18. Assembly

• the mating of parts to give a combined operation.

• In previous centuries, and before, fit in assemblies was often not considered, or when it did matter, each piece was custom fitted.

• Modern methods of mass production means that some fundamental methods of fitting are necessary.

• The three basic methods of fitting are,

Fitting: One part is made to size, and then a second part is made to fit it.

Selective Assembly: parts are made to loose general tolerances, and then the results are sorted into tolerance ranges. (e.g. bearings, solar cells, etc)

Interchangeable Assembly: Parts are made to tighter tolerances, and as a result assembly of randomly selected parts will yield a good fit. This is essential for modern assembly lines.

• The basic categories of fits are,

Clearance: a gap is always present between parts

Transitional: the parts will have a gap sometimes, other times the parts will touch

Interference: both parts will always be in full contact

18.1 The Basics of Fits

• A set of reasonable fits is suggested below,

shafts rotating under 600 rpm with ordinary loads; >RC5

shafts rotating over 600 rpm with heavy loads; < RC5

shafts sliding freely; approx. LC

push fits with keyed shafts and clamp, no fitting; LT

parts assembled with some basic fitting; LN

Permanent assembly with no freely moving parts; FN1

permanent assembly with severe loading effects; FN3

permanent assembly with press needed for assembly; FN5

18.1.1 Clearance Fits

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• A clearance fit always has a gap between the two mating parts.

• The diagram below shows a clearance fit between a shaft and a hole

 

18.1.2 Transitional Fits

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• This type of fit may result in interference, or clearance

• This type of fit can be used for items such as snap fits

• The figure below illustrates this condition for a hole shaft pair

 

18.1.3 Interference Fits

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• Interference fits always overlap and are used mainly for press fits where the two parts are pushed together, and require no other fasteners

• The figure below shows an interference fit for a hole shaft pair

 

18.2 CSA B97-1 1963 Limits and Fits

• Five types of fits are allowed, with a variety of classes

 

 

• This system is hole based (it uses the “H” hole)

 

• There are 22 diameter steps from 0” to 200”

• A fit is classified as a grade of “H” hole and a grade of shaft

• For example,

 

• To go on further with a more complicated example, consider the hole shaft pair with a bushing

 

 

 

18.3 CSA Modified Fits

 

18.4 CSA Limits and Fits

 

 

 

18.5 The ISO System

• Basic features of this system were,

several diameter steps

22 holes and 22 shafts

16 tolerance grades

18.6 Problems

Problem 18.1 On a Ferris wheel we have a 3.5” running journal that is to be pressure lubricated. The fit selected for this application is RC4. Use a tolerance diagram to determine the tolerances required on a final drawing. Sketch the hole, and shaft using appropriate drafting techniques.

Problem 18.2 2. Do complete drawings for a 3.000” hole shaft pair if they have a RC3 fit.

Problem 18.3 Clearance fits are found in,

a) fitted assembly.

b) interchangeable assembly.

c) selective assembly.

d) all of the above.

Problem 18.4 Which statement is more true?

a) production errors cause tolerances.

b) there are no standard tolerances.

c) both a) and b) are completely true.

d) neither a) or b) is true.

Problem 18.5 Given the diagram below, what will the average interference/clearance be?

a) 0.008”

b) 0.020”

c) 0.032”

d) none of the above

Problem 18.6 Briefly describe the relationship between tolerance and accuracy.

Problem 18.7 A hole shaft pair uses a bushing. We know that the fit between the shaft and bushing is LC5, with a nominal diameter of 7” and the fit between the bushing and outer hole is 8” with an FN3 fit.

a) Draw the tolerance diagrams.

b) Draw the final parts with dimensions and tolerances.

c) What will the gap between the shaft and the bushing be?

 

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