On Hirth Ring Couplings: Design Principles Including the Effect of Friction

Rings with Hirth couplings are primarily used for the accurate positioning of axial-symmetric components in the machine tool industry and, generally, in mechanical components. It is also possible to use Hirth rings as connection tools. Specific industries with special milling and grinding machines are able to manufacture both tailor made and standard Hirth rings available on stock. Unfortunately, no international standard (for instance ISO, DIN or AGMA) is available for the production and the design of such components. In the best-case scenario, it is possible to find simplified design formulae in the catalogue of the suppliers. The aim of this work is to provide some accurate formulae and computational methods for design to provide better awareness on the limitations and the potential of this type of connection. The work consists of five parts: (i) a review of the base calculation derived mainly from the catalogues of manufacturers; (ii) an improved calculation based on a new analytical method including the friction phenomenon; (iii) an experimentation run for validating the method; (iv) a case study applied to a machine tool; and, (v) a closed form formulation to determine an upper threshold for friction, thus ensuring the Hirth coupling regular performance

A methodology for the lightweight design of modern transfer machine tools

This paper deals with a modern design approach via finite elements in the definition of the main structural elements (rotary table and working unit) of an innovative family of transfer machine tools. Using the concepts of green design and manufacture, as well as sustainable development thinking, the paper highlights the advantages derived from their application in this specific field (i.e., the clever use of lightweight materials to allow ruling out high-consumption hydraulic pump systems). The design is conceived in a modular way, so that the final solution can cover transfers from four to 15 working stations. Two versions of the machines are examined. The first one has a rotary table with nine divisions, which can be considered as a prototype: this machine has been studied in order to set up the numerical predictive model, then validated by experimental tests. The second one, equipped with a rotary table with 15 divisions, is the biggest of the range: this machine has been entirely designed with the aid of the previously developed numerical model. The loading input forces for the analyses have been evaluated experimentally via drilling operations carried out on a three-axis CNC unit. The definition of the design force made it possible to accurately assess both the rotary table and the working units installed in the machine

INFLUENCE OF THE STIFFNESS AND FRICTIONAL CHARACTERISTICS ON THE SHANK TORQUE OF SCREWS IN BOLTED JOINTS

This work aims at determining the influence of tribological and stiffness characteristics of a bolted joint on the residual shank torque of the screw. Even if it is commonly accepted to consider such a residual torque equal to half the torque at the thread, the literature lacks experimental data about the topic. The residual shank torque combines with the axial preload and the external loads to bring about the overall stress on the screw. Hence, the higher the residual torque, the lower the admissible external load for given size and class of the screw. From there stems the need for an analytical tool allowing the designer to calculate the residual torque as a function of the key parameters of the joint

Design Improvement of clamped joints in front motorbike suspension based on FEM analysis

This paper aims at defining the tensile state in shaft hub joints realised between the clamp (hub) located at the end of the suspension leg of front motorbike suspensions and the wheel pin (shaft). The clamp under investigation has an asymmetric shape and a slot useful for the assembly operation: one or two bolts (depending on the vehicle performances) are tightened in order to generate the coupling pressure which is necessary to lock the pin. The fundamental objective of this work is to define a mathematical model to calculate the maximum stress generated on the clamp by the tightening forces. The mathematical model was defined by comparing the De Saint Venant theoretical bending stress (the maximum stress on the clamp) with several FEM results. Thus a sort of theoretical stress concentration factor was calculated in function of the dimension and of the location of the spot facings of the clamp. The new developed model is useful to optimize and to verify, in a very short time, this type of coupling without performing any complex numerical analysis. The validity of the model has been demonstrated by several FEM results and by some experimental tests carried out on a high performance vehicle

Finite Element modelling of rotary transfer machines

Vibration monitoring and control are central topics for machine tools, since high vibration levels reduce the quality of machined surfaces and shorten the tool life. In order to predict potential vibration issues since the early design stage, it is necessary to implement ad hoc numerical models for modal analysis. This requires significant efforts and possible conflicts with tight production scheduling of companies. This work focuses on a specific family of rotary transfer machines for the manufacturing of parts related to lock&keys industry. It investigates the possibility to achieve an acceptable estimation of the elastodynamic behavior of the machine tools through limited modifications of the Finite Element (FE) models used for structural analysis, which are generally available in the early phases of the design process. The structural FE model of a new machine tool is implemented and validated through experimental tests performed on a prototype. Then, the elastodynamic FE model is derived and simulated. The accuracy of the numerical results will be assessed through Experimental Modal Analysis (EMA)

