Multi-Disciplinary Engineering Design Conference

Kate Gleason College of EngineeringRochester Institute of Technology

Rochester, New York 14623

Project Number: P13505BEARING HEALTH TEST FIXTURE

Stephen RuggMechanical Engineer

William CraigMechanical Engineer

Andrew ShumanMechanical Engineer

Kevin AlbinoMechanical Engineer

Tyler HillMechanical Engineer

Lauren KaczorMechanical Engineer

William NowakProject Guide

Abstract: A bearing health test fixture was designed to quantify bearing health through the collection of

vibration data on rotating radial ball bearings. The vibration data is collected using two accelerometers mounted radially to the bearing clamp. During testing, the outer race of the bearing is held static, while the inner race is rotated without any radial load applied. By collecting and analyzing data for a wide range of operating procedures, the best testing procedure was converged upon. Measurement System Analyses were conducted and the test fixture was found to be able to accurately and repeatedly measure bearing vibration.

Introduction:The Xerox Corporation is a leader in the copier and printer industry, with little competition when

it comes to the quality and durability of their products. To achieve this quality, numerous systems need to work together. The system of interest in this project is the fuser roll assembly in the iGen printer system. The fuser roll assembly contains a number of different rollers that help apply ink to the page. Specifically of interest are the two bearings that the fuser roll operates on.

These bearings operate under a load of 600lb and at high temperatures. Additionally, Xerox’s current specifications for the bearings require them to be checked every 200,000 prints and replaced after 1,000,000 prints. However, Xerox currently has no method of testing that yields meaningful quantitative results about the bearings. Instead, there are a few tests that are conducted on the bearings to try to determine bearing health. The bearings are spun to see if they spin more than 1 time, the bearings are checked for wear marks and rust, and the bearings are checked to see if they have gone through 1,000,000 prints. Xerox has calculated that the bearings should last up to 800,000 prints, but has found that many bearings were being scrapped before reaching that life. In one test, 18 failed bearings were sent to another company to be tested and it was found that only 3 of the 18 bearings were bad. Therefore, it was discovered that their testing methods were not adequate.

Xerox has pursued several avenues of interest to try and fix this problem. One avenue was to purchase an acoustic test fixture, and another was to give the task of designing a test stand to an RIT senior design team. The first senior design team that worked on the project was team P11511 during the winter of 2010 and spring of 2011. This team built a test stand and reached the point of data collection, but was not able to determine if the data that was collected was meaningful. The test stand has not been used by Xerox in the past two years. Additionally, Xerox did purchase a machine from Flextronic, but it was unable to pass a Measurement System Analysis (MSA) and Xerox is not confident in its accuracy. Therefore, Xerox has asked our group to build another test stand. The goal of this test stand is to give Xerox the capability to quantitatively pass or fail bearings.

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Customer Needs:At the beginning of the design process a list of customer needs was generated and agreed upon by

both parties. Passing or failing bearings was the most important need. It was assumed that this need would be met and the rest of the customer needs were designed around this assumption. Several other key needs included data collection, operation efficiency, and analysis on Xerox’s acoustic machine.

Concept Selection:With the customer needs and specifications created, the design process began. A number of

iterations of concept development were conducted. Pugh matrices were used to measure the suitability of an idea. After the first brainstorming session, where full concepts were presented, the actions that would be required in the design were broken down and analyzed separately. These included topics such as how to hold the bearing on the inner and outer race, how to spin the bearing, what kind of motor to use, what kind of vibration measurement to use, etc. During the design process there were two key needs that drove most of the design. The first was that sources of vibration other than the bearings needed to be eliminated or reduced, which resulted in the system design being very simple. The second key need was that the system needed to be easy to use. Keeping these key concerns in mind, the design below was created.

This concept had a two piece bearing clamp with quick releases used to apply radial pressure. It had a T-rail alignment system so that the bearing and mandrel could be easily aligned. A DC brushless motor and rigid coupling was used to decrease vibration, and a mandrel would be used to allow a smaller shaft to spin the bearing. Finally, accelerometers were used to measure the vibration.

Figure1: Concept drawing of the system

CAD Design:Once the design was approved, a CAD model was generated in Creo. When creating the CAD

model several unforeseen issues were discovered, which forced the design to be changed to remove or reduce the items of concern. The method of alignment and the arbor press assembly were changed drastically between the initial design and the final concept. The final design looks as follows:

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Figure 2: Final System Design

Alignment Method:During the design process it was decided that an 80/20 rail system would be used for the

alignment system. There were several reasons for this decision. First, by using 80/20 rails we gained a number of degrees of freedom when assembling the system. This was beneficial because one of the main concerns with trying to create an alignment system was that the tolerance stack-up for the parts would be too great to allow for accuracy. By building in degrees of freedom in several directions this concern was eliminated because the system could adjust for any unexpected differences.

