DE1172861B - Method and device for determining the natural frequencies and relative deflections of mechanical systems with the help of investigations on a model - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school Kl .: G Ol h

German class: 42 c-42

Number: 1 172 861

File number: N 17868 IXb / 42 c

Filing date: February 10, 1960

Opening day: June 25, 1964

The review of systems, the torsional vibrations or longitudinal or transverse vibrations can perform, has so far been done by determining the natural frequency for individual types of vibration, whereby to determine additional mechanical stresses on shafts or other components relative deflections can also be determined. These values have to go through lengthy calculations or can be determined with the help of analogy considerations in electrical oscillation circuits.

There are already various methods for calculating the natural frequencies to be checked Systems known. But even when using simplified and abbreviated mathematical methods is for a complicated system, e.g. B. for a working within a wide speed range Shaft with additional masses, such as crankshaft with connecting rods and pistons, flywheel, clutch and the like, the required calculation is quite time consuming and sometimes takes several weeks. Simplifications of mathematical methods often lead to considerable errors, which then inevitably turn into affect practice.

After the calculations have been carried out, certain corrections are often required that need to be repeated make the whole lengthy calculation necessary.

One known method of detection that is based on an electrical oscillatory circuit analogy is quite complicated and places great demands on even a skilled low-voltage person skilled in the art. When using this method, errors arise due to exposure to scattering fields and feedback that affect the end result. The object of the invention is to avoid these disadvantages and to create a simple method with the aid of which the natural frequencies and deflections can be determined on a model in a simple manner and with sufficient accuracy even in the case of complicated mechanical systems. This object is achieved according to the invention in that measurements are made on a mechanical model by first simulating the actual system with the help of calibrated components in a simplified but vibration-equivalent manner and then excited to vibrate with the help of a vibration generator, whereupon it is maintained of the vibrations, the vibration frequency and the relative deflections of the parts of the simulated system can be measured.
To carry out the new method, the method and device for determining the
Natural frequencies and relative deflections
mechanical systems with the help of
Investigations on a model

Applicant:

NAVIKA, närodni podnik, Prague
Representative:

Dipl.-Ing. A. Spreer, patent attorney,
Göttingen, Groner Str. 37

Named as inventor:
Jan Smolik, Prague

Claimed priority:
Czechoslovakia February 11, 1959 (824)

Invention before a mechanical model, which has at least one shaft and one or more on the shaft adjustable and optionally replaceable drive plates, to which additional weights can be attached to change the moments of inertia, that these items are calibrated for predetermined moments of inertia and that a vibration generator, a frequency measuring device and a test device for determining the relative deflection are provided.
A mechanical oscillation model is already known which consists of a multiple pendulum carried by a stand with adjustable suspension points, adjustable pendulum lengths and optionally exchangeable weights of the individual pendulums, one of which is attached to the other. This model serves only as a teaching device for demonstrating mechanical vibration processes of a complicated type. This known device is not suitable for determining the natural vibrations of mechanical systems. It is also known to investigate the bending stress of load-bearing structures in lightweight construction with the aid of models.

With the method and the device according to the invention it is possible to adjust the natural frequency in a short time and to determine the relative deflections of the mechanical system to be tested. at It is the use of the simplified model assembled according to the modular system

409 627/124

3 4

still possible, individual moments of inertia is absolutely necessary that the rigidity of the shafts and the rigidity of the shaft can be changed quickly and easily and connections of the system to the same extent that suitable natural frequencies of the rod are reduced; And the downsizing of the system under test can be achieved to ensure masses can be in a different but for all masses that the critical speeds are not within the 5 same scale. This allows the operating frequencies of the system in question to be put together in any model. This method can advantageously be of different sizes. With the same measure for quick and accurate determination of rods of rigidity (of shafts and connection) relative deflections of different systems and the masses results in a natural frequency of the turns, z. B. with internal combustion engines without an io model that corresponds to that of the system to be measured in or with driven devices, and also in terms of actual size and design,
various units, e.g. B. with motor-blower, when different to scales of rigidity and motor-pumps, motor-displacement units, door masses is when the stiffnesses (rigidity) and binenaggregaten and different wavelengths, moments of inertia of the corresponding z. B. in ships or other devices in which 15 components are in the same ratio, the individual components by different, given by frequency by the following formula:
Couplings subject to torsion with one another.
are bound. It is also possible with the help of this ^{Uann means}
Procedure to generate transverse or longitudinal vibrations _ τ. '<■ «- !,« · © "
average if appropriate analogies are used. 20 Ic ''

