Article
citation information:
Homišin, J. Contribution and
perspectives of new flexible shaft coupling types – pneumatic couplings. Scientific Journal of Silesian University of
Technology. Series Transport. 2018, 99,
65-77. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2018.99.6.
Jaroslav HOMIŠIN[1]
CONTRIBUTION AND
PERSPECTIVES OF NEW FLEXIBLE SHAFT COUPLING TYPES – PNEUMATIC COUPLINGS
Summary. The contribution briefly approaches the
author’s profile in the field of scientific research during his work at
TU in Košice. More precisely, it presents a selected, specific area of
torsional oscillation of mechanical systems concerning the characteristics,
research and application of new elements, i.e., so-called pneumatic tuners of
torsional oscillation (pneumatic couplings), a field that the author has long
been devoted to. In the process, the article informs the reader about the
development of new types of flexible shaft couplings, i.e., tangential and
differential pneumatic couplings (also with autoregulation), presents the
results of static and dynamic measurements made on certain couplings with the
interconnection of pneumatic flexible elements and draws attention to the
conditions involved in the application of these coupling types in torsionally
oscillating mechanical systems.
Keywords: torsionally oscillating
mechanical systems; pneumatic flexible shaft couplings – characteristic
1. INTRODUCTION
Since my arrival at the
Department of Machine Parts and Mechanisms, Faculty of Mechanical Engineering
of Technical University of Košice (1981) (then known as VŠT), my
scientific research was focused upon the clarification of the problematic regulation of
dangerous torsional oscillations in mechanical systems, especially in torsionally
oscillating mechanical systems (TOMS). As it was proven that dangerous
torsional oscillations can be regulated via the application of suitable
flexible elements, via the concrete application of pneumatic flexible shaft
couplings, I increasingly devoted my attention to these issues and to the
development of and research into new types of flexible shaft couplings.
Indeed, theoretical and practical analysis (development of and research into
coupling prototypes) was at the core of my dissertation, entitled The Impact
of Pneumatic Flexible Shaft Couplings upon the Torsional Oscillations of
Mechanical Systems [1].
In the following period,
I focused my attention on the narrower specification and concretization of the issue
concerning the functioning of pneumatic flexible shaft couplings in TOMS.
The obtained results in this area were summarized in my habilitation thesis, Application of Pneumatic Couplings in
Modelled Torsionally Oscillating Mechanical Systems [2].
Further, I was dealing
with the issue of tuning TOMS during
steady-state operations, i.e., in states of continuous tuning, as one of
the methods of TOMS tuning, which resulted in successfully completing a
grant-funded project called “Research
of Torsional Oscillations of Mechanical Systems with Regard to Possibilities of
Their Regulation” (2000-2002). The results of this project pointed
to five modes of so-called continuous tuning - tuning during the operation of
TOMS, as follows:
a)
Tuning via a regulatory system for securing
continual change in the characteristic of pneumatic couplings [11]
b)
Tuning via a regulatory system for the
realization of the continuous tuning of mechanical systems [4]
c)
Tuning via the application of pneumatic
coupling with an added regulatory system [12]
d)
Tuning via static optimization on the basis
of an extremal regulation method [3,4]
e)
Tuning via the application of pneumatic
coupling with autoregulation [9,10]
Each of the suggested
methods for the continuous tuning of TOMS (a, b, c, d, e) is presently a topic of interest in postgraduate
studies. The proposed method for the continuous tuning of TOMS (c) is currently
a part of the FENA Katowice Company’s offer.
Optimizing TOMS via a
new method for the regulation of dangerous torsional oscillations is narrowly
interconnected with the diagnostics of given systems. For this reason, I consider it
necessary in future to focus upon the total diagnostics of TOMS from the aspect of
the size of torsional oscillations and mechanical oscillations caused by them.
With the aim of
preparing a general and up-to-date characterization of my scientific research
over the last four decades at TU in Kosice, I can say that it was mainly
focused on reduction issues, especially the regulation of dangerous torsional
oscillations in mechanical systems. In the sense of a time sequence, as I was
progressing my research, it is possible to select and specify the following
thematic areas:
o
Development of and
research into new flexible shaft coupling types
o
Development of and
research into the regulation of dangerous torsional oscillations
o
Application of
pneumatic flexible shaft couplings in TOMS via suggested five methods of
continuous tuning
o
Optimizing TOMS
with an emphasis upon the regulation of dangerous torsional oscillation size
o
Diagnostics of TOMS
from the aspect of the size of torsional oscillations and mechanical
oscillations caused by them
The research results,
which I obtained for each of stated areas, were subsequently published in
domestic and foreign professional papers, presented at scientific conferences
and summarized in a monograph and textbooks. In the process of researching
these issues, a total of 52 patents was granted.
