Gheorghe Stan

The Influence of Kinematic Joints Clearances on the Positioning Accuracy of 3DOF Delta Parallel Robots

During the latest years the robots with Delta 3DOF parallel structure began to be used more and more in high accuracy applications, due to their advantages: high stiffness, availability for usage at high rates of speed and acceleration [1, . Obviously, for the high accuracy applications the elaboration of new analysis methods of the positioning accuracy is imposed. Further to the deviations of the element dimensions in the structure of the kinematical couplings or wear, perturbing influences may occur on the positioning accuracy. This work approaches the influence of the maximum clearance in the kinematical couplings on the positioning accuracy and its repartition on the work space of the robots with Delta 3DOF parallel structure, by using a method that has numerical solution.

Design of an Automatic Tool Changer System for Milling Machining Centers

In the field of manufacturing processes it is observed that the trend is to produce more and more fast, efficiently parts with high complexity, which involves using a high number of tools in the machining process. One of the main solutions for high speed and efficient manufacturing is based on the full automation of the entire manufacturing process. The automatic changing of the tools involved in the manufacturing process is carried out by the automatic tool changing mechanism, thus the auxiliary non-productive time consumed with the tool change is highly minimized. In this paper we present a novel automatic tool changer which is both simple and compact, and any milling machining center provided with chain or disc tool magazine can be equipped with. Also by adopting the use of this tool changing mechanism other subassemblies of the tool changing system, such as the tool transfer mechanism and the waiting position, are substituted by this changing mechanism. The auxiliary movements needed to bring the tool from the magazine into the waiting position are overlapped with the machining time, so that the total time for exchanging the tool in the spindle with the tool from the magazine is minimized.

A Comparative Approach Regarding the Dynamic Behaviour of Planetary Transmissions Used in Industrial Robots

The dynamic characteristics of planetary transmissions have a direct influence over the possibilities of industrial robots regarding the imposed trajectory. Also, the conditions specific to the achievement of the transient regime have a significant influence over the positioning accuracy of the robots. In this paper, a comparative approach is proposed, applied to four types of planetary transmissions, concerning the gyration torque distribution and computation. Based on the computation equations presented in this paper, some optimization strategies concerning the response times of the overall kinematic chains are proposed.

Research Regarding Backlash Improvement for Planetary Speed Reducers Used in the Actuation of Industrial Robots

The backlash concept includes the deflections resulting from the gearing loads and the flank plays that exist between the flanks of the sun, planet and ring gears, forming the planetary transmission. Therefore, it is important that planetary transmissions used for industrial robots should have low backlash, in order to achieve a good positioning accuracy. In this paper, a computation method for the backlash of planetary transmissions is presented, applied for a two-stage planetary transmission having a ratio of i=36. Based on the results, strategies can be formulated, allowing the minimization of the backlash for planetary transmissions, improving the positioning accuracy.

Floating Hydrostatic Bearings for Numerically Controlled Machine Tools

The use of hydrostatic guideways in CNC machine-tools is very common as a result of their low friction coefficient. The major problem of hydrostatic guideways represents the recovery of the fluid used in guideways, where the fluid should be protected against mechanical impurities and then led towards the hydraulic aggregate. The floating support described in this paper resolves the collection of fluid by using a closed circuit, without exposing the fluid to mehanical impurities and to a gravitational collecting.

Research Regarding Improvement of Torsional Stiffness for Planetary Speed Reducers Used in the Actuation of Industrial Robots

