BACKGROUND OF INVENTION

The invention relates in general to electric parking brakes for use with disc brake assemblies and in particular to an improved gearing for such an electric parking brake.

A typical disc brake assembly for a vehicle includes a brake disc which is secured to a wheel of the vehicle for rotation therewith and non-rotating brake linings that are operable between non-braking and braking positions. Each of the brake linings is supported on a brake shoe. In the non-braking position, the brake linings do not slow rotation of the brake disc and vehicle wheel. In the braking position, the brake linings are in frictional engagement with the brake disc to slow rotation of the brake disc and vehicle wheel.

The disc brake assembly further includes a brake piston and a sliding brake caliper. The brake linings are moved into the braking position by the brake piston and the sliding brake caliper. For example, hydraulic pressure may linearly actuate the brake piston to displace the brake linings to frictionally engage the brake disc. This provides service braking. Typically, the brake piston displaces an inboard brake lining directly and an outboard brake lining via the brake caliper.

The disc brake assembly may also be used to provide a parking brake function. The disc brake assembly provides the parking brake function by first using the pressure to move the brake linings into the braking position and then using an electric parking brake (EPB) to clamp or otherwise support the brake piston in the braking position. An actuator of the EPB may have a rotationally restrained spindle nut threaded onto a spindle driven by an electric motor. As the spindle is rotationally driven, the spindle nut axially translates to clamp the brake piston on the brake linings in the braking position.

Generally, as more brake pistons are provided for the single disc brake assembly, a brake force produced by the disc brake assembly increases. To produce a required brake force for some vehicles, such as pickup trucks, twin—i.e., two—brake pistons are commonly provided for each of the disc brake assemblies. When the disc brake assembly with twin brake pistons provides the parking brake function, the EPB clamps both of the twin brake pistons. To clamp both of the twin brake pistons, a separate motor is typically provided for each of the twin brake pistons. However, control of the two separate motors must be coordinated for smooth application of the EPB. Coordinating control of the two separate motors adds complexity to the EPB. Thus, it would be desirable to have a disc brake assembly with twin brake pistons and an EPB having a single motor.

SUMMARY OF INVENTION

This invention relates to electric parking brakes for use with disc brake assemblies, wherein the electric parking brake has an improved gearing.

According to one embodiment, a vehicle disc brake assembly having a hydraulically actuated service brake and an electrically actuated parking brake may comprise, individually and/or in combination, one or more of the following features: A brake disc, a first brake piston having a first inner portion and a first spindle nut rotationally restrained on a first spindle and having a first outer portion. The first outer portion is configured to clamp against the first inner portion to produce a first clamp force when the first spindle is rotated and the first clamp force engages the first brake piston against the brake disc. A second brake piston having a second inner portion and a second spindle nut rotationally restrained on a second spindle and having a second outer portion. The second outer portion is configured to clamp against the second inner portion to produce a second clamp force when the second spindle is rotated and the second clamp force engages the second brake piston against the brake disc. An electric motor configured to produce a torque to actuate and apply the parking brake. A first torque limiting device configured to, during an application of the parking brake, fully transmit a first portion of the torque to the first spindle when a first threshold value exceeds the first clamp force and not fully transmit the first portion of the torque to the first spindle when the first clamp force exceeds the first threshold value. A second torque limiting device configured to, during the application of the parking brake, fully transmit a second portion of the torque to the second spindle when a second threshold value exceeds the second clamp force and not fully transmit the second portion of the torque to the second spindle when the second clamp force exceeds the second threshold value.

According to this embodiment, the first and second threshold values may be equal.

According to this embodiment, when the first clamp force exceeds the first threshold value and the second threshold value exceeds the second clamp force, the second torque limiting device may transmit the first portion of the torque that exceeds the first threshold to the second spindle.

According to this embodiment, the electric motor may be a single electric motor.

According to this embodiment, a first total quantity of torque limiting devices may be equal to a second total quantity of brake pistons.

According to this embodiment, during a release of the parking brake, the first torque limiting device may fully transmit the first portion of the torque to the first spindle and the second torque limiting device may fully transmit the second portion of the torque to the second spindle.