A second benefit of the 80/20 rails was that multiple rail systems could easily attach to one another. This was beneficial because a second accelerometer was going to be mounted on the bottom of the bearing clamp, which meant that the clamp needed to be elevated off of the ground.

Thirdly, 80/20 had side mount carriages which have several key benefits over other rail systems. First, the side mounting was important because it was desirable to have as much surface area to mount the bottom accelerometer as possible, and it made attaching the bearing clamp to the rail system much simpler. Second, the carriages were capable of having hand clamps attached to them, which was ideal because the system needed to be rigid when operating since all excess vibration needed to be prevented. Also, Tyler Hill had tested similar clamps and attested that they would have enough clamping force to keep everything rigid. Finally, the bearing surfaces on the carriages were easily adjustable. 80/20 provides shims with the carriages so that the operator can adjust the bearing fit according to his or her needs. This was desirable because it had been noticed that other linear carriage systems that had loose bearing fits did not have any way to tighten the tolerances.

Press Fit and Slip Fit Mandrel:In the original design a slip fit mandrel was going to be used to hold the inner race of the bearing

while it was rotating, because that is what Xerox’s acoustic machine used. However, as testing of the acoustic machine was conducted, it was found that the bearing would slip on the mandrel while it was rotated. This was undesirable because, if the bearing slipped, it would generate inconsistent vibration signals. Therefore, it was decided that two mandrels would be created. One mandrel would be a press-fit mandrel with .0005 inches of interference and the other mandrel would have .0005 inches of clearance and would have an o-ring that the mandrel would compress when it slid onto the mandrel. It was hoped that the compression of the O-ring would keep the bearing from slipping.

The two different mandrels were created because of concerns about how well each method would meet the established customer needs. It was expected that the press fit would work very well, but it would take much longer to test a single bearing since the bearing would need to be pressed onto and off of the mandrel and, for each bearing, the mandrel would need to be removed from the shaft. Additionally, to make the press fit possible, an arbor press assembly needed to be manufactured. In contrast, an operator could just slide the bearing onto and off of the slip fit mandrel by hand.

During testing it was found that the press fit worked well and the bearing spun smoothly without slipping; whereas, the slip fit didn’t completely stop the bearing from slipping and the slippage wore down the o-ring. Also, the system vibrated much more with the slip-fit than the press fit.

Arbor Press Assembly:

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To make the press fit mandrel a feasible option a press fit assembly was designed. The assembly was that it needed to align the bearing and mandrel and support the bearing so it is not damaged while the mandrel is pressed on. These two tasks were accomplished with the assembly shown below. The base aligns the bearing with the inner surface of the lip. Then the alignment cup aligns with the outer surface of the lip and the mandrel slides into the hole in the alignment cup. Once everything is in place, the arbor press, shown on the left, is used to apply load to the top of the mandrel and force it into the bearing. When the bearing needs to be pressed out it is placed on the base upside down, so that the mandrel will fall through the bottom hole, and the mandrel is pressed out.

Figure 3: Arbor Press Assembly

DAQ Selection:The DAQ device that was chosen for this project was the National Instruments USB-6210. This

DAQ was chosen because it is capable of reading small input voltages accurately and it was not overly expensive. Specifically, for the USB-6210’s largest voltage range (-10-10V) it is capable of maintaining a 2.69mV accuracy, and at the smallest range (-0.2-0.2V) it has an accuracy of 91.6µV. This was important because data received from Xerox indicated that tested bearings would, at a minimum, generate a 4mV signal. However, since testing, it has been found that the bearings generate significantly stronger signals.

Motor Test:In order to ascertain the required torque for the desired operating range, two tests were conducted.

First, a mass was attached to fishing line, the fishing line was wrapped around the outside of a bearing, and the mass was dropped from a predetermined height. By recording the critical velocity, the torque required to spin the bearing at 42 rpm was determined. However, this test was not able to generate information about the torque at higher rpm’s. Therefore, using a motor and motor controller from Xerox, two bearings were run at a variety of speeds while the torque was monitored. It was found that as speed increased the required torque began to level out. Using this information, a motor was properly sized for the desired operating conditions. The motor that was chosen can apply a torque up to 5 in*lbs.

