Continuously variable transmission

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Continuously variable transmission

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The continuously variable transmission ( CVT ) is a transmission in which the ratio of the rotational speeds of two shafts, as the input shaft and output shaft of a vehicle or other machine, can be varied continuously within a given range, providing an infinite number of possible ratios.

The continuously variable transmission should not be confused with the power split transmission (PST), as used in the Toyota Prius and other hybrid vehicles that use two or more inputs with one output, despite some similarities in their function.

A CVT need not be automatic, nor include zero or reverse output. Such features may be adapted to CVTs in certain specific applications.

Other mechanical transmissions only allow a few different discrete gear ratios to be selected, but the continuously variable transmission essentially has an infinite number of ratios available within a finite range, so it enables the relationship between the speed of a vehicle, engine, and the driven speed of the wheels to be selected within a continuous range. This can provide better fuel economy than other transmissions by enabling the engine to run at its most efficient speeds within a narrow range.

CVTs have been refined over the years and are much improved from their origins.

Types

Infinitely Variable Transmission (IVT)

A specific type of CVT is the infinitely variable transmission ( IVT ), which has an infinite range of input/output ratios in addition to its infinite number of possible ratios; this qualification for the IVT implies that its range of ratios includes a zero output/input ratio that can be continuously approached from a defined higher ratio. A zero output implies an infinite input, which can be continuously approached from a given finite input value with an IVT. Low gears are a reference to low ratios of output/input which have high input/output ratios that are taken to the extreme with IVTs, resulting in a neutral, or non-driving low gear limit.

Most continuously variable transmissions are not infinitely variable.

Most (if not all) IVTs result from the combination of a CVT with an epicyclic gear system (which is also known as a planetary gear system) termed power split device that facilitates the subtraction of one speed from another speed within the set of input and planetary gear rotations. This subtraction only needs to result in a continuous range of values that includes a zero output; the maximum output/input ratio can be arbitrarily chosen from infinite practical possibilities through selection of extraneous input or output gear, pulley or sprocket sizes without affecting the zero output or the continuity of the whole system. Importantly, the IVT is distinguished as being infinite in its ratio of high gear to low gear within its range; high gear is infinite times higher than low gear.

The IVT is always engaged, even during its zero output adjustment.

The term Infinitely Variable Transmission does not imply reverse direction, disengagement, automatic operation, or any other quality except ratio selectability within a continuous range of input/output ratios from a defined minimum to an undefined, infinite maximum. This means continuous range from a defined output/input to zero output/input ratio.

Additionally, IVT offers better efficiency when compared to other CVTs as most of the power flows through planetary gear and not the controlling CVT, and torque transmission capability is increased. There’s also possibility to stage power splits for further increase in efficiency, torque transmission capability and expanded gear ratios range.

Ratcheting CVT

The Ratcheting CVT is a transmission that relies on static friction and is based on a set of elements that successively become engaged and then disengaged between the driving system and the driven system, often using oscillating or indexing motion in conjunction with one-way clutches or ratchets that rectify and sum only forward motion. The transmission ratio is adjusted by changing linkage geometry within the oscillating elements, so that the summed maximum linkage speed is adjusted, even when the average linkage speed remains constant. Power is transferred from input to output only when the clutch or ratchet is engaged, and therefore when it is locked into a static friction mode where the driving driven rotating surfaces momentarily rotate together without slippage.

These CVTs can transfer substantial torque because their static friction actually increases relative to torque throughput, so slippage is impossible in properly designed systems. Efficiency is generally high because most of the dynamic friction is caused by very slight transitional clutch speed changes. The drawback to ratcheting CVTs is vibration caused by the successive transition in speed required to accelerate the element which must supplant the previously operating decelerating, power transmitting element.

An Infinitely Variable Transmission (IVT) that is based on a Ratcheting CVT and subtraction of one speed from another will greatly amplify the vibration as the IVT output/input ratio approaches zero.

