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Equipment Reliability
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Rather than review for you what articles comprise this May issue, I'm going to let you explore for yourself. Just click on the links below. "Defining Fluid Temperature & Viscosity Limits for Maximum Hydraulic Component Life" - by Brendan Casey "EH, ED or RS?" (part 3) - by Wayne Tustin (this has to do with three kinds of shakers). You perhaps know that I teach about vibration and shock testing. And that I wrote a text "Random Vibration in Perspective" in 1984. I'm working hard to update that book, and hope to announce (in our August issue) a prepublication sale. Publication is scheduled for the Fall, so that we can ship your copy before December 31. The full title of the new book is "A minimal-mathematics introduction to the fundamentals of Random Vibration and Shock Testing, HALT, ESS and HASS, also Measurements, Analysis and Calibration, with applications in the fields of aeronautical, automotive, seismic and shipboard testing". Hope you enjoy this issue. Best wishes, |
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Defining
Fluid Temperature & Viscosity Limits for Maximum Hydraulic Component
Life Many factors can reduce the service life of hydraulic components. Incorrect fluid viscosity is one of these factors. To prevent low (or high) viscosity from cutting short component life, an appropriate fluid operating temperature and viscosity range must first be defined and then maintained on a continuous basis. Before I discuss this in detail, let me explain the interrelationship of fluid temperature and viscosity, and how they impact upon hydraulic component life. Temperature/Viscosity Relationship of
Hydraulic Fluid Manufacturers of hydraulic components publish permissible and optimal viscosity values, which can vary according to the type and construction of the component. As a general rule, operating viscosity should be maintained in the range of 100 to 16 centistokes (460 to 80 SUS), however viscosities as high as 1000 centistokes (4600 SUS) are permissible for short periods at start up. Optimum operating efficiency is achieved with fluid viscosity in the range of 36 to 16 centistokes (170 to 80 SUS) and maximum bearing life is achieved with a minimum viscosity of 25 centistokes (120 SUS). Hydraulic Fluid Viscosity Grades Determining the Correct Viscosity Grade
In most cases, the machine manufacturer will specify the correct viscosity grade. It is important to understand that the machine manufacturer's recommended viscosity grade should change as the ambient temperature conditions in which the machine operates change. I say this because several years ago I was involved in the analysis of several premature component failures from a mobile hydraulic machine. The machine was designed and built in the Northern Hemisphere, but was operating in high ambient air temperatures in the Southern Hemisphere. The components had failed due to inadequate lubrication, because of low fluid viscosity. Investigation revealed that the fluid in the system was ISO VG 32. While this viscosity grade is suitable for cooler climates found in parts of the Northern Hemisphere, it was not suitable for the high ambient temperatures in which this machine was operating. The machine owner confirmed that the manufacturer's fluid recommendation was indeed ISO VG 32. The machine manufacturer had not altered their fluid viscosity recommendation to take into account the higher ambient temperatures in which this particular machine was operating. This oversight resulted in several premature component failures because of low fluid viscosity. The machine manufacturer's viscosity grade recommendation can be checked using the viscosity/temperature diagram shown in exhibit 1, assuming the minimum starting temperature and the hydraulic system's maximum operating temperature are known. For example, let's consider an application where the minimum ambient temperature is 15°C, the system's maximum operating temperature is 75°C, the optimum viscosity range for the system's components is between 36 and 16 centistokes and the permissible, intermittent viscosity range is between 1000 and 10 centistokes.
