Vibration Survey of Blades in 19XB Axial-Flow Compressor 2: Dynamic Investigation Page: 4 of 22
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NACA RM No, E7D09
The compressor, tested separately from the other parts of the
engine, was driven through a variable-speed magnetic coupling and
speed increasers by a 2500-horsepower .constant-speed drive
motor (fig. 5). The compressor intake was open to the atmosphere
but the compressed air was exhausted into a controlled pressure
system. The compressor pressure ratio was varied by means of a
butterfly valve in the exhaust line.
Compressor-rotor speed was varied until vibration signals were
observed at which time photographic data.were recorded. The magni-
tude of the vibratory stresses was evaluated by comparison with
stress signals of known magnitude supplied from similar strain gages
mounted on the bar of a calibrating instrument.
The operating range of the equipment was limited to a compressor
speed of 14,000 rpm by lack of driving power, although the compressor
is rated at' 17000 rpm. Unfortunately, conclusive test data on the
last five stages were not obtained because some of the lead wires
were thrown out by centrifugal force during development ofthe
instrument tation', The lead-wire failures were attributed to poor
adhesion and faulty void filling by the ceramic, cement.
RESULTS AND DISCUSSION
The results of the static frequency measurements inade on blades
of the 19XB compressor indicated that failures experienced in the
seventh and tenth stages could have been caused by first bending-
mode vibrations .reslting from fourth- and sixth-order excitation
of the rotative speed, respectively. Critical-speed diagrams
illustrating this :possibility are included: in reference 1. The
sources of the,fourth- and sxth-order exciting forces have not been
learned. Four bearing supports interrupt.the inlet-air stream; it
is doubtful, however, that the effect of: these supports would carry
as far as the seventh Lstage. No condition exists within the
compressor that would cause six equally spaced discontinuities in
the air flow.
During compressor operation at low pressure ratios (atmospheric
inlet and exhaust pressure), sharply resonant blade vibrations were
observed as the rotative speed was changed. Analysis revealed that
the critical speeds were exact multiples of the first bending-mode
frequencies of the blades. Actually after two resonant speeds were
known, a critical-speed diagram was drawn from which other critical
speeds were predicted (fig. 6), Oscillograph records were taken at
each critical speed and were analyzed to determine frequency, order,3
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Meyer, André J., Jr. & Calvert, Howard F. Vibration Survey of Blades in 19XB Axial-Flow Compressor 2: Dynamic Investigation, report, April 9, 1947; (https://digital.library.unt.edu/ark:/67531/metadc65447/m1/4/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.