A self-locking telescoping device including an outer tube, an inner tube telescoped into the outer tube having a cone-shaped ramp at an inboard end, and a plurality of metal spheres between the ramp and the outer tube. The metal spheres wedge between the ramp and the outer tube when the inner tube is thrust into the outer tube in a collapse direction thereby locking the tubes together. When the thrust is attributable to a severe impact, the spheres plastically deform the outer tube by plowing tracks therein thereby to absorb energy. The self-locking telescoping device further includes an actuator rod, a driver which translates the actuator in the collapse direction and in an expansion direction, a first clutch which translates the inner tube with the actuator rod in the expansion direction, a second clutch which translates the inner tube with the actuator rod in the collapse direction, and a tubular retainer on the actuator rod having a plurality of closed-ended slots around the metal spheres. The closed ends of the slots prevent the spheres from becoming wedged between the ramp and the outer tube when the second clutch translates the inner tube with the actuator rod in the collapse direction.

TECHNICAL FIELD
 This invention relates to a self-locking telescoping device capable of
 functioning under impact as an energy absorber.
 BACKGROUND OF THE INVENTION
 A motor vehicle typically includes a bumper bar and an energy absorber
 which supports the bumper bar on a body of the motor vehicle for
 translation though a relatively short energy-absorbing stroke in response
 to a low speed impact on the bumper bar. During the energy-absorbing
 stroke, a fraction of the kinetic energy of the impact is converted by the
 energy absorber into work. In a high speed impact on the bumper bar,
 however, its short energy-absorbing stroke is quickly traversed and most
 of the kinetic energy of the impact is converted into work by plastic
 deformation of body structure of the motor vehicle behind the bumper bar.
 As motor vehicles have become more compact, the energy-absorbing
 capability of their body structures has decreased due to the smaller span
 between the vehicle's passenger compartment and bumper bar. A telescoping
 device described in U.S. Pat. No. 5,370,429 supports a bumper bar close to
 a body of a motor vehicle except when sensors on the vehicle detect an
 impending impact. Then, the telescoping device extends the bumper bar out
 from the body to maximize the energy-absorbing stroke of the bumper bar.
 During the energy-absorbing stroke, hydraulic fluid is throttled through
 an orifice of the telescoping device to absorb a fraction of the kinetic
 energy of the impact. The telescoping device described in the aforesaid
 U.S. Pat. No. 5,370,429 is not "self-locking", i.e., does not become
 structurally rigid in compression under any circumstances, and requires a
 fluid reservoir and fluid seals which may leak during the service life of
 the device. Accordingly, manufacturers continue to seek improved
 telescoping devices which are self-locking and which are also suitable for
 use as bumper energy absorbers.
 SUMMARY OF THE INVENTION
 This invention is a new and improved self-locking telescoping device
 including a stationary outer tube, an inner tube telescoped into the outer
 tube having a cone-shaped ramp at an inboard end thereof, and a plurality
 of metal spheres between the cone-shaped ramp and the outer tube. The
 metal spheres become wedged between the cone-shaped ramp and the outer
 tube when the inner tube is thrust into the outer tube in a collapse
 direction corresponding to a decrease in the length of the telescoping
 device thereby locking the inner and outer tubes together and rendering
 the telescoping device structurally rigid in the collapse direction. When
 the thrust is attributable to a severe impact on the inner tube, the
 spheres plastically deform the outer tube by plowing tracks therein
 thereby to convert into work a fraction of the kinetic energy of the
 impact. The self-locking telescoping device further includes an actuator
 rod, a driver which translates the actuator in the collapse direction and
 in an opposite expansion direction corresponding to an increase in the
 length of the telescoping device, a first clutch which translates the
 inner tube as a unit with the actuator rod in the expansion direction, a
 second clutch which translates the inner tube as a unit with the actuator
 rod in the collapse direction, and a tubular retainer on the actuator rod
 having a plurality of closed-ended slots around respective ones of the
 metal spheres. The ends of the slots prevent the spheres from becoming
 wedged between the cone-shaped ramp and the outer tube when the second
 clutch translates the inner tube as a unit with the actuator rod in the
 collapse direction.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring to FIGS. 1-3, a self-locking telescoping device 10 according to
 this invention includes a stationary outer tube 12 having an inside
 cylindrical wall 14 and an inner tube 16 telescoped into the outer tube
 through an end 18 of the latter. An end fitting 20 rigidly attached to the
 inner tube constitutes an inboard end thereof in the outer tube and
 includes an outside cylindrical wall 22 bearing against and cooperating
 with the inside cylindrical wall 14 of the outer tube in supporting the
 inner tube on the outer tube for translation in an expansion direction "E"
 corresponding to an increase in the length of the device 10 and in a
 opposite collapse direction "C" corresponding to a decrease in the length
 of the device each parallel to a longitudinal centerline 24 of the outer
 tube.
