The document summarizes a study on reducing vibration from the blast fan of an air-cooled Volkswagen Beetle engine. Vibration readings were taken and found to be severe, exceeding IEEE standards. The root cause was identified as defective bearings in the alternator through a risk analysis. The defective bearings were causing mechanical looseness. The housings were re-machined and sleeves added to properly fit new bearings. Vibration readings after the correction showed reduction in vibration levels.

1. Reducing the Vibration Level of the Blast Fan of the Air-cooled Engine of a Volkswagen Beetle Harland Emmanuel C. Machacon, M.Eng. Department of Mechanical-Industrial Engineering College of Engineering, University of San Carlos

2. Vibration is practically the most important indicator of the mechanical integrity of rotating machinery. Like the human heartbeat, vibration within rotating machinery tells a great deal about the health of that machinery. This study focuses on a case of severe mechanical vibration coming from the blast fan assembly of the 1975 Model Volkswagen Beetle owned by the author, identifying the root cause of such vibration and the corrective actions taken.

3. The Volkswagen Beetle The Volkswagen Beetle was designed to be as simple as possible, thus making it reliable. Its air-cooled 1500cc 40 hp (25kW) engine proved especially effective in actions of the German “ Afrika Korps ” in Africa’s desert heat. This is due to the built in oil cooler.

4. Possible Sources of Vibration in Volkswagen Beetle Engines excitations that are inherent in the slider-crank mechanism of the engine itself, vibrations coming from the mating phenolic gears that drive the valves, vibrations coming from the valve clearances that may have been improperly set, vibrations caused by loosely mounted air cleaner assembly, and vibrations from the blast fan assembly. In this study, the excessive vibration was noticed at the blast fan even without taking the readings.

5. The Rear-Mounted Engine of the Volkswagen Beetle BLAST FAN SHROUD ALTERNATOR DRIVEN PULLEY 9.5x1175 V-BELT DRIVE PULLEY

6. Schematic Diagram Showing the Alternator and the Blast Fan PULLEY BLAST FAN 6202-2Z BALL BEARINGS BEARING HOUSING ALTERNATOR WINDINGS BEARING HOUSING

7. Vibration Readings Against IEEE Standard 1068 Paragraph 4.1.5.3 Vibration velocity readings are supposed to be taken on six directions on the alternator, three on the pulley side bearing location (vertical, horizontal and axial directions) and similarly three on the fan side bearing location using the BALMAC 205 Digital Vibration Meter. However, due to space limitations, only the vertical and horizontal directions on each side are accessible. Readings were taken at different engine speeds, the usual idle speed (1200rpm), 2000rpm, and 3000rpm. This was done by manipulating the accelerator adjustment screw until the desired engine speed is attained as displayed in the tachometer.

8. Average Vibration Velocity Readings ( zero-to-peak ) of the Alternator at Engine speed of 1200rpm (idling) - - - - - - - - - - 13.208 0.52 11.176 0.44 FAN - - - - - - - - - - 10.414 0.41 8.890 0.35 PULLEY mm/sec in/sec mm/sec in/sec mm/sec in/sec SIDE AXIAL AXIAL HOR HOR VERT VERT LOCATION

9. Average Vibration Velocity Readings ( zero-to-peak ) of the Alternator at Engine speed of 2000rpm - - - - - - - - - - 13.970 0.55 10.668 0.42 FAN - - - - - - - - - - 10.160 0.40 9.144 0.36 PULLEY mm/sec in/sec mm/sec in/sec mm/sec in/sec SIDE AXIAL AXIAL HOR HOR VERT VERT LOCATION

10. Average Vibration Velocity Readings ( zero-to-peak ) of the Alternator at Engine speed of 3000rpm - - - - - - - - - - 12.700 0.50 11.430 0.45 FAN - - - - - - - - - - 10.668 0.42 8.890 0.35 PULLEY mm/sec in/sec mm/sec in/sec mm/sec in/sec SIDE AXIAL AXIAL HOR HOR VERT VERT LOCATION