Ratcheting CVTs are distinguished from Variable Diameter Pulleys (VDPs) and Roller-based CVTs by being static friction-based devices, as opposed to being dynamic friction-based devices that waste significant energy through slippage of twisting surfaces.

Variable-diameter pulley (VDP) or Reeves Drive

In this system, there are two V-belt pulleys that are split perpendicular to their axes of rotation, with a V-belt running between them. The gear ratio is changed by moving the two sections of one pulley closer together and the two sections of the other pulley farther apart. Due to the V-shaped cross section of the belt, this causes the belt to ride higher on one pulley and lower on the other. Doing this changes the effective diameters of the pulleys, which changes the overall gear ratio.

The distance between the pulleys does not change, and neither does the length of the belt, so changing the gear ratio means both pulleys must be adjusted (one bigger, the other smaller) simultaneously to maintain the proper amount of tension on the belt.

Roller-based CVT

(marketed as the Traction CVT, Extroid CVT, Nuvinci CVP, or IVT)

Consider two almost-conical parts, point to point, with the sides dished such that the two parts could fill the central hole of a torus. One part is the input, and the other part is the output (they do not quite touch). Power is transferred from one side to the other by one or more rollers. When the roller’s axis is perpendicular to the axis of the almost-conical parts, it contacts the almost-conical parts at same-diameter locations and thus gives a 1:1 gear ratio.

The roller can be moved along the axis of the almost-conical parts, changing angle as needed to maintain contact. This will cause the roller to contact the almost-conical parts at varying and distinct diameters, giving a gear ratio of something other than 1:1. Systems may be partial or full toroidal.

Full toroidal systems are the most efficient design while partial toroidals may still require a torque converter (e.g. Jatco Extroid), and hence lose efficiency.

Hydrostatic CVTs

Hydrostatic transmissions use a variable displacement pump and a hydraulic motor. All power is transmitted by hydraulic fluid. These types can generally transmit more torque, but can be sensitive to contamination. Some designs are also very expensive. However, they have the advantage that the hydraulic motor can be mounted directly to the wheel hub, allowing a more flexible suspension system and eliminating efficiency losses from friction in the drive shaft and differential components.

This type of transmission is relatively easy to use because all forward and reverse speeds can be accessed using a single lever.

This type of transmission has been effectively applied to a variety of inexpensive and expensive versions of ridden lawn mowers and garden tractors. Many versions of riding lawn mowers and garden tractors propelled by a hydrostatic transmission are capable of pulling a reverse tine tiller and even a single bladed plow.

One class of riding lawn mower that has recently gained in popularity with consumers is zero turning radius mowers. These mowers have traditionally been powered with wheel hub mounted hydraulic motors driven by continuously variable pumps, but this design is relatively expensive. A company called Hydro-Gear, a joint venture between Sauer-Danfoss and Agri-Fab, Inc. of Sullivan, Illinois, created the first cost-effective integrated hydrostatic transaxle suitable for propelling consumer zero turning radius mowers. An integrated hydrostatic transaxle (IHT) uses a single housing for both hydraulic elements and gear-reducing elements.

As of May 9, 2007, Hydro-Gear remains the only company producing integrated hydrostatic transaxles for consumer zero turning radius mowers in North America.

Some heavy equipment may also be propelled by a hydrostatic transmission; e.g. agricultural machinery including foragers and combines, but not anything that works the ground because the transmission cannot transmit enough torque.

Hydristor IVT

The Hydristor torque converter is a true IVT in that the front unit connected to the engine can displace from zero to 27 cubic inches per revolution forward and zero to -10 cubic inches per revolution reverse. The rear unit is capable of zero to 75 cubic inches per revolution. The common kidney port plate between the two sections communicates the hydraulic fluid under pressure and suction return in a serpentine-torodial flow path between the two Hydristor internal units.