 An annular groove 26 in the outside cylindrical wall 22 of the end fitting
 20 includes a bottom 28, a small diameter end 30, and a big diameter end
 32. The bottom 28 of the groove annular flares outward, i.e., toward the
 inside cylindrical wall 14, from the small diameter end 30 to the big
 diameter end 32 and constitutes a cone-shaped ramp 34 on the inner tube at
 the inboard end thereof. A plurality of hard steel spheres 36 are disposed
 in the annular groove 26.
 During translation of the inner tube 16 in the expansion direction "E", the
 spheres 36 are cupped in the annular groove 26 against the small diameter
 end 30 thereof, FIG. 2, where they slide along the inside cylindrical wall
 14 of the outer tube without obstructing translation of the inner tube.
 Conversely, at the onset of translation of the inner tube in the collapse
 direction "C", the spheres roll up the cone-shaped ramp 34 and quickly
 become wedged between the cone-shaped ramp and the inside cylindrical wall
 14 of the outer tube thereby effectively locking the inner and the outer
 tubes together and rendering the self-locking telescoping device
 structurally rigid in the collapse direction "C".
 When the thrust on the inner tube in the collapse direction "C" is
 attributable to an extreme impact on the inner tube, the telescoping
 device 10 functions as an energy absorber. That is, with the steel spheres
 36 wedged between the cone-shaped ramp and the inside cylindrical wall of
 the outer tube, and the self-locking telescoping device therefore
 structurally rigid in the collapse direction "C", the steel spheres
 plastically deform the outer tube 12 by rolling tracks therein when the
 thrust attributable to the extreme impact exceeds the yield strength of
 the material from which the outer tube 12 is constructed. Such plastic
 deformation absorbs energy by converting into work a fraction of the
 kinetic energy of the impact.
 In a modified self-locking telescoping device 37 according to this
 invention, FIG. 9, the inner and outer tubes 16,12 are interrupted by a
 plurality of perforations 39. The interstices between the perforations 39
 constitute crush initiators. With the steel spheres 36 wedged between the
 cone-shaped ramp and the inside cylindrical wall of the outer tube, and
 the self-locking telescoping device therefore structurally rigid in the
 collapse direction "C", the outer tube plastically deforms at the crush
 initiators when the thrust attributable to the extreme impact exceeds the
 yield strength of the material from which the outer tube 12 is
 constructed. Such plastic deformation absorbs energy by converting into
 work a fraction of the kinetic energy of the impact.
 The self-locking telescoping device 10 further includes an actuator rod 38
 telescoped into a second end 40 of the outer tube 12 and into a bore 42 in
 the end fitting 20 on the inner tube. The actuator rod has a rack gear 44
 thereon which meshes with a pinion gear 46. The pinion gear 46 is
 connected by a pinion shaft 48 to a prime mover in the form of an electric
 motor 50 so that the motor, the pinion gear, and the rack gear constitute
 a drive means operable to translate the actuator rod back and forth in the
 expansion and collapse directions "E", "C" of the inner tube.
 A tubular hub 52 is rigidly attached to the actuator rod 38 and supports
 the actuator rod in the bore 42 in the end fitting 20 for translation
 relative to the inner tube in the direction of the longitudinal centerline
 24 of the outer tube. A ring 54 is rigidly attached to the hub 52 at the
 end thereof facing the rack gear 44 on the actuator rod and cooperates
 with the inside cylindrical wall 14 of the outer tube in supporting the
 actuator rod on the outer tube for back and forth translation in the
 expansion and collapse directions "E", "C" of the inner tube. An annular
 flange 56 on the end of the hub 52 opposite the ring 54 faces an annular
 shoulder 58, FIG. 3, on the end fitting 20 around the bore 42. A
 compression spring 60 seats against the ring 54 and against the end
 fitting 20 and biases the end fitting and the actuator rod in opposite
 directions until the annular flange 56 seats against the annular shoulder
 58.
