Traction Drive Overview - Fallbrook Tech Drive... · Traction Drive Overview ... information is accurate but does not warrant its accuracy and is not ... the engine speed must increase

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Traction Drive Overview Fallbrook Technologies Inc., the developer of the NuVinci® continuously variable planetary traction drive, provides this information to readers of the textbook Fundamentals of Machine Component Design 6th Edition by Robert C. Juvinall, Kurt M. Marshek, published by Wiley and available from major booksellers and to others interested in learning about traction drives. The material presented here is not meant as an exhaustive source. Fallbrook has attempted to ensure that the information is accurate but does not warrant its accuracy and is not responsible for any errors or omissions. For more information on NuVinci technology, please go to www.fallbrooktech.com.

Traction Drive Overview

Traction drive technology has been steadily advancing since C.W. Hunt patented a toroidal traction drive in 1877, including major development efforts by General Motors in the 1920s through 1940s and NASA in the 1970s and 1980s. While Kopp variators have been used in industrial applications since the 1970s, introduction of specially developed traction fluids in the latter half of the 20th century have enabled traction drive development to accelerate and become more practical for applications such as automotive transmissions.

Figure1 – Traction drives transmit forces between input and output rolling surfaces through shearing of a thin fluid film.

Traction drives transmit power through a thin fluid film existing between the rolling contact interface between bodies. (see Figure 1). This fluid film transfers the force between bodies while also separating the bodies to prevent surface wear. A special fluid, commonly known as traction fluid, is used to increase the coefficient of traction between the bodies and increase the ability to transmit power. The forces are transmitted through this fluid through the shearing of the fluid. The more resistant the fluid is to shear forces, the more force that is transmitted through the interface.


The force between the rolling elements is defined by the equation Fx=μFz (see Figure 2). This equation is true for both friction (surfaces sliding relative to each other) or traction (surfaces not sliding relative to each other). The term mu (μ) can be called the coefficient of friction or the coefficient of traction. If the driving force exceeds the maximum frictional force generated at the interface, sliding will occur, resisted by the frictional force. However, if the driving force is less than the maximum frictional force, then the bodies will not slide relative to one another, as they remain in traction. This can occur for both dry and wet surfaces. Automobile tires rolling on a roadway is a commonly understood example of traction.

Figure 2 – The traction force is a function of the normal force and the coefficient of traction at the interface of the rolling elements


Fluid / Contact Patch Traction Fluid

Traction fluid is an elastohydrodynamic fluid specifically designed for use in traction drives. Elastrohydrodynamic lubrication (EHL) is a regime of lubrication that occurs in rolling contact areas. In this regime, shear resistance of the fluid is several orders of magnitude greater than classical hydrodynamic theory would predict. In EHL regimes, the fluid in the contact patch is under high pressure (>1 GPa) and behaves like an elastic solid over a short duration of time (≤1msec). This regime occurs in ball bearings and between gear teeth, as well as in traction drives.

Early traction drives relied on hydrodamic lubrication, which limited the amount of force that could be transferred across the contact patch. In hydrodynamic lubrication, normal forces pressing the rolling elements together are relatively small, the fluid molecules are disordered and tangled, and the force transferred across the fluid film is also small. In EHL, normal forces are high, the fluid gap is orders of magnitude smaller, and the fluid molecules assume a stacked arrangement that makes the fluid behave like an elastic solid capable of transferring large amounts of force through the contact patch. The development of elastohydrodynamic traction fluids allows current traction drives to transfer large amounts of torque.

Traction oil consists of synthetic oil with additives that significantly increase the coefficient of traction under high pressure. In addition to transferring power through traction, the fluid also lubricates surfaces, bearings and gears and transfers heat created by power losses. The traction coefficient of this fluid is non-linear and is dependent on a number of factors, including Hertz pressure, fluid temperature, fluid properties, entrainment velocity, and relative motion between the rolling bodies, or creep. A sample traction curve is shown in Figure 3.

The traction coefficient dependence on temperature is of special significance for automotive applications. Traction oils have historically had high viscosity at -40C, making them difficult to pump to the transmission. In addition, the traction performance falls off at elevated temperatures common to the engine compartment. Recent developments of traction fluids have improved the viscosity at -40C while also decreasing the sensitivity of traction performance to elevated temperatures.


Figure 3 – Typical traction curve for traction oil. Mu is a function of % creep, or the difference in speed between the driving element and the driven element

Contact Patch

The contact patch is the small area of contact between the elements of the traction drive. Typically, the rolling elements are ball-on-cone (forming an elliptical contact patch) or cone-on-cone (forming a cylindrical contact patch). The size of the patch is determined by the geometry of the rolling elements combined with the contact pressure, which causes elastic deformation of the two elements. The contact pressure is distributed through the contact patch according to Hertz theory (see Figure 4).


