Setup for Studying the Preliminary Displacement of Press-Fit Joints
Abstract Generated abstract
This paper describes an apparatus for investigating preliminary displacements in press-fit joints under simple and combined loading. The authors motivate the work by noting that conventional design calculations must account for many uncertain factors and often rely on increased safety factors, while displacement and energy dissipation measurements may support more accurate strength calculations. The setup uses a torsion-testing machine, disk springs for axial loading, and a capacitive displacement sensor with submicron scale divisions to measure relative motion in shaft-bushing specimens. Preliminary tests on specimens with 30 mm diameter working sections and an 8 micrometer interference fit indicated a displacement versus torque behavior similar to that observed in flat contact, and the apparatus was found to be reliable, convenient, and relatively simple to use.
Full Text
Apparatus for Investigating the Preliminary Displacement of Press-Fit Joints
B. F. Sovetchenko, V. I. Maksak
(Presented by the scientific seminar of the Department of Strength of Materials)
Among the various joints of machine parts in mechanical engineering, press-fit joints occupy an important place. Their strength and stiffness are determined by many factors: the mechanical properties of the material, the magnitude of the interference fit, the quality of machining of the mating surfaces, their frictional characteristics, and the conditions of pressing-in.
Fig. 1. Diagram of the apparatus
Taking the totality of these factors into account presents considerable difficulties; therefore, modern calculations are based on increasing the safety factor, which leads to an increase in the weight of mechanisms, inertial loads, and additional costs.
Studying the strength of press-fit joints from the standpoint of preliminary displacement and the energy dissipated in the process will make it possible to obtain more accurate methods of calculation.
The paper considers an apparatus (Fig. 1) that makes it possible to study preliminary displacements under both simple and complex loading. Shaft 1 is installed in the centers 2 of a 150 kg·m torsion-testing machine; one end of the shaft is secured by wedges 3 in the machine grips. Torque is transmitted along the shaft through bushing 4 and pins 5. The axial loading of the joint is produced by disk springs 6, by means of nut 7 through thrust bearing 8. Displacements are measured by capacitive sensor 9, one of whose plates is mounted on the shaft by means of clamp 10, and the other on holder 11, which is bonded with epoxy resin to bushing 4. The signal from the sensor is recorded by an indicator built into amplifier 12, which is supplied by rectifier 13 with an electronic stabilization circuit. The indicator has a scale with division values of 0.01, 0.02, and 0.04 μm, depending on the position of the switch, with a sensor-plate area of 2 cm². Screw 14 serves to clamp the clamp on the shaft, and micrometer screw 15 serves to adjust the gap between the sensor plates.
Fig. 2. Graph of the dependence of displacement on moment
For carrying out preliminary tests, specimens were selected with a working-part diameter of 30 mm and a length of 5 mm. The mating surfaces had an eighth-grade roughness.
The dependence of displacement on the magnitude of the torque for a specimen with an interference fit equal to 8 μm is shown in Fig. 2; from it one can see that the preliminary displacement for a press-fit joint has the same character as for flat contact.
Testing of the apparatus and the preliminary tests showed its good reliability in operation, ease of use, and that the experiment itself is comparatively not labor-intensive.