Numerical determination method of crankshaft neck stresses of marine engines.

Moroianu, Corneliu ; Iurea, Neculai

Abstract: The crankshaft of internal-combustion engines endures the highest and the most complex stresses. The tensile compression, bending and twisting stresses appear under the action of gas pressure forces and inertic forces in the elements of the crankshaft. One to the bending stresses, the crankshaft is the strained compromising the coaxility of necks and bearing bushes. Taking into account these points (considering the points), the determination of the stresses to witch it is put, becomes a defining element for designing and testing to strength. This work presents a numerical modelling (MathCad program) for determining the stresses to which the crankshaft is put for the marine two-stroke engines

Key words: numerical modeling, marine internal-combustion engine, crankshaft, stresses

1. INTRODUCTION

One to the installing clearances, high speed of the rise of pressure during the combustion and change of application direction of forces, the stress of the crankshaft of the internal combustion engine is like a shock. The variable forces induce the fatigue phenomenon dangerous especially to the passing from the arm to the necks and the torsional vibration stress is also dangerous. Knowing the state values of the motive fluid in the characteristical points of the duty cycle, we can determine the values of the shown and effective parametres of the duty cycle as well as the values of the main building dimensions of the engine (Buzbuchi, 1997). We obtain the duty cycle diagram based on the data obtained for the real volumes of the motive fluid in the characteristical points of the duty cycle (Alexandru, 1991).

[FIGURE 1 OMITTED]

2. THE DETERMINATION OF KINEMATIC VALUES OF PISTON AND CONNECTING ROD

Having established the type of crank and connecting rod assembly, determined the crank star and chosen the firing sequence we shall develop the kinematic calculation of driving mechanism taking into account the characteristical values of piston and connecting rod (shift, speed, acceleration), (Grunwald, 1980).

[FIGURE 2 OMITTED]

3. THE DYNAMIC CALCULATION OF DRIVING MECHANISM

The dynamic calculation of driving mechanism assumes the determination of gas pressure forces, of inertic forces of masses being in rotating and translation motion as well as of their components. In figure 3, 4 and 5. I presented the diagrams obtained for a marine Sulser RTA 84 engine.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

4. THE DETERMINATION OF STRESSES OF THE CRANKSHAFT NECKS

The calculation of the direction of resulted force straining the crankpin neck is made according to the positive or negative values of the involved forces and at the values so obtained it was added 180[degrees] to achieve the correlation of force variation with the neck surface which takes over those forces. In Figure 6 the polar diagram of stress is presented. The diagram shores that the highest stresses of crankpin neck are recorded in the scale II for [P.sub.M] = 90[degrees] ... 180[degrees], while the lowest stresses are in the scale IV for [P.sub.M] = 270[degrees] ... 360[degrees].

[FIGURE 6 OMITTED]

To determine the stresses of crankshaft bearing, it is necessary to specify the planes on which the forces determining these stresses act. The total loads of forces acting on a crankshaft bearing are determined taking into account the arrangement of cylinders, the position of contiguous cranks round about the axis of rotation and the firing sequence (Dragalina, 1993). The determination is made based on the moment equations of pressure forces and inertia forces of masses with rotating and translation motions being also included the forces generated by counterweights. In Fig. 7 and 8 there are the diagrams of forces loading the bearing neck as well as the polar diagram of force loading the base bearing (Bobescu, 1997).

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

5. CONCLUSION

The paper presents a type of numerical modeling represented by a program written in MathCad program for determining the stressed at which the crankshaft neck of marine engines are loaded, but not only. Such a program is useful for designing the engines as well as for testing the strength of the crankshafts. The program also can be used for computer simulation to design marine engine moving parts.

6. REFERENCES

Buzbuchi, N. (1997). Motoare diesel. Procese si caracteristici, Editura Didactica si Pedagogica, Bucuresti

Alexandru, C. (1991). Masini si instalatii navale de propulsie, Editura Tehnica, Bucuresti

Grunwald, B. (1980). Teoria, calculul si constructia motoarelor pentru autovehicule rutiere, Editura Didactica si Pedagogica, Bucuresti

Dragalina, Al. (1993). Aplicatii de calcul ale motoarelor diesel navale, vol.I si II, Academia Navala "Mircea cel Batran", Constanta

Bobescu, G. (1997). Motoare pentru automobile si traetoare--indrumar de proiectare, Universitatea "Transilvania"