Internal CVP shift mechanismTilting ball variator schematic. The rotating input disk drives rotation of the balls, which drive the rotating output disk. The rotation axis of the balls can be changed in order to change the ratio of input to output speed
The NuVinci Continuously Variable Planetary Transmission is a type of roller-based continuously variable transmission (CVT) manufactured and marketed by the American company Fallbrook Technologies Inc. The design saw its initial market application as a bicycle gearing system, first available in December 2006 in the Netherlands and United States. NuVinci CVP technology is also currently under development for other applications, including wind turbines, light electric vehicles, outdoor power equipment, and automotive front-end accessory drives.
Mechanical variators have existed since the 1800s, and they have been used successfully in machinery, particularly the Kopp tilting ball variator. Various attempts have been made to implement them in vehicle transmissions, but commercial success has been elusive.[1]
NuVinci has been rebranded to Enviolo as part of a bankruptcy restructuring. The former model N380 was renamed to TR for trekking. N330 is now the CI variant for city (CO for commercial renting systems is the same, but may offer different color schemes). Stronger models SP (sportive) and CA (cargo) have been added since.
Design
The NuVinci CVT gear system uses a set of rotating and tilting balls positioned between the input and output discs of a transmission. Tilting the balls changes their contact diameters and varies the speed ratio.[3][4] As a result, the NuVinci CVT system offers seamless and continuous transition to any ratio within its range. The gear ratio is shifted by tilting the axles of the spheres in a continuous fashion, to provide different contact radii, which in turn drive input and output discs. The system has multiple "planets" to transfer torque through multiple fluid patches. The spheres are placed in a circular array around a central idler (sun) and contact separate input and output traction discs. This configuration allows input and output to be concentric and compact. The result is the ability to sweep the transmission through the entire ratio range smoothly, while in motion, under load, or stopped.
Two factors allow the NuVinci CVT to provide a continuously variable ratio range in a compact package:
The first is the geometric configuration of the drive, which is based on differing contact ratio of a sphere. Contacting a rotating sphere at two different locations relative to the sphere's rotational axis will provide a “gear ratio”, which can range from underdrive to overdrive depending on the location of the contact points for input and output torque and speed.[3]
The second factor is elastohydrodynamic lubrication (EHL).[5] Transmissions that use EHL to transfer power are known as traction drives. A traction drive transmission operates utilizing a traction fluid that, under normal circumstances, provides lubrication for the drive. When this fluid undergoes high contact pressures under rolling contact between the two very hard elements, the spheres and the discs, the fluid undergoes a near-instantaneous phase transition to an elastic solid. Within this patch of traction the molecules of the fluid stack up and link to form a solid, through which shear force and thus torque can be transferred. Note that the rolling elements are actually not in physical contact.[3]
Comparison to conventional transmissions
The NuVinci CVT system has a small number of parts.[3][4] Most CVTs have lower mechanical efficiency than competitive conventional transmissions.
Since any CVT may allow a power plant, human or motorized, to operate at (or closer to) its speed of optimal efficiency, output torque or output power, the NuVinci CVT may improve a system's overall efficiency or performance compared to a 'conventional' geared transmission, but only if such gain in the efficiency or performance of the power plant exceeds any loss in efficiency or performance that may be introduced by replacing the conventional transmission with the NuVinci CVT. For example, if the NuVinci CVT were only 90% as efficient as the conventional transmission it replaced, the gain in operating efficiency from the power plant would have to be 11% (i.e., 1/0.90 - 1 = 0.11 or 11%) before the overall efficiency improved. The company does not provide any numbers relating to efficiency of their transmissions, and has declined to do so when requested.[6]
The NuVinci CVT further offers the ability to accept multiple inputs while varying speed and ratio, managing torque and providing single or multiple power outlets. By supporting a torque-demand rather than a speed-demand control solution, the NuVinci CVT solves the low-speed acceleration problem inherent in some torque-demand vehicles.[3]
The NuVinci CVT's simple design and low part count make it easily scalable, with tooling that can be used across a wide variety of applications.
Comparison to other CVTs
The NuVinci uses rolling traction to transfer torque, just as do toroidal transmissions. However, unlike toroidal CVTs, it distributes the transmitted torque over several spheres, thus lowering total clamping force required.
This arrangement allows the NuVinci CVT to combine the smooth, continuous power transfer of a CVT with the utility of a conventional planetary gear drive.