Figure 4– Contact force is distributed through the contact patch according to Hertz theory. Ball on cone geometry shown.

The shear force created by the driving member on the contact patch is resolved into traction force as well as losses. The traction force consists of the sum of the forces in the rolling direction. These are the forces that are transferred from the driving member to the driven member. Losses can be divided into three primary sources (see Figure 5).

• Slip: speed variation between rolling elements • Side Slip: relative motion orthogonal to the rolling direction • Spin: relative speed variation within the contact

These losses occur as the fluid film is sheared. This shearing can occur in the direction of rolling (slip) as well as in non-rolling directions. Some slip is normal in the operation of a traction drive and is related to how much torque is being transferred. The side slip and spin terms are related to the geometry of the contact patch as well as the different axis of rotation for the two rolling bodies.


The energy lost through the shearing of the fluid is converted into heat in the fluid. Some of the heat transfers into the rolling elements, and the remainder of the heat is retained by the fluid and exits the contact patch.

Because the traction performance of the fluid is degraded with elevated temperature, the fluid is typically sent through a heat exchanger to remove some of this heat prior to resupply to the drive.

Figure 5 – Contact patch traction losses. Direction of rolling is along the X-axis


Traction Drive Applications Traction drives can be used for both fixed ratio drives as well as variable ratio drives. Most frequently, they are used as variable ratio drives, or continuously variable transmissions (CVTs). This application utilizes one of the primary advantages of traction drives, their ability to provide seamless ratio changes. Because they avoid the stepped ratio changes inherent in a geared transmission, they allow more control of input or output speeds. Depending on the needs of the application, this can allow the driving source to operate more efficiently, or it can provide more refined output speed control. Some of the ways that CVTs can be controlled are described below.

Constant Input Speed, Variable Output Speed A CVT can vary the output speed of a device while accepting a constant input speed. For example, in an automotive transmission, a CVT can allow the engine to run at its most efficient operating speed, while the CVT ratio can be varied to generate the desired vehicle speed. In a traditional geared transmission, the engine speed must increase proportionally to the change in wheel speed when between gear changes. The same is true with an electric motor driving a variable speed component. For example, in an application requiring a variable pump, the electric motor driving the pump can be kept at its most efficient speed while the CVT varies the speed of the pump.

Variable Input Speed, Constant Output Speed A CVT can also accept a variable input speed and output a constant speed. Some automotive components operate at peak performance or efficiency at a constant speed, for instance an alternator. However, these devices are typically driven at a fixed ratio to engine speed, so their speed varies from idling to peak engines speeds. A CVT can increase the accessory input speed when the engine speed is low and additional performance is required, and can decrease the accessory input speed when performance is not required to offer better fuel efficiency.

Refined Output Speed Control The stepless ratio changes provided by a traction drive can offer very fine control of speed. This can be a significant advantage in certain applications. For example, a forklift requires a very fine level of speed control to smoothly and accurately position the vehicle in relation to the load. A CVT can aid in allowing the driver to smoothly decelerate into the proper position without the need for braking.

Disadvantages of Traction Drives Major disadvantages that have kept traction drives from being more widely used include the cost of manufacturing, the sensitivity to temperature, and the high clamp loads required. Manufacturing costs are driven by the need for high quality bearing steels and the need for precision surfaces for the rolling contact elements. New traction drive designs and technology advances are working to overcome these issues. These advances have led to a renewed interest in this technology.


Traction Drive Designs Toroidal and Half-Toroidal Designs

The toroidal drive was invented by C.W. Hunt in 1877 (see Figure 6). The toroidal transmission has been used periodically since then, and is still in development today. General Motors and Nissan are two companies that have contributed to the development of the toroidal transmission.

Toroidal drives transfer power from input disk to power rollers and then to an output disk. By tilting the roller axis, the point of contact to the input and output disks can be varied. This causes the circumference of the contact track to be different from input to output, thereby changing the drive ratio.

Figure 6– US Patent 197,472, showing the Hunt toroidal transmission. The tilting transfer wheel (E) tilts to change the rolling contact radii with toroids (B and D). This creates a stepless means of varying the ratio between input and output speed. [Hunt, Charles W., “Counter-shaft for driving machinery.” Patent 197,472. 27 November 1877].


There are two main types of toroidal transmissions, the half toroidal and the full toroidal (see Figure 7). The two differ in their geometry, with each version having advantages and disadvantages. A major difference is that the half toroidal unit requires thrust bearings on the power rollers, thus making the shifting mechanism more complex and space-consuming. The full toroidal avoids power roller thrust bearings by virtue of its geometry, but suffers from reduced torque overload margins.

Toroidal transmissions offer the benefits of a continuously variable ratio range, high torque capacity, better efficiency than competing non-traction drive continuously variable technologies (such as belt CVTs), and fast shifting. The drawbacks of this design include the need for a high clamp force and the cost of manufacturing the toroidal components.

