The bimetal arm on this thermal valve controls fluid flow delivered to clutch packs in automatic transmissions, ensuring smooth shifting. In designing these valves for transmission fluid control applications, engineers take advantage of the flexural changes that occur in bimetal devices with variations in temperature. In automatic transmissions, temperature changes cause both the viscosity and density of the fluid to vary, making possible two thermal valve applications. Transmissions change gears by diverting fluid from clutch pack to clutch pack, and viscosity changes can affect smooth performance. To counter viscosity variations, a thermal valve may be inserted into the servo system that feeds or exhausts a clutch pack in the transmission.
Cold fluid flows slower than hot and needs a large flow cross section to minimize pressure buildup.
The thermal valve responds to the lower temperature and opens up. As the fluid increases to normal operating temperature and viscosity decreases, the flow cross-section area needs to be reduced. The rising temperature causes the thermal valve to progressively close the orifice gate. At normal driving temperatures, the thermal valve control orifice is completely closed, and the fluid passes only through a second, smaller orifice that is constantly open. The thermal valve maintains constant pressure during gear changes, and so helps to avoid the rough spots in the transmission functioning.