Rivet's are used for permanent fastening.

When two non-parallel or intersecting, but coplaner shafts are connected by gears, then these gears are called bevel gears and the arrangement is known as bevel gearing. The bevel gears, like spur gears, may also have their teeth inclined to the face of the bevel, in which case they are known as helical bevel gears.

Relative density, or specific gravity, is the ratio of the density of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest; for gases, air at room temperature is the reference.
Relative Density has no Units.