 A tubular retainer 62 of the telescoping device 10 surrounds the
 compression spring 60 and overlaps the gap between the end fitting 20 and
 the ring 54. The retainer includes a hooked end 64, FIG. 2, seated in a
 corresponding annular groove in the ring 54 whereby the retainer is
 rigidly attached to the ring and, therefore, to the actuator rod 38. The
 tubular retainer has a plurality of slots 66, FIG. 1, parallel to the
 longitudinal centerline 24 of the outer tube each of which terminates at a
 closed end 68. Each slot receives a corresponding one of the spheres 36
 and has a length calculated to locate its closed end 68 close to the
 corresponding sphere when the spring 60 thrusts the annular flange 56 on
 the hub 52 against the annular shoulder 58 on the end fitting 20, FIG. 2.
 The ring 54 and the spring 60 constitute a first clutch which effects
 unitary translation of the actuator rod and the inner tube in the
 expansion direction "E" in response to corresponding rotation of the
 pinion gear 46. That is, when the pinion gear rotates clockwise, FIGS.
 2-3, the thrust applied to the actuator rod is transferred to the end
 fitting 20 through the ring 54 and the spring 60 and urges the inner tube
 in the expansion direction "E". At the same time, the spheres 36 remain
 cupped against the small diameter end 30 of the annular groove 26 where
 they slide along the inside cylindrical wall 14 of the outer tube without
 interfering with translation of the outer tube. If the actuator rod
 translates in the expansion direction "E" relative to the inner tube
 because of friction between the inner and outer tubes, the closed ends 68
 of the slots 66 in the retainer 62 separate harmlessly from the spheres 36
 until the thrust on the inner tube exceeds the friction.
 Conversely, the annular flange 56 on the hub and the annular shoulder 58 on
 the end fitting 20 constitute a second clutch which effects unitary
 translation of the actuator rod and the inner tube 16 in the collapse
 direction "C" in response to corresponding rotation of the pinion gear 46.
 That is, when the pinion gear rotates counterclockwise, FIGS. 2-3, the
 thrust applied to the actuator rod 38 is transferred directly to the end
 fitting through the flange 56 and the annular shoulder 58 and urges the
 inner tube in the collapse direction "C". At the same time, the ring 54
 translates with the actuator rod in the collapse direction "C" so that the
 retainer 62 and the end fitting 20 translate as a unit in the same
 direction. In that circumstance, the closed ends 68 of the slots 66
 prevent the spheres 36 from rolling up the cone-shaped ramp 34 and thus
 prevent the spheres from becoming wedged between the end fitting 20 and
 the outer tube 14 and interfering with translation of the inner tube in
 the collapse direction "C".
 Referring to FIGS. 4-5, a pair of the self-locking telescoping devices 10
 are illustrated in a bumper energy absorber application on a schematically
 represented motor vehicle 70 having a frame 72 and a bumper bar 74. The
 outer tubes 12 are rigidly attached to the frame 72 on opposite sides of
 thereof and the inner tubes 16 are rigidly attached to the bumper bar. An
 electronic control module (ECM) 76 on the motor vehicle is connected to
 each of the electric motors 50 and to a transducer 78 which provides
 electronic signals to the ECM corresponding to the velocity of the motor
 vehicle. When the ECM 76 turns on the electric motors to rotate the pinion
 gears 46 in the expansion direction "E" of the inner tubes, the bumper bar
 74 is translated by the actuator rods and the inner tubes from a retracted
 position to an extended position, illustrated respectively in solid and
 broken lines in FIG. 4, in which the bumper bar protrudes further in front
 of the frame 72. When the ECM turns on the electric motors to rotate the
 pinion gears in the collapse direction "C" of the inner tubes, the bumper
 bar is translated by the actuator rods and the inner tubes from its
 extended position back to its retracted position.
 With the electric motors 50 turned off and the bumper bar in its extended
 position, a severe impact on the bumper bar 74 initiates translation of
 the inner tubes 16 of the devices 10 in the collapse direction "C"
 relative to the outer tubes and the actuator rods. The end fittings 20
 plunge toward the rings 54 against the resistance of the springs 60 while
 the closed ends 68 of the slots 66 in the tubular retainers separate from
 the spheres 36, FIG. 3. The spheres then roll up the cone-shaped ramps 34,
 become wedged against the inside cylindrical walls 14 of the outer tubes,
 and commence plowing tracks in the outer tubes to convert into work a
 fraction of the kinetic energy of the impact on the bumper bar.
