Part 2: CVTs for Farm Tractors
The previous article in this series discussed the features and benefits of CVTs. Article 2 examines CVTs for farm tractors and details how they can help increase efficiency.
Like other industries, agriculture is looking for efficiency gains wherever it can find them. Farmers need methods to minimize their operating costs – such as more efficient work processes and the consumption of fewer resources – and CVTs can help make this happen.
A farm tractor working in a field might have one or two attachments that require a certain engine speed. Because a power take off (PTO) is a very simple gearbox and often only has three gears, the operator has to use a certain amount of revs in order to get the required speed for the equipment. This then dictates the ground speed of the tractor. By using a more flexible transmission such as a Continuously Variable Transmission. An automatic transmission that replaces conventional gear ratios with a chain or belt and a system that adjusts diameters on either side of two pulleys to generate seamless transitions of speed and torque., operators can get the ground speed that they need while also saving fuel.
Another benefit involves torque delivery. Drivers of farm tractors normally deal with four gears and four splitters. With a conventional tractor using a synchro box, an operator needs to use both hands to change gears to keep the momentum up. But with a CVT, when the joystick is pushed forward and the accelerator foot is kept on the floor, there is minimal alteration of engine speed. This saves fuel and makes the machine much easier to drive.
For a wide range of farm tractor applications, CVTs are a popular choice. When a tractor is used in a yard, there is a lot of low-speed shuttling – moving backwards and forwards, loading and unloading. A CVT allows operators to complete these tasks smoothly, quickly and efficiently. With a previous generation tractor, the driver would have to select reverse gear to go backwards. With a CVT, on the other hand, the driver just has to move the joystick and can do it without coming to a complete stop.
The development of CVTs for farm tractors dates back to 1973, when a Fendt engineer patented what became the company’s Vario transmission. By 1996, Fendt had launched the Vario for large tractors. Prior to that, Steyr began development of the S-Matic transmission, which was introduced for Steyr and Case tractors in 2000.
The early 2000s saw ZF’s offering – the ZF Eccom – which could be found in Deutz-Fahr and John Deere products. More recently, in 2011, Dana and Bosch Rexroth formed a joint venture to develop a hydromechanical variable transmission, designed specifically for wheel loaders, motor graders, heavy material handling and forestry skitters. Claiming a 25 percent increase in fuel efficiency over torque converter transmissions, it transmits power only hydrostatically until the ground speed matches the mechanical transmission speed. At that point, it introduces mechanical power transmission in parallel until, eventually, power transfer becomes entirely mechanical.
There are two different design principles when it comes to farm tractor CVTs – planetary output coupled and planetary input coupled. The former sees power from the engine split to both hydrostatic and mechanical paths, and is a popular approach with Fendt, Massey-Ferguson, Valtra and Challenger.
The planetary input coupled approach sees power merge from hydrostatic and mechanical paths to the output drive wheels. Examples of this design are Steyr’s S-Matic and the ZF Eccom.
In both principles, an onboard computer controls the engine and transmission to achieve the most efficient combination of power and fuel savings. Aside from Fendt, it is believed that all of the OEMs have developed both instances. Case, for example, tends to have a mix of types, depending on the power output of the tractor.
While traditional powershift transmissions remain common, CVTs are an increasingly popular option for many farmers. The greater flexibility throughout the workload range is one major advantage for CVTs, even though this capability is delivered in a subtly different way. The engine’s specified maximum power and torque might not coincide at the same revs, which means drivers may sometimes not feel the same sense of direct mechanical connection that a traditional manual gearbox provides. Nevertheless, for most operators the efficiency benefits will outweigh the potential drawbacks in subjective feel.
The development of CVTs for farm tractors has afforded a number of benefits for operators, and these are set to continue as the products continue to be refined. With additional levels of control and ease of operation being delivered by the latest transmission designs, it would not be a surprise to see more and more CVTs added to fleets throughout the sector – at the expense of more traditional gearbox technologies.
The third and final article in this series explores CVTs in the passenger vehicle market. Read Part 3 now.