A comparative performance assessment of a hydrodynamic journal bearing lubricated with oil and magnetorheological fluid

Highlights

• Properties of magnetorheological (MR) lubricant were experimentally determined.

• Comparative study for bearing lubricated with MR fluid and oil was performed.

• Theoretical investigations were positively verified experimentally.

• MR lubricated bearing performed better than oil lubricated at low speed conditions.

Abstract

This work presents the investigation results of a journal bearing lubricated with magnetorheological fluid that is activated by a local constant magnetic field to vary both the local flow resistance and pressure. The bearing performance is assessed via Finite Element Modelling (FEM) and results are corroborated by experiments. The FEM model uses the Bingham model to describe the fluid film. A dedicated test rig is used to assess the hydrodynamic behavior of the bearing with magnetorheological fluid, and the results are compared with the same geometrical bearing lubricated with oil. Thicker fluid films at low speeds, beneficial pressure distribution, and higher friction losses under all operating conditions are observed in the bearing with magnetorheological fluid compared to the oil lubrication.

Introduction

Oil-lubricated sliding bearings are commonly employed in high-speed rotating and reciprocating machines such as turbines and compressors due to their good load-carrying capacity and durability [1], [2].

Over the years, bearings have been improved since their initial conception [3], but the losses of energy owing to viscous friction and system failure are yet considerable. The lubricated bearing in its simplest form consists of two parts in relative motion with a lubricant in between. Depending on the application and its working velocity range, one can choose a hydrodynamic bearing for high-speed applications to gain benefits in system alignment, friction, and damping [4], [5], [6]. This bearing configuration supports the rotating shaft through a pressurized oil film [7]. However, at low speeds, physical contact between bearing surfaces might occur as a result of the lubrication film incapability to withstand anymore the shaft load, thus causing extreme friction and wear and ultimately compromising the integrity of the system.

In case the system operates at low speeds, hydrostatic bearings are a suitable option since they are externally pressurized, ensuring a separation gap between the bearing surfaces [8]. Hydrostatic bearings use failure-sensitive high-pressure supply pumps to pressurize the lubricant and lift up the shaft at critically low speeds. Once the pump fails, high wear in the bearing is generated as its performance in the hydrodynamic regime is very poor due to the unfavorable geometry related to the presence of pockets in the sliding surface [9], [10]. Despite the wide selection of lubricated bearings available nowadays, the optimization of surface textures and lubricants has led to only small improvements, and relevant technical limitations remain. Interesting developments are seen in the field of tribotronics [11] with the introduction of smart fluids to actively control vibrations and damping in dampers [12] and bearings [5], [13], [14], [15], [16], [17] as well as in brake applications [18]. Smart fluids like magnetorheological (MR) fluid, electrorheological (ER) fluid, and ferrofluid (FF) contain magnetic iron particles dispersed in a carrier oil, but they differ in particle size and the way in which they respond to an imposed excitation. Both MR and ER fluids are referred to as pseudo-plastic or Bingham since they act like solid materials at low stresses but flow as viscous fluids at high stresses when excited respectively by an electric filed or by a magnetic field. In these fluids, suspended particles tend to sediment over time, but their viscosity can vary by several orders of magnitude in millisecond, making them more appealing than conventional lubricating fluids for applications like lubricated bearings [19], [20], [21], [22]. On the other hand, FFs are colloidal liquids that contain ferromagnetic nanoparticles coated with surfactant to keep the particles in suspension by Brownian motion and show non-Newtonian effects in the presence of a magnetic field [23], [24]. Among all the smart fluids discussed, MR fluids seem to be the most promising for heavy-duty rotary machines due to their ability to generate greater fluid forces [25], since the yield stress is about tenfold higher than the yield stress in ER fluids [22]. Besides, ER fluids must be degassed and pressurized to avoid cavitation, but this problem is far less prominent in MR fluids [26]. FFs do not meet the requirement of viscosity change for lubricated bearings since only small viscosity changes occur under the imposition of a magnetic field [5], [19], [27]. It is worth mentioning that only a limited number of works dealing with bearings lubricated with MR fluid are available in the literature [25]. In [28], an MR fluid was used in a hydrostatic bearing to keep the gap constant when the payload was varied. The work proved that the system reacted faster to changes in applied loads than the external valve-based system, since the MR fluid acts right inside the gap. Reference [5] compared the performance of a hydrodynamic bearing lubricated with MR fluid against one lubricated with FF. The authors concluded that the MR fluid is a better candidate for active control than the FF one due to the greater change in viscosity under a magnetic field. A numerical study [25] on hydrodynamic bearings confirmed the positive contribution of MR lubricant to the bearing load-carrying capacity under the influence of a magnetic field. Conventional bearings change their flow resistance through local changes in the geometry of the surface, or known as local texture, which cannot be varied during operations [10]. On the contrary, bearings lubricated with MR fluid are able to tune the viscosity during operations by the imposition of a magnetic field, thus creating a local alteration in flow resistance, called the MR texture [29]. A negative aspect associated with the use of smart fluids as lubricants is the increase in friction, which causes higher torques and therefore greater system power consumption [5], [25].

According to the literature provided, there are only a few works investigating the performance of hydrodynamic bearings lubricated with MR fluids; hence there is still a gap in the knowledge concerning their operational performance, especially when trying to make a direct comparison to other bearing types.

The goal of this research is to prove that smart fluids add an extra degree of freedom in the bearing system than mineral oils since the lubricant viscosity can be locally controlled through a magnetic field during operations, thus ensuring the presence of a lubricating film at critical operating conditions when boundary or mixed lubrication is normally expected.

To this aim, a full-size hydrodynamic journal bearing under operation with two types of lubricants, MR fluid and conventional mineral oil, is investigated. An experimental setup to verify the effects of using an MR fluid in a hydrodynamic journal bearing during operation at different speeds and loads was used. The results of the experimental research (e.g. eccentricities, bearing friction losses, film pressures), complemented with numerically calculated bearing parameters, allowed a direct comparison of the bearing performance depending on the lubricant used. Based on the analysis of results, the main differences between the operational behavior of the bearing lubricated with MR fluid and with mineral oil were identified and discussed in detail.