机械设计准则-外文文献

Some Rules of Mechanical DesignDesigning starts with a need ,real or imagined.Existing apparatus may need some improvements in durability,efficiently,weight,speed,or cost.New apparatus may be needed to perform a function previously done by men,such as computation,assembly,or servicing.With the objective wholly or partly defined ,the next step in design is the conception of mechanisms and their arrangements that will perform the needed functions.For this ,freehand sketching is of great value ,not only as a record of ones thoughts and as an aid in discussion with others ,but particularly for communication with ones own mind ,as a stimulant for creative ideas.When the general shape and a few dimensions of the several components become apparent ,analysis can begin in earnest.The analysis will have as its objective satisfactory or superior performance,plus safety and durability with minimum weight,and a competitive cost.Optimum proportions and dimensions will be sought for each critically loaded section ,together with a balance between the strength of the several components. Materials and their treatment will be chosen.These important objectives can be attained only by analysis based upon the principles of friction ;of dynamics for inertia ,acceleration,and energy;of elasticity 。Of years manufacture and service ,the design is likely to undergo changes as new ideas are conceived or as further analysis based upon tests and experience indicate altertions .Sales appeal,customer satisfaction,and manufacture cost are all related to design ,and ability ,and ability in design is intimately involved in the success of an engineering venture .To stimulate creative thought ,the following rules are suggested for the designer .1.Apply ingenuty to utilize desired physical properties and to control undeired ones .The performance requiement of a machine are met by utilizing laws of nature or properties of matter ( e g ,flexibility ,strength ,gravity ,inertia ,buoyancy centrifugal force ,principles of the lever and inclined plane ,friction ,viscosity ,fluid pressure,and thermal expansion ) ,also the many electrical,optical ,thermal ,and chemical phenomena.However,what may be useful in one application may be detrimental in the next.Flexibility in the valve springs but not in the valve camshaft ;friction is deired at the cluch face but not in the cluch bearing.Ingenuity in design should be applied to utilize and control the phsical proerties that are desired and to minimize those that are not deired .2.Provide for favorable stress distribute and stiffiness with minimum weight.Oncomponents subjeted to fluctuating stress,particlar attention is given a reduction in stress concentration ,and to an increase of strength at fillets ,theads,holes ,and fits.Stress reduction are made by modification in shape ,and strengthening may be done .by prestressing treatments such as surface rolling and shallow hardening.Hollow shafts and tubing and box sections five a favorable stress distribution,together with stiffness and minimum weight .Sufficient stiffiness to maintain alignment and uniform pressure between contacting surfaces should be provided to crank ,cam ,and gear shafts and for enclosures.and frames containing bearing supports .The stiffiness of shafts and other components must be suitable to avoid resonant vibrations.3.Use basic equations to calculate and optimize dimensions .The fundamental equations of mechanics and the other sciences are the accepted bases for calculations They are sometimes rearrannged in special forms to facilitate the determination or ptimization pf dimensions ,such as the beam and surface stress equations for determination for determining gear-tooth size .Factors may be added to a fundamental equation for conditions not analtically deteminable .e.g,on thin steed tubes an allowance for corrosion added to the thickness based on pressure . On thin steel tubes ,an allowance for corrosion added to the thickness based on pressure.When it is necessary to apply a fundamental equation to shapes ,materials,or conditions which only approximate the assumptions for its derivation,it is sone in a mannerwhich gives results on the safe side .In situations where data are incomplete ,equations of the science may be used as propotioning guides to extend a satisfactory design to new capacities .4.Choose material for a combination of properties.Materials should be chosen for a combination of pertinent properties ,not only for stengths ,hardness,and weight,but sometimes for resistance to impact ,corrosion, and low or high temperatures Cost and fabrication properties are factors ,such as weldability,machinability,sensitivity to variation in heat-treating temperatures ,and required coating.5.Select carefully between stock and integral components. A previously developed component is frequently selected by a designer and his company from the stocks of parts manufactures,if the important meets the performance and reliability requirements and is adaptable without additional development costs to the particular machine being designed.However ,its selection should be carefully made with a full knowledge of its properties ,since the reputation and liability of the company suffer if there is a failure in any one of the machines parts .In other cases the strength ,reliability,and cost requirement are better met if the designer of the macine also designs the component , with the particular advantage of compactness if it designs integral with other components ,e.g.,gears to be forged in clusters or integral with a shaft .6.Provide for accurate location and non-interence of parts in assembly.A good design provides for the correct locating of parts and for easy assembly and repair .Shoulders and pilot surfaces give accurate location without measurement during assembly Shapers can be designed so that parts cannot be assembled backwards or in the wrong place .Interference ,as between screws in tapped holes ,and between linkage must be avoided ,or provision must be made to minimize any resulting detrimental displacements and stresses.