东大机械设计考试样卷

(1) For a tight bolted connection with transverse load, the bolt is carried by .

A. shear stress B. tensile stress

C. shear stress and tensile stress D. shear stress or tensile stress

(2) For a tight bolted connection with axial tensile load F and preload , the resultant tensile force in the bolt is .

A. B. C. D.

(3) If the performance grade of a bolt is 6.8, then the yield limit stress of the bolt material is MPa.

A. 600 B. 800 C. 680 D. 480

(4) The lubricating method of a journal bearing is dependent on .

A. average intensity of pressure p B. circle velocity of journal v C. pv D.

(5) For a hydrodynamic radial journal bearing, the valid method for increasing is to .

A. increase relative clearance B. increase oil supply

C. decrease D. replace lubricating oil with lower viscosity

(6) For a hydrodynamic radial journal bearing, the relative clearance is the ratio of .

A. diametral clearance and journal diameter B. radial clearance and journal diameter

C. minimal clearance and diametral clearance

D. minimal clearance and journal diameter

(7) The axial preloading for rolling bearings is for _______.

A. increasing loading capacity B. decreasing noise

C. improving running precision D. avoiding shaft displacement

(8) The inner ring and outer ring of the bearing are separable.

A. deep-groove ball B. self-aligning ball C. tapered roller D. self-aligning roller

(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 _______.

A. B. C. D. uncertain

(10) For a cylindrical helical compressive spring, the maximal shear stress on spring wire happens at the ______.

A. inner side of spring coil B. outer side of spring coil

C. center of spring wire D. arbitrary position on spring wire

(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 _____.

A. decreased to 3/4 of the original value B. decreased to 9/16 of the original value

C. not changed D. increased to 4/3 of the original value

(12) For a static balanced rigid rotor, its mass center is located at the rotating axis.

A. certainly B. not always C. certainly not

(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 _________.

A. static imbalance B. static balance C. complete imbalance D. dynamic balance

(14) The installation of flywheel in machinery is to .

A. eliminate speed fluctuation B. realize steady running

C. decrease speed fluctuation D. balance the inertia forces

(15) In order to decrease the flywheel weight, the flywheel should be installed on the 。

A. equivalent link B. crankshaft with lower rotating speed

C. crankshaft with higher rotating speed D. spindle of machinery

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.

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.

(1) Determine the loads carried by the shaft and shaft types in each structure scheme.

(2) Determine the stresses on the shaft and their changing characteristics in each structure scheme.

(3) Compare the strength difference of the four shafts.

(a) (b) (c) (d)

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.

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 .

(1) Find the axial forces on the two bearings: , .

(2) If , find the equivalent dynamic loads of the two bearings: , .

(3) Find the bearing lives: , .

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 .

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 .

(1) Find the equivalent driving moment of force.

(2) Find the coefficient of speed fluctuation , maximal rotating speed and minimal rotating speed of the equivalent link.

(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.

8. (15 points) Point out all the mistakes in the shaft system shown in the figure and explain them with brief words.

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