Module - Lubrication - Part A Flashcards
The 3 types of friction are?
- Sliding
- Rolling
- Fluid
What is Friction?
Friction is the resistance to movement of two surfaces in contact with each other.
What are 3 negative effects of friction?
- Increased power consumption
- Causes wear.
- Generates heat.
What are 4 positive effects of friction?
- Provides a means of polishing.
- Provides a means of starting, stopping, most machines.
- Accurate tracking in bearing raceways.
- Provides traction. Ensures rolling instead of skidding in anti-friction bearings.
What are 5 functions of a lubricant?
- To reduce friction.
- Protect against corrosion.
- Carry away heat.
- Carry away contaminants.
- Help seal.
What are the three types of Lubricant Films?
- Full film lubricaiton
- Elasto-Hydrodynamic film lubrication
- Boundary film lubrication
Define: Full Film Lubrication.
Full film lubrication is thick enough to completely separate the surfaces with a continuous film.
+ offers the least friction resistance
+ eliminates wear on the moving surfaces
+ generates the least amount of heat
Define: Elasto-Hydrodynamic Film Lubrication
An elasto-hydrodynamic film completely separates two surfaces and is therefore a type of full film lubrication. However, in this case the two surfaces are loaded to the extent that deformation takes place in the loaded area.
+ fluid film in extremely thin
+ fluid film offers low friction resistance
+ type of film forms at high speeds as well as high pressures.
+ example: Hydroplaning
Define: Boundary Film Lubrication
A boundary film does not completely separate the surfaces, so some of the tips of the asperities touch.
+ forms at low speeds
+ forms when the viscosity of the lubricant is too low to support the load
+ forms if there is not enough lubricant
Why does a reduction in clearance increase the load capacity of a journal bearing?
A reduction in clearance will increase the arc of the load supporting film.
+ Smallest total clearance gives largest support area.
Define: Viscosity
Viscosity is a measurement of fluid friction (resistance to flow). Fluids that flow easily, like water, have low viscosity. Fluids that flow very slowly, such as tar, have high viscosity.
Define: Viscosity Index
Viscosity index (VI) is a number that identifies the amount of change in the viscosity of a lubricant over a temperature range. This means that an oil that shows very little change in viscosity as it is heated from 40C to 100C would have a high viscosity index.
Define: Pour Point
Pour point is the lowest temperature at which an oil is observed to flow.
Define: Flash Point
Flash point is the lowest temperature at which the vapour above an oil will ignite when a flame is passed over it.
What 5 things does viscosity affect in the performance of a lubricant?
- load capacity
- flow rate
- fluid friction
- heat generation
- sealing effect