Bearings 1 Flashcards
Types of Bearings
- Rolling Element Bearings (Rolling Motion) 2. Plain Bearings (Sliding Motion).
What is the function of a bearing?
A bearing allows two parts to move independently of each other.
What motion do most bearings allow?
Most bearings allow rotational motion, but some allow linear motion.
Operating Mechanism of Plain Bearings
Sliding between surfaces, which can generate friction and wear.
What are the loading types for plain bearings?
Some take radial load only, others thrust load (axial load), and some can take both radial and thrust loads.
Types of Lubrication for Plain Bearings
- Partial Lubrication (Boundary, Dry, None) 2. Film Fluid Lubrication (Hydrostatic, Hydrodynamic).
Characteristics of Boundary Lubrication
Grease/oil film applied, carries part of the load, surfaces still in contact leading to wear. Suitable for moderate loads/speeds.
Examples of Boundary Lubrication Applications
Linkages, children’s bicycle wheels.
Features of Dry Lubrication
Uses graphite powder, suitable for low loads/speeds, surfaces in contact leading to wear.
Applications of Dry Lubrication
Locks, sliding mechanisms.
Hydrodynamic Lubrication in Plain Bearings
Uses a wedge of lubricant film, no surface contact, suitable for high loads/high speeds.
Examples of Hydrodynamic Lubrication Usage
Crankshafts, turbine shafts.
Hydrostatic Lubrication Characteristics
Pressurized lubricant film, requires external pump, no surface contact, suitable for high loads/low to moderate speeds.
Applications of Hydrostatic Lubrication
Machine tools, measuring instruments.
Materials used in Plain Bearings
Shaft - hardened steel; Bearing - soft materials like white metals, copper alloys, aluminum alloys, polymers, porous sintered metals.
Design Considerations for Plain Bearings
Magnitude and direction of forces, relative velocity, misalignment, operating environment, bearing length relative to shaft diameter.
Calculating Pressure x Velocity (PV) Value in Bearing Design
Determine the mean bearing pressure and sliding velocity, then double the PV value for safety.
Example Calculation in Bearing Design
Consider radial load, shaft diameter, bearing length, and running speed to calculate PV value and select suitable bearing material.
Summary of Plain Bearings
Sliding motion generates wear and heat, selection based on shaft speed and load, calculate PV value and consult supplier’s chart.
Factors in Selecting Bearing Material
PV value, maximum load capacity, operating temperature, coefficient of friction and expansion, application needs.
No Lubrication
Polymer used as the bearing surface
* No maintenance (i.e. no re-lubrication)
* Surfaces are still in contact
Low Loads/Low Speed
