Near zero shock and momentum transfer selectively releasable separation nut

A selectively releasable separation nut for securing a payload and/or deployable equipment (hereafter “second body”) to a rocket, missile, or aircraft or spacecraft (hereafter “first body”) by way of a preloaded bolt, or other fastener, and releasing them on command. The separation nut may have magnetic eddy current damping components that dissipate as heat the strain energy stored in the separation nut, the bolt, and surrounding first body and second body structures during the bolt preload release. Energy not dissipated as heat during preload release may be stored as kinetic energy and dissipated as heat after the bolt mechanical release. The bolt acceleration and velocity are controlled throughout the release cycle. The bolt kinetic energy post release is less than 0.01% of the stored strain energy pre-release. Shock, impulse, and momentum transfer to the released second body are near zero.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a separation nut that generates near zero shock and momentum transfer to a released payload or deployable equipment for launch vehicle, missile, and spacecraft applications.

2. Background of the Invention

In launch vehicle, missile, and spacecraft applications it is sometimes desirable to hold down payloads or deployable equipment during launch and then release them on command. The hold down function is typically through a preloaded bolt, or other fastener, that connects the payload or deployable equipment, hereafter second body, to a launch vehicle, missile, or spacecraft, hereafter first body, by way of a separation nut. The bolt is typically withdrawn from the separation nut by a bolt catcher that may incorporate a spring to withdraw and capture the bolt and a deformable pad to damp the impact of the bolt within the bolt catcher. The hold down operation results in the storage of strain energy in a preload force loop proportional to the hold down preload force and the deflections of the separation nut, bolt, first body, second body, and bolt catcher. During the release operation the stored strain energy is converted to kinetic energy in the form of ½ mass times velocity squared of each of the deflected components. The released kinetic energy is manifested as impulse, or shock, during acceleration and stopping of the moving components within the separation nut, velocity of the bolt post release, and as momentum transfer to the released second body by the moving bolt that may be transferred to and captured by the released second body. Shock may occur when a fast-moving released bolt is stopped within the bolt catcher. Shock may damage sensitive electronics such as clock oscillators, alignment sensitive optics such as telescopes and star finders, or less robust mechanical mechanisms. Impulse conducted through the first body and momentum transfer to a released second body may adversely affect attitude-sensitive or formation flying satellites or the accuracy of released weapons.

The present invention is a separation nut that effectively dissipates virtually all, greater than 99.99%, of the stored strain energy in the separation nut and the preload force loop as heat. Shock caused during release is predicted to be less than 20 g on a standard test fixture, or less than 10% of that of the best mechanisms of the prior art and less than 2% of that of the typical mechanisms of the prior art. Momentum transfer is predicted to be less than 1% that of prior art mechanisms used in launch vehicle and missile applications. The present invention can be reset in situ and does not require refurbishment between operations. Its performance will not degrade over time.

Some separation nuts incorporate pyrotechnic actuators. Pyrotechnic actuators generally utilize an electrically ignited NASA Standard Initiator to, in-turn, ignite a high-pressure chemical gas generator. The high-pressure gas drives a piston to affect release of the mechanism load. Pyrotechnically actuated release mechanisms generally cause high shock and impulse due to the high acceleration and deceleration of the moving piston and may transmit the pyrotechnically generated momentum and impulse to the released payload through the surrounding structure. Momentum may be transmitted to the released payload by way of the released bolt mass times velocity and shock generated when the moving bolt is stopped. Pyrotechnically actuated release devices are typically used once and are not refurbished for re-use.

“Missile Stage Coupler,” U.S. Pat. No. 4,002,120, 1/1997, Swales, assigned to The United States of America as represented by the Secretary of the Navy, is an example of a pyrotechnic release device and bolt catcher for separating stages of a missile, such a booster stage and a re-entry vehicle. A stated object of the invention is “minimizing the possibility of tipoff (re-entry body angular velocity induced during the separation operation) or other flight perturbation.” The separation nut assembly ejects “ . . . separation bolt from the separation nut assembly with great force. The bolt travels upward within bolt catcher chamber at high velocity, exerting considerable separation force on the cover. . . . In practice, virtually no time lag exists between the transmission of the (release) signal to the pyro squib and the release of the separation bolt.” Analyses conducted during development of the present invention on similar pyrotechnic separation nuts in multi-separation nut re-entry body release systems showed that release simultaneity errors between the multiple separation nuts of just a few micro-seconds, when combined with the high impulse of pyrotechnic separation nuts and high momentum transfer of high velocity separation bolts, can cause a significant increase in the tipoff velocity, and reduction in targeting accuracy, of the released re-entry body.

