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3 Torsional Vibration
3-1
3 Torsional
Vibration
Crankshaft torsional vibration has been a problem
with aircraft engines since before World War I.
Crankshaft torsional vibration happens because each
power stroke tends to slightly twist the shaft. When
the power stroke subsides, the crankshaft untwists.
One would think that something as massive as a
crankshaft would not twist significantly, but any piece
of metal always deflects a bit when a force is applied,
and when large amounts of power are generated, the
forces can become huge indeed. The effects of
torsional vibration can be amplified by a phenomenon
called torsional resonance. Each crankshaft design
has a natural torsional frequency like the note of a
ringing bell or sound of a vibrating guitar string. If this
natural frequency coincides with the torsional
frequency of the crankshaft, the effects can be
devastating, resulting in broken crankshafts, lost
propeller blades, sheared accessories, and stripped
gear trains.
One of the first major scandals in British aviation
began in April of 1917 and involved torsional
vibration. Granville Bradshaw, chief designer of ABC
Motors, Ltd., secured a production contract from the
British Air Board for a new engine, the Dragonfly.
Bradshaw was a better salesman than engine
designer. The Dragonfly had not even run at the time
it was procured. When it did run, it was a miserable
failure because Bradshaw had managed to design its
crankshaft with a resonance exactly in the operating
range. By the time the contract was cancelled, 1147
of the engines had been built. This episode upset
British air-cooled engine development for years.
1
The problem of crankshaft torsional vibration in
American radial engines appeared almost
simultaneously in Curtiss-Wright, Pratt & Whitney,
and Lycoming radial engines. This was due to the
use of controllable-pitch propellers that were heavier
than previous wood and fixed-pitch metal propellers.
This increased the effective propeller inertia and
brought the crankshaft resonant frequency down into
the engine operating range. Lieutenants Howard
Couch, Orval Cook and Turner A. Sims, working at
Wright Field in Dayton, Ohio, first identified the
difficulty.
The problem became really serious in 1934 when the
geared Wright R-1820 began breaking propeller
shafts. E. S. Taylor of Massachusetts Institute of
Technology became involved in the problem and in
1934 and proposed the puck-type damper to
Curtiss-Wright. This damper, depicted in Figure 3.1,
has a thick disk resembling a hockey puck rolling
inside a large hole in the fixed counterweight.
Figure 3.1 Puck-type Damper
(Pratt & Whitney)
Curtiss-Wright employed Roland Chilton, a prolific
designer of many aviation engine and accessory
mechanisms. Chilton immediately designed a
pendulum mechanism that was vastly superior to
Taylors puck-type damper. See Figure 3.2. Chilton
received a U. S. patent for his design, which is called
variously the Chilton damper or bifilar damper.
Three months after Taylor proposed the damper to
Curtiss-Wright, they were delivering engines
equipped with it.
Figure 3.2 Chilton Damper
(Pratt & Whitney)
The patent situation, however, turned out to be most
involved since two French engineers, Salomon and
Sarazin, working independently, were earlier in
conception. According to Taylor, "Salomon was the
first to understand the principle of the pendulum
damper." Also, "Sarazin had designed a device
almost identical with Chilton's and was in contact with
Hispano-Suiza."
2
The Chilton damper had much better vibration-
reducing characteristics, but this would not be
evident for years. Since Curtiss-Wright held the
patent for the Chilton damper, Pratt & Whitney was
left with the Salomon or puck-type damper. This was
suitable for the earlier, smaller radials but would be
pushed to its limits in the R-2800 and eventually
replaced entirely.
Just as E. S. Taylor became the principal vibration
consultant to Curtiss-Wright, another M.I.T.
Professor, J. P. Den Hartog, became a consultant to
Pratt & Whitney. Den Hartog who would later literally
write the book on mechanical vibrations, contributed
both theoretical knowledge and instrumentation
experience. Den Hartog also insisted that the correct
terminology was tuned absorber instead of
damper. A damper converts movement to heat,
while a tuned absorber temporarily stores energy, 3-2
and then later returns it to the system without
producing any significant heat.
When work began on the R-2800, torsional vibration
was becoming better understood. The Army had
even issued a Torsional Vibration Specification that
set a maximum value of 0.50 degrees. Engine
designers had learned to make crankshafts large
enough so that natural resonance would fall outside
the engine operating range. But as engine power
increased, even a small percentage of total engine
power that became resonant could do damage.
Initially, the R-2800 design lacked any mechanism for
addressing torsional vibration. One can only guess
that the designers chose the simplest configuration,
hoped for the best, but were prepared to redesign if
necessary. And redesign they did. Trouble appeared
almost immediately.
Robert E. Bob Gorton got in on the ground floor of
R-2800 vibration problems. Gorton was born
December 5, 1915 in Norwich, New York where he
grew up and attended Norwich High School. Like
many of boys of his era, Gorton had been inspired by
Charles Lindberghs solo flight from New York to
Paris in 1927. Gorton had a keen interest in aviation
and built model airplanes in high school. Also like
many boys of his era, Gorton was faced with real
challenges when it came time for college the
country was in the midst of the Great Depression.
Fortunately, Gorton placed well in the Regents
examination and was awarded a tuition scholarship
to Rensselaer Polytechnic Institute.
Toward the end of his senior year at RPI, Gorton was
again faced with a shortage of money. He had a
summer job at Pratt & Whitney, but needed support
to complete his Masters degree. Gorton did
something that was unprecedented for the time he
convinced Pratt & Whitney to finance his Masters
study in vibration, and in return, agreed to a work-
study program. Pratt & Whitney got its very first
engineer with actual college training in vibration
issues. The relationship was destined to be long and
fruitful. Gortons diligent testing and instrumentation
contributed greatly to getting all of Pratt & Whitneys
reciprocating engines developed. He and his team
invented new types of instrumentation to meet the
challenge of each new problem. When jets arrived,
Gorton continued to develop innovative approaches
to instrumentation of turbine wheels and other gas
turbine components.
3
Gorton initially worked with W. H. Sprenkle in the
Test and Instrumentation Department. When
Sprenkle moved on to other things in 1939, Gorton
took over the department and grew it into a large
organization. Test engineers had to be quite creative
in the design and implementation of vibration
instrumentation. It was a science in its infancy, and
the problems had to be solved as they went along.
Gorton joined Pratt & Whitney at the same time it
acquired a Sperry-MIT torsiograph, serial number 2.
4
The torsiograph, depicted in Figures 3.3 and 3.4,
consisted of a lightweight axle that was attached
directly to the vibrating shaft, usually at the rear end
of the engine crankshaft. Suspended on ball bearings
around the axle was a heavy seismic element that,
except for very light springs, was free to rotate. The
relative angle between the axle and seismic element
was measured electrically. Once in motion, the
seismic element tended to stay in constant motion. If
the axle were undergoing torsional vibration, the
positional difference between the axle and seismic
element would be recorded on a 35mm filmstrip.
5
Later analysis of the record could isolate individual
frequency and amplitude of torsional vibration. A
typical statement from this analysis would be
something like a 4.5X torsional resonance of +/-1.36
degrees was detected at 2000 RPM. This means
that when the engine was run at 2000 RPM, the
crankshaft twisted 1.36 degrees back and forth at a
frequency four and one-half times the rate of
crankshaft revolution.
Figure 3.3 Torsiograph Mechanical Components
(Draper