Designers and engineers can choose to use zinc-based hydraulic fluids because of their ability to resist oxidization and wear, as well as their performance. However, there is a belief within the manufacturing community that using zinc-based hydraulic fluids can lead to an increase in the production of varnish and sludge formation. This will then increase the maintenance costs associated with the equipment that uses the zinc-based hydraulic fluid as filters can become blocked and decrease the lifespan of the oil.

There are some specific tests that have been created in order to discover which oils allow sludge to form. Some of these include:

  • The thermal stability test (ASTM D2070)
  • The high-temperature Indiana stirring oxidation test (ISOT)
  • The Bosch Rexroth A2F10 piston pump test (JCMAS P 045)

Until recently, a lot of zinc-based hydraulic oils haven’t performed particularly well in these examinations, particularly the Bosch one. Shell’s engineers have used these various tests to show that their new product, a type of zinc-based hydraulic fluid, can decrease the formation of sludge and prevent oxidization and wear.

Sludge can adversely affect equipment, particularly engines that run with such low tolerances. Source: Adobe StockSludge can adversely affect equipment, particularly engines that run with such low tolerances. Source: Adobe Stock

What is it made of?

This new fluid is created from Gas-to-Liquids (GTL) technology. It changes natural gas, which happens to be the fossil fuel that burns the cleanest, into a quality product that would be made from crude oil in other situations. Products that are created using GTL are odorless and colorless, and also have almost none of the impurities that is typically found in crude oil, like nitrogen, sulfur and aromatics.

Other products made using GTL include hydraulic and motor oil, fuel for transportation, and certain ingredients that go into general everyday items that are made from detergents, cosmetics and plastics.

The tests that this new oil had to pass were rigorous and plentiful, and it was mandatory to pass all of them in order to prove that it can reduce varnish and sludge formation and lead to a longer lifespan for the oil. These tests focused on testing the oil in situations where zinc-based oils have fallen down in the past.

A2F10 piston pump test

The A2F10 piston pump test from Bosch examined the hydraulic fluid’s oxidation stability while under operation conditions. Mobile equipment’s operation life is generally based on this test and it can accurately calculate an oil’s resistance to sludge formation and is one of the few pump tests that is able to do that. If a zinc-based hydraulic fluid performs well in this test, it would indicate that the formula of the liquid impacts sludge formation.

The test is carried out by sending a pressurized fluid at up to 5000 psi (350 bar) through a relief valve. The testing tank holds 13 L of oil at 80° C, and fluid oxidation is accelerated by a copper specimen in the tank. Oxidation is also helped by air that is pumped in at 1 L/hr.

Samples of the fluid were taken at regular intervals and examined for parameters that include an increase in the acid number and foaming, viscosity, sludge levels and viscosity change. The volume of the fluid reduces over time; therefore the fluid’s oxidative stress artificially increases.

The new zinc-based hydraulic fluid from Shell not only passed but surpassed the A2F10 test even after 2,00 hours of testing, which would give an estimate of 9,600 hours (or more) for the operational lifespan of the oil.

High temperature test

The Indiana stirring high temperature oxidation test evaluates the fluid’s sludge formation and oxidation features while under conditions specifically set to promote oxidation. It blows in hot (165° C) dry air along with catalysts for oxidation. Samples of the fluid are passed through paper filters, and from this the sludge build up is calculated.

To pass this test a fluid must have great, if not excellent, thermal stability.

Compressor lubricants are one of the fluids that are typically tested using this method. Fluids that are zinc-free have always performed better in the past and produce less sludge, so this test is specifically referred to as one that is favorable for “zinc-free” fluids.

Thermal stability test

The ASTM D2070 test is one that examines an oil’s ability to not break down under very high temperatures. A sample of the fluid in question will have catalysts added to it and heated to above 1,350° C. No oxygen or air is added in to help oxidation, and the sample will not be agitated, just heated. The sludge deposits are weighed after the test along with the weight lost by the catalysts.

This specific test is used to recreate conditions where corrosion and wear can be a problem because of the steel and copper surfaces coming into contact and rubbing off of one another. It is classed as an important test as it can identify a certain lubricant’s ability to resist sludge, which is very important in applications such as bronze slipper pads in axial piston pumps.

Varnish test

This is a test that Shell carries out in-house and is proprietary to Shell. It speeds up the formation of varnish on aluminum rods with temperature cycling. This was seen as a must-have assay by Shell so they created it themselves. The rods are weighed both before and after the test, and the difference is the amount of varnish that formed. Varnish can wear vital hydraulic parts prematurely and cause failures in equipment. It may also cause some systems to overheat, make their valves stick, and plug filters requiring costly maintenance. It is very important to prevent varnish formation before it occurs rather than deal with the consequences afterwards.

After going through all of these tests and more, it was determined that Shell’s new GTL technology can be utilized to manufacture effective and efficient zinc-based hydraulic fluids.

So, what do you think of this new zinc-based hydraulic fluid from Shell? Will it be used by the major equipment manufacturers, and in turn, increase the lifespan of their equipment? Engineering360 would love to hear your thoughts and opinions in the comments below!

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