EHL squeeze at pin–pulley interface in CVTs: Influence of lubricant rheology

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Abstract

In this paper we analyze the influence of fluid rheology on the strongly non-stationary squeeze process of an oil film sandwiched between the chain-pin and pulley in continuously variable transmission. As recently demonstrated [Carbone G, Scaraggi M, Soria L. The lubrication regime at pin–pulley interface in chain CVT transmissions. ASME Journal of Mechanical Design 2009;131(1)], the spatial pressure distribution is characterized by a non-central annular pressure peak, which first appears in the external region of the contact region and moves toward the center of the pin with rapidly decreasing speed. In this paper we show that the non-Newtonian viscoelastic rheology of the lubricant plays a crucial role in determining the actual value of pressure peaks and leads to a strong reduction of such pressure spikes in comparison to a perfect Newtonian lubricant. Even more, if the threshold value of shear stress τl, which characterizes the transition from Newtonian to non-Newtonian behavior of the lubricant, is sufficiently small the annular pressure peak may even disappear. In this case the squeeze process occurs faster, the film thickness distribution is reduced and the lubricant may not be able to avoid direct asperity contact between the two approaching surfaces.

Introduction

A correct understanding of the tribological phenomena occurring at the pin–pulley interface in chain CVT transmissions, is the starting point to predict, control and design CVTs with optimized mechanical efficiency and traction performances. Indeed, the mechanical efficiency of CVT transmissions depends on CVT running conditions, and, in the particular case of chain CVTs depends on the amount of slip between the chain pin and the pulley, and on the actual value of actuators pressure. Often, an over clamping of about 30% is considered acceptable to limit the pin–pulley slip to safety values, cope with unexpected torque load increase, which often occurs in normal working conditions, and prevent damage to the system. But an excessive reduction of the slip may also strongly decrease the mechanical efficiency of the transmission [2]. This suggests that a decrease of the overclamping can be beneficial to achieve better overall performances. Therefore, novel slip control techniques [2] and advanced models of the CVT mechanical behavior [3], [4] have to be conceived and developed to accurately control clamping forces and improve CVT mechanical efficiency and traction performances. It should be noticed that understanding what is the actual consequence of a change of clamping force on traction performances and efficiency of chain CVTs requires a deep analysis of the lubrication conditions at the pin–pulley interface. In a previous paper [1], some of the authors investigated the Hard-EHL squeeze contact between the pin and the pulley for the Gear Chain Industrial (GCI) chain (see Fig. 1), to understand whether direct contact between the two approaching metal surfaces can occur. In Ref. [1], we have shown that the time the pin spends in the pulley groove (Δt0.01s) is not enough to bring the two metallic surfaces into contact. In fact, the lubricant remains trapped in between the two approaching surfaces as a consequence of the strong increase of viscosity due to the piezo-viscous effect. Ref. [1] also shows that pressure distribution strongly differs from the Hertzian pressure distribution and presents an annular region of a very high pressure peak, which may exceed the value of 10 GPa. These pressure values, in turn, appears to be too high to be compatible with the mechanical resistance of the material the pin and pulley are made of, and have been thought to be caused by the assumption of a Newtonian behavior of the lubricant. In this paper we analyze in detail this aspect of the EHL squeeze process, by investigating what is the effect of the non-Newtonian rheology of the lubricant and its viscoelastic response. Indeed the very small thickness of the lubricant and the relatively high speed of the squeeze process lead to very large strain rates. As a consequence, the resulting shear stresses cannot be correctly described by a Newtonian model of the fluid. Moreover the strongly time-dependence of the squeeze process does not allow to neglect the elastic response of the lubricant and, therefore, should be properly taken into account. The scientific literature is still lacking of a clear physical understanding of lubricant behavior in such high-loaded EHL squeeze contacts, although automotive companies strongly ask for a deep comprehension of such a problem. Most of the existing research investigations focus, indeed, on the steady-state EHL lubrication analysis [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. This lack of investigations in EHL squeeze contact is partially justified by the computational difficulties encountered in designing efficient and fast numerical schemes able to deal with strongly time-dependent high pressure distribution in a fluid with complicated rheology. In this paper we give our personal contribution to achieve a better description and understanding of the influence of non-Newtonian rheology and viscoelastic response of the lubricant on pressure and oil thickness time evolution and spatial distribution in fast closing squeeze contacts.

Section snippets

Formulation

In the case of chain CVTs the squeeze of lubricant sandwiched between the pin and the pulley surface occurs as soon as the pin enters the pulley groove as a consequence of the sudden step variation from zero (on the free strand of the chain) to a finite value of the normal force acting on the pin (see Fig. 2). The actual value of the squeezing normal force F0 at the pin–pulley contact can be calculated by means of theoretical models (see e.g. Refs. [3], [4]) and is of order 1 kN. By taking into

Results

Reynolds equations (19), (24) need, in order to be solved, the following boundary and initial conditions:prr=0=0p(r=a,t)=0h(r,t=0)=h0+r22Rand with the equilibrium conditionF=2π0ap(r)rdrWe have employed, with some modifications, the numerical procedure already described in Ref. [1], which the reader is referred to for details about the numerical scheme. Here, we simply present results for a constant squeeze force F and initial film thickness h0(t=0)=10μm, and discuss these results in

Conclusion

In this paper we have investigated the effect of fluid rheology on the squeeze process at the pin–pulley interface. The effect of lubricant viscoelasticity and its non-Newtonian viscous behavior have been analyzed separately. We have shown that in both cases the annular pressure peak is strongly reduced of about 50% and decreases more if the threshold shear stress of the oil is significantly reduced. However, if the threshold shear stress is not to small, and apart from the peak position, the

Acknowledgments

The authors would like to thank Gear Chain Industrial B.V.—Neunen (NL) and the JTEKT Corporation—Nara (Japan), for having supported this research activity.

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