Joints and connections – Screw or cam
Reexamination Certificate
1999-08-24
2001-05-29
Browne, Lynne H. (Department: 3629)
Joints and connections
Screw or cam
C403S230000, C403S282000, C411S166000, C411S114000, C092S100000
Reexamination Certificate
active
06238132
ABSTRACT:
This invention relates to spring brake actuators of the type utilized to actuate the brakes of heavy vehicles. More particularly, the present invention relates to the push rod and plate assembly of such spring brake actuators and an improved method of securing the plate to the push rod which eliminates the requirement for a thread adhesive and reduces or eliminates deformation of the exterior surface of the push rod which may cause leakage.
BACKGROUND OF THE INVENTION
Pneumatic braking systems are commonly used on large commercial vehicles, such as trucks, trailers and buses. The considerable inertial mass of these heavy-duty vehicles in combination with the high speeds at which they travel requires a braking system which responds rapidly with substantial braking power. One system component of pneumatic braking systems is the spring brake actuator. The brake actuator provides the force necessary when braking the vehicle. There are several types of spring brake actuators including piston type spring brake actuators and diaphragm spring brakes. The service chamber of the spring brake actuator includes a housing defining a pneumatic chamber connected to the pneumatic system of the vehicle, a piston or push rod having a head or plate which reciprocates within the housing under pneumatic pressure and a return spring. In a diaphragm type spring brake actuator, a cup-shaped flexible diaphragm is supported within the chamber and the push rod plate is received against the diaphragm, such that one end of the push rod is driven through an opening in the housing when the pneumatic pressure flexes the diaphragm. A dual diaphragm spring brake actuator of the type disclosed herein includes an upper emergency chamber and a lower service chamber. The emergency chamber includes a power spring which is normally held in a compressed state by pneumatic pressure acting against the diaphragm in the upper chamber. When the pneumatic system of the vehicle fails or the parking brakes are applied, the power spring expands, driving the diaphragm and push rod in the upper chamber to brake the vehicle. The service chamber includes a second diaphragm and push rod assembly which actuates the braking system of the vehicle. Such spring brake actuators are well known in the art.
The push rod is normally secured to the end plate or end plates by a convention screw. The end plate includes an opening therethrough and the end of the push rod includes an internally threaded bore which receives the screw. However, there are several problems associated with the present method of attaching the end plate to the push rod. First, the connection between the push rod and the end plate must not loosen during use, particularly under the extreme temperature variations and vibration requirements. Second, the overall length of the push rod and plate assembly must be accurately controlled to the control the stroke of the push rod. Thus, the plate must be rigidly secured to the push rod. This is presently accomplished by using a thread adhesive such as Loctite® on the threads of the screw. As will be understood, however, applying a thread adhesive in a blind tapped hole cannot be fully controlled, particularly where the thread adhesive is applied by hand, and therefore at least some of the installations will fail. Further, non-destructive testing cannot assure that there will not be failures in the field. Finally, the exterior surface of the cylindrical push rod in the emergency chamber must be very accurately machined and coated with a hard protective coating to avoid leakage because of the pressure differential between the emergency and service chambers. A seal is provided between the emergency and service chambers and the push rod reciprocates through the seal. Therefore, any lack of concentricity or damage to the exterior surface of the push rod in the emergency chamber of a dual diaphragm spring brake actuator will result in leakage between the chambers. In the disclosed embodiment of the dual diaphragm spring brake actuator, an end plate is attached to both ends of the push rod. It is not therefore practical to forcefully grip the push rod of the emergency chamber during the connection of the end plates to the push rod which would cause deformation damage to the push rod and leakage between the chambers.
There is therefore a longstanding need to develop an effective method of attaching the end plate or end plates to the push rod of a spring brake actuator which assures a rigid permanent interconnection and which does not loosen during use. Further, a method of interconnecting the end plate to the push rod cannot result in damage to the push rod, particularly the push rod in the emergency chamber to avoid leakage between the emergency chamber and the service chamber. Finally, the method of attaching the end plate to a push rod cannot significantly increase the cost of the assembly. The improved brake actuator, particularly the improved push rod and plate assembly and method of attaching the end plate to a push rod of this invention accomplishes these objects.
SUMMARY OF THE INVENTION
As set forth above, this invention relates to a brake actuator having an improved push rod and plate assembly and a method of attaching an end plate to a push rod which assures a rigid and permanent assembly while avoiding any damage to the push rod. The brake actuator includes a housing which defines one or a pair of chambers, a push rod having an end plate which reciprocates within the housing and a return spring. In a diaphragm type spring brake actuator, a cup-shaped diaphragm is supported within the housing and the end plate of the push rod is biased against the midportion of the diaphragm. As set forth above, the push rod of an emergency chamber reciprocates through an annular seal.
The push rod and plate assembly includes a cylindrical push rod having a longitudinal axis, at least one end portion having an internally threaded bore, an end plate having an opening therethrough extending generally perpendicular to the longitudinal axis of the push rod and a screw having a threaded shank portion extending through the opening in the end plate and threadably received in the internally threaded bore of the push rod. The screw includes a head portion which retains the end plate to the push rod.
In the improved push rod and plate assembly of this invention, the shank portion of the screw adjacent the threaded portion includes a plurality of radial lobes each having a maximum circumferential diameter greater than the internal crest diameter of the threaded bore which deforms the crests of the threads of the internal bore, thereby securely retaining the end plate to the end portion of the push rod in flush relation and significantly increasing the torque required to unthread the screw. Stated another way, the circumferential diameter of the lobes is greater than the root diameter of the threaded shank portion. The longitudinal width of the lobes is preferably equal to one to seven threads of the threaded shank portion or more preferably two to five threads, such that the locking occurs only in threading the last few threads of the threaded shank in the threaded bore. Thread forming screws in an unthreaded bore has also been tried; however, thread forming screws require too much torque resulting in deformation or distortion of the push rod used in the emergency chamber of the brake actuator. As set forth above, the cylindrical push rod in the emergency chamber is accurately ground and preferably hard coated. An aluminum push rod is preferred to reduce weight. Further, an aluminum push rod in the emergency chamber is preferred to avoid rust which will interfere with and wear the seal between the emergency chamber and the service chamber. The most preferred embodiment of the push rod is an aluminum push rod having a hard zinc anodized coating. Alternatively, the push rod may be formed of steel having a hard rust resistant coating.
In the most preferred embodiment of the push rod and end plate assembly, the screw is designed such that the torque require
Plantan Ronald S.
Schultz Thomas O.
Browne Lynne H.
Cottingham John R.
Howard & Howard
Indian Head Industries Inc.
LandOfFree
Push rod and plate assembly for brake actuators and method... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Push rod and plate assembly for brake actuators and method..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Push rod and plate assembly for brake actuators and method... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2567810