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A-Z of Tribology Concepts

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | XY | Z

A

Additive

Additive to lubricants, corrosion protection and maintenance products to achieve certain properties.

Aging

Chemical transformation of substances due to the effects of heat, light and oxygen throughout the running time.

B

Basic oil

Carrier fluid for pastes, greases, and oils.

Bleeding

The lubricating oil contained in the lubricating grease is separated from the soap frame. The bleeding of the grease at the lubricating point can mean that the grease is not sufficiently operatively stable and/or temperature resistance.

Boundary friction

Arises if the lubricating film is penetrated or destroyed during the friction action. The material pairs come into contact due to friction conditions, such as pressure or speed. The boundary friction area cannot be avoided before the formation of a supporting lubricating film when starting and stopping or when changing the direction of rotation.

Boundary lubrications

Is given when two surfaces sliding on top of each other partially come into contact in the presence of a lubricant.

C

Classification of consistency

The consistency of a lubricating grease is measured in accordance with DIN ISO 2137 with a penetrometer, whereby the grease is worked prior to the measurement in order to imitate the stress in a bearing. The penetration depth of a cone permits the assignment into a consistency class in accordance with NLGI (National Lubricating Grease Institute) in accordance with DIN 51818.

Code designation

The code designation in accordance with DIN 51502 provides information about the type of lubricating grease, the suitability, consistency classes (NLGI) and the application temperature range of lubricants.

Corroders

Arise when the lubricating film is penetrated and surface roughness peaks thus weld together.

D

Differences between Oil, Grease and Paste

Oil: Liquid lubricant made from one or more types of oil for rapidly moving parts with relatively low surface pressures. Grease: Mass of oil and thickener (soap matrix). The oil stored in the thickener is separated by means of pressure and dynamics. In the stationary phase, the thickener absorbs the oil once again. For medium to fast moving parts with higher surface pressure. Paste: Very high-viscous mass of solid lubricants, carrier oil, and thickener for static and slow moving parts subject to a high level of stress with an extremely high surface pressure.

Drop point

The drop point (in °C) for a lubricating grease is the temperature at which liquefaction arises in accordance with DIN ISO 2176. It is a great deal higher than the recommended upper limit of the application temperature. However, certain grease thickeners do not liquefy, i.e. they do not have a drop point.

E

Emergency running lubrication

Is reached with solid lubricants if a deficiency of lubrication arises with fat or oil lubricants.

Evaporation loss

In accordance with DIN 58397 this is investigated over a stipulated time at high temperatures. The loss of evaporated oil as a percentage of weight should be as low as possible.

F

FDA

“Food and Drug Administration” in the United States, responsible for laws and regulations in this area.

Flash point

In the case of combustible fluids, the flash point is a measured quantity that makes it possible to estimate the fire risk. Depending on the product type and the level of the expected flash point, the most common measuring methods are with a closed (according to DIN 51755) or open (according to DIN ISO 2592) cup.

Friction

Friction is the mechanical resistance against the relative movement of two surfaces. Friction is unwanted in the area of lubrication technology as energy losses, friction heat, and wear are linked to it. During the movement of material pairs against each other, a distinction is made between the following friction states:
Dry friction – boundary friction (starting friction, solid-to-solid friction, visible surface friction)
Semi-fluid friction – mixed friction (from boundary friction to fluid friction)
Fluid friction – liquid friction (hydrodynamic friction)

Friction figure or coefficient of friction µ

The formula (based on Coulomb) friction µ = FR (frictional force = tensile force) is used to determine the friction. FN (normal force = weight). The types of friction can be divided up into sliding friction, boring friction, rolling friction, and combined sliding and rolling friction. µ = coefficient of friction.

Frictional corrosion

Corrosion that arises at fits subject to oscillations with micro-frictional movements. Immediate formation of rust on rubbed-off steel particles.

H

HT lubricant

Suitable for permanent temperatures in excess of +140°C.

Hydrodynamic lubrication

The individual sliding partners are separated from each other by a liquid lubricating film.

I

Industrial lubricants

Lubricating oils and lubricating greases for industrial aggregates and machines (DIN 51 502, DIN ISO 6743 Section 0).

Inhibitors

Aging protecting agents (inhibitors), which delay or prevent certain reactions.

L

Low temperature lubricant

Suitable for permanent temperatures of below -20 °C.

Lubricating greases

Lubricating greases are consistent compounds of thickening agents and oils. Metal soap lubricating greases (calcium, Al,Ba,Li,Na,Pb-and complex soap lubricating greases), which are comprised of fatty acids and alkaline solutions as metal soaps (thickening or swelling agents) and lubricating oils;
Metal soaps, lubricating oils and the manufacturing process determine the structure, consistency, usability, type of application etc. Soap-free lubricating greases with inorganic gel binders (silica gel, betonite etc.) or organic thickening agents (PE, PP, polycabarmides etc) and lubricating oils.
Synthetic lubricating greases, which are comprised of organic and inorganic thickening agents and synthetic oils (ester, silicon, polyglycol, polyphenyl distilled oils).

