Limits and Fits

Introduction• In engineering you are usually

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

assembly

• 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.

What is a FIT in engineering terms?

• There are three types of fit used in engineering:

1. Clearance Fit2. Transition Fit3. Interference Fit

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

Called the Clearance

GAP

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

NO GAP

Called the Transition

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

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

• 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

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.

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?

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.

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.

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.

Zero lineA B

The Fundamental Deviation

HK

M

The Fundamental Deviation

The Fundamental Deviation

Nominal Size

The Holes

The Shafts

Nominal Size

Zero Line

Fundamental Deviation

Fundamental DeviationNominal Size

a

b g m e

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

British Standards Tolerance zones

Tolerance Zone

BS4500 use numbered tolerance zones (01 to 16)

• 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

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.

• Here then is the complete fit:

• The Hole The shaft

H9 f7

BS 4500

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

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.

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)

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

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.

When do Tolerances become important

• Tolerances do not affect the function here

When do Tolerances become important

• Tolerances are important here !

When do Tolerances become important

• What about the tolerance levels in this mechanism?

Straightness

Geometric tolerance

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.

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

Plug gauge

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

workpie

Replaceable thread and plug gauges

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

Other gauges