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Chapter 3

Q. 3.1

The main bearing of an engine is shown in Fig. 3.25. Calculate
(i) the maximum and minimum diameters of the bush and crank pin; and
(ii) the maximum and minimum clearances between the crank pin and bush.
Suggest suitable machining methods for both.

Step-by-Step

Verified Solution

Step I Maximum and minimum diameters of the bush and crank pin
The tolerances for the bush and crank pin from Tables 3.2 and 3.3a are as follows:

Table 3.2 Tolerances for holes of sizes up to 100 mm (H5 to H11)

H Diameter
steps in mm
5-11 11 10 9 8 7 6 5
ei es to over
0 +60 +40 +25 +14 +10 +6 +4 3 0
0 +75 +48 +30 +18 +12 +8 +5 6 3
0 +90 +58 +36 +22 +15 +9 +6 10 6
0 +110 +70 +43 +27 +18 +11 +8 18 10
0 +130 +84 +52 +33 +21 +13 +9 30 18
0 +160 +100 +62 +39 +25 +16 +11 50 30
0 +190 +120 +74 +46 +30 +19 +13 80 50
0 +220 +140 +87 +54 +35 +22 +15 100 80

Table 3.3a Tolerances for shafts of sizes up to 100 mm (from d to h)

h g f e d Diametersteps in
mm
10 9 8 7 6 5 5-10 7 6 6-7 8 7 6 6-8 9 8 7 6 6-9 11 10 9 8 8-11
ei es ei es ei es ei es ei es to over
-40 -25 -14 -10 -6 -4 0 -12 -8 -2 -20 -16 -12 -6 -39 -28 -24 -20 -14 -80 -60 -45 -34 -20 3 0
-48 -30 -18 -12 -8 -5 0 -16 -12 -4 -28 -22 -18 -10 -50 -38 -32 -28 -20 -105 -78 -60 –48 -30 6 3
-58 -36 -22 -15 -9 -6 0 -20 -14 -5 -35 -28 -22 -13 -61 -47 -40 -34 -25 -130 -98 -76 –62 -40 10 6
-70 -43 -27 -18 -11 -8 0 -24 -17 -6 -43 -34 -27 -16 -75 -59 -50 -43 -32 -160 -120 -93 –77 -50 18 10
-84 -52 -33 -21 -13 -9 0 -28 -20 -7 -53 -41 -33 -20 -92 -73 -61 -53 -40 -195 -149 -117 –98 -65 30 18
-100 -62 -39 -25 -16 -11 0 -34 -25 -9 -64 -50 -41 -25 -112 -89 -75 -66 -50 -240 -180 -142 –119 -80 50 30
-120 -74 -46 -30 -19 -13 0 -40 -29 -10 -76 -60 -49 -30 -134 -106 -90 -79 -60 -290 -220 -174 –146 -100 80 50
-140 -87 -54 -35 -22 -15 0 -47 -34 -12 -90 -71 -58 -36 -159 -126 -107 -94 -72 -340 -260 -207 –174 -120 100 80

Bush:

20^{\overset{+0.013}{_{+0.000}}}         \text { or } \frac{20.013}{20.000} mm .

Crankpin:

20^{\overset{-0.040}{_{-0.061}}}         \text { or } \frac{19.960}{19.936} mm             (i).

Step II Maximum and minimum clearances
Maximum clearance = 20.013 – 19.939 = 0.074 mm
Minimum clearance = 20.000 – 19.960 = 0.040 mm      (ii).

Step III Machining methods
The bush (H6) is usually machined by precision boring, honing or hand reaming methods. The crank pin (e7) is machined by high precision turning or broaching.