## Relationship to functional performance

A journal bearing in a car engine is a convenient example of the necessity of carefully defining tolerances. If a journal bearing is designed to operate at high rotational speeds, the diamentral clearance is very important. If the clearance is too small, the bearing will seize whereas if the clearance is too large, the journal will vibrate within the bearing, creating noise, wear, vibration and heat. There is therefore an optimum clearance which is associated with smooth running. However, because variabilities are always present, an optimum range has to be specified rather than an absolute value. TTie left-hand drawing in Figure 5.1 shows a sketch of a journal bearing of nominal diameter 20mm, which has been designed to run at speed. The tolerances associated with the shaft and bearing are 19,959/19,980 and 20,000/20,033. These are the 'limits' of size. They have been selected from special tables that relate certain performance situations to tolerance ranges (BS 4500A and B).

When the shaft and bearing are manufactured to these values the journal bearing will operate satisfactorily at speed without vibration or seizure. The tolerance ranges given in Figure 5.1 refer to a 'close-running fit'. The word 'fit' is used specifically here because it describes the way that the journal fits in the bearing in terms of the dimensional relationships. For a 'close-running' fit, the tolerance ranges are given the designation: H8/f7. The standard tables show that the minimum diameter for the f7 shaft is 19,959mm and the maximum diameter is 19,980. With respect to the H8 hole, the minimum allowable diameter is 20,000mm and the maximum is 20,033. Thus, the average clearance is 47um, the minimum is 20um and the maximum is 74um. This means that if the clearance in the journal bearing is less than 20um, it will seize and if it is greater than 74um, wear and vibrations will result. Under these 'close-running fit' tolerances, the shaft and bearing will perform satisfactorily.

The right-hand sketch in Figure 5.1 shows a 'sliding fit'. This would apply to, say, a spool valve in which a shaft translates and/or rates at slow speed. The 'sliding fit' class corresponds to tolerance grades H7 and g6. The H7 tolerance applies to the hole and is 21um (i.e. 20,021-20,000). The shaft tolerance is g6 and is 13um (i.e. 19,993-19,980). These tolerance bands mean that the maximum clearance is 41um, the minimum clearance is 7um and

020,00 H8/f7

020,00 H7/g6

020,00 H8/f7

020,00 H7/g6

020,00 H8/f7

I Close-running fit

020,00 H7/g6

Figure 5.1 Examples of two different types of bearings and their tolerances

020,00 H8/f7

I Close-running fit

020,00 H7/g6

| Sliding fit

Figure 5.1 Examples of two different types of bearings and their tolerances the average is 24um. These are about half the values of the 'close-running' fit of the left-hand sketch in Figure 5.1.

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### Responses

• anonymous
what is the clearance for H7g6
9 years ago
• sophie
How to show tolerance h7 in drawing?
9 years ago
• Myrtle
WHAT MEANS H7g6 TOLERANCE?
7 years ago
• lobelia sandyman
HOW TO SHOW H7 CLEARANCE ON DRAWING?
7 years ago