his
patent in 1926; Peugeot’s automobiles acquired the
license of Erban’s drives for use in Peugeot’s 25 HP.
Frank Hayes received his patent for a dual toroidal
traction drive, which was installed in a bus in 1929.
Approximately 600 cars fitted with a hydraulic
84
Nabil Abdulla Attia, et al. / An overview on research developments of toroidal…
automatic control via Austain of England in 1928 were
sold. GM’s “ New Departure Division” modified the
drive, which was named Transitorq and was powered
by constant speed electric motor. More than 2500 units
were sold to machine tool manufacturers during 19351937. In 1937, Perbury in England developed a dual
toroidal drive with rollers steered for ratio change to
balance engine speed with load. Charles E. Kraus
developed a 90 HP toroidal traction drive with the ratio
responding to the torque load independent of speed.
His patent was received in 1958, and this unit was
installed in a Rambler in 1959 [5].
Wright Aeronautical with support from American
motors began developing a half toroidal automotive
transmission in 1943. In June, 1959, he devised the half
toroidal CVT. Wright acquired a patent from Charles E.
Kraus in 1959 and developed the toroidal CVT in 1963.
In 1973, engineers at Tractor in Austain, Texes, used a
Ford Pinto 85-HP as the test vehicle to check out the
control and operation of the traction drive. They used a
single toroidal element with cone rollers and a
hydrostatic bearing for necessary roller contact
pressure. The transmission ratio of the CVT was from
2.65:1 to 0.6:1, their results implied that the CVT was
capable of realizing good acceleration, high fuel
efficiency and cleaner vehicle emissions [1,2].
3.2 Modern developments
In addition to the continued efforts of Charles E.
Kraus to develop a successful automotive application
for the toroidal drive CVT, others also conscientiously
pursued the development of the toroidal drive for
automotive use from 1980s to 1990s. British Leyland is
working with a modified Perbury roller toroidal drive
mated with a planetary box [5].
NSK started studying the half toroidal CVT in 1978,
conducted tests of its prototypes, and completed a halftoroidal CVT. The powertoros unit using unique
materials and technologies were accumulated through
the development of rolling bearings [2]. NSK et al.
started joint research of a new concept of half toroidal
CVT system and expected to apply this system to
actual automotive uses. A prototype of the new toroidal
CVT was manufactured in 1981 and was installed in a
SUBARU 1600cc FF car. It was a half toroidal and
single cavity system with drive ratio from 2.0:1 to 0.5:1
[1]. The development history of NSK prototypes is
listed in Ref [2].
Vol. 2 No. 1
3.3 New developments
In recent years, improvements in toroidal CVT have
been introduced in automobile engineering to develop
high system power. A noted example is the Torotrak
transmission as shown in Fig.3, which was designed by
Torotrak development Ltd in 1991 [6], and was labeled
a double sided (or double cavity), full toroidal drive.
Chain drive sprocket
Input disk
Power roller
End load
hydraulic chamber
Input
Input disk
Output disks
Fig. 3. Schematic Torotrak full toroidal variator
The first installation was a Rover 820 Si, 2.0-liter, 4cylinder, 16-valve, multi point fuel injected engine
producing a maximum of 107 kW (140 HP) at
6000 r/min, built in a front wheel drive passenger car.
It had direct acting double hydraulic cylinders to cope
with torques in input and output directions instead of
mechanical roller control linkages in the aircraft
alternator drive, which carried unidirectional torque.
The analysis of contact losses was reported by
theoretical techniques [7].
4. Inherent advantages and benefits
Certainly, shift shock transmission is familiar to all
drivers. By contrast, toroidal CVTs are perfectly smooth
at most operating conditions such that the driver or
passenger feels steady state acceleration. Moreover,
toroidal CVTs are a technology for achieving a friendly
environment, fuel economy, improved efficiency and
performance. The overall efficiency of traction drive is
quite flat, over most of the torque-loading regime,
providing quick ratio response and high efficiency in
the practical speed range. Nissan took a dramatic step
with its Extroid CVT in the Gloria and Cedric luxury
sedans Cars with a half toroidal double cavity variator
from NSK in the Japanese market in November 1999 as
shown in Fig. 4 [2].
