7/00 mechanical engineering: input output

input/output

Smooth shifting

by Associate Editor John DeGaspari

Automatic transmission fluids must function at temperatures as low as -40°F and as high as 300°F. One way to keep these fluids flowing smoothly despite temperature extremes is a thermostat bimetal valve. Thermostat metals are composite metal strips or sheets that are made up of two or more layers that are permanently bonded together. The layers of the strip have different thermal expansion coefficients, causing the strip to bend in precise and reproducible ways when exposed to temperature changes.

Now a manufacturer is looking at other places besides the transmission where the valves can serve an automobile.

Thermostat bimetals are well established in temperature controls, where they are commonly used to make and break electrical connections. These same properties make them a natural for fluid control in automotive transmissions, according to Paul Galipeau, senior product applications engineer with Texas Instruments' engineered materials division in Attleboro, Mass., which has supplied valves to major automotive manufacturers.

Galipeau said the advantages of bimetal valves are low cost and high reliability. "The thermostat metals have never failed due to corrosion or any other reason," he noted.

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. When the engine is shut off, the thermal valve opens as the liquid cools and so the way is cleared for efficient flow of a high-viscosity liquid, which completes the cycle.

The thermal valve maintains constant pressure during gear changes, and so helps to avoid the rough spots in the transmission functioning. The thermal valve may be used for forward, neutral, or reverse gears, and may be inserted in either the supply side or exhaust side of the gear servo systems, according to Texas Instruments.

The other application is to help maintain a constant transmission fluid level in the transmission's sump, regardless of fluid volume changes. Sump levels that are too high or too low can keep the transmission from functioning properly. Sump fluid level depends on the transmission's demand for fluid as well as fluid density. The thermal valve, which is calibrated to fluid density, senses the fluid temperature and opens to the right position to maintain fluid level.

Bimetal thermal valves can be supplied in packages that are flat, rectangular, or tubular, according to Texas Instruments. Either a cantilever blade or a sliding gate controls the orifice cross section. The design choice depends on the forces involved in the control action.

The automatic servo system valve uses a sliding gate. A cantilever blade is used for the sump fluid level control. Depending on the design, the gate can be actuated with a thermostat bimetal blade, or a helical or spiral coil.

The gear change system uses a sliding gate design, because fluid operating pressures are high, varying from 30 to 120 pounds per square inch, according to Texas Instruments. A sliding gate minimizes actuation force by keeping the valve from working directly against the system's fluid pressure. Instead, the gate valve sees only friction forces that are easier to overcome.

Thermal valves have been used in automatic transmissions for several years, according to Texas Instruments. The company is also developing several new applications: fuel delivery system flow regulation; automotive gas control, to speed warmup and reduce emissions; hydraulic power flow control; and oil cooling heat exchangers, to maintain constant cooling capacity over an operating temperature range and provide a bypass at low ambient temperatures.