Electromechanical strut

An electromechanical strut is provided for moving a pivotal lift gate between an open position and a closed position relative to a motor vehicle body. The electromechanical strut includes a housing connected to one of the lift gate and the motor vehicle body. An extensible shaft is slidably mounted to the housing. The extensible shaft is connected to the other of the lift gate and the motor vehicle body. A drive mechanism includes a rotatable power screw. The drive mechanism converts rotary motion of the power screw into linear motion of the extensible shaft to move the extensible shaft between a retracted position corresponding to the closed position of the lift gate and an extended position corresponding to the open position of the lift gate. A power spring includes one end connected to the extensible shaft and another end connected to the housing for providing a mechanical counterbalance to the weight of the lift gate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electrically-driven mechanical strut. More particularly, the present invention relates to an electromechanical strut used to raise or lower an automotive lift gate.

2. Description of Related Art

Lift gates provide a convenient access to the cargo areas of hatchbacks, wagons, and other utility vehicles. Typically, the lift gate is hand operated, requiring manual effort to move the lift gate between the open and the closed positions. Depending on the size and weight of the lift gate, this effort can be difficult for some users. Additionally, manually opening or closing a lift gate can be inconvenient, particularly when the user's hands are full.

Attempts have been made to reduce the effort and inconvenience of opening or closing a lift gate. One solution is to pivotally mount gas struts to both the vehicle body and the lift gate, reducing the force required for opening the lift gate. However, the gas struts also hinder efforts to close the lift gate, as the struts re-pressurize upon closing, increasing the effort required. Additionally, the efficacy of the gas struts vary according to the ambient temperature. Furthermore, the use of gas struts still requires that the lift gate is manually opened and closed.

U.S. Pat. No. 6,516,567 to Stone et al. (hereafter referred to as the '567 patent) provides a power actuator that works in tandem with a gas strut. The '567 power actuator comprises a motor mounted within the vehicle body coupled to a flexible rotary cable by a clutch. The flexible rotary cable drives an extensible strut that is pivotally mounted to both the vehicle body and the lift gate. Thus, the motor can raise or lower the lift gate conveniently without manual effort. A controller to engage and disengage the motor can be connected to a remote key fob button or a button in the passenger compartment, providing additional convenience.

The power actuator described in the '567 patent is not without its disadvantages. The power actuator is comprised of multiple parts, each of which needs to be assembled and mounted to the vehicle separately, increasing costs. The vehicle body must be specifically designed to provide a space to house the motor. Due to the limited space available, the motor is small and requires the assistance of the gas strut. Additionally, because the power actuator described in the '567 patent is designed to work in tandem with a gas strut, the gas strut can still vary in efficacy due to temperature. Thus, the motor provided must be balanced to provide the correct amount of power with varying degrees of mechanical assistance from the gas strut.

It is therefore desired to provide a means for raising and lowering a vehicle lift gate that obviates or mitigates at least one of the above-identified disadvantages of the prior art.

SUMMARY OF THE INVENTION

According to an embodiment of the invention, an electromechanical strut is provided for moving a pivotal lift gate in a motor vehicle body between a closed and an open position. The electromechanical strut comprises a housing, pivotally mountable to one of the motor vehicle body and the lift gate; an extensible shaft, one end of the shaft being slidably mounted to the housing, and the other end of the shaft being pivotally mounted to the other of the motor vehicle body and the lift gate; a drive mechanism, comprising a power screw, for converting rotary motion into linear motion of the extensible shaft in order to move it between a position corresponding to the closed position of the lift gate and an extended position corresponding to the open position of the lift gate; and a power spring, connected to the power screw within the housing, which assists the power screw.

The present invention provides an electromechanical strut using an inline motor coupled to an inline planetary gear that are both mounted in the housing. The motor-gear assembly drives a power screw and nut assembly in the upper housing, extending or retracting an extensible shaft. Additionally, a power spring mounted coaxially around the power screw urges the extensible shaft to the extended position and provides a mechanical counterbalance to the weight of a lift gate on the shaft. As the shaft extends, the power spring uncoils, assisting the motor-gear assembly in raising the lift gate. Retracting the shaft recoils the spring, storing potential energy. Thus, a lower torque motor-gear assembly can be used, reducing the diameter of the housing.

In another embodiment of the invention, an electromechanical strut is provided for moving a pivotal lift gate between an open position and a closed position relative to a motor vehicle body. The electromechanical strut includes a housing connected to one of the lift gate and the motor vehicle body. An extensible shaft is slidably mounted to the housing. The extensible shaft is connected to the other of the lift gate and the motor vehicle body. A drive mechanism includes a rotatable power screw. The drive mechanism converts rotary motion of the power screw into linear motion of the extensible shaft to move the extensible shaft between a retracted position corresponding to the closed position of the lift gate and an extended position corresponding to the open position of the lift gate. A power spring includes one end connected to the extensible shaft and another end connected to the housing for providing a mechanical counterbalance to the weight of the lift gate.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now toFIGS. 1 and 2, an embodiment of the invention mounted to a motor vehicle is shown generally at10. Electromechanical strut10includes a lower housing12, an upper housing14, and an extensible shaft16. A pivot mount18, located at an end of lower housing12is pivotally mounted to a portion of the vehicle body that defines an interior cargo area in the vehicle. A second pivot mount20is attached to the distal end of extensible shaft16, relative to upper housing14, and is pivotally mounted to the lift gate of the vehicle.

