Research on the Dynamics of Crankshaft Under the Condition of Operating Mode Based on LS-DYNA

(cid:726) A model of crankshaft under working condition is established based on LS-DYNA. The stress distributions of crankshaft and its regularities of the stress along surface of crankshaft and layer depth at the fillet are studied. The results show that the obvious stress concentration exist in the upper end of fillet on main journal and the lower end of fillet on rod journal when the piston arrives at top dead center, and the maximum equivalent stress occurs at the fillet on rod journal, so the fracture of crankshaft is most likely to occur in the lower end of fillet on rod journal. With the increase of depth, the stress on the surface of fillet reduces correspondingly, and the equivalent stress begins to stabilize when the depth reaches up to 2.8 mm. The study of maximum working stress provides the theory basis for the selection of residual stress.


Introduction
Crankshaft is one of the most important parts of the engine. Due to the impact of cyclic stress, the permanent deformation will occur at the crankshaft [1] .How to ensure the crankshaft working life and prevent the occurring of fracture has been the research topic over the years. In The crankshaft fillet rolling strengthening is an ideal surface strengthening method. Compared with the traditional methods, rolling strengthening has many advantages like low cost, short processing time, obvious strengthening effect [2] etc. After rolling strengthening, the crankshaft fatigue strength can increase by 30%-200% [3] .
However, how much the residual stress produced in the process of rolling strengthening is uncertain, and it's very hard to confirm the value of residual stress. This paper finds that the maximum working stress can provide the theory basis for the selection of residual stress.
According to the piston displacement formula, it deduces the velocity and acceleration:

Gas force and inertia force of connecting rod mechanism
The force of the cylinder is not only related to the size of the cylinder, but also to the pressure between inside and outside of the cylinder. The gas force formula is shown as follows: ‫ܨ‬ ൌ ሺܲ ଵ െ ܲ ሻߨ‫ܦ‬ ଶ Ͷ ‫ݏܿ‬ ߙǤ Τ ሺͶሻ Ƚ :swing angle of connecting rod; ‫ܦ‬ :cylinder diameter; ܲ ଵ :absolute pressure of gas in cylinder; ܲ :absolute pressure of gas inside the box.
In the theoretical analysis, the whole weight of connecting rod is translated into large mass ݉ ଵ and small mass ݉ ଶ by the static equivalent principle, ݉ ଵ is heavier than ݉ ଶ . Reciprocating inertia force is generated by the piston and small mass. It is considered that the weight of piston assembly concentrates on the center of the pin. ‫ܯ‬ ൌ σ ݉ . The reciprocating inertia force is shown as follows:

Solution of load
There're some parameters of crankshaft showed in table 1.   The loading force is not constant, when the gas bursts, the pressure in the cylinder increases rapidly to the maximum. As the piston moves down, the gas volume increases and the pressure begins to fall.

Figure.4 Force-time curve
In order to make the load and the actual gas pressure closer, the force-time curve is established in figure 4.
With the increase of rotation angle of crankshaft, the load decreases rapidly. When the time is 0.009s, the load decreases to 0. The terminal time in this paper is 0.009s.

Figure.6 Paths in two directions
The path L1 is from A to B in the connecting rod journal along the circumferential direction, the path L2 is from C to D in the connecting rod journal along the rolling direction. The equivalent stress values and the stress distributions along path L1 and path L2 are obtained from the simulation which is shown in Figure   7a and 7b. In Figure 7a, it can be seen a parabolic form of the fillet surface equivalent stress curve, the maximum value of equivalent stress in the fillet of connecting rod journal is 888MPa and the point of maximum equivalent stress locates in between A and C. In Figure 7b, it can be seen that the stress in the fillet of the connecting rod journal gradually reduces with the increase of the depth, when the depth approaches 2.8mm, the equivalent stress is beginning to stabilize, and the maximum equivalent stress on the surface is 860MPa.