Lecture 2 Flashcards
What is friction?
Tribology: “the study of friction, wear, lubrication, and the design of bearings; the science of interacting surfaces in relative motion”
What are the two types of friction?
Static friction
Kinetic/dynamic friction
What is static friction?
Cuased by the microscopic roughness of surfaces
What is cold welding?
A molecular process that takes place when two sufaces meet - to break or deform the process a force needs to be applied
What is kinetic friction?
Once an object moves, force to overcome abrasion must be applied
What is abrasion?
When an object starts moving, a process known as abrasion occurs
How can the coefficient of friction be calculated empirically?
μ = F/Fn
where:
Fn = force of the object onto the surface
F = the force required to move an object from static state or keep the object moving in dynamic state
What are the two motions of bearings?
Linear and rotary
What are the mechanical contact in bearings?
Sliding
Rolling
Flexing
Why are non-contact used in bearings?
Low noise
High speed
High load capability
What are features of pressurised (hydrostatic) bearings?
High pressure lubricant injected into the structure
Contactless bearing
What are features of non-pressurised (hydrodynamic) bearings?
Film of oil around the shaft
What are slide bearings?
Bearings placed between shafts and housings to reduce friction, wear, and to facilitate rotary and/or translation motion.
Can be used in both linear and rotary application
What are the advantages of slide bearings?
Operational noise is low
Cheap
Small space requirement
What are the disadvantages of slide bearings?
Can have high friction between surfaces resulting in low energy efficieny if not hydrostatic
Susceptible to damage from impurities in lubricant
Have stringent lubricant requirements
Must have sufficient pressure in hydrostatic form
What are the properties of jewel bearings?
Low friction
Thermally stable
High hardness (low wear)
The bearings are stronger than the shaft
Where are jewel bearings used?
Watches
Compasses
Precision instrumentation
What are the advantages of sliding/plain bearings in linear application?
Self lubricating therefore maintenance free
Low friction
Contaminant particulates do not adhere to the bearing
Resistant to corrossion
Low noise
Light weight
Safe in specialised environments
How do dovetail linear slides work?
Lubricated gib slotted between
Screws on the maile and tightening the screws causes the gib to push up agains the surface
Tightening screws create greater force - friction controlled by torque of the screws
What are the advantages of the dovetail slide?
Adjustable tension
Precision can be defined by adjustments
What are the disadvantages of the dovetail slide?
Regular maintenance
Undefined Fs
What are the two types of loading for a rolling bearing?
Axial/ thrust load - perpendicular to the axis of the shaft
Radial load - perpendicular to the shaft
What are the types of rolling bearings?
Ball - Radial or Thrust
Roller - Radial or Thrust
What is the advantage and disadvantage of the sperical roller thrust bearing?
Advantage - help with misalignment
Disadvantage - expensive to produce
