Seal for a joint or juncture – Seal between relatively movable parts – Circumferential contact seal for other than piston
Reexamination Certificate
2000-12-27
2003-11-25
Knight, Anthony (Department: 2178)
Seal for a joint or juncture
Seal between relatively movable parts
Circumferential contact seal for other than piston
C092S168000, C277S909000
Reexamination Certificate
active
06651988
ABSTRACT:
BACKGROUND
The present invention relates generally to a hydraulic actuator and, more particularly, to an improved hydraulic actuator for use in an active mount for a vibrating component in a system for reducing vibration and noise transmission from the vibrating component to a support structure.
Hydraulic actuators are used in numerous environments to induce movement of one object with respect to another object. A hydraulic actuator generally includes a cylinder and a moveable piston inside the cylinder. A piston rod is connected to the piston and extends outwardly from one end of the cylinder where the rod end is attached to the first object. The other end of the cylinder is mounted, directly or indirectly, to the second object. The means for mounting the piston rod and cylinder to the objects may incorporate flexible bearing assemblies to provide some “softness” to the attachment to allow for possible misalignment. Such bearing assemblies preferably comprise elastomeric material. Pressurized hydraulic fluid is introduced into the interior of the cylinder on one or both sides of the piston to effect longitudinal movement of the piston in the cylinder so that the objects are moved relative to one another.
Hydraulic actuators may be used as a component of an active mount in a system for reducing vibration and noise transmission from a vibrating component to a support structure. For example, hydraulic actuation systems are used for actively reducing the vibratory and acoustic loads on aircraft, particularly rotary wing aircraft such as helicopters. A primary source of vibratory and acoustic loads in a helicopter is the main rotor system. The main rotor system of a helicopter includes rotor blades mounted on a vertical shaft that projects from a transmission, often referred to as a gearbox. The gearbox comprises a number of gears that reduce the rotational speed of the helicopter's engine to the much slower rotational speed of the main rotor blades. The gearbox has a plurality of mounting “feet” which are connected directly to structure in the airframe that supports the gearbox. The main rotor lift and driving torque produce reaction forces and moments on the gearbox. All of the lift and maneuvering load torques are passed from the main rotor blades to the airframe through the mechanical connection between the gearbox feet and the airframe. The airframe structure that supports the gearbox is designed to react to these primary flight loads and safely and efficiently transmit the flight loads to the airframe.
In addition to the nearly static primary flight loads, the aircraft is also subjected to vibratory loads originating from the main rotor blades and acoustic loads generated by clashing of the main rotor transmission gears. The vibratory and acoustic loads produce vibrations and audible noise that are communicated directly to the helicopter airframe via the mechanical connection between the gearbox and the airframe. This mechanical connection thus becomes the “entry point” for the unwanted vibration and noise energy into the helicopter cabin. The vibrations and noise within the aircraft cabin cause discomfort to the passengers and crew. In addition, low frequency rotor vibrations are a primary cause of maintenance problems in helicopters.
Active vibration and noise reduction systems in aircraft utilize sensors to monitor the status of the aircraft, or the vibration producing component, and a computer-based controller to command actuators to reduce the vibration and noise. The sensors are located throughout the aircraft and provide signals to the adaptive controller. The controller provides signals to the hydraulic actuation system, including a plurality of actuators located at strategic places within the aircraft. The actuators produce controlled forces or displacements that attempt to minimize vibration and noise at the sensed locations.
Two methods of actuator placement are frequently used in the active system: (1) distribution of actuators over the airframe, or (2) co-location of the actuators at, or near, the vibration or noise entry point. When applied to the main rotor system of a helicopter, the co-location approach places the actuators at or near the structural interface between the transmission and airframe stopping vibration and noise near the entry point before it is able to spread out into the aircraft. This has the advantage of reducing the number of actuators and the complexity of the control system. Active systems using co-location to counteract vibration and noise employ actuators mounted in parallel (across) or in series (between) the transmission gearbox feet and airframe support structure.
When the actuator is mounted in series with the vibrating component and its support structure, six possible degrees of freedom exist between the two objects. However, only the degree of freedom along the principle load-carrying axis is actively controlled for vibration and noise reduction. The remaining degrees of freedom must remain unconstrained to prevent vibration and noise from reaching the support structure. The longitudinal axis of the actuator is aligned with the principle load carrying axis. Further, since the elastomeric bearing is located between the piston rod and the attachment point to the vibrating component, the bearing must provide high static and dynamic stiffness along this active, load-carrying axis so that motions of the piston translate directly into unattenuated motions at the attachment point. To ensure that motions at the attachment point along the five non-active degrees of freedom do not create vibration and noise, the stiffness between the attachment point and actuator along these directions must be low. However, the need for the elastomeric bearing to be stiff along its principle load-carrying axis, yet soft about the other five degrees of freedom, can cause the elastomeric bearing to be unstable under load. Also, transverse and rotational motions at the attachment point become transverse and rotational forces through the stiffness of the elastometric bearing. These forces are transmitted to the piston and can induce high loads between the piston and cylinder that may cause the piston to bind.
Moreover, since hydraulic actuators operate under high pressure, leakage of hydraulic fluid often occurs. This leads to maintenance problems as well as environmental concerns. Additionally, escaping hydraulic fluid can damage the elastomeric material of the bearing.
Examples of conventional seals used between a piston and a cylinder include elastomeric seals, spring-energized seals, and piston rings. Elastomeric seals tend to wear rapidly in actuators serving as active mounts as the result of excessive friction between the elastomeric material and the cylinder mating surface. The friction may be characterized as “interlocking,” which increases with roughness of a mating surface, and “adhesive,” which increases with an increase in the contact area with the mating surface. Even when the smoothness of an elastomeric seal's mating surface is increased, the resulting decrease in interlocking friction is insufficient to offset the increase in the adhesive friction. A low friction seal is vital to reducing noise and vibration. Since the piston will move within the cylinder at the frequencies of the noise and vibration, a high seal friction will partially regenerate the shaking forces in the cylinder wall that are desired to be reduced. This is especially true for noise, which is characterized by already small disturbance forces.
Conventional spring-energized seals include a U-shaped jacket, often made of a low-friction polymer, and a U-shaped metal spring device disposed in the jacket. While the friction between the seal and the cylinder is low, in many actuator applications where transverse loads are applied to the piston rod, the seal stiffness is inadequate to prevent the piston from contacting the cylinder. When the piston contacts the cylinder, the cylinder can become abraded. The abrasions in the cylinder increase the wear rate of the seal, leadi
Terpay Gregory Waston
Zipfel George Gustave
General Dynamics Advanced Information Systems Inc.
Hutton William D.
Knight Anthony
Morre & Van Allen, PLLC
Witsil Matthew W.
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