Base

Alignment Cup Lip

Mandrel

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Figure 4: Motor Test Graph

Accelerometers:During the design process a device had to be chosen to measure the bearing vibration. There were

several options including proximity probes, velocity transducers, and accelerometers. From these options accelerometers were chosen because they have the largest range of operation (0-10,000Hz) and could be mounted easily to a number of different surfaces to help with initial vibration diagnostics. The large range of operation was important because research had shown that the bearing vibration that is usually generated by radial ball bearings is at or above 5000Hz.

Once the accelerometers were chosen a mounting technique needed to be selected. The standard methods to mount accelerometers are using magnets, wax, or a stud. The benefit of the magnet and wax are that they allow accelerometers to be mounted easily without modifying the system; however, by mounting using these methods the range of operation is decreased. Based on manufacturer data, it was found that both the wax and stud mounting would be acceptable. Therefore, in the early stages of testing, wax was used. It was soon discovered that the bearing test stand, when operating for long periods of time, got hot enough to melt the wax. Therefore, the system was modified to allow for stud mounting.

Signal Conditioner:When researching and collecting information on the components that were going to be required

for the data collection system, it was found that the accelerometers would require a signal. However, it was found that the most basic signal conditioner would cost roughly $500, which raised the system cost significantly above the target budget. Therefore, a decision was made to design, build, and test a signal conditioner before buying one. It was estimated that the full cost of a homemade signal conditioner would be $150, a cost saving of $350.

The key characteristics required of the signal conditioner was that it needed to provide the accelerometers with a constant current between 2 and 10mA and a voltage between 18 and 30V. Providing the constant current was achieved through the use of constant current diodes which automatically regulate the output current to a certain value. Achieving the 18V output was not as straightforward since it was undesirable to use batteries since they are cumbersome and require replacing periodically. Instead, it was decided that USB power would be used to power the signal conditioner, which meant that the 5V output by the USB needed to be increased to 18V. This was accomplished with two integrated circuits, the ADM660ANZ and the LTC1144. When assembled properly, these circuits output 18V.

When the signal conditioner was tested, it was found that the accelerometer data was correct, but there was a DC offset to the data. Therefore, a digital filter was placed into the programming to remove this offset.

Figure 5: Signal Condition Schematic

LabVIEW:To collect the vibration data and be able to conduct the analysis that was desired, a custom

LabVIEW interface was created. This interface allows the user to import two channels of data and scale each channel according to the calibration of the accelerometers. Additionally, the data is processed through a band-pass filter that extends from 1,000Hz to 7,500Hz. The lower frequency was chosen because the

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research that was conducted indicated that the visible bearing vibration would be seen above 1,000Hz and the higher frequency was chosen because that is when the accelerometers start to deviate from unity gain. Finally, averaging of the overall RMS values was built into the system. The reason for this feature is that the data fluctuates and it is hard to collect accurate data points, therefore by averaging, a much more consistent value is achieved. The amount of time LabVIEW averages can be controlled by the user.

For the user interface a number of options are displayed including the channels to import data from, the sampling rate, the accelerometer calibration, and the overall RMS values. Also, the front panel displays the time waveform and the FFT of the data. The FFT has averaging capabilities that are easily specified by the user. Finally, the capability to set a pass fail criterion was added, so that LabVIEW automatically assesses if a bearing is good or bad and displays the result.

Figure 6: LabVIEW Front Panel

Accelerometer and Signal Conditioner Testing:Once the system was fully assembled the purchased accelerometer and manufactured signal

conditioner were compared against a B&K accelerometer and signal conditioner. This test was conducted to verify the accuracy of the signal being acquired since the signal conditioner was home made. Both accelerometers were placed on a shaker table, which simulated random noise. By looking at the correlation between both readings it was found that the purchased accelerometer and manufactured signal conditioner were accurate to about 7500Hz. This is a conservative number because there were several factors with the accelerometer mounting that may have impacted the readings.

Testing:Once the stand was fully assembled testing was conducted to try and pass a measurement system

analysis (MSA). By passing an MSA it would certify that the test stand produces repeatable and reproducible results.

A number of MSA’s were conducted to try and converge on the best operating procedures. Each time a new MSA was conducted, only one testing condition was changed. Through the testing, it was found that operating at 1000rpm for 15seconds was best. Running the bearing for longer periods of time before taking data was not as good because each bearing had a different amount of grease and would heat up at different rates. As the bearings heated up the vibration levels changed. Therefore, by operating them for a short period of time, there was little chance that the vibration levels would change greatly from the original, which improved consistency.

