An improved braking cable-payout mechanism has been developed. Like other such mechanisms, this mechanism can be used as a braking or shock-absorbing device for any of a variety of purposes — for example, enabling a person to descend from an upper floor of a burning building at a safe speed, capturing and slowing a moving vehicle, or limiting the shock load generated by opening of a parachute. Whereas other such mechanisms operate at payout speeds that vary with the length of payout, this mechanism operates at approximately constant payout speed, regardless of the length of cord that has already been paid out.
In a prior mechanism of this type, a cord is paid out from a spool on a shaft connected to a centrifugal brake. Because the payout radius on the spool decreases as cord is paid out, the speed decreases by a corresponding amount.
The present mechanism (see figure) includes a spool, a capstan assembly, and centrifugal brakes. The spool is used to store the cord and, unlike in the prior mechanism, is not involved in the primary braking function. That is, the spool operates in such a way that the cord is unwound from the spool at low tension. The spool is connected to the rest of the mechanism through a constant-torque slip clutch. The clutch must slip in order to pay out the cord.
As the cord leaves the spool, it passes into the capstan assembly, wherein its direction is changed by use of the first of three idler sheaves and it is then routed into the first of three grooves on a capstan. After completing less than a full circle in the first groove, the cord passes over the second idler sheave, which is positioned to enable the cord to make the transition to the second groove on the capstan. Similarly, a third idler sheave enables the cord to make the transition to the third groove on the capstan. After traveling less than a full circle in the third groove, the cord leaves the capstan along the payout path. The total wrap angle afforded by this capstan-and-idler arrangement is large enough to prevent slippage between the cord and the capstan.
The capstan is connected to a shaft that, in turn, is connected to a centrifugal brake. Hence, the effective payout radius, for purposes of braking, is not the varying radius of the remaining cord on the spool but, rather, the constant radius of the grooves in the capstan. The payout speed is determined primarily by this radius and by the characteristics of the centrifugal brake. Therefore, the payout speed is more nearly constant in this mechanism than in the prior mechanism.
This work was done by Tommaso P. Rivellini, Donald B. Bickler, Bradford Swenson, John Gallon, and Jack Ingle of Caltech for NASA’s Jet Propulsion Laboratory.
NPO-40109
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Improved Descent-Rate Limiting Mechanism
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Overview
The document discusses the Improved Descent-Rate Limiting Mechanism (NPO-40109) developed by NASA's Jet Propulsion Laboratory (JPL) to address the limitations of a commercial descent rate limiter used in the Mars Exploration Rover (MER) mission. The original device, a Descent Rate Limiter (DRL) from Capewell Corporation, was designed for human weight and could not handle the increased loads of the MER lander, leading to failures in the payout line and brake pads.
The primary issue with the commercial DRL was its reliance on a flat steel ribbon that wound around a center shaft connected to centrifugal braking elements. As the ribbon was extracted, it created drag, but the device's capacity was exceeded due to the MER's mass growth. Attempts to enhance performance by using thicker steel ribbons were limited by the device's housing size and would alter the descent rate, leading to further complications. Additionally, the variable rate of descent caused excessive wear on the brake pads, necessitating a more robust solution.
To overcome these challenges, the new design introduced a capstanning mechanism that utilized a multiple groove high friction capstan wheel and a low tension storage spool for a high-strength synthetic cord (specifically zylon). This design allowed for a constant rate of descent, independent of the total length of the cord deployed, which was a significant improvement over the variable rate of the previous device.
The capstanning mechanism operates by routing the cord over the capstan wheel, ensuring that it maintains a no-slip condition while controlling the descent rate. A constant torque slipping clutch was added to the storage spool to provide the necessary back tension, preventing slippage and ensuring reliable operation.
The novelty of this improved descent-rate limiting device lies in its ability to maintain a consistent descent rate for falling objects, regardless of the length of the cord, thus enhancing the safety and reliability of descent operations in aerospace applications. This advancement not only addresses the specific needs of the MER mission but also has broader implications for future aerospace technologies. The document serves as a technical support package, providing insights into the development and potential applications of this innovative mechanism.
