COMMON TERMINOLOGY USED WITH RESPECT 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 theoretical size arrived at designing. 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 dimension as measured on a manufactured part.

Deviation : It is the algebraic 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 vice-versa.

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

Mean deviation : It is the arithmetic 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 negative 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., tolerance 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 clearance or maximum interference between mating parts.

Unilateral limits : In this type of method of presenting the limits, both the limits of size 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: .. 

FIT OF A COMPONENT SPECIFICATION

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

a) The basic size, and

b) The tolerance 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 toleranced.

.The tolerance 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. 

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 slowly 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 rolls against a smooth inner or outer metal surface (race). The balls or rollers take up the load, allowing the device to spin.   

PRINCIPLE OF FRICTION

          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.

Comparison of 304 or 316 and 304L or 316L type welding electrodes

As American AISI basic grades, the only practical difference between 304 or 316 and 304L or 316L is carbon content. ... The 1.4301 (304) and 1.4307 (304L) have carbon ranges of 0.07% maximum and 0.030% maximum, respectively. The chromium and nickel ranges are similar, nickel for both grades having an 8% minimum. Low carbon rods, which provides extra corrosion resistance.

--> ER 308 stands for MIG wire

MAKE	SIZE  DxL (MM)	Amps.	/CARTON
SS - Supron	2.50X350	60-90	260
MS -Manglam	3.15X350	100-140	150
	3.15X450	100-140	150