The IVT ratio is determined by the ratio of input displacement to output displacement. Therefore, the theoretical range of Hydristor IVT ratios is 1/infinity to +-infinity/1 but real-world ratios are constrained by physics.

Simkins’ Ratcheting CVT

This transmission is an example of a Ratcheting CVT, prototyped as a bicycle transmission, protected under U.S. Patent #5516132. The input is the crank with a round hub integrated with it, and an array of twelve arms that are pivotally mounted to pins in the hub circle. Each arm has a pinion gear mounted on a one way clutch that allows only clockwise rotation of the pinion relative to the arm.

All of these pinions are engaged with a large ring gear that is integrated with the chainwheel as the output, and the ring gear/chainwheel assembly is mounted to a mechanism that enables it to be adjusted from a position of concentricity with the crank hub to various amounts of eccentricity with the crank hub. Adjustment of this eccentricity variably changes the output/input ratio from 1:1 to 2.6:1 as the ring gear/sprocket assembly is moved from a position concentric with the crank hub to an eccentric position.

The eccentricity control mechanism is connected to a spring that pushes the transmission into its eccentric high gear position. The largest spread of the arms is indicative of the gear ratio because the spreading arms are the only arms whose pinions (and one-way clutches) are locked and driving the ring gear/chainwheel assembly. Strong pedaling torque causes this mechanism to react against the spring, moving the ring gear/chainwheel assembly toward a concentric, lower gear position. When the pedaling torque relaxes to lower levels, the transmission self-adjusts toward higher gears, accompanied by an increase in transmission vibration.

This transmission behaves according to the definition of a Ratcheting CVT.

Variable toothed wheel CVT

Variable toothed wheel transmission relies on a toothed wheel positively engaged with a chain where the toothed wheel has the ability to add or subtract a tooth at a time in order to alter its ratio with relation to the chain it is driving. The toothed wheel can take on many configurations as is listed below in the patent specifications and include ladder chains, drive bars and sprocket teeth. This type of CVT is not a true CVT that can alter its ratio in infinite increments but rather approaches CVT capability by having a large number of ratios, typical 49, as is described in the VW owned German patent application DE10010741A1. The huge advantage of this type of CVT is that it is a positive mechanical drive and thus does not have the frictional losses and limitations of the Roller based or VDP CVT s. The challenge in this type of CVT is to add or subtract a tooth from the toothed wheel in a very precise and controlled way in order to maintain synchronized engagement with the chain.

This type of transmission has the potential to change ratios under load because of the large number of ratios resulting in the order of 3% ratio change differences between ratios, thus a clutch or torque converter is only necessary for pull away. None of this type of CVT is in commercial use probably because of above mentioned development challenges. Other examples of this type of CVT can be found in the following patent specifications: US5406863, WO9404411, US2669885, and WO2005036028.

Diagram and video clip

Girotto iCVT

The Girotto iCVT is a mechanical transmission with a variable transmission ratio between an input rotor and an output rotor coupled together by means of a coupling freewheel for transmission of the rotary movement, characterized in that it comprises between the input rotor and the output rotor one or more free wheels arranged in cascade with the coupling free wheel and able to transmit to one of the two rotors the rotary movement supplied by the coupling free wheel. Rotor is understood as meaning clearly any rotating mechanical part able to transmit, receive or transfer rotary movement. Publication number: EP1688645

Advantages and drawbacks

Compared to hydraulic automatic transmissions.

CVTs can smoothly compensate for changing vehicle speeds, allowing the engine speed to remain at its level of peak efficiency. They might also avoid torque converter losses. This improves both fuel economy and exhaust emission s. However, some units (e.g.

Jatco Extroid) also employ a torque converter. Fuel efficiency advantages as high as 20% over four-speed automatics can be obtained.

CVTs have much smoother operation. This can give a perception of low power. because many drivers expect a jerk when they begin to move the vehicle. The expected jerk of a non-CVT can be emulated by CVT control software though, eliminating this marketing problem.

