机械原理(双语)试卷2答案

《机械原理（双语）》课程试卷2答案

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

(20 points 2 points/each )

1．The links within a mechanism can have determined relative motions if and only if the DOF of the mechanism is greater than zero and equal to the number of the driving links ( T )

2. During the group dividing of mechanism, if the DOF of the group is zero, then the group is called Assur group ( F )

3. Any three independent links in general plane motion must have three instant centres, the three instant centres may lie on a common straight line. ( F )

4.If a mechanism comply with the Grashof’s criterion , then the mechanism must have a crank. ( F )

5. A rotor is statically balanced if it has been dynamically balanced. ( T )

6.In the velocity polygon ,The vector connecting the any two points except point p represents relative velocity between them and the direction is opposite to the velocity’ symbol ( T )

7. The analysis with dynamic static is analysis of the mechanism force with exception of the inertia force, applying into the case of low velocity mechanism. ( F ) 8. In a crank-rocker mechanism, the rocker DC reaches its two limiting positions DC1 and DC2. if the crank AB is a driver, then the limiting positions are called dead points . ( F ) 9. The pressure angle remains the same at all points on the tooth profiles of the rack (a k=a), which is also called the normal pressure angle. ( T ) 10. The transmission ratio of worm gearing (worm is driver) can be calculated by the formula i12=d2/d1 ( F )

二. Calculate the degree of freedom of the mechanism shown in Fig

(10 points) solution:

A: passive DOF

B B: compound hinge

n=9, P L=12, P H =2

F=3n-2P L-P H (3 points)

=3×9-2×12-2

=1 (7 points)

三. For the plate cam mechanism with translating inline roller follower as shown in Fig, the cam contour is circle(the radius r =100mm), the radius of roller r R=10mm, OC=20mm, the cam rotates at anticlockwise .

1). Draw out the pitch curve of the cam mechanism

2). Find out the radius of prime circle r p=?

3). Find out the lift of follower h=? Array

4).

(20 points)

solution:

1). the pitch curve of the cam mechanism

As shown in Fig dotted circle (2 points)

2). r p=r-OC+r T=90mm (2 points)

3). h=2 OC=40mm (2 points)

4). As shown in Fig (2 points)

5).As shown in Fig (2 points)

四. Design an offset slider-crank mechanism by graphic method. The offset e is 16mm. The coefficient K of travel speed variation is to be 1.2. The working stroke H of the slider is to be 50mm

solution:

θ=1800(k-1)/(k+1)

=1800(1.2-1)/(1.2+1)

=16.40 (3 points)

Select u L =2, Design the offset slider-crank

mechanism by graphic method (8 points)

then ：L BC -L AB =AC1. x u l =36

L BC +L AB =AC2 x u l =86

L AB =25mm

L BC =61mm

(4 points) 五. A pair of standard spur involute gears has a module of 10mm, pressure angle α=200, transmission ratio i 12=2. the numbers of teeth z 1=30. 1. Find out reference diameters d 1,d 2,

2. Tooth thickness s and spacewidth e .

3. Find out reference centre distance a

3. If working centre distance a’= a+1, find out working pressure angle α’ (15 points)

solution:

1 i 12=

2 Z 2= Z 1x2=80

d 1=m Z 1=300mm ，d 2=m Z 2=600mm ， (4 points)

2. P=πm=31.4mm

S=e=πm/2=15.7mm (4 points)

3. a=（d 1+d 2）/2=450mm (3 points)

4. acos α= a’cos α’

450cos200=451cos α’ α’=20.30 (4 points) P

A C2

C1 θ

e