Limits and Fits • Need for different types of fit • Limits of size and Tolerance • Principal Terms Explained • Definition of different types of Fit
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.
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
Nominal Size, Unilateral Tolerance and Bilateral Tolerance • Nominal/Basic Size:- The size by which the component the component feature is known e.g. 20.00mm, 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 +0.01 /20 +002 (20.01 / 20.02) • Bilateral Tolerance:- One where the tolerance zone is in both directions and crosses the basic size e.g. 20 +0.01 / 20 – 0.01 (20.01 / 19.99)
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 Maximum Material Condition (MMC) and Least Material Condition (LMC) • 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
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 Clearance 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. Interference Fit
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