COMPARATIVE ANALYSIS OF FLUID COOLING SYSTEMS IN MOTORIZED SPINDLES

This paper analyzes geometrical approaches to optimize the fluid cooling circulation of motorized spindles. The spindle fluid cooling’s effectiveness, efficiency and influence on the machine’s precision are analyzed through observations of the stator temperature, pressure drop and thermal asymmetry, respectively. The observation is based on a validated coupled thermal/fluid mechanical simulation model. The widely used helix and meander shape stator cooling sleeves are primarily investigated. Additionally, a so-called S-meander shape was developed, which combines the advantages of the formerly mentioned sleeves. In order to understand the nonlinear thermal interactions properly, width and height of the cooling channels were varied separately and simultaneously. While keeping the flow rate identical, the average stator temperature could be decreased by 2.3 K solely with geometrical optimizations. Interestingly, the motor temperature is not continuously decreased by raising the fluid velocity through a reduction of the cooling channels size. For the helix and the S-meander, the temperature actually increases after passing a certain geometrical sweet spot. Additionally, this optimum is different for the helix, meander and S-meander cooling sleeve. The results imply that the geometrical optimization of fluid cooling channels in motorized spindles has a significant potential. Furthermore, the developed cooling sleeves are trans-ferable to any electric motor with fluid cooling.

THERMAL ASYMMETRY ANALYSIS OF MOTORIZED SPINDLES

This paper presents a method to quantify and reduce thermal asymmetry of motorized spindles. Thermal asymmetry leads to angular and radial deflections at the tool center point. In contrast to simple thermal expansion issues, these effects are harder to compensate. Therefore, the causes of the asymmetries should preferably be evident in the construction phase. This paper introduces a newly developed mathematical formulation to quantify thermal asymmetry. Thermal asymmetry is observed along the longitudinal axis of a motorized spindle. The formulation quantifies thermal asymmetries as a difference of a geometrical centroid and a newly introduced thermal centroid. For this analysis, several motor spindles with different fluid cooling circulation systems were observed. In order to show the legitimacy of the formulation, the spindle’s calculated thermal asymmetries are compared with their respective radial tool center point displacements. The results show that the asymmetries correlate with the displacements. Furthermore, the quantification of the thermal asymmetry actually allows to locate its causes. In motor spindles the asymmetry is mostly caused by the complex fluid circulation system. The spindle with the worst cooling circulation showed a radial displacement of 26,32 µm. Through thermal asymmetry optimization of the circulation’s heat transfer, the displacement could be reduced to 0,66 µm. The developed method is not limited to motorized spindles. It will be investigated further to develop a generally valid formulation.

A Fast Engineering Method for Estimating Iron Losses in Induction Type Motor Spindles Based on Equivalent Magnetic Circuit

The iron losses in the motor of motorized spindles have a significant effect on the heat generation, working efficiency, and speed-torque characteristics of motorized spindles as well as on their thermal deformation and machining accuracy. The existing finite element and analytical methods based on Maxwell’s equations are too complicated to be suitable for engineering designers. A fast engineering method for estimating iron losses in the spindle motor is presented based on equivalent magnetic circuit (EMC) where the problem of solving a complex electromagnetic field inside the spindle motor is simplified into a simple magnetic circuit calculation by the assumption that the magnetic flux density distribution of any cross section along the magnetic flux direction in the spindle motor is uniform. The EMC is combined with the Boglietti’s model. They are integrated into a developed program by compiling source codes to achieve the analysis and prediction of iron losses in the spindle motor. The results obtained from the proposed method are compared with the prototype experiment data to verify its validity. With the purpose of ensuring accurate experiment results of iron losses, a method of no load running combined with a sudden loss of power supply is proposed to eliminate the braking torque and electromagnetic losses of the spindle motor, namely to achieve the separation of the mechanical loss from the total losses.