The present invention minimizes tipoff velocity of released bodies by transferring near zero impulse and momentum to the released body. The increased simultaneity error between multiple units of the present invention, estimated at less than 0.5 milliseconds, is more than offset by the near zero impulse and momentum transfer of the present invention. Analysis predicts that the present invention causes less than 1% of the tipoff velocity of identical re-entry bodies than separation nuts of the prior art.

“Flywheel Nut Separable Connector and Method,” U.S. Pat. No. 5,603,595, Nygren, Jr., assigned to Martin Marietta Corp., claims an estimated 90% conversion of the strain energy in a connecting member into rotational kinetic energy in a rotating flywheel by way of a long pitch thread on the connecting member and internal to the flywheel. Stated flywheel rotational speed “may exceed 5,000 rpm” and thread lead is one inch per revolution. No energy dissipation method other than bearing and thread friction is incorporated. The connecting member does not separate from the flywheel until after the connecting member strain energy has been converted to kinetic energy. The connecting member velocity, kinetic energy, and momentum at release are a function of the flywheel rotational velocity and the thread pitch plus the potential energy stored in the retractor housing spring. Based on the given parameters, calculated connecting member velocity at release will exceed 83 inches per second. Significant momentum transfer to the released “second surface” and shock from stopping the connecting member within the retractor housing can be expected. In contrast, in the present invention when preloaded to 10,000 pounds, the bolt, including spring spacer or cup, velocity post release is calculated to be less than three inches per second.

The “Reduced Shock Separation Fastener,” U.S. Pat. No. 6,352,397, O'Quinn, et al, assigned to Hi-Shear Corporation (now Chemring Energetic Devices) is pyrotechnically released. It attempts to reduce shock by the incorporation of a limited rotation rotating ring that converts a portion of the bolt preload strain energy to heat through friction and to kinetic energy in the ring during release. Compliant pads stop the fast-moving pyrotechnically driven piston. U.S. Pat. No. 7,001,127, Tuszynski, also assigned to Hi-Shear Corporation, is a similar device that uses an electrical actuator to drive the initial release mechanism. Both mechanisms rely upon friction, created by the preload force, to both ensure load retention and ensure release.

A common initial release device used in electromechanical separation nuts is a fusible link. In these mechanisms, redundant load retaining wire links are electrically heated until they fuse, fail and release the bolt preload carrying mechanisms within the release device. One such device is U.S. Pat. No. 5,221,171, Rudoy et al, assigned to G & H Technology (now Eaton) that releases a split nut retention device when either of two fusible links is fused. This device does not incorporate bolt energy dissipation elements and hence causes both high shock and high momentum transfer to released payloads. U.S. Pat. No. 6,433,990, Rudoy et al, (assigned to NEA Electronics, Inc) uses redundant fusible links to release one end of a restraining wire, or strap, wrapped around a split spool. When the wire is released, it uncoils from around the split spool which releases a stud that carries the preload. Some reduction in shock is afforded by the energy dissipation that occurs while the wire uncoils and the split spool spreads. The fusible links must be replaced if the units are to be refurbished and reused.

Another type of separation nut incorporates shape memory alloy (SMA) actuators to release the preload carrying bolt. Shape memory alloys are formulated and processed so that when heated to their transformation temperatures they change phase and revert back to their “memorized” size or shape. Some SMA's change phase at 75 to 80 degrees C. and may change shape and cause premature release due to solar heating of spacecraft in which they may be utilized. One such mechanism is “Resettable Separation Mechanism With Antifriction Bearings,” U.S. Pat. No. 6,450,064, Christiansen, et al, (assigned to Starsys Research Corporation). This mechanism uses an SMA wire, with relatively short actuation stroke and low force, to release a cascaded mechanism that releases a split nut that releases the preload carrying bolt. Another SMA released mechanism, U.S. Pat. No. 7,544,257 B2, Johnson, et al (assigned to TiNi Alloy Company) uses an SMA cylinder, that when heated expands to increase the stress in a notched preload carrying bolt until the stress at the notch exceeds the ultimate strength of the bolt material, the bolt fails structurally, and the load is released. Neither of these SMA-release mechanisms incorporates strain energy dissipation elements and the undissipated energy is manifested as high bolt shank acceleration during release, high bolt velocity following release, high shock when the bolt is stopped, and high momentum transfer to a released payload or deployable equipment.