 A flow chart 80, FIG. 5, depicts an algorithm according to which the ECM 76
 turns the electric motors 50 on and off including a start block 82
 initiated when the electrical system of the motor vehicle is turned on
 with the bumper bar in its retracted position. From the start block 82,
 the algorithm monitors the velocity of the motor vehicle through an
 electrical signal from the transducer 78 and asks at a decision block 84
 whether the velocity of the motor vehicle is in a high range, e.g., above
 15 miles per hour (MPH), in which a high speed impact is possible. If the
 answer is no, the ECM does not turn on the electric motors and the bumper
 bar remains in its retracted position. If the answer is yes, the algorithm
 turns on the electric motors through the ECM to translate the bumper bar
 74 to its extended position more remote from the frame 72 where it affords
 increased protection against a high speed impact.
 With the bumper bar in its extended position, the algorithm monitors the
 velocity of the motor vehicle through the electrical signal from the
 transducer 78 and asks at a decision block 86 whether the velocity of the
 motor vehicle is in a low range, e.g., less than 10 MPH, in which a high
 speed impact is improbable. If the answer is no, then the algorithm
 repeats the interrogation of vehicle velocity between the decision blocks
 84,86. If the answer is yes, the algorithm interrogates vehicle velocity a
 second time after a delay of about three seconds and asks at a decision
 block 88 whether vehicle velocity is still in the low range. If the answer
 is no, then the algorithm repeats the interrogation of vehicle velocity
 between the decision blocks 84,86. If the answer is still yes, the
 algorithm turns on the electric motors 50 through the ECM to translate the
 bumper bar back to its retracted position.
 Referring to FIGS. 6-8, another modified self-locking telescoping device 90
 according to this invention is identical to the self-locking telescoping
 device 10 described above except as now recited. Structural elements
 common to the device 10 and the modified device 90 are identified in FIGS.
 6-8 with primed reference characters. In place of the compression spring
 60 in device 10, the modified device 90 includes a retaining ring 92, an
 annular wave spring 94, and a thrust washer 96, FIG. 8, which constitute a
 preload means of the modified device. The retaining ring 92 is supported
 on the end fitting 20' on the inner tube 16' and constitutes the small
 diameter end of the annular groove in the outside cylindrical surface 22'
 of the end fitting. The thrust washer 96 loosely encircles the cone-shaped
 ramp 34' between the retaining ring 92 and the spheres 36'. The wave
 spring encircles the cone-shaped ramp between the retaining ring 92 and
 the thrust washer 96.
 The pinion gear 46' translates the inner tube 16' of the modified
 self-locking telescoping device 90 in the collapse direction "C" through
 the actuator rod 38', the annular flange 56' on the hub 52', and the
 annular shoulder 58' on the end fitting 20'. At the same time, the closed
 ends 68', FIG. 8, of the slots 66' in the tubular retainer 62' prevent the
 spheres 36' from rolling up the cone-shaped ramp 34' and becoming wedged
 between the end fitting and the outer tube, FIG. 6, while maintaining the
 wave spring flexed in compression between the thrust washer and the
 retaining ring. When the pinion gear 46' rotates in the opposite direction
 to translate the actuator rod in the expansion direction "E", the inner
 tube 16' and the end fitting 20' remain stationary due to friction until
 the ring 54' on the actuator rod seats against the end fitting, FIG. 7.
 The ring and the end fitting thus constitute the aforesaid first clutch of
 the modified device 90 which translates the inner tube as a unit with the
 actuator rod in the expansion direction "E".
 When the pinion gear 46' is stationary, thrust on the inner tube in the
 collapse direction "C" initiates translation of the end fitting in the
 same direction relative to the actuator rod while the closed ends of the
 slots in the retainer 62' separate from the spheres 36'. At the same time,
 the annular wave spring 94 separates the retaining ring 92 and the thrust
 washer 96 to positively and substantially instantly thrust the spheres 36'
 up the cone-shaped ramp 34' into wedging engagement between the end
 fitting and the inside cylindrical wall of the outer tube. The spheres 36'
 thus render the modified self-locking telescoping device 90 structurally
 rigid in the collapse direction "C" unless the Thrust is attributable to a
 severe impact on the inner tube. Then, the spheres plastically deform the
 outer tube by plowing tracks therein to convert into work a fraction of
 the kinetic energy of the impact.