Since the CVT keeps the engine turning at constant RPMs over a wide range of vehicle speeds, pressing on the accelerator pedal will make the car move faster but doesn’t change the sound coming from the engine as much as a conventional automatic transmission gear-shift. This confuses some drivers and, again, leads to a mistaken impression of a lack of power.

Most CVTs are simpler to build and repair.

CVT torque handling capability is limited by the strength of their belt or chain, and by their ability to withstand friction wear between torque source and transmission medium for friction-driven CVTs. CVTs in production prior to 2005 are predominantly belt or chain driven and therefore typically limited to low power ed car s and other light duty applications. More advanced IVT units using advanced lubricants, however, have been proven to support any amount of torque in production vehicles, including that used for buses, heavy trucks, and earth moving equipment.

History

Leonardo da Vinci. in 1490, conceptualized a stepless continuously variable transmission. 1 The first patent for a toroidal CVT was filed in 1886. 2

From the 1950s. CVTs have been applied to aircraft electrical power generating systems.

A CVT, called Variomatic. was designed and built by the Dutchman Hub van Doorne, co-founder of Van Doorne’s Automobiel Fabriek (DAF), in the late 1950s, specifically to produce an automatic transmission for a small, affordable car. The first DAF car using van Doorne’s CVT, the DAF 600. [1] was produced in 1958. Van Doorne’s patents were later transferred to a company called VDT (Van Doorne Transmissie B.V.) when the passenger car division was sold to Volvo ; it’s CVT was used in Volvo 340.

The Ford Fiesta and Fiat Uno were the first mainstream European cars to be equipped with steel-belted CVT (as opposed to the less robust rubber-belted DAF design), in 1987.

In the 1980s and 1990s, the Subaru Justy was offered with a CVT. While the Justy saw only limited success, Subaru continues to use CVT in its keicars to this day, while also supplying it to other manufacturers.

Nissan first introduced CVT in the 1992 Nissan March with a unit sourced from Subaru. In the late 1990s, Nissan designed its own CVT that allowed for higher torque, and includes a torque converter. This gearbox was used in a number of Japanese market models. Nissan is also the only car maker to bring roller-based CVT to the market in recent years.

Their toroidal CVT, named the Extroid, was available in the Japanese market Y34 Nissan Gloria and V35 Skyline GT-8. However, the gearbox was not carried over when the Cedric/Gloria was replaced by the Nissan Fuga in 2004.

After studying pulley-based CVT for years, Honda also introduced their own version on the 1995 Honda Civic VTi. Dubbed Honda Multi Matic, this CVT gearbox accepted higher torque than traditional pulley CVTs, and also includes a torque converter for creep action. The CVT is also currently employed in the Honda City ZX that is manufactured in India.

Toyota introduced the E-CVT in the 1997 Prius. and all subsequent Toyota and Lexus hybrids sold internationally continue to use the system (marketed under the Hybrid Synergy Drive name). Although sold as a CVT it is in fact not such a device as the gear ratios are fixed and the transmission is actually a Power Split Transmission (PST), allowing either the electric motor or the ICE (internal combustion engine ) or both to propel the vehicle. The response of the complete system (under computer control) is similar in feel to a CVT in that the ICE speed is relatively low and constant under low power or high and constant under high power.

BMW used a belt-drive CVT as an option for the low and middle range MINI in 2001, forsaking it only on the supercharged version of the car where the increased torque levels demanded a conventional automatic gearbox. The CVT could also be manually shifted if desired with software simulated shift points.

General Motors designed a CVT for use in small cars, which was first offered in 2002. After just three years, however, this transmission will be phased out in favor of conventional planetary automatic transmissions.

Audi has, since 2000, offered a chain -type CVT as an option on some of its larger-engine models, for example the A4 3.0 L V6.

