NO-BACK BRAKE FUNCTIONALITY MONITOR
A no-back device usable in a Horizontal Stabilizer Trim Actuator (HSTA) includes a ratchet and pawl brake mechanism in which a pivot pin supporting the pawl includes a sensor for directly measuring torque developed by the brake mechanism. A signal generated by the sensor may be evaluated to determine the apparent operational integrity of the no-back device.
1 . A no-back device for an actuator having a ball screw subject to an axially directed load, the no-back device comprising:
a housing arranged to receive a portion of the ball screw, wherein the ball screw is mounted for rotation in first and second opposite rotational directions relative to the housing; and
a first brake mechanism responsive when the axial load is in a first load direction, the first brake mechanism acting between the housing and the ball screw to produce a first torque resisting rotation of the ball screw in the first rotational direction and not substantially resisting rotation of the ball screw in the second rotational direction, wherein the first brake mechanism includes a first ratchet wheel and a first pawl, the first pawl being pivotally mounted to the housing by a first pivot pin, wherein the first pawl engages the first ratchet wheel to prevent rotation of the first ratchet wheel relative to the housing when the ball screw rotates in the first rotational direction and the first pawl permits rotation of the first ratchet wheel relative to the housing when the ball screw rotates in the second rotational direction;
wherein the first pivot pin includes a first sensor generating a signal representative of the first torque produced by the first brake mechanism.
2 . The no-back device of claim 1 , wherein the first sensor includes at least one strain gauge embedded in the first pivot pin.
3 . The no-back device of claim 1 , further comprising signal processing electronics connected to the first sensor for evaluating the signal generated by the first sensor.
4 . The no-back device of claim 1 , wherein the first brake mechanism includes exactly one first pawl.
5 . The no-back device of claim 1 , further comprising:
a second brake mechanism responsive when the axial load is in a second load direction opposite the first load direction, the second brake mechanism acting between the housing and the ball screw to produce a second torque resisting rotation of the ball screw in the second rotational direction and not substantially resisting rotation of the ball screw in the first rotational direction, wherein the second brake mechanism includes a second ratchet wheel and a second pawl, the second pawl being pivotally mounted to the housing by a second pivot pin, wherein the second pawl engages the second ratchet wheel to prevent rotation of the second ratchet plate relative to the housing when the ball screw rotates in the second rotational direction and the second pawl permits rotation of the second ratchet wheel relative to the housing when the ball screw rotates in the first rotational direction;
wherein the second pivot pin includes a second sensor generating a signal representative of the second torque produced by the second brake mechanism.
6 . The no-back device of claim 5 , wherein the first load direction is a compression load and the second load direction is a tension load.
7 . The no-back device of claim 5 , wherein the first sensor includes at least one strain gauge embedded in the first pivot pin and the second sensor includes at least one strain gauge embedded in the second pivot pin.
8 . The no-back device of claim 5 , further comprising signal processing electronics connected to the first sensor and to the second sensor for evaluating the respective signals generated by the first and second sensors.
9 . The no-back device of claim 5 , wherein the first brake mechanism includes exactly one first pawl and the second brake mechanism includes exactly one second pawl.
10 . A method for testing operational integrity of a no-back device having a brake mechanism configured to apply a torque resisting rotation of a ball screw in a braked rotational direction and not substantially resisting rotation of the ball screw in a freewheeling rotational direction opposite the braked rotational direction, the method comprising:
measuring the torque produced by the brake mechanism when the ball screw is rotated in the braked rotational direction, wherein the torque is measured using a sensor associated with a structural member of the brake mechanism, the sensor generating a braking torque signal representative of the torque produced by the brake mechanism when the ball screw is rotated in the braked rotational direction; and
evaluating the braking torque signal to determine operational integrity of the no-back device.
11 . The method according to claim 10 , wherein the step of evaluating the braking torque signal includes comparing the braking torque signal to a braking threshold value corresponding to a minimum required braking torque.
12 . The method according to claim 10 , wherein the ball screw is driven to rotate by a motor, and the step of evaluating the braking torque signal includes monitoring the braking torque signal over time and correlating the braking torque signal with electric current or hydraulic pressure supplied to energize the motor.
13 . The method according to claim 10 , wherein the method further comprises:
measuring the torque produced by the brake mechanism when the ball screw is rotated in the freewheeling rotational direction, wherein the torque is measured using the sensor, the sensor generating a freewheeling torque signal representative of the torque produced by the brake mechanism when the ball screw is rotated in the freewheeling rotational direction; and
evaluating the freewheeling torque signal to further determine operational integrity of the no-back device.
14 . The method according to claim 13 , wherein the step of evaluating the freewheeling torque signal includes comparing the freewheeling torque signal to a freewheeling threshold value corresponding to a maximum allowed freewheeling torque.
15 . The method according to claim 13 , wherein the ball screw is driven to rotate by a motor, and the step of evaluating the freewheeling torque signal includes monitoring the freewheeling torque signal over time and correlating the freewheeling torque signal with electric current or hydraulic pressure supplied to energize the motor.
16 . The method according to claim 10 , wherein the structural member of the brake mechanism is a pivot pin rotatably supporting a pawl of the brake mechanism.