Vibration analysis of shot wheel in abrasive blasting

. Today, abrasive blasting is experiencing rapid development, due to two important factors. The first one is related to the introduction of the throwing wheel as an element that sets the granular media in motion directing them to the treated surface, while the second factor is related to the production of modern machining media. Together, these factors make blasting one of the most efficient mechanical surface preparation methods. Unfortunately, the high erosiveness of the abrasive stream, so appreciated in the process of preparing the treated surface, also contributes to significant wear of the elements in its immediate vicinity. The undertaken issues focus on the possibilities of classifying various states of the throwing wheel of a rotary machine tool based on the analysis of its dynamic properties.


Introduction
Abrasive blasting is currently a commonly used method for preparing surfaces not only of metal products, but also of plastics, ceramics and other similar materials [1].Of particular interest are machine tools with one or a set of throwing rotors.
The essence of the machining process is to create and give the right direction of the abrasive stream to the machined surface in order to e.g.[1,2,3]: ➢ giving the appropriate functional properties of the native material, ➢ surface preparation before the next stages of the technological process, ➢ surface finishing treatment.The above-mentioned issues, to varying degrees, make up a very complex machining process, the effect of which mainly comes down to obtaining the appropriate surface quality.
The efficiency of abrasive blasting will certainly depend on the properties of the machining medium used and the state of the blast wheel, which has a significant impact on such parameters of the abrasive stream as [4]: ➢ mass flow rate of the machining medium, ➢ speed and kinetic energy, ➢ flux concentration, ➢ reception, ➢ rake angle, ➢ grain size distribution of the machining medium.
Undoubtedly, the degradation of the blast wheel in rotary machine tools is closely related to the obtained quality of the machined surface.It is therefore advisable to strictly control this process, for example by monitoring the dynamic properties of the throwing wheel.
The wide possibilities of using the analysis of dynamic properties based on vibration signals are presented in numerous publications from various areas of knowledge, including the area of manufacturing techniques [5,6,7,8,9].The versatility of vibroacoustic techniques encourages you to try to use them in the case of abrasive blasting.

Construction of the research facility
In the construction and operation of modern rotary machine tools, two systems are distinguished (fig.1): ➢ machining medium circulation, ➢ air circulation.
The proper functioning of these two circuits is a prerequisite for the good operation of rotary machine tools, while due to the issues raised, the research is focused on the first system.In the shot cycle, the following elements can be distinguished: the working chamber, the unit for moving the workpieces, the units for transporting the processing medium, the medium separator, the medium feeder and the working rotor turbine.
The working rotor turbine is the basic working unit of the abrasive blasting machine.Its task is to change the flow direction of the machining medium and give it the appropriate outlet velocity.The basic elements of the tested blasting turbine are shown in photos 2 and 3.The working space of the turbine is limited from the outside by a body made of thick-walled sheets.The body is a supporting structure for movable elements and cover plates constituting its internal lining.These plates are made of wear-resistant materials, most often of special chromiummolybdenum alloy cast iron.The basic internal element of the shot blasting turbine is the blast wheel as well as the regulating sleeve and the distribution rotor.The throwing wheel assembly includes such elements as: a throwing wheel with a disc and blades, a reduction sleeve and a distribution impeller.The main distinguishing feature of the throwing wheels is their diameter and the number of blades installed.The detailed technical specification of the tested throwing wheel is presented in the table 1.The design solutions of the rotors differ mainly in the system of fastening the blades to the rotor discs, the construction of the distribution rotor and the adjustment sleeve.As a rule, the distribution impeller has the same number of holes as the number of blades in a given throwing wheel.There are eight of them in the examined object.
The distribution rotor changes the direction of the shot flow, gives the abrasive grains initial acceleration and, through the outlet opening of the regulating sleeve, supplies the medium in portions to the throwing blades.The recesses in the impeller (fig.4) are intended for fixing the blades.Spacer sleeves setting the distance of the wheels from each other also act as locks protecting the blades from slipping out of the sockets.The method of attaching the blades depends primarily on their shape and is solved in a different way depending on the selected manufacturer.The blades (fig.5) are usually made of alloy cast iron with a high content of chromium, and in the case of turbines used in rotary machine tools working with the hardest steel shot, they should be made of tungsten carbide [3].The shape and properties of the stream of the machining medium depend directly on the construction of the throwing wheel.The principle of operation of the blasting wheel together with the diagram of the formation of the stream of the machining medium is shown in the figure 6.

Wear of the elements of the shot turbine
The rotor blades are the element of the blasting wheel that is most exposed to the erosive action of the abrasive stream, but it is not necessarily the element that undergoes the fastest degradation processes.It turns out that in many cases the body of the blasting wheel, in which the blades, the distribution impeller and the adjustment sleeve are mounted, is the element that wears out much earlier.Examples of signs of wear are shown in the photo 7.

