Extreme pressure oil additives can destroy worm-gears!

Extreme pressure oil additives are vital for the effective performance of any Gear oil, but have you ever thought of the negative effects these have; or even what damage they cause to certain parts? Extreme pressure oil additives offer a wide range of benefits, but in some circumstances they harm the equipment they are supposed to protect.

A good example of extreme pressure oil additives causing damage to a gearbox:

A very common industrial transmission is the worm gearbox. Commonly,
these have gears composed of yellow metal (typically bronze). Certain extreme pressure oil additives can chemically react with these softer metals, causing premature wear and even failure.

Worm gearboxes are mainly comprised of two units: the worm and the worm wheel. The worm is what actually drives the worm wheel. It is a rod with a helical ridge on its surface that allows it to mesh with the teeth of the worm wheel to provide rotary motion.

These gearboxes are great for achieving high reduction ratios as well as high torque. And in order to increase either of these values, the worm wheel is made larger in diameter. The larger the circumference the worm wheel has, the greater the speed reduction and the greater the torque imparted through the exit shaft.

Obviously the loading and pressure on these components can be very high, prompting many people to opt for extreme pressure oil additives.

The reason certain extreme pressure oil additives cannot be used for this application.

Generally, the worm is made of steel, while the worm wheel is made of a yellow metal. However, in some cases, both the worm and worm wheel are steel; or they both may be yellow metals: And the worm is always harder than the wheel.

Yellow metals, as the name suggests, are yellowish in color. They are alloys that contain copper. A standard definition would be a type of brass having about 60 percent copper and 40 percent zinc. Bronze is another type of yellow metal. These metals have been used for centuries to form gears and other components of simple machines.

The problem is that many extreme pressure oil additives contain a form of Sulphur, which will react with the yellow metal.

An easy way to determine which form of sulfur is being utilized in your extreme pressure oil additives

Two different types of sulfur may be used within these additives. The first type is active sulfur. Sulfur in its active state readily reacts with metal surfaces to form a ductile metal soap that is sacrificial and allows opposing surfaces to contact one another with minimal damage. Active sulfur is chemically aggressive, and with yellow metals being softer than steel, they can begin to pit and form spalls due to this chemical attack.

The second type of sulfur used within extreme pressure oil additives is inactive sulfur. It is less likely to bond to surfaces and react chemically.

The best way to judge whether the form of sulphur in the extreme pressure oil additives is likely to cause damage, is to look at the results of the copper strip corrosion test (ASTM D130). In this test, a strip of copper is immersed in the fluid to be tested at 40 degrees C and again at 100 degrees C. The strip is removed after each test and checked for staining of the copper. The results range from, very little to no staining, all the way to very dark stains. If the results are in the darkish area, then the yellow metals in your worm gearboxes could be at risk of chemical attack.

Rising temperatures can increase the rate at which this reaction takes place. This is explained by the Arrhenius rate rule, which states that the rate of a chemical reaction doubles for every increase of 10 degrees C (18 degrees F) in oil operation temperature.

Active sulfur in some extreme pressure oil additives react with the copper within the brass or bronze. Sulfur, when in contact with copper along with the presence of heat, forms copper sulfide.

This simple chemical reaction can have devastating repercussions on the reliability of machines. In extreme pressure situations, copper disulfide can be formed. Both of these crystalline forms of copper are very hard and can cause abrasive damage to soft machine surfaces.

By understanding some simple chemistry and reading the product data sheets of the EP gear oil you put into your gearboxes, you can increase reliability.

For peace of mind, contact Habot Synthetic Lubricants about selecting synthetic gear oils. Or, if you have any questions about extreme pressure oil additives in gear oils.