The transmission of power for a half toroidal CVT is
required to have an efficiency of 90 % to 92 % [3]. To
meet the demand for efficient CVTs, a power split
Journal of Chongqing University-Eng. Ed.
Vol. 2 No. 1
system with a single cavity has confirmed a power
transmission efficiency of 96 % [8]. The theoretical
efficiency of the power split double cavity half toroidal
CVT is 97 % at a high speed [9]. Ongoing research and
development will inevitably expand the efficiency and
capacity of toroidal CVT to a much broader range of
engines and automobiles.
Double cavity variator
Front
Rear
85
traction fluids were confirmed [10]. The developed
superior traction fluid of higher traction coefficient is
expected for use at a higher temperature
5.2 Contact force
A larger contact force enables a larger traction force.
The contact force was analyzed and the improvement
of loading cam was done to minimize the frictional
losses [11]. Increasing the contact force influences
rolling fatigue life of rolling elements. Longer fatigue
life of rolling materials was confirmed by using case
hardened steel with extra-pure specifications and
carbonitriding heat treatment [12]. The development of
better materials is expected to bear high contact
pressure at rolling contact points.
5.3 Number of power roller
Preload system
Output disks
Input disks
Fig. 4. Gloria and Cedric Cars with a half toroidal double cavity
variator from NSK
5. Key technologies for development
To overcome the limited torque capacity between
traction drive rollers of toroidal CVTs, the following
technologies can be used.
5.1 Traction fluid
Power transmission is accomplished by special
traction oil that displays high shear resistance under a
state of high contact pressure based on elastohydrodynamic analysis of oil film behavior. The
thickness of the fluid film can be calculated based on
the theory of Hamrock-Dowson. Johnson et al.
formulated the mechanism of traction generation using
the elastic-plastic model of oil that classifies the elastohydrodynamic lubrication (EHL) into three kinds, slip,
sideslip and spin. The traction fluid under a high
pressure can be considered as a non-linear Maxwell
Rheology model based on the Eyring theory. Tanka
used those theories and theoretically analyzed the
transmission efficiency of the traction drive power
system. The traction fluid (Santotrac) was developed
successfully by Monsanto as synthetic cycloaliphatic
hydrocarbons, which is of a higher traction coefficient
than naphthenic petroleum, and the maximum traction
coefficient is 0.095. In 1987, Hata and Aoyama of
Idemitsu Kosan measured the traction coefficient of
various oils under 40 ºC to 140 ºC [1,2]. In 1991, new
Increasing the number of power rollers leads to
compact size and higher power transmission. The new
concept for a three-roller system is shown in Fig. 5.
The optimum number for power rollers to get high
power density was presented in Ref [13]. A roller
suspension to achieve quick and stable control was
presented in Ref [14], which leads to a more compact
and low cost variator.
Power roller 3
Power roller 1
Power roller 2
Fig. 5. Three roller half toroidal CVT
5.4 Rolling radius
It is proper that the larger radius transmits the higher
torque. But for automotive transmissions, there is a size
limitation, so it is impossible to make the rolling radius
larger arbitrarily.
5.5 Ratio control
The Ratio changes of a toroidal CVT can be
achieved smoothly by using side-slip, which is
generated by a small offset at the rolling contact point.
The first computer ratio control of a half toroidal CVT
was applied in 1983 [1]. The stability studies were
performed with single or dual feedbacks, which
Nabil Abdulla Attia, et al. / An overview on research developments of toroidal…
86
founded that a control strategy with dual feedbacks on
both the swing and translation of the variator CVT was
effective and stable [15]. In November 1999, Nissan
developed a control system as shown in Fig. 6 for
accurately synchronizing the gear ratios for the power
rollers of a dual-cavity, Gloria and Cedric cars [16].
capacity, and level of reliability that meets requirements
of automotive design so that the toroidal plays a great
role in the field of power transmission CVT.
