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GOOD TO THE LAST DROP after mounting unting on the 250° Fahren- s required to d sometimes S. System of mensioned in of any cylin- ft are usu- gree of ted as ance. cating Dimensions for holes to be drilled may be stated in inch fractions or in decimals. For drilled holes, tolerance is usually stated as a decimal inch. Holes to be drilled 3/4" in diameter might carry a tolerance of plus or minus .005 inch. To show .005", or five thousandths of an inch as a fraction results in 1/200". There is no convenient way of measuring 1/200 of an inch but .005 can easily be measured using a micrometer. Because tolerances are more easily shown in decimal form for smaller measurements, fractional inch dimen- sions are usually shown in decimal form. In the preced- ing example, 3/4" hole + or - .005" would probably be shown as .750" hole + or - .005". By stating dimension and tolerance in decimal form, the upper and lower limit can easily be determined. Adding .005 to 3/4" is difficult, but adding .005 to .750 gives .745 minimum and .755 maximum without needing a calculator. Where maximum and minimum dimensions are given on a drawing, they may be labeled MAX and MIN according to ANSI Standards. Escalators are installed to much closer tolerances than elevators. Where 1/64" is usually close enough for distance between elevator guide rails, escalator tracks must be 460 663 5 of a 161 417 620 GLASS 8MM more perfectly aligned. Track alignment is critical on escalators to prevent excessive wear and noisy operation. Observe an escalator in operation. Note how closely the step treads mesh with the comb plates. For these parts to mesh silently, they must be precisely aligned. Other unseen parts of the escalator require the same tight clear- ances. Figure 8 is a cutaway view of a section of an escalator truss and balustrade. Standard escalator widths are 32" or 48", although other widths are manufactured. On this drawing the dimensions with diamonds are for 48" width, and those with triangles are for 32" width. In the lower right corner, note the tolerance table. Fig- ures given here tell the accuracy expected from the installing mechanic. Where the upper and lower limits of tolerance are shown as plus and minus measurements, tolerance is said to be bilateral. Bilateral tolerance means the part can be a certain size larger or smaller than standard. All of the pre- vious examples have shown bilateral tolerance. Figure 9 illustrates three different means of showing the same bilateral tolerance. In some cases, conditions dictate that a part can be smaller than standard, but no larger. In this case, tolerance may be indicated plus .000" and minus .003". For instance, the diameter of a shaft might be indicated on a drawing as 2.500" plus or minus .000/.002. This means the diameter must not be larger than 2.500", but it may be .002" undersize and still pass inspection. That is, mini- mum shaft size is 2.498". Alternate ways of showing tol- erance in one direction only are 2.500" plus zero/minus .002", or "shaft size 2.500" to 2.498". Tolerance in one direction only is called unilateral tolerance. Unilateral tolerances are common in mating parts. When one part must be made to fit into another, a negative tolerance may while peed you- een lly tor ay ΤΟ d, 0 DIM'S FOR 32 STEP FOR 48 STEP 305 508 STEP CHAIN 329.4 A532.6 HANDRAIL DRIVE 1387 ▲ 590.2 MAIN DRIVE CHAIN 463 ▲ 666.2 579 782 GENERAL TOLERANCE MM DEC. INCHES 406 1.016 FIGURE 8 Use of Metric on Escalators 11 1.522 1.518 (a) 1.520 (b) +.002 -002 -1.520 ±.002 (c) FIGURE 9 Bilateral Tolerances