Momentum transfer by way of a released bolt, and its' adverse effect on released body tipoff velocity, from many electromechanical release devices of the prior art may be similar in magnitude to that of pyrotechnic separation nuts.

U.S. Pat. No. 5,248,233, No Shock Separation Mechanism, Webster, describes a release mechanism wherein the preload is carried as compression in an SMA column. When the SMA column is heated, it shrinks in length, relieving the tension on the preload carrying bolt, allowing a spring-loaded retainer to move and open a split nut so that the preload carrying bolt can be withdrawn. It may release when exposed to environmental shock if the preload is low.

Release mechanisms of the prior art may dissipate energy only through friction. However, friction is highly unpredictable due to changes in force between contacting parts, wear of the contacting parts, and lubricant viscosity changes. As a result, if the friction is too high the mechanism may not release and if it is too low the mechanism may release, or partially release, due to environmental shock, vibration, and temperature. If the friction is too low less than optimal energy may be dissipated resulting in high shock and momentum transfer during and following release.

SUMMARY OF THE INVENTION

One embodiment of the present invention is a selectively releasable separation nut for holding, by way of a preloaded bolt, or other fastener, payloads and/or deployable equipment secure against vibration during launch by launch vehicles (aircraft, rockets, or missiles) and releasing them on command. During bolt preload release a portion of the strain energy stored in the separation nut, bolt, and surrounding structure preload force loop is dissipated as heat and residual strain energy is stored as kinetic energy within the separation nut so that the bolt potential and kinetic energy at release are essentially zero. Following bolt release the stored kinetic energy within the separation nut is dissipated as heat. The separation nut may have elements that dissipate kinetic energy as heat, control the velocity of the bolt preload release, and compensate for changes in temperature, friction, and aging.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the present invention is a separation nut1,FIG. 1, for holding by way of a preloaded bolt13,FIG. 2, a payload and/or deployable equipment (hereafter second body) securely to an aircraft, a launch vehicle, or spacecraft (hereafter first body) securely against vibration and shock and releasing the second body upon an electrical command signal. Strain energy stored in a preload force loop comprising the separation nut, the preloaded bolt, the first body, and the second body, is converted to kinetic energy within the separation nut and converted to heat by an eddy current damper and friction both during preload release and following the bolt mechanical release.

The present invention embodies a multi-stage release and energy dissipation cycle comprising:

A locked condition in which a structural preload force loop secures a second body to a first body by way of the separation nut and the preloaded bolt.

An initial actuation in response to an externally supplied selectable electrical command signal.

A preload reduction to zero, at a controllable rate, of the forces within the structural preload force loop, dissipation of a portion of the stored strain energy as heat within the hold down and release mechanism, and storage of residual strain energy stored in the preload force loop that is not dissipated as heat, as kinetic energy within the separation nut.

A mechanical release of the bolt.

A dissipation of the stored kinetic energy as heat following the mechanical release of the bolt.

The separation nut1,FIG. 2, may have elements that store and dissipate kinetic energy as heat and comprise a feedback system that controls the acceleration and velocity of the bolt13during preload release. During bolt13preload release the preferred embodiment of the invention dissipates a portion of the stored strain energy as heat through a combination of electromagnetic eddy current damping and mechanical friction in the release mechanism1. The residual un-dissipated preload energy is stored as kinetic energy in the linearly moving rotor assembly7and rotating rotor8during preload release. When the bolt13has fully released its preload and its strain energy has been released, the strain energy has either been dissipated as heat or stored as kinetic energy in the rotor assembly7and dynamically balanced rotor8. When the bolt13preload is zero, the radial bolt preload thread reaction force of the split nut segments11on lock ring18is also zero, and lock ring18is free to rotate and allow split nut segments11to separate and mechanically release bolt13,FIG. 4. Following bolt13mechanical release from the split nut segments11, eddy current damping and mechanical friction in the preferred embodiment convert the rotor assembly7and dynamically balanced rotor8kinetic energy to heat and stop the rotor assembly7and dynamically balanced rotor8rotation and translation.

