FITS AND CLEARANCES

Question 1

is one having limits of size defined such that a clearance always results when mating parts are assembled

Answer 1

· Clearance Fit

Question 2

is one having limits of size so prescribed that an interference always results when mating parts are assembled

Answer 2

· Interference Fit

Question 3

is one having limits of size so prescribed that either a clearance or an interference may result when mating parts are assembled.

Answer 3

· Transition Fit

Question 4

To ensure consistent specifications, various standard fits have been devised around the world. _______devised by the British Standards Institute (BSI) has a comprehensive system designed to cater for all classes of work.

Answer 4

The British Standards System (BS 1916-1 & 2:1953 and 3:1963)

Question 5

The system provides for 21 types of holes designated by ___1____, and 21 types of shaft designated by ____2____., from nominal diameters of ____3____. Each type of hole or shaft is provided with ____ grades of accuracy designated by numbers 1-16.

Answer 5

1)capital letters A,B,C,D, etc..
2) small letters:- a,b,c,d, etc
3 ) 0.04 inch up to 19.69 inches
4) 16

Question 6

_____ is very similar to that of the BS, except that the system provides for _____ types for holes and shafts with ____ grades of accuracy.

Answer 6

ISO 286

28

20

Question 7

In the early _____ hole-based system of limits, the holes are classified as Class A and Class B fits. Class A holes are manufactured to a _______ than are Class B holes. The below figure shows how the shafts are classified, using the letters F, P, D, X, Y, and Z.

Answer 7

NEWALL SYSTEM
closer tolerance
F force
P push
D driving
XYZ running

Question 8

The British Institute has now produce a _________ . This is a system of limits based on standard sized holes and the various fits are obtained by using shafts of varying sizes. This permits the use of standard hole manufacturing tools i.e. drills, reamers, etc. The varying shaft sizes are easily obtained by normal manufacturing processes.

Answer 8

British Standard Specification 4500

Question 9

British Standard Specification 4500
It can be seen that in an Interference Fit, the upper and lower limits of the shaft are _______ and, thus, force is necessary to achieve the fit.

Answer 9

greater than the corresponding limits of the hole

Question 10

British Standard Specification 4500
In the Transition Fit, the differences in the upper and lower limits of both items are ___________ is required to insert the shaft into the hole.

Answer 10

negligible so that only light effort

Question 11

British Standard Specification 4500
The upper and lower limits of the shaft, in a Clearance Fit, are always _________, so that the shaft moves easily within the hole

Answer 11

less than those of the hole

Question 12

· The ‘_______’ is the largest size allowed.

Answer 12

upper limit

Question 13

· The ‘_________’ is the smallest size allowed.

Answer 13

lower limit

Question 14

· The ‘________’ is the difference between the upper and lower limit

Answer 14

tolerance

Question 15

Where variation either side of the nominal dimension can occur, the tolerance is called _____1____. Where one tolerance is zero, the tolerance is said to be ______2____.

Answer 15

1. bilateral
2. unilateral

Question 16

· _______: the difference between a size and the corresponding basic or nominal size.

Answer 16

Deviation

Question 17

·________: the working condition between a mating shaft and hole.

Answer 17

Fit

Question 18

· _______: shaft always smaller than the hole -allows movement.

Answer 18

Clearance

Question 19

· _______: may provide either clearance or interference - keys and keyways.

Answer 19

Transition

Question 20

_______: shaft always bigger than the hole.

Answer 20

Interference

Question 21

· _______: numbers which for a particular IT number has the same relative level of accuracy but vary depending upon the nominal or basic size

Answer 21

International Tolerance Grade (IT)

Question 22

· _________: a system of fits relating to basic or nominal hole size. Fundamental deviation is “H”.

Answer 22

Hole Basis

Question 23

· _________: a system of fits relating to basic or nominal shaft size. Fundamental deviation is “h”.

Answer 23

Shaft Basis

Question 24

Deviations for holes are indicated by ____1____ and deviations for shafts by ___2____.

Answer 24

1. capital letters
2. lower case

Question 25

Tolerance position letters’ are used, ____1____and ____2___ and these indicate the ‘fundamental deviation’ which locates the tolerance zone relative to the basic or nominal size

Answer 25

1. capitals for hole (internal) dimensions
2. lower case for shaft (external) dimensions

Question 26

An _________ is a difference in dimension that is necessary to give a particular ‘class of fit’ between two parts, for example, a shaft required to locate within a corresponding hole in a component. To assist in the economy of manufacture, either the hole or the shaft is made as accurately as possible to the nominal size and an allowance is applied to the associated item

Answer 26

allowance

Question 27

_____1____ is when there is an amount of dimension variation above and below the design size. ____2___ is when the design size can only vary in one direction.

Answer 27

1. Bilateral tolerance
2. Unilateral tolerance

Question 28

This usually occurs as a result of the surface wearing through friction or linear movement. Ovality can apply equally to holes and shafts.

Answer 28

LIMITS FOR OVALITY

Question 29

Holes may be tested for ovality, using such instruments as ________

Answer 29

Go/No-Go gauges, internal micrometers, or calipers,

Question 30

_________ relates to the size of the part when new, and will show the relevant tolerances.

Answer 30

Dimension New

Question 31

________ refers to the size to which a part may wear before it must be rejected as unserviceable. Parts, which are not worn beyond this size, can be used again, providing a suitable mating part is chosen to keep the clearance within the permissible figure. This will frequently involve choosing a new part to mate with the worn part

Answer 31

Permissible Worn Dimension

Question 32

If the center line of the shaft is not straight, then the item is ______.

Answer 32

bowed

Question 33

If the twist disappears, as a result of removing the force, then the shaft will have been loaded ______.

Answer 33

below its elastic limit

Question 34

If the shaft remains twisted, after removal of the load, then it has been loaded _________.

Answer 34

above its elastic limit

Question 35

Solid or tubular shafts that have to be checked for twist will possibly have witness_______. The shafts can be checked, by mounting the shaft in V blocks and then locating these marks in the horizontal position.

Answer 35

marks or lines engraved or etched at each end of the shaft

Question 36

CLEARANCE MEASURED BY FEELER GAUGES
Over the length of a member in general a maximum bow of _______ is normally acceptable unless otherwise stated in the repair manual. During normal servicing or overhauling of aircraft components, dimensional checks may be carried out with the aid of various types of precision measuring instruments for example; micrometer, vernier caliper, and dial indicator. Wear limits may be found in the appropriate repair or overhaul manual

Answer 36

1 in 600