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White Paper 3 – Friction in Bearings

Friction in Bearings

Rolling bearings are called anti-friction bearing. They have high loading capacity and exhibit very low rolling friction torques.   The friction torques are similar or lower than ideally designed plain bearing operating under conditions of thick film lubrication.  Rolling bearing has low starting torques.

  • Friction component and influence factor

 

The idling friction is depends on the quantity of lubricant, operating speed, viscosity of lubricant during operation, seal and operating conditions in which bearing is running.

The bearing friction torque Mr = F. f. (d/2) or

The bearing friction torque Mr = F. fm. (Dm/2)

  • Mr = Friction torque (N mm)
  • F = Radial (or axial load) (N)
  • f = coefficient of friction of rolling bearing.
  • fm = coefficient of friction of rolling bearing based on mean diameter
  • d = Diameter of the bore of the bearing (Shaft diameter)(mm)
  • D = Outside diameter of the bearing (mm)
  • Dm = (d+D)/2 (mm)

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White Paper – Friction in Bearings

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White Paper 2 : Bearings Fatigue Life & Reliability

Fatigue Life & Reliability

Where any part failure may result in damage to the entire system and repair of damage is not possible, in such application greatly increased reliability is demanded of each component. Aircraft, satellites, or missiles are good example. This concept is being applied generally to durable consumer goods and may also be utilized to achieve effective preventive maintenance of machines and equipment.

The rating fatigue life of a rolling bearing is the gross number of revolutions or the gross rotating period when the rotating speed is constant for which 90% of a group of similar bearings running individually under similar conditions can survive without suffering material damage due to rolling fatigue. In other words, fatigue life is normally defined at 90% reliability. There are other ways to describe the life. For example, the average value is employed frequently to describe the life span of human beings.

However, if the average value were used for bearings, then too many bearings would fail before reaching the average life value. On the other hand, if a low or minimum value is used as a criterion, then too many bearings would have a life much longer than the set value. In this view, the value 90% was chosen for common practice. The value 95% could have been taken as the statistical reliability, but nevertheless, the slightly lower reliability of 90% was taken for bearings empirically from the practical and economical viewpoint. However, 90% reliability is not acceptable for parts of aircraft or electronic computers or communication systems these days and a 99% or even 99.9% reliability is demanded in some of these cases.

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White Paper – Bearing Fatigue Life and Reliability

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White Paper 1 : Concept of Bearing Life and Bearing Load Carrying Capacity

Concept of Bearing Life and Bearing Load Carrying Capacity

 

  • Concept of fatigue theory

The concept of the fatigue theory depends on the calculation s in the ISO 281 with the accordance of the Lundeberg and Palmgren’s theory for the calculation of the bearing rating life.

The actual bearing operational life is however more than the calculated life. Thus the life adjustment factor comes in to the role for the calculation of the bearing life.

The life adjustment factor is influenced by the following factors,

  • The bearing load
  • The fatigue limit of the material
  • The extent to which the surfaces are separated by the lubricated
  • The cleanliness in the lubrication gap
  • Additives in the lubricants
  • The internal load distribution
  • Frictional condition in the bearing

 

  • Dynamic load carrying capacity of bearing and its dependency on bearing life

The basic dynamic load rating of rolling bearings is defined as the constant load applied on bearings with stationary outer rings that the inner rings can endure for a rating life (90% life) of one million revolutions. Radial load on the center of the bearing defines the basic load rating of radial bearings and an axial load of constant direction and magnitude defines the basic load rating of the thrust bearings.

The dynamic load carrying capacity is described in terms of the basic dynamic load rating and the basic rating of bearing life.

The fatigue life of the bearing depends on the following factors,

  • Load acting on bearing
  • Operating speed of bearing
  • Statistical probability of the first appearance of the bearing failure

The basic dynamic load rating is defined by C. Constant radial load for radial bearing is Cr and for axial load is Co.

 

  • Bearing life calculation

Bearing application decides the various function included in the rolling bearings. These functions should work defect free for the maximum number of period. Bearings will eventually fail to perform satisfactorily due to an increase in noise and vibration, loss of running accuracy, deterioration of lubricant, or fatigue flaking of the rolling surfaces even if bearings are properly mounted and correctly operated.

Bearing life is defined as, satisfactory performance of bearing function even after the continuously operated. Bearing life depends on the factor of evaluation of noise life, grease life abrasion life or rolling fatigue life.