The initial release mechanism41,FIG. 4, components of the present invention preferred embodiment, with the exception of the sears5, are dynamically balanced and rotate about their centers of gravity axes. Linear vibration and shock, as defined in typical customer separation nut specifications, will not act upon the present invention's dynamically balanced release components to cause rotation and premature release. Prior to initial release the low-mass sears5are mechanically locked by release armature17and bearings21and are not susceptible to shock and vibration.

The present invention preferred embodiment incorporates an initial release mechanism41incorporated into cover6,FIG. 6, and housing2,FIG. 4. A direct current electric motor19,20,FIG. 6, is incorporated into cover6,FIG. 2, to affect initial release. Ten motor magnet segments20are bonded to the outside diameter of release armature17,FIG. 2, 6. The magnet segments20alternate radial polarity, so that every other magnet segment has its north pole on its outside diameter and each adjacent magnet segment has its south pole on its outside diameter. Primary and redundant motor windings19and their leads30,FIG. 2, are bonded to the inside surface of cover6so that the leads30are not flexed due to environmental shock or vibration or during operation. Each motor winding19is comprised of five rectangular coils wound and formed to fit the inside radius of cover6. As shown inFIG. 6, the portion of each coil where the current flow is within the fields of magnets20and parallel to the motor19,20rotational axis generates torque that causes release armature17to rotate counterclockwise. Electrically shielded leads30supply an electrical release signal from the first body to the primary and redundant windings19and are routed out of the release mechanism1through the port31,FIG. 2, in cover6and conductive epoxy strain relief32.

Release armature17is supported and aligned by rolling element guide bearings22,FIG. 2, and rolling element bearings between release armature17and sears5. The rolling element bearings minimize friction and the torque requirement for the electric motor19,20.

As shown inFIG. 6, each winding19coil spans two oppositely polarized magnet segments20so that the tangential forces, and torque, generated by each side of the coils are in the same direction. The nominal full-torque tangential stroke of the motor19,20equals the magnet segment20tangential width minus the winding19coil tangential width. The motor19,20, and release armature17, nominal full torque angular stroke, in radians, is then (the tangential stroke)/(the winding19mean radius). The mechanism may be designed so that release armature17travel is greater than the full-torque motor angular stroke so that motor19,20torque will decrease when the winding19coils are over the gaps between the magnet segments20and reverse direction when the coils approach the next oppositely polarized magnet segment20. The release armature17will stop when the reverse torque plus the return springs42,FIG. 4, torques equals the motor19,20driving torque. This non-impact means of stopping release armature17further reduces the shock generated by the separation nut1during release.

Cover6and release armature17,FIG. 2andFIG. 6, comprise the magnetic circuit for the motor19,20. They may be composed of multiple alloys or coatings to satisfy structural, magnetic performance, and anti-corrosion requirements. Magnet segments20may be fabricated from neodymium boron iron or other magnet material at the discretion of the designer. Magnetic flux flows radially outward from the north poles of five first magnet segments20, across a mechanical clearance gap38, through the motor windings19, clockwise and counterclockwise through the cover6, radially inwards through the motor windings19, across the gap38, into the south poles of the five alternately polarized magnet segments20, clockwise and counterclockwise through the release armature17, and then back into the south poles of the five first magnet segments20.

Torque developed by the motor19,20is calculated by the equation
T=BlirN
where T, torque, is in newton-meters, B, magnetic gap flux, is in Tesla, l, active coil length in the magnetic gap per magnet, is in meters, i, motor current, is in amps, r, winding19coil mean radius, is in meters, and N, is the number of magnet segments20. The metric units may be converted to English units, or vice-versa, for consistency in the calculations at the discretion of the analyst. Alternative motor configurations may occur to those skilled in the art.

The preferred embodiment of the present invention primarily uses an eddy current damper,FIG. 3, to dissipate the strain energy stored in the preload force loop as heat. Energy dissipation through friction is unavoidable. However, friction is highly unpredictable due to changes in force between contacting parts, wear of the contacting parts, and lubricant viscosity changes. The present invention incorporates rolling element bearings in all highly loaded mechanical interfaces to minimize friction and the inherent feedback system minimizes the effects of friction variation.

