Module 7 (Maintenance Practices) Sub Module 7.6 (Fits and Clearances) (Repaired).pdf

ISO 9001:2008 Certified For Training Purpose Only

PIA TRAINING CENTRE (PTC) Module 7 - MAINTENANCE PRACTICES Category A/B1 Sub Module 7.6 Fits and Clearances

PTC/CM/B1.1 Basic/M7/02 Rev.00

7.6 Mar 2014

MODULE 7

Sub Module 7.6

FITS AND CLEARANCES



PTC/CM/B1.1 Basic/M7/02 Rev.00

7.6 - i Mar 2014

Contents

DRILL SIZES FOR BOLT HOLES ---------------------------------------------------- 4

CLASSES OF FITS -------------------------------------------------------------------- 5

COMMON SYSTEM OF FITS AND CLEARANCES ----------------------------- 10

SCHEDULE OF FITS AND CLEARANCES FOR AIRCRAFT AND ENGINES- 12

LIMITS FOR BOW ------------------------------------------------------------------ 13

LIMITS FOR TWIST ----------------------------------------------------------------- 13

LIMITS FOR WEAR ----------------------------------------------------------------- 14

STANDARD METHODS FOR CHECKING SHAFTS, BEARINGS AND OTHERS ------------------------------------------------------------------------------ 16



PTC/CM/B1.1 Basic/M7/02 Rev.00 7.6 - 1 Mar 2014

FITS AND CLEARANCES For ease of manufacture and replacement, it is essential that the components of similar mechanisms should be interchangeable. For this reason limits (tolerances and allowances) are imposed on the manufacturing procedures. The limits on dimensions ensure that, if any two mating parts are manufactured to the tolerances and allowances, stated on the drawing, then they will assemble without the need of further hand working or machining to achieve the required standard of fit. Because shafts are much easier than holes to machine small diameters, the main systems of Limits and Fits, for engineering purposes, is usually the hole-based system. The holes are created to a certain tolerance and the sizes of the shafts are altered to provide the required class of fit between the two items. DIMENSIONS Mass production has long been the basis of the approach to the most economic methods of manufacturing and the complete replacement of a defective item is common practice in the maintenance of aircraft and aerospace components.

For this reason, limits are imposed on the manufacturing processes, to ensure that, if any two mating parts are manufactured to the dimensions as stated on the relevant drawings, then the parts will assemble without need of further major adjustments and in the least time possible. The limits are based on the allowances and tolerances imposed on the dimensions of the manufactured parts. These dimensions will be given the accuracy required by the designer of the respective parts. ALLOWANCES An allowance is a difference in dimension that is necessary to give a particular class of fit between two parts. If, for example (and using a typical limit system), a shaft were required to locate with a corresponding hole in a component. Then, to assist in the economy of manufacture, either the hole or the shaft is made as accurately as possible to the nominal size and an allowance is applied to the associated item. The term shaft also includes bolts and pins. If the shaft is constant and the hole varies in size, then the system used is said to be shaft-based. If the hole is constant and the shaft varies in size, then the system is hole based. The hole-based system is the one in more general use.




The item dimensioned to include the allowance also has high and low limits and, therefore, a tolerance. The correct allowance would be the difference between the high limit of the shaft and the low limit of the hole. TOLERANCES The tolerance on a dimension is the variation tolerated and may be considered as a numerical expression of the desired quality of workmanship. It is the difference between the high and low limits of size for that dimension (refer to Fig. 1). Thus, a part that should be exactly 25 mm nominal diameter, will be accepted for a certain purpose if it is within the limits 25.1 mm, (the high limit); and 24.9 mm, (the low limit). The difference between the two (0.2 mm) is the tolerance.

High limit of shaft

Low limit of shaft

Low limit of hole

High limit of hole

Shaft/Hole Tolerance Terms Fig. 1




Table 1 shows that tolerances may be stated in one of three ways, using a hole that has a nominal size of 100 mm diameter and a tolerance of 0.02 mm as an example. Table 1 TYPES OF TOLERANCES

Bilateral 100 mm 0.01 mm Unilateral 100 mm + 0.02 mm

100 mm 0.00 mm

Limits 100.01 mm 99.99 mm

With sheet material, such as patch plates, used in certain repairs, the dimensions quoted in the repair scheme usually have a tolerance in one direction only, the nominal size being the lower limit. In effect the patch plate must never be below the nominal size, although it can be slightly over, in accordance with the repair scheme in the manual.




