PRINCIPLE OF FRICTION IN BEARINGS

          If you've been asked to move one ton, smoothly polished block which is placed on ground (another surface) from one place to another location. During initial attempt to move the block, the two surfaces in contact (the base of the block and the ground) resist movement. This is called Static friction. If we applied some more force, which is enough so that the surface begin to slide against one another. Once in motion, the resisting force is from kinetic or slippery friction, instead of static friction.

          If beneath of that very same block we placed equally spaced rollers, the force which is required to move the block is considerably reduced. Do you know why? The rollers, in contact with the surfaces of ground and block, still encounter friction, but the rotating action of the rollers carries the block and have eliminated the resisting force of Kinetic friction; the friction encountered is now classified as Rolling friction. Rolling element bearings are designed based on this principle. They eliminate sliding friction and utilize the efficiency of rolling friction to carry load.

BEARINGS

          "A bearing is a machine element that constrains relative motion between moving elements to solely offer the desired motion". Bearings are designed to provide free linear movement of the moving elements or free rotation around a fixed axis or it may prevent motion by controlling the vectors of normal forces that acting on the moving element. They are mainly used as friction-reducing devices between moving parts.

          When a metal contacts with another metal produces a large amount of friction. Due to this friction the metals wear and tear, produce grinding that slowely degrades the metals that are in contact. Bearing reduces friction by allowing one moving part to glide past another moving part. Bearings consists of a smooth metal ball or roller that rolles against a smoth inner or outer metal surface (race). The balls or rollers take up the load, allowing the device to spin. 

INSTRUCTIONS FOR MECHANICAL SEAL INSTALLATION // INSTALLING MECHANICAL SEAL IN CENTRIFUGAL PUMPS

LIMIT GAUGES AND THEIR APPLICATION

          Gauges are scale-less inspection tools. A limit gauge is not a measuring gauge, they are just used for inspection purpose. Two sets of limit gauges widely used for checking limit of dimension of various parts. There are two gauges : 'GO' limit gauge, and 'NOT GO GAUGE'.

          'GO GAUGE' should pass through or over a part while 'NOT GO GAUGE' should not pass through or over the part.

1. Go Limit:

          The GO limit applied to that of the two limits of size corresponds to the maximum material condition i.e., (1) an upper limit of shaft, and  (2) the lower limit of a hole. This is checked by GO gauge.

2. Not Go Limit:

          The Not Go Limit applied to that of the two limits of size corresponds to the minimum material condition, i.e., (1) lower limit of a shaft, and (2) the upper limit of a hole. This is checked by the Not Go gauge.

ACHIEVABLE TOLERENCE LEVELS BY VARIOUS MACHINING PROCESSES

COMMON APPLICATIONS OF VARIOUS FITS

Some common applications of various fits are shown in below:

Fundamental Tolerance Chart

By K-code-g - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=48589598

FIT OF A COMPONENT SPECIFICATION

          The components of the tolerenced dimension shall be indicated in the following order: 

a) The basic size, and

b) The tolerence symbol.

          If, in addition to the symbols it is necessary to express the values of the deviations or the limits of size, the additional information shall be shown in brackets.

Permissible deviation:

.The upper deviation or the upper limit of size shall be written in the upper position and the lower deviation or the lower limit of size in the lower position, irrespective of whether a hole or shaft is tolerenced.

.The tolerence symbol for the hole shall be placed before that for the shaft or above it, the symbols being preceded by the basic size indicated once only. 

International tolerence (IT) grade

          IT Grade refers to the International Tolerance Grade of an industrial processes defined in ISO 286. This mechanical tolerence grade identifies what tolerences a given proces can produce for a given dimension.

The specific tolerence for a particular IT grade is calculated by the following formula

Where :

               . T is the tolerence in micrometers {µm}

               . D is the geometric mean dimension in millimeters [mm]

               . ITG is the IT grade, a positive integer.

The larger the ITG, the looser the tolerence that can be achieved.

Application: 

          Almost all manufacturing processes have an IT grade associated with, indicating how precise it is. IT grade offer guidence for typical manufacturing method capability or how precise one can expect manufacture of a specific option or feature.

          When designing a part, an engineer will typically determine a key dimention (D) and Tolerence on that dimension. Using this formula, the engineer can determine what IT Grade is necessary to produce the part with those specifications. For example, if injection molding has an IT Grade of 13 and a part needs an IT Grade of 5, one cannot injection mold that part to those specifications. It is useful in determining the processes capable of producing parts to the needed specification.

COMMON TERMINOLOGY USED WITH RESOECT TO FITS & LIMITS

The terminology used in Fits & Tolerances is shown below.

Basic size : It is also called as "Nominal size". It is the exact theoritical size arrived at desiging. This is the size which is obtained by calculation for strength. It is the size from which limits or deviations are assigned.

Actual size : Actual size is the dimention as mesured on a manufactured part.

Deviation : It is the algebric difference between the basic size and the hole or shaft size.

Upper deviation : It is the difference between the basic size and permitted maximum size of the component. It is a positive quantity when the maximum limit of size is greater than basic size and viceversa.

Lower deviation : It is the difference between the basic size and the minimum permitted size of the part.

Mean deviation : It is the arithmatic mean deviation between the upper deviation and lower deviation.

Zero line : It is the line of zero deviation and represents the basic size. When the zero line is drawn horizontally, positive deviations are shown on above and negetive deviations are shown on below the line.

Fundamental deviation : It is the deviation closest to the basic size.

Limits of size : These are the maximum and minimum permissible sizes of the part.

Tolerance : It is the difference between permitted maximum and minimum sizes of the part.

Tolerance zone : It is the zone bounded by the two limits of size of the parts and defined by its magnitude i.e., tolerence and by its position in relation to the zero line.

Allowance : It is an international difference between maximum and minimum limits of mating parts. It is the minimum clearence or maximum interference between mating parts.

Unilateral limits : In this type of method of presenting the limits, both the limits of sie are on the same side of zero line. Ex:  or  e

Bilateral limits : In this type of representation, one of the limits of size one side of the zero line and the other limit of size is on the other side of the zero line. Ex: ..