Lesson 3 Flashcards
One of the most useful and most common mechanisms is the four-bar linkage. Where link 1 is the frame, links 2 and 4 are the cranks, and link 3 is called the __________.
coupler
There are two positions during the cycle when the linkage is not con-strained. These are the positions where the follower, link 4, is colinear with link 3.
Dead points or dead center
what conditions must exist in a crank and rocker mechanism?
O₂B + BC + O₄C > O₂O₄
O₂B + O₂O₄ + O₄C > BC
O₂B + BC - O₄C < O₂O₄
BC - O₂B + O₄C > O₂O₄
A four-bar linkage in which the shortest link is fixed is called____________.
drag-link mechanism
If one crank rotates at a constant speed, the other crank will rotate in the same direction at a varying speed. The proportions of the links must be as follows:
BC > O₂O₄ + O₄C - O₂B
BC < O₄C - O₂O₄ + O₂B
A modification of the slider-crank linkage is known as an _____________. The crank consists of a circular disk with center B, which is pivoted off-center at O₂ to the frame. The disk rotates inside the ring end of rod 3.
eccentric mechanism
it is a variation of the slider-crank mechanism. It is used in testing machines to simulate vibrations having simple harmonic motion.
Scotch yoke
it is the equivalent of a slider crank having an infinitely long connecting rod.
scotch yoke
__________________ are used in machine tools such as shapers and power-driven saws for the purpose of giving the reciprocating cutting tool a slow cutting stroke and a quick-return stroke with a constant angular velocity of the driving crank.
Quick-return mechanisms
The ratio of the time required for the cutting stroke to the time for the return stroke is called the _______________ and is greater than unity.
time ratio
This mechanism employs an inversion of the slider-crank linkage which shows the arrangement in which link 2 rotates completely and link 4 oscillates. If the driver. link 2, rotates counterclockwise at constant velocity, slider 6 will have a slow stroke to the left and a fast return stroke to the right.
Crank-shaper
time ration of crank shaper
θ₁/θ₂
This mechanism is obtained by making the distance O₂O₄ less than the crank length O₂B. Both links 2 and 4 rotate completely. If the driver, crank 2, rotates counterclockwise with constant angular velocity, slider 6 will move from D’ to D’’ with a slow motion while 2 rotates through angle θ₁ Then as 2 rotates through the smaller angle θ₂ slider 6 will have a quick-return motion from D’’ to D’ The time ratio is θ₁/θ₂
whitworth
This mechanism where links 1, 2, 3, and 4 comprise a drag-link mechanism. If link 2, the driver, rotates counterclock wise with constant angular velocity, then slider 6 makes a slow stroke to the left and returns with a quick stroke to the right. The time ratio is θ₁/θ₂.
drag link
The slider-crank mechanism can be designed with an offset y so that the path of the slider does not intersect the crank axis. It is then a quick-return mechanism, though not a very effective one since the time ratio θ₁/θ₂ is only a little larger than 1.
offset slider crank
_______________ are linkages having a point that moves along a straight line, or nearly along a straight line, without being guided by a plane surface.
Straight-line mechanisms
__________________ produces approximate straight-line motion. Point P traces a figure-eight-shaped path, a considerable portion of which is approximately a straight line.
Watt’s mechanism
The _______________ gives the exact straight-line motion of point P. Length AC = BC = CP A variation of this mechanism is shown in Fig. 3-15, where the slider is replaced by crank BD. In this linkage, point P has an approximate straight-line motion.
Scott-Russell mechanism
_________________ produces approximate straight-line motion. Point P moves very nearly along line AB. Length AC = CP = PD = DB and CD= AP = PB.
Robert’s mechanism
________________ gives approximate straight-line motion. Point P. the midpoint of CB, moves very nearly along line CB. Length AB = CD = 1.25AD and AD = 2CB
Tchebyshef’s mechanism
_________________ produces exact straight-line motion for point P. Peaucillier, a Frenchman, developed this mechanism in _______. The following relationships must hold: AB = AF, BC BD, and PC = PD = CE = DE. It will be proved that point P moves in a straight line.
Peaucillier’s mechanism, 1864
These are linkages which give parallel motion.
parallel mechanisms
___________ are used for reducing or enlarging drawings and maps. They are also used for guiding cutting tools or cutting torches to duplicate complicated shapes.
Pantographs
___________ are used whenever a large force acting through a short distance is required.
Toggle mechanisms