DRILL SIZES FOR BOLT HOLES The size of hole to be drilled depends upon the purpose of that hole. A hole drilled for a rivet with a specific diameter would differ from those drilled to take a screw thread, or the plain shank of a bolt, of the same diameter. Similarly the size of a hole which is to accommodate a shaft will depend on the size of the shaft and on the manner in which the hole/shaft combination is to be used. Additionally, if the hole is to be reamed, then it must be drilled slightly smaller than its nominal size, to allow for the metal removed by the reamer. Drill sizes (as discussed in the Tools topic) are fixed and can be found on charts that list each standard drill size, together with other columns such as clearance and tapping sizes. These charts may also include equivalent sizes displayed in metric, fractional, letter and in the number/letter system.




CLASSES OF FITS There are three principal classes of fit, between shafts and holes, and they are the: Interference Fit: where the shaft is larger than the hole Transition Fit: where the shaft and hole are approximately

the same size Clearance Fit: where the shaft is smaller than the hole.

HOLE-BASED SYSTEM

Transition Fit Interference Fit Clearance Fit

British Standards System of Fits Fig. 2




EXAMPLE 1 The limits shown on a drawing for a mating hole and shaft are:

Find what type of fit will exist between the hole and shaft. The smallest hole is 10.000 + 0.000 = 10.000 mm. The largest shaft is 10.000 - 0.006 = 9.994 mm. Hence the shaft is always smaller than the hole and a clearance fit will be obtained.

EXAMPLE 2 The limits shown on drawing for a mating hole and shaft are:

Find the type of fit that exists between the hole and shaft. The largest hole is 40.000 + 0.025 = 40.025 mm The smallest shaft is 40.000 + 0.043 = 40.043 mm The shaft is always larger than the hole and hence we have an interference fit.




EXAMPLE 3 The limits shown on a drawing for a mating hole and shaft are:

Find the type of fit that exists between the hole and shaft. The largest shaft is 30.000 + 0.018 = 30.018 mm The smallest hole is 30.000 + 0.000 = 30.000 mm This combination gives an interference fit. The smallest shaft is 30.000 + 0.002 = 30.002 mm The largest hole is 30.000 + 0.025 = 30.025 mm This combination gives a clearance fit. Hence, since we can have either interference or clearance, we have a transition fit.




Newall system In the early Newall hole-based system of limits, the holes are classified as Class A and Class B fits. Class A holes are manufactured to a closer tolerance than are Class B holes. Table 2 shows how the shafts are classified, using the letters F, P, D, X, Y, and Z.

Table 2 Newall System of Fits




British standards system The British Standard (BS 4500) hole-based system of fits (refer to Fig. 2), also uses the basic terminology for holes and shafts. The terminologies are similar to those used in the Newall system (and several other systems), the holes and shafts are identified by a more extensive alpha/numeric identifier. It can, however, be seen that, in an Interference Fit, the upper and lower limits of the shaft are greater than the corresponding limits of the hole and, thus, force is necessary to achieve the fit. In the Transition Fit, the differences in the upper and lower limits of both items are negligible so that only light effort is required to insert the shaft into the hole. The upper and lower limits of the shaft, in a Clearance Fit, are always less than those of the hole, so that the shaft moves easily within the hole.




COMMON SYSTEM OF FITS AND CLEARANCES Limits It is impossible to make anything to an exact size. One way of getting over this difficulty is to define an acceptable margin for every dimension. The extreme permissible values of a dimension are known as limits. The larger dimension is called the upper limit and the smaller dimension the lower limit. Nominal size The dimension of an object when variations in size are disregarded Basic size Basic Size is the theoretical exact size from which limits of size are determined by the application of allowances and tolerances. Deviation Deviation is the algebraic difference between a size and the corresponding basic size. Upper deviation Upper deviation is the algebraic difference between the maximum limit and the corresponding basic size. Lower deviation Lower deviation is the algebraic difference between the minimum limit and the corresponding basic size. Upper limit - The 'upper limit' is the largest size allowed. Lower limit - The 'lower limit' is the smallest size allowed.