Ford introduced a chain-driven CVT known as the CFT30 with a maximum torque capacity of 221 ftlbf (300 Nm) in their 2005 Ford Freestyle. Ford Five Hundred and Mercury Montego. The transmission was designed in cooperation with German automotive supplier ZF Friedrichshafen and was produced in Batavia, Ohio at Batavia Transmissions LLC (a subsidiary of Ford Motor Company) until 3/22/07.

The Batavia plant also produced the belt-driven CFT23 CVT which went in the Ford Focus C-MAX and Escape Hybrid and still produces the 4-speed automatic (CD4E) for the Ford Escape and Mazda Tribute. Ford also sold Escort (European version) and Orion models in Europe with CVTs in the 1980s and 1990s.

ZF Friedrichshafen supplied its belt drive VT-1 CVT unit to BMW for use in some versions of the European Mini Cooper.

The 2007 Dodge Caliber 3 and the related Jeep Compass employ a CVT using a variable pulley system as their optional automatic transmission.

Contract agreements were established in 2006 between MTD and Torotrak for the first full toroidal system to be manufactured for outdoor power equipment such as jet skis, ski-mobiles and ride-on mowers.

Examples

Many small tractors for home and garden use have simple hydrostatic or rubber belt CVTs. For example, the John Deere Gator line of small utility vehicles (used by many parks, stadiums, colleges, and other places where miscellaneous items must be displaced by laborers) use a belt with a conical pulley system. They can deliver a lot of power but can also build up speed to 10-15 MPH, all without need for a clutch or gearshift.

Most snowmobiles use CVTs. Most new motorscooters today are equipped with CVT. Virtually all snowmobile and motor scooter CVTs are rubber belt/variable pulley CVTs.

Some combine harvesters have CVTs. The machinery of a combine is adjusted to operate best at a particular engine speed. The CVT allows the forward speed of the combine to be adjusted independently of the engine speed.

This allows the operator to slow down and speed up as needed to accommodate variations in thickness of the crop.

CVTs have been used in SCCA Formula 500 race cars since the early 1970s.

More recently, CVT systems have been developed for go-karts and have proved to increase performance and engine life expectancy. The Tomcar range of off-road vehicles also utilizes the CVT system.

Some older drill presses contain a pulley-based CVT where the output shaft (which the chuck is connected to) has a pair of manually-adjustable conical pulley halves to which a wide drive belt from the motor loops through. The pulley on the motor, however, is usually fixed in diameter, or may have a series of given-diameter steps to allow a selection of speed ranges. A handwheel on the drill press, marked with a scale corresponding to the desired machine speed, is mounted to a reduction gearing system for the operator to precisely control the width of the gap between the pulley halves. This gap width thus adjusts the gearing ratio between the motor’s fixed pulley and the output shaft’s variable pulley, changing speed of the chuck; a tensioner pulley is implemented in the belt transmission to take up or release the slack in the belt as the speed is altered.

However, the drill press’ speed almost always cannot be changed without the motor running.

New automobiles equipped with CVT

Old automobiles equipped with CVT

Notes

1. ^ Hilton Holloway, Martin Buckley (2002). 20th Century Cars . Carlton. ISBN 1-84222-835-8.

External links

range of input/output ratios in addition to its infinite number of possible ratios; this qualification for the IVT implies that its range of ratios includes a zero output/input ratio that can be continuously approached from a defined ‘higher’ ratio.

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A sequential manual transmission (or sequential manual gearbox ) is a type of manual transmission used on motorbikes and high-performance cars or auto racing, where gears are selected in order and random access to specific gears is not possible.

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automatic transmission (commonly abbreviated as AT) is an automobile gearbox that can change gear ratios automatically as the vehicle moves, thus freeing the driver from having to shift gears manually (similar but larger devices are also used for railroad locomotives).