References
1
2
Turnnion
Power roller
Disc
3
Small offset
4
Hydraulic
servo piston
5
6
Precision
cam
7
Ratio change
control valve
Stepper motor
8
Line pressure
CVT controller
9
Fig. 6. Ratio change control system in Gloria and Cedric cars
The performance simulation of vehicles equipped
with a half toroidal single cavity traction drive CVT
was presented by a computer model [17]. In addition, a
computer model was used to analyze and simulate the
traction drive CVT control system [18]. The
development control system to achieve quick, smooth,
stable ratios in all operating condition is expected to
improve the vehicle performance. The basis for the
design of a high power system is to optimize these
aspects of technologies efficiently.
6. Conclusion
Currently, the use of toroidal CVTs in low power
systems and high power systems are feasible for
automotive applications and other industrial applications. The benefits and applications increase based on
research and development of the key technologies such
as traction fluid, number of power rollers, material for
bearing elements and ratio control system. The
development situation of the toroidal CVTs gives a
bright prospect of attaining a specific size, torque
Vol. 2 No. 1
10
11
12
13
14
15
16
17
18
Machida H. Traction drive CVT up to date. In: Proc.
CVT99. Eidnhoven: [s.n.], 1999. 71-76.
Machida H, Murakami Y. Developments of the
powertoros unit half toroidal CVT. NSK Technical J,
2000 (9): 15-26.
Imanishi T, Machida H. Development of powertoros unit
half-toroidal CVT (2): comparison between half-toroidal
and full-toroidal CVTs. NSK Technical J, 2001 (10): 1-8.
Hunt CW. Counter shafts for driving machinery. USP
197,472, Nov. 27, 1877
Frederick W III, Heilich SEE. Traction drives: selection
and application. New York: Marcel Dekker Inc, 1983.
Thomas GF, Greenwood CJ. The design and development
of an experimental traction drive CVT for a 2.0-liter FWD
passenger car. SAE, 1991 (910408): 535-544.
Patterson M. The full-toroidal variator in theory and in
practice. SAE, 1996 (9636394): 95-100.
Miyata S, Machida H. Development of the half-toroidal
CVT powertoros unit (3): Development of the power-split
system. NSK Technical J, 2001 (11): 11-18.
Miyata S, Liu D, Machida H. Study of the control
mechanism of a half-toroidal CVT during load
transmission. In: MPT’2001. Fukuoka: [s.n.], 2001.844848.
Hata H, Tsubouchi T, Shirasawa H. New traction fluids
for automotive use. SAE, 1996 (9636484): 147-150.
Yamamoto T, Osidari T, Nakano M. Improvement of
loading cam performance in a toroidal CVT. JSAE
Review, 2002, 23 (4): 481- 487.
Machida H, Abe T. Fatigue life analysis of a traction drive
CVT. SAE, 1996 (9636402): 101-106.
Imanish T, Machida H, Tanaka H. Development of high
power-density double cavity half-toroidal CVT. In:
MPT’2001. Fukuoka: [s.n.], 2001. 860-863.
Tenberge P, Mockel J. Toroidal CVT with compact roller
suspension. VDI-BERICHE, 2002 (1709): 623-637.
Tenberge P. Ratio stability of toroidal traction drives. In:
Proc. CVT99. Eidnhoven: [s.n.], 1999. 77-84.
Nissan Motor Co. Ltd. Extroid CVT for application to
rear- wheel drive cars powered by large engines- Nissan’s
CVT Technologies. Technical note, Oct. 1999.
Zhang Y, Tobler W, Zhang Y, et al. Performance
simulation of vehicles equipped with traction drive CVTs.
In: ICMT’2001. Chongqing: [s.n.], 2001. 496-502.
Zhang Y, Tobler W, Zhang Y, et al. Ratio response
stability of traction drive CVT control. In: ICMT’2001.
Chongqing: [s.n.], 2001. 503-507.