Avoiding the Bubbles

Date: 17.10.19

Close collaboration between Trelleborg Sealing Solutions and the manufacturers of lubricants is vital. In this article, Ulf Rieper, who is OEM Manager at Shell Lubricants, discusses the development they have undertaken to avoid aeration of lubricant in compact hydraulic systems. -By ULF RIEPER, OEM Manager at Shell Lubricant

TODAY’S MORE COMPACT MOBILE FLUID POWER SYSTEMS

have smaller hydraulic fluid reservoirs and can be more susceptible to aeration. Because the fluid volumes are low, reservoir residence times can be as short as 30 seconds, which leaves little time for air release. Fluid aeration reduces the efficiency and responsiveness of hydraulic systems, as well as increasing their noise. Collapsing air bubbles on the high-pressure side can also cause erosion, which can lead to pump failure and unplanned downtime. Aeration can also accelerate oil oxidation, thereby shortening oil life and contributing to poor precision in hydraulic operations.

It’s therefore important to understand the aeration characteristics of lubricants. Shell Lubricants evaluated four ISO viscosity grade 46 fluids formulated with different base oils; Group I mineral oil, Group II mineral oil, Group III gas-toliquids (GTL) synthetic fluid and Group IV polyalphaolefin (PAO) synthetic fluid.

In our tests, we showed that the gas-to-liquids synthetic fluid and the PAO synthetic fluid demonstrated a significantly faster air release than mineral-based oils, which resulted in less entrained air and up to eight percent greater volumetric efficiency. We also recorded a 50% reduction in perceived sound.

BIOGRAPHY

Ulf Rieper studied mechanical engineering at the Technical University of Hamburg. Since 1995, he has been employed at Shell Lubricants in various functions. In his current role of OEM Manager, he coordinates technical relations with OEMs in mechanical engineering and for industrial transmissions, hydraulic applications and greases in various sectors such as wind, general manufacturing, mobile applications and steel equipment. His main task is to transfer requirements from the markets to product development so that new products meet the requirements and receive approvals from key OEMs.

Group	Air Release Time, Minutes (ASTM D3427)
A I (mineral)	5.03
B II (mineral)	1.51
C III (GTL)	0.17
D IV (PAO)	0.17

Evaluating efficiency and noise

We used the ASTM standard test method D3427 to measure the air release properties of each fluid. In this test, compressed air was blown through the test oil at +50°C. The time required for finely dispersed air in the oil to decrease to 0.2% by weight was then measured using a density balance.

All the fluids evaluated were inside the 10-minute maximum air release time allowed by the industry standard, but they did not all perform equally well, as can be seen by the table below.

We then evaluated the performance of these fluids in a dynamometer fitted with a reservoir with an inlet aerator and an outlet mass flowmeter using a modified ISO 4409:2007 procedure.

The hydraulic circuit configuration, instrumentation and operating conditions were kept constant; only the fluids were changed. Twelve different combinations of speed, pressure and aeration state were evaluated a minimum of three times, which generated more than 150,000 data points. This large dataset ensured that the air ingress and release rates had reached equilibrium and the pump performance was in a steady state.

Greater efficiency

Although the mechanical efficiency of the pump slightly improved when the fluid was aerated, this improvement was less than the concurrent decrease in volumetric efficiency. So, aeration reduced the overall pump efficiency. As volumetric efficiency affects hydraulic power transmission and productivity, fluids with a lower air content should improve machine performance.

The average volumetric efficiency of the pump was approximately 93% when the aerator was off. With the aerator on, this efficiency decreased by 9.4 and 4.5% for mineral-oil-based fluids I and II respectively. Both syntheticbased fluids reduced the efficiency by 2.0%, which means that these have up to and eight percent greater volumetric efficiency than mineral-based oils.

Less noise

Fluid aeration causes cavitation and contributes to broadband noise generation by a pump. In our tests, pump sound emissions were evaluated at several speeds, at 10 cm away, with the fluid in aerated and non-aerated states. Aeration increased the broadband high-frequency sound levels to between 2,500 and 4,000 Hz, which accounts for the harsh noise associated with pump cavitation.

Group I mineral-based oil was the most susceptible to aeration and exhibited the largest sound increase relative to the baseline value. The mean sound level of the pump running with the fluid was about 6 dB(A) greater than that of the other fluids, which corresponds to a human perception of the noise being 50% louder.

KEY TAKEAWAYS

Hydraulic fluids formulated with GTL (Group III) and PAO (Group IV) base oils display very fast air release times in the standard air release test and low levels of aeration in the dynamometer.
Fluids formulated with Group l and Group II base oils met the standard requirements for air release time but exhibited high levels of aeration in the dynamometer.
GTL-based fluids have significantly faster air release compared with the mineral-based oils, which means less entrained air and up to 8% greater volumetric efficiency.
The mean sound level of the aerated Group I fluid was about 6 dB(A) greater than that of the three other fluids. This corresponds to a human perception of the noise being 50% louder.
GTL-based fluids such as Shell Tellus S4 offer the performance advantages of PAO-based fluids but more cost-effectively.

GTL VERSUS PAO – WHICH IS BETTER?

Fluids C and D, based on GTL (Group III) and PAO (Group IV) base oils respectively, performed equally well, so which is better? The answer comes down to simple economics. PAO oils are expensive to produce, so our ability to manufacture lubricants, such as Shell Tellus S4, formulated with GTL base oils on a global scale means that we can offer operators PAO benefits more cost effectively.

Optimum sealing in lubricants

“Trelleborg Sealing Solutions works in close collaboration with lubricant manufacturers, such as Shell. This is vitally important, because to ensure the extended life of a hydraulic system, the sealing material must be matched to the lubricant within the system.

“Shell’s recommendation to its customers to avoid any issues with aeration in compact hydraulic applications is to use a GTL-based fluid, such as Shell Tellus S4. We as a seal supplier must know which se aling materials will work best in this lubricant and have test results to back our conclusions up.

“The situation is complex when we consider the interaction of seal with lubricant. Looking at tribology, for instance, this is influenced by mating surfaces, oil (pressure medium) and the seal. We have a maximum of one third of the influence on the function of the tribology system, which is why a mutual exchange of information is imperative.

“In our latest sealing developments, Lubrication Management and Radial K, we use the seal’s interaction with pressure medium to reduce the wear on seals and mating surfaces. We can only achieve this by working hand-in-hand with lubricant suppliers.”

Holger Jordan, Director Global Technical Management for Trelleborg Sealing Solutions

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