Dynamic Journal Bearing Loading System

Transcription

1 Dynamic Journal Bearing Loading System Abstract A journal bearing is a supporting sleeve which allows the formation of a lubrication film, creating a low friction surface in which a shaft can freely rotate. Journal bearings are utilized in modern rotating equipment solutions, in the oil, gas, power and transportation industries worldwide. Additionally, journal bearings contribute to the longevity and efficiency of rotational systems. Therefore, it is important to understand failure modes. Currently, seed of fault testing is conducted on full scale equipment to gain an understanding of the effects of damage and contamination on the degradation of journal bearings. Data collected during the testing provides opportunities for fault detection and system design improvements. Institute of Technology ( ), supported by, researches performance and instrumentation of the reciprocating compressor. To conduct the seed of fault testing a deliberately damaged journal bearing must be perpetually replaced to identify various failure conditions. This replacement can take over six hours to accomplish. To reduce compressor down time a series of capstone projects have been commissioned by, Assistant Professor of Mechanical Engineering at, to design a stand alone test rig capable of simulating compressor environments. The alpha prototype accomplished a working journal bearing similarity test rig capable of static loading conditions. The beta prototype, currently in development, will fit the the alpha prototype with dynamic loading capabilities using electromechanical actuators (EMA) donated by., to more accurately replicate compressor characteristics. The beta prototype will also focus implementation of sophisticated data acquisition equipment for the study of film thickness, flow conditions, fluid properties, and vibrations. Furthermore, the methodologies of design, testing and evaluation will be explored in this presentation.

2 Introduction designs, manufactures, and services a wide variety of products for use in the oil, gas, process, and power industries. In 2011, donated an reciprocating compressor to to be used for graduate and undergraduate research. Currently, much of the research is based in the areas of measurement, controls and extended life/fault testing. A journal bearing test rig was designed and fabricated by an Multidisciplinary Senior Design team during the academic year to simulate s reciprocating compressor. The prototype became the alpha version of the test rig. This project serves as a continuation of the project, improving the alpha version to the beta version. The reciprocating compressor test rig allows for rapid testing of the crank journal bearing found in the full scale compressor. Comparatively, disassembling the full scale compressor can take many hours while disassembling the test rig can take approximately a sixth of the time. The rapid disassembly allows for testing and monitoring of the crank journal bearing while reducing time between tests. The alpha version is fitted with static loading due to time and budgetary constraints. However, this does not accurately simulate the environment found in the compressor. The goal of this project is to successfully adapt the alpha version with dynamic loading while meeting bearing loads of approximately 1900 lb at 6 Hertz. Both the dynamic loading components and the base test rig will be fitted with data acquisition sensors for condition monitoring. The resulting design will add both dynamic loading as well as data acquisition without sacrificing ease of use or safety. Needs and Specifications In order to properly simulate the loading profile, there are many characteristics that needed to be met. To ensure the project provided an efficient and usable solution, we mapped the given customer requirements to engineering requirements. This identified a path to solution while focusing efforts to the most important aspects of the problem. Furthermore, the engineering requirements were measured against the customer requirements and judged for their assistance in meeting the final goal. That is, properly simulating the load profile of a reciprocating air compressor while not increasing the time required to change the bearing. The raw scores provided for each requirement were then analyzed and organized using the pareto analysis to prioritize requirements. Additionally, in order to document the testing and calibration procedures, the team created standard operating procedures. These procedures provide step by step instructions on how to perform the required calibration and testing procedures in simple terms and with detailed pictures.

