FIGURE 7 Design curves for Mf/(µMp) for φ1=10°. r/R ratios for the upper, external brake, curves are: 1—r/R=0.2; 2—r/R=0.4; 3—r/R=0.6; 4—r/R=0.8. r/R ratios for the lower, internal brake, curves are: 5—r/R=1.2; 6—r/R=1.4; 7—r/R=1.6; 8—r/R= 1.8.

Since pmax, (sin Φ)max, and µ cancel out when equation (1-11) is divided by the product of µ and equation (1-9), the ratio Mf/(µMp) is a function of only three quantities: r/R, Φ1, and Φ2. Thus, Mp/(µMp) may be plotted as a function of Φ2 for fixed values of r/R and Φl, as in Figures 7 and 8. Criterion (3.4) also can be included in these graphs by noting that 1/µ>0 pertains to external drum brakes and 1/µ<0 pertains to internal drum brakes, so these values may be shown on the left-hand ordinate of these graphs by relating them to the limiting values of Mf/(µMp) according to relation (3.4), namely, that at the lower limit,

and that at the upper limit,

Consequently, the ordinates on the right-hand sides of the graphs in Figures 7 and 8 are the reciprocals of the ordinates on the left-hand sides. Thus, we may read directly from these graphs that to be non-self-locking, the Mf/(µMp) ratio must fall below the 1/µ value for external drum brakes, and it must fall above the −1/µ value for internal drum brakes.