Moravec, J.

Stach, E.

Stejskal, M.

Sulitka, M.

DSpace at University of West Bohemia

The effect of feed drive axes dynamics on the wearingof linear guidewaysJ. Moraveca, E. Stacha, M. Stejskala, M. SulitkaaaDepartment of Production Machines and Equipment (RCMT), Faculty of Mechanical Engineering, Czech Technical University in Prague,Horská 3, 128 00 Praha 2, Czech Republic1. IntroductionThis paper treats a method developed by the authors which helps to evaluate loading linearguideways used in NC machine tools. Research was motivated by the industrial problem withtotal damage of linear guideways which occured during the high productive manufacturing ofsmall parts. Method links two mechanical influences on failure of linear guideways. The countof contacts between the guideway static part (rail) and its moving part (bearing block) alongsidethe rail and the level of acceleration at these points. Method facilitate insight into non-constantload of linear guideway alongside its rail and takes into accounts the influence of dynamics ofall three feed drive axes X, Y and Z on axis X where damaging occurs. Method helps to findappropriate set of maximal accelerations and jerks of feed drive axes X, Y and Z with lowerdynamic impact on the load of guideway and with minimal penalty on machining time.2. Outline of the problemFig. 1 (left) shows schema of linear guideway commonly used in machine tools. It consists ofrail, bearing block and rolling element which could be ball shaped or roller shaped. In our caseroller type was used. Bearing block is filled by a long life grease and equipped by seal elementsto wiping down chips from machining. Damaged locations on rails are marked by the red points.Our task was to clarify the causes of damage of linear guideways used in highly productiveproduction of duralumin parts. Two significant influences with impact on linear guidewaysdamage were supposed. Malfunction of greasing and impact of inertial forces given by frequentacceleration and deceleration of moving axes. Both flows from extreme conditions during thepart production. Cycle time of one machined part was approx. 4.5 minutes and machine wasused in three-shift operation seven days in week. For the complete machining of part severaltools were used. Tool changes requires fast movements to a magazine located at the back endof Y-axis (Fig. 3). Inertial and frictional forces from rapid feeds such causes additional stress tolinear guideways beside the cutting forces and inertial forces from machining. Workpiece wasmachined under intensive flow of cooling emulsion.3. Causes of linear guideways damageDamaged sides of rails are shown in Fig. 1 (right). Case A shows constant wear, case B showswear with surface fatigue but cases C and D show sharply bounded traces from rolling elementsof bearing block. Inside the bearing block high degradation of seals and mixture of metal126Fig. 1. Schema of linear guideway (left); damaged sides of rails (right)chips with grease were observed. Such conditions complicate soft movement and causes furtherdamage rollers and rails.By this hypothesis it would be possible to interpret flat regular traces in cases A and Bbut short sharp, regular and relatively deep traces in cases C and D would suggest rather localforce shocks. It would agree with previously noted dynamical conditions of Y-axis movement.Therefore evaluation method was based on analysis of mechanical loading from inertial forces.Many research topics related to linear guideways from the view in many aspect were published,for example experimental analysis of the wear coefficient [4], effect of grease characteristics onsound and vibration [1] or model for wear prediction [3]. Analysis of drive dynamics is in [2].4. Graphical evaluation of influence of moving axes dynamics on linear guideways loadingPresented analysis is based purely on processing of data acquired from NC machine tool controlsystem by its internal oscilloscope. Data for analysis are recorded as desired and real positionsand velocities of feed drive axes X, Y and Z. Accelerations of axes was computed by a nu-merical derivation of velocities. Even though proper approach would be based on computationwith forces which directly impact on wear of linear guideways only computation with axes ac-celerations is done. The reason is that no information about moving masses and position ofcenter of gravity is available. So, computations with forces are imitated by computation withaccelerations.Some locations on rails were more damaged than other. This led to examine accelerationnot only in time domain but also in geometrical domain along the rail. Moreover, during theworking cycle some locations could be in more numerous contact between rail and bearingblock than others. Count of appearance on particular coordinate is therefore taken into accountby a histogram plot.So, level of acceleration along the rail forms first 2D graph for identification where localextremes of acceleration acts (top set of Fig. 2). Count of accelerations at the partial rail pointsforms second 2D