11. IEEE Standard 1068 Machinery Vibration Chart for Rotating Equipment 0.003 inch Large Machines 0.0025 inch Small and Medium Machines 0.150 inch 999 0.0025 inch Large Machines 0.002 inch Small and Medium Machines 0.150 inch 1000-1499 0.002 inch Large Machines 0.0015 inch Small and Medium Machines 0.100 inch 1500-2999 0.100 inch 0.001 inch 3000 and above (Zero-to-peak) (Peak-to-peak) (rpm) MAXIMUM VELOCITY MAXIMUM DISPLACEMENT MACHINE SPEED

12. IEEE Standards are good for a start until machine history is developed. However, the most efficient and reliable method for evaluating vibration severity is comparing the most recent overall reading versus previous readings for the same measurement. Once a history of readings is plotted out, a trend comparison between current and past readings are easier to analyze. The actual vibration velocity values show higher readings on the horizontal direction as compared to those on the vertical direction due to gravity. Both the vertical and horizontal values are roughly in the level of four to five times the IEEE standard. The vibration can be ranked as severe as confirmed by the alternator malfunction indicator lamp on the dashboard. This lamp indicates that there is insufficient charging from the alternator.

13. Measuring Vibration Velocity on the Pulley Side of the Alternator in the Vertical Direction BALMAC 205 VIBRATION METER MAGNETIC PROBE VERTICAL MEASUREMENT PULLEY SIDE

14. The Root Cause of the Severe Mechanical Vibration on the Alternator Identifying the root cause of the vibration is easier using a Fast Fourier Transform analyzer such as the SKF Microlog Vibration Analyzer. A Fast Fourier Transform analyzer converts a vibration signal in time domain to a vibration signature in the frequency domain using algorithms. Since most of the machine vibration wave forms are close to sine waves, observing the frequency at which the vibration occurs is an effective guide as to the cause of the excitation. Since only a digital vibration meter is available, we are limited to trend plots and analysis will have to be through “ Kepner-Tregoe ” problem analysis tool and common sense. The simple design of the Volkswagen Beetle’s blast fan yields to the easy identification of the root cause.

15. The possible causes of excessive vibration in the blast fan assembly are unbalance, misalignment, bent shaft, mechanical looseness, and bearing defects. In identifying which of these factors is the contributor to the vibration problem, a systematic tool of prioritizing based on a risk priority number, which of the items are to be verified first.

16. Prioritizing Among Which Possible Cause of Vibration to Check Very likely to occur 63 5 9 7 Visual Check Bearing Defects Checked vs. bearing 49 5 7 7 Measurement Mechanical Looseness Unlikely to occur 14 2 2 7 Mount on lathe Bent Shaft Easy to Check 49 8 7 7 Visual Check Misalignment Visual Check Unlikely to occur 14 2 2 7 Balancing Unbalance (SxO) REMARKS RPN D O S METRICS POSSIBLE CAUSES

17. where; Metric – the test made to detect or measure the cause, effect or failure. Severity (S) – an assessment of the seriousness of the effect related to a qualitative scale of 1 to 10. Occurrence (O) – the likelihood that a specific cause exists. Detection (D) – is the assessment of the ability of the possible cause to be detected. Risk Priority Number (RPN) – the product of the severity and occurrence rankings. The RPN can be used to compare overall risk. Although bearing defect has the highest Risk Priority Number , the pulley alignment can be visually checked even before removing the bearings. Visual inspection confirmed that the pulleys were well aligned. Meanwhile, mechanical looseness can be checked upon inspecting the bearings.

18. The V-belt that drives the alternator was removed. Inspection on the alternator without the belt revealed that there was considerable mechanical looseness on the 6202-2Z deep-groove ball bearing on the pulley side. It was also confirmed later that the bearing on the fan side was also defective. In fact, there are instances when the alternator rotor rubs against some portions on the stator when the shaft was turned by hand without the belt. This is the condition that restricts the motion of the alternator. The defective bearing is only an effect of the root cause of the vibration problem.