The invention will be on hand schematic drawings''->'·>.

voltages at a plurality of exemplary embodiments fo _r (j _as Model:

explained It shows / '= frequency, &" = mass inertia

F 1 g. 1 schematically a simplified, after the moment

Modular system compiled model for 25 _c < = rigidity (stiffness) of the shaft between

Implementation of the method _{according to the invention · the masses mh the} moments of inertia

F 1 g. 2 schematically shows a contactless sensor q · _unc j ß for determining the frequencies and

Fig. 3 schematically shows another embodiment for the actual system:

shape of the adjustable drive plate. 30 f - frequency, Q _n = mass inertia

To put together a simplified, at the moment,

The modular system built model of the test _{Cn in =} rigidity of the shaft between the masses system is the inventive "for determination" with the moments of inertia 6> _w _i and 6>". The device 35 serving the natural frequencies and the relative swings of the system to be tested, the simplified model of the elastic is equipped with a series of calibrated waves. The shafts can be made of bright-drawn stainless steel. Forces, the magnitude of which changes over time, or from another, the actual system over at certain time intervals a reduced wave 1, which replaces one or more 40 recurring impulses up to periodic excitation waves of the actual system, one whose course is repeated regularly,
Sufficiently small internal damping guaranteed. The excitation device 3, which the model in vibration

The device also consists of a number of staggered movements, which can also be of various types

calibrated, adjustable drive plates 2 can be designed with simple mechanical vibrating

various, also calibrated additional weights 11, 45 exciters with cams up to compli-

the smaller flywheels, ornate centrifugal or electromagnetic, electro-

Replace clutches, crankshafts, pistons, etc. dynamic, electrostatic, piezoelectric, ma-

These components can be made of rolled sheet steel, gnetostrictive and similar excitation devices,

or another suitable material. Suitable for generating torsional vibrations

will. 50 In the present case, it is best to consider such

From these three types of components, a model can be put together that does not reduce the damping of the system, enlarges it, has a small internal rigidity and an arbitrarily given actual system has a small mass of the movable, with the oscillatory dynamics equivalent, all for the system have the components connected to the system and characteristic values such as wave rigidity, size 55, a large frequency bandwidth, good efficiency of the mass moments of inertia, etc., are retained. and accuracy as well as simple operation and the model is designed in such a way that the driver ensures noiselessness,
disks 2 on the shaft can be adjusted as desired. In some systems, these requirements can be changed, as a result of which the rigidity of the shaft is changed by means of an electrodynamic process. At the same time, the number of driver excitation 60 can be met, as shown in FIG. 1 is shown, where even disks and their masses are changed by additional weights 11 consisting of coils 13 exciter 3 with alternation, whereby in any current from an audio frequency generator 7 and a way whose moments of inertia are influenced amplifier 6 is fed, whose output voltage can. with a voltmeter and the current with an ampere

In the model of a system to be measured, 65 meters are measured, both in the device 10 below it is not necessary that one is brought with the when measuring. In the air gap between the pole pieces Frequency of the real system to be measured 14, 15 of the coils is a permanent magnet rotatable matching frequency is measured, but it is stored. The excitation forces arise through action

of the coil fields that mutually influence each other.

For other elastic systems, e.g. B. with a higher natural frequency, preferably a piezoelectric, magnetostrictive or similar excitation device can be used.

The determination of the natural frequency of the model set in oscillation can be carried out when using of the method according to the invention take place in different ways with different means. At the the simplest would be to find the natural frequency of the simplified model by directly reading the to determine the frequency values set on the audio frequency generator. However, this method achieves for itself alone does not provide the required accuracy and in particular does not guarantee correct measurement results, if the model also has lower or higher harmonic frequencies.

There are other ways to determine the natural frequency, for. B. in a comparison of the frequency of the model and a known one, on a film strip or on the screen of an oscilloscope recorded frequency. An analyzer can also be used to measure the frequency, which enables frequencies to be determined almost immediately with a relatively high degree of accuracy and at the same time an approximate attestation of the relative intensity of individual measured resonances allowed to carry out.

An electromagnetic contactless sensor 4 can advantageously be used for this purpose F i g. 2, which is preferably set in the location of an expected antinode. The sensor consists of a permanent magnet, on the pole pieces of which coils 13 are attached. Of the magnetic circuit of the permanent magnet is via an adjustable drive plate 2 on one with a Recess 12 provided point closed. This recess 12 is located and dimensioned so that the End face of one pole piece 14 opposite the full circumferential surface 16 of the adjustable drive plate 2 stands, while the center of the end face of the other pole piece 15 is an edge 17 of the Recess 12 is opposed. The coils 13 are connected in such a way that their voltages add up will. During the measurement, the recess 12 moves in relation to the adjustable drive plate 2 the electromagnetic sensor 4, which also changes the magnetic field accordingly will.

