Q & A
"I suffer cold start-up problems in the morning with respect to the filters. How can I overcome this?"

Typically, the problems associated with cold start-ups include the filter differential pressure exceeding the limit owing the higher viscosity to the cooler oil. This results in the pressure differentialindicator tripping and the filter going into bypass mode allowing unfiltered oil to pass through.

If the system has a very short warm-up period, and there are no critical components, it may be worth considering allowing the filter to go into bypass mode for a short period, but remember to install an indicator that either has a thermal lockout or has an automatic reset function. As the filter blocks-up in the normal course of events, the period of bypass will increase, and this can be monitored by the length of time the auto-reset takes to activate. However, regular particle counting is essential to highlight a problem with the bypass valve sticking in the open position.

However, on critical systems, possibly at an outdoor hydraulic plant with a longer warm-up period, ensure the filter is selected according to the worst-case viscosity conditions as a result of the lowest ambient temperature likely to be encountered. This will result in a larger than required element at operating conditions, it should result in a longer element life requiring less frequent changes.

Today's Tip

Avoid the use of suction line filters because they can cause problems such as cavitation in the pump. However, ensure that the suction line at least has a protection strainer of 200 µm rating to help keep small hard objects from damaging or jamming the pump. Always check with the pump manufacturer before fitting any devices immediately upstream of the pump as these can possibly affect the performance of the pump and cause problems downstream in work-end components.

Book Bits

From the book "Wear Debris Analysis":
Trend Analysis is the process by which a change in machine condition is determined from an examination of changes in specific sensors or output.

Different systems exhibit different trends. For instance, the wear debris generated from a system involving gears and bearings may well be quite high initially, particularly if no attempt has been made to 'run-in' the system. Thus there is a gradually reducing level of debris until the system settles in. Then comes the acceptable very low wear rate associated with well lubricated surfaces, which perhaps increases just slightly. Finally, the machine begins to show signs of fatigue or fracture and particle generation increases at an ever increasing rate. These three stages of wear are shown in the classical 'Bath-tub' curve.

Another changing feature of wear with time, is that the wear rate begins to fluctuate. After the 'running-in' period, the wear rate is expected to be reasonably uniform, although it may rise slightly. However, as conditions begin to become less favorable wear may be less or more in terms of particle size generation as well as with total quantity. Scatter is thus seen as a trend indicator.

The third sort of trend indication is that associated with the shape of particle - its morphology. This is less easy to measure than the gravimetric level or 'size' of particulate; but, by using the analysis mentioned later in this chapter for defining particle shape, two or more possible specific indicators of shape may be trended to detect the point at which a serious change in debris particle shape occurs.

Q & A

"About a year ago we ran a lubricant too long in a high-duty gearbox and it oxidized and threw sludge. Ever since that incident, we've been watching the oil more closely with oil analysis. My problem is that we are now seeing acid numbers rise and oil darken after only one month of service--the lubricant used to last a full year. We keep changing the oil but the problem just repeats. Why does our gear oil have such such a short life?"

Its sounds like the gearbox was not thoroughly flushed after the oil oxidized the first time. Typically, a simple drain will leave more than 15 percent of the old oil behind - occluding to machine surfaces and trapped within the casing. This also leaves a host of reactive chemicals (pro-oxidants) behind that rapidly deplete antioxidant additives, leaving the base oil unprotected.

You refer to the gearbox as high duty, which probably means high temperature and high wear metal production. The temperature and wear particles also accelerate the rate of oxidation, especially when sludge and other pro-oxidants are in the mix. I suggest you do a thorough flushing of the gearbox. You might refer to the article on flushing that will be featured in the July - August issue of Machinery Lubrication magazine.

Today's Tip

Things that can extend the oil drain interval in engines: high oil consumption (makeup), synthetics, highway miles, clean oil, dry oil, low sulfur fuels, low blowby, minimal idling and lugging, few stops/starts, no coolant leaks, properly serviced air, fuel, oil and sump filters, and proper rack and air/fuel settings.

