There is a range of applications that can be loosely classified as “power transfer units” – hydrostatic transmissions, CVT and powershift transmissions, gearbox and axle applications and even power steering. The common thread that ties these applications together is hydraulic oil. It’s used for lubrication and cooling, and it’s vitally important to keep it clean with effective filtration.
Types of Transmissions
Hydrostatic Transmissions
The role of hydrostatic transmissions is to transfer hydraulic energy into kinetic energy. These transmissions transfer power from the engine to pumps that push oil to a hydraulic motor driven by the power of the fluid. They can be used in a variety of applications but are commonly used in off-road and stationary equipment.
CVT and Powershift Transmissions
Continuously variable transmissions (CVT) provide a continuous band of power via a sheave and a power band, whereas powershift transmissions use a series of gears (often planetary) and clutches to gradually increase rotational speed and mechanical advantage. These transmissions are commonly used for equipment mobility in both on- and off-road applications.
Gearbox, Axle and Differential
These applications include rotational gears that transfer power from transmissions to drive mechanisms, which are often wheels or tracks. These components need to be cooled and lubricated in order to maintain effective function.
Power Steering
Typical power steering systems include a hydraulic pump that supplies pressure to both sides of a steering cylinder. These systems need to transmit a lot of fluid to respond to driver inputs, allowing heavy-duty equipment to move where intended.
E-Drive Systems
In e-drive motors, there’s a need for cooling and lubrication – with either single or multiple fluids per drive motor. We’re seeing greater interest in e-drive system filtration. It’s still early days, but requirements for these systems will be long-life, for both oils and filtration.
Types of Circuits
Transmission Control (Mid-Pressure)
Mid-pressure (30-40 bar / 450-600 psi) circuits are often used with controls and clutches. The high-pressure fluid puts pressure on the clutch to transfer mechanical power, and the higher pressure increases power transfer output. This type of system is most often used in powershift, CVT and hydrostatic transmissions and power steering systems. These systems typically have tight clearances and can be susceptible to damage from contamination.
Lower-Pressure Circuits
Low-pressure circuits (1-10 bar / 15-150 psi) require high oil flow to ensure continuous lubrication of gears and shafts. This type of system is most often used in gearbox, axle and differential applications.
Maintaining Clean Fluid Is Essential
In all of these systems, fluid cleanliness needs to be maintained in order to support proper function – lubrication and cooling. Transmissions typically run at higher temperatures than other hydraulic systems, making effective cooling even more important. Most systems are closed, but powershift and CVT transmissions will typically have a vent to compensate for thermal expansion and contraction. This venting, if not done properly, can become a source of external contamination. Contamination can also come from wear of internal components, including fibers and graphite (high-friction clutch material).
Maintaining Clean Fluid Is Essential
In all of these systems, fluid cleanliness needs to be maintained in order to support proper function – lubrication and cooling. Transmissions typically run at higher temperatures than other hydraulic systems, making effective cooling even more important. Most systems are closed, but powershift and CVT transmissions will typically have a vent to compensate for thermal expansion and contraction. This venting, if not done properly, can become a source of external contamination. Contamination can also come from wear of internal components, including fibers and graphite (high-friction clutch material).
The Right Filter for the Job
Filter adaptations need to be made to allow them to perform in this higher-temperature environment. This includes seals, such as Viton, and media that are designed specifically to perform in these harsh conditions.