Fundamental deviation Fundamental deviation is either the upper or the lower deviation, depending on which is closer to the basic size. The fundamental deviation locates the tolerance zone relative to the basic size. Tolerance Tolerance is the difference between the upper and lower limit. Where variation either side of the nominal dimension can occur, the tolerance is called bilateral. Where one tolerance is zero the tolerance is said to be unilateral. International tolerance grade (IT grade) number International tolerance grade numbers are numbers which for a particular IT Numbers have the same relative level of accuracy but vary depending upon the nominal or basic size. British Standard Limits and Fits (BS 4500) - Governs limits and fits used for holes and shafts in industry. Clearance Clearance is the measured difference in dimensions of two mating parts when the inner member is smaller than the outer member. Here the reference is to the inner dimensions of the outer part and the outer dimensions of the inner part. Interference Interference is the measured difference in dimensions of two mating parts when the inner member is larger than the outer member, where the outer dimensions of the inner part and the inner dimensions of the outer part is considered.




Allowance The minimum stated clearance or the maximum stated interference for mating parts.




SCHEDULE OF FITS AND CLEARANCES FOR AIRCRAFT AND ENGINES Wear occurs at any time that there is motion between two parts. This motion can be intentional, such as when a shaft rotates in a plain (journal) bearing or when a roller moves back and forth over a track. Wear can also be accidental, where two parts, that should be immovable, chafe together. If the parts are intended to move together, then the maintenance documentation will have a Schedule of Fits and Clearances, based on the limit system, issued for each mechanism, used on the aircraft. If the parts are not intended to move together, it will depend upon inspection procedures to discover the problem and repair schemes will be initiated, in an attempt to prevent recurrence. The Schedule of Fits and Clearances contains tables, which specify the limits on wear and other characteristics such as:

Ovality (of a hole or shaft) Bow of a shaft

Twist of a shaft.




LIMITS FOR BOW When dealing with shafts and tubes, it is vital that not only are the ends square with each other, but that the centreline of the complete shaft or tube is straight. If the centre line of the shaft is not straight, then the item is bowed. When the shaft or tube is rotating, especially at a high speed in a bowed state, there is the risk of vibration, which can lead to mechanical failures, loosening of fasteners and (most critical of all) fatigue. All cylindrical items, both tubular and solid, can be given a limit to the amount of bow permitted. For example a drive shaft, which rotates about 1500 rpm, may have a limit of 0.25 mm (0.01 in) bow over the length of the shaft. This ensures that, within the limits of production, the drive shafts are effectively straight, giving the least possible vibration.

LIMITS FOR TWIST Twist is the result of applied torsion on circular or square-sectioned shafts. If the twist disappears, as a result of removing the force, then the shaft will have been loaded below its elastic limit. If the shaft remains twisted, after removal of the load, then it has been loaded above its elastic limit. The action of a shaft (of whatever section), carrying a torque load is to twist in proportion to the torque applied. The result of cyclic loading of shafts is that, at certain times, the shafts have to be checked for permanent twist. If the shaft has a square section, it can be checked for twist on a surface table using a DTI mounted on a surface gauge. Solid or tubular shafts that have to be checked for twist will possibly have witness marks or lines engraved or etched at each end of the shaft. The shafts can be checked, by mounting the shaft in V blocks and, then, locating these marks in the horizontal position. It is possible to measure the amount of twist, to which a shaft is subjected, whilst in operation or rotation, by the use of strain gauges. These emit varying amounts of electric current when under strain, giving an indication (on a calibrated instrument) of the load being applied.




The designer of the aircraft or equipment will set all limits, with regards to the distortion of parts and set them down in the relevant manuals. The methods used to measure the distortion will either be standard procedures, such as using a DTI and surface table etc., or will have a special procedure included in the manuals.