Exhibit 1 From the viscosity/temperature diagram in exhibit 1 it can be seen that to maintain viscosity above the minimum, optimum value of 16 centistokes at 75°C, an ISO VG 68 fluid is required. At a starting temperature of 15°C, the viscosity of VG 68 fluid is 300 centistokes, which is within the maximum permissible limit of 1000 centistokes at start up. If the machine manufacturer's recommendation was ISO VG 32 fluid under the same conditions, I would question it. A word of warning here - do not change the fluid viscosity grade in a system without consulting the equipment manufacturer. Doing so may void the manufacturer's warranty and/or cause damage to the system's components. Defining Operating Temperature Limits By referring back to the viscosity/temperature curve for VG 68 fluid in exhibit 1, it can be seen that an optimum viscosity range of between 36 and 16 centistokes will be achieved with a fluid temperature range of between 55°C and 78°C. The minimum viscosity for optimum bearing life of 25 centistokes will be achieved at a temperature of 65°C. The permissible, intermittent viscosity limits of 1000 and 10 centistokes equate to fluid temperatures of 2°C and 90°C, respectively. Going back to our example, this means that with an ISO VG 68 fluid in the system, the optimum operating temperature is 65°C and maximum operating efficiency will be achieved by maintaining fluid temperature in the range of 55°C to 78°C. If cold start conditions at or below 2°C are expected, it will be necessary to preheat the fluid to avoid damage to system components. Intermittent fluid temperature in the hottest part of the system, which is usually the pump case, must not exceed 90°C. Note that fluid temperatures above 82°C (180°F) damage seals, reduce the service life of the hydraulic fluid and in most cases, will cause the viscosity limits of the fluid to be exceeded. This means that the operation of any hydraulic system at temperatures above 82°C (180°F) is detrimental and should be avoided. Preventing Damage Caused by High Temperature
Operation Continuing to operate a hydraulic system when the fluid is over-temperature is similar to operating an internal combustion engine with high coolant temperature. Damage is almost guaranteed. Therefore, whenever a hydraulic system starts to overheat, shut down the system, find the cause of the problem and fix it! Brendan Casey has more than 16 years
experience in the maintenance, repair and overhaul of mobile and
industrial hydraulic equipment. He is the founder of HydraulicSupermarket.com,
the most popular independent hydraulics web site on the Internet*
and the author of 'Insider Secrets to Hydraulics', the most comprehensive
guide to reducing hydraulic equipment operating costs ever published.
For more information on reducing the operating cost and increasing
the uptime of your hydraulic equipment, visit his website: www.InsiderSecretsToHydraulics.com
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EH, ED or RS?
(part 3) In the November 2002 newsletter I described EH or electrohydraulic (sometimes called servohydraulic) shakers. Then in February 03 I discussed ED or electrodynamic shakers, widely used for sine and random vibration (and for some shock) environmental testing. Now I will discuss pneumatic RS (repetitive shock) "bangers". These offer a less expensive way to obtain multi-axis broadband excitation for HALT and HASS. Pneumatic vibrators such as the unit shown in Figure 1 are used for HALT, ESS and for HASS. Operating off compressed air, these RS or repetitive-shock devices, often combined with fast-ramping thermal chambers, are widely used.
Figure 1 - Pneumatic vibrator Piston tips are often plastic. This somewhat limits the frequency range to say 5,000 Hz rather than the perhaps 25,000 Hz that would result from steel banging against steel. RS units are attached to the bottom (see Figure 2) of a softly-sprung horizontal platform, as in Figure 3. The devices to be tested or screened are attached topside.
Figure 2 - Pneumatic vibrators attached to platform The platform translates in X, Y and Z motions and rotates in a, b and g.
The purpose of hammering is to excite all possible resonances inside the unit being screened. In my "live" vibration and shock courses, I pass around the class the unit shown here as Figure 4. I encourage everyone to "rap" it with a knuckle and to note that each of the reeds (each has a different natural frequency) quivers briefly. This convinces participants that mechanical shock briefly excites all structural resonances. Let us know if you'd like us to attach a video clip to an e-mail message. These analyzers are available from H.H. Sticht Co. in New York, NY; see http://www.stichtco.thomasregister.com/olc/stichtco/reed.htm.
I strongly endorse ESS and HASS being conducted right in the production area, so that little time elapses between a worker soldering a connection (for instance) and learning that he/she "didn't do it right". We want the worker to get this "feedback" before soldering too many more connections.
Investigations (see Figure 5) directly on the tables of new RS machines show tremendous variations in RMS acceleration. Vertical (Z) axis readings were taken every 4 inches. No doubt the spectra also differ greatly from one location to the next. A fixture covering an appreciable table area probably has an averaging effect.