Regarding the restricted
size of the paper, but primarily due to the extent of my research results in
the individual thematic areas, it is impossible to present a more detailed
analysis in any of stated issues (each would deserve it). However, I will try
to focus my attention on one of the selected areas of my research work, which I
consider essential and which is serving as a basis for the development of others.
I will concentrate on a brief presentation about new types of flexible shaft
couplings - tangential and differential pneumatic flexible shaft couplings
(also with autoregulation). At the same time, I will present some results of
realized laboratory measurements and define the conditions for the application
of certain types of couplings in TOMS.
2. BRIEF CHARACTERISTICS OF
FLEXIBLE SHAFT COUPLINGS
Based on the general
characteristics of flexible shaft couplings, it is valid to state that, besides
adjusting the axial, radial and angle misalignment of shafts, these devices
serve as a very efficient tools for restricting the occurrence of resonance in
a range of operating speeds in TOMS. In other words, suitably selected flexible
shaft couplings, as described by several researchers [5,6,7,8], represent
simple and relatively cheap devices for restricting the occurrence of resonance
in the operating speed range and reducing the dynamic torsional load of all
elements in the system. Usually, this involves shifting the natural frequency
of torsional oscillations of the system and, consequently, critical
revolutions, from individual harmonic components of torque load into the area
of lower-frequency revolutions. As a result, during speedy acceleration or braking,
unacceptable torsional accelerations will not occur, as coupling will be able
to absorb them.
In the case of
mechanical systems offering a broad range of operating speed, the regulation of
dangerous torsional oscillations is a very demanding task [12]. The difficulty
is caused by the need for a coupling that is able to withstand all demands in
terms of dynamic load during the whole operating range (minimal - manoeuvring,
maximal - full load) of revolutions, but also under the demanding conditions of
starting, stopping and reversing the system. The requirement of the secure
functioning of the system across the whole operating range is achieved via the
use of flexible coupling with non-linear progressive characteristics.
Consequently, it is the
aim of every designer, firstly, to suitably adjust the dynamic properties of
flexible coupling to the dynamic properties of the mechanical system, to the
maximum degree, in order to control the dangerous torsional oscillations found
in TOMS.
The long-lasting operation,
which causes wear and fatigue in the material, especially in flexible shaft
couplings, as well as accidental occurrences mainly caused by a change in the
properties of a driving or driven piston device, negatively impacts the smooth
operation of a mechanical system from the aspect of dangerous torsional
oscillations [5].
As a result of these
negative circumstances, the preliminary tuned mechanical system becomes
mistuned. In turn, its tuning element - a flexible shaft coupling - is not able
to reduce, or completely eliminate, growing dangerous torsional oscillations in
the system.
With the aim to reduce
these dangerous torsional oscillations and therefore secure the tuning of TOMS,
it is appropriate to apply newly developed pneumatic flexible shaft couplings,
i.e., pneumatic tuners of torsional oscillations, whose characteristics I will
discuss in the following section of this paper.
3. TANGENTIAL PNEUMATIC FLEXIBLE SHAFT COUPLINGS
Tangential pneumatic
flexible shaft couplings (Fig. 1)
consist of driving (1) and
driven (2) parts, with a
compression space inserted in-between.
a)
b) Fig. 1. Tangential pneumatic flexible shaft coupling
with the complete mutual interconnection of pneumatic flexible elements: a)
graphic representation, b) design solution
A compression space is
created by four pneumatic flexible elements, positioned along the circumference
(tangentially) in one row (3),
with a diameter of 130 mm, marked
4-1/130-T. A pneumatic flexible element consists of bellow-shaped rubber cord
material filled with a gaseous medium (in our case, air). During the
transmission of torque, the two pneumatic flexible elements are pressed and, at
the same time, two other elements are pulled. In this way, the construction of
two-sided coupling is secured and a valve (4) filling the compression space of coupling with a gaseous
medium is executed. The design solution for pneumatic coupling enables the
mutual interconnection of individual pneumatic flexible elements with the help
of interchangeable throttling nozzles (5)
and hoses (6). In turn, three
basic pneumatic coupling option are created:
v Pneumatic coupling without the mutual
interconnection of pneumatic flexible elements (4-1/130-T-A) when the carrying units
of a compression space of tangential pneumatic coupling are not mutually
interconnected
v Pneumatic coupling with the mutual
interconnection of pneumatic flexible elements (4-1/130-T-B) when a compression
space of tangential pneumatic coupling consists of two independent flexible
units,
v Pneumatic coupling with the full mutual
interconnection of pneumatic flexible elements (4-1/130-T-C; see Fig. 1) when a compression space of tangential pneumatic
coupling consists of two mutually interconnected flexible units
In my research, the
tangential pneumatic coupling underwent static and dynamic measurements
[1,2,7]. For illustration purposes, the results gained by measuring pneumatic
coupling with the complete interconnection of pneumatic flexible elements (4-1/130-T-C)
are presented.
