Abnormal behavior of a hydrodynamic lubrication journal bearing caused by wall slip

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Abstract

Reynolds lubrication theory assumes that there is no wall slip on the interfaces between the solids and lubricant. During recent years, however, it is found that wall slip often happens. The present paper analyzes the wall slip occurring in a hydrodynamic lubrication journal bearing. If the two surfaces have the same adhesion property wall slip always decreases the oil film load support capacity. If there is wall slip over all of the lubricated surfaces, the hydrodynamic effect of the journal bearing vanishes, and no load support exists. If the two lubricated surfaces have different adhesion properties, the wall slip effect is more complex and may cause the journal bearing to operate in an instable manner. In order to avoid the wall slip, the limiting shear stress at the bearing surface should be higher than that at the journal surface.

Introduction

The classic Reynolds lubrication theory assumes that there is no slip between the solid surface and the lubricant, i.e. the lubricant speed at the solid surface equals the solid speed. Although the no-slip boundary condition has been used in many technical books and papers, the sudden occurrence of wall slip under certain conditions is a challenging problem in both fluid mechanics and lubrication mechanics. During recent years it has been found that wall slip often occurs not only in polymer flow [1], [2], but also in hydrodynamic [3], [4] and elasto-hydrodynamic lubrication [5]. Although many mechanisms for wall slip have been proposed, the experimental manifestation of wall slip is the existence of a critical or limiting wall-shear stress. In lubrication mechanics, if the lubricant has a limiting shear yield stress, for example a viscoplastic fluid [6], [7], grease [8] or lubricant at high pressure [9], [10], wall slip will occur at the wall/lubricant interfaces when wall shear stress is sufficiently high. Although there are reports of wall slip observations, no detailed theoretical studies has been reported for a journal bearing. Strozzi [11] gave a theoretical analysis for one wall slip situation using a complementarity method. Spikes [12] analyzed wall slip occurring at a slider bearing using a difference method.

In a journal bearing, it is known that if the journal and the bearing are made of the same material, the system will easily break down. Different materials of bearings have different limiting rotational speeds. Most explanations for this involve the heat induced by viscous flow or the strength of the bearing materials. This paper reports a new finding that wall slip in a journal bearing can give rise to many abnormal phenomena, including journal instability, vibration and even oil film collapse.

Section snippets

Control equations for wall slip

A parametric quadratic programming (PQP) method, which is described in detail in references [13], [14], is used to study the wall slip problems in the present paper The lubricant velocities at surfaces a and b, u¯a and u¯b, can be expressed by (see Fig. 1)u¯α=uα+up(α=a,b)where ua and ub denote the velocities of solid surfaces a and b, respectively, and uap and ubp are the corresponding wall slip velocities. If uαp>0 (α=a, b) the wall slip goes in the positive x direction. Otherwise it goes in −x

Wall slip analysis of journal bearing

A journal bearing rotating with velocity ω is shown in Fig. 2, where θ=x/R, c=R0−RiR0=R., ε=e/c. The unit length load support in the x and y directions are:wy=0θoutpRcosϑdϑ

wx=0θoutpRsinϑdϑ

The total film load support is:w=wx2+wy2and the load support angle is:ϕ=tan1(wxwy)

In the present paper, we use the dimensionless parameters: P=pc2/(ηωR2), H=h/c, T=τc/(ηωR), T=τc/(ηωR), Tα=ταc/(ηωR), Uα=uα/U, Kα=kαR/c, Λα(i)=λα(i)/U(i=1,2;α=a,b), T=tU/R, Q=qs/(cU), where U=ua+ub. W=wc2/(ηωR3).

Discussions

The present paper describes a general method to analyze wall slip at lubricated surfaces. Numerical solutions show that if the two lubricated surfaces have the same limiting shear stress, wall slip may first occur at the outlet zone on the bearing surface. The dimensionless initial limiting shear stress at surface α is defined as T=τc/(ηωR). Consequently a decrease in τ and an increase in ω will give rise to a small T. If the surface limiting shear strength is unchanged, increasing the

Conclusions

  • (1)

    If the two surfaces have exactly the same adhesion property with a lubricant, wall slip always makes the oil film load support capacity decrease. If there is wall slip in opposite directions over all of both lubricated surfaces, the hydrodynamic effect of the journal bearing vanishes and no load support exists.

  • (2)

    If the two lubricated surfaces have different adhesion properties, the wall slip effect is more complex. In order to avoid wall slip, the limiting shear stress at the bearing surface

Acknowledgements

This work was partly supported by NSFC (10272028) and by the Foundation of Doctor Discipline of Education Ministry.

References (15)

  • S.Q. Wang

    Molecular transitions and dynamics at polymer/wall interfaces: origins of flow instabilities and wall slip

    Adv Polym Sci

    (1999)
  • L. Leger et al.

    Surface-anchored polymer chains: their role in adhesion and friction

    Adv Polym Sci

    (1999)
  • V.S.J. Craig et al.

    Shear-dependent boundary slip in an aqueous Newtonian liquid

    Phys Rev Lett

    (2001)
  • E. Bonaccurso et al.

    Surface roughness and hydrodynamic boundary slip of a Newtonian fluid in a completely wetting system

    Phys Rev Lett

    (2003)
  • M. Kaneta et al.

    Observation of wall slip in elasto-hydrodynamic lubrication

    ASME, J Tribol

    (1990)
  • B.O. Jacobson et al.

    Non-Newtonian fluid model incorporated into elastohydrodynamic lubrication of rectangular contacts

    ASME, J Lubr Technol

    (1984)
  • W.D. Wilson et al.

    Viscoplastic behavior of a silicone oil in a metalforming inlet zone

    ASME, J Tribol

    (1989)
There are more references available in the full text version of this article.

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