Keywords

1 Introduction

The Tilting Pad Journal Bearings (TPJBs) [1] are fundamental machine elements for load supporting in rotating machinery with good performance under extreme operational conditions of load and rotational speed. However, the steadily demanding operational conditions deplete the bearing capability of dissipating energy. Incorporating elements of mechatronics such as vibration sensors, hydraulic servo valves, control systems and laws to inject lubricant in a controlled way, has been proposed as a solution to tackle high vibration amplitudes by acting directly over the bearing behaviour and expand the machine operational limits. Santos [2] reviews trends in fluid film bearings linked to mechatronic redesign. In [3] the author aims at adding flexibility to the pivot point of a TPJB via membrane chambers. In [4] the same author presents the active lubrication, the controlled injection of high-pressure lubricant to the shaft load supporting fluid film through a centered bore at the pad. Active lubrication is modelled through a thermo-elasto-hydrodynamic approach based on the modified Reynolds equation. More recently, Varela et al. [5] proposed and studied theoretically and experimentally a mechatronic version of a Leading Edge Groove TPJB, obtaining good results in terms of its applicability. TPJBs can feature different types of pivots such as rocker, spherical, flexible, and fluid. Authors incorporate the pivot flexibility determined by the hertzian contact theory [6] by assuming a pad radial movement as a new degree of freedom [7]. It is demonstrated that the TPJBs behaviour is highly affected by the pivot design [8], specially at demanding operational conditions where the pivot stiffness becomes larger than the one of fluid film, therefore decreasing the capability to dissipate energy.

1.1 The Mechatronic Fluid Pivot Journal Bearing

This work proposes a new design that focuses on the fluid pivot dynamics. The proposed design is inspired in the granite fountain [9] to incorporate a lubricated mechanism at pivot able to support a high load with small amount of lubricant flow. It is a mechatronic approach of a TPJB with a spherical pivot to support the pad [10]. This spherical pivot has been modified into an injector to provide, in a controlled way, high pressure lubricant in between surfaces of the pivot and pad seat. In this way, a fluid pivot with controllable lubricant injection is obtained. The proposed bearing design aims at becoming a mechatronic alternative to improve the pivot dynamics. As results of this work, the conceptual design and the design of a first mechatronic Fluid Pivot Journal Bearing (FPJB) prototype are presented. In the state-of-the-art review a FPJB was found [11]. However, it can be considered as non-mechatronic bearing.

2 Bearing Design

2.1 Bearing Fundamentals

The bearing concept surges from the granite ball fountains observations, also known as ball fountain [9], in which a thin fluid layer of water allows it to develop a hydrostatic pressure distribution able to support a high load, the granite ball weight. The same principle can be imposed at the pivot-pad socket gap aiming at this extra source of lubricant can: 1) to avoid the hertzian contact between surfaces and to remove the friction resistant moment, 2) to provide an additional source of damping and 3) to endow the pivot dynamic properties with the possibility of being controlled. A first isothermal model is considered although the literature establishes a thermo-elasto-hydrodynamic model to properly describe the phenomena in tilting pad journal bearings [12].

2.2 Bearing Design

The standard recommendations for designing hydrodynamic bearings were attended besides further considerations from a mechatronics viewpoint.

Design Parameters.

a) Standardization: optimized and standardized sizes to reduce space and cost of production are considered. b) Lubrication: immersion lubrication is considered with an ISO VG22 lubricant. c) Material: bronze is utilized instead of babbitt, fulfilling the same fuse function to protect the shaft in case of surfaces contact. d) Specific Load: it is necessary to avoid low and high values due to instability and heat transfer and babbitt fatigue problems. For the bearing is approximately 1 MPa. e) Tangential Shaft Surface Velocity: it increases with the shaft diameter and shaft rotational speed and for the bearing is approximately 52 m/s. It is a critical design parameter because it can develop a turbulent flow. f) L/D ratio: it is defined as 1 to be utilized in the available test rigs having lower specific load, hence lower stiffness, and good damping properties. g) Clearance: the narrowed the tolerances, the higher the cost of producing it. In this case a value of 1,1 mils per inch in diameter is selected. h) Pivots: a partial contact of a spherical pivot with the pad socket is utilized in this design.

Mechatronic Design Parameters.

They are considered as: i) Actuator. Two electrohydraulic servo valves for injecting high pressurized lubricant to provide a plane control force. ii) Sensors. Displacement sensors for measuring the shaft positioning and feedback the control laws. iii) Injector. An injector orifice in the pivot center to create a fluid film between the pivot and pad socket surfaces. The Table 1 summarizes the main design parameters of the mechatronic fluid pivot journal bearing.