As with other traction-type CVTs, transmission of torque through the NuVinci CVT involves some relative sliding between the torque-transmitting contact patches.[7] This is because, for any given contact patch, parts of the ball are going in a slightly different direction and at slightly different speeds than the disc (this phenomenon of traction-type CVTs is referred to as "contact spin"). "The spin velocity (or drill speed) is defined as the difference in the rotational speed of the driving and driven rollers in a direction perpendicular to the contact patch plane. It is caused by the relative difference in surface speeds of both elements across the contact patch and is a major source of power loss in traction drive CVT’s."[7] In all traction-type CVTs, this relative sliding necessarily occurs between surfaces which are under the very high clamping pressures required to ensure torques are transmitted reliably. This relative sliding under high pressures cause transmission losses (inefficiency). Fallbrook Technology refuse to publish any efficiency data for the NuVinci CVT.[8] However, the NuVinci is a variant on the "tilting-ball drive" type of continuously variable transmission (CVT), and the efficiency of "tilting-ball drive" type CVTs is typically in the range of 70% to 89%.[9] However, its geometry does differ significantly from the Kopp type of tilting ball variator in the reference in that the NuVinci has its torque transfer contacts on the outside diameter rather than the inside diameter, which puts the idler (an element that reacts clamping load) in compression rather than tension, and because the idler contact surface is not conformal as in the Kopp design. In general because of the way the CVT is set up, it is more efficient at a 1:1 ratio compared to maximum overdrive or underdrive positions. Independent test results have shown that at 1:1 it is actually more efficient than comparable bicycle internal gear hubs, while at the ratio extremes it is slightly lower.[citation needed]
Torque inputs can be summed or divided, just as in a conventional planetary.[clarification needed] Ratio control is stable, and can be actuated down the center line of the transmission, which again is similar to the proven planetary transmission. In most applications, there is no need for high-pressure hydraulics.
Other advantages shared with similar CVT designs include that the input and output shafts may be either in-line, offset, or in a U configuration (input and output both coaxial and coplanar), making the transmission simple, small, light and easy to package.
Fallbrook claims the transmission control is stable, linear and does not require a major control system development effort.
The NuVinci CVT may be applicable to many different products using a mechanical power transmission. The only application currently commercialized is the internally geared bicycle hub. The NuVinci CVT replaces derailleurs and other internally geared hubs, and provides a gear ratio range of about 350%, similar to that on bicycles with 8- or 9-speed hub gears, or with double chain rings (most racing or fast road bicycles).[10] This range is somewhat less than the 526% range found on bicycles with a Rohloff 14-speed hub gear or most mountain/hybrid/touring bicycles with triple chainrings. By conventional standards, a NuVinci bicycle hub is considered heavy. The second generation model N171B weighs between 3.85 and 3.95 kilograms (8.5 and 8.7lb), including freewheel and mounting hardware.[11] The N360 model announced early September 2010 weighs 2.45kg (5.4lb).[12] That same article points out that "the combined weight of Shimano SLX front and rear derailleurs, shifters, freehub and cassette comes out to about 1.3kg (2.9lb)". However, the NuVinci CVT is targeted towards the less weight-conscious (cruiser-, comfort-, and commuter) segments with the latest product introductions the N330 and N380 also being targeted at city, urban, trekking and eMTB (SUV) bikes. Their biggest advantage is the durability, while shifting under load and continuously, which is especially important with the powerful eBike drive systems.
The company is also developing products based on the CVT unit for automotive applications.
Awards and publicity
The system and associated bicycles have been the recipients of several awards. These include:
Bicycling Magazine's Editor's Choice for Commuter Bike of the Year[13]
Technology of the Year and Bike of the Year in The Netherlands[14]
Bicycles equipped with NuVinci transmissions subsequently won iF Design EUROBIKE awards in 2010 and 2011.[16]
Ellsworth Handcrafted Bicycles, a manufacturer of bicycle frames and wheels designed a bicycle around the Nuvinci CVT. In 2007, The Ellsworth Ride received top honors in Popular Science magazine's annual Best of What's New review of significant new products.[17]
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for this is the law of the lever. Machine components designed to manage forces and movement in this way are called mechanisms. An ideal mechanism transmits power without adding to or subtracting from it. This means the ideal machine does not include a power source, is frictionless, and is constructed from rigid bodies that do not deflect or wear. The performance of a real system relative to this ideal is expressed in terms of efficiency factors that take into account departures from the ideal.
A hub gear, internal-gear hub, internally geared hub or just gear hub is a gear ratio changing system commonly used on bicycles that is implemented with planetary or epicyclic gears. The gears and lubricants are sealed within the shell of the hub gear, in contrast with derailleur gears where the gears and mechanism are exposed to the elements. Changing the gear ratio was traditionally accomplished by a shift lever connected to the hub with a Bowden cable, and twist-grip style shifters have become common.