LIMITS FOR WEAR The four dimensions typically covered in wear tables are:

Dimension New

Permissible Worn Dimension

Clearance New

Permissible Worn Clearance. Dimension New relates to the size of the part when new, and will show the relevant tolerances. Permissible Worn Dimension refers to the size to which a part may wear before it must be rejected as unserviceable. Parts, which are not worn beyond this size, can be used again, providing a suitable mating part is chosen to keep the clearance within the permissible figure. This will frequently involve choosing a new part to mate with the worn part. Clearance New is the desired clearance in limit form. Interference fits are quoted as negative clearances. Permissible Worn Clearance refers to the maximum allowable clearance when reassembling the component.




Limits for Ovality This usually occurs as a result of the surface wearing, through friction or linear movement. Ovality and can apply equally to holes and shafts (refer to Fig. 3). Holes may be tested for ovality, using such instruments as Go/No-Go gauges, internal micrometers, or callipers, as were previously discussed in the Tools topic of this course. A shaft may be tested for ovality, by the use of snap gauges, external callipers and micrometers, which were, again, discussed in the Tools topic. It is important to test for ovality of a shaft, before testing it for bow, as the results may be suspect if bow is done first. Bow in a shaft can be determined, in a workshop, by utilising V blocks, a surface gauge and a DTI (in conjunction with a surface table).

HOLE

Wear

Ovality of a Hole or a Shaft Fig. 3

SHAFT

Wear




STANDARD METHODS FOR CHECKING SHAFTS, BEARINGS AND OTHERS Iso system of limits The International Federation of National Standardizing Associations (ISA) system has recently been revised by the International Organization for Standardization (ISO) which is postwar successor to ISA, and the revised system is given in ISO Recommendation R 286 dated December 1962.

In the system the 27 possible holes are designated by Uppercase letters ABCDEetc, and the shafts by Lowercase letters covering the same range. (Figure A and B)

Hole basis and Shaft basis fits are available fundamental

deviation in such situations will be H for the hole in the hole basis system and h for the shaft in the shaft basis system.

The 18 accuracy grades are covered by the numerals

01, 0, 1, 2 16.

For specifying any particular hole or shaft the rule is to write the letter followed by the numeral denoting the tolerance grade, e.g. H7 for a hole or f7 for a shaft. A fit involving these two elements is written as H7 f7 or H7/f7.

For ordinary engineering practice the H holes and

particularly H7, H8, H9 and H 11 are recommended as being satisfactory for most purposes.

The British Standards Institution has revised BS 1916 1953 to include these additions and modifications. Thus the ISO system of limits is set out in BS 4500 1969 which allows for 27 types of fits and 18 grades of tolerance, covering a size range of zero to 3150mm.




Fig. A Shafts used in ISO hole basis system

Fig. B Holes used in ISO shaft basis system




Fig. C - Fundamental Deviations for ISO Holes

Basic Size Fundamental Deviation forshafts

UpperDeviation Lower Deviation

a b c cd d f g h j k m n p r s t u v x y z za zb zc

3 -270 -140 -60 -34 -20 -6 -2 0 -4 0 +2 +4 +6 +10 +14 - +18 - +20 - +26 +32 +40 +60

3-6 -270 -140 -70 -46 -30 -10 -4 0 -4 +1 +4 +8 +12 +15 +19 - +23 - +28 - +35 +42 +50 +80

6-10 -280 -150 -80 -56 -40 -13 -5 0 -5 +1 +6 +10 +15 +19 +23 - +28 - +34 - +42 +52 +67 +97

10-14 -290 -150 -95 - -50 -16 -6 0 -6 +1 +7 +12 +18 +23 +28 - +33 - +40 - +50 +64 +90 +130

14-18 -290 -150 -95 - -50 -16 -6 0 -6 +1 +7 +12 +18 +23 +28 - +33 +39 +45 - +60 +77 +108 +150

18-24 -300 -160 -110 - -65 -20 -7 0 -8 +2 +8 +15 +22 +28 +35 - +41 +47 +54 +63 +73 +98 +136 +188