3.1. Results of static measurements executed on tangential pneumatic coupling
with the complete interconnection of pneumatic flexible elements
The gained results
indicated that, due to a change in the pressure of the gaseous medium,
pneumatic coupling is able to operate with variable characteristics (Fig. 2), that is, it is able to
operate with various properties (torsional stiffness and damping coefficient).
Based on Fig. 2, it is possible to state that
the static characteristics of pneumatic coupling are moderately non-linear.
Their courses can be expressed by the following simple equation:
(1)
where the constants a0
and a3 are determined from the measured curves via the least
squares method.
From Eq. (1), using the method of equivalent
linearization, the equivalent static torsional stiffness kest
is determined (2). For the
static characteristics of coupling (Fig.
2), in terms of pressure (pS=0÷700 kPa), based on Eq. (2) the values of kest
are computed. Fig. 3 shows
which courses depend on pressure:
(2)
By changing the
gaseous medium in the pneumatic coupling, the values of its static torsional
stiffness (Fig. 3) are also
changed; at the same time, to a considerable degree, the size of its
non-linearity ε=a3/a0 is
influenced. On the basis of the calculation, it is possible to state that,
during the increase in pressure from 100
kPa to 700 kPa, the
coefficient of non-linearity decreases in the range ε=15÷1.2 (Fig. 4). The results point to the conclusion that, in the range
of the gaseous medium pS=200÷700
kPa, pneumatic coupling can be defined as linear. This statement is
supported by research reports [5,6], which confirm that, in the case of ε<10, flexible coupling is
considered to be linear.
3.2. Results of dynamic
measurements realized from tangential pneumatic coupling Type 4-1/130-T-C
To determine the dynamic
properties of pneumatic coupling, in our research, a dynamic method of free
oscillations with the preload was applied. On the basis of the recorded free
oscillations, the resulting dynamic characteristics were determined (Fig. 5) [1], which are expressed by
Eq. (3).
(3)
On the basis of Eq. (2),
equivalent values of dynamic torsional stiffness ked of pneumatic coupling for various
pressures of the gaseous medium were established (Fig. 6). The research reports [5,6] indicate that, due to the
ratio between dynamic and static torsional stiffness, the growth in flexible
torque, which is dependent on the coupling oscillations’ frequency, is
respected. In our case, it is possible to agree with this argument almost
entirely.
This
leads to the conclusion that, given the relation of dynamic and static
torsional stiffness (Fig. 7), a
growth in the flexible element of pneumatic coupling is respected, which is
similar to the construction according to the equation below:
(4)
Through the further
elaboration of free oscillations records, the values of pneumatic coupling,
with an equivalent damping coefficient b* under various pressures, were defined [1]. The results
of our measurements confirm the claim [5,6] that the values of the damping
coefficient of flexible couplings depend on the preload, amplitude and
temperature to smaller degree, while they depend upon the frequency of
oscillations ω to a
greater degree. Based on this conclusion, the impact of the frequency upon the
damping coefficient can be expressed by Eq. (5), when the coefficient of
absorption constant b* [1] (Fig. 8)
refers to the given preload, amplitude and temperature, which are approximately
constant, that is:
(5)
Fig. 6. The course of dependency of
the equivalent dynamic torsional stiffness ked on the pressure of gaseous medium pS in tangential
pneumatic coupling
Fig. 5. The courses of the dynamic characteristics of tangential pneumatic
coupling; courses a, b, c, d
correspond to the pressures of gaseous medium pS=100, 300, 500 and 700 kPa
4. DIFFERENTIAL PNEUMATIC FLEXIBLE SHAFT
COUPLINGS
In the sphere of
development of and research into differential pneumatic couplings in our department,
attention has been given to:
§
Differential
pneumatic flexible shaft couplings of Type 3-1/130-D (Fig.
