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One princple aim of kinemarics is to creat the designed motions of the subject mechanical parts and then mathematically compute the positions, velocities ,and accelerations ,which those motions will creat on the parts. Since ,for most earthbound mechanical systems ,the mass remains essentially constant with time,defining the accelerations as a function of time then also defines the dynamic forces as a function of time. Stress,in turn, will be a function of both applied and inerials forces . since engineering design is charged with creating systems which will not fail during their expected service life,the goal is to keep stresses within acceptable limits for the materials chosen and the environmental conditions encountered. This obviselyrequies that all system forces be defined and kept within desired limits. In mechinery , the largest forces encountered are often those due to the dynamics of the machine itself. These dynamic forces are proportional to acceletation, which brings us back to kinematics ,the foundation of mechanical design. Very basic and early decisions in the design process invovling kinematics wii prove troublesome and perform badly.36293
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Any mechanical system can be classified according to the number of degree of freedom which it possesses.the systems DOF is equal to the number of independent parameters which are needed to uniquely define its posion in space at any instant of time.
A rigid body free to move within a reference frame will ,in the general case, have complex motoin, which is simultaneous combination of rotation and translation. In three-dimensional space , there may be rotation about any axis and also simultaneous translation which can be resoled into componention along three axes, in a plane ,or two-dimentional space ,complex motion becomes a combination of simultaneous along two axes in the plane. For simplicity ,we will limit our present discusstions to the case of planar motion:
Pure rotation the body pessesses one point (center of rotation)which has no motion with respect to the stationary frame of reference. All other points on the body describe arcs about that center. A reference line drawn on the body through the center changes only its angulai orientation.
Pure translation all points on the body describe parallel paths. A reference line drawn on the body changes its linear posion but does not change its angular oriention. Complex motion a simulaneous combination of rotion and translationm .any reference line drawn on the body will change both its linear pisition and its angular orientation. Points on the body will travel non-parallel paths ,and there will be , at every instant , a center of rotation , which will continuously change location
Linkages are the bacis building blocks of all mechanisms. All common forms of mechanisms (cams , gears ,belts , chains ) are in fact variations of linkages. Linkages are made up of links and kinematic pairs.
A link is an (assumed)rigid body which possesses at least two or more links (at their nodes), which connection allows some motion, or potential motion,between the connected links.
The term lower pair is used to describe jionts with surface contact , as with a pin surrounded by a hole. The term higher pair is used to describe jionts with point or line contact ,but if there is any clerance between pin and hole (as there must be for motion ),so-called surface contact in the pin jiont actually becomes line contact , as the pin contacts actually has contact only at discrete points , which are the tops of the surfaces’ asperities. The main practical advantage of lower pairs over higher pairs is their better ability to trap lubricant between their envlopingsurface. This ie especially true for the rotating pin joint. The lubricant is more easily squeezed out of a higher pair .as sresult , the pin joint is preferred for low wear and long life .
When designing machinery, we must first do a complete kinematic analysis of our design , in order to obtain information about the acceleration of the moving parts .we next want te use newton’s second law to caculate the dynamic forces, but to do so we need to know the masses of all the moving parts which have these known acceletations. These parts do not exityet !as with any design in order to make a first pass at the caculation . we will then have to itnerate to better an better solutions as we generate more information.