Planetary speed reducers are used in the actuation of the revolute kinematic joints thanks to their specific advantages. However, in order to achieve a good positioning accuracy for the kinematic chains, these must have reduced backlash and high torsional stiffness. Therefore the deformations of the elements composing the mechanical structure under the action of gearing forces should be as low as possible, thus leading to backlash reduction. This also becomes important since the presence of backlash in the structure of the planetary speed reducer affects the stability of the overall kinematic chain. In this paper, a novel computation method for bearing deflections is proposed, starting from the kinematic diagram of the planetary speed reducer. Having known the kinematic diagram, it becomes possible to compute the gearing forces, keeping in mind the non-uniform power distribution on the planetary gears. The bearing reactions were computed in two cases: when the planetary gears are free to spin on the planetary carrier and when the planetary gears are fixed on the planet carrier, being supported at the extremities. Based on the computed values for bearing reactions, the corresponding deflections are determined. Once the values of the bearing reactions and deflections had been determined, the load-deflection and stiffness-deflection diagrams were elaborated for ball and roller bearings, allowing the determination of the bearing type that ensures low deflections and high stiffness. Based on the results, some recommendations are made, allowing the reduction of bearing deflections, and increasing the torsional stiffness.

Optimization Strategies of Peripheral Speeds for Planetary Speed Reducers Used in the Actuation of Industrial Robots

Planetary speed reducers are used as the actuating module of the revolute joints for the industrial robots as a result of their specific advantages given by high transmission ratios, efficiencies and reliability. However, planetary speed reducers are currently used with variable high speed electric motors, which leads to increased peripheral speeds for the gears and planet carrier, leading in turn to increased noise during operation. Therefore it becomes necessary to manufacture the gears and planet carrier in an upper precision class, leading to significantly increased costs for the entire planetary transmission. Throughout this paper, novel solutions for planetary speed reducers are proposed, characterized by reduced peripheral speeds of the moving elements. For all the structural variants proposed, the summation method (Svamp’s rule) was applied in order to establish mathematical expressions for the transmission ratios. Subsequently, expressions for the peripheral speeds, corresponding to each moving element were derived, and expressed as a function of the number of revolutions of the input shaft. The method described in this paper was applied for each structural variant proposed, the results obtained being compared and analysed, leading to the optimal variant, characterized by a reduced level of noise and vibrations. Based on the mathematical expressions previously derived, a series of graphical representations of peripheral speeds as a function of the number of revolutions for the input shafts were elaborated for each structural variant proposed. At the same time, some recommendations useful for both designers and manufacturers of planetary speed reducers were made, based on the obtained results.

Overall Effectiveness Improvement for CNC Machine Tools

Overall effectiveness (OE) for CNC machine tools is a metric sistem for assessing the production obtained performance. To optimize OE is necessary to determine the performance indicators [1,2], identifying and correcting the non-values [3,4,5]. In this paper we propose new methods for CNC machine tools OE improvement, through IFA (Information Focus Action) analysis of production losses, during milling machine Wissner Wi3020 type. Comparison of real OE diagram with the theoretical one, provides information about the real losses associated with sub-processes. Also, root-cause analysis of performance losses, leading to the generation of new ways to improve the CNC machine tools OE.

Maintenance Strategy Optimization for Computer Numerical Control Machine Tools

In this paper are proposed two new approaches to maintenance strategies for Computer Numerical Control (CNC) machine tools. The analysis is done for different families of CNC machine tools from S.C. Elmet Bacau, a company specialized in aviation. In maintenance actions applied to CNC machine tools is very important to know the evolution of defects and critical state of electrical and mechanical components. The results of this analysis concludes that maintenance actions can be judged by the developing time period diagram, between failure appearance and interruptions in operation. It is also analyzed the financial impact, revealed from known maintenance strategies adopted on CNC machine tools, resulting in a positive approach of condition based maintenance.

Braking Solution for a Hydraulically Driven Linear Pallet Changer Mechanism

The use of machining centres is predominant in the case of small and medium size batch manufacturing, their advantages being materialised in high productivity. The productivity is owed, mainly, to the small auxiliary times consumed with the tool and material change, times which are served by the automatic tool changer system and the automatic pallet changer. The purpose of this article researches aims to determine the kinematic parameters of an hydraulically driven linear pallet changer in two cases (when the deceleration is carried out through a throttle and when the deceleration is made through a new solution). Thus, an experimental set-up, which matches an hydraulically driven linear pallet changer, was designed and built. The experiments aimed toward using a classic braking method of hydraulic cylinders and a new two-stage braking with counter-pressure limitation.