9)
§
Differential
pneumatic shaft couplings with autoregulation of Type 3-1/130-D/A (Fig. 10)
A differential pneumatic
coupling (Fig. 9) consists of a
driving part (1), a driven part
(2) and a compression space
filled with a gaseous medium (in our case, air) in-between. A compression space
is created by three interconnected differential elements, marked 3-1/130-D, positioned along the
circumference. Each differential element consists of a pressed (3) and a pulled pneumatic flexible
element (4) with an outside
diameter of 130 mm. The mutual
interconnection of differential elements is secured by interconnecting hoses (5), while a valve (6) fills the compression space of a
coupling, which affects a change in the pressure of the gaseous medium inside
it.
A differential pneumatic flexible
shaft coupling with autoregulation (Fig. 10),
of which the basic principle is elaborated in granted patent claims [9,10],
has, compared with differential pneumatic coupling, a common construction base.
The main difference is the absence of a valve and the insertion of a regulator
(6), which maintains the
constant twist angle of the coupling. The basic characteristic of coupling is
the ability to autoregulate the twist angle change caused by an actual change
in load torque to the preliminary set constant twist angle value φk. In this way,
autoregulation of the gaseous medium pressure value in the pneumatic coupling
compression space in relation to the actual value of load torque will be
secured.
4.1. The results of
laboratory measurements realized for differential pneumatic flexible shaft
coupling and differential pneumatic flexible shaft coupling with autoregulation
In the frame of
laboratory tests, measurements on a differential pneumatic coupling were
realized. The following results were obtained (Figs. 11-13).
The values of the dynamic torsional
stiffness of pneumatic coupling (Fig.
11, Course b) were
determined via Eq. (4) on the
basis of static values, according to Fig.
11, Course a. Fig. 11 shows that the courses
of torsional stiffness in pneumatic coupling increase the dependence on gaseous
medium pressure.
The courses of dynamic
torsional stiffness in the dependence on load torque (Fig. 12a-g) are constant.
Fig.
12 provides
information about the range of dynamic torsional stiffness in differential
pneumatic coupling, while, at the same time, providing information about the
suitability of the given coupling to operate in a particular mechanical system.
Autoregulation of the
gaseous medium pressure value and the preliminary set value of the constant
twist angle influence the dynamic properties of pneumatic flexible shaft
coupling with autoregulation to a considerable degree. Fig. 13 illustrates the courses of coupling dynamic
torsional stiffness in the dependence on the load torque. One course of
torsional stiffness marked as a, b, c,
d corresponds to each constant twist angle φk=2°, 4°, 6° and 8°.
The stated courses are
limited by the minimal and maximal value of torsional stiffness, which
corresponds to the pressure of the gaseous medium in the range of pS=100÷700
kPa. At the
same time, the courses are shown by a broken line consisting of pre-regulatory
(A), regulatory (B) and
above-regulatory (C) zones. As shown on the scheme, via a change in φk, the length of the
pre-regulatory and regulatory zones, and primarily the value of torsional
stiffness, are impacted. This means that pneumatic coupling with an increased
constant twist angle and the same mean load torque will operate with decreased
torsional stiffness. In this way, via a change in the constant twist angle with
a value of φk=8°,
relatively stiff coupling, for example, when φk=2°, operating with maximal torsional stiffness (ked=17000N.m.rad-1), will
result in highly flexible coupling due to load torque MS=592N.m,
which will
reach the maximal value of dynamic torsional stiffness (ked=17000N.m.rad-1),
but only
when torque MS=2375N.m.
5. REQUIREMENTS PLACED ON
PNEUMATIC COUPLINGS FOR THEIR APPLICATION IN TORSIONALLY OSCILLATION MECHANICAL
SYSTEMS
Pneumatic tuners of
torsional oscillations in TOMS must fulfil the following conditions:
v
Alignment of axial, radial and angular misalignments
of shafts caused by manufacturing imperfections.
Ø During the transfer of
load torque, the alignment of axial, radial and angular misalignments between
driving and driven shafts is secured by a flexible compression space.
v
The securing of stable dynamic properties and
stable flexible transmission of load torque during the lifespan of mechanical
system.
