Reliability of bearing knots
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FUNCTION OF A BEARING
The main function of a rotating shaft is to transmit power from one end of the line to the other.
It needs a good support to ensure stability and frictionless rotation. The support for the shaft is known as “bearing”.
The shaft has a “running fit” in a bearing. All bearing are provided some lubrication arrangement to reduced friction between shaft and bearing.
Bearings are classified under two main categories:
Plain or slider bearing : -
In which the rotating shaft has a sliding contact with the bearing which is held stationary . Due to large contact area friction between mating parts is high requiring greater lubrication.
Rolling or anti-friction bearing : -
Due to less contact area rolling friction is much lesser than the sliding friction , hence these bearings are also known as antifriction bearing.
Bearing used to constrain, guide, or reduce friction in rotary applications.
Work by means of sliding action as opposed to the rolling action used by ball, roller and needle bearings.
Proper lubrication is required to reduce wear and friction.
Sleeve Bearing Materials
Relative softness (to absorb foreign particles), reasonable strength, machinability (to maintain tolerances), lubricity, temperature and corrosion resistance, and in some cases, porosity (to absorb lubricant).
A bearing element should be less than one-third as hard as the material running against it in order to provide embedability of abrasive particles.
Whitemetal - usually used as a lining bonded to bronze, steel or cast iron.
Copper Based alloys - most common alloys are copper tin, copper lead, phosphor bronze: harder and stronger than whitemetal: can be used un-backed as a solid bearing.
Aluminium based alloys - running properties not as good as copper based alloys but cheaper.
Sintered bronze - Sintered bronze is a porous material which can be impregnated with oil, graphite or Ptfe. Not suitable for heavily loaded applications but useful where lubrication is inconvenient.
Nylon - used only in very light applications.
Types of Lubrication
h > Rz1+Rz2.
Advantages and disadvantages of the plain bearing
Plain bearing are cheap to produce and have noiseless operation. They can be easily machined, occupy small radial space and have vibration damping properties. Also they can cope with tapped foreign matter.
Disadvantages are they require large supply of lubricating oil, they are suitable only for relative low temperature and speed; and starting resistance is much greater than running resistance due to slow build up of lubricant film around the bearing surface.
Ball and roller bearings
due to low rolling friction these bearings are aptly called “antifriction” bearing.
Frictional resistance considerably less than in plain bearings
Rotating – non-rotating pairs separated by balls or rollers
Ball or rollers has rolling contact and sliding friction is eliminated and replaced by much lower rolling friction.
In plain bearing the starting resistance is much larger than the running resistance due to absence of oil film.
In ball and rolling bearings the initial resistance to motion is only slightly more than their resistance to continuous running.
Hence ball and rolling bearing are more suitable to drives subject to frequent starting and stopping as they save power.
Owing to the low starting torque, a low power motor can be used for a line shaft running in ball bearing.
Races and balls are high carbon chrome steel (to provide resistance to wear) machined and ground to fine limits of 0.0025 mm, highly polished and hardened.
The cages are made of low-carbon steel, bronzes or brasses, though for high temperature application case-hardened and stainless steels are used.
Ball and Roller bearing
Inner ring or race which fits on the shaft.
Outer ring or race which fits inside the housing.
Ball and roller arranged between the surfaces of two races. These provide rolling action between the races.
the radius of the track for balls is slightly greater 5 to 10 % than that of the ball themselves.
Note that the rotating surfaces rotate in opposite directions.
Cage which separates the balls or rollers from one another.
The disadvantage of the ball and roller bearings are high cost, they cannot be used in half, and greater noise.
The ball and roller bearing consists of following parts:
Tapered roller bearing (TRB):
TRB can take both radial and axial loads and used for gear boxes for heavy trucks, bevel-gear transmission, lathe spindles, etc.
Thrust ball bearing:
It can take only thrust loads.
Thrust ball bearing are used for heavy axial loads and low speeds.
Needle roller bearing:
It use small diameter of rollers. They are used for radial load at slow speed and oscillating motion.
They have the advantage of light weight and occupy small space.
They are used in aircraft industry, live tail stock centers, bench-drill spindles, etc.
APPLICATIONS OF ROLLER BEARINGS
During the work with rotation bearings lack of pollution is a factor of paramount importance. Finishing purity of a surface of contact surfaces of rings of bearings and elements of rotation is usually measured by the tenth shares of microns (1/10 µм, or 0,0001 mm). Such smooth surface is extremely sensitive to the damages put with pollution. The greasing layer between contact surfaces usually has thickness from 0,2 to 1 µм. With a size of particles it is more than pollution, than at a greasing layer, are rolled between rotation elements, creating local tension in steel of bearings and eventually causing premature fatigue of material. The sizes of particles of usual household dust reach to 10 µм so it always damages bearings.
Pollution can get stuck in a greasing layer between a ring and elements of rotation of the bearing. It can lead to damage of bearings
Damage of a surface (jag),
caused by pollution
Bearing life depends on:
Load and revolutions
From experimentation: For a given R, linear log behavior
Reliability = % chance that the part will function as expected
a = 3 for ball bearings
a = 3.333 for roller bearings
Bearing life at rated reliability
How to handle combined loading
So far we have only considered radial loading…
What about combined radial, Fr, and axial loading , Fa?
Use an equivalent load, Fe, that does the same amount of damage.
Where V = 1.2 for outer ring rotation and 1 for inner ring
This has to do with the fact that outer ring fails more often
Xi and Yi are a function of the
Axial load, Fa
Static load rating, Co
For instrument bearings, certain special considerations should be emphasized:
Heavy press fits should be avoided.
Accuracy of mounting surfaces should be equal to accuracy of mating bearing surface.
Misalignment for low torque and running accuracy should not exceed 1/4°. Loading across the bearing during assembly should be avoided.
During cold mounting of a roller bearing,it must be made sure that the mounting forces are always applied to the ring with the interference fit. Mounting forces should never go through the rolling elements.
The danger of damaging raceways can be
minimized by the use of the specifically
designed BALTECH fitting tools
INDUCTION HEATERS BALTECH HI SERIES
The induction heaters Heater Induction Series are intended to perform heating of bearings, gearwheels, half-couplings and metal rings for their interference fit on a shaft. The heating process is automatically controlled by a temperature sensor, which is installed on the inner bearing ring. The induction heaters automatically demagnetize bearings (parts) after heating to a defined temperature. The induction heaters heat only parts, while when using an oil bath the energy is consumed on heating a bath and oil, and often no temperature control is performed. This leads to underheating or overheating, which results in improper part mounting. The heating can be conducted based on the temperature control or time control. All the induction heaters are delivered with thermal-resistant gloves and yokes. Large size heaters are delivered with trolleys
Distinctive features of the induction heaters
Energy saving and environmental friendliness
Safety and ease of use
Automatic demagnetization of bearings after heating to a defined temperature
Automatic time and temperature control
Universal use (bearings, sleeves, couplings, etc.)
Kronsforder Allee 45, 23560, Lübeck
Tel: +49 (0) 451-370-87-700,
Fax: +49 (0) 451-370-87-707,
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