Mechanical Basis Resit revision Flashcards
Degree of freedom
F = 3N - 2PL - PH
the following mechanism and application
crank rocker mechanism - the pitching mechanism of radar antenna, feet operated sweing machine
double crank mechanism - a pair of scale, the inertial vibrosieve
double rocker - a crane, steering operating mechanism
variations
- replacing the revolute pair with sliding pair
- taking different links as the frame
- enlarging a revolute pair
four inversion of planar
- rotating guide bar mechanism
- swing guide bar mechanism
- crank and oscillating block mechanism
- translating guide bar mechanism
quick return characteristics
if crank rotates at a constant speed, oscillating links or reciprocating slider moves slowly forward and returns quickly. this characteristics is called quick return. such mechanism is called quick return mechanism.
obviously, the average angular velocity of the rocker CD of 2 strokes of the follower will be different due to the different Ø1 and Ø2.
the ratio of the faster average angular velocity w2 than the slower one w1 is called the coefficient of travel speed for time ratio, denoted as K
the crank angle between 2 limitating position
where t1 and t2 is the time duration for the faster stroke and the slowly stroke, respectively.
furthermore, by geometry, quick return characteristics can be identified by <C1AC2, which is called the crank angle between 2 limitating position, denoted as Ø
shape of the follower end
- knife-edge follower
- roller follower
- flat-faced follower
soft impulse/soft jerk
in acceleration plot, the acceleration jumps and cause abrupt changes and undesired vibration in inertia. inertial forces is finite, rather than infinite cause it will cause finite impulse/jerks. therefore, this is called soft impulse/soft jerks
failure modes of gear
(FPWSP)
- Fracture of teeth (breakage)
- Pitting
- Wear of teeth (abrasive wear)
- Seizure of teeth (gluing)
- Plastic deformation (ridging)
Stress
During the belt drives working time, there are 3 types of stress.
1) when tight side tension f1 and slack side tension f2 produce tension stress σ1 and σ2
2) σc is the centrifugal-force-induced stress, for the part of the belt that is in contact with the pulley
3) bending stress exists due to the bending deformation
adv of belt drive
- can be used for long center distance
- can absorb large amount of shock, can operate smoothly and silently
- slipping may occur due to overload applied to the system, which protects other element from damage
- construction is simple, cost relatively low
dis of belt drive
- angular velocity is non constant
- construction is incompact
- can neither be used in high speed/combustible cases
- short service life when used in high speed
structure of v belt
- standard ordinary v belt is made up of jointless ring
- configuration composed of top rubber, tensile body, bottom rubber and cloth
- 2 types of tensile are fabric core v belt & rope inner core v belt
main parameters
1) major diameter - coinciding with the external thread crest
2) minor diameter d1 - coinciding with the external thread root
3) pitch diameter - where the tooth thickness is equal to the width between the tooth
4) thread pitch - distance between 2 corresponding adjecent thread
5) number of thread n - n<4
6) lead L - distance of the mating thread with one revolution
7) lead angle - angle between tangent helix and plane perpendicular to the helix
8) tooth angle a - angle between 2 sides of the thread
9) tooth angle B - angle between flank and plane perpendicular to the helix
10) working height h - radial height between 2 crests
woodruff key connection
- the keyway of the shaft is milled by a disc shape milling cutter with the same radius of the woodruff key. this is so that the key can swing around its geometric center to accomodate the inclination of the key and the keyway on the hub when the woodruff key is working