Ø The twisting of
pneumatic coupling enables the compression of a gaseous medium in its
compression space, which is adequate for the load; and, in this way, the
flexible transmission of load torque in TOMS is realized. The stable flexible
transmission is secured by the use of flexible material in a coupling, which is
a gaseous medium (in our case, air). Air has a dominant impact upon the basic
properties of pneumatic tuner [11] and, during its whole operation, it is
resistant to wear and tear. As a consequence, the pneumatic coupling does not
lose its original characteristic properties and it is stabile during whole
lifespan of TOMS.
v
The capacity to suitably tune TOMS, that is,
the ability of tuning to adjust its dynamic properties to the dynamics of the
systems.
Ø On the basis of a
gaseous medium pressure pS change in the compression space of
pneumatic coupling, the dynamic torsional stiffness ked of the
coupling is tuned. This has a decisive influence on the natural frequency ΩO
of the system, when Ired is the reduced mass moment of inertia of
the system.
(6)
Ø Thus, the principle of
the suitable tuning of TOMS via pneumatic couplings, as explained above, is
based on the adjustment of the natural frequency of the system ΩO to the exciting
frequency ω in a manner
preventing the state of resonance in the operating range of the system (ΩO=ω).
Consequently, this results in the prevention of dangerous torsional
oscillations.
6. CONCLUSION
The control of dangerous
torsional oscillations in mechanical systems is currently resolved by the use
of highly flexible couplings with suitably selected courses of linear or
non-linear characteristics. Only certain types of flexible couplings are able
to fulfil this need, because not all types of couplings are capable of
achieving satisfactorily low torsional stiffness and, at the same time,
satisfactory strength. The torsional stiffness and strength are dependent upon
the shape of the flexible element and the material used for manufacturing the
flexible element. It is also appropriate to mention that each linear or
non-linear flexible coupling presently used is defined only by one
characteristic. To change the characteristics of the flexible coupling, in
order to suitably adjustment its dynamic properties to the dynamic of the
system, it is necessary to use a different element of flexible coupling, or to
use a different flexible shaft coupling. In addition, it is unacceptable to
overlook the fatigue and wear of flexible materials, which, in the end, impact
the original dynamic properties. The instability of dynamic properties of
flexible couplings caused by the wear and fatigue of these flexible elements,
together with the frequent malfunction of other parts of the system, results in
the mistuning of preliminary TOMS. In this case, the tuning part, i.e., the
flexible shaft coupling, has no capacity to remove or reduce increasing
dangerous torsional oscillations.
By taking these facts
into consideration with the aim of tuning up or tuning TOMS and limiting the
dangerous torsional oscillations, a proposal was made to apply the newly
developed flexible shaft couplings as so-called pneumatic tuners of torsional
oscillations. These pneumatic couplings have, at their disposal, a whole array
of characteristics (not just one) and, at the same time, a range of
characteristic properties. The properties of these couplings are influenced by
changes in gaseous medium pressure, namely, in differential pneumatic flexible
shaft couplings, and the selection of constant twist angles of couplings with a
parallel change in gaseous medium pressure, namely, in differential pneumatic
flexible shaft couplings with autoregulation.
Based on the results of
experimental measurements performed on newly developed pneumatic couplings, we
can say that, due to a gaseous medium pressure change in a compression spaces,
the dynamic torsional stiffness of a coupling is also changed (tuned), which
has a decisive impact upon the natural frequency of the system. Thus, the basis
of the principle of tuning TOMS via pneumatic tuners is the adjustment of the
natural frequency of mechanical system to the existing frequency in a manner
whereby a state of resonance and, consequently, dangerous torsional
oscillations in the operating range are prevented.
In the context of
dangerous torsional oscillations of mechanical systems, the development of and
research into various types of flexible couplings are highly topical and, from
the aspect of limiting dangerous torsional oscillations, unquestionably
necessary. It has been shown that one of the types of shaft couplings, which
are exceptionally suitable for the fulfilment of this aim, are pneumatic shaft
couplings, as they function as pneumatic tuners of torsional oscillations.
In summarizing my
results to date in the area of the torsional oscillation of mechanical systems
research, it is possible to state that this work has made a contribution by
broadening and enriching knowledge in this field. This scientific contribution
can be evaluated primarily from the two perspectives:
Ø
Elaboration of new elements - pneumatic
couplings, tuners of torsional oscillations
Ø
Implementation of new control methods for
torsional oscillations via several innovative approaches
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Received 17.03.2018; accepted in revised form 04.06.2018
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