Book Bits

From the book "Turbine Lubrication in the 21st Century":
For turbine oils, the greatest advantage Group II finished oils provide over Group I oils is superior oxidation stability. Severe hydroprocessing of feedstocks greatly reduces the aromatic, sulfur and nitrogen-containing "impurities" to give virtually impurity-free Group II base oils. These impurities are responsible for the lower oxidation stability of Group I stocks because they readily oxidize. These impurities oxidize to form benzylic, sulfuric, and nitric acids and other oxidized species. Group II base oils typically contain only trace amounts of these impurities and the finished oils can exhibit outstanding oxidation stability properties when formulated with the proper additives. The superior oxidation stability of Group II finished turbine oils is demonstrated in their RPVOT and TOST values.

For gas turbines, such as the General Electric (GE) Frame 7 series, a turbine oil with excellent oxidation stability is very desirable. The GE Frame 7 series has three journal bearings with a center bearing that can be subjected to 500 degrees Fahrenheit. Since a turbine oil also acts as a coolant, it will be subjected to high temperatures from the center bearing and it must be oxidatively and thermally stable. A turbine oil with excellent oxidation stability is necessary in many gas turbine applications.

Q & A

"Apart from the data on the multi-pass filter test, what other ISO standards exist for the evaluation of filter performance that I can use to objectively select a filter?"

The ISO standard for multi-pass testing (ISO 16889) has recently changed to require filter manufacturers to publish their data for the Beta Ratios across a wide size range, that is, at 2, 20, 75, 100, 200 and 1000 microns. Historically filter beta ratios were published at either 75 or 200 with an absolute rating (per ISO 4572). The new standard gives a better interpretation of the filter's overall performance, and can be better compared against competitor offerings.

In addition to the new multi-pass data, consider the following data:

ISO 2941 - 1974 - Verification of collapse/burst resistance.

ISO 2942 - 1994 - Verification of fabrication integrity and determination of the first bubble point.

ISO 2943 - 1998 - Verification of material compatibility with fluids.

ISO 3723 - 1976 - Method for end load test.

ISO 3724 - 1976 - Verification of flow fatigue characteristics.

ISO 3968 - 1981 - Evaluation of pressure drop versus flow characteristics.

These additional tests are useful for comparison where the installation may be an unusual lubricant, an extreme contaminant sensitive workpiece, a high-pressure fluid power system or a safety critical application. When selecting for a noncritical system with no unusual requirements, evaluation of filter suppliers the multi-pass data, in addition to other commercial criteria, will normally suffice.

Today's Tip

Always ensure the correct lifting and carrying equipment is available. Avoid lowering the barrel onto any small, sharp objects. These objects can pierce the barrel and cause leakage, that if unnoticed, could pose a safety risk to the person moving the barrel as well as other colleagues.

Book Bits

From the book "Synthetic Lubricants and High Performance Functional Fluids":
The use of PAO-based gear oils in industrial settings can lead to important savings in energy consumption, as well as decreased downtime and lower maintenance requirements. The wide range of operating temperatures allows the use of less viscous oils, which results in greater energy efficiency. The relatively low coefficient of friction for PAOs reduces the amount of internal friction created by the normal shearing of an oil film during operation.

Improved scuffing performance for gear/circulating oils has been demonstrated by Jackson et al., who studied the influence of lubricant traction characteristics on the load at which scuffing occurs. The study compared low traction PAO-based lubricants with mineral oils in additive-free, antiwear, and extreme pressure (EP) formulations. Benefits of 25-220% were observed for the PAO-based synthetic lubricants over mineral oils. The investigators found that low traction PAO-based lubricants uniformly gave higher scuffing loads per unit width than the mineral-based fluids tested at both high and low specific film thickness. PAO-based gear and circulating oils outperformed mineral oil-based gear and circulating oils, respectively. PAOs were also shown to be very responsive to additives.

The advantages of PAOs as lubricants in conveyor applications has been demonstrated by Paton et al. Gearboxes lubricated with a fully synthetic poly(a-olefin)-based gear oil (75W-90) was studied. An all-season PAO-based fluid was chosen for pulley shaft bearing lubrication.