LIMIT FITS AND TOLERANCE

INTRODUCTION

In the modern world of competition,the sole aim of the production engineer is to manufacture the components or parts in true shape and size at the economical rate with the best quality. For this,production drawing are prepared. A production drawing is the drawing of the orthographic views of the machine component which is to be manufactured. A production drawing must contain the following information besides dimensioning of parts and specification of materials:

1. Type of raw material.2. Quantity to be produced.3. Manufacturing methods.4. Surface quality.5. Geometrical tolerances.

Geometrical deviation: The deviation of parts from its true geometrical shape is called geometrical deviation.

Dimensional tolerance: The difference between maximum and minimum limits of size is called dimensional tolerance.

Limits system: The system in which deviation are accepted is called limit system.

Basic size : Basic size is the basic dimension from which allowable maximum and minimum limits are decided.

Deviation: It is the algebraic difference between the actual measured size and the corresponding basic size

Fit: The assembly of two mating parts is called fit.

Basic hole: The hole whose lower deviation is zero is called basic hole.

Basic Shaft: The shaft whose upper deviation is zero is called basic shaft.

Allowance: It is the intentional difference between the maximum material condition of mating parts.

Limits: the maximum and minimum sizes indicated by a toleranced dimension are called limits.

Types of fits

Clearance fit, Transtion fit, Interference fit.

Clearance fit: In this fit, minimum hole diameter is larger than maximum shaft diameter. In this fit ,both maximum and minimum clearance are always positive.

Transtion fit: In this fit, the tolerance zones of shaft and hole overlap. In this fit, maximum clearance is positive, while minimum clearance is negative. The negative clearance is called interference

Interference fit: In this fit, maximum hole diameter is always smaller than minimum shaft diameter. In this fit, both maximum and minimum clearances are always negative. Negative clearance is called interference .

CH – 2 ….. COUPLING

• COUPLING

Mild steel shafts are generally available in 6 to 7 meters length. in order to have shafts of larger length, two or more pieces of shafts are joined together with the help of coupling . The couplings also transmits motion or power from one shaft to another . The process of joining or connecting two or more pieces of shafts together for the transmission of motion or power is known as shaft coupling.

• NECESSITY OF A COUPLING

Shaft coupling plays an important role due to several purpose , the most common of which are as under :

1. Mild steel shafts are available in length upto 7 meters. Sometime , it

becomes difficult to use a shaft of large length due to space limitation and transport problem. So a long line shaft consists of a number of small shafts put together end to end by coupling supported by bearings.

2. To provide for misalignment of the shaft and to provide or introduce mechanical flexibility.

3. Coupling provides protection against overloads.

4. Coupling alters the vibration characteristics of rotating units.

• UNIVERSAL COUPLING

• TYPES OF COUPLING S…..

CHAPTER -3 BEARING

Bearing: Bearing may be defined as the support and guide for a rotating shaft.

Types of bearing:1. Journal bearing2. Pivot or foot step bearing.3. Thrust bearing

Foot step bearing:Foot step bearing is used to support the lower end of a rotating shaft.

Plummer Block:plummer block is just like a journal bearing. When load varies in direction and the speed of the rotating shaft is too high,plummer block is used.