โ Scribed by William W. Shugarts Jr.
No coin nor oath required. For personal study only.
Recent changes in the instrumentation associated with the High-Speed Friction Testing Machine show promise of revealing rapidly fluctuating friction phenomena with a resolution heretofore impossible. This machine, designed and built by the Laboratories staff some years ago under the sponsorship of the Army Ordnance Corps, produces dry friction betweeri solid materials at sliding velocities from 150 to 2000 fps. It is capable of applying normal loads Of 10 to 100 lb. (pressures of 2000 to 30,000 psi.).
The friction occurs when small samples of the test material are applied to both faces of the rim of a rotating disk 2 ft. in diameter. As the samples are applied at the rim they are given a radial motion with just enough velocity to make them trace a spiral path without overlapping on successive disk revolutions. Thus a non-retracking test surface is possible for distances up to 40 ft. For tests where retracking information is wanted, either from one test or from successive tests, radial motion of the samples is not used. The chamber housing the disk and its associated equipment is evacuated to a pressure of about 30 mm. of mercury.
As the problem of wear is closely associated with dry friction, wear measurements are made of the change in linear dimension of the sample as well as its change in weight. Also intimately connected with both friction and wear is temperature, particularly the instantaneous temperature at the mating surfaces. When a thermocouple is uged to measure the rapidly fluctuating temperature at the friction surface of a sample whose diameter is approximately 0.100 in., care must be taken if the temperature readings are to be reliable. Faithful response to rapid changes in temperature can be obtained only with a couple whose size is a minimum. The small size is also necessary to minimize the contamination of the sample surface by thermocouple materials. If possible, the choice of these materials should include at least one that is thermoelectrically similar to one of the two friction materials. In addition, from the work of others, it appears that a temperature gradient may exist across the surface of the sample, hence the smallest thermocouple possible must be placed at the proper location, if peak tempera-
๐ SIMILAR VOLUMES
The spin dynamics of an S(1/2)IN system during the CP mixing time of continuous wave and variable amplitude cross-polarization magic angle spinning (CWCPMAS and VACPMAS) experiments is discussed. The signal enhancement of a low abundant S spin, coupled to a set of N = 6 coupled spins with I = 1/2, i