| The connecting rod is one of the key transmission mechanisms in an automotive engine and connects the piston to the crankshaft.During operation,the connecting rod is in a complex state of stress and therefore has high requirements for dimensional accuracy,shape accuracy and positional accuracy.Pneumatic gauges have the advantages of being contactless,efficient and fast and are widely used in the field of precision measurement.However,existing buoy-type pneumatic gauges are subject to unstable measurement values and probe wear during the measurement process,resulting in inaccuracies in the actual dimensional and positional tolerances of the measured work piece.This paper analyses the flow field of the pneumatic gauge probe and the flow field in the glass tube of the buoy display,and proposes a new pneumatic gauge probe and a new buoy structure,which can achieve more accurate values and improve the measurement stability of the pneumatic gauge in the measurement process.This paper focuses on the analysis of the measurement stability of pneumatic gauges by conducting the following research work.(1)Numerical analysis of the existing probe flow field of pneumatic gauges.Based on the fluent finite element numerical calculation method to analyse the probe flow field,establish the mathematical model and control equations,analyse the internal velocity and pressure flow field of the pneumatic gauge probe,determine the influence of the probe flow field on the measurement stability,and propose a double E type pneumatic gauge probe,based on the premise of reducing the probe air resistance and flow field turbulence,the pneumatic gauge double E type probe exhaust slot structure is optimized.A comparison of the pressure distribution between the existing probe and the double-E probe,together with an analysis of the transient flow field and velocity convergence time,reveals that the double-E probe has less turbulence,smoother airflow and lower air resistance.(2)Numerical calculation of the flow field of the pneumatic gauge buoy display.In response to the oscillation instability of the pneumatic gauge buoy,the flow field in the tapered glass tube of the buoy display is analyzed.Firstly,the fluid calculation model and boundary conditions are determined,the flow field between the buoy and the tapered glass tube is calculated based on the Ansys fluent calculation method,and the influencing factors of the buoy instability phenomenon in the tapered glass tube are determined,and a pneumatic gauge buoy with a guide surface is proposed.The results show that the increase in the length of the guiding surface increases the stiffness of the air film within the gap between the outer surface of the buoy and the inner surface of the glass tube,reducing the effect of the wake vortex on the buoy.The angle of the guiding surface increases,the air flow along the curved surface of the outer surface of the buoy is influenced by the Coanda effect,and the low pressure area outside the guiding surface appears,while the exit of the buoy cavity shrinks and the accompanying vortex inside the cavity increases,the cyclone in the tail of the buoy moves towards the guiding surface,and the trailing symmetric vortex interferes more with the tail of the buoy.(3)Experimental study of pneumatic gauges.The experimental platform of pneumatic gauges was built,the double E type pneumatic gauging probe and different parameters of pneumatic gauging buoys were produced and machined,24 calibrated connecting rod bore diameters were selected for experimental measurement,through repeated measurement of the same connecting rod bore roundness 10 times found: the measurement error of the double E type probe was 0.5 μm,while the existing probe measurement error was stable within 1μm.The experiments were carried out on different parameters of the float,and the deflection angle of the float decreased from8° to 0.5° when the length of the guide surface was increased to 2 mm,and exceeded 10° when the angle of the guide surface was increased beyond 10°.In summary,it was shown that improving the structure of the probe exhaust slot and the structure of the display float can effectively improve the detection accuracy and stability of the pneumatic gauge. |
