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Design of machine and machine elements
Machine design
Machine design is the art of planning or devising new or improved machines to accomplish specific purposes. In general, a machine will consist of a combination of several different mechanical elements properly designed and arranged to work together, as a whole. During the initial planning of a machine, fundamental decisions must be made concerning loading, type of kinematic elements to be used, and correct utilization of the properties of engineering materials. Economic considerations are usually of prime importance when the design of new machinery is undertaken. In general, the lowest over-all costs are designed. Consideration should be given not only to the cost of design, manufacture the necessary safety features and be of pleasing external appearance. The objective is to produce a machine which is not only sufficiently rugged to function properly for a reasonable life, but is at the same time cheap enough to be economically feasible.
The engineer in charge of the design of a machine should not only have adequate technical training, but must be a man of sound judgment and wide experience, qualities which are usually acquired only after considerable time has been spent in actual professional work.
Design of machine elements
The principles of design are, of course, universal. The same theory or equations may be applied to a very small part, as in an instrument, or, to a larger but similar part used in a piece of heavy equipment. In no ease, however, should mathematical calculations be looked upon as absolute and final. They are all subject to the accuracy of the various assumptions, which must necessarily be made in engineering work. Sometimes only a portion of the total number of parts in a machine are designed on the basis of analytic calculations. The form and size of the remaining parts are designed on the basis of analytic calculations. On the other hand, if the machine is very expensive, or if weight is a factor, as in airplanes, design computations may then be made for almost all the parts.
The purpose of the design calculations is, of course, to attempt to predict the stress or deformation in the part in order that it may sagely carry the loads, which will be imposed on it, and that it may last for the expected life of the machine. All calculations are, of course, dependent on the physical properties of the construction materials as determined by laboratory tests. A rational method of design attempts to take the results of relatively simple and fundamental tests such as tension, compression, torsion, and fatigue and apply them to all the complicated and involved situations encountered in present-day machinery.
In addition, it has been amply proved that such details as surface condition, fillets, notches, manufacturing tolerances, and heat treatment have a market effect on the strength and useful life of a machine part. The design and drafting departments must specify completely all such particulars, must specify completely all such particulars, and thus exercise the necessary close control over the finished product.
As mentioned above, machine design is a vast field of engineering technology. As such, it begins with the conception of an idea and follows through the various phases of design analysis, manufacturing, marketing and consumerism. The following is a list of the major areas of consideration in the general field of machine design:
① Initial design conception;
② Strength analysis;
③ Materials selection;
④ Appearance;
⑤ Manufacturing;
⑥ Safety;
⑦ Environment effects;
⑨ Reliability and life;
Strength is a measure of the ability to resist, without fails, forces which cause stresses and strains. The forces may be;
① Gradually applied;
② Suddenly applied;
③ Applied under impact;
④ Applied with continuous direction reversals;
⑤ Applied at low or elevated temperatures.
If a critical part of a machine fails, the whole machine must be shut down until a repair is made. Thus, when designing a new machine, it is extremely important that critical parts be made strong enough to prevent failure. The designer should determine as precisely as possible the nature, magnitude, direction and point of application of all forces. Machine design is mot, however, an exact science and it is, therefore, rarely possible to determine exactly all the applied forces. In addition, different samples of a specified material will exhibit somewhat different abilities to resist loads, temperatures and other environment conditions. In spite of this, design calculations based on appropriate assumptions are invaluable in the proper design of machine.
Moreover, it is absolutely essential that a design engineer knows how and why parts fail so that reliable machines which require minimum maintenance can be designed. Sometimes, a failure can be serious, such as when a tire blows out on an automobile traveling at high speeds. On the other hand, a failure may be no more than a nuisance. An example is the loosening of the radiator hose in the automobile cooling system. The consequence of this latter failure is usually the loss of some radiator coolant, a condition which is readily detected and corrected.
The type of load a part absorbs is just as significant as the magnitude. Generally speaking, dynamic loads with direction reversals cause greater difficulties than static loads and, therefore, fatigue strength must be considered. Another concern is whether the material is ductile or brittle. For example, brittle materials are considered to be unacceptable where fatigue is involved.
In general, the design engineer must consider all possible modes of failure, which include the following:
① Stress;
② Deformation;
③ Wear;
④ Corrosion;
⑤ Vibration;
⑥ Environmental damage;
⑦ Loosening of fastening devices.
The part sizes and shapes selected must also take into account many dimensional factors which produce external load effects such as geometric discontinuities, residual stresses due to forming of desired contours, and the application of interference fit joint.
Selected from” design of machine elements”, 6th edition, m. f. sports, prentice-hall, inc., 1985 and “machine design”, Anthony Esposito, charles e., Merrill publishing company, 1975.
Mechanical properties of materials
The material properties can be classified into three major headings: (1) physical, (2) chemical, (3) mechanical
Physical properties
Density or specific gravity, moisture content, etc., can be classified under this category.
Chemical properties
Many chemical properties come under this category. These include acidity or alkalinity, react6ivity and corrosion. The most important of these is corrosion which can be explained in layman’s terms as the resistance of the material to decay while in continuous use in a particular atmosphere.
