Design of Machine Element HW # 2



22.425 Design of Machine Element HW # 5

Ninth Edition

8–4 A single-threaded 25-mm power screw is 25 mm in diameter with a pitch of 5 mm. A vertical load on the screw reaches a maximum of 5 kN. The coefficients of friction are 0.06 for the collar and 0.09 for the threads. The frictional diameter of the collar is 45 mm. Find the overall efficiency and the torque to “raise” and “lower” the load.

8–9 Find the power required to drive a 1.5-in power screw having double square threads with a pitch of 1/4 in. The nut is to move at a velocity of 2 in/s and move a load of F = 2.2 kips. The frictional coefficients are 0.10 for the threads and 0.15 for the collar. The frictional diameter of the collar is 2.25 in.

8–60 The section of the sealed joint shown in the figure is loaded by a force cycling between 4 and 6 kips. The members have E = 16 Mpsi. All bolts have been carefully preloaded to Fi = 25 kip each.

(a) Determine the yielding factor of safety.

(b) Determine the overload factor of safety.

(c) Determine the factor of safety based on joint separation.

(d) Determine the fatigue factor of safety using the Goodman criterion.

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8–61 Suppose the welded steel bracket shown in the figure is bolted underneath a structural-steel ceiling beam to support a fluctuating vertical load imposed on it by a pin and yoke. The bolts are 1/2 -in coarse-thread SAE grade 8, tightened to recommended preload for nonpermanent assembly. The stiffnesses have already been computed and are kb = 4 Mlb/in and km = 16 Mlb/in.

(a) Assuming that the bolts, rather than the welds, govern the strength of this design, determine the safe repeated load that can be imposed on this assembly using the Goodman criterion with the load line in Fig. 8–20 and a fatigue design factor of 2.

(b) Compute the static load factors based on the load found in part (a).

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