东大机械设计考试样卷

(1)     For a tight bolted connection with transverse load, the bolt is carried by     . 4ju=5D];  
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A. shear stress             B. tensile stress kuX{2h*`  
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C. shear stress and tensile stress   D. shear stress or tensile stress -G|?Kl  
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(2)     For a tight bolted connection with axial tensile load F and preload , the resultant tensile force in the bolt is       . F4R0A6HL  
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(3)     If the performance grade of a bolt is 6.8, then the yield limit stress of the bolt material is       MPa. ^J]_O_ee$  
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A. 600       B. 800       C. 680       D. 480 (p?7-~6|:  
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(4)     The lubricating method of a journal bearing is dependent on     . `|2p1
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A. average intensity of pressure p   B. circle velocity of journal v   C. pv   D. uH.1'bR?a  
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(5)     For a hydrodynamic radial journal bearing, the valid method for increasing is to     . {^jRV @  
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C. decrease     D. replace lubricating oil with lower viscosity "?n ;dXYSi  
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(6)     For a hydrodynamic radial journal bearing, the relative clearance is the ratio of     . ,bRYqU?#0  
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A. diametral clearance and journal diameter   B. radial clearance and journal diameter 2E$K='H:,  
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C. minimal clearance and diametral clearance m2x=Qv][@c  
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D. minimal clearance and journal diameter ;/R\!E   
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(7)     The axial preloading for rolling bearings is for _______. NxGSs_7  
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A. increasing loading capacity     B. decreasing noise a
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C. improving running precision   D. avoiding shaft displacement m> YjV>5  
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(8)     The inner ring and outer ring of the     bearing are separable. E06)&tF  
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A. deep-groove ball   B. self-aligning ball   C. tapered roller   D. self-aligning roller Xwz'h;Ks_  
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(9)     In an important application, if the reliability of rolling bearings is increased to 95%, then the relation between the dynamic load rating relative to 95% reliability to the basic dynamic load rating C is _______. ,\'E<O2T  
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(10)   For a cylindrical helical compressive spring, the maximal shear stress on spring wire happens at the ______. fI"OzIJV  
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C. center of spring wire     D. arbitrary position on spring wire X 3(CY`HH[  
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(11)   For a cylindrical helical compressive spring with working coils being 12, if the number of working coils is decreased by 3, then spring stiffness is _____. L 4Sa,ZL  
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A. decreased to 3/4 of the original value   B. decreased to 9/16 of the original value N+CcWs!E  
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(12)   For a static balanced rigid rotor, its mass center is     located at the rotating axis. 7$A=|/'nSA  
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(13)   Shown in the figure is a cylindrical cam with uniform material, accurate manufacture and installation. When it rotates round the axis A-A, the cam is under the state of _________. Ur,{ZGm  
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A. static imbalance   B. static balance   C. complete imbalance   D. dynamic balance 5z@QAQ  
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(14)   The installation of flywheel in machinery is to       . k5
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A. eliminate speed fluctuation     B. realize steady running 2UrE>_  
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C. decrease speed fluctuation     D. balance the inertia forces { PS|q?  
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(15)   In order to decrease the flywheel weight, the flywheel should be installed on the       。 }ZkGH}K_}  
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A. equivalent link               B. crankshaft with lower rotating speed KYC<*1k  
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C. crankshaft with higher rotating speed   D. spindle of machinery mExJ--}  
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2.     (7 points) In a tight bolted connection with axial tensile load, both the preload and the axial working load are constant. If the metal gasket between the connected components is replaced by a leather one, explain, with force-deformation diagram, the changes of the resultant tensile force in the bolt and residual compressive force on the connected components. #p/'5lA&j  
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3.     (10 points) Shown in the figure are the four structure schemes of the moving pulley shaft system in a lifting device. In each structure scheme, the lifting load is constant; the shaft diameter, shaft material and method of heat treatment are the same, respectively. 5H |< h  
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(1)   Determine the loads carried by the shaft and shaft types in each structure scheme. dX;G[\  
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(2)   Determine the stresses on the shaft and their changing characteristics in each structure scheme. C;.+ kE  
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(3)   Compare the strength difference of the four shafts. SM
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4.     (8 points) Shown in the figure is a transmission layout for a belted conveyer. The arrangement for speed reduction is as follows: 1(motor)→2(chain drive)→3(gear reducer)→4(belt drive)→5(conveyer). Point out the unreasonable aspects of the arrangement and give a correct order represented with the drive number. N}2xt)JZz  
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5.     (15 points) Shown in the figure, a helical gear shaft is supported by a pair of angular contact ball bearings (7208AC) with ; the radial loads acting on the bearings are , ; the externally axial load on the shaft is ; the rotating speed of the shaft is . 4x JOPu
 
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(1)   Find the axial forces on the two bearings: , . n/3g x4.g  
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(2)   If , find the equivalent dynamic loads of the two bearings: , . tT;=l[7%  
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(3)   Find the bearing lives: , . fYzO T,c  
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6.     (15 points) In a gear train shown in the figure, , , , , , , , , , the rotating direction of the gear 1 is as shown in the figure. Find both the magnitude and the direction of . C9
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7.     (15 points) Shown in Fig. (a) is the dynamically equivalent model of a mechanical system. The equivalent resistant moment of force is a given function of the rotating angle shown in Fig. (b) with the period . The equivalent driving moment of force is constant. The equivalent moment of inertia of the system is (constant). The average rotating speed of the equivalent link is . l_u1 ~K  
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(1)   Find the equivalent driving moment of force. QQ97BP7W  
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(2)   Find the coefficient of speed fluctuation , maximal rotating speed and minimal rotating speed of the equivalent link. jh`&c{#*)M  
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(3)   If the allowable coefficient of speed fluctuation is , find the minimum moment of inertia of the flywheel on the crankshaft A of the equivalent link. &oYX093di  
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8.     (15 points) Point out all the mistakes in the shaft system shown in the figure and explain them with brief words.