Factors which cause bearing failure other than the above mentioned factors are seizure due to heat, fracture, scoring of the rings, damage of seals or cage, or any other damage occurs.

As a result of errors in bearing selection, improper design or manufacture of the bearing surroundings, incorrect mounting, or insufficient maintenance; if such condition occurred then failure should not be interpreted as normal bearing failure.

 

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White Paper – Concept of Bearing Life and Bearing Load Carrying Capacity

 

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Shaft and Housing Fits

Planning for proper fit between the bearing and the shaft or housing is as important as bearing design and cleanliness. Improper bearing fits or incorrect installation can damage the bearing assembly.

The ideal mounting for a bearing is line to line on both shaft and housing. Such a fit has no interference or looseness. For random fitting the fit tolerances may need to be increased to meet the lot variances. For selective fitting the bearing inner and outer diameter should be accurately measured, then the shaft and housing machined to suit.

Interference fits should be used with care as they can distort the raceway and reduce radial internal clearance. In preloaded pairs, reducing the internal radial clearance increases the preload. If excessive, the results can be significantly reduced speed capability and higher operating temperatures which will ultimately reduce life.

Some applications require interference fits such as:

  • Heavy radial loading
  • Intense vibration
  • Lack of axial clamping

Radial internal clearance is reduced by approximately 80% of the interference fit.

Interference fits are usually applied to the rotating ring.

Loose fits may be suggested when:

  • Axial clamping is possible, such a snap rings, locking collars or adhesive
  • Ease of assembly
  • Axial movement is required for spring preload or thermal movements

Fits are often overlooked and is arguably the most common bearing handling mistake.

The table below is simply a guideline as there are many influencing factors to be considered such as.

  • Load, speeds and temperatures
  • Ease of assembly and disassembly
  • Rigidity and accuracy requirements
  • Machining tolerances

Therefore the appropriate fit may fall somewhere in between.

ROTATING RING

APPLICATION DESIRED FIT TYPE FIT
(inches)
Low speed, or spring preload. Loose .0001L to .0005L
Medium speed Line to Line .0002L to .0002T
High speed Light press .0000 to .0004T
High speed, high load Tight press .0002T to .0006T

 

L = Loose fit, T = Tight fit, d = Bearing I.D., D = Bearing O.D.

Selecting the right bearing for a specific application requires a review of performance requirements and material operating limitations. Certain design considerations must be met in the device and bearing to address such factors as load, speed, temperature, environment, method of lubrication and fit. Often times sacrifices must be made in one area to satisfy another to achieve optimal life.

Many factors come into play when selecting a bearing or creating a new design in which a ball bearing will be employed. Most important are speed, load and temperature.

Load

Speed Temperature Environment

Shaft / Housing

Direction
– Radial
– Axial
– Shock
– Combined
Constant  Variable
Hi or Low
dN> 250,000? *
Average
Max. Operating
Ambient
Vacuum
Gas or Air
Contaminants
Fits
Geometry
Tolerances
Mounting

*d=bore in mm, N=RPM

 

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Keep your elevators running smoothly with NIBL bearing solutions

21

An elevator or lift  are  vertical transportation that moves people or goods between floors of a building, vessel, or other structure. Elevators are powered by electric motors that either drive traction cables or counterweight systems like a hoist, or pump hydraulic fluid.

In agriculture and manufacturing, elevator like conveyor device used to lift materials in a continuous stream into bins or silos. Example, chain and bucket elevator

The most important bearing types for Elevator Systems are as follows :

Deep Groove Ball Bearing(DGBB) :

  • Suitable for robust operation and require little maintenance.
  • Uninterrupted raceway shoulders.
  • Non Separable design & can support radial and axial loads.
  • Different sealing options like seals or shields available.
  • Application used Traction Machines

 

Spherical Roller Bearing (SRB) :

  • Self-aligning design helps to withstand extreme conditions to compensate for drive shaft deflection, which can lead to edge stresses.
  • Internal design enables them to withstand High Radial load and able to accommodate axial loads acting in both directions.
  • Most types have circular groove and lubricating holes in the outer ring, for effective lubrication.
  • Application: Traction Machines.

