Ask the Expert

the Expert

Q:

Can you provide an explanation regarding the temperature capability of some standard high temperature greases and how to interpret and compare the values typically listed for each product? Drop point, Flash point, maximum operating temperature. There is some debate here about how to apply a product and what the different product descriptors indicate.

A:

The maximum operating temperature of lubricating greases is reported differently by different suppliers. First, we will discuss the terms that you have listed:

Dropping Point is basically the temperature at which the thickener in a grease melts, as determined in various test methods. The melting of the thickener results in the grease becoming fluid enough to run out of the dropping point test cup. Greases should not be used at temperatures approaching the dropping point.

Flash Point is a property of the fluid component of a grease. The flash point is the temperature at which the vapors of the fluid momentarily ignite when an ignition source is applied. The flash point is typically well above the operating temperature of the grease.

The maximum operating temperature of a grease is typically determined by how the grease performs in bearing tests at elevated temperature. Such tests include ASTM D3336, ASTM D3527, FAG FE-9, etc.

It is recommended that you discuss the specific application and temperature range with your grease supplier so that the supplier understands the requirements and can supply a product with the appropriate temperature range capabilities for the operating conditions.

Q:

What types of greases are used in the following mills: steel mills, lumber mills, sugar mills & paper mills?

A:

Steel mills – The steel industry originally used clay greases, as they were the only high-temperature products of the time. The industry moved to aluminum complex when it was introduced. Some steel mills continue to use aluminum complex grease, while others have moved to lithium complex products for general lubrication. Some applications use polyurea greases with synthetic base fluid.

Lumber mills – This application does not have the extreme conditions found in the steel or paper industries. The lumber industry has typically used lithium, lithium/ calcium, or lithium complex greases.

Sugar mills – The sugar industry is characterized by heavy-loaded, slow-moving plain bearings. The greases used in the sugar industry have historically been calcium soap or clay thickened, with high (ISO 1000 or higher) base oil viscosity and lubricating solids, such as molybdenum disulfide.

Paper mills – The typical paper machine has both high water (wet end) and high temperature (dry end) applications. In general, lithium complex greases are used, while some specialized applications require products such as polyurea grease with a synthetic base fluid.

Q:

What type of grease is the thickest but still is a lubricant to take up or make up the gear mesh?

A:

The use of a grease in place of gear oil in a worm gear will not make up for excessive slack in the gear set.

Although grease has consistency (stiffness), it will still flow under conditions of shear, and will not fill the gap in a poorly matched worm and wheel set. Worms and their mating wheels are typically machined together as a matching set. If the slack between the worm and wheel is such that vibration or shudder is present when the gearbox is operated, the gearbox should be replaced or the worm/wheel pair re-machined to make them match better.

Most worm gear units are lubricated with a gear oil that is specifically formulated for that application. Lubricants for worm gears include high viscosity (ISO 460 or 680 viscosity grades) compounded mineral oils, synthetic non-EP (non extreme pressure) lubricants based on polyalphaolefin (PAO) fluids in the ISO 220, 320, or 460 viscosity grades, or synthetic non-EP lubricants based on polyalkylene glycol (PAG) fluids in the ISO 150, 220, 320, 460, or 680 viscosity grades. Although grease is sometimes used to lubricate worm gears, it is uncommon, as the EP additives used in many greases can be aggressive toward the bronze wheel. The EP additives in gear oils designed for use in spur gears can also be problematic. Those types of lubricants should be avoided. Also, if the bronze alloy contains aluminum, PAG based worm gear lubricants should be avoided.

We are trying to manufacture Lithium based grease with high transparency. Any advise on it?

A:

Transparent, or at least translucent, lithium soap greases tend to be made with light-colored base oils and additives that do not cause them to become opaque. The degree of translucency is related to the amount of thickener in the product. A product with less thickener will be more translucent, since it is the thickener that causes the oil to become opaque. Less thickener (better yield) in the grease is controlled by the base oil characteristics (chemical make-up, solvency) as well as the manufacturing procedure and conditions. A product that contains a significant portion of naphthenic base oil (relatively low aniline point) will have a better yield (contain less thickener) than a product based only on paraffinic base oils. In particular, if the soap is formed in naphthenic base oil during saponification, the yield will be significantly improved. A word of warning, though - if the thickener content is too low, the oil separation, mechanical stability, and water resistance properties of the grease can be affected. Grease formulation is a balance, and striving for transparency should not be allowed to compromise other performance properties.