Patent Application
20050026732
1. Field of the Invention
The present invention relates to limited slip differential assemblies for motor vehicles.
2. Description of the Prior Art
Differentials are very well known in the prior art. Differential assemblies are arranged in the power transmission system of a motor vehicle to allow a pair of output shafts operatively coupled to an input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction while the vehicle is turning. The differential distributes the torque provided by the input shaft between the two output shafts. The most basic and common type of differential is known as an open differential. Although open differentials may be adequate for most driving conditions, they are generally unsuitable for slippery conditions where one wheel experiences a much lower coefficient of friction than the other wheel. If one wheel of a vehicle is located on a patch of ice or mud and the other wheel is on dry pavement, the wheel experiencing the lower coefficient of friction loses traction. If even a small amount of torque is directed to the wheel without traction, the torque will cause the wheel to “spin out”. Since the maximum amount of torque which can be developed on the wheel with traction, is equal to torque on the spinning wheel without traction, the engine is unable to develop any torque and the wheel with traction is unable to rotate. To address this problem, a differential has been developed that re-allocates the torque so that the torque is directed to the wheel that has traction. Such differential assemblies are typically called limited slip differentials.
Although limited slip differentials are common options for rear wheel vehicles, they are generally rare in FWD vehicles. Designing a limited slip differential for a FWD vehicle is extremely difficult because of space limitations and economic considerations. Currently there are no active (i.e. controllable) limited slip differentials available for FWD transaxles from original equipment manufactures.
FWD vehicles typically have transaxles with open differentials. Although some high performance torque-sensing FWD limited slip differentials are commercially available, they comprise a small segment of the overall market and are prone to technical problems. Torque sensing limited slip differentials in front axle applications have historically been unsatisfactory due to excessive torque steer and handling problems, unless substantially ‘detuned’ to limit performance. However, given the increased emphasis on high performance FWD vehicles, there is an increasing need for a controllable limited slip differential for front transaxles.
In order to increase reliability, decrease costs, and improve performance, the present invention has been developed. In operation, the invention significantly improves FWD vehicle mobility and minimizes wheel spin, while avoiding problems with torque steer and inadequate mobility. The current invention gives FWD vehicles performance approaching that of an all wheel drive system at a fraction of the cost.
The present invention is a limited slip differential assembly, preferably for a FWD vehicle. The invention allows an operator to manually or automatically control the limited slip function. The differential control and actuation assembly is located external to the differential housing and differential case. The assembly includes an electro-magnetic actuator, preferably an electric solenoid that modulates hydraulic pressure produced by a hydraulic pump. When the limited slip differential control is in the ON position, hydraulic pressure produced by the hydraulic pump engages a friction clutch pack, which couples the differential case to the front axle output shaft. When the limited slip function is in the OFF position, the electrical solenoid does not allow the hydraulic pump to generate sufficient hydraulic pressure to actuate the clutch pack. When the differential control is in an intermediate position, the specific control setting, and the rotational speed of the axle output shaft relative to the rotational speed of the differential case determine differential engagement. In addition to the ability to manually control the limited slip function, an alternative embodiment includes a computer processor/logic unit that can electronically control the function.
As indicated in
The friction clutch pack (8) is comprised of sets of alternating outer friction plates (18a) and inner friction plates (18b). The outer circumference of the outer plates (18a) and inner friction plates (18b). The outer circumference of the outer friction plates (18a) have projections that non-rotatably engage corresponding grooves formed in the differential controller housing (3). Similarly, an inner circumference of the inner friction plates (18b) has projections that non-rotatably engage corresponding grooves formed in the clutch sleeve (11), which in turn is splined to a front axle output shaft (9). Both the outer friction plates (18a) and the inner friction plates (18b) are slideable in the axial direction. The outer clutch plates (18a) frictionally engage the inner clutch plates (18b) to form a torque coupling between the differential case (1) and the axle output shaft (9). When the clutch plates (8) are engaged, torque is transferred from the differential case (1) to the axle output shaft (9).
The clutch pack (8) is selectively actuated by a hydraulic clutch actuator system. The clutch actuator system includes a hydraulic displacement pump (6), and a piston assembly (7) for axially loading the clutch pack (8).
