1. Field of the Invention
The present invention relates generally to pneumatic brake boosters, and more particularly to a pneumatic brake booster with an air valve and a collapsing plunger.
2. Background of the Invention
In the design and manufacture of automobiles brake systems, decreasing the stopping distance of the vehicle when the brakes have been applied has frequently been a focus. Systems have been developed to generate maximum braking power within a fraction of a second, depending on the input rate from the driver.
Typically, panic devices located within the brake booster are used to accomplish this function. The brake booster amplifies the input force applied by the driver""s foot on the brake pedal. A brake booster is comprised of front and rear chambers which are housed within a sealed volume. The front and rear chambers are separated by a flexible diaphragm membrane which prevents any type of pressure communication between the two chambers. In addition, brake boosters contain an input member of which the vehicle operator uses to engage the brake booster. When the braking system is at-rest, i.e., the driver""s foot is off the brake pedal, both front and rear chambers are evacuated and in equilibrium. When the driver actuates the brake pedal, a mechanical valve is opened which allows air to flow into the rear chamber creating a pressure differential across the membrane.
This pressure differential acts over the membrane area and thus creates an output boost force which is generated on the piston reaction surface. The output force of the booster is the input force (from the brake pedal) plus the boost force. In this manner, the force of the driver""s foot on the brake pedal is boosted to improve the force required of the driver for a deceleration.
Modern pneumatic brake boosters create a feedback force which acts in the opposite direction of the output force. The feedback force is transmitted through the brake pedal and gives the driver an indication of the current brake force. The feedback force is generated from the pressure and extrusion of the elastomer reaction disc acting in such a way as to close off the working diaphragm chambers from atmospheric pressure and allows the chambers to evacuate, thus reducing the output force. The feedback force is dependent upon the output force and increases as the output force increases. Such a system works sufficiently for average brake apply conditions. However, in an emergency condition where a short braking distance is critical, the feedback force may act to increase the braking distance by creating additional force required by the driver of the vehicle.
Several systems have been designed to reduce or eliminate the feedback force in emergency situations. For example, U.S. Pat. No. 6,186,042 issued Feb. 13, 2001 to Roland Levrai et al (hereafter xe2x80x9cLevraixe2x80x9d) discloses a xe2x80x9clatchingxe2x80x9d type brake booster. The Levrai booster allows a braking force to be maintained even if the operator terminates the input force (removes his/her foot from the brake pedal) by locking the air valve position relative to the power piston if an emergency condition is detected. This, however, is undesirable since it removes control from the driver and gives no feedback with respect to status of the brake system.
U.S. Pat. No. 6,109,164 issued Aug. 29, 2000 to Hisakazu Okuhura et al (hereafter xe2x80x9cOkuhuraxe2x80x9d) discloses a brake booster which reduces the feedback force in the case of an emergency by varying the angles of loading surfaces which changes the amount of force being transferred to the power piston.
PCT Application WO 00/30946 published Jun. 24, 1999 and naming as an inventor Christopher Voss (hereafter xe2x80x9cVossxe2x80x9d) discloses a brake booster which reduces the feedback force in the case of an emergency by modifying the size of an orifice through which a reaction disk transmits the feedback force.
However, both of these types of systems are difficult and expensive to manufacture. The present invention is aimed at one or more of the problems identified above.
In one aspect of the present invention, a pneumatic brake booster is provided. The pneumatic brake booster includes a generally cylindrical shaped piston having a first piston end and a second piston end. The piston defines a chamber at the first piston end and a piston bore at the second piston end separated by an air flow aperture. The piston bore has a plunger aperture located at the second piston end. The pneumatic brake booster also includes a plunger having a first plunger end and a second plunger end. The plunger is moveable between first and second positions and is located within the plunger aperture. An air valve has a first valve end and a second valve end and is moveable between first and second positions and defines a valve bore for receiving the first plunger end. A collapsing spring is located within the valve bore and is adapted to couple the plunger and the air valve. The plunger and the air valve are spaced apart a first relative distance in response to the pneumatic brake booster being in a regular apply mode and spaced apart a second relative distance in response to the pneumatic brake booster being in a panic apply mode.
In another aspect of the present invention, a pneumatic brake booster, is provided. The pneumatic brake booster includes a generally cylindrical shaped piston having a first piston end and a second piston end. The piston defines a chamber at the first piston end and a piston bore at the second piston end separated by an air flow aperture. The piston bore has a plunger aperture located at the second piston end. The pneumatic brake booster further includes a plunger, an air valve, and a sleeve. The plunger has a first plunger end and a second plunger end and is moveable between first and second positions. The second plunger end is located within the plunger aperture. The plunger includes a plunger trench which is located near the first end of the plunger. The air valve has a first valve end and a second valve end. The second valve end includes a bearing aperture and defines a valve bore for receiving the first plunger end. The sleeve includes a sleeve bore for receiving the second valve end and includes a sleeve trench located around an interior surface of the sleeve. The pneumatic brake booster further includes at least one ball bearing located within the bearing aperture and being adapted to fit alternatively in the plunger trench or the sleeve trench.