Referring now toFIG. 2, the interior of lower housing12is shown in greater detail. Lower housing12provides a cylindrical sidewall22defining a chamber24. Pivot mount18is attached to an end wall26of lower housing12proximal to the vehicle body (not shown). Upper housing14provides a cylindrical sidewall32defining a chamber34that is open at both ends. A distal end wall28of lower housing12includes an aperture30so that chamber24and chamber34communicate with each other. Upper housing14has a smaller diameter than lower housing12. However, it is contemplated that lower housing12and upper housing14can also be formed as a single cylinder or frusto-cone. Other form factors for lower housing12and upper housing14will occur to those of skill in the art. Upper housing14can be integrally formed with lower housing12, or it can be secured to lower housing12through conventional means (threaded couplings, weld joints, etc). A motor-gear assembly36is seated in chamber24.

The motor-gear assembly36includes a motor42, a clutch44, a planetary gearbox46, and a power screw40. Motor42is mounted within chamber24near end wall26. Motor42is secured to at least one of cylindrical sidewall36and end wall26to prevent undesired vibrations or rotation, Motor42is a direct current bidirectional motor. Electrical power and direction control for motor42is provided via electrical cables that connect into the vehicle body through apertures (not shown) in end wall26. The clutch44is connected to an output shaft48on motor42. Clutch44provides a selective engagement between the output shaft48of motor42and the planetary gearbox46. Clutch44is an electromechanical tooth clutch that engages planetary gearbox46when motor42is activated. When clutch44is engaged, torque is transferred from motor42through to planetary gearbox46. When clutch44is disengaged, torque is not transferred between motor42and planetary gearbox46so that no back drive occurs if the lift gate is closed manually.

Planetary gearbox46is a two-stage planetary gear that provides torque multiplication for power screw40. A ring gear50is driven by the teeth of clutch44. In turn, a number of planetary gears52transfer power from ring gear50to power screw40, which is centrally journaled within planetary gearbox46, providing the desired gear ratio reduction to power screw40. In the present embodiment, planetary gearbox46provides a 47:1 gear ratio reduction. Other gear ratio reductions will occur to those of skill in the art. Power screw40extends into upper housing14.

Extensible shaft16provides a cylindrical sidewall54defining a chamber56and is concentrically mounted between upper housing14and power screw40. As described earlier, pivot mount20is attached to the distal end of extensible shaft16. The proximal end of extensible shaft16is open. A drive nut58is mounted around the proximal end of extensible shaft16relative to lower housing12and is coupled with power screw40in order to convert the rotational movement of power screw40into the linear motion of the extensible shaft16along the axis of power screw40. Drive nut58includes two splines60that extend into opposing coaxial slots62provided on the inside of upper housing14to prevent drive nut58from rotating. The length of slots62defines the retracted and the extended positions of extensible shaft16. Alternatively, a ball screw assembly could be used in lieu of drive nut58without departing from the scope of the invention. An integrally-formed outer lip64in upper housing14provides an environmental seal between chamber34and the outside.

A spring housing38is provided in lower housing12and is defined by cylindrical sidewall22, end wall28, and a flange66. Within spring housing38, a power spring68is coiled around power screw40, providing a mechanical counterbalance to the weight of the lift gate. Preferably formed from a strip of steel, power spring68assists in raising the lift gate both in its powered and un-powered modes. One end of power spring68attaches to power screw40and the other is secured to a portion of cylindrical sidewall22. When extensible shaft16is in its retracted position, power spring68is tightly coiled around power screw40. As power screw40rotates to extend extensible shaft16, power spring68uncoils, releasing its stored energy and transmitting an axial force through extensible shaft16to help raise the lift gate. When power screw40rotates to retract extensible shaft16, power spring68recharges by recoiling around power screw40.

Power spring68stores sufficient energy when coiled to drive power screw40to fully raise the lift gate, even when motor gear assembly36is not engaged (typically by unlatching the lift gate to raise it manually.) In addition to assisting to drive power screw40, power spring68provides a preloading force that reduces starting resistance and wear for motor42. Furthermore, power spring68provides dampening assistance when the lift gate is closed. Unlike a gas strut, power spring68is generally not affected by temperature variations, nor does it unduly resist manual efforts to close the lift gate. Although the present embodiment describes power spring68that uncoils to assist in raising a lift gate and recoils to lower a lift gate, it has been contemplated that a power spring68could be provided that uncoils when lowering a lift gate and recoils when raising a lift gate.

