Investigation of surging in helical valve-springs
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Date
1963Author
Derbyshire, Geoffrey
Metadata
Abstract
This investigation is concerned
with surging in helical valve-springs in internal combustion
engines. The main object of the work was to
observe and measure surging in an actual engine, benchmounted,
whilst the cam-shaft was motored.
A considerable amount of development
work was carried out in order to estabAsh a reliable
method for measuring the amplitude of surging and the
cam-shaft speed at which it occurred. The method ultimately
used was to pull .a smoked-glass slide through
a small rig so that a trace was made by a small pointer
attached to the central coil of the valve-spring. A
second pointer vibrating at a constant known frequency 1
left a further trace from which the cam-shaft rotational
speed could accurately be found.
Surging became increasingly pronounced
as the cam-shaft speed increased and~ response
curve was drawn -showing surging amplitude plotted
against cam-shaft speed. Harmonic analysis showed how
the amplitude of surging is roughly proportional to the
amplitude of the exciting harmonic.
Here, as in all practical vibrating
systems the damping factor controls the amplitude of
surging for a given cam-valve-spring configuration.
The magnitude Qf the damping factor was obtained :from
the peaks of the response curve. Because of its relevance
to this phenomenon damping was further investigated
in a separate experiment, in an attempt to discover how
it was affected by the precompression of the spring.
The results showed that the damping was higher at large
and small precompressions than at intermediate precompressions.
It was apparent that no advantages would be
obtained by increasing the precompression of the spring
within the engine. However early traces taken from the
engine with increased spring precompression,(use having
been made of the tappet-adjusting screw,) indicated
that the damping had increased -for that part of the
trace drawn when the valve was closed, showed a considerable
dying-away of the surging amplitude.
Further traces showed how, at a
different surging speed the amplitude of vibration increased
in that part of the cycle when the valve was
closed. It was realised that two frequencies causing
longitudinal motion were being excited and that beating
occurred. Careful replotting of the enlarged traces
showed that the two frequencies were separated by about
12 c~p.s.
A further experiment was devised
to explain this additional phenomenon. The spring was
vibrated at various precompressions and the three natural
modes of vibration, torsion, bending and pure longitudinal,
were noted. It was apparent from this experiment that
during bending vibration longitudinal motion occurred.
The results revealed that the original work on the engine
had been performed with the longitudinal and bending
frequencies virtually coincident and in the later work
at increased precompression the two frequencies were
separated by 15 c.p.s. Thus careful observation of the
form of the vibration throughout a complete cycle of the
cam rotation revealed that two frequencies cause longitudinal
motion and each contributes to valve-spring
surging.
Quite a large proportion of the
work has been concerned with drawing out theoretical
and actual surging cycles for one complete cam-rotation
at various surging speeds. It is shown theoretically
how by increasing the damping factor ten-fold surging
can be virtually eliminated.