1. Limits and Fits Need for different types of fit
Limits of size and Tolerance
Principal Terms Explained
Definition of different types of Fit

2. Need for Different Types of Fit
Machines and Plant such as lathes, compressors, etc. consist of many different components assembled together.
Some components need to rotate or slide whilst others need to be fixed, therefore the tightness with which they fit together varies.
The difference in size of the components determines the type of fit.
There are three types of fit sub-divided into different classes these are Clearance, Transition and Interference.

3. Principal Terms of Limits and Fits

4. Limits of Size and Tolerance
The largest acceptable size for a component is known as the Upper Limit of Size
The smallest acceptable size for a component is Known as the Lower Limit of Size
The difference between the Upper Limit of Size and the Lower Limit of Size is Known as the Tolerance or Tolerance Zone

5. Nominal Size, Unilateral Tolerance and Bilateral Tolerance
Nominal/Basic Size:- The size by which the component the component feature is known e.g mm, it is the exact functional size by which the limit of size are developed (sometimes referred to as the line of zero deviation)
Unilateral Tolerance:- One where the tolerance zone is in one direction and lays to one side of the basic size e.g / (20.01 / 20.02)
Bilateral Tolerance:- One where the tolerance zone is in both directions and crosses the basic size e.g / 20 – (20.01 / 19.99)

6. Examples of Unilateral and Bi-Lateral Tolerance

7. Maximum Material Condition (MMC) and Least Material Condition (LMC)
With respect to shafts
Maximum Material condition (MMC) refers to the largest shaft (upper limit of size) i.e. least material removed
Least Material Condition (LMC) refers to the smallest shaft Lower limit of size) i.e. most material removed
With respect to holes
Maximum Material condition (MMC) refers to the smallest hole (lower limit of size) i.e. least material removed
Least Material Condition (LMC) refers to the largest hole (upper limit of size) i.e. most material removed

8. Clearance Fit
Where movement is required between mating parts a clearance fit is necessary.
With a clearance fit the upper and lower limits of size for each component are set so that a clearance always exists between the two components (the upper limit of the shaft is equal to or less than the lower limit of the bearing
With a clearance fit assembly requires little or no effort
Clearance fits are sub-divided depending on the amount of clearance e.g. Free running clearance, close running clearance location clearance etc

9. Interference Fit
An Interference fit is required where movement between mating components is not permitted due to the function of the components e.g. a gear mounted on a shaft.
With an Interference fit the upper and lower limits of both component are set so that Interference always occurs
The amount of interference varies and assembly may require a press fit ,a force fit, or in extreme cases a shrink fit, This is where the bearing is heated and the shaft cooled, the two components are then assembled and returned to normal temperature resulting in a permanent fixture which is near impossible to dismantle e.g. railway wagon wheels fitted to axles.

10. Transition Fit
A Transition fit is required when two mating components are not required to rotate or slide but may need to be dismantled easily for example a pulley mounted on a shaft
With a transition fit the upper and lower limits of size for each component are set so that either a small clearance or a small interference could occur
If the shaft is at its lower limit and the bearing is at its upper limit a clearance is present
If the shaft is at its upper limit and the bearing at its lower limit then interference is present
Transition fits can be a light press or push fit