Table 1. Design parameters of mechatronic FPJB.
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2.3 Design Stages: A System Approach

The hydrodynamic bearing is designed under a constructive approach which allows unique functional subsystems to be identified. This implies that the system is compounded by a minimum number of pieces easily of machining and installing in the laboratory test rigs. Furthermore, it is also flexible as the pieces shall be adjustable and interchangeable to study additional configurations of the bearing. The design stage considered are: 1) Pivot system: it is compounded by the pad and the spherical pivot. Pivots are installed in an oil distributor ring. The pivots can be adjusted to change the pad-pivot clearance. 2) Oil distributor system: It comprises the oil supply channel from servo valves to the spherical pivots mainly compounded by a distributor ring. The return lines are also part of the system. 3) Sealing ring: It is meant for sealing the distribution ring to avoid leakages. 4) Housing and accessories: It comprises the protecting housing, oil deposit, servo valve mounting block, seals, bolts and foundation bolts. The housing is considered stiff, and it allows the simple lubricant recovering. Sensors mounting structures are also considered.

3 Results

3.1 The Mechatronic Fluid Pivot Journal Bearing

The Fig. 1 a) and b) show the designed bearing assembled and exploited respectively. A total weight of 109 kg is estimated with the CAD software considering bronze as pad material and steel for other pieces. Geometrical parameters are stablished in Table 1. Metric bolts between M3 and M20 are utilized. Seals are used in caps, servo valve blocks and pivots. Two radial shaft seals are also considered to minimize leakages between shaft and bearing. The Table 2 summarizes the main bearing components.

The following characteristics are highlighted for the proposed mechatronic FPJB: 1) It allows operating the bearing in load-between-pads and load-on-pads configurations. 2) It allows adjusting the pad-pivot clearance due to the type of o-ring utilized, which can be deformed up to 25% of its original size. This affects the whole bearing clearance. 3) Simple and flexible oil and heat extraction thanks to its lower oil collector with two oil outlets. The collector volume is calculated based on the oil flow rate. 4) It allows bearing alignment thanks to ovals holes for mounting bolts. 5) The insertion percentage of the spherical pivot in the pad socket is 25%. 6) It allows an actuating active control force in two perpendicular directions. 7) The injected oil flow, by the nozzle in the pivot, reaches the pad socket or seat. It is expected that the bottom pads also feature immersion lubrication. 8) A high-pressure pump is needed to supply the lubricant.

Fig. 1.
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Mechatronic Fluid Pivot Journal Bearing. a) Assembled. b) Exploited.

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Table 2. Components List of the Mechatronic FPJB.
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3.2 Bearing Simulations

The simulations are performed by solving the isothermal model with the finite difference method. The correct implementation and convergence study of the method has been validated in cylindrical journal bearings and TPJBs found in literature. Afterwards, the method is adapted to the mechatronic FPJB. It is simulated with data of Table 1 and with the operational conditions reported in the graph legends.

Figures 2a) and 2b) depict the results for the eccentricity and pad tilt. As the rotational speed increases, the eccentricity decreases. Besides, there is lower eccentricity at higher thickness \({h}_{pi}\) for a fixed rotational speed. Conversely, occur with the load \(W\). For the case of pad tilt, it can be seen in Fig. 2b) that as the rotational speed increases, the pad tilt also decreases, as in the previous case.

Fig. 2.
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a) Eccentricity v/s rotational speed, b) Pad tilt v/s rotational speed.

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Figure 3a) depicts the hydrodynamic pressure distributions at the Shaft-Pad surface and Fig. 3b) depicts the hydrostatic pressure distribution at the Pad-Pivot surface. The difference in magnitudes between them is mainly because the surfaces are different in size. By comparing Figs. 3a) and 3b), it can be observed that there is some symmetry in the pressure fields because a constant hydrostatic thickness \({h}_{pi}\) is considered.

Fig. 3.
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a) Hydrodynamic and b) hydrostatic pressure distributions.

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4 Conclusions

The present work has presented a prototype of a new design of fluid pivot journal bearing with a mechatronic approach, the mechatronic FPJB. This approach is applied aiming at influencing the bearing properties by means of controlling the physics that dominates the modified spherical pivot behaviour for injecting lubricant behind of each pad. The proposed design incorporates some industrial aspects, and it can be incorporated into test rigs for further experimental tests. An isothermal model is used to simulate the mechatronic FPJB. It is expected that this bearing can provide an extra and controllable source of damping when compared with Hertzian contact pivot bearings.