TABLE OF CONTENTS 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. The 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...
