Research on Thermal Stiffness of Machine Tool Spindle Bearing under Different Initial Preload and Speed Based on FBG Sensors

As a key parameter, stiffness determines the dynamic characteristics of the bearing and the spindle unit. However, the interaction of the spindle speed, initial preload, and cutting force will change the bearing stiffness characteristics during the running process cause the thermal expansion of spindle unit components. Aiming at real-time online monitoring of the thermal characteristics of machine tool spindle bearing stiffness, this paper proposed a fiber Bragg grating (FBG) sensors network. And under different axial loads, combined axial and specific radial load, the bearing temperature rise, thermally induced preload, and thermal stiffness are studied. The results show that the thermal stiffness of the bearing decreases with the increase of the spindle speed, and there is an optimal initial preload at the speed to maximize the thermal stiffness of the bearing, and the thermally induced preload has an additional pre-tightening effect on the bearing.

VIBRATORY ACTIVITY INVESTIGATION OF GRINDING MACHINE ELECTRIC SPINDLES

The purpose of this work is to support dynamic properties of spindle units in grinding machines. For this there are problems under solution for the definition of the origin of the constituents in the spindle unit vibratory activity by means of the linear increase of electric spindle rotation frequency, obtaining and analyzing a vibratory acceleration signal for the possibility to determine a preload. The vibratory acceleration signal was investigated through a spectrum analysis method. A scientific novelty of investigation consists in the substantiation of possibility to determine a preload by means of the spectrum analysis of a vibration acceleration signal at the linear increase of spindle rotation frequency that is at starting. It gives, in its turn, a possibility for the automated estimate of the spindle unit state before cutting beginning. In the experimental way there are obtained temporal realizations of the vibratory acceleration signal at different efforts of the preload. A high-speed grinding motor-spindle is as a basic element of the bench, which was investigated through the methods of testing diagnostics in the operation. In the bench design there were made some alterations. The bench was supplemented with the systems essential to support motor-spindle full operation, in particular: with systems of lubrication, cooling and drive control. There was revealed a large number of harmonics multiple to 50 Hz, which tells of the connection with the frequency of power supply circuit. Their coincidence with the own frequencies of the spindle unit results in the considerable increase of their amplitudes. To increase dynamic quality one should avoid the cases of the coincidence of switching frequencies and circuit harmonics with own frequencies of the electric spindle. It is also necessary to bring a form of power voltage to a pure harmonic oscillation to decrease the impact of a drive electromagnetic field upon dynamic characteristics of the spindle unit.

RADIAL STIFFNESS ANISOTROPY OF BODY BORING FOR LATHE SPINDLE BEARINGS

In the paper there are considered problems connected with the radial stiffness anisotropy of body borings for lathe spindle bearings. Factors having an influence upon stability of its operation characteristics of a machine-tool during idling are considered. The analysis of force factors having an influence upon bearings of a spindle unit during lathe idling is carried out. The work purpose is to determine a quantitative and qualitative situation in the changes of boring forms for spindle bearings under the impact of loads mentioned. As an object of investigations there is chosen the lathe “Vector” manufactured at the Middle-Volga machine-tool plant. A finite element modeling is carried out for deformation investigations in the body boring of the spindle unit under the impact of forces caused by weight and imbalance of the rotating spindle. There is shown a geometrical model of a spindle carrier body, the peculiarities of a finite element model formation and a procedure for the fulfillment of a numerical experiment are described, and approaches of boundary conditions setting are also considered. As a result of a finite element there are obtained values of finite element unit displacements lying on the surface of the body boring of the spindle carrier under the impact of the mentioned power factors. The results found are presented for illustration in the form of solid and flat graphical dependences. The analysis of numerical experiment results has confirmed the presence of the stiffness anisotropy in spindle unit bearings. The phenomenon of stiffness anisotropy in bearings was already under consideration in investigations carried out earlier. But the results of the numerical experiments carried out by us have shown that along with other factors the imbalance forces together with design peculiarities of body parts have an influence upon the constancy of elastic characteristics of bearings. The matter obtained can be used during development of measures for design changes in a spindle unit body with the purpose of stiffness anisotropy decrease in the borings for bearing.

