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
六. The numbers of teeth of the gears in the gear train shown in Fig are given as Z 1=20, Z 2=40, Z 2’=20 Z 3=30and Z 4=80.
Calculate the train ratio i lH · (15
points) solution:
1．Dividing the gear train
gear train with fixed axes :
gear1_gear2
planetary gear train:
gear 2’_gear3_carrier H_ gear4 (5 points)
2. Calculate the train ratio i lH
gear train with fixed axes :
i 12=n 1/n 2=-z 2/z 1=-2
n 2=- n 1/2 (1) (2 points)
planetary gear train:
4/10'24'2'24'24'2−=−=−=−−=−−=Z Z n n n n n n n n n i H
H H H H H
(4 points) 2'2n n = H n n 52= (2) (2 points)
(2 points)
102/15112−==−==H
H H n n i n n n。