解题，将解题步骤写的更详细一点，所关联的知识点也要详细讲解，解题步骤要非常详细，计算过程也要详细---Extraction Content: **Problem Description:** 73. A steel plate with thickness $\delta=12 \text{ mm}$ is fastened to a cast iron support with thickness $\delta_1=30 \text{ mm}$ using 4 bolts. The height of the pressure surface (under the bolt head or nut) is $h=8 \text{ mm}$. The bolt material is Q235. The allowable normal stress for bolts is $[\sigma]=95 \text{ MPa}$, the allowable shear stress for bolts is $[\tau]=96 \text{ MPa}$. The allowable bearing stress for the steel plate is $[\sigma_{bs}]=320 \text{ MPa}$, and for the cast iron is $[\sigma_{bs}]_1=180 \text{ MPa}$. The friction coefficient between the mating surfaces is $f=0.15$. The reliability coefficient is $K_s=1.2$. The load is $F_\Sigma=12000 \text{ N}$. The dimensions are $l=400 \text{ mm}$, $a=100 \text{ mm}$. The bolt arrangements are shown in figures (a) and (b). **(1)** Compare which bolt arrangement scheme is reasonable. **(2)** According to the reasonable bolt arrangement scheme, respectively determine the bolt diameter when using bolts in clearance holes and when using bolts in reamed holes (fitted bolts). **Diagram Description:** Two diagrams, (a) and (b), are shown, each depicting a bolted joint connecting a steel plate to a cast iron support. Each diagram includes a top view and a side view. * **Side View (Same for both (a) and (b)):** Shows a rectangular steel plate of thickness $\delta$ resting on a stepped cast iron support of thickness $\delta_1$. A bolt assembly is shown passing through the plates, consisting of a bolt head, a bolt shank, and a nut. The height of the pressure surface $h$ is indicated. A downward force $F_\Sigma$ is applied to the free end of the steel plate. The distance $l$ from the bolt group's centroid (labeled O in the top view) to the point of force application is indicated. * **Top View (Different for (a) and (b)):** Shows the layout of the 4 bolts on the surface of the steel plate, which is L-shaped in this view. Point O is marked as the centroid of the bolt group. The distance from the left edge of the plate to the vertical line passing through O is labeled as $2a$. The dimensions $2a$ indicating the spacing between bolts are shown. The total length $l$ from the centroid O to the end where the load is applied is indicated. * **Diagram (a):** The top view shows the 4 bolts arranged in a rectangular pattern centered at O. The horizontal distance between the centers of the bolts in each row is $2a$. The vertical distance between the centers of the bolts in each column is $2a$. Thus, the bolts are at positions relative to O (assuming O is (0,0)) approximately at (+a, +a), (+a, -a), (-a, +a), (-a, -a). * **Diagram (b):** The top view shows the 4 bolts arranged in a diamond/cross pattern centered at O. Two bolts are located on the horizontal line through O, and two bolts are located on the vertical line through O. The horizontal distance between the centers of the two horizontal bolts is $2a$. The vertical distance between the centers of the two vertical bolts is $2a$. Thus, the bolts are at positions relative to O (assuming O is (0,0)) approximately at (+a, 0), (-a, 0), (0, +a), (0, -a).