19. Possible causes of bearing failure are ineffective lubrication, overloading, improper installation, defective bearing seats on shaft and in housing, vibration while bearing is not rotating, and passage of electric current through the bearing. Further inspection during the disassembly of the alternator showed that the bearing seat on the stationary housing on both sides of the alternator is defective. The bearing seat was already loose versus the outer race of a new 6202-2Z ball bearing by as much as 0.010” as measured by a micrometer.

20. Measurements on Actual Housing Fits on the Alternator Bearings (6202-2Z) Defective Fit 1.3786 1.3780 1.386 1.378 Fan Side Defective Fit 1.3786 1.3780 1.389 1.378 Pulley Side Inches Inches Inches Inches Maximum Minimum Maximum Minimum REMARKS SPECIFIED BORE SPECIFIED BORE HOUSING BORE HOUSING BORE LOCATION

21. Measurements on Actual Shaft Fits on the Alternator Bearings (6202-2Z) Acceptable Fit 0.5909 0.5906 0.591 0.591 Fan Side Acceptable Fit 0.5909 0.5906 0.5905 0.5905 Pulley Side Inches Inches Inches Inches Maximum Minimum Maximum Minimum REMARKS SPECIFIED DIAMETER SPECIFIED DIAMETER SHAFT DIAMETER SHAFT DIAMETER LOCATION

22. Corrections Done on the Loose Bearing Housing Fit of the Alternator The loose stationary housing bore was sent to the machine shop for re-machining of the bore to oversize it to an inside diameter of 1.500 to 1.5005 inches while maintaining its depth of 0.432 inch. The step on the housing perimeter was made reference in locating the exact center during the process of re-machining the bore in an engine lathe. An aluminum sleeve measuring 1.5000 to 1.5005 inches outside diameter and 1.3780 to 1.3786 inches inside diameter and 0.432 wide was fabricated and then inserted into the re-machined housing bore. With the aluminum sleeve pressed into the oversized bore, the resulting dimension was then in proper fit with the outside diameter of the 6202 deep groove ball bearing.

23. This was done to both stationary housing bores both on the pulley and on the blast fan side. The defective 6202 ball bearings were replaced with new ones. The alternator was then reassembled.

24. Average Vibration Velocity Readings ( zero-to-peak ) of the Alternator at Engine speed of 1200rpm (idling) after Correction - - - - - - - - - - 1.778 0.07 1.651 0.065 FAN - - - - - - - - - - 2.159 0.085 1.778 0.070 PULLEY mm/sec in/sec mm/sec in/sec mm/sec in/sec SIDE AXIAL AXIAL HOR HOR VERT VERT LOCATION

25. Average Vibration Velocity Readings ( zero-to-peak ) of the Alternator at Engine speed of 2000rpm after Correction - - - - - - - - - - 1.651 0.065 1.143 0.045 FAN - - - - - - - - - - 1.651 0.065 1.524 0.060 PULLEY mm/sec in/sec mm/sec in/sec mm/sec in/sec SIDE AXIAL AXIAL HOR HOR VERT VERT LOCATION

26. Average Vibration Velocity Readings ( zero-to-peak ) of the Alternator at Engine speed of 3000rpm after Correction - - - - - - - - - - 1.524 0.060 1.379 0.055 FAN - - - - - - - - - - 1.778 0.070 1.651 0.065 PULLEY mm/sec in/sec mm/sec in/sec mm/sec in/sec SIDE AXIAL AXIAL HOR HOR VERT VERT LOCATION

27. Conclusions Results show that after correcting the mechanical looseness on the alternator bearings the vibration velocity readings reduced drastically. Now the readings were within the acceptable levels of the IEEE Standard 1068. This means that dynamic balancing the blast fan impeller is no longer necessary.

28. Recommendation A consistent condition monitoring of the vibration readings can generate a trend plot showing the history of readings making this a convenient reference in predicting the health condition of this piece of rotating machinery. A horizontal line with a magnitude of 1.5 times the recommended zero-to-peak vibration velocity standard can be incorporated in such graph to represent the “warning” level.