Chapter 4 Test Facilities
4.4.2 Oil Degradation Tests
Three different oil degradation tests were seeded, water contaminated oil, oil viscosity changes and less lubrication referred to as oil level changes. In the following the procedure followed and value used for each fault type when such faults were tested.
4.4.2.1 Water Contaminated Oil
As stated in [129] the typical acceptable water content for transmission oils are in the range of 1 to 2% (10 to 20 kppm). Therefore, the contamination levels tested were derived from values both below and above this range, which allows a variety of different underlying measurements to be examined in a wide range for defining their corresponding detection performances. Tests were performed using four incremental water contents: 4.0kppm, 7.0kppm, 20kppm, 30.0kppm, and 60.0kppm, which respectively correspond to 0.4%, 0.7%, 3% and 6% water content of the name plate oil quantity.
4.4.2.2 Oil with Different Viscosities
The British Standard PD ISO/TR 18792:2008 [180] details guidelines on the selection of the lubricant viscosities for different types of gears while the BS 4231:1992 ISO 3448: 1992 [181] provides viscosity classification for industrial liquid lubricants. However, as stated in the American National Standard Institute/American Gear
Manufacturers Association standard number (ANSI/AGMA 9005-E02) “the
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selecting a gear lubricant.” Also, standards request manufacturers to provide general
information about the recommended oil suppliers.
The viscosity is principally specified during the design stage. Generally the speed at the pitch line of gears is used for determining the required viscosity. Hence, the viscosity can be derived using the following empirical equation [182]:
= . (8.7)
Where is the lube kinematic viscosity at 40 oC in (cSt) and V is the operating pitch
line velocity in (ft/min) and can be derived from [182]:
= .
(8.8)
where d is the operating pitch diameter in inches, and n is the pinion speed in (rev/min).
For the gearbox utilised in this study the manufacturer recommends the EP 320, with Millers Oil Ltd one of the recommended suppliers. Consequently the oil used for regular operation in this study is the EP 320 from this supplier. However, this can be confirmed from the standards and Equations 8.7 and 8.8. The two stage helical gearbox has two pinions; the maximum input shaft speed is 1500 rpm transferred into the second shaft by a rate of (1.234). Hence the highest speed of the second shaft is 1851.1 rpm. Considering the highest speed side pinion and from the gearbox specifications, the pitch line diameter is d=1.275 inches hence:
V=0.262*1.275*1851.1=618.36
The required viscosity at 40 oC therefore is:
= / . . = . S
By following guidelines in the BS 4231:1992 ISO 3448:1992, the recommended viscosity recommended therefore is ISO VG 320, i.e. EP320.
To study the effect of varying oil viscosity in a gearbox, four different oils having different viscosities, EP 100, EP 320, EP650 and EP 1000 respectively, were tested. The gearbox manufacturer recommends the EP 320. The EP 650 was made in the laboratory by mixing 61% of EP 1000 with 39% of EP 100. While the other types have
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been provided from a supplier who was recommended by the gearbox manufacturer, with key specifications listed in Table 12.
4.4.2.3 Oil with Different Levels
Three different oil volumes were tested, namely the standard level (BL = 2.6 litres), 600ml less (LL-600 = 77% of the full recommended volume) and 1100ml less (LL- 1100 = 57.7% of the full recommended volume).
4.4.2.4 Oil Degradation Test Procedures
Each set of oil tests have been performed separately. However, tests have been carried out with the same test profiles and procedures. This allows variation of different underlying measurements to examine a wide range of faults, with different severities, for defining the corresponding detection methods. Oil degradation tests were carried out on GB1.
The test rig was firstly aligned at the lowest possible misalignment using the dial indicator (this was at 0.04±0.01 mm). Oil is added and removed from GB1 without affecting the alignment condition using the top and drainage holes on the gearbox. The rig is operated at three speed settings: 50%, 75% and 100% of the full motor speed, under four incremental load settings: 0%, 30%, 70%, and 100% of the system full load for each speed cycle. This was aimed at investigation of the detection under variable speed and load operations, which are common scenarios in real applications. Each load setting operated for a period of two minutes and was automatically changed to the next step by the PLC controller. In total, each load cycle lasted 8 minutes. The VSD was set
Table 12. Specifications of the oils used for tests.
Oil type Specific Gravity (at 15°C) Kinematic Viscosity at (100°C, c.St) Kinematic Viscosity at (40°C, c.St) Viscosity Index Pour Point (°C) Flash Point (°C) EP 100 0.885 10.95 100 93 -9 200 EP 320 0.901 23.5 320 92 -9 200 EP 1000 0.927 71.0 1000 140 -6 200
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under sensorless control mode for evaluating the detection capability under this particular mode.
To ensure data quality and reliability, each speed/load cycle was operated five times consecutively for each oil type. During these repeated operations, the lube temperature in GB1 and GB2 was observed and recorded on-line and reached to around 43°C-45°C when the system operating parameters also became stabilised. Thirty seconds length of dynamic data was collected at every load setting. In the meantime, the static data from the VSD was also logged for the entire speed/load cycle. In addition samples of oil were taken after each oil viscosity and water in oil test to measure their viscosity values.
Data Processing
Dynamic data, i.e. vibration, current, voltage and power signals from sensors, is converted to the frequency domain by implementing the Fast Fourier transform in the Matlab environment. The principle of computing fast Fourier transform (FFT) in a Matlab programme is based on:
= ∫∞ −
−∞ (4.1)
= ∫−∞∞ (4.2)
where i= 0, 1, 2, …..n, n number of samples.
The FFT decomposes a series of values into different frequency elements. It is obtained by the following formula:
= [ ] = ∑+∞
−∞ −
(4.3) The performance of each signal is then examined by extracting the amplitudes at the corresponding feature frequencies. Amplitudes are then compared for detecting any changes that may occur due to faults.
To investigate vibration changes, vibration signals are applied with a time synchronous average (TSA) procedure and subsequent order spectrum analysis to suppress noise influences which are not time aligned to the second shaft, and hence to obtain vibration
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which is more associated with gear meshing dynamics. Brief information on the TSA and order analysis is given in Appendix I. The TSA algorithm is developed by one of research groups at the Centre for Efficiency and Performance Engineering in the University of Huddersfield. Thanks to Dr. Fungshou Gu the algorithm was made available for this study. Meanwhile a direct comparison of the static data is made between the baseline and different cases to examine their detection performance in line with the results from dynamic data.