Mechanical Components

Retaining Rings for Industrial Fastening Applications

Retaining rings are selected based on material, finish, and a variety of application parameters. A discussion of retaining rings inevitably must begin with a debunking of myths; namely, that one style of retaining ring will function better than all other types in all instances. No one retaining ring style is better than another. Rather, the parameters of an application actually determine which retaining ring is best to use, and this can vary from assembly to assembly. Selecting the correct type of retaining ring based on variables such as installation/removal requirements, anticipated thrust load, and end-play take-up can ensure the retaining ring chosen will perform reliably, while significantly reducing fastener costs.

Posted in: Mechanical Components, Briefs

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Simulation and Testing of Maneuvering of a Planetary Rover

A report discusses the development of a computational model of a Mars Explorer Rover maneuvering across terrain under varying conditions. The model is used to increase understanding of the rover dynamics. Increased understanding is helpful in planning further tests and in extending the operational range of the rover to terrain conditions that would otherwise have to be avoided in a conservative approach. The model is implemented within MSC.ADAMS®, a commercial suite of computer programs for simulating a variety of automotive and aeronautical mechanical systems. Following its initial formulation, the model has been successively refined in an iterative process of simulation, testing on simulated terrain, correlation of simulation results with test results, and adjustment of model parameters to increase degrees of matching between simulation and test results. In particular, three aspects of the model have been refined, as follows:

Posted in: Mechanical Components, Briefs, TSP

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Update on Controlling Herds of Cooperative Robots

A document presents further information on the subject matter of “Controlling Herds of Cooperative Robots” (NPO-40723), NASA Tech Briefs, Vol. 30, No. 4 (April 2006), page 81. To recapitulate: A methodology for controlling a herd of cooperative and autonomous mobile robots exploring the surface of a remote planet or moon (specifically, Titan or Titan-like) is undergoing development. The proposed configuration of mobile robots consists of a blimp and a herd of surface sondes. The blimp is the leader of the herd, and it commands the other robots to move to locations on the surface or below the surface to conduct science operations. Once a target is chosen, the sondes cooperatively aim sensors at the target to maximize scientific return. This hierarchical and cooperative behavior is necessary in the face of such unpredictable factors as terrain obstacles and uncertainties in the model of the environment.

Posted in: Mechanical Components, Briefs, TSP

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Pseudo-Waypoint Guidance for Proximity Spacecraft Maneuvers

A paper describes algorithms for guidance and control (G&C) of a spacecraft maneuvering near a planet, moon, asteroid, comet, or other small astronomical body. The algorithms were developed following a model- predictive-control approach along with a convexification of the governing dynamical equations, control constraints, and trajectory and state constraints. The open-loop guidance problem is solved in advance or in real time by use of the pseudo-waypoint generation (PWG) method, which is a blend of classical waypoint and state-of-theart, real-time trajectory-generation methods. The PWG method includes satisfaction of required thruster silent times during maneuvers. Feedback control is implemented to track PWG trajectories in a manner that guarantees the resolvability of the open-loop-control problem, enabling updating of G&C in a provably robust, model-predictive manner. Thruster firing times and models of the gravitational field of the body are incorporated into discretized versions of the dynamical equations that are solved as part of an optimal-control problem to minimize consumption of fuel or energy. The optimal- control problem is cast as a linear matrix inequality (specifically a secondorder cone program), then solved through semi-definite-programming techniques in a computationally efficient manner that guarantees convergence and satisfaction of constraints.

Posted in: Mechanical Components, Briefs, TSP

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Simple, Compact, Safe Impact Tester

Cushioned impact decelerations up to hundreds of normal Earth gravitation are easily produced. An apparatus has been designed and built for testing the effects, on moderatesized objects, of cushioned decelerations having magnitudes ranging up to several hundred g [where g  = normal Earth gravitational acceleration ( ≈9.8 m/s2)]. The apparatus was originally intended for use in assessing the ability of scientific instruments in spacecraft to withstand cushioned impacts of landings on remote planets. Although such landings can have impact velocities of 20 to 50 m/s, the decelerations must not exceed a few hundred g. This requires the deceleration to occur over a distance of as much as 50 cm in a time of tens of milliseconds. This combination of conditions is surprisingly difficult to simulate on the ground. The apparatus could also be used for general impact testing.

Posted in: Mechanical Components, Briefs, TSP

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Six-Message Electromechanical Display System

This system would overcome the three-message limit of prior such systems. A proposed electromechanical display system would be capable of presenting as many as six distinct messages. This system would be a more capable and more complex successor to the proposed system reported in “Four-Message Electromechanical Display System” (MFS-31368), NASA Tech Briefs, Vol. 24, No. 4 (April 2000), page 32. In contrast to the now-proposed six-message system and the previously proposed four-message system, a typical conventional electromechanical display system is limited to three messages.

Posted in: Mechanical Components, Briefs, TSP

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Compact, Recuperated, Binary-Fluid Rankine-Cycle Engine

The primary advantages of this engine are compactness and relatively high energy-conversion efficiency. A Rankine-cycle engine that contains a binary working fluid (ammonia + water) and a recuperative heat exchanger has been built and tested. This engine is a prototype of "bottoming"-cycle engines that would be used to extract additional useful power from the exhaust heat of gas turbine engines. Its advantages are well suited to vehicles, where volume and weight are important constraints.

Posted in: Machinery & Automation, Mechanical Components, Briefs

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