3 Chart 1 Pareto Analysis Table 1 Requirements Concept Selection In efforts to find the most effective solution the team generated multiple concepts at each level of design. Starting at the systems level, a functional decomposition was developed to lay out all of the key functions the test rig. This allowed the team to focus on the subsystems that ranked highest on the Pareto chart while understanding their connections to the other parts of the system. The critical subsystems are shown in orange in figure 2

4 Figure 2 Functional Decomposition Further decomposing the subsystems into individual components followed similar brainstorming process. Morphological charts were used to visually display possible options that were then turned into potential unique solutions. This lead to the identification of the benefits and risks of each potential solution. Furthermore, minimizing risks and combining multiple ideas lead to concept selection. Concepts were then evaluated using a Pugh Chart, which is a comparative chart that displays each concept based on categories specific to the engineering requirements and design considerations. Furthermore, combining certain aspects of the concepts produced new concepts; these combinations were added to the Pugh Chart and analyzed. This became an iterative process allowing us to narrow our concepts down to four. One using piezoelectric actuators, the second using hydraulic actuation, the third using pneumatic actuation, and the final using an electromechanical actuator (EMA). Each concept was examined further until the EMA was chosen due to its accuracy and cost. While piezo had excellent micro motion abilities it was limited in its ability to displace large distances while also being very costly. Hydraulic was able meet force and speed actuation but required many additional components while also being very costly. Pneumatic actuation struggled to meet the required force without a large quantity of air on demand.

5 Figure 5 Pugh Chart The EMA concept consists of three main components; the actuator, the power supply and the controller. EMA actuation is controlled with a displacement feedback loop from the pitch of a ball screw and it s rotational position. In order to control the actuator using force feedback a secondary control loop must be created to adjust position based on applied and required force to adequately replicate the desired load profile. Another design challenge is false brinelling which occurs from lubrication being pushed out of a loaded region during small oscillatory movements that occur with micro displacements. One way to mitigate this is to do periodic macro displacements with one or more full screw revolutions to to re distribute lubrication within the rolling elements. To ensure testing won t exceed manufacturer's recommended limits of micro cycles the following calculation was completed. Mounting Design When deciding how to mount the EMA s various design aspects were taken into consideration. These include both budget and structural features. Additionally to simulate a sinusoidal loading profile actuation has to occur in two directions x any y. Parallel to the test rig surface was determined to be the x direction while parallel to the test rig legs is the y direction. Mounting in the x direction simply required holding the EMA to the table while placing the shaft in the center of the bearing housing. The specifics of the x direction mounting can be seen below in Figure 6.

6 Figure 6 However, mounting in the y direction was more difficult because the EMA has to sit perpendicular to the surface. Thus the EMA had to either be mounted above the bearing block or hang from below the table surface. Mounting above the bearing block was quickly, eliminated because it would restrict access to the bearing housing. Thus we utilized the Pugh method of concept selection to determine which of our concepts would be the best design to attach to the bottom of the table surface. The analysis allows us to see concept B was the best option. Furthermore, concept B is the x direction mount flipped upside down. The ultimate considerations for the mounting design were stress and deflection. Because we were actuating at such low displacements the deflections had to be small enough to overcome with our control. Lastly, the total stress had to be low enough to ensure the endurance limit of our components allowed for infinite life of steel. Vibration Analysis

7 To analyze the vibrations of our test rig, the focus was directed towards the components which created a frequency: the motor and the shaft. While the shaft is known to spin at 360rpm (6Hz) the motor drive rpm was unknown. Using simple pulley ratios, the rpm and frequency of the motor drive was determined to be rpm and 21.43Hz respectfully. With the frequencies of both major components on the table, Leissa s vibrations of plates analysis can be performed to determine the vibration of the plate itself, using the Simply Supported of the four corners of the plate as the worst case scenario [4]. Table 4.89 is used to find ƛ, which gives values of the desired wavelengths determined by the length ratio. Frequency was found using the following relationship and input parameters Because our ratio is between two of the parameters found in Table 4.89, we know the frequencies found with these two values will be the limits of our actual vibration. Frequency Excerpt from Table 4.89 Length Ratio = 1.0 Length Ratio = rev/s rev/s Frequency 88.2 Hz Hz Typically, the frequencies will be considered safe to avoid resonance between them if they are outside their values multiplied by 4. The actual value will come out to approximately 100Hz, but cannot be directly interpolated since the relationship of the data isn t linear. Data Acquisition Although the primary focus of this project is the ability of dynamic actuation, data acquisition capabilities account for many of the secondary objectives. In terms of condition monitoring and seed of fault research, the ability to obtain accurate data at the time of failure is crucial. Knowing the applied load to the system, is important to both the actuation system, to provide feedback to the EMAs, and to the data acquisition system, to understand at what load the bearing fails. load cells capable of 2000 lbs of force will be implemented between the EMAs and the bearing housing. The load cells work with strain gauges that output millivolt