The magnetic eddy current damper,FIG. 3, consists of fourteen damper magnet segments4bonded, or otherwise affixed, to the inside diameter of housing2,FIG. 2. The damper magnet segments4alternate radial polarity, so that every other magnet segment has its north pole on its inside diameter and each adjacent magnet segment has its south pole on its inside diameter. Housing2and rotor8,FIG. 2andFIG. 3, comprise the magnetic circuit for the eddy current damper. They may be composed of multiple alloys or coatings to satisfy structural, magnetic, and anti-corrosion requirements. Damper magnet segments4may be fabricated from neodymium boron iron or other magnet material at the discretion of the designer. Magnetic flux flows radially inward from the north poles of seven first magnet segments4,FIG. 3, across a mechanical clearance gap15, through the conductive damper ring12, clockwise and counterclockwise through the rotor8, radially outward through the conductive damper ring12, across the gap15, into the south poles of the seven alternately polarized damper magnet segments4, clockwise and counterclockwise through the housing2, and then back into the south poles of the seven first damper magnet segments4. Other numbers of magnet segments4can be used, or the magnets can be affixed to the rotor8and the damper ring12affixed to the housing1, at the discretion of the designer.

Eddy current damping is linearly proportional to the relative velocity between the magnet segments4and the conductive damper12,FIG. 2. Damping torque and energy dissipation are maximum when the split nut segments11, thread end of bolt13, and the rotor assembly7linear velocity and rotor8rotational velocity are maximum and are zero when their velocities are zero. Unlike mechanical friction damping, magnetic eddy current damping has no stick-slip characteristic where the damping coefficient changes between when there is no relative motion between the magnet segments4and the conductive damper12and when there is relative motion. The separation nut1,FIG. 1, will not stall and fail to release due to friction inconsistencies if the bolt13preload is very low and release is driven solely by spring23,FIG. 2.

When the separation nut1is released the dynamically balanced rotor8rotates in ball screw assembly3and the radial magnetic flux moves tangentially and downwards through the damper ring12so that a circulating electric current is induced in the plane of the damper ring12normal to the radial magnetic flux. The magnetic fields of the induced currents in damper ring12oppose the magnet segment4magnetic fields which results in damping forces that oppose the rotation of the rotor8.

Following bolt13mechanical release the residual energy that was not dissipated as heat through eddy current damping and mechanical friction is stored as kinetic energy in the linearly moving rotor assembly7and the rotating dynamically balanced rotor8,FIG. 2. Dynamically balanced rotor8continues to rotate downwards on the ball screw3until all kinetic energy is dissipated as heat from eddy current damping and mechanical friction. Any residual energy at the end of the rotor8travel is dissipated by friction with and compression of resilient pad29.

In the preferred embodiment of the present invention the instantaneous eddy current damping torque on the rotor8can be calculated from the equation

Tdamping=B2⁢l2R⁢ω⁢⁢r2⁢N
where Tdamping, instantaneous damping torque, is in Newton-meters, B, magnetic flux density in the conductive damper ring12, is in Tesla, l, damper ring12active electrical circuit length between adjacent magnet segments, is in meters normal to the magnetic gap flux and the relative velocity of the rotating magnetic flux B, R, electrical resistance of the damper ring12total electrical circuit between adjacent magnet segments4, is in ohms, ω, the instantaneous rotational velocity of the rotor8, is in radians/second, r, radius of the damper12, is in meters, and N is the number of magnet segments4. The total energy dissipated by the eddy current damper ring12is then
Edamping=∫Tdampingdθ
where Edamping, dissipated energy, is in Newton-meters and d⊖, rotor8differential rotation angle, is in radians. The contribution to energy loss due to the linear velocity of rotor8is calculated similarly, though it is relatively low because both the linear velocity and linear distance traveled by rotor8are low. The metric units may be converted to English units, or vice-versa, for consistency in the calculations at the discretion of the analyst. The present invention analyses can be readily performed by anyone ordinarily skilled in physics and magnetics design and analysis. Alternative magnetic eddy current damper configurations may occur to those skilled in the art.

