Original scientific paper
Review of mixed lubrication in concentrated contacts: thinning, mixed and partial films, classical, modern and future modes
Yongbin Zhang
; Hangzhou Gear Box Company, Zhejiang Province, P. R. CHINA
Abstract
The mixed lubrication in concentrated contacts i.e. mixed elastohydrodynamic lubrication (mixed EHL) in both line and point contacts developed in the past time is reviewed. Due to the fluid non-Newtonian shear thinning, contact surface roughness, and frictional heating effects, in mixed EHL especially in severe operating conditions, the fluid film can be severely thinned and the fluid film thickness in the Hertzian zone can be reduced to molecular scale and even disappearance. As a result of this, the oxidized chemical boundary layer lubrication or/and dry contact may occur between opposing asperities of the contact surfaces where the fluid film disappears. Actually, the mixed EHL in concentrated contacts may be transited gradually from the mixed EHL of relatively thick overall
fluid film to chemical boundary layer lubrication mainly occurring in the Hertzian zone and then further to the destruction of the contact surfaces, i.e. the removal of the oxidized chemical boundary layer from the contact surfaces or dry contact between asperities mainly occurring in the Hertzian zone where the fluid film disappears, when the severity of the operating condition is gradually increased, i.e. the carried load of the contact, the sliding speed of the contract or/and the bulk fluid temperature are gradually increased. Even on the stage of the mixed EHL of relatively thick overall fluid film, in usual operating conditions, small areas of oxidized chemical boundary layer lubrication and instant dry contact can actually locally occur between opposing asperities of the contact surfaces. This boundary lubrication and dry contact respectively have important impacts on the performance and the transition of the mixed EHL of relatively thick overall fluid film and can quickly result in the transition of this mixed EHL to chemical boundary layer lubrication mainly occurring in the Hertzian zone. These show that the lubricating film and dry contact are usually mixed in an actual mixed EHL and even the lubricating films in an actual mixed EHL are also mixed due to different rheological behavior lubricating films respectively, occurring simultaneously in different areas of the mixed EHL contact. These mixed lubricating films occurring simultaneously in an actual mixed EHL include viscoelastic continuum (relatively thick) fluid film, viscoplastic continuum (relatively thick) fluid film, non-continuum fluid film i.e. physical adsorbed layer boundary lubrication film, chemical boundary layer, and vanishing lubricating film, i.e. dry contact. Each of these mixed lubricating films may only be locally present and may thus only be partially present in an actual mixed EHL contact. According to the development and the future tendency, the mode of mixed EHL in concentrated contacts is classified into three kinds i.e. classical, modern, and future modes of mixed EHL in concentrated contacts respectively. The classical mode of mixed EHL in concentrated contacts refers to the mode of mixed EHL in concentrated contacts where the fluid film between the contact surfaces is overall relatively thick. Thus continuum across the fluid film thickness contacts in the whole mixed EHL. The modern mode of mixed EHL in concentrated contacts refers to the mode of mixed EHL in concentrated contacts where the fluid film between the contact surfaces is molecularly thin. Thus non-continuum across the fluid film thickness on some separate locations of the contact or dry contact of asperity occurs on some separate locations of the contact, while in the other zones of the contact, the fluid film is relatively thick and so that
continuum across the fluid film thickness and the fluid behavior is Newtonian, (shear-thinning) viscoelastic or viscoplastic depending on the operating condition. The future mode of mixed EHL in concentrated contacts refers to the mode of mixed EHL in concentrated contacts where the fluid film between the contact surfaces may be molecularly thin, i.e. non-continuum across the fluid film thickness on some separate locations of the contact. At the same time chemical boundary layer lubrication and dry contact may occur respectively on some other separate locations of the contact, while in the other zones of the contact, the fluid film is relatively thick and thus continuum across the fluid film thickness and the fluid behavior is shear-thinning viscoelastic or viscoplastic depending on the
operating condition. It is found that the future mode of mixed EHL in concentrated contacts best reflects the mode of mixed EHL in real concentrated contacts among these three modes of mixed EHL in concentrated contacts. It is the direction of the future research of mixed EHL. It is also found that in the study of the future mode of mixed EHL in concentrated contacts, detailed, fine, and time-dependent results for mixed EHL are of purpose and the mixed EHL contact is more technical and real. In this mixed EHL study, the fluid model needs to be non-Newtonian, the contact surface roughness needs to be taken as technical and real, and the frictional heating effect both within the fluid film and in dry contact of asperity needs to be incorporated. In the study of the future mode of mixed EHL, the lubrication stage transition occurring in real mixed EHL contacts should be studied under the operating condition. The friction, wear, and contact surface destruction respectively, occurring in real mixed EHL contacts in dependence on the operating condition should be studied as well.
Keywords
Hrčak ID:
295850
URI
Publication date:
15.12.2005.
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