Figure 7– Full and half toroidal geometries


Kopp Variator

The Kopp Variator was invented in the 1940s by Jean Kopp (see A). This was the first commercial introduction of a cone and roller traction drive, and is still in use for controlling industrial machinery. This design is one of the most commercially successful traction drives of the 20th century.

A.

B.

Figure 8– (A) The Kopp variator has been widely used in industrial application (Kopp, Jean, “Stepless variable change-speed gear with roller bodies.” Patent 2,469,653. 10 May 1949.) (B) The Kopp variator uses a tilting planet axis to change drive ratio.


The Kopp Variator uses a set of planets that are rotating on an axle. The planets contact the outer diameter of a pair of cones attached to the center shaft. One of these cones is attached to the input, and the other to the output. As the input cone is rotated, it drives the planet, which drives the output cone. By tilting the planet axis, the ratio between the input and output is changed due to the difference in contact radii (see Figure 8B).

The Kopp Variator provides a relatively low cost variator with a 9:1 ratio range and stable ratio under static conditions. However, its drawbacks are that it has low torque density and cannot shift under load.

Milner Continuously Variable Transmission

The Milner CVT is a traction drive that utilizes three or more balls (planets) as the rolling elements. The planets are in four-point contact with two inner races and two outer races. The power is transmitted to the planets through the center shaft and one of the inner races which is fixed to the shaft. The planets orbit the inner race and transmit power to an output carrier that includes follower rollers between the planets. One of the outer races is fixed to ground. The other outer race is moved axially to force the planets in or allow them to move out. The second inner race moves in reaction to the movement of the planets due to the actuation of the outer control race (see Figure 9). These four races keep the planet fully supported. As the planets move radially in or out with the outer control race, the location of four contact points on the planet change, thereby changing the ratio between input and output speed.

Figure 9– Milner design uses 4-point contact on a planet to enable ratio changes.


NuVinci® CVP Technology NuVinci Continuously Variable Planetary (CVP) The NuVinci continuously variable planetary (CVP) is based on a set of rotating, tilting planets fitted between two rings. Figure 10 illustrates the basic components of the NuVinci CVP technology.

• an input disc, or ring, driven by the power source. • an output disc, or ring, connected to the CVP output • a set of balls (planets), each rotating on its own axle and fitted between the input

ring, output ring and a central sun to help maintain the planets in position

Torque from an engine, motor or other input source is transmitted from the input ring to the planets through a thin layer of traction fluid. The torque is then transmitted from the planets to the output ring via another thin layer of traction fluid. The input ring and output ring are “clamped” onto the planets tightly so that the requisite amount of clamping force is provided for the amount of torque being transmitted. However, as indicated above, torque is transmitted through the traction fluid, which prevents destructive metal to metal contact between the planets and ring while providing traction for the planets and rings and lubrication for bearings and other components.

The speed of the output ring compared to the speed of the input ring, or speed ratio, is controlled by the angle of the planet axles relative to the axis of the transmission. Tilting the planet axles shifts the transmission from low to high (as shown left to right), or from high to low, or to any ratio in between. The number of planets used depends on several factors including torque and speed requirements, operational requirements and space considerations. This design is scalable from small application such as a bicycle transmission up to the size of a wind turbine or commercial vehicle transmission. The animation in Figure 11, provides a visualization of how this design functions.

Figure 10– NuVinci CVP uses planets to transfer torque between an input ring and output ring. Tilting of the planet axis changes the ratio by varying the contact radii (click image to enlarge).


Figure 11– Animation of NuVinci CVP.

To see the animation, go to https://youtu.be/kN8CCY1vFC8

This design is analogous to a planetary gear set. Because this design is a continuously variable planetary drive, power can be input or output on either ring, the sun, or the carrier, just as with a planetary gear train. This allows the CVP to be used to sum power from multiple sources (such as an engine and motor in a hybrid vehicle) or split power to drive multiple devices (such as the drive wheels and a generator for battery charging). This is done through the selection of which elements are driven, fixed, or free. Compared to a toroidal drive, the NuVinci CVP provides the benefit of a simpler geometry, reducing cost of manufacturing, and lower clamp forces.

Fallbrook Technologies is the inventor of the NuVinci CVP technology. The NuVinci CVP is currently commercially available as a bicycle transmission, providing a continuously variable ratio solution that replaces the traditional fixed ratio derailleurs. The NuVinci design is also currently being developed for use as a primary transmission in automobiles (by Dana Incorporated), commercial vehicles (by Allison Transmission) and light utility and recreational vehicles (by TEAM Industries, Inc.).

https://youtu.be/kN8CCY1vFC8

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Traction Drive Overview - Fallbrook Tech Drive... · Traction Drive Overview ... information is accurate but does not warrant its accuracy and is not ... the engine speed must increase