Referring toFIGS. 4 and 5, wherein primed reference numerals represent similar elements as those set forth above, the electromechanical strut10′ according to another embodiment includes the lower housing12′ having the cylindrical sidewall22′ defining the chamber24′, and the upper housing14′ having the cylindrical sidewall32′ defining the chamber34′. It is appreciated that the lower12′ and upper14′ housings may be formed as a single housing.

The electromechanical strut10′ also includes the extensible shaft16′ movable between a retracted position, shown inFIG. 4, corresponding to a closed position of the lift gate and an extended position, shown inFIG. 5, corresponding to an open position of the lift gate.

The motor-gear assembly36′ is seated within the chamber24′. The motor-gear assembly36′ includes the motor42′, the planetary gearbox46′, and the power screw40′. The planetary gearbox46′ includes the planetary gears52′ that transfer power from the ring gear50′ to the power screw40′. In the current embodiment, the planetary gearbox46′ provides a 20:1 gear ratio reduction.

The extensible shaft16′ extends between opposing first70and second72ends. The first end70of the extensible shaft16′ is open and the second end72of the extensible shaft16′ is closed off by an end wall76. The second end72of the extensible shaft16′ is connected to the pivot mount20′.

The extensible shaft16′ includes an outer cylindrical wall78, and an inner cylindrical wall80spaced apart inwardly from the outer cylindrical wall78. One end of the inner cylindrical wall80is connected to the end wall76. The outer cylindrical wall78and the inner cylindrical wall80define a toroidal chamber82therebetween. One end of the toroidal chamber82is closed off by the end wall76and an opposing end of the toroidal chamber82defines an opening84. The inner cylindrical wall80further defines a cylindrical chamber86inward of the toroidal chamber82. The cylindrical chamber86is separated from the toroidal chamber82by the inner cylindrical wall80.

The drive nut58′ is rigidly mounted in the cylindrical chamber86of the extensible shaft16′. The drive nut58′ is coupled with the power screw40′ in order to convert the rotational movement of the power screw40′ into linear motion of the extensible shaft16′ along a longitudinal axis88of the power screw40′. The power screw40′ rotates in situ, that is, during rotation of the power screw40′ there is no linear motion of the power screw40′ relative to the lower housing12′ and the upper housing14′. As such, the rotation of the power screw40′ effects linear movement of the extensible shaft16′ relative thereto.

The power spring68′ is seated within the toroidal chamber82. The power spring68′ includes one end90connected to the second end72of the extensible shaft16′, and another end92connected to the upper housing14′ adjacent the lower housing12′. The power spring68′ is a coil spring that uncoils and recoils as the extensible shaft16moves relative to the upper14and lower12housings. It is, however, appreciated that the particular type of spring may vary.

In powered operation, torque provided by the motor42′ is transferred via the planetary gearbox46′ to the screw40′, causing linear motion of the extensible shaft16′ as described above. For manual operation, the motor42′ and the planetary gearbox46′ must be back driven. The friction in the system due to the direct engagement of the motor42′ and the planetary gearbox46′ with the power screw40′ allows the lift gate to remain still in any intermediate position between the open and closed positions. The electromechanical strut10′ thus provides stable intermediate positions for the lift gate (useful, for example, for garages with low ceilings) without power consumption by using the internal friction of the motor-gear assembly36′.

The power spring68′ provides a mechanical counterbalance to the weight of the lift gate. The power spring68′, which may be a coil spring, assists in raising the lift gate both in its powered and un-powered modes. When the extensible shaft16is in the retracted position, the power spring68′ is tightly compressed between the extensible shaft16′ and the lower housing12′. As the power screw40′ rotates to extend the shaft16′, the power spring68′ extends as well, releasing its stored energy and transmitting an axial force through the shaft16′ to help raise the lift gate. When the power screw40′ rotates to retract the extensible shaft16′, or when the lift gate is manually closed, the power spring68′ is compressed between the shaft16′ and the lower housing12′ and thus recharges.

In addition to assisting in driving the power screw40′, the power spring68′ also provides a preloading force for reducing starting resistance and wear of the motor42′. Furthermore, the power spring68′ provides dampening assistance when the lift gate is closed. Unlike a gas strut, the power spring68′ is generally not affected by temperature variations, nor does it unduly resist manual efforts to close the lift gate.

It is appreciated that a ball screw assembly, as known in the art, could be used in lieu of the drive nut58′. Also, although reference has been made specifically to a lift gate, it is also appreciated that the invention may be applied to a variety of other closure panels such as trunks or deck lids.

The above-described embodiments of the invention are intended to be examples of the present invention and alterations and modifications may be effected thereto, by those of skill in the art, without departing from the spirit of the invention.