An automatic transmission is a multi-speed transmission used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions.
An epicyclic gear train consists of two gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates the planet and sun gears mesh so that their pitch circles roll without slip. A point on the pitch circle of the planet gear traces an epicycloid curve. In this simplified case, the sun gear is fixed and the planetary gear(s) roll around the sun gear.
A torque converter is a type of fluid coupling that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the power source to the load. It is usually located between the engine's flexplate and the transmission. The equivalent location in a manual transmission would be the mechanical clutch.
Variomatic is the continuously variable transmission (CVT) of the Dutch car manufacturer DAF, originally developed by Hub van Doorne. It is a stepless, fully-automatic transmission, consisting of a V-shaped drive-belt, and two pulleys, each of two cones, whose effective diameter can be changed so that the "V" belt runs nearer the spindle or nearer the rim, depending on the separation of the cones. These are synchronized so that the belt always remains at the same optimal tension.
A transmission is a mechanical device which uses gears to change the speed or direction of rotation in a machine. Many transmissions have multiple gear ratios, but there are also transmissions that use a single fixed gear ratio.
A continuously variable transmission (CVT) is an automatic transmission that can change through a continuous range of gear ratios. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. The flexibility of a CVT with suitable control may allow the engine to operate at a constant RPM while the vehicle moves at varying speeds.
Hybrid Synergy Drive (HSD), also known as Toyota Hybrid System II, is the brand name of Toyota Motor Corporation for the hybrid car drive train technology used in vehicles with the Toyota and Lexus marques. First introduced on the Prius, the technology is an option on several other Toyota and Lexus vehicles and has been adapted for the electric drive system of the hydrogen-powered Mirai, and for a plug-in hybrid version of the Prius. Previously, Toyota also licensed its HSD technology to Nissan for use in its Nissan Altima Hybrid. Its parts supplier Aisin Seiki Co. offers similar hybrid transmissions to other car companies.
Multitronic is a stepless transmission launched by AUDI AG in late 1999, jointly developed and manufactured by LuK. The capitalization used is multitronic and is a registered trademark of AUDI AG.
Chain drive is a way of transmitting mechanical power from one place to another. It is often used to convey power to the wheels of a vehicle, particularly bicycles and motorcycles. It is also used in a wide variety of machines besides vehicles.
A gear train is a machine element of a mechanical system formed by mounting gears on a frame so the teeth of the gears engage.
Motor drive means a system that includes a motor. An adjustable speed motor drive means a system that includes a motor that has multiple operating speeds. A variable speed motor drive is a system that includes a motor and is continuously variable in speed. If the motor is generating electrical energy rather than using it – this could be called a generator drive but is often still referred to as a motor drive.
Bicycle gearing is the aspect of a bicycle drivetrain that determines the relation between the cadence, the rate at which the rider pedals, and the rate at which the drive wheel turns.
A friction drive or friction engine is a type of transmission that utilises two wheels in the transmission to transfer power from the engine to the driving wheels. The system is naturally a continuously variable transmission; by moving the two disks' positions, the output ratio changes continually. Although once used in early automobiles, today the system is most commonly used on scooters, mainly go-peds, in place of a chain and gear system. It is mechanically identical to a ball-and-disk integrator, but intended to handle higher torque levels.
Hybrid vehicle drivetrains transmit power to the driving wheels for hybrid vehicles. A hybrid vehicle has multiple forms of motive power.
The Turboglide is a Chevrolet constant torque, continuously variable automatic transmission first offered as an option on Chevrolet V8 passenger cars for 1957. It consisted of a turbine-driven planetary gearbox with a 'switch pitch' dual-pitch torque converter stator. It had a die-cast aluminum transmission case, like Packard's Ultramatic of 1956. Turboglide cost about $50 more than the Powerglide 2-speed automatic. It was available in all V8-powered 1957-1961 Chevrolet models except the Corvette. General Motors produced 646,000 of these transmissions during its production.
The Beier variable-ratio gear or Beier variator is a mechanical drive offering a continuously variable gear ratio between input and output.
Bicycle drivetrain systems are used to transmit power on bicycles, tricycles, quadracycles, unicycles, or other human-powered vehicles from the riders to the drive wheels. Most also include some type of a mechanism to convert speed and torque via gear ratios.
A motorcycle transmission is a transmission created specifically for motorcycle applications. They may also be found in use on other light vehicles such as motor tricycles and quadbikes, go-karts, offroad buggies, auto rickshaws, mowers, and other utility vehicles, microcars, and even some superlight racing cars.
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