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Tiptronic is a type of discrete automatic transmission developed by Porsche and used in its vehicles and those of its licensees. A Tiptronic transmission can operate just as the common type of automatic transmission, but it also allows the driver to override the automatic mode by

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A semi-automatic transmission (also known as clutchless manual transmission . dual-clutch transmission . automated manual transmission . e-gear . or paddle shift gearbox

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A twin-clutch gearbox or dual clutch transmission (DCT) is a semi-automatic transmission with separate clutches for odd and even gears. The outer clutch drives the odd numbered gears and reverse, while the inner clutch drives the even numbered gears.

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Saxomat was a type of automatic clutch available as an option on Fiat 1800, Saab 93, Volkswagen Beetle, Borgward, Goliath, DKW, BMW, Opel, NSU, Glas, Trabant and Wartburg. Opel sold it as Olymat; Trabant and Wartburg named the system Hycomat.

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A non-synchronous transmission is a form of transmission based on gears that do not use synchronizing mechanisms. They are found commonly in various types of agricultural, commercial, and military vehicles.

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Variomatic is the stepless, fully automatic transmission of the Dutch car manufacturer DAF, using a drive belt and two pulleys. It was the first continuously variable transmission (CVT) (as opposed to shifting between separate gears).

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In 2000 Audi launched a new stepless transmission called Multitronic . Based on the principles of continuously variable transmission (CVT) popularised by DAF, multitronic offers a stepless automatic transmission in which the ratio between the input shaft and output shaft can be

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gearing on a bicycle is the selection of appropriate gear ratios for optimum efficiency or comfort. Different gears and ranges of gears are appropriate for different people and styles of cycling. Multi-speed bicycles allow gear selection to suit the circumstances, e.g.

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Derailleur gears are a variable ratio transmission system commonly used on bicycles, consisting of a chain, multiple sprockets and a mechanism to move the chain from one sprocket to another.

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Hub gears or internal-gear hubs are a type of gear system used on bicycles. Hub gears are used mostly on utility bikes and various types of small wheeled bicycle, such as folding bikes.

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Toyota Hybrid System

305 N·m (225 ft·lbf) @ 0 rpm

For other types of Hybrid Transportation, see .

A hybrid vehicle (HV) is a vehicle that uses two or more distinct power sources to propel the vehicle such as:

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The gear ratio is the relationship between the number of teeth on two gears that are meshed or two sprockets connected with a common roller chain, or the circumferences of two pulleys connected with a drive belt.

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Fuel economy in automobiles is the amount of fuel required to move the automobile over a given distance. While the fuel efficiency of petroleum engines has improved markedly in recent decades, this does not necessarily translate into fuel economy

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An engine is something that produces an output effect from a given input. The origin of engineering however, came from the design, building and working of (military engines) because before such devices came to be employed in battles there were very few mechanical devices used.

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In mathematics, a continuous function is a function for which, intuitively, small changes in the input result in small changes in the output. Otherwise, a function is said to be discontinuous .

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Epicyclic gearing or planetary gearing is a gear system that consists of one or more outer gears, or planet gears, rotating about a central, or sun gear.

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In geometry, a torus (pl. tori ) is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle, which does not touch the circle. Examples of tori include the surfaces of doughnuts and inner tubes.

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A variable displacement pump is a device that converts mechanical energy to hydraulic (fluid) energy. Some of these devices can also be reversible, meaning that they can act as a hydraulic motor and generate mechanical energy from fluid energy.

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Hydraulic fluids are a large group of mineral oil, water or water-based fluids used as the medium in hydraulic systems. These fluids are found in machinery and equipment ranging from brakes, power steering, and transmissions to backhoes, excavators, garbage trucks and industrial

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differential is a device, usually consisting of gears, that allows each of the driving wheels to rotate at different speeds, while supplying equal torque to each of them.

Purpose

A vehicle’s wheels rotate at different speeds, especially when turning corners.

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A lawn mower or lawnmower is a piece of equipment that has one or more revolving blades to cut grass or other plants of a lawn at an even length.

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