Mechanical friction is difficult to predict accurately due to its dependence upon variables including, but not limited to, surface contact normal force, surface finish, wear, lubricant viscosity, and the difference between static and dynamic friction coefficients. In the preferred embodiment of the present invention friction is minimized by rolling element bearings3and9,FIG. 2, at high load interfaces in the mechanism to ensure release under worst case conditions. Prior to bolt13preload release the moving parts within the release mechanism1are mechanically retained against premature release caused by shock, vibration, other environmental impacts, or low preload rather than held in place by friction.

The stored energy converted to heat through mechanical rolling element friction is largely dependent upon the bearing geometry and the bearing force between the moving surfaces during the release cycle. The bearing force is in turn a function of the bolt13and spring23preload forces at any time during the release cycle. Friction is a maximum when the bolt13and spring23preloads are greatest at the start of preload reduction and minimum after the bolt13contracts to its unloaded length and its preload is zero. The instantaneous friction torque in the separation nut1rolling element bearings3,9is calculated from the equation
Tfriction=(Fμr)
where T friction, instantaneous torque, is in inch-pounds, F, instantaneous bearing force, is in pounds, μ is the bearing coefficient of friction, and r, the radius of the normal force, is in inches. The energy converted to heat by friction during the release cycle is calculated by the equation
Efriction=∫(Fμr)dθ
where Efriction, dissipated energy, is in inch-pounds, and d⊖, differential angular rotation, is in radians. In the preferred embodiment of the present invention friction is minimized by rolling element bearings3and9. As a result, the friction torque at any time during the release cycle is very low compared to the dynamically balanced rotor8torque developed from the remaining bolt13and spring23preloads and the release mechanism1will not stall during release. The total energy dissipated as heat by the separation nut throughout the release cycle is
Eheat=Edamping+Efriction
and is predicted to equal greater than 99.99% of the stored strain energy in the preload force loop pre-release.

The instantaneous kinetic energy of the linearly moving rotor assembly7, including the dynamically balanced rotor8and conductive damper ring12, split nut segments11, and lock ring18,FIG. 2, is proportional to the square of its linear velocity per the equation

KE⁢⁢linear=12⁢m⁢v2
where KE linear, instantaneous kinetic energy, is in in-lbs, m, mass of the rotor assembly7, is in lb-second2/inch, and v, velocity, is in inches/second.

The dynamically balanced rotor8and damper ring12are also rotating. The instantaneous kinetic energy of the rotating dynamically balanced rotor8and damper ring12,FIG. 2andFIG. 3, is proportional to the square of their rotational velocity per the equation

KE⁢⁢rotational=12⁢I⁢⁢ω2
where KE rotational, instantaneous kinetic energy, is in inch-pounds, I, the rotor mass moment of inertia, is in inch-pound-seconds2, and ω, rotational velocity, is in radians/second.

The total instantaneous kinetic energy stored is the sum of the instantaneous linear and rotational kinetic energies of the rotor assembly7and the dynamically balanced rotor8and damping ring12.

The combination of the eddy current damper4,12and bearing3,9friction energy dissipation and rotor assembly7and dynamically balanced rotor8and damper ring12,FIG. 2, energy storage characteristics results in a self-regulating feedback system that controls the mechanism1release acceleration and velocity. If friction energy dissipation increases, the release velocity decreases, the energy dissipated by the eddy current damper decreases, the kinetic energy stored in the rotor assembly7and dynamically balanced rotor8and damping ring12decreases, and the mechanism1release velocity stabilizes at a value that ensures that the energy being released by the bolt13and spring23equals the energy dissipated by friction and eddy current damping plus the stored kinetic energy at any time during release. For reasonable variations in friction torque, eddy current damping torque, and kinetic energy storage the time from bolt13preload maximum at the instant of preload release to bolt13preload equals zero is essentially constant for a given design. The bolt13, rotor assembly7, and dynamically balanced rotor8velocity, time, distance and angle traveled, and energy dissipation can be solved for by numeric integration of the equations or by simulation software.

During bolt13preload release,FIG. 2, the remaining bolt13preload force, separator27spring23force, and resulting rotor assembly7torque, are always several times greater than the torque necessary to overcome bearings3and9friction plus the eddy current damper4,12torques so that the separation nut1will not stall during release. In the event that the bolt13preload is lost prior to preload release the separator27spring23has sufficient force to overcome friction and damping forces and ensure that the separator spring23, segment separator27, and torsion springs33will open the split nut segments11and the separation nut1will always release the bolt13.

