FITS.ppt engineering graphics and drawings

engineering drawing

1. NETA PowerPoint® Slides
to accompany
Prepared by
Ursula Westelaken,
Lambton College
1Copyright © 2015 Nelson Education Ltd.

2. Unit 20
This unit describes the fit between mating parts,
such as
•inch fits
•description of fits
•standard inch fits
Copyright © 2015 Nelson Education Ltd. 2

3. Inch Fits
–Fit describes the range of tightness or looseness
resulting from the application of a specific
combination of allowances and tolerances in the
design of mating parts.
–Fits are of three general
types:
•Clearance
•Interference
•Transition
Copyright © 2015 Nelson Education Ltd. 3

4. Clearance Fits
–Clearance fits have limits of size prescribed, so a clearance
always results when mating parts are assembled.
–Clearance fits are intended for accurate assembly of parts
and bearings.
Copyright © 2015 Nelson Education Ltd. 4

5. Interference Fits
–Interference fits have limits of size so prescribed that an
interference always results when mating parts are assembled.
–Parts are usually pressed together using an arbor press.
Copyright © 2015 Nelson Education Ltd. 5

6. Transition Fits
–Transition fits have limits of size so prescribed that either a
clearance or interference may result when mating parts are
assembled.
–Transition fits are a compromise between clearance and
interference fits.
Copyright © 2015 Nelson Education Ltd. 6

7. Description of Fits
•Running and Sliding Fits
–Represent a special type of clearance fit.
–Are intended to provide a similar running
performance, with suitable lubrication allowance,
throughout the range of sizes.
–See Table 17 of the Appendix for tolerances and
clearances.
Copyright © 2015 Nelson Education Ltd. 7

8. Description of Fits cont’d
•Locational Fits
–Locational fits are intended to determine only the
location of the mating parts.
–They may provide rigid or accurate location, as
with interference fits, or some freedom of
location, as with clearance fits.
–Accordingly, they are divided into three groups:
•Clearance fits
•Transition fits
•Interference fits
Copyright © 2015 Nelson Education Ltd. 8

9. Description of Fits cont’d
•Drive and Force Fits
–Drive and force fits constitute a special type
of interference fit, normally characterized by
maintenance of constant bore pressures
throughout the range of sizes.
Copyright © 2015 Nelson Education Ltd. 9

10. Standard Inch Fits
–Standard fits are designated for design purposes in
specifications and on design sketches by means of the symbols.
–The letter symbols used are as follows:
•RC Running and sliding fit
•LC Locational clearance fit
•LT Locational transition fit
•LN Locational interference fit
•FN Force or shrink fit
Copyright © 2015 Nelson Education Ltd. 10

11. 11
Standard Inch Fits cont’d
–Following the letter symbols are numbers which
represent the class of fit.
–See Appendix Tables 17–21.
Copyright © 2015 Nelson Education Ltd.

12. Running and Sliding Fits
•RC1 Precision Sliding Fit
–This fit is intended for the accurate location of
parts that must assemble without perceptible
play for high-precision work, such as gauges.
•RC2 Sliding Fit
–This fit is intended for accurate location, but
with greater maximum clearance than class RC1.
–Parts made to this fit move and turn easily but
are not intended to run freely.
Copyright © 2015 Nelson Education Ltd. 12

13. Running and Sliding Fits cont’d
•RC3 Precision Running Fit
–This fit is about the closest fit that can be
expected to run freely, and is intended for
precision work for oil-lubricated bearings at slow
speeds and light journal pressures.
Copyright © 2015 Nelson Education Ltd. 13

14. 14
Running and Sliding Fits cont’d
•RC4 Close Running Fit
–This fit is intended chiefly as a running fit for
grease- or oil-lubricated bearings on accurate
machinery with moderate surface speeds and
journal pressures, where accurate location and
minimum play are desired.
Copyright © 2015 Nelson Education Ltd.

15. Running and Sliding Fits cont’d
•RC5 and RC6 Medium Running Fits
–These fits are intended for higher running speeds
and/or where temperature variations are likely to
be encountered.
•RC7 Free Running Fit
–This fit is intended for use where accuracy is not
essential, and/or where large temperature
variations are likely to be encountered.
Copyright © 2015 Nelson Education Ltd. 15

16. Copyright © 2015 Nelson Education Ltd. 16
Running and Sliding Fits cont’d
•RC8 and RC9 Loose Running Fits
–These fits are intended for use where materials
made to commercial tolerances are involved, such
as cold-rolled shafting, tubing, etc.