Measurement methodology
The operation of the blast wheel in the grinding machine is characterized by a very high and variable load.This condition causes irregular wear of its individual elements, which makes it impossible for the maintenance services to plan downtimes and carry out appropriate maintenance and repair activities.Due to the nature of the operation of the rotor unit, an attempt was made in this work to use vibrodiagnostics as a method of monitoring the technical condition.By analyzing the speed of the blasting wheel, the direction of rotation and the nature of the types of wear of surfaces cooperating with the abrasive, the measurement range was set at 0.7 -12800 Hz.Measurements were made at two points: A and B, shown in photo 8. Mounting the piezoelectric sensor at point A made it possible to record the vibration signal in the axial direction, and at point B in the radial direction of the projection circle.The measurement was made in two operating states of the machine tool.In the first state, the vibration signal generated by the rotor assembly without the abrasive material load was registered, while in the second state, the vibrations of the assembly subjected to the load with this factor were registered.The DIAMOND 401A device was used for the measurement, while the data analysis was performed in the MBJLab software.The vibration signal was recorded using a piezoelectric sensor by IMI PCB, type 622B01, with a range of +/-50 g.The sensor was fixed with a magnetic base.For other issues related to the measurement, the PN -ISO 10816 standard was used, referring to the possibility of evaluating machine vibrations on the basis of measurements on non-rotating parts.In the recorded signal, the characteristic frequencies of individual elements included in the rotor assembly were searched.The recorded results are presented in the next chapter in the form of amplitudefrequency graphs.

A B 6 5 Results
Due to the complexity of the tested technical object and the high probability of vibration signal disturbances during the operation of the system under load, it was decided to first record the vibration signal of the test object without load, i.e. no abrasive was supplied to the blasting wheel.The no -load measurements of the throwing wheel at A are shown in FFT charts 9 -11.The no -load measurements at B are shown in FFT charts 12 -14.

Conclusion
The obtained results of the preliminary tests allow to conclude that the vibrodiagnostic method can be an effective tool for monitoring the technical condition of the blast wheel of the abrasive blasting machine.Two significant areas of vibration emission can be observed in the presented graphs.The first in the range of approx.400 -4000 Hz, and the second in the range of approx.5000 -10000 Hz.The first area of the observed vibrations concerns mainly the shaft and the throwing wheel of the rotor assembly, while in the second area the observed vibrations come from the bearings on which the shaft driving the throwing wheel is mounted.Analysis of the graphs in the axial direction without and under load clearly shows that the abrasive material supplied to the blast wheel on the impeller blades contributes to their growth.This would explain why the body of the throwing wheel itself is subject to much more wear.In turn, analyzing the radial direction without and under load, one can see a natural phenomenon in the case of the operation of rolling bearings.Supplying the abrasive material to the blasting wheel causes a significant increase in the load on the wheel, which is transferred not only by the elements of the rotor assembly being in direct contact with the abrasive, but also by the bearings on which the wheel is mounted.When this additional load appears, the vibration amplitude in the characteristic frequency of the bearings almost doubles.
Vibrodiagnostic tests are an excellent tool for monitoring the technical condition of various technical objects as well as technological processes.Further tests should be carried out in order to determine the limit states of individual elements of the rotor assembly, and then it is possible to undertake the optimization of the abrasive stream on the basis of the registered vibration signal in order to extend the service life of the body of the blasting wheel. 10

Fig. 2 .
Fig.2.Shot blasting turbine mounted in a rotary machine tool.

Fig. 5 .
Fig.5.An example of a paddle of a throwing wheel.

Fig. 6 .
Fig.6.The principle of operation of the blast wheel and the diagram of the formation of the stream of abrasive.

Fig. 7 .
Fig.7.Local areas of wear on the body of the throwing wheel.The abrasive stream thrown by the deformed surfaces of the blasting wheel has a negative effect on the uniformity of surface treatment and the repeatability of surface characteristics after treatment.

Fig. 8 .
Fig.8.Location of measurement points along with directions of vibration measurement.

Fig. 9 .
Fig.9.First vibration measurement at point A in the axial direction of the throwing wheel without load.

Fig. 10 .
Fig.10.Second vibration measurement at point A in the axial direction of the throwing wheel without load.

Fig. 11 .Fig. 12 .Fig. 13 .
Fig.11.Third vibration measurement at point A in the axial direction of the throwing wheel without load.

Fig. 14 . 8 Fig. 15 .
Fig.14.Third vibration measurement at point B in the radial direction of the throwing wheel without

Fig. 16 .
Fig.16.Second vibration measurement at point A in the axial direction of the throwing wheel under load.

Fig. 17 .Fig. 18 .
Fig.17.Third vibration measurement at point A in the axial direction of the throwing wheel under load.

Fig. 19 .
Fig.19.Second vibration measurement at point B in the radial direction of the throwing wheel under load.

Fig. 20 .
Fig.20.Third vibration measurement at point B in the radial direction of the throwing wheel under load.

Table 1 .
Technical specification of the throwing wheel.