CVT: 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. This can be achieved in a number of ways; after all, the concept of CVTs is not new to mankind..Da Vinci was the first one to design a CVT system. A few types of CVTs in use are:
Infinitely Variable Transmission (IVT) A specific type of CVT is the infinitely variable transmission (IVT), which has an infinite range of input/output speed 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 speed ratio that can be continuously approached from a defined "higher" ratio. A zero output speed with a finite input speed implies an infinite input-to-output speed ratio, which can be continuously approached from a given finite input value with an IVT. Low gears are a reference to low ratios of output speed to input speed. This ratio is taken to the extreme with IVTs, resulting in a "neutral", or non-driving "low" gear limit, in which the output speed is zero, although, unlike neutral in a normal automotive transmission, the output torque may be non-zero: the output shaft is rigidly fixed at zero speed rather than being freely rotating. Most continuously variable transmissions are not infinitely variable.
Most IVTs result from the combination of a CVT with an epicyclic gear system (which is also known as a planetary gear system, and is termed a power split device in this applicaton) which enforces an output shaft rotation speed which is equal to the difference between two other speeds (perhaps multiplied by some gear ratio, depending on implementation). If these two other speeds are the input and output of a CVT, there can be a setting of the CVT that results in an output speed of zero. The maximum output/input ratio can be chosen from infinite practical possibilities through selection of additional input or output gear, pulley or sprocket sizes without affecting the zero output or the continuity of the whole system. 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.
IVTs can in some implementations offer better efficiency when compared to other CVTs as in the preferred range of operation, most of the power flows through the planetary gear system and not the controlling CVT. Torque transmission capability can also be increased. There's also possibility to stage power splits for further increase in efficiency, torque transmission capability and better maintenance of efficiency of a wide gear ratio 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
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.
Versatile Manufacturing first introduced hydrostatic drive in 400 swathers. Hydrostatic quickly became popular with swather and combines. Attempts to use hydrostatic in tractors however were less successful. IH hydro suffered from exxcessive cabin noise and many operators complained of not being able to rest their feet on the cab floor due to heat from the pump.
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. Some heavy equipment may also be propelled by a hydrostatic transmission; e.g. agricultural machinery including foragers combines and some tractors. However, Hydrostats are usually not used for extended duration high torque applications due the heat that is generated by the flowing oil.
Next post, I'll try writing on the i-DSI engine...
