Design and Development of a Wet Clutch with Variable Cooling and Very Low Drag, 03-R8100

Principal Investigators
George E. Bailey
Bryan J. Williams
Christopher L. Wray

Inclusive Dates:  09/28/09 – 09/28/10

Background - Clutches are used to shift from one gear ratio to another within a transmission, or they can be used to start the vehicle's forward motion from a stopped position. Often, the two sides of the clutch are not rotating at the same speeds prior to engaging the clutch. In a typical automatic transmission, the clutch is engaged by a hydraulically actuated apply piston, and the speeds are matched by dissipating power in the friction interface of the clutch and turning that power into heat. In a wet clutch, the heat is removed by oil that flows through the clutch. In the multi-plate wet clutches of automatic transmissions, oil is typically sprayed into the center of the rotating clutch at atmospheric pressure. The oil is thrown outward by centrifugal force and it flows through the facing and cools the clutch. This oil is often supplied from a flow circuit that lubricates bearings, and therefore it flows constantly. To remove the heat generated by an engagement, cooling flow is only needed in the clutch during and just after the engagement. At all other times, oil flow through the clutch is not necessary. It becomes a penalty when the clutch is open, because it must be sheared between the open plates of the clutch.

Approach - To increase efficiency of automatic transmissions, SwRI engineers conceptualized a unique method to supply cooling oil. In this concept, the oil is delivered through a set of restrictions in the clutch apply piston. Therefore, oil will only be delivered when pressure is applied to the piston to engage the clutch and no oil will be delivered when the clutch is open. Because no oil is flowing through the open clutch pack with this concept, the drag caused by the shearing of the oil will be reduced or eliminated. Having the ability to select the correct number and size of restrictions in the apply piston will be necessary to provide a design that reduces power consumption without sacrificing durability.

Accomplishments - An analytical model was developed that predicts the amount of cooling flow required for the clutch under a variety of conditions. The model is capable of providing an excellent starting point for designing clutch pack hardware and predicting lubrication requirements. The model has been transformed into a Windows™-based executable program for distribution to development engineers within SwRI, allowing the use of this tool on future clutch development projects. The concept cooling method was developed and demonstrates comparable durability to that of conventional clutch cooling methods. A variety of measurement techniques were used to evaluate the durability in an accelerated test program. The concept cooling method also showed a significant reduction in the amount of open pack clutch drag, and it appears to completely eliminate the parasitic drag caused by the shearing of oil in the clutch interface. Because the concept is a valid method to reduce parasitic losses without adversely affecting or significantly changing the durability of the clutch friction material, the concept proves to be feasible for application in automatic transmissions.

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