Lifetime Analysis of Motorized Spindle Bearings Based on Dynamic Model

The traditional probabilistic-based lifetime evaluation methods for motorized spindles neglect the effects of load dynamic and structure difference. Hence, a dynamic-model-based lifetime estimation method combining these effects is proposed to improve the estimating results for motorized spindles, especially in the design stage. Considering the bearings lifetime has dramatically influenced the reliability of motorized spindles, this paper establishes a shaft-bearing-toolholder based on a dynamic model to estimate the bearing group lifetime. The proposed dynamic model is closer to the actual structure in spindles, indicating the stiffness of bearings and contact surface conditional on the inputting radial-and-axial forces is nonlinear. The stiffness model is verified by finite element analysis and experiment. The load applied to bearings is accurately calculated through the dynamic model. Then, the load is introduced to a well-known bearing lifetime model, thereby calculating the lifetime of each bearing and its group. The bearing lifetime results under different conditions of preload, clamping force, and cutting force are discussed.

Optimization Design of Step Stress Accelerated Degradation Test for Motorized Spindle Based on Ds-Optimality

In the accelerated degradation test (ADT) of motorized spindles, it is necessary to apply a variety of stresses to simulate real working conditions. However, the traditional accelerated test scheme optimization method does not consider the weight of various stresses in the test, resulting in the evaluation accuracy of important stress parameters in the model being too low. In order to solve this problem, an optimal design method of the step stress accelerated degradation test (SSADT) scheme for motorized spindles is proposed based on Ds-optimality. Firstly, the fault tree analysis (FTA) method is used to analyze the collected fault data of motorized spindles and screen the main stress. Then, the accelerated degradation model is established by using drift Brownian motion. Based on the Ds-optimality, the optimization variables and constraints in the test are determined, and the optimization model is established with the objective of minimizing the estimated variance of the main stress parameters in the acceleration model; additionally, the optimization steps are given. Finally, an example is given to verify the effectiveness of the method. Sensitivity analysis of the optimization results shows that the method has good robustness.

Dynamic Support Stiffness of Motorized Spindle Bearings under High-speed Rotation

The rotor operating stiffness of high-speed motorized spindles (HSMSs) is key to machining accuracy. Because HSMSs are difficult to load due to their high speeds, a contact loading device was developed to test rotor operating stiffness. The dynamic support stiffness of the front/rear bearings (DSSB) is the main factor affecting the rotor operating stiffness. Two novel experimental schemes for measuring the DSSB are proposed: 1) indirect measurement—by analysing deformation displacements at two points on the external loading rod of the HSMS, and 2) direct measurement—by eddy current sensors installed near the front/rear bearings. Based on the experimental device and two experimental schemes, the influences of working-condition parameters on the DSSB were tested. The results show that the proposed experimental device and two experimental schemes can effectively and accurately measure rotor operating stiffness and DSSB at speeds of up to 30,000 rpm. However, because the tapered connection gap between the loading rod and rotor increases the measured deformation displacement, the DSSB measured by the indirect measurement scheme was relatively small. The DSSB decreases with speed and increases with radial force and working temperature. This study provides a new experimental basis for the quality inspection of finished HSMSs and the verification of theoretical bearing stiffness models.

Condition-Based Maintenance Optimization for Motorized Spindles Integrating Proportional Hazard Model with SPC Charts

Reliability of motorized spindles has a great effect on the performance and productivity of computer numerical control (CNC) machine tools for intelligent manufacturing. Condition-based maintenance (CBM) is an efficient method to prevent serious failures, to improve system reliability, and to reduce management costs for motorized spindles. However, owing to various degradation features acquired during condition monitoring, the challenge is to propose an appropriate feature to evaluate the reliability level of motorized spindles and to set up optimal CBM policies. Based on the motivation, a three-stage approach is proposed in this paper. In the first stage, proportional hazard model (PHM) is developed to describe the reliability considering failure events together with multiple degradation features. Next, statistical process control (SPC) charts are constructed for condition monitoring and anomaly detection in order to achieve early detection of potential failures. At last, a CBM schedule is modeled in consideration of maintenance cost minimization; the maintenance plan is optimized by determining the optimal control limits of SPC charts.