US Patent 10132401 Thermostatic bypass valve

A thermostatic bypass valve has an intentional flow resistance through a bypass passageway when fluid is at a normal operating temperature. This flow resistance, of between 10 and 20 psi at a flow rate of 10 liters per minute, is achieved by limiting a radial clearance within the bypass passageway to less than 1.0 mm. The flow rate through the bypass passageway is at least half of the flow resistance of the cooler. This bypass flow resistance is beneficial because it limits the flow rate of lubrication fluid during bypass, reducing the parasitic losses of the gearbox.