《机械原理》_机械原理(双语)试卷

答案

一、Checkpoint (mark your answer on the answer sheet by True or False)

(20 points 2 points/each )

1. The grade of a mechanism is defined as the highest grade of Assur group in the mechanism. ( T )

2. Any mechanism which has determined motions can be assembled from

a basic mechanism by connecting Assur group to the determined links using outer pairs, group by group. This is the composition principle of mechanism ( T )

3. Inversion of a mechanism can change the relative motion between its links ( F )

4. If a mechanism has quick return characteristics, then we should set the faster stroke as working stroke. ( F )

5. In a cam mechanism, the sine acceleration motion curve would produce soft impulse ( F )

6. A rotor is dynamically balanced if it has been statically balanced. ( F )

7.If crank- rocker mechanism(rocker is driver) reach it’s dead point, then transmission angle γis equal to zero . ( T )

8. If the efficiency of mechanismη≤0, then the mechanism is self-lock mechanism. ( T )

9. The pressure angle remains the same at all points on the tooth profiles of the rack (ak=a), which is also called the normal pressure angle. ( T )

10. The proper meshing conditions for external helical gear is mn1=mn2= mn,

an1=an2=an and β1=－β 2

( T )

二．Calculate the degree of freedom of the mechanism as shown in Fig (10 points )

Solution:

A ：passive DOF

n=8, pl=11, ph=1

F=3n-2Pl-Ph (3 points )

=3x8-2x11-1

=1 (7 points )

三.In the cam mechanism with inline translating Flat-faced follower shown in Fig, the cam is a circular disk (The radius of the circular disk r=50mm ). lA0=202mm Supposing that the angular velocity

ω1 of the cam is known, ω1=10rad/s, the velocity V2 of the follower 2 is to be found when θ=45°by instant center method.

（15 points ）

Solution:

Because point c is instant centre between

Cam 1 and follower 2, (6 points )

then:

VC2= VC1=ωC=10x202x 2/2=200mm/S (6 points ) Because follower 2 is a translating follower

Then:

V2= VC2=200mm/S (3 points )

四. An offset slider-crank mechanism is as shown in Fig. Supposing that the length a=120mm ，b=600mm ，offset e=120mm . find out the coefficient K of travel speed variation and The working stroke H by graphic method (15 points)

Solution:

Select ul=1/10, draw out two limiting positions (AC1, AC2)of slider-crank mechanism

AC1=AB+BC

AC2=BC-BC (6 points )

THEN θ=∠C1AC2=4.90 (3

points )

K=（180+θ）/（180-θ）=1.06 (3 points )

H=C1C2/ul=245 mm

(3 points ) A C1

C2 e