Continuously Variable Transmission: How a CVT Works
A guide to the continuously variable transmission: its pulleys, its belt or chain, and how the ratio changes.
Design diagram only, not a product photograph.
The name, unpacked
Continuously variable transmission, shortened to CVT, is a transmission that changes its ratio continuously rather than in fixed steps. A conventional automatic transmission moves between a set of gear ratios: first, second, third, and so on, each a fixed relationship between input speed and output speed. A CVT does not step between fixed ratios. It varies the ratio across a range, smoothly, by changing the working diameters of its two pulleys. That single sentence is the definition, and everything else in this guide is an unpacking of it.
The word continuously matters. It does not mean infinitely, and it does not mean without limits. The pulleys have a smallest and a largest working diameter, and the ratio range is bounded by those diameters. What continuously means is that, within that range, the transmission is not forced to sit at one of a handful of preset ratios. It can hold an intermediate ratio, and it can move gradually from one ratio to another without the step change a gear shift produces.
The two pulleys
A CVT has two pulleys. The parts table names them the primary pulley, also called the drive pulley, and the secondary pulley, also called the driven pulley. The primary pulley is the input pulley: power enters there. The secondary pulley is the output pulley: power leaves there. The names primary and drive describe the same row, and the names secondary and driven describe the same row, and a buyer who meets either name in a catalogue should treat them as one row, not two products.
Each pulley is, in effect, a pair of conical faces that can move closer together or further apart. When the faces move together, the belt or chain rides further out, at a larger working diameter. When they move apart, the belt or chain rides further in, at a smaller working diameter. The ratio at any moment is set by the relationship between the working diameter at the primary pulley and the working diameter at the secondary pulley. Change one diameter, or both, and the ratio changes.
The belt or the chain
Between the pulleys runs the transmission element: in most CVTs on this site, a steel belt, and in a minority, a chain. The parts table is explicit about both rows. The assembly row is described as pulley-and-steel-belt type, which makes the belt the standard element for the CVT family on this site. The chain row carries the note that a minority of CVTs use chain drive, which makes the chain the exception to name, not the default to assume.
The table also warns that the market often calls a belt a chain. That warning belongs in a guide about how a CVT works, because a reader who believes every CVT element is a chain, or every chain reference means a belt, will misunderstand both the part and the fault descriptions that follow. Belt and chain are different rows, and the guide pages on this site keep them separate for exactly that reason.
The pulley cylinder: what moves the pulleys
Pulleys do not change diameter by themselves. Something has to move the conical faces, and in a CVT that something is hydraulic pressure applied to the pulley cylinder. The parts table lists the CVT pulley cylinder as its own row, with the note that it is the CVT core pressure-test part. The process documents give that note its meaning: in step 6 of the ten-step process, CVT units receive a dedicated pulley-cylinder pressure test, checking for internal leakage and pressure loss.
The logic is straightforward. If a cylinder is found to leak internally or to lose pressure, a technician can check what that means for the pulley diameter and the ratio on that unit. That chain, cylinder pressure to pulley diameter to ratio, is the hydraulic heart of a CVT, and it is why pressure behaviour and ratio behaviour are described together in this guide. The pressure-test guide on this site explains that chain in full, and this guide does not repeat it beyond the summary here.
The valve body and the step motor, in brief
Pressure has to be directed, and direction is the valve body's job. In a CVT, the valve body directs hydraulic pressure to the parts that need it, including the pulley cylinder. The step motor is the CVT-linked control row inside that story: the parts table places it in the valve-body section with the note CVT valve-body step motor. A step motor moves in steps to control a hydraulic function, and in the CVT case that function sits in the control chain that reaches the cylinder.
This guide keeps that description deliberately general. The table gives no valve-body specification, no step-motor specification and no control map, and a guide that invented one would be a specification sheet in disguise. The step-motor guide on this site takes the control relationship further, still in guide language, and still without a specification.
What a CVT is not
A CVT is not a gearbox with very many gears. It has no gear set in the sense that an automatic transmission with planetary gears has one, and the planetary rows that appear elsewhere in the company parts table belong to a different transmission family, not to this guide. A CVT is not a belt drive of the kind used for accessories, and it is not a scooter transmission, even though scooters also use a belt-and-pulley idea; the CVT on this site is the automotive transmission in the company parts table, with its pulley cylinder, its step motor and its pressure test.
A CVT is also not a maintenance-free device. Over time, parts in a transmission can wear, and the fault-signs guide on this site describes symptoms in plain words for a technician to check. None of those symptoms, on its own, diagnoses a vehicle, and this guide does not use them that way. It uses them to show why the parts exist: each symptom can be described alongside the rows it may relate to, and a technician can narrow the row from there.
Reading the rest of this site from here
From this guide, the natural next pages are the part pages themselves. The pulley set page shows the two pulleys as a working pair. The belt page and the chain page separate the two transmission elements. The pulley cylinder page explains the pressure part. The valve-body and step-motor pages explain the control side, with the CVT limit stated on each. The selection guide shows how to turn understanding into an inquiry that names a row, and the inquiry checklist shows every question that inquiry should contain.
One final caution for the reader who arrived here from a search engine. This guide describes the CVT family in the company parts table. It does not describe every CVT ever made, and it does not claim that every CVT uses the same parts. Where this guide says a CVT has a pulley cylinder, it means the CVT family in that table. A different manufacturer's CVT may differ, and a buyer ordering for a specific unit should confirm the row with the supplier rather than assuming it from this guide.