M

Mixed friction

Is a state of friction during which boundary friction and fluid friction occur at the same time or following each other. The surface roughness (roughness peaks) are separated in some cases, sometimes there is also contact so that wear arises. Or a lubricating state in which solid body friction sometimes is present alongside hydrodynamic lubrication.

MoS²

Chemical formula for molybdenum sulphide (mineral molybdenum). A sold lubricant with a layer lattice structure. If worked into the metal surface correctly this solid body with a laminated structure can reduce the friction value of the surface for a period of time.

N

NSF

NSF International is an independent organisation based in the USA that carries out tests according to fixed standards in order to determine the suitability of products for the use, e.g., in the food industry. It is the successor organisation of the former USDA/FDA.

O

Oil separation

Oil separation as a percentage of weight is measured in accordance with DIN 51817 whereby the lubricating grease to be inspected is admitted with pressure and temperature.

Oxidation

Oxidation is a combustion process. The addition of oxygen to certain elements or molecules occurs in this process. In the case of hydrocarbons, jellification, the formation of lacquer, polymers, corrosive radicals, and so on arise.

P

Penetration

Measured quantity for the determination of the consistency (ductility) of lubricating greases. In the case of lubricating greases, it is the distance that a cone of certain dimensions penetrates vertically into the sample to be investigated under stipulated conditions (DIN ISO 2137 or DIN 51 804).
Static penetration: Penetration of lubricating grease sample that was not pre-treated in the grease shaper, measured at +25°C.
Worked penetration: By the term worked penetration, we mean the cone penetration, which is determined immediately following the treatment of the sample with 60 double cycles within a minute in the grease shaper at +25°C.

Press-Fit-Test

The Press-Fit-Test provides information about the behaviour and the adhesion of solid lubricants at very high pressure and at a low sliding speed. The friction figure µ is measured and it is determined whether stick slip arises. Both results are important for the applications in the event of installation work (e.g. press manufacture) or in the case of gliding channels and guides (e.g. tool machines).

R

Room temperature (RT)

Room temperature; defined in accordance with DIN 50014 = +23°C with 50% relative air humidity.

S

Salt spray test

The salt spray test simulates a salty climate in accordance with DIN 50017, whereby coated metal plates are exposed to a defined salt spray. The number of hours that pass until traces of rust arise is monitored.

SKF EMCOR procedure

Is applied to assess the corrosion-preventing properties of roller bearing lubricants. In this process water is added to the grease and examined for evidence of corrosion in self-aligning ball bearings with a defined running duration and stop periods in accordance with DIN 51802. If no corrosion emerges during the visual inspection of the test rings then the corrosion level is 0. In the event of very extensive corrosion the maximum mark is 5.

Solid lubricant

Generally only used and required for lubrication tasks under extreme conditions (e.g. during operation in mixed friction areas). The most well-known ones are graphite, molybdenum sulphide, various plastics (e.g. PTFE), heavy metal sulphides, etc. The solid lubricants are determined according to DIN 51 831 and DIN 51 832.

Speed characteristic value

The speed characteristic value is a guiding value stating at which peripheral speeds lubricant can be used in roller bearings. Results on grease inspection machines, the basic oil viscosity and practical experience form the basis for the statement of the value.

SRV test (OFW test = oscillating friction wear)

Test of the effectiveness of assembly pastes in regard to the coefficient of friction, wear, and frictional corrosion (tribocorrosion).An upper test specimen is oscillated on a lower test specimen that usually has a round, lapped, or ground surface. A few grams of mounting paste is applied between these test specimens.

Stick slip

Stick slip arises if the lubricant does not have a sufficient separation grade as the initial friction is higher than the motional friction.

Synthetic oils

In contrast with natural oils, such as mineral, vegetable, and animal oils, these oils are gained by means of chemical processes. In this way, specific advantages can be gained such as the low tendency to coke, a low pour point, good resistance to chemicals, an often excellent viscosity temperature behaviour. Synthetic hydrocarbons, esters, polyglycols, fluorinated oils, and silicon oils, for example, are used as lubricants.

T

Thread friction

Thread friction is determined on a screw test stand. According to DIN 946, you obtain the coefficient of frictionµ of a screw connection when tightening screws and nuts. The thread dimension, material, and type of surface must be specified.

Tribology

The area of tribology comprises the scientific research and technical application of friction, wear and lubrication whilst taking account of the construction, material science, authority regulations etc.

U

USDA

United States Department of Agriculture
USDA-H1: Designation class for tested and authorised lubricants, which can come into occasional, technically unavoidable, contact with foods.
USDA-H2: Designation class for lubricants which may not come into contact with foods.

V

VKA

VKA is the abbreviation for “Vierkugelapparat” or a four-ball tester for luboils, by means of which the welding load and wear is measured in the event of punctual contact. DIN 51350 describes the test process. The welding load [N] is the applied load at which the individual balls are welded together. The wear characteristic [mm] is the average diameter of the spherical caps which is formed at a constant load following a defined testing time.
Spherical cap value = depth of wear on the surface of the ball.

W

Water resistance

There is a static and a dynamic test to test the behaviour of lubricating greases towards water. It is investigated how the water influences the lubricating grease at various temperatures (DIN 51 807).

Wear

Arises after penetrating the lubricating film if the sliding partner touch one another and rub against one another.