Spring devices – Coil – Including internal brace
Reexamination Certificate
1999-06-29
2002-01-29
Oberleitner, Robert J. (Department: 3613)
Spring devices
Coil
Including internal brace
C267S174000, C267S178000, C267S252000
Reexamination Certificate
active
06341767
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to springs requiring a high ratio of stored energy to moving mass, so that the spring can move a payload through a specified distance in a very short time. The invention is applicable in the field of automotive valve springs, and especially to high performance springs used to restore electric valve actuation solenoids to a central position between two holding electromagnets.
2. Description of the Prior Art
Springs used to control fast motions with high accelerations must be able to exert a force through a distance, i.e. to transfer energy, while contributing minimally to the moving mass of the system. While a high performance spring will accelerate a payload mass through a specified stroke distance (e.g., the stroke of an electric valve actuator) in some specified short time period (e.g., 3 milliseconds), a poorly designed spring cannot even move its own mass through the specified distance in the specified time, with no payload at all. In a spring-and-payload system, some fraction of the effective moving mass ends up being spring inertia, with the remaining mass being the true payload. It is generally an advantage to maximize the payload fraction of the total moving mass, but in valve applications for internal combustion engines, and especially in the design of electric valve actuators for internal combustion engines, a high payload mass fraction is especially critical to overall performance. When the payload mass fraction is low, the spring mass and total mass of the system necessarily go up, in order to make the spring big enough to accelerate and move the valve payload through a specified stroke in a specified time. With an inefficient spring (i.e. a spring with a low ratio of exchanged elastic energy to effective moving mass), an increase in actuation force must accompany the increase in moving spring mass, implying more mechanical work performed per stroke, with a larger mechanism and at increased energy losses. In engines with a mechanical valve drive train, the actuation mechanism is a rotating cam, whose mass is not part of the mass to be accelerated with the valve and spring. In electric valve actuators, by contrast, the actuation mechanism includes an armature whose mass adds to the payload to be accelerated and decelerated quickly, by spring action, in transit between holding positions at full-open and full-closed. When moving spring mass is added, spring force must to be added to keep acceleration at a specified level. The increase in spring force calls for an increase in electromagnetic holding force, which in turn calls for an increase in armature mass. One sees that excess moving spring mass propels an upward spiral of mass addition to satisfy engineering requirements. Conversely, a reduction in moving spring mass for the same valve mass, stroke, and transit time, propels a spiral of mass reductions until the designer is faced with a desirable pair of design alternatives: either to make a faster valve, or to keep valve transit speed the same while transferring spring mass savings over into additional armature magnetic material, permitting achievement of increased electromagnetic efficiency.
Practical considerations for many high performance springs, particularly for electric valve actuation, usually include operation without fatigue and with minimal mechanical wear, and also compactness of the spring. Surface wear in highly stressed regions of a spring must be avoided, since wear accelerates stress-related failure. Attachment to a wire spring is prone to create localized stress concentrations, especially if the spring is attached where it undergoes significant bending or torsional moments. By far the most common solution to these multiple design challenges has been the helical compression spring. The wire in a helical spring experiences mostly torsion when the spring is compressed (or stretched, in the case of a tension spring design). It is well known that spring wire will store more energy per unit mass in a torsional mode than in a bending mode, lending an advantage to the helical compression or tension spring approach. In high performance compression spring designs, the end of the spring generally flattens out into a holding cup with a rolling motion causing no rubbing. A smooth transition is achieved from working spring wire to supported spring wire, minimizing stress concentrations. Compression springs whose ends are ground flat achieve a very small mass fraction that is non-working end mass. While it is an effective design, the traditional helical compression spring with flat ends leaves room for improvement, especially when used for electric valve actuation. In the electric actuation context, the valve is not preloaded to a mechanical stop, but instead sits at a neutral position roughly midway between its travel limits, until magnetic forces move the valve away from that neutral position either to a full-open or full-close position. The overall spring configuration must therefore exert force in two directions, toward the neutral position from either side. This bi-directional force is achieved, in the present art, by pre-loading a pair (or more than one pair) of compression springs against one another, so that one spring does most of the pushing from one side of center, while the other spring does most of the pushing from the opposite side of center. One finds that each compression spring stores two components of elastic energy: a variable energy component that contributes to the bi-directional centering restoration force, and a fixed energy component that provides compression preload but does not contribute to the bi-directional restoration. This mechanical fixturing preload serves to keep the ends of the spring seated firmly in their cups, since the end attachment is designed to push but not pull. By contrast, a built in “preload” of compressive surface stress, as achieved by shot peening of a finished wire spring, can help the surface resist crack propagation, thus extending fatigue life. Mechanical fixturing preload, as used to stabilize spring material with a net unidirectional force toward a confining surface, is a disadvantage if it creates a stress bias in material that is also highly cyclically stressed by spring operation. The functional price paid for the preload is that the spring wire must store substantially more total energy in relation to the “working” energy that cycles in and out of the metal with each stroke. While metal fatigue is associated most strongly with the cyclic component of stored elastic energy, the static preload energy component takes its toll on the design, cutting significantly into the capacity for cyclic energy storage. Hence, one might inquire whether springs without static preload, operating over a range including tension and compression, might offer improved performance over paired preloaded compression springs. The invention to be described below embodies an affirmative response to this query.
It is not easy to design an end attachment for bi-directional push-pull operation, especially if the spring wire must be gripped at the radius of the helix, where high torsion forces tend to twist the wire in its attachment and cause wear and fatigue. The desirable action whereby a compression spring flattens smoothly into an end cup is lost when one attempts to design for both tension and compression. In tension spring designs, an approach to reducing wear and stress at the end attachment point is to bend or spiral the end of the wire inward toward the center-axis, thus reducing or eliminating the force-times-radius couple that puts the spring wire in torsion. Wire spiraled inward to a center-axis attachment need only be gripped for force transfer, avoiding the formidable problem of wear-free gripping of a wire subjected to variable torsional stress. Existing tension spring designs achieve this objective, but adaptations of this kind of approach for combining tension and compression are lacking.
An example of a helical spring used in both tension an
Bergstrom Gary E.
Seale Joseph B.
Atwood Pierce
Caseiro Chris A.
Oberleitner Robert J.
Scanlon Patrick R.
Williams Thomas J.
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