24-30 -300 -160 -110 - -65 -20 -7 0 -8 +2 +8 +15 +22 +28 +35 +41 +48 +55 +64 +75 +88 +118 +160 +218

30-40 -310 -170 -120 - -80 -25 -9 0 -10 +2 +9 +17 +26 +34 +43 +48 +60 +68 +80 +94 +112 +148 +200 +274

40-50 -320 -180 -130 - -80 -25 -9 0 -10 +2 +9 +17 +26 +34 +43 +54 +70 +81 +97 +114 +136 +180 +242 +325

50-65 -340 -190 -140 - -100 -30 -10 0 -12 +2 +11 +20 +32 +41 +53 +66 +87 +102 +122 +144 +172 +226 +300 +405

65-80 -360 -200 -150 - -100 -30 -10 0 -12 +2 +11 +20 +32 +43 +59 +75 +102 +120 +146 +174 +210 +274 +360 +480

80-100 -380 -220 -170 - -120 -36 -12 0 -15 +3 +13 +23 +37 +51 +71 91 +124 +146 +178 +214 +258 +335 +445 +585

100-120 -410 -240 -180 - -120 -36 -12 0 -15 +3 +13 +23 +37 +54 +79 +104 +144 +172 +210 +254 +310 +400 +525 +690

120-140 -460 -260 -200 - -145 -43 -14 0 -18 +3 +15 +27 +43 +63 +92 +122 +170 +202 +248 +300 +365 +470 +620 +800

140-160 -520 -280 -210 - -145 -43 -14 0 -18 +3 +15 +27 +43 +65 +100 +134 +190 +228 +280 +340 +415 +535 +700 +900

160-180 -580 -310 -230 - -145 -43 -14 0 -18 +3 +15 +27 +43 +68 +108 +146 +210 +252 +310 +380 +465 +600 +780 +1000

180-200 -660 -340 -240 - -170 -50 -15 0 -21 +4 +17 +31 +50 +77 +122 +166 +236 +284 +350 +425 +520 +670 +880 +1150

200-225 -740 -380 -260 - -170 -50 -15 0 -21 +4 +17 +31 +50 +80 +130 +180 +258 +310 +385 +470 +575 +740 +960 +1250

225-250 -820 -420 -280 - -170 -50 -15 0 -21 +4 +17 +31 +50 +84 +140 +196 +284 +340 +425 +520 +640 +820 +1050 +1350

250-280 -920 -480 -300 - -190 -56 -17 0 -26 +4 +20 +34 +56 +94 +158 +218 +315 +385 +475 +580 +710 +920 +1200 +1550

280-315 -1050 -540 -330 - -190 -56 -17 0 -26 +4 +20 +34 +56 +98 +170 +240 +350 +425 +525 +650 +790 +1000 +1300 +1700

315-355 -1200 -600 -360 - -210 -62 -18 0 -28 +4 +21 +37 +62 +108 +190 +268 +390 +475 +590 +730 +900 +1150 +1500 +1900

355-400 -1350 -680 -400 - -210 -62 -18 0 -28 +4 +21 +37 +62 +114 +208 +294 +435 +530 +660 +820 +1000 +1300 +1650 +2100




Basic Size Fundamental Deviation for holes

Upper Deviation Lower Deviation

A B C CD D F G H J K M N P R S T U V X Y Z ZA ZB ZC

0-3 +270 +140 +60 +34 +20 +6 +2 0 -4 0 2 -4 -6 -10 -14 - -18 - -20 - -26 -32 -40 -60

3-6 +270 +140 +70 +46 +30 +10 +4 0 -4 -1 -4 -8 -12 -15 -19 - -23 - -28 - -35 -42 -50 -80

6-10 +280 +150 +80 +56 +40 +13 +5 0 -5 -1 -6 -10 -15 -19 -23 - -28 - -34 - -42 -52 -67 -97

10-14 +290 +150 +95 - +50 +16 +6 0 -6 -1 -7 -12 -18 -23 -28 - -33 - -40 - -50 -64 -90 -130

14-18 +290 +150 +95 - +50 +16 +6 0 -6 -1 -7 -12 -18 -23 -28 - -33 -39 -45 - -60 -77 -108 -150

