Ask the Expert

the Expert

Q:

We have used uroflax type gear coupling in Boiler feed pump - booster pump rotate at 1480rpm and couple between motor & booster pump. We used molycoat 1000 type grease for lubrication purpose, but after some running grease dry and it effect erosion & corrosion of gear tooth and damaged the tooth. So please suggest the proper grease type for lubrication purpose.

A:

Euroflex (UK) and Euroflex India reportedly manufacture non-lubricated flexible disc couplings.

Dow Corning Molykote (R) 1000 is an anti-seize paste containing solid lubricants and metal powders, not a coupling grease. Grease-lubricated gear couplings require specialized products that do not readily separate base oil in service. Such products typically contain high concentrations of polymers. Coupling greases are typically not suitable for use in bearings or other applications where greases are normally used.

Coupling gear tooth corrosion and damage may indicate inadequate performance of the lubricant, or an excessively long relubrication interval. The interval between grease changes on a grease-lubricated coupling should be adjusted so that the performance of the product does not significantly degrade over the service period. Factors such as elevated temperature can significantly shorted relubrication intervals.

Seizing of Threaded Couplings

A frequent complaint expressed about stainless steel fasteners an other threaded couplings is the problem of seizing. The problem is frequently not that of the material itself but more than likely caused by mismatched threads, Nonuniform threads, an dirt on threaded surfaces. Reasonable care should be exercised in the handling of fasteners to prevent damage and to keep threads clean.

Fasteners made in accordance with nationally recognized standards, such as published by the American National Standards Institute, Inc. (ANSI), will assure that nuts and bolts are uniformly threaded. Torque should also be considered for a properly fastened joint. Suggested maximum torque values for stainless steels are published in the booklet “Stainless Steel Fasteners A Systematic Approach to Their Selection.”, copies of which are available from the Committee of Stainless Steel Producers. Nevertheless, galling may occur in clean, carefully machined threads, and it may be desirable to use a lubricant if another alloy material either cannot be used or is not available.

If a lubricant is going to be used with threaded fasteners, tests should be conducted to determine torque

requirements and to evaluate the compatibility of the lubricants to the environment. Among the popular lubricants are those containing substantial amounts of molybdenum disulfide, graphite, mica, talic, copper or zinc fines, or zinc oxide. Zinc-bearing lubricants are not recommended for use with stainless steels at elevated temperature.

Q:

Can a blend of fumed silica and bright stock be called a silicon grease?

A:

In general, greases that are claimed to be “silicone” (not silicon) are those based on a silicone fluid (some form of siloxane fluid). Some of those products are thickened with fumed silica, but other thickeners are also possible in greases based on silicone fluids. In the case of the composition described, it would be proper to call the product “silica-thickened grease,” but not simply “silicon grease,” since the majority of the composition would be mineral oil (bright stock), which contains no silicon.

Q:

We are trying to manufacture Lithium based grease with high transparency. Can you give us any advice?

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.