What is Continuously Variable Transmission?

Published by Carl Wilson on

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

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.

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.

Roller-based CVT

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 call 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.

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Categories: Articles

Carl Wilson

You won't believe it, I'm native Scotsman. Enthusiast. Car lovers. Almost finished rebuilding my Reliant Saber 🔥

1 Comment

Infinitely Variable Transmission (IVT) · 28 February 2021 at 3:20 pm

[…] also our CVT section for more information on […]

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