7.6: Fits and Clearances Flashcards
is not possible to maintain constantly
i.e A one inch shaft - nominal size. Not possible to manufacture exactly 1.00000’’ so a tolerance must be used, and also integrated with fits and clearances
The total permitted variation above or below the nominal size
A bi-lateral tolerance
Bilateral tolerance is the amount of variation above or below the design size Example 3.75 +/- 0.05
Unilateral tolerance is the amount of variation in one direction either above or below the design size Example 3.75 +0.05/-0.00
May require force to assemble parts
Clearance Fit - a clearance fit is one having limits of size defined such that a clearance always results when mating parts are assembled.
Interference fit - an interference fit is one having limits of size so prescribed that an interference always results when mating parts are assembled.
Transition fit - a transition fit is one having limits of size so prescribed that either a clearance or an interference may result when mating parts are assembled
Go/ No Go gauge
Ovality
This usually occurs as a result of the surface wearing, through friction or linear movement. Ovality can apply equally to holes and shafts.
Holes may be tested for ovality, using such instruments as Go/No-Go gauges, internal micrometers, or callipers, as were previously discussed in the tools module.
Testing roundness of a shaft may be achieved by placing the shaft to be tested on a V-block and rotating under a scribing block or a dial test indicator (this same method may be used to checking shafts for bow)
Yes by means of a strain gauge
'’Solid or tubular shafts that must be checked for twist will possibly have witness marks or lines engraved or etched at each end of the shaft. The shafts can be checkedm by mounting the shaft in V blocks and, then, locating these marks in the horizontal position.
It is possible to measure the amount of twist, to which a shaft is subjected, whilst in operation of rotationm using strain gauges. These emit varying amounts of electric current when under strain, giving an indication (on a calibrated instrument) of the load being applied.’’
Holes can be drilled and reamed using standard size drill reamers
If the shaft is constant and the hole varies in size, then the system is said to be ‘shaft-based’.
If the hole is constand and the shaft varies in size, then the system is ‘hole based’.
The hole based system is the one in more general use
Allow for interchangeablility between parts
1 in 600
'’In general, a maximum bow of 1 in 600 is normally acceptable unless otherwise stated in the Repair Manual’’
What is it not possible to do with a Nominal size component?
Nominal size of a component is not possible to maintain constantly
Will a Transition fit require force to assemble?
Transition fit, may require force to assemble
How can Ovality of a hole be checked?
Ovality in a hole can be checked with a Go / No Go gauge
Max bow in a shaft is?
Max bow in a shaft is 1 in 600
Where is the Hole base system of fits and clearances more common and why?
Hole based system of fits and clearances is more common in Europe because holes can be drilled and reamed using standard size drills and reamers
Option C: Dimensional and geometric
Option C: 1:600
Option A: a DTI and ‘V’ blocks
Option C: 1 in 600
Option B: 0.748 inch
Option B: Tolerance
Option C: 2 inches +- 0.002
Option B: a permitted variation on a dimension to allow for inaccuracy of equipment
Option C: the same size as the hole
Option B: 20 times its depth