Non-coherence - chaos Table vibratory motion can be described as spatially non-coherent. I've heard it called "chaotic". There is little similarity between signals from accelerometers that are located close together. And little between axes. Acceleration magnitudes can vary 30:1. For the intended purposes of such tables, this seems tolerable. But it certainly surprises first time users of these platforms. Are these wide variations from location to location a reason to disdain RS machines in favor of ED shakers? I don't think so. In Chapter 15 of my 2003 text I will tell you of an ED shaker investigation of vibration intensity patterns. Dry sand was sprinkled on the table. It collected in nodal rings. Random vibration spectral detail on ED shakers also varies from location to location. Place an accelerometer close to each of the mounting points of a "black box" on an all-too-typical fixture on an all-too typical ED shaker. Compare the several spectra. You will find wide variations. Momentary peaks can reach 15x or 20x the indicated acceleration (usually shown in RMS g units), whereas random vibration tests on electrodynamic shakers rarely exceed 3 s (3 "sigma" clipping). Displacements are very small, usually invisible to the naked eye. Most energy is above 1,000 Hz. Highest frequencies can reach 10 kHz. 25 kHz is available on special order. Spectral peaks can reach 1g2/Hz. The "basic vibrator" operates at a fixed repetition rate, unfortunately producing a line spectrum rather than the continuous spectrum that many prefer. "Smearing" of the line spectrum is accomplished in different ways by the several HALT/ESS/HASS manufacturers. Some authorities prefer to use the shock response spectrum (SRS - beyond the scope of this article) to describe RS platform motion, rather than (1) the RMS g's mentioned in Figure 5 or (2) PSD/ASD (power spectral density/acceleration spectral density - also beyond the scope of this article). Certainly the resulting impulses contain mostly high frequency energy. Not much spectral adjustment is possible. Table motion has little velocity and little displacement. Manufacturers are trying to generate more low frequency force and motion. Some low-frequency energy appears as a sort of "beating" whenever two vibrators have similar repetition frequencies. Low-frequency content Wayne Tustin, ERI's president, can be reached by e-mail or phone (805) 564-1260. Read more about Wayne at ERI's website. |
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Questions our readers have asked... This section of our newsletter was created for you, reader! Feel free to send questions or suggestions to the webmaster. One of our specialists will respond. Q: How many accelerometers do I need?
Wayne Tustin, ERI's president, can be reached by e-mail or phone (805) 564-1260. Read more about Wayne at ERI's website. |
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Laurel, Maryland attendees From left to right: 1. Walt Ferguson of Hughes Network Systems, Germantown, MD; 2. Dan Mumma, SAIC, Sterling, VA; 3. Ben Robbins, NASA Wallops, VA; 4. Boiris Bayevsky, Talla-Com Industries at FL; 5. Brian Tribelhorn, CSIR, South Africa and Wayne Tustin on the bottom center. |
Hello,
everyone!
Figure
3 - Softly-sprung platform
Figure
4 - Mechanical spectrum analyzer
Figure
5 - RMS g varies greatly 
| Laurel-MD class |
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Free web-based training coming up |
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| Electronic Failures |
Sources of contamination and the adverse effects of contamination on high-rel electronic systems are some of the topics that will be discussed in David Douthit's next course. "Contaminants and Moisture can Disrupt your Electronics" will meet June 16-18, 2003. |
| Electronic Circuit Cards |
A three-day interactive workshop aimed at shortening the time required for electronics design, vibration testing and (when weaknesses are found) corrective action will meet next June 23-25. "Optimizing HALT, ESS and HASS of Electronic Circuit Cards" will be presented again by John Starr at Santa Barbara, CA. |
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Vibration and Shock in Switzerland |
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On the same dates, Wayne will offer similar training at Newport, Rhode Island. Details can be found on our websites. |
| Vibration and Shock courses coming up |
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Bohemia,
New York, Seattle, Washington, August 12-14, 2003 Santa Barbara, California, August 26-28, 2003 Detroit,
Michigan |
| Announcements |
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AST Meeting Shock and Vibration Symposium Reliability Symposium |
| Contact information |
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Wayne Tustin tustin@equipment- Webmaster webmaster@equipment Websites Copyright © 2000-2003 Equipment Reliability Institute. All rights reserved. |
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