A compact conical roller clutch can be either (1) engaged to obtain both positive locking against rotation in one sense and freewheeling in the opposite sense or (2) disengaged to obtain free rotation in both senses. This clutch was devised to enhance the functionality and increase the durability of the orthotic knee brace described in "Knee Brace Would Lock and Unlock Automatically" (MFS-28991), NASA Tech Briefs, Vol. 19, No. 12 (December 1995), page 28. In the knee brace, the clutch can be actuated selectively to prevent undesired flexion when weight is applied or to allow both flexion and extension when weight is not applied or when it is desired to exercise leg muscles.
Previously, the knee brace contained a clutch in which locking was achieved by exploiting friction between padded mating conical surfaces. One disadvantage of that design was that the relatively soft padding deteriorated with repeated actuation. Another disadvantage is that when engaged, the clutch locked against rotation in both senses (preventing both extension and flexion), instead of desirably preventing flexion while allowing extension. The present clutch achieves locking by jamming (wedging) of rollers in tapered pockets, as described in more detail in subsequent paragraphs. The hard roller and pocket surfaces withstand repeated actuation much better than did the soft padding of the previous clutch.
The figure shows the knee-joint mechanism equipped with the present releasable conical roller clutch. The mechanism includes an upper and a lower housing connected via a needle bearing that allows free rotation.
The conical roller clutch includes rollers in tapered pockets in a conical inner surface of the upper housing. A cage retains the rollers in the pockets. Together, the pockets and cage keep the rollers in their nominal positions and orientations in loose contact with a conical surface of the lower housing. The cage and rollers are retained by clips that are attached to the upper housing by screws. Return springs in the cage bias the rollers toward the wide ends of the tapered pockets (the free-rotation position).
A cam in the enclosed space between the upper and lower housings rotates about a pin affixed to the lower housing. The cam includes an elongated lever portion that passes through a slot in an actuation rod and rests against a back plate attached to the upper housing. The actuation rod is mounted in a passage in the lower housing. The movement of the actuation rod is controlled by a cable that can be pulled, for example, by a mechanism sensitive to a load applied to the wearer's foot, described in "Heel-Strike Mechanism for Rehabilitative Knee Brace" (MFS-28992), NASA Tech Briefs, Vol. 20, No. 11 (November 1996), page 72.
When the cable is pulled downward, the actuation rod pulls downward on the lever portion of the cam, forcing the back plate and upper housing to the right (as depicted in the front view) and thereby forcing the rollers into contact with both the cone of the lower housing and the walls of the tapered pockets in the upper housing. In this condition, if one attempts to rotate the lower housing counterclockwise relative to the upper housing as depicted in the side view, then the rollers become jammed between the cone and the pocket walls; the resulting wedging force prevents further relative rotation. On the other hand, if one attempts to rotate the lower housing clockwise in the side view, then the rollers are moved toward the wide ends of the tapers; there is no jamming, and consequently rotation can continue freely.
When the cable is not pulled downward, a return spring pushes the actuation rod upward, causing the nose on the left (as depicted in the front view) end of the cam to push the upper housing to the left. This action disengages the rollers from the cone. As a result, there is no opportunity for jamming, and the lower housing can rotate freely, either clockwise or counterclockwise, relative to the upper housing.
The present clutch can be modified and adapted to different applications. For example, if it were to contain equal numbers of tapers in opposite senses of rotation, then the clutch would lock against rotation in both senses during engagement, yet would still allow free rotation in both senses during disengagement.
The present clutch occupies less space than does an equivalent conventional friction clutch. Inasmuch as it can be disengaged to allow free rotation in both senses, the present roller clutch offers an advantage over other roller and sprag clutches that allow free rotation in one sense but always lock against rotation in the opposite sense.
This work was done by Neill Myers of Marshall Space Flight Center.
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
Marshall Space Flight Center at (256) 544-0021
Refer to MFS-31258.