Limits and Fits
Introduction In engineering you are usually concerned with a number of parts or components fitting together to make an: assembly
To 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.
What is a FIT in engineering terms? There are three types of fit used in engineering: 1. Clearance Fit 2. Transition Fit 3. Interference Fit
1. The Clearance Fit. This is where the shaft slides through the hole GAP Called the Clearance
2. The Transition Fit. This is where the shaft slides through the hole very tightly. O 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 in another whilst there is no force being applied to the shaft The Clearance Fit
A Force Fit which needs a big Hammer or preferable a power press to assemble the shaft and hole. The Interference Fit A Push Fit which means the hole and the shaft can be push together or perhaps tapped gentle with a soft hammer. The Transition Fit
How 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.
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 much interference 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 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.
British Standards 4500 Uses 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.
BS4500 The 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.
The Holes The Fundamental Deviation A B Zero line Nominal Size H K M The Fundamental Deviation The Fundamental Deviation
The Shafts Fundamental Deviation a Zero Line Nominal Size b g m e Nominal Size Fundamental Deviation
Any combination of hole and shaft can be use So be careful when A choosing your fits they don t all work e a M M/a fit A/e fit
British Standards Tolerance zones Tolerance Zone BS4500 use numbered tolerance zones (01 to 16)
British Standards Tolerance zones 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. 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 H9 The shaft 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
Today's task Housing Spacer Pulley 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.
Pulley Spacer creates another type of fit Clearance fit: - H7 hole (width of bush) and g6 for the shaft (length to first shoulder) Choice is: - H7 Hole in both cases and for the shaft either g6 or h6 Shaft must be allowed to slide so pulley and bush can rotate Fit should be Clearance Fit Bush pressed in using force. Fit is Interference H7 Hole P6 Shaft
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