Study on the Stiffness, Longevity and Reliability of the Spindle Unit of Eclipse 300 CNC Lathe with Different External Load

The stiffness of the spindle assembly of machine tools in general and also CNC machines in particular is an important criteria to evaluate the quality of machining. The initial stiffness of the spindle assembly is due to pre-load. During operating, the spindle assembly is wear under the influence of external loads including cutting force and speed. It reduces the stiffness of the spindle assembly, as well as decreasing machining accuracy. This study presents the results of various stiffness of the spindle assembly under external loads in laboratory conditions. Based on the relationship between the stiffness and the amount of axial wear of the spindle assembly according to the working time, as well as the permissible stiffness limit to predict remaining time, reliability and time of adjusting the pre-load to maintain stability and machining accuracy.

MEASURING OF THE MINIMUM VALUE OF TENSION PRELOAD IN A BEARING TO ENSURE THE RIGIDITY OF A SPINDLE UNIT DURING REPAIR OF THE “BEARING OUTER RING–CASE” JOINT

The article proposes a method for determining minimum tension preload when restoring stiffness of a joint between a case and an outer ring of a bearing. Stiffness is essential prerequisite for functioning of a spindle assembly in a metal-cutting machine. To large extent, stiffness of an assembly is affected by deformation of its supports, mainly the frontal one. During work, this influence increases due to weakening and displacement of “outer ring of a bearing–case” fit. Fretting corrosion wear is believed to be the main cause of this phenomenon. Usage of polymer compositions in restoration of joints increases stiffness due to contact area enlargement. Main task is to determine required minimum value of tension preload to fully restore stiffness of a fit. This experimental study includes finding stiffness of “outer ring of a bearing–case” joint directly and finding stiffness of a spindle assembly as a whole. Method proposed helps to define minimum tension preload in an adjustable front bearing with different values of hole wear in a case which have been restored with polymer composition. Minimum tension preload is defined at the intersection point of a bearingʼs stiffness-tensile curve with the lower boundary of the stiffness parameter range. This method can be used for fit and for a spindle assembly as a whole.

DETERMINATION OF VIBRATION POWER OF VIBRATION SOURCES OF SPINDLE UNITS OF METAL CUTTING MACHINES

Monitoring the state of spindle assemblies of modern metal-cutting machines using CIP methods implies the possibility of installing control sensors in places with maximum vibration information content. In this regard, the assessment of the informativeness of the vibration field of the spindle assembly, which can be carried out in advance, taking into account the design features, geometric and dimensional characteristics, is an urgent task. Based on the energy approach, using the example of the spindle assembly of a universal lathe, a computational model is proposed, built along the median planes of the walls. On the basis of the calculated model, the equations of the energy balance were compiled taking into account the conditions for the transfer of vibration power between the walls of the housing. The method for determining the vibration power of the main sources in the spindle assemblies is given: a spindle with residual unbalance, a drive gear and bearings. It has been established that the most significant factors determining the vibration power introduced into the spindle unit housing are the operation of the bearings, the weight of the spindle and the location of its center of mass. The result obtained makes it possible to detect incipient defects in the bearings, which, as practice shows, are the defining elements of the parametric reliability of spindle assemblies. The resulting model of the vibration field can be used to determine the reference values of vibration velocities that are formed from sources in the absence of defects.

Informativity of the Vibration Signal of the Spindle Unit of the Metal Cutting Machine

Monitoring the condition of the spindle units of modern metal-cutting machines by methods of non-selective diagnostics involves the possibility of installing monitoring sensors in places with maximum vibration information. In this regard, the assessment of the informative value of the vibration field of the spindle unit, which can be carried out in advance, taking into account the design features, geometric and dimensional characteristics, is an urgent task. Based on the energy approach, a computational model based on the median planes of the walls is proposed using the example of a universal lathe spindle unit. On the basis of the calculated model, the energy balance equations are drawn up taking into account the conditions for the transmission of vibration power between the walls of the housing. The dependences that allow us to calculate the coefficients that take into account the absorption of vibration energy by the walls of the housing are given. The solution of the energy balance equations made it possible, based on the level of the vibration power flow, to rank the walls of the spindle unit body by information content,. The resulting model of the vibration field can be used to determine the reference values of vibration velocities that are formed from sources in the absence of defects. This will allow for non-selective diagnostics to detect the occurrence of a defect at an early stage, and in the presence of a defect to assess the level of its development.

Recommendations on Diagnostics of Support Condition of High-Speed Spindle Units

In the course of the study, amplitude-frequency characteristics of the responses of vibrational acceleration of the spindle unit were obtained experimentally with a short-term impact of a force directly on its spindle. By analyzing the frequency composition of amplitude spectra, the peaks of natural frequencies of the spindle unit were determined at various values of the preload, and, accordingly, the operating modes of the spindle unit were selected, which would reduce its vibration activity and increase processing accuracy.