8 signals corresponding to the experienced load. These signals are then interpreted by signal conditioners that convert it to a zero to ten volt scale that is then interpreted using s programming methods. We are using load cells and signal conditioners donated by, for the first iteration which have limited sampling capabilities of 60 samples a second. Testing will determine if this is sufficient. Determining the position of journal in respect to the journal bearing allows the systems eccentricity and film thickness to be determined. The diametric clearance of 90 microns represents the total displacement the system can undergo, therefore linear variable differential transformers (LVDT) were chosen due to their ability to measure such small displacements. Similar to the load cell, the signals are converted using a signal conditioner and processed using a. The also interprets data from an pressure transducer located at the oil inlet of the bearing housing along with vibration, angular position and temperature data. The vibrations of the system are analyzed by a Kistler accelerometer and ( ) usb 4431 capable of 102.4kS/s. A quadrature encoder is used to analyze the angular position which is sent to the actuation controller to apply the proper load profile and to the data acquisition system to give rotational velocity (RPM) data. Finally the thermocouples (TC) acquire temperature data from the journal bearing, oil outlet and oil reservoir and are imported to using an TC module. Oil Flow Problem Solving As part of our customer requirements we were tasked with acquiring the oil flow rate through the journal bearing. The system that was used in the alpha prototype was unsuccessful due to significantly smaller flow rates than anticipated. To understand the cause of this reduced flow we utilized our problem solving process. First we identified that the issue was perpetuating from the bearing housing by checking oil flow at each major component. We then analyzed the oil flow path through the bearing housing and identified potential restriction. With the assistance of Dr. we developed a theoretical model for the oil flow, Q p, under no load and no rotation using the equation given by Martin and Lee [6]. Where D is the bearing diameter, C is the radial clearance between the journal and bearing, P f is the groove supply pressure, μ is the dynamic viscosity, ε is the journal eccentricity ratio, L is the overall bearing length, and a is the groove width. From this equation we were able to identify the parameters with the most influence and check our assumed values with experimentation. Such as measuring the pressure drop induced by the journal bearing s orifice like feed hole which was 20 psi. The measured radial clearance between the journal and journal bearing was about 30 percent smaller than specified by the alpha team s design. After the analyses the theoretical flow rate were gpm when the journal eccentricity ratio is 1 and a value of gpm when the journal eccentricity ratio is 0. Our measured value of gpm correlates well. From these finding we are able to verify the low flow rate as adequate and that

9 the alpha prototypes flow meter was over sized. Next we plan to test flow rates with journal rotation and dynamic loading both analytically and experimentally. References [1] Budynas, Richard G., J. Keith. Nisbett, and Joseph Edward. Shigley. Shigley's Mechanical Engineering Design [2] Fox, Robert W., Alan T. McDonald, and Philip J. Pritchard. Introduction to Fluid Mechanics. Hoboken, NJ: Wiley, Print. [3] Holzenkamp, Markus. Modeling and Condition Monitoring of Fully Floating Reciprocating Compressor Main Bearings Using Data Driven Classification. Rochester Institute of Technology, [4] Leissa, A. W., 1969, Vibration of Plates, NASA Report SP 160 [5] Manring, Noah. Hydraulic Control Systems. Hoboken, NJ: John Wiley, Print. [6] Martin, F. A., and Lee, C.S., 1983, Feed Pressure Flow in Plain Journal Bearings, ASLE Transactions, 26, pp [7] Palm, William J. System Dynamics. Boston, MA: McGraw Hill, Print. [8] Parker Hannifin Corporation. Parker Pneumatic. Cataloug PDE2600PNUK, Print. [9] "Piezo Nano Positioning." Physik Instrumente (PI) Gmb H and Co. KG, Web. [10] Rippel, Harry C.,1960. Cast Bronze Bearing Design Manual, Cast Bronze Bearing Institute Inc.