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the loosest fit, having the greatest positive allowance. Class 8 is the tightest fit, having the most negative allowance. Class 4 is zero allowance - shaft and bore diameters are the same. Figure 7 illustrates. BORE +ALLOWANCE ZERO SHAFT 1 2 SHAFT ALLOWANCE BORE 3 4 5 6 7 8 FIGURE 7 ASA Fits Figure 7 is somewhat of an exaggeration. Class 1 is a loose fit, often found in agricultural and mining machin- ery. Class 2 is a precision free fit, used in gasoline and diesel engine shafts. Class 3 is the snuggest possible pos- itive allowance. It is found in precision machine tools and sliding parts, such as automotive valve hydraulic lifters and automatic transmission parts. Class 4 is zero allowance; shaft and bore diameters are the same. It is the closest fit that can be assembled by hand. In common parlance it is called a "slip fit with no slop." Classes 5 and 6 are used to fit ball or roller bearings for medium duty, and are often found together. As an exam- ple, the bearing may be fitted on the shaft at Class 6, and in the housing at Class 5. You will find this to be true dur- ing bearing replacement. The bearing will usually slide out of its housing with light pressure, but you will need a puller to remove it from the shaft. Class 6 is often called a "light press fit." Class 7 is a medium force fit, and Class 8 is a heavy force fit. These classes are often assembled by heating the outer part, causing it to expand. In addition, the shaft may be cooled with dry ice or other means, causing it to con- tract or shrink. Therefore these two classes are called shrink fits. Shrink fits produce considerable internal stresses in the outer part, so the part must be designed to withstand these stresses. Regular grey cast iron is somewhat brittle, and cannot be fitted tighter than Class 7. High carbon steels and alloy steels can withstand Class 8 fits. Ball or roller bearings used on gearless elevator machine main shafts are usually shrink fit at Class 7. It should be evident that this will stretch the inner bearing race, reducing internal clearance. However, bearings are designed in such a way that if the recommended shaft fits are used and operating temperatures are taken into account, the internal clearance remaining after mounting will be sufficient. Bearings must often be heated for mounting on the heit - 120° Celsius. Extreme pressure is required to they must be cut off carefully using a torch. remove bearings mounted in this way, and sometimes Review Questions Check your answers on page 24 17. Measurements on elevator layouts are given in the U.S.System of measurement, however many escalator prints are dimensioned in measurements. 18. One kilometer is equal to 19. One centimeter is equal to one .20.Fit refers to the meters. of a meter drical part and the bore intended to receive it. 21. There are numbered classes of fits. between the outside diameter of any cylin- 22. Ball and roller bearings on a gearless elevator main shaft are usu- ally installed with a Class TOLERANCES fit. Tolerance is a numerical way of stating the degree of accuracy required in a measurement. It may be stated as a measurement followed by a plus or minus allowance. An alternative means of stating tolerance is by indicating limits, i.e. the maximum and minimum dimensions of a part. Certain measurements require extreme accuracy, while others are not so critical. Bearing journals on high-speed rotating shafts may require accuracy within one ten-thou- sandth of an inch. On the other hand, distance between main guide rails on an elevator installation are usually held to plus or minus 1/64". Tolerances on certain escalator parts may be much closer. The title block on an elevator layout drawing may include a statement that all fractional dimensions are to be held within plus or minus 1/64". If tolerance is not stated, it is understood to be plus or minus 1/64". A tolerance of plus or minus 1/64" gives a range of 1/32" between the max- imum and minimum dimension. As an example, distance between main guide rails might be shown as 5'6" plus or minus 1/64". Using this tolerance, distance between guides can be 5'647/64" mini- mum and 5'649/64" maximum. The larger dimension is called the upper limit and the smaller dimension is called the lower limit. In Module 5, Guide Rails, you will learn to assemble a gauge to maintain this tolerance. Dimension: inch fractions is usually state in diameter m inch. To show fraction resul measuring 1/20 using a micro Because to form for sma sions are usu ing example shown as .75 By stating the upper a Adding .005 gives .745 m a calculator. are given or MIN accord Escalator elevators. W between ele 0 DIM'S FOR FOR 33 Galion 10