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.  The information may also be of interest 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.

Figure 1 – Traction drives transmit forces between input and output rolling surfaces through shearing of a thin fluid film (click image to enlarge).

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 (click image to enlarge).

 

Additional Traction Drive information is available by clicking  "Next Section" below or by clicking the topic links in the right-hand column.

All of the traction drive material is also available in a single document (PDF) by clicking here).

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