Experimental vibration analysis of a rotary transfer machine for the manufacture of lock components

This study deals with vibrations in machine tools featuring rotary transfer architecture. The machine tool manufacturer aims at two long-term achievements, namely developing a reliable virtual testing tool to aid the design process of new products, and implementing a real-time system for condition monitoring and diagnostics of the cutting tool and the machining units to be equipped on-board. A seven-station rotary transfer machine with multi-spindle CNC machining units is investigated as a starting point of the research. An experimental campaign is carried out to assess the machinery vibration response. Several different experiments are conducted to estimate the modal parameters of the machine as well as to identify the elastodynamic effects induced by the nominal working cycle and by each machining operation. The result analysis permitted to identify potentially critical issues and to define the corresponding strategies to overcome them. In particular, limited modifications of the machining parameters and a partial redesign of just one machine component are suggested

A Methodology for the Lightweight Design of Modern Transfer Machine Tools

This paper deals with a modern design approach via finite elements in the definition of the main structural elements (rotary table and working unit) of an innovative family of transfer machine tools. Using the concepts of green design and manufacture, as well as sustainable development thinking, the paper highlights the advantages derived from their application in this specific field (i.e., the clever use of lightweight materials to allow ruling out high-consumption hydraulic pump systems). The design is conceived in a modular way, so that the final solution can cover transfers from four to 15 working stations. Two versions of the machines are examined. The first one has a rotary table with nine divisions, which can be considered as a prototype: this machine has been studied in order to set up the numerical predictive model, then validated by experimental tests. The second one, equipped with a rotary table with 15 divisions, is the biggest of the range: this machine has been entirely designed with the aid of the previously developed numerical model. The loading input forces for the analyses have been evaluated experimentally via drilling operations carried out on a three-axis CNC unit. The definition of the design force made it possible to accurately assess both the rotary table and the working units installed in the machine

The Influence of Material, Hardness, Roughness and Surface Treatment on the Frictional Characteristics of the Underhead Contact in Socket-Head Screws

The present paper aims at determining the effect of several design and manufacturing parameters on the frictional response of the underhead contact in a bolted joint comprising a socket-head M6 class 12.9 screw. The rationale of the research moves from the need to understand the frictional behavior of screw joints used in machine tools as a means to join roller monoguides with the moving carriages of three-axis tooling units. The experimentation takes into account different underhead materials (Steel, Cast Iron, Aluminum), different roughness levels (Ra=0.8, 1.6, 3.2), different surface treatments (No treatment, oxidised, DLC), as well as repeated tightening operations. The experimentation has been run by a specifically developed specimen, consisting of two parts: an instrumented sleeve, equipped with a double array of strain gauges capable of sampling both the axial preload and the underhead torque, and interchangeable underhead washers, with an anti-rotation device, whose material and surface treatment are suitably changed among the different sets. The tightening torque is recorded by a digital torque wrench. The collected data are processed by ANoVa tools, to investigate the significance of each factor, as well as related interactions. The friction coefficients associated with the different operating parameters are a useful tool to support the design task of bolted joints

Experimental Measurement of the Shank Torque As a Function of the Stiffness and Frictional Characteristics of the Bolted Joint

During tightening, the amount of torque given by the difference between the tightening torque, which is directly applied by the torque wrench, and the underhead torque, flows through the screw shank towards the threaded portion. This torque combines with the axial preload to bring about the overall stress state of the screw at tightening. Upon release of the torque wrench, a certain amount of the shank torque is released due to the elastic springback of the screw-plates system. In the literature [1], this phenomenon is just briefly treated by a few authors: they generally agree that approximately a half of the initial shank torque is released just a few seconds after torque wrench removal. This indication is given regardless of the frictional [2] and stiffness [3] parameters, which govern the joint. The present contribution aims at assessing, if there is any effect of the following parameters on the amount of shank torque being released after wrench removal: (i) the ratio between the torsional stiffness of the screw and of the plates, (ii) the friction coefficients in the underhead and in the thread. The experimentation has been run on a M20 8.8 class socket head screw, which has been instrumented by a double array of strain gauges, to simultaneously measure both the axial preload and the torque acting on its shank. Two different types of joined members have been examined: a cylindrical sleeve whose diameter is twice the screw diameter (compliant joint) and a rectangular plate whose transverse dimensions are more than ten times larger than the screw diameter (stiff joint). The underhead and thread friction coefficients have been controlled by properly selecting lubrication conditions. The main outcome of the work is that the torsional stiffness of the joined members does have an impact on the residual shank torque. A simple mathematical model has also been implemented, in order to predict the residual shank torque during the design phase