Mechanical properties
Mechanical properties include in the strength properties like tensile, compression, shear, torsion, impact, fatigue and creep. The tensile strength of a material is obtained by dividing the maximum load, which the specimen bears by the area of cross-section of the specimen.
This is a curve plotted between the stress along the This is a curve plotted between the stress along the Y-axis(ordinate) and the strain along the X-axis (abscissa) in a tensile test. A material tends to change or changes its dimensions when it is loaded, depending upon the magnitude of the load. When the load is removed it can be seen that the deformation disappears. For many materials this occurs op to a certain value of the stress called the elastic limit Ap. This is depicted by the straight line relationship and a small deviation thereafter, in the stress-strain curve (fig.3.1)
. Within the elastic range, the limiting value of the stress up to which the stress and strain are proportional, is called the limit of proportionality Ap. In this region, the metal obeys hookes’s law, which states that the stress is proportional to strain in the elastic range of loading, (the material completely regains its original dimensions after the load is removed). In the actual plotting of the curve, the proportionality limit is obtained at a slightly lower value of the load than the
elastic limit. This may be attributed to the time-lagin the regaining of the original dimensions of the material. This effect is very frequently noticed in some non-ferrous metals.
Which iron and nickel exhibit clear ranges of elasticity, copper, zinc, tin, are found to be imperfectly elastic even at relatively low values low values of stresses. Actually the elastic limit is distinguishable from the proportionality limit more clearly depending upon the sensitivity of the measuring instrument.
When the load is increased beyond the elastic limit, plastic deformation starts. Simultaneously the specimen gets work-hardened. A point is reached when the deformation starts to occur more rapidly than the increasing load. This point is called they yield point Q. the metal which was resisting the load till then, starts to deform somewhat rapidly, i. e., yield. The yield stress is called yield limit Ay.
The elongation of the specimen continues from Q to S and then to T. The stress-strain relation in this plastic flow period is indicated by the portion QRST of the curve. At the specimen breaks, and this load is called the breaking load. The value of the maximum load S divided by the original cross-sectional area of the specimen is referred to as the ultimate tensile strength of the metal or simply the tensile strength Au.
Logically speaking, once the elastic limit is exceeded, the metal should start to yield, and finally break, without any increase in the value of stress. But the curve records an increased stress even after the elastic limit is exceeded. Two reasons can be given for this behavior:
①The strain hardening of the material;
②The diminishing cross-sectional area of the specimen, suffered on account of the plastic deformation.
The more plastic deformation the metal undergoes, the harder it becomes, due to work-hardening. The more the metal gets elongated the more its diameter (and hence, cross-sectional area) is decreased. This continues until the point S is reached.
After S, the rate at which the reduction in area takes place, exceeds the rate at which the stress increases. Strain becomes so high that the reduction in area begins to produce a localized effect at some point. This is called necking.
Reduction in cross-sectional area takes place very rapidly; so rapidly that the load value actually drops. This is indicated by ST. failure occurs at this point T.
Then percentage elongation A and reduction in reduction in area W indicate the ductility or plasticity of the material:
A=(L-L0)/L0*100%
W=(A0-A)/A0*100%
Where L0 and L are the original and the final length of the specimen; A0 and A are the original and the final cross-section area.
Selected from “testing of metallic materials”
Quality assurance and control
Product quality is of paramount importance in manufacturing. If quality is allowed deteriorate, then a manufacturer will soon find sales dropping off followed by a possible business failure. Customers expect quality in the products they buy, and if a manufacturer expects to establish and maintain a name in the business, quality control and assurance functions must be established and maintained before, throughout, and after the production process. Generally speaking, quality assurance encompasses all activities aimed at maintaining quality, including quality control. Quality assurance can be divided into three major areas. These include the following:
①Source and receiving inspection before manufacturing;
②In-process quality control during manufacturing;
③Quality assurance after manufacturing.
Quality control after manufacture includes warranties and product service extended to the users of the product.
Source and receiving inspection before manufacturing
Quality assurance often begins ling before any actual manufacturing takes place. This may be done through source inspections conducted at the plants that supply materials, discrete parts, or subassemblies to manufacturer. The manufacturer’s source inspector travels to the supplier factory and inspects raw material or premanufactured parts and assemblies. Source inspections present an opportunity for the manufacturer to sort out and reject raw materials or parts before they are shipped to the manufacturer’s production facility.
The responsibility of the source inspector is to check materials and parts against design specifications and to reject the item if specifications are not met. Source inspections may include many of the same inspections that will be used during production. Included in these are:
①Visual inspection;
②Metallurgical testing;
③Dimensional inspection;
④Destructive and nondestructive inspection;
⑤Performance inspection.
Visual inspections
Visual inspections examine a product or material for such specifications as color, texture, surface finish, or overall appearance of an assembly to determine if there are any obvious deletions of major parts or hardware.