However, the sensor can also be arranged in other ways, e.g. B. instead of a recess a projection 18 may be provided which moves towards and away from the pole pieces 14, 15 etc. (Fig. 3).

A tube voltmeter 9 be used. If this device is used in conjunction with an oscilloscope, it is possible to when measuring natural vibrations, the maximum amplitude of the resonance vibrations is proportionate great accuracy.

To eliminate an incorrect voltage curve when measuring natural vibrations it is also advantageous to use an oscilloscope 8 because sensors of all types can, under certain circumstances, have erroneous voltage curves that the do not correspond to the actual oscillation movement. Making the course visible of the measured voltages with the help of an oscilloscope allows quick control and eliminated all sources of error.

After starting the excitation device, the simplified, according to the modular system built model by the action of the electromagnetic excitation device 3 in vibrations offset that are close to the resonance frequency. The rotating dial of the audio frequency generator 7 is set approximately in a position corresponding to the greatest sound effect. The one in the generator 7 generated frequency is amplified in amplifier 6. Its amplitude is on the measuring instrument 10 readable.

After selecting an appropriate measuring range of the tube voltmeter 9, the maximum is the measured amplitude precisely determined by changing the excitation frequency. This frequency then represents represents the natural frequency of the vibrating system.

The agreement of the two frequencies is confirmed with the aid of the oscilloscope 8, with on the one pair of deflection plates the excitation vibration, on the other pair of deflection plates with the help of the Electrodynamic sensor 4 recorded frequency of the model is placed.

The agreement between the two frequencies is assessed with the aid of the well-known Lissajous figures. By further slowly increasing the frequency of the audio frequency generator 7, in the same way the frequency of the two-, three- and multi-node oscillations can be determined.

Before starting the test device, all adjustable driver disks 2 with a Mark a line that is at the same height on all panes. Proportional relative Swings at the resonance frequency can then be determined with the aid of an optical device 5 will. The optical device 5 is adjustable parallel to the axis of the model, and the amplitude of the Line mark vibrations at resonance frequencies with constant excitation is on the scale of the optical device, separately for each adjustable drive plate.

During this measurement, the tube voltmeter 9 is used to monitor whether the swiveling for all readings and the given resonance frequency remains constant. A simultaneous relative Reading can be done with the help of the reflex method using slit lamps, wherein the oscillating beam is projected directly onto scales on which relative swings can be read. Is each adjustable drive plate with its own mirror, slit lamp and scale, then the relative swings instead of a subjective reading can also be photographed.

Claims (4)

Patent claims: 1. Procedure for determining the natural frequencies of torsional, lateral or longitudinal vibrations and the relative deflections of mechanical systems through measurements on a model, characterized in that the actual system is calibrated with the help of Components is simulated as a model in a simplified, but vibration-equivalent manner, the model by means of a vibration generator excited to vibrate and the vibration frequency and the relative deflections of the individual parts of the system can be measured. 2. Device for carrying out the method according to claim 1, characterized in that the model, which is composed in a manner known per se from several mutually adjustable individual parts, has at least one shaft (1) and one or more adjustable and possibly exchangeable driver disks (2) on the shaft has, on which additional weights (11) can be attached to change the moments of inertia, that these items are calibrated for predetermined moments of inertia and that a vibration generator (3), a frequency measuring device (4) and a test device (S) are provided for determining the relative deflection. 3. Apparatus according to claim 2, characterized in that the frequency measuring device has a sensor (4) for scanning the simplified model in the form of a permanent magnet, on the pole pieces of which coils (13) are attached which are connected in such a way that their voltages are added, and the magnetic circuit of this permanent magnet via a recess (12) an adjustable drive plate (2) is closed, the end face of one Pole shoe (14) is opposite the surface (16) of the adjustable drive plate (2) and an edge (17) of the recess (12) of the adjustable drive plate (2) of the center of the end face of the other pole piece (15) faces. 4. Apparatus according to claim 3, characterized in that that the adjustable drive plate (2) is provided with a projection (18), wherein the permanent magnet (4) with the coils (13) below or above the projection (18) is stored. Considered publications:
Austrian Patent No. 162 851;
VDI magazine, 102 (1960), p. 92. 1 sheet of drawings 409 627/12 * 6.64 © Bundesdruckerei Berlin