2assembly Introduction In engineering you are usually concerned with a number of parts or components fitting together to make an:assembly
3To assemble components together engineers must control the sizes of the components to make sure that they fit together correctly.If they don’t fit together correctly then the components will not function properly.
4What is a FIT in engineering terms? There are three types of fit used in engineering:1. Clearance Fit2. Transition Fit3. Interference Fit
5The Clearance Fit. This is where the shaft slides through the hole GAPCalled the Clearance
62. The Transition Fit. This is where the shaft slides through the hole very tightly. NO GAPCalled the Transition
7Called the Interference 3. The Interference Fit. This is where the shaft slides through the hole and shears the metal has it goes in.SHEARINGCalled the Interference
8Other Names Used for the Fits Sliding Fit allows one part to slide in another whilst there is no force being applied to the shaftThe Clearance Fit
9The Interference Fit The Transition Fit A Force Fit which needs a “big Hammer” or preferable a power press to assemble the shaft and hole.The Interference FitA Push Fit which means the hole and the shaft can be push together or perhaps tapped gentle with a soft hammer.The Transition Fit
10How do we decide which fit to use? First consider what is required of the fit.Then decide how much clearance or interference you are going to give your fit.Then determine what nominal size you are going to fit together.
11What are Nominal sizes?They are basically any size, it maybe any whole number you like: -,000Now you have the size, how much interference or clearance do we use?
12We 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 speeds up decisions on sizes to give particular fits.Assurance of correct fit - actual fit will be as required even if design size changes.Spare Parts – manufacture of these is made easier as they can be made to the same standard.
13British Standards 4500Uses the nominal size for both holes and shafts.This size is determined by strength considerations.Calculations of material properties and strengths are used to obtain the nominal size.Remember: The nominal size is the same for both hole and shaft.
14Fundamental deviation. BS4500The British Standard System uses the letters of the alphabet to describe how much bigger or smaller a hole or a shaft will deviate from the nominal 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.
15The Holes The Fundamental Deviation B H Nominal Size K M A Zero line
16The Shafts Nominal Size Nominal Size Zero Line Fundamental Deviation a bgFundamental Deviationme
17Any combination of hole and shaft can be use So be careful when choosing your fits they don’t all workeM/a fitA/e fit
18British Standards Tolerance zones BS4500 use numbered tolerance zones (01 to 16)
19British Standards Tolerance zones The most accurate tolerance zones are numbered and 03.Whereas for more general engineering work we use 06, 07 and 08.Finally, rough work is done at 14,15 and 16.In engineering we don’t use the zeros
20Round up H7, K8 and M9. 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.
21Here then is the complete fit: The Hole The shaftH9f7
23Tolerance is max deviation minus min deviation Limits of sizeLook on the chartMaximum and minimum allowances are shown in thousandths of a millimetreMinimum clearance and minimum interference is minimum hole size minus maximum shaft sizeTolerance is max deviation minus min deviation
24Today's task Pulley Spacer Housing Shaft Standard washer Bush 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.
25PulleyShaft must be allowed to slide so pulley and bush can rotateSpacer creates another type of fitFit should be Clearance FitClearance fit: -H7 hole (width of bush) and g6 for the shaft (length to first shoulder)Bush pressed in using force. Fit is InterferenceChoice is: -H7 Hole in both cases and for the shaft either g6 or h6H7 HoleP6 Shaft
26When does Tolerances become important Assemblies: Parts will often not fit together if theirdimensions do not fall with in a certain range of values
27When do Tolerances become important InterchangeabilityIf a replacement part is used it must bea duplicate of the original part within certain limits of deviation.
28When do Tolerances become important Tolerances do not affect the function here
29When do Tolerances become important Tolerances are important here !
30When do Tolerances become important What about the tolerance levels in this mechanism?
33Measuring and gaugingMeasuring 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.
34Measuring and gaugingGauging is checking that a component fits within tolerance limitsTolerances for Go gauges are placed within working limitsTolerances for No Go gauges are outside working limits