17. Locational Clearance Fits
–Locational clearance fits are intended for parts that
are normally stationary but can be freely assembled
or disassembled.
Copyright © 2015 Nelson Education Ltd. 17

18. Locational Clearance Fits cont’d
•LC1 to LC4
–These fits have a minimum zero clearance, but in
practice the probability is that the fit will always
have a clearance.
•LC5 and LC6
–These fits have a small minimum clearance intended
for close location fits for non-running parts.
Copyright © 2015 Nelson Education Ltd. 18

19. Copyright © 2015 Nelson Education Ltd. 19
Locational Clearance Fits cont’d
•LC7 to LC11
–These fits have progressively larger clearances
and tolerances, and are useful for various loose
clearances for assembly of bolts and similar parts.

20. Locational Transition Fits
–Locational transition fits are a compromise between
clearance and interference fits, for application where
accuracy of location is important.
–But either a small amount of clearance or interference
is permissible.
Copyright © 2015 Nelson Education Ltd. 20

21. Locational Transition Fits cont’d
•LT1 and LT2
–These fits average a slight clearance, giving a light
push fit.
•LT3 and LT4
–These fits average virtually no clearance, and are
for use where some interference can be tolerated.
•LT5 and LT6
–These fits average a slight interference, although
appreciable assembly force will be required.
Copyright © 2015 Nelson Education Ltd. 21

22. Locational Interference Fits
–Locational interference fits are used where accuracy
of location is of prime importance, and for parts
requiring rigidity and alignment with no special
requirements for bore pressure.
Copyright © 2015 Nelson Education Ltd. 22

23. Locational Interference Fits cont’d
•LN1 and LN2
–These are light press fits, with very small minimum
interference, suitable for parts such as dowel pins,
which are assembled with an arbor press in steel,
cast iron, or brass.
Copyright © 2015 Nelson Education Ltd. 23

24. Copyright © 2015 Nelson Education Ltd. 24
Locational Interference Fits cont’d
•LN3
–This is suitable as a heavy press fit in steel and
brass or a light press fit in more elastic materials
and light alloys.
•LN4 to LN6
–While LN4 can be used for permanent assembly
of steel parts, these fits are primarily intended as
press fits for soft materials.

25. Force or Shrink Fits
–Force or shrink fits constitute a special type of
interference fit.
–The interference varies almost directly with diameter,
and the difference between its minimum and
maximum values is small to maintain the resulting
pressures within reasonable limits.
Copyright © 2015 Nelson Education Ltd. 25

26. Force or Shrink Fits cont’d
•FN1 Light Drive Fit
–This fit requires light assembly pressure and
produces more or less permanent assemblies.
•FN2 Medium Drive Fit
–This fit is suitable for heavier steel parts or as a
shrink fit on light sections.
Copyright © 2015 Nelson Education Ltd. 26

27. Force or Shrink Fits cont’d
•FN3 Heavy Drive Fit
–This fit is suitable for heavier steel parts or as a
shrink fit in medium sections.
•FN4 and FN5 Force Fits
–These fits are suitable for parts that can be highly
stressed.
Copyright © 2015 Nelson Education Ltd. 27

28. Basic Hole System
–The basic hole system is recommended for general
use where the basic size will be the design size for
the hole and the tolerance will be plus.
–The design size for the shaft will be the basic design
size minus the minimum clearance, or plus the
maximum interference, and the tolerance will be minus.
Copyright © 2015 Nelson Education Ltd. 28

29. Basic Shaft System
–Fits are sometimes required on a basic shaft system,
especially in cases where two or more fits are required
on the same shaft.
–For design purposes, the letter S follows the fit symbol;
for example, RC7S.
–Tolerances for holes and shafts are identical with those
for a basic hole system, but the basic size becomes the
design size for the shaft, and the design size for the
hole is found by adding the minimum clearance or
subtracting the maximum interference from the basic
size.
Copyright © 2015 Nelson Education Ltd. 29