A shaft carrying an idler sheave for a belt drive system is to have a nominal size of 2.00 in. The sheave must rotate reliably on the shaft, but with the smoothness characteristic of accurate machinery. Specify the limits of size for the shaft and the sheave bore, and list the limits of clearance

Question

A shaft carrying an idler sheave for a belt drive system is to have a nominal size of 2.00 in. The sheave must rotate reliably on the shaft, but with the smoothness characteristic of accurate machinery. Specify the limits of size for the shaft and the sheave bore, and list the limits of clearance that will result. Use the basic hole system.

Accepted Answer

An RC5 fit should be satisfactory in this application. From Table 13–3, the hole tolerance limits are +1.8 and -0. The sheave hole then should be within the following limits:
Sheave Hole

2.0000 + 0.0018 = 2.0018 in (largest)
2.0000 - 0.0000 = 2.0000 in (smallest)
Notice that the smallest hole is the basic size.
The shaft tolerance limits are -2.5 and -3.7. The resulting size limits are as follows:
Shaft Diameter
2.0000 - 0.0025 = 1.9975 in (largest)
2.0000 - 0.0037 = 1.9963 in (smallest)

Figure 13–5 illustrates these results.
Combining the smallest shaft with the largest hole gives the largest clearance. Conversely, combining the largest shaft with the smallest hole gives the smallest clearance. Therefore, the limits of clearance are
2.0018 - 1.9963 = 0.0055 in (largest)
2.0000 - 1.9975 = 0.0025 in (smallest)
These values check with the limits of clearance in Table 13–3. Notice that the total tolerance for the shaft is 0.0012 in, and for the hole 0.0018 in, both relatively small values.

TABLE 13–3 Clearance Fits (RC)
Nominal size range (in)	Class RC2			Class RC5			Class RC8			Nominal size range (in)
Nominal size range (in)	Limits of clearance	Standard limits		Limits of clearance	Standard limits		Limits of clearance	Standard limits		Nominal size range (in)
Over To	Limits of clearance	Hole	Shaft	Limits of clearance	Hole	Shaft	Limits of clearance	Hole	Shaft	Over To
0–0.12	0.1	+0.25	-0.1	0.6	+0.6	-0.6	2.5	+1.6	-2.5	0–0.12
	0.55	0	-0.3	1.6	-0	-1.0	5.1	0	-3.5
0.12–0.24	0.15	+0.3	-0.15	0.8	+0.7	-0.8	2.8	+1.8	-2.8	0.12–0.24
	0.65	0	-0.35	2.0	-0	-1.3	5.8	0	-4.0
0.24–0.40	0.2	+0.4	-0.2	1.0	+0.9	-1.0	3.0	+2.2	-3.0	0.24–0.40
	0.85	0	-0.45	2.5	-0	-1.6	6.6	0	-4.4
0.40–0.71	0.25	+0.4	-0.25	1.2	+1.0	-1.2	3.5	+2.8	-3.5	0.40–0.71
	0.95	0	-0.55	2.9	-0	-1.9	7.9	0.0	-5.1
0.71–1.19	0.3	+0.5	-0.3	1.6	+1.2	-1.6	4.5	+3.5	-4.5	0.71–1.19
	1.2	0	-0.7	3.6	-0	-2.4	10.0	0	-6.5
1.19–1.97	0.4	+0.6	-0.4	2.0	+1.6	-2.0	5.0	+4.0	-5.0	1.19–1.97
	1.4	0	-0.8	4.6	-0	-3.0	11.5	0	-7.5
1.97–3.15	0.4	+0.7	-0.4	2.5	+1.8	-2.5	6.0	+4.5	-6.0	1.97–3.15
	1.6	0	-0.9	5.5	-0	-3.7	13.5	0	-9.0
3.15–4.73	0.5	+0.9	-0.5	3.0	+2.2	-3.0	7.0	+5.0	-7.0	3.15–4.73
	2.0	0	-1.1	6.6	-0	-4.4	15.5	0	-10.5
4.73–7.09	0.6	+1.0	-0.6	3.5	+2.5	-3.5	8.0	+6.0	-8.0	4.73–7.09
	2.3	0	-1.3	7.6	-0	-5.1	18.0	0	-12.0
7.09–9.85	0.6	+1.2	-0.6	4.0	+2.8	-4.0	10.0	+7.0	-10.0	7.09–9.85
	2.6	0	-1.4	8.6	-0	-5.8	21.5	0	-14.5
9.85–12.41	0.7	+1.2	-0.7	5.0	+3.0	-5.0	12.0	+8.0	-12.0	9.85–12.41
	2.8	0	-1.6	10.0	-0	-7.0	25.0	0	-17.0

Question 13.1: A shaft carrying an idler sheave for a belt drive system is ...

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