Flex wedges have been proposed for use in brakes and clutches like those in which, heretofore, basic wedges have been used. Flex wedges (see Figure 1) offer advantages of superior braking and clutching performance and less weight, relative to basic wedges.
The upper part of the figure depicts part of a typical brake or clutch that contains a basic wedge that mates with the side walls of a groove in a fixture. To obtain braking or clutching action, one pushes the wedge into the groove to obtain friction between the wedge and the fixture. A large frictional force is obtained by utilizing the mechanical advantage afforded by the wedge/groove geometry to multiply the perpendicular-to-the-surface contact force between the wedge and the fixture.
The mechanical advantage, and thus the effectiveness of braking or clutching in the case of a basic wedge, is greater for wedge faces and groove walls that are more nearly parallel. Unfortunately when these contact surfaces are fabricated more nearly parallel, there is a greater tendency for a wedge to become jammed in a fixture once it has been pushed in; as a result, the force needed to remove the wedge from contact with the fixture is larger and less predictable, and there is an increasing tendency for unjamming action to jerk the brake or clutch mechanism.
The lower part of the figure depicts part of a typical brake or clutch similar to that of the upper part of the figure, except for the use of a flex wedge instead of a basic wedge. This mechanism functions similarly to the basic-wedge mechanism, except that it offers enhanced holding performance, reliability, and predictability. Less actuation force is needed for both insertion and removal of the wedge; even unjamming requires little force and hence gives rise to little or no jerk.
The flex wedge (see Figure 2) is regarded as having been fabricated by machining away most of the material from a basic wedge. The flex wedge includes wedge shoes connected, via shoe flexures, to a wedge compliance flexure. The wedge shoes mate with the side walls of the groove in the fixture, in the same manner as that of the basic wedge. The wedge compliance flexure is made flexible with respect to motion in the x direction to accommodating small motions needed to align the wedge shoes with the side walls of the groove in the fixture. However, the wedge compliance fixture is stiff along the axis of insertion and removal (the y axis) and along the axis of the relative motion (the z axis) that the brake or clutch was meant to resist when engaged.
The shoe flexures are perpendicular to the wedge shoes, affording an additional mechanical advantage (beyond that of the basic wedge geometry) for both engagement and disengagement of the brake or clutch: Once the flex wedge was inserted as far as it could go, further pushing of the flex wedge into the fixture gives rise to a slight bend of the shoe flexures with consequent pushing of the wedge shoes into tighter contact with the side walls of the groove. Hence, the perpendicular-to-the-surface contact force (and thus the desired frictional braking or clutching force) for a given insertion force is greater than that for a basic wedge.
Even if the wedge shoes were pushed very tightly against the side walls, as described in the preceding paragraph, disengagement does not depend on the application of a large unjamming force. This is because pulling on the wedge compliance flexure in an effort to disengage the brake or clutch would first cause the shoe flexures to bend oppositely to the way they bent during insertion and this bend pulls the shoes out of contact with the side walls of the groove. This disengagement action is characterized by a mechanical advantage that depends only on geometry (and not on the coefficient of friction). Hence, the force needed for extraction or disengagement would be small and predictable.
The shoe flexures are made thin and flexible to allow bending in the parallel-to-the-shoe directions, yet stiff as needed in the perpendicular-to-the-shoe directions. They are also made long in the z direction, as needed for strength and rigidity in holding against the relative motion that the brake or clutch is meant to resist when engaged.
This work was done by John M. Vranish of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Goddard Space Flight Center; (301) 286-7351. Refer to GSC-14006.