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Unformatted text preview: KINEMATICS FUNDAMENTALS (O) (b) FlGURE 2-25 seiwafie-e t ; z V. MM Compliant bistable silicon micromechonism in WC posiiions Courtesy of Professor Larry L Howell, Brigham Young University 2.17 PRACTICAL CONSIDERATIONS There are many factors that need to be considered to create good-quality designs. Not all of them are contained within the applicable theories. A great deal of art based on experience is involved in design as well. This section attempts to describe a few such practical considerations in machine design. Pin Joints versus Sliders and Half Joints Proper material selection and good lubrication are the key to long life in any situation, such as a joint, where two materials rub together. Such an interface is called a bearing. Assuming the proper materials have been chosen, the choice of joint type can have a sig- nificant effect on the ability to provide good, clean lubrication over the lifetime of the machine. REV'OLL'TF. (PIN) JOINTS The simple revolute or pin joint (Figure 2-263) is the clear winner here for several reasons. it is relatively easy and inexpensive to design and build a good quality pin joint. In its pure form—a so—called sieeve or journal bearing— the geometry of pin-in-hole traps a lubricant film within its annular interface by capil- lary action and promotes a condition called hydrodynamic lubrication in which the parts are separated by a thin film of lubricant as shown in Figure 2-27. Seals can easily be Oil ,— Stationary journal Shaft rotating rapidly - hydrodynamic lubrication - no metal contact - fluid pumped by shaft - shaft iags bearing centerline F | G U R E 2 A 2 7 1'?" ‘5‘ ’ :55. = Hydrodynamic lubrication in o sleeve bearing—clearance and motions exoggeroted (a) Pinioirtt (o) Slider iomt (c) Holtjoim FIG” oins of oriu typ s DESIGN OF MACHINERY CHAPTER 2 (a) Eccem‘ric crank-rocker ; 5\ (b) Eccem‘ric siidericrank FIGURE 2-33 FIGURE 2-34 Should r screw Eccenh'ic cranks Coun‘esy of Cordovo Boh‘ mo, Bueno Park, CA KINEMATICS FUNDAMENTALS (a) Singfe rod in bushing (b) Platform on Two rods Wfimwmmmmafivmwmmmmfi Bearing ratio iii: EW,%§W& ...
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