Metallurgical testing
Metallurgical testing is often an important part of source inspection, especially if the primary raw material for manufacturing is stock metal such as bar stock or structural materials. Metals testing can involve all the major types of inspections including visual, chemical, spectrographic, and mechanical, which include hardness, tensile, shear, compression, and spectr5ographic analysis for alloy content. Metallurgical testing can be either destructive or nondestructive.
Dimensional inspection
Few areas of quality control are as important in manufactured products as dimensional requirements. Dimensions are as important in source inspection as they are in the manufacturing process. This is especially critical if the source supplies parts for an assembly. Dimensions are inspected at the source factory using standard measuring tools plus special fit, form, and function gages that may required. Meeting dimensional specifications is critical to interchangeability of manufactured parts and to the successful assembly of many parts into complex assemblies such as autos, ships, aircraft, and other multipart products.
Destructive and nondestructive inspection
In some cases it may be necessary for the source inspections to call for destructive or nondestructive tests on raw materials or p0arts and assemblies. This is particularly true when large amounts of stock raw materials are involved. For example it may be necessary to inspect castings for flaws by radiographic, magnetic particle, or dye penetrant techniques before they are shipped to the manufacturer for final machining. Specifications calling for burn-in time for electronics or endurance run tests for mechanical components are further examples of nondestructive tests.
It is sometimes necessary to test material and parts to destruction, but because of the costs and time involved destructive testing is avoided whenever possible. Examples include pressure tests to determine if safety factors are adequate in the design. Destructive tests are probably more frequent in the testing of prototype designs than in routine inspection of raw material or parts. Once design specifications are known to be met in regard to the strength of materials, it is often not necessary to test further parts to destruction unless they are genuinely suspect.
Performance inspection
Performance inspections involve checking the function of assemblies, especially those of complex mechanical systems, prior to installation in other products. Examples include electronic equipment subcomponents, aircraft and auto engines, pumps, valves, and other mechanical systems requiring performance evaluation prior to their shipment and final installation.
Selected form “modern materials and manufacturing process”
Electro-hydraulic drum brakes
Application
The YWW series electro-hydraulic brake is a normally closed brake, suitable for horizontal mounting. It is mainly used in portal cranes, bucket stacker/reclaimers’slewing mechanism.
The YKW series electro-hydraulic brake is a normally opened brake, suitable for horizontal mounting, employing a thruster as actuator. with the foot controlling switch the operator can release or close the brake. It is mainly used for deceleration braking of portal cranes’slewing mechanism. In a non-operating state the machinery can be braked by a manual close device.
The RKW series brake is a normally opened brake, which is operated by foot driven hydraulic pump, suitable for horizontal mounting. Mainly used in the slewing mechanism of middle and small portal cranes. When needed, the brake is activated by a manual closed device.
Main design features
Interlocking shoes balancing devices (patented technology) constantly equalizes the clearance of brake shoes on both sides and made adjustment unnecessary, thus avoiding one side of the brake lining sticking to the brake wheel. The brake is equipped with a shoed autoaligning device.
Main hinge points are equipped with self-lubricating bearing, making high efficiency of transmission, long service life. Lubricating is unnecessary during operation.
Adjustable bracket ensure the brake works well.
The brake spring is arranged inside a square tube and a surveyor’s rod is placed on one side. It is easy to read braking torque value and avoid measuring and computing.
Brake lining is of card whole-piece shaping structure, easy to replace. Brake linings of various materials such as half-metal (non-asbestos) hard and half-hard, soft (including asbestos) substance are available for customers to choose.
All adopt the company’s new types of thruster as corollary equipment which work accurately and have long life.
Hydraulic Power Transmission
The Two Types Of Power Transmission
In hydraulic power transmission the apparatus (pump) used for conversion of the mechanical (or electrical,thermal) energy to hydraulic energy is arranged on the input of the kinematic chain ,and the apparatus (motor) used for conversion of the hydraulic energy to mechanical energy is arranged on the output (fig.2-1)
The theoretical design of the energy converters depends on the component of the bernouilli equation to be used for hydraulic power transmission.
In systerms where, mainly, hydrostatic pressure is utilized, displacement (hydrostatic) pumps and motors are used, while in those where the hydrodynamic pressure is utilized is utilized gor power transmission hydrodynamic energy converters (e.g. centrifugal pumps) are used.
The specific characteristic of the energy converters is the weight required for transmission of unit power. It can be demonstrated that the use of hydrostatic energy converters for the low and medium powers, and of hydrodynamic energy converters of high power are more favorite (fig.2-2). This is the main reason why hydrostatic energy converters are used in industrial apparatus. transformation of the energy in hydraulic transmission.
1. driving motor (electric, diesel engine);
2. mechanical energy;
3. pump;
4. hydraulic energy;
5. hydraulic motor;
6. mechanical energy;
7. load variation of the mass per unit power in hydrostatic and hydrodynamic energy converters
1、hydrostatic; 2.hydrodynamic
Only displacement energy converters are dealt with in the following. The elements performing converters provide one or several size. Expansion of the working chambers in a pump is produced by the external energy admitted, and in the motor by the hydraulic energy. Inflow of the fluid occurs during expansion of the working chamber, while the outflow (displacement) is realized during contraction. Such devices are