According to this embodiment, each of the first and second torque limiting devices may have a first gear, a drive plate, pockets in an end face of at least one of the first gear and the drive plate, force transmitting members, and a spring member producing a spring force on the drive plate. During the application of the parking brake, the spring force holds the force transmitting members compressed in the pockets to transmit a force between the first gear and the drive plate when the spring force exceeds a portion of a clamp force and does not hold the force transmitting members in the pockets, and the force is not transmitted between the first gear and the drive plate, when the portion of the clamp force exceeds the spring force. For the first torque limiting device, the first gear may be configured to receive the first portion of the torque, the drive plate may be configured to transmit the first portion of the torque to the first spindle, and the spring force may hold the force transmitting members compressed in the pockets when the spring force exceeds the portion of the first clamp force. For the second torque limiting device, the first gear may be configured to receive the second portion of the torque, the drive plate may be configured to transmit the second portion of the torque to the second spindle, and the spring force may hold the force transmitting members compressed in the pockets when the spring force exceeds the portion of the second clamp force. The first threshold value may be equal to the spring force of the first torque limiting device and the second threshold value may be equal to the spring force of the second torque limiting device. The force transmitted by the first torque limiting device may be the first portion of the torque and the force transmitted by the second torque limiting device may be the second portion of the torque.

According to another embodiment, an electrically actuated parking brake for a vehicle disc brake assembly having first and second pistons and a brake disc may comprise, individually and/or in combination, one or more of the following features: A single electric motor configured to produce a torque and first and second torque limiting devices. The torque actuates and applies the parking brake. The first torque limiting device is configured to, during an application of the parking brake, fully transmit a first portion of the torque to a first spindle until a first clamp force produced by the parking brake overcomes a first spring force of the first torque limiting device. The first clamp force is configured to engage the first brake piston against the brake disc. The second torque limiting device is configured to, during the application of the parking brake, fully transmit a second portion of the torque to a second spindle until a second clamp force produced by the parking brake overcomes a second spring force of the second torque limiting device. The second clamp force is configured to engage the second brake piston against the brake disc. When the first clamp force overcomes the first spring force and the second spring force overcomes the second clamp force, the first torque limiting device does not fully transmit the first portion of the torque to the first spindle and the first portion of the torque that exceeds the first spring force is instead transmitted to the second spindle by the second torque limiting device.

According to this embodiment, during a release of the parking brake, the first torque limiting device may fully transmit the first portion of the torque to the first spindle and the second torque limiting device may fully transmit the second portion of the torque to the second spindle.

According to yet another embodiment, a torque limiting device for an electrically actuated parking brake producing a clamp force to engage a brake piston of a vehicle disc brake assembly against a brake disc of the vehicle disc brake assembly may comprise, individually and/or in combination, one or more of the following features: A first gear configured to receive a torque from an electric motor, a drive plate configured to transmit the torque to a spindle of the parking brake, pockets in an end face of at least one of the first gear and the drive plate, force transmitting members configured to, when held compressed between the first gear and the drive plate in the pockets, transmit the torque between the first gear and the drive plate and, when not in the pockets, to not transmit the torque between the first gear and the drive plate, and a spring member producing a spring force on the drive plate. The torque is configured to actuate and apply the parking brake. During an application of the parking brake, the spring force holds the force transmitting members compressed in the pockets when the spring force overcomes a clamp force produced by the parking brake and does not hold the force transmitting members in the pockets when the clamp force overcomes the spring force. The force transmitting members exit the pockets when not held in the pockets by the spring force during the application of the parking brake.

According to this embodiment, the first gear may be a sun gear.

According to this embodiment, the torque limiting device may further include a second gear rotationally fixed to the drive plate, wherein the drive plate transmits the torque to the spindle through the second gear. The second gear may be a pinion gear.

According to this embodiment, the pockets may be in the first gear, the drive plate, or both the first gear and the drive plate.

According to this embodiment, the pockets may further include ramp surfaces that are transverse to a longitudinal axis about which the first gear and drive plate are configured to rotate, stop surfaces parallel to the longitudinal axis, and pocket surfaces connecting the ramp surfaces and the stop surfaces. The force transmitting members may move on the ramp surfaces to exit the pockets when the clamp force overcomes the spring force.

According to this embodiment, when the clamp force overcomes the spring force, the force transmitting members may exit the pockets when the first gear is rotated in a first direction and do not exit the pockets when the first gear is rotated in a second direction opposite the first direction.

According to this embodiment, the force transmitting members may be ball bearings.

An advantage of an embodiment is an electric parking brake having a single electric motor for a disc brake assembly with twin brake pistons. Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.