graph for identification how many contacts between bearing block and railwas done during the working cycle (middle set of Fig. 2). Both graphs jointed together form3D histogram with position of particular machine axis on one axis acceleration on second axisand count of acceleration on partial rail point on third axis. For better readibility, 3D graph isconverted to 2D where the third axes is represented by the color scale (bottom set of Fig. 2). Letus note the graphs that count of contacts between both parts of guideway was evaluated at 5 mmsections as a summation of values inside this interval (comparisons of histograms for counts onelementary points and for 5 mm ones are for respective axes in middle row of Fig. 2).The most damaged guideways were on Y-axis where the high exposure of coolant and ac-celeration frequency from rapid feed acts together. For example, during the working cycle of127Fig. 2. Level of acceleration along rail (top row); count of acceleration at partial rail points (middle row);3D histograms count of acceleration on partial rail distances (bottom row). Axes X, Y and Z are groupedin respective columns. Limit of axes accelerations are 7 m/s2 for X, 10 m/s2 for Y and 12 m/s2 for Z,maximal axes jerks are 25 m/s3 for X, 50 m/s3 for Y and 300 m/s3 for Z. Machining time is 250 s250 s more than 30 accelerations (or decelerations) were to a maximum speed of 75 m/min. Forthis reason the main attention is paid to the analysis of Y-axis.Fig. 3 shows a diagram of the machine in the YZ plane. From the figure, it is clear that theforces from the acceleration of Y-axis, together with the forces from acceleration in the Z-axis,create a tilting moment Mx around the X-axis. This causes stress on the guideways of Y-axis.Fig. 3. Schema of machine with coordinates (left); correlation of acceleration extremes with damagedrail sections on Y-axis (right)128Therefore, in Fig. 2 (top row), two acceleration values are plotted. Reds are the accelerationsin a given axis and greens are the summations of accelerations in the X, Y and Z axes whichtogether contributes to the loading of guideways. Black lines on graphs in Fig. 2 (top row)shows limit values of axes accelerations. It can be seen that sum of accelerations significantlyoverlaps limit values for axes X and Y. This imply the influence of other axes to particular axiswhich results to high forces acting to the guideway. Moreover, histograms from the middle andbottom rows shows that contacts in the Y and Z axes are less regular than in Y-axis. Color mapsfurther detects that higher levels of accelerations are concentrated at particular coordinates. Itwould clarify why are rails such damaged at these coordinates. Detail graph with correlation ofacceleration extremes with damaged rail sections on Y-axis is in Fig. 3 (right).Based on these analysis several measurements with different accelerations and jerks of axesX, Y and Z were evaluated and the most proper one, also with regard to minimal time penalty,was appicated into the control system of real NC machine tool. This set is graphically shown inFig. 4. It is clear that sum of the accelerations X, Y and Z axes has lower level and more regularcharacter then in the previous case. Local extremes which probably cause sharp traces on railsdiminish.Fig. 4. Optimal set of axes accelerations and jerks with limits 4.2 m/s2 for X, 6 m/s2 for Y and 12 m/s2for Z, maximal axes jerks are 25 m/s3 for X, 50 m/s3 for Y and 300 m/s3 for Z. Machining time is 251.3 s5. ConclusionsPrimary cause of linear guideway damage was probably influence of cooling emulsion whichdegrades seal and originate mixture of grease and metal chips which significantly gets worsefriction in linear guideways. Another cause of guideways damage sources from high forceshocks given by accelerating of all three axes in one moment. Analysis shown relativelly highinfluence of dynamics of neighbouring axes Z and X to the load of axis Y. Developed analysisbrings results which correlates level of accelerations with damaged sections on the rails. Moresuitable levels of axes accelerations and jerks were chosen and applied into the NC machinetool in industry.References[1] Ohta, H., Tsuruoka, T., Fujinami, Y., Kato, S., Effects of grease characteristics on sound andvibration of a linear-guideway type recirculating ball bearing, Journal of Tribology 138 (2) (2016)No. 021101.[2] Souček, P., Servomechanisms in production machines, Czech Technical University in Prague,Prague, 2004. (in Czech)[3] Tao, W., Zhong, Y., Feng, H., Wang, Y., Model for wear prediction of roller linear guides, Wear305 (1-2) (2013) 260-266.[4] Wang, X.-Y., Zhou, C., Ou, Y., Experimental analysis of the wear coefficient for the rolling linearguide, Advances in Mechanical Engineering 11 (1) (2019), doi: 10.1177/1687814018821744.129

The effect of feed drive axes dynamics on the wearing of linear guideways

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The effect of feed drive axes dynamics on the wearing of linear guideways

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