TRADE OF HEAVY VEHICLE MECHANIC - eCollege

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Module 1 – Unit 4 Bench Fitting and Drawing There would be a gap, or technically clearance, between the diameter of the hole and the shaft. Alternatively you might want the shaft to be a tight fit in the hole and machine the shaft slightly larger than the hole –the method of determining the actual size will be discussed later! In this case you would need to force the shaft into the hole. The hole and shaft diameters interfere with each other. Notice that it was suggested carrying the fit by varying the size of the shaft. Because the hole is the fixed size this system is based on the hole and is known as a hole basis. The hole in our example could just as easily be a slot, groove, gap or similar internal feature whilst the shaft could be a tongue, tenon or similar external feature. You could, however, decide on a shaft size and vary the hole sizes to give various fits. This would be known as a shaft basis. A particular base is a shaft having the same basic size throughout, which is designed to carry various parts required to assemble a shaft with different fits. Such variations in fit required by bearings, couplings, collars sliding members or other parts would involve expensive machining of the shaft in order to vary its diameter and to adjust the bore of each part to give the required fit. Motor Mechanic – Phase 2 2 Revision 1.0 September 2007
Module 1 – Unit 4 Bench Fitting and Drawing 3.0 S I System A set of units known as the metric system is used internationally by most nations. It was introduced by the French National Assembly late in the sixteenth century and was adopted by Ireland in the early 1970s. One of the main characteristics of the system is its decimal nature; therefore, the conversion between smaller and larger units is made by moving the decimal point to the left of right. The SI system of units (Systeme International d’Unites), developed from the metric system, and has been defined and recommended as the system of choice for scientific use worldwide. The primary units in the SI system which are of interest to the motor mechanic are as follows: QUANTITY UNIT SYMBOL Length Meter m Mass Kilogram kg Capacity Litre l Temperature Degree Celsius oC Temperature Degree Kelvin K Time Second s Force Newton N Heat Joule j Power Watt w Motor Mechanic – Phase 2 3 Revision 1.0 September 2007
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