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Limits and Fits

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Page 1: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Limits and Fits

Page 2: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Introduction• In engineering you are usually

concerned with a number of parts orcomponents fitting together to makean:

assembly

Page 3: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

• To assemble components togetherengineers must control the sizes ofthe components to make sure thatthey fit together correctly.

• If they don’t fit together correctlythen the components will not functionproperly.

Page 4: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

What is a FIT in engineering terms?

• There are three types of fit used in engineering:

1. Clearance Fit2. Transition Fit3. Interference Fit

Page 5: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

1. The Clearance Fit.This is where the shaft slides through the hole

Called the Clearance

GAP

Page 6: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

2. The Transition Fit.This is where the shaft slides through the hole very tightly.

NO GAP

Called the Transition

Page 7: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

3. The Interference Fit.This is where the shaft slides through the hole and shears the metal has it goes in.

SHEARING

Called the Interference

Page 8: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Other Names Used for the Fits

• Sliding Fit allows one part to slide inanother whilst there is no forcebeing applied to the shaft

The Clearance Fit

Page 9: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

• A Force Fit which needs a “bigHammer” or preferable a power pressto assemble the shaft and hole.

The Interference Fit

• A Push Fit which means the hole andthe shaft can be push together orperhaps tapped gentle with a softhammer.

The Transition Fit

Page 10: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

How do we decide which fit to use?

• First consider what is required of the fit.

• Then decide how much clearance orinterference you are going to give your fit.

• Then determine what nominal size you aregoing to fit together.

Page 11: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

What are Nominal sizes?

• They are basically any size, it maybe any whole number you like: -

• 16 12 40 60 1200 230,000

• Now you have the size, how muchinterference or clearance do we use?

Page 12: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

We need a system to help!

Advantages of having a system:Interchangeable - parts made at different times or

places fit together.

Time saving - reference to a standard system speedsup decisions on sizes to give particular fits.

Assurance of correct fit - actual fit will be as requiredeven if design size changes.

Spare Parts – manufacture of these is made easier asthey can be made to the same standard.

Page 13: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

British Standards 4500

• Uses the nominal size for both holes andshafts.

• This size is determined by strengthconsiderations.

• Calculations of material properties andstrengths are used to obtain the nominalsize.

• Remember: The nominal size is the samefor both hole and shaft.

Page 14: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

BS4500• The British Standard System uses the letters of

the alphabet to describe how much bigger orsmaller a hole or a shaft will deviate from thenominal size.

• This deviation is called the:-

Fundamental deviation.

• Holes are described by capital letters H, G etc.• Shafts are described by lower case letters h, g

etc.

Page 15: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Zero lineA B

The Fundamental Deviation

HK

M

The Fundamental Deviation

The Fundamental Deviation

Nominal Size

The Holes

Page 16: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

The Shafts

Nominal Size

Zero Line

Fundamental Deviation

Fundamental DeviationNominal Size

a

b g m e

Page 17: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

M

A

Any combination of hole and shaft can be use

e

A/e fit

aSo be careful when choosing your fits they don’t all work

M/a fit

Page 18: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

British Standards Tolerance zones

Tolerance Zone

BS4500 use numbered tolerance zones (01 to 16)

Page 19: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

• The most accurate tolerance zones are numbered 01 02 and 03.

• Whereas for more general engineering work we use 06, 07 and 08.

• Finally, rough work is done at 14,15 and 16.

British Standards Tolerance zones

In engineering we don’t use the zeros

Page 20: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Round up• You now have a letter and a number.

• Put them together and you have the type of fit and the tolerance that will achieve it. Examples are:

»H7, K8 and M9.»For holes.

• For shafts we use:• g6, h7 and f7.

Page 21: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

• Here then is the complete fit:

• The Hole The shaft

H9 f7

Page 22: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

BS 4500

Page 23: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Limits of size• Look on the chart

• Maximum and minimum allowances are shown in thousandths of a millimetre

• Minimum clearance and minimum interference is minimum hole size

minus maximum shaft size

• Tolerance is max deviation minus min deviation

Page 24: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Bush

Shaft

Pulley

SpacerHousing

Standard washer

Today's task

Decide what fits are required between the shaft and bush and the bush and pulley. Give reason for your choices in not less than 600 words.

Page 25: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Bush pressed in using force. Fit is Interference

Shaft must beallowed to slideso pulley andbush can rotate

Pulley

Spacer creates another type of fit

H7 Hole

P6 Shaft

Fit should be Clearance Fit

Choice is: -H7 Hole in both cases and for the shaft either g6 or h6

Clearance fit: -

H7 hole (width of bush) and g6 for the shaft (length to first shoulder)

Page 26: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

When does Tolerances become important

• Assemblies: Parts will often not fit together if their

• dimensions do not fall with in a certain range of values

Page 27: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

When do Tolerances become important

• Interchangeability

• If a replacement part is used it must be

• a duplicate of the original part within certain limits of deviation.

Page 28: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

When do Tolerances become important

• Tolerances do not affect the function here

Page 29: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

When do Tolerances become important

• Tolerances are important here !

Page 30: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

When do Tolerances become important

• What about the tolerance levels in this mechanism?

Page 31: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Straightness

Page 32: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Geometric tolerance

Page 33: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Measuring and gauging• Measuring is a direct reading process

in which the inspection instrument consists of (or incorporates ) a continuous series of linear measurement units, commonly known as a scale.

Page 34: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Measuring and gauging• Gauging is checking that a component

fits within tolerance limits

• Tolerances for Go gauges are placed within working limits

• Tolerances for No Go gauges are outside working limits

Page 35: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Plug gauge

Page 36: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside
Page 37: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Snap go/no go gauge for the OD of a cylindrical

workpie

Page 38: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Replaceable thread and plug gauges

Page 39: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Plug Gauge operation

Hole Diameter (mm) Dia of Go End Dia of No Go end

a, 40 ± 0.02 39.98 40.02

b, 46 + 0.03 46.00 45.03

C, 30 - 0.2 29.80 30.00

Page 40: Limits and Fits - Physics Champion and Fits ken.pdffits within tolerance limits •Tolerances for Go gauges are placed within working limits •Tolerances for No Go gauges are outside

Other gauges