The hydraulic displacement pump (6) generates a selectively variable amount of hydraulic pressure to actuate the clutch pack (8). Preferably, the hydraulic displacement pump (6) is a gerotor-type pump (6). The gerotor pump (6) includes an outer ring member (16a), an outer rotor (16b), and an inner rotor (16c). The inner rotor (16c) is drivingly coupled to the output axle shaft (9), and the outer ring member (16a) is secured to the differential case (1) via the differential controller housing (3) spline joint (4). The inner rotor (16c) has one less tooth than the outer rotor (16b) and when the inner rotor (16c) is driven, it will drive the outer rotor (16b), which can freely rotate within the outer ring member (16a) eccentrically with respect to the inner rotor (16c), thus providing a series of decreasing and increasing volume fluid pockets by means of which fluid pressure is created. As a result, when relative motion takes place between differential case (1) and the axle output shaft (9), the inner rotor (16c) of the gerotor pump (6) generates hydraulic fluid pressure. Consequently, the hydraulic pressure generated by the pump (6) is substantially proportional to a rotational speed difference between the output axle (9) and the differential case (1).
Although a gerotor pump is specifically discussed, any type of hydraulic pump functionally capable of generating hydraulic pressure in response to the relative rotation between the specified components is within the scope of the present invention.
The piston assembly (7) serves to compress the clutch pack (8). The piston assembly (7) is comprised of a piston pressure chamber (17a) and an actuating piston (17b). The gerotor pump (6) supplies pressurized hydraulic fluid to actuate the piston assembly (7) and engage the clutch pack (8). In operation, when a speed difference between the output shaft (9) and the differential case (1) exists, the gerotor pump (6) pumps the pressurized fluid into the piston pressure chamber (17a), which causes the actuating piston (17b) to exert pressure on the clutch pack (8). As the speed difference increases, the pressure generated by the gerotor pump increases. The pressurized fluid in the piston pressure chamber (17a) exerts an axial force on the piston (17b), urging the piston (17b) to engage the clutch pack (8). Engaging the clutch pack (8) (as described above) allows torque to be transferred between the differential case (1) and the axle output shaft (9).
In order to control the fluid pressure in the piston pressure chamber (17a) and, subsequently, the output torque distribution of the limited slip differential assembly, a variable pressure relief valve assembly (12) is provided. The variable pressure relief valve may be of any configuration known in the art, but is preferably an electro-magnetic ball-and-seat valve assembly controlled by the electro-magnetic actuator (5). The electro-magnetic actuator (5) may be any appropriate electro-magnetic device known in the art, such as a solenoid.
As best illustrated in
When a maximum current is applied to the solenoid (5), the retaining force of the pressure relief valve (12) is at its maximum, and as a result, the pressure relief valve (12) allows a relatively high pressure to build in the piston pressure chamber (17a). In this configuration, the pressure is sufficient to fully actuate the hydraulic clutch pack (8), which results in fully enabling the limited slip function. This condition results when an operator manually places the differential control in the “ON” position.
When a minimum current is applied to the solenoid (5), the retaining force of the pressure relief valve (12) is at its minimum, and the relief valve (12) bleeds off hydraulic fluid at a relatively low pressure and does not allow pressure to build in the piston pressure chamber (17a). In this configuration, the pressure attainable is not sufficient to actuate the hydraulic clutch pack (8), which results in fully disabling the limited slip function. This condition results when an operator manually places the differential control in the “OFF” position.
In between the “ON” and “OFF” conditions, the release pressure of the relief valve (12) may be set at any value by modulating the current applied to the solenoid (5). Modulating the solenoid current results in a variable amount of pressure to engage, disengage, or partially engage the limited slip function. The ability to control the limited slip function enables the operator to adjust vehicle performance to match personal preferences, or to address various operating conditions and performance profiles.
In addition to the ability to manually control the limited slip function, a computer processor/logic unit can also electronically control the function. As best shown in
One or more active vehicle management systems, such as a programmable yaw stability control system, can be used with the present invention. Active vehicle management systems may incorporate a processor (13) that can interface with the electrical solenoid (5) and pressure relief valve (12) to optimize vehicle traction, maneuverability, and stability.
For the foregoing reasons, it is clear that the invention provides a traction system for FWD vehicles. The invention may be modified in multiple ways and applied in various technological applications. The current invention may be modified and customized as required by a specific operation or application, and the individual components may be modified and defined, as required, to achieve the effect and result. Although the materials of construction are not described, they may include a variety of compositions consistent with the function of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