18-24 +300 +160 +110 - +65 +20 +7 0 -8 -2 -8 -15 -22 -28 -35 - -41 -47 -54 -63 -73 -98 -136 -188

24-30 +300 +160 +110 - +65 +20 +7 0 -8 -2 -8 -15 -22 -28 -35 -41 -48 -55 -64 -75 -88 -118 -160 -218

30-40 +310 +170 +120 - +80 +25 +9 0 -10 -2 -9 -17 -26 -34 -43 -48 -60 -68 -80 -94 -112 -148 -200 -274

40-50 +320 +180 +130 - +80 +25 +9 0 -10 -2 -9 -17 -26 -34 -43 -54 -70 -81 -97 -114 -136 -180 -242 -325

50-65 +340 +190 +140 - +100 +30 +10 0 -12 -2 -11 -20 -32 -41 -53 -66 -87 -102 -122 -144 -172 -226 -300 -405

65-80 +360 +200 +150 - +100 +30 +10 0 -12 -2 -11 -20 -32 -43 -59 -75 -102 -120 -146 -174 -210 -274 -360 -480

80-100 +380 +220 +170 - +120 +36 +12 0 -15 -3 -13 -23 -37 -51 -71 91 -124 -146 -178 -214 -258 -335 -445 -585

100-120 +410 +240 +180 - +120 +36 +12 0 -15 -3 -13 -23 -37 -54 -79 -104 -144 -172 -210 -254 -310 -400 -525 -690

120-140 +460 +260 +200 - +145 +43 +14 0 -18 -3 -15 -27 -43 -63 -92 -122 -170 -202 -248 -300 -365 -470 -620 -800

140-160 +520 +280 +210 - +145 +43 +14 0 -18 -3 -15 -27 -43 -65 -100 -134 -190 -228 -280 -340 -415 -535 -700 -900

160-180 +580 +310 +230 - +145 +43 +14 0 -18 -3 -15 -27 -43 -68 -108 -146 -210 -252 -310 -380 -465 -600 -780 -1000

180-200 +660 +340 +240 - +170 +50 +15 0 -21 -4 -17 -31 -50 -77 -122 -166 -236 -284 -350 -425 -520 -670 -880 -1150

200-225 +740 +380 +260 - +170 +50 +15 0 -21 -4 -17 -31 -50 -80 -130 -180 -258 -310 -385 -470 -575 -740 -960 -1250

225-250 +820 +420 +280 - +170 +50 +15 0 -21 -4 -17 -31 -50 -84 -140 -196 -284 -340 -425 -520 -640 -820 -1050 -1350

250-280 +920 +480 +300 - +190 +56 +17 0 -26 -4 -20 -34 -56 -94 -158 -218 -315 -385 -475 -580 -710 -920 -1200 -1550

280-315 +1050 +540 +330 - +190 +56 +17 0 -26 -4 -20 -34 -56 -98 -170 -240 -350 -425 -525 -650 -790 -1000 -1300 -1700

315-355 +1200 +600 +360 - +210 +62 +18 0 -28 -4 -21 -37 -62 -108 -190 -268 -390 -475 -590 -730 -900 -1150 -1500 -1900

355-400 +1350 +680 +400 - +210 +62 +18 0 -28 -4 -21 -37 -62 -114 -208 -294 -435 -530 -660 -820 -1000 -1300 -1650 -2100

Fig. C - Fundamental Deviations for ISO Holes




Iso tolerance grade numbers The magnitude of the tolerance zone is the unilateral variation in part size and is the same for both the internal and the external dimensions. The tolerance zones are specified in international tolerance grade numbers, called IT numbers. The smaller grade numbers specify a smaller tolerance zone. This range from IT 01 to IT 16, but only grades IT06 to ITII are needed for the preferred fits.




Fig. A

Module 7 (Maintenance Practices) Sub Module 7.6 (Fits and Clearances) (Repaired).pdf

Documents

Transcript of Module 7 (Maintenance Practices) Sub Module 7.6 (Fits and Clearances) (Repaired).pdf