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Additionally, it was found that the accelerometer on the top of the bearing clamp yielded a more consistent vibration result than the accelerometer on the bottom. It was also found that vibration through the clamp was acceptable. Therefore, only one accelerometer is needed on the top of the clamp.

Eventually, we began decreasing the number of samples and increasing the number of operators. Due to the non-normal distribution of sample bearings, it was difficult to find a set of bearings which spanned the desired range well. The final test involved three bearings and seven operators. The bearings read approximately 0.15, 0.25, and 0.45 g’s. With this set of conditions, the system finally passed an MSA.

Once the system passed an MSA, the grease was removed from several different bearing to see if the vibration generated with grease corresponded to the vibration generated without grease. This test was conducted because there was a concern that the grease may inconsistently affect the vibration of the bearing. It was found that the grease acted as damping, and that the good bearings remained good and the bad bearings remained bad. This is desirable because it indicates the system is measuring vibration from defects in the mechanical parts of the bearing and not the grease.

In an attempt to validate the acoustic machine, a range of bearings were tested in order to compare values against the vibration stand. To accomplish this, a range of bearings that spanned the full vibration spectrum, on the designed test fixture, were tested on the acoustic. The chosen ten bearings were tested 4 times each, and every bearing passed every time.

However, it had been noticed earlier that the bearing slipped on the mandrel during operation, which could have been a cause for some of the error in the measurements. Therefore, the ten bearings were tested again, and the slippage was restricted through the use of doubled sided tape. Once again, all the bearings passed every time. Therefore, it is believed that the acoustic machine is not an adequate testing fixture.

Budget:Xerox gave us a total budget of $3000 with a target spending of $1500.  In order to keep the costs

low, they provided us with some of the more expensive components, such as a DAQ, motor, and motor controller.  For raw materials, machining and purchased parts, we spent a total of $1439.03 including shipping.

Results: The most important result from the testing conducted was that the system passed an MSA. This

means that the system is capable of measuring vibration differences between bearings. Specifically, it was found that the system has 13 ranges of resolution from 0.1 to 0.7 g’s.

Figure 7: MSA Results

Conclusions and Recommendations:

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Reaching the project goal required an interesting spread of skills and forced learning about many new subjects. Senior Design I taught a lot about the concept selection process and forming a highly functional team. Senior Design II taught machining, MSAs, signal processing, LabVIEW, and time management.

While the system was successful, there are many improvements that can be made. A proper pass/fail threshold can be established by testing a comprehensive range of bearings on the test stand, then giving the bearings to Arnprior for professional analysis and comparing pass/fail results. It is also recommended that any future clamp be made out of steel with enlarged dowel pins, and that hardened steel be used for the shaft.

While the system has been somewhat automated, further automation could expedite the process, as well as increase precision. This would involve a new motor controller that could accept inputs from LabVIEW, insuring the exact same motor speed and rotational acceleration each test. Given the difficulty of the bearing press fit, it would also be helpful to purchase a larger arbor press that would require less effort, a change that would be mandatory if the machine is used all day long. Additionally, a new signal conditioner would also improve the data by decoupling the signal and removing a false peak just above 4,300 Hz. Also, an actuator could be added to apply a constant axial force to the top and bottom of the bearing clamp. Finally, since the top accelerometer proved capable of measuring the vibration, the rigidity of the system could be improved by removing the bottom accelerometer and dropping the rails alignment rails to the base and lowering the motor accordingly.

With regards to the acoustic fixture, it is recommended that some efforts be taken to try and acquire numerical data from it so that it can be better understood so that its results can be verified as trustworthy or untrustworthy.

AcknowledgementsWe would like to thank Bill Nowak for guiding us through senior design, taking us out to lunch,

and helping with the accelerometer testing. Thank you to Erwin Ruiz for giving us this project, teaching us about MSA’s, and giving us a tour of Xerox. Thank you to Dr. Kolodziej for helping us with motor and accelerometer selection, and giving us constructive criticism on our design. Thanks to Dr. Boedo for giving us instruction of bearings and potential avenues to look into when testing. Thank you to Rob Kraynik, Dave Hathaway, and Jan Maneti for helping us greatly with the machining of the parts. Thank you to the senior design lab staff for putting up with our constant requests for different pieces of equipment.

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