The preferred embodiment of the present invention separation nut1, shown inFIG. 2, is supported by a steel housing2that provides structural support, an outer raceway for the low pitch recirculating ball screw3, a magnetic return path for the eddy current damper12magnet segments4magnetic flux, reaction force support structure for the initial release system sears5, and the cover6. The ball screw3ball return path cover36is secured to housing2with screws37. Screws34secure the cover6to the housing2and dowel pins44,FIG. 4, or other features, align the cover6and react the bolt13tightening torque carried through the cover6and cover lugs50. The housing2may be composed of multiple alloys or coatings to satisfy structural, magnetic performance, and anti-corrosion requirements. The magnet segments4may be fabricated from neodymium boron iron or other magnetic material and bonded in place using a suitable adhesive.

The bolt13preload reaction force is carried from the housing2,FIG. 2, through the ball screw3and bearings35, through the rotor8, through the roller thrust bearings9, through the upper thrust bearing race10, to the split nut segments11. The roller thrust bearings9, the upper thrust bearing race10, and the lower thrust bearing race in the rotor8may have spherical or conical surfaces to permit angular misalignment between the bolt13and the release mechanism1. The split nut segments11may slide relative to the upper thrust bearing race10to permit radial misalignment between the bolt13and the release mechanism1. The bolt13preload force loop is completed through the nut segments11, through the bolt13, to the attached second body, through the second body structure and first body structure, and back to the housing2.

In the preferred implementation,FIG. 2, the outer race for ball screw3may be machined into housing2. The dynamically balanced rotor8may be machined to function as the ball nut for the ball screw3, support for the eddy current electrically conducting damper ring12, the magnetic circuit return path from the magnets4through the damper ring12, the locking notches14,FIG. 4andFIG. 5, that interface with the sears5,FIG. 4, and the lock ring18reset surfaces47,FIG. 4andFIG. 5. The damper ring12may be fabricated as a cylinder from copper or other conductive material to improve damping efficiency or reduce weight. The dynamically balanced rotor8may be composed of multiple alloys and coatings to satisfy structural, magnetic, and anti-corrosion performance requirements. The damper ring12is bonded using a suitable adhesive or otherwise secured to the rotor8to prevent their relative motion. Gap15provides mechanical clearance between the magnets4and the damper12. Polygonal socket49in the base of dynamically balanced rotor8interfaces with reset tool53,FIG. 7.

The preferred implementation shows four split nut segments11,FIG. 4andFIG. 5. Other split nut configurations may be used to interface with alternative bolt, or fastener, configurations. The split nut segments11are keyed to the cover6by segment notches28,FIG. 2, and cover lugs50,FIG. 2andFIG. 4andFIG. 4, to prevent their rotation about the mechanism rotational axis when the bolt13,FIG. 2, is tightened and to ensure they do not rotate under vibration or shock loading and cause bolt13to lose preload. InFIG. 2the cover lugs50, split nut segments11, and split nut segment notches28are rotated into the plane ofFIG. 2for clarity.

FIG. 4shows the locked condition of the present invention initial release mechanism41. The sears5are held into their mating notches14in the dynamically balanced rotor8by the rolling element bearings21and release armature17. The sears5are supported by the plates43that may be screwed, or otherwise fastened, to housing2. The contact angle between the sears5and the rotor notches14is selected to ensure that the interface will slip and the sears5will rotate outwards away from the rotor8notches14when the rotor8is forced to rotate down the ball screw3by the bolt13preload force and/or the spring23force. The sear5to rotor8contact angle is also selected to minimize the radial reaction force on the sears5, rolling element bearings21, and release armature17to minimize the friction that the motor19,20must overcome to release the separation nut1. Springs42hold the release armature17in its locked position until the electrical release signal is applied. Release armature17is dynamically balanced to reduce its susceptibility to vibration and shock.

FIG. 2andFIG. 5, a section looking down just below the level of the sears5rolling element bearings21, show how the lock ring18prevents the nut segments11from opening under the bolt13preload force and features that prevent rotation of the lock ring18when the rotor8,FIG. 4, is locked by the sears5. When the separation nut1is locked by the sears5the axial splines internal to the lock ring18and external to the nut segments11are aligned radially and prevent the nut segments11from opening due to the bolt13preload force reacting through the bolt13thread angle, the separator spring23force acting on the segment separator27, and the springs33,FIG. 2, radial forces.