 

Angular Contact Ball Bearing (ACBB) :

  • Sustain significant unidirectional axial load together with radial load.
  • Bearings with suffix E are designed for higher load ratings and better performance.
  • Inner, outer races and balls are manufactured from high quality bearing steel for optimum performance.
  • Single row angular contact ball bearings are self-retaining units.
  • Rigidity of single row angular contact bearings increases by preloading,
  • Application :Worm Gear Unit

 

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Parameters of Bearing Selection

Allowable Bearing installation Space

To install a bearing in target equipment, the allowable space for a rolling bearing and its adjacent parts is generally limited so the type and size of the bearing must be selected within such limits. In most cases, the shaft diameter is fixed first on the basis of its rigidity and strength by the machine designer; therefore, the bearing is often selected based on its bore size. There are numerous standardized dimension series and types available for rolling bearings and the selection of the optimum bearing from them is an important task.

Load and Bearing Types

Load magnitude, type and direction of applied load are to be considered in bearing type selection. The axial load carrying capacity of a bearing is closely related to the radial load capacity in a manner that depends on the bearing design.

Permissible Speed and Bearing Types

Bearings to be selected with response to rotational speed of equipment in which bearing is to be installed; the maximum speed of rolling bearings varies depending, not only the type of bearing, but also its size, type of cage, loads on system, lubrication method, heat dissipation, etc. Assuming the common oil bath lubrication method, the bearing types are roughly ranked from higher speed to lower.

Misalignment of Inner/Outer Rings and Bearing Types

 The inner and outer rings are slightly misaligned because of deflection of a shaft caused by applied loads, dimensional error of the shaft and housing, and mounting errors. The permissible amount of misalignment varies depending on the bearing type and operating conditions, but usually it is a small angle less than 0.0012 radian. When a large misalignment is expected, bearings having a self-aligning capability, such as self-aligning ball bearings, spherical roller bearings and bearing units should be selected.

Rigidity and Bearing Types

When loads are imposed on a rolling bearing, some elastic deformation occurs in the contact areas between the rolling elements and raceways. The rigidity of the bearing is determined by the ratio of bearing load to the amount of elastic deformation of the inner and outer rings and rolling elements. The higher the rigidity that bearing possess, better they control elastic deformation. For the main spindles of machine tools, it is necessary to have high rigidity of the bearings together with the rest of the spindle. Consequently, since roller bearings are deformed less by load, they are more often selected than ball bearings. When extra high rigidity is required, bearings negative clearance. Angular contact ball bearings and tapered roller bearings are often preloaded.

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The correct way to mount a bearing

Industry Challenges: A loose fit for a bearing will lead to relative movement between mating components, resulting in fretting, smearing, scoring, wear, excessive heat, and/or fracture. A extra-tight fit will reduce the bearing’s internal clearance, which will increase the operating temperature of the bearing in operation. Increase in mounting forces required, but can lead to fracture of bearing’s.

Introduction : When a bearing is mounted without proper care — without utilising the correct knowledge and tools — the bearing’s service life will be cut short. As per the industry estimates, estimated 16 percent of all premature bearing failures are due to poor fitting and the lack of knowledge) of the right fitting tools. The  best practices for mounting a bearing can avoid future problems and extend bearing performance and service life. This will be visible, in the efficiency and productivity of an operation with an increase in machinery uptime.

Methods : The primary methods for proper mounting of a bearing are classified as “cold” or “hot,” as per the shaft diameter or bearing inner diameter. Cold mounting, or mechanical mounting, generally is recommended for small- and medium-sized bearings.

Hot mounting will be for larger bearings; and hydraulic techniques should be used when mounting especially large bearings. Tools have been developed to take care of  each particular bearing needs.

In cold mounting, the wrong practice of using a hammer and pipe for the job is not recommended as will damage the raceways. This can also allow contamination to enter the bearing or, if not done properly, a pipe can slip and impact the rolling element of the bearing. The best practice recommended is to use fitting tools to eliminate the possibility of damage from force, not acting in line with the shaft and bearing position. Fitting tools ensure the proper force is applied to both bearing rings and isolates the rolling elements from impact force, thus assuring a more reliable installation and limiting the risk of damage.

Hot mounting, where the bearing is pre-heated, provides a fool proof solution to allow for a bearing’s inner ring expansion and subsequently easier installation, while maintaining specified interference fit or tolerances after the job is completed. It also saves time , free from any risk of lower flash point of the used oil being used , free from dirt entering the rolling elements and acting as abrasives.

For example, conventional hot oil baths raise the probability of contamination (and premature bearing failure) and flashing. Other techniques (such as exposure to an open flame) fall short, too, and can be potentially dangerous. Induction heating is highly recommended, which involves heating bearings to be mounted prior to installation to allow for a high degree of control, efficiency and safety.

mounting

 

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