Industrial Gas Turbines

Lubricants are particularly important in conjunction with bolt connections. Massive damages and costs occur through careless handling of apparently similar materials.

Lubricants on threads influence not only the efficiency of the thread ( "Ill. 4.2.3.1-1"), i.e., the connection between the clamping torque and bolt prestress critical for dynamic stability, but can also contribute to material damage due to corrosion in cold or hot areas or, in extreme case, produce, through diffusion, a drop in strength, cracks and fracture ( "Ill. 4.2.3.1-2" and "Ill. 4.2.3.1-3").

Lubricants (grease materials) that are implemented to prevent bolt (screw) connections from seizure at high temperatures have always been a difficulty. Although many compounds (pastes), mainly with metal powder filling (inter metallic phases, copper, nickel), hard materials (boron carbide), metal oxides (chrome oxide, nickel oxide) or pastes with hexagonal boron nitride are offered on the market, a universally implemented product is not present. It is best to employ the manufacturer’s suggestions. If proved inadequate, it should be conveyed to him.

Objects that are especially deceptive or consciously exchanged mistakenly are the almost similar compounds with MoS2 – or graphite supplements. Due to the outer deceivable appearance, these lubricants are not to be exchanged except under explicit instructions from the manufacturer, as they function very differently. This has less to do with clamping torque and pre stress as with behavior during operation. Different disintegration temperatures and decomposable products influence the components (example 4.2-9).

A further problem is sulfidation ( "Ill. 3.4-2"), a dangerous form of corrosion in Ni alloys, induced through small traces of sulfur, emanating from the disintegrating lubricant at high temperatures.