When the mechanism1is preloaded, friction between the lock ring18and the split nut segments11radial splines prevents lock ring18from rotating and permitting the nut segments11to open,FIG. 4andFIG. 5. If the preload force is reduced for any reason, the surfaces25on lugs45of the lock ring18,FIG. 4, contact mating surfaces47on the locked rotor8and prevent the lock ring18from rotating, unlocking nut segments11, and releasing the bolt13even if the bolt13preload force is zero,FIG. 4andFIG. 5. When the rotor8rotates during release its surfaces47move out of contact with surfaces25on lugs45of lock ring18and free lock ring18to rotate when the tension in the preloaded bolt13, and the split nut segments11to lock ring18friction, both reach approximately zero. Lock ring rotation18is stopped when its lug45surfaces46,FIG. 4andFIG. 5, contact the cover6lugs50surfaces24,FIG. 4. When lock ring18is stopped its internal splines align with the spaces between the split nut segments11external splines, the split nut segments11are free to move outward radially, and bolt13can be withdrawn from the separation nut1,FIG. 1, by a bolt catcher affixed to the second body.

There are two or more torsion springs33,FIG. 2, one for each split nut segment11. The top end of each spring connects to the top flange of the lock ring18and the bottom end of each spring connects to one segment of the split nut11. The springs33provide torque to rotate lock ring18counterclockwise to misalign the lock ring18and split nut segment11splines so that the split nut segments11can open and release bolt13. Springs33are wound such that their lower ends where they attach to the split nut segments11are compressed radially inwards during installation. The radial force facilitates opening the split nut segments11after the lock ring18has rotated following preload release. The springs33are wound such that they are axially aligned with each other and fit together in the manner of a multi-lead thread.

As shown inFIG. 2andFIG. 4, during bolt13release the release armature17rotates, sears5are permitted to retract radially from the dynamically balanced rotor8, and the preloaded bolt13and separator spring23forces cause the dynamically balanced rotor8to roll counterclockwise downwards on the ball screw3relative to the housing2. The ball screw3has minimum pitch to maximize the rotational velocity of the dynamically balanced rotor8, maximize eddy current damping energy dissipation, maximize rotor assembly7kinetic energy storage, and minimize dynamically balanced rotor8reaction forces against the sears5when the separation nut1is locked and preloaded. In the preferred embodiment of the present invention the dynamically balanced rotor8rotates counterclockwise, viewed from the top of the separation nut1, during release so that torqueing bolt13will seat all of the moving components in their lowest energy states and time, vibration, shock, or thermal inputs will not cause mechanical shifting that would partially relieve the bolt13preload. Other rotation configurations might be used to satisfy specific application requirements.

After release, the separation nut1,FIG. 1, must be reset to prepare it to have the bolt13re-inserted and re-torqued. Separation nut1does not need to be refurbished. To reset the separation nut1the reset tool53,FIG. 7, is inserted into the base of the separation nut1until the polygonal shank of reset tool53engages the polygonal socket49in dynamically balanced rotor8and the round end of reset tool53pushes the segment separator27clear of the split nut segments11. The reset tool53is then turned counterclockwise, looking from the base of the separation

nut1, until the dynamically balanced rotor8engages the sears5and is locked in place. The return springs42rotate the release armature17clockwise, viewed from the top, and force the rolling element bearings21against the sears5so that the sears5are held in the locked position shown inFIG. 4. Counterclockwise rotation of dynamically balanced rotor8with reset tool53, viewed from the bottom, engages rotor8surfaces47with lock ring18stop lugs45surfaces25,FIG. 4andFIG. 5, and rotates lock ring18counterclockwise, viewed from the base of separation nut1, against the torque of torsion springs33,FIG. 2. The interfacing beveled faces51on the lock ring18internal splines and the nut segments11external splines force the nut segments11radially inwards into their locked positions,FIG. 5. The surfaces47of the locked rotor8and surfaces25of lock ring18maintain the alignment of the lock ring18splines with the split nut11splines so that the split nut11remains closed, the bolt13can be threaded into split nut11, and ensures that the bolt13cannot be released until the separation nut1is commanded to release by a control signal from the first body.