FABRICATION OF PESTICIDES BICYCLE SPRAYER MACHINE

                          

  BACKGROUND  OF THE PROJECT

       In Tanzania about 80% of population is directly or indirectly depends upon the farming. Hence it is said that Tanzania is an agricultural based country. But till now our farmers are doing farming in same traditional ways. They are doing seed sowing, fertilizers and pesticides spraying, cultivating by conventional methods. There is need of development in this sector and most commonly on fertilizers pesticides spraying technique, because it requires more efforts and time to spray by traditional way.
         Most of African nations are at developing stage and they are facing the problem of high population and as compared to that agricultural productivity is much lower as compared to developed nations. Tanzania is one of the nations who is facing the same problem. This is caused due to low level farms, insufficient power availability to farms and poor level of farm mechanization
.         In order to meet the requirement of food of growing population and rapid industrialization, there is a need of the modernization of agriculture sector. On many farms production suffers because, delay in sowing, improper distribution suffer because delay in sowing, improper distribution of pesticides and fertilizers, harvesting. Mechanization solves all the problems which are responsible for low production. It conserves the input and precision in work and get better and equal distribution. It reduces quantity needed for better response, prevent the losses and wastage of input applied. It get high productivity so that cost of production will reduced.
      To reach the requirement of production Agriculture implement and machinery program of the government take steps to increase availability of implement, pumps, tractors, power tillers, harvester and other power operated machines. Special emphasis was laid on the later as more than 35% of the farmers fall in small and marginal category. Generally mechanization of small forms is very difficult and non-affordable but in some developed countries make it happens. They are by proper mechanization they did farming and get more production than Tanzanian. They are using the modern time saving machine of required sizes to get more production. Developed countries led agriculture to new heights
           As we have seen in Tanzania mechanical sprayer farmers need to pump manually, as a result, spraying remains difficult task with these sprayers. Today, we decided to design bicycle sprayer which is much

PROBLEM STATEMENTS

Spraying of pesticides and other chemicals in the far is a tedious and laborious task. The conventional knapsack sprayers available in the market require manual labor to operate, and nowadays labor is difficult to find due to movement of farm laborers toward cities. The small farmers cannot afford to buy the power operated sprayer or tractor-mounted sprayers available in the market, as these are very costly and are of not much use to small farmers due to small land holdings.

Spraying of pesticides and other chemicals in the far is a tedious and laborious task. The conventional knapsack sprayers available in the market require manual labor to operate, and nowadays labor is difficult to find due to movement of farm laborers toward cities. The small farmers cannot afford to buy the power operated sprayer or tractor-mounted sprayers available in the market, as these are very costly and are of not much use to small farmers due to small land holdings. easier to operate than its predecessor as it is powered by bicycle.

Group member

Filed Under: really-story on Thursday, 14 July 2016

AutoCad

Attribute extraction template file
A text file that determines which attributes are extracted and how they are formatted when written to an attribute extraction file. See also attribute extraction file.
Attribute prompt
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Filed Under: really-story on Monday, 11 July 2016

COMMON SOURCES OF ACCIDENTS

A large number of revolving, rotating, reciprocating and moving parts of machinery can be said as the sources of danger and require guarding for protection against accidents. Extensive studies reveal that some characteristic groups of dangerous parts are acting as common sources of accidents in workshops. Many such major sources are as under.

1. Revolving parts, viz. pulley, flywheels, worms, worm wheel, fan, gears, gear trains, gear wheels etc.

2. Projecting fasteners of revolving parts; like bolts, screws, nuts, key heads, cotters and pins etc.

3. Intermittent feed mechanisms, viz., tool feed of planer; table feed of a shaper, ram feed of power presses and similar other applications.

4. Revolving shafts, spindles, bars, mandrels, chucks, followers and tools like drills, taps, reamers, milling cutters, and boring tool etc.

5. Rotating worms and spirals enclosed in casings, such as in conveyors and revolving cutting tool, like milling cutters, circular saw blade, saw band, circular shears and grinding wheels, etc.

6. Reciprocating tools and dies of power presses, spring hammer, drop hammers, and reciprocating presses, reciprocating knives and saw blade such bow saw, shearing and perforating machines and the cutting and trimming machine and power hacksaws etc.

7. Moving parts of various machines, like those of printing machines, paper-cutters and trimmers, etc.

8. Revolving drums and cylinders without casing, such as concrete and other mixers, tumblers and tumbling barrels, etc.

9. High speed rotating cages such as in hydro-extractors.

10. Revolving weights, such as in hydraulic accumulator or in slotting machines for counter-balance.

11. Nips between meshing racks and pinions of machine parts

12. Nips between reciprocating parts and fixed components, such as between shaper table and the fixture mounted on it or a planer table and table reversing stops, etc.

13. Nips between crank handles for machine controls and fixed parts

14. Projecting nips between various links and mechanisms, like cranks, connecting rods, piston rods, rotating wheels and discs, etc.

15. Projecting sharp edge or nips of belt and chain drives; via belt, pulleys, chains pockets and belt fasteners, spiked cylinders etc.

16. Nips between revolving control handles and fixed parts traverse gear handles of ashes, millers, etc.

17. Moving balance weights and dead weight, hydraulic accumulators, counter-balance weight on large slotting machines, etc.

18. Revolving drums and cylinders uncased, tumblers in the foundry, mixers, varnish mixers etc.

19. Nips between fixed and moving parts such as buckets or hoppers of conveyors against tipping bars, stops or parts of the framework.

20. Nips between revolving wheels or cylinders and pans or tables, sand mixers, crushing and incorporating mills, mortar mills, leather carrying machines, etc.

21. Cutting edges of endless band cutting machines, wood working, and log cutting metal find stone-cutting band saws, cloth-cutting band knives, etc.

22. Nips between gears and racks strips, roller drives, presses, planning machine drives, etc.

Human Causes

1. Accidents may occur while working on unsafe or dangerous equipment's or machineries possessing rotating, reciprocating and moving parts.

2. Accidents occur while operating machines without knowledge, without safety precautions, without authority, without safety devices.

3. Accidents generally occur while operating or working at unsafe speed.

4. Accidents may occur while working for long duration of work, shift duty etc.

5. Accidents commonly occur during use of improper tools.

6. Accidents may occur while working with mental worries, ignorance, carelessness, nervousness, dreaming etc.

7. Accidents occur because of not using personal protective devices.

Environmental Causes

1. Accidents may occur during working at improper temperature and humidity causes fatigue to the workers so chances of accidents increases with workers having fatigue.

2. The presence of dust fumes and smoke in the working area may causes accidents.

3. Poor housekeeping, congestion, blocked exits, bad plant layout etc. may cause accidents.

4. Accidents occur due to inadequate illumination.

5. Improper ventilation in the plant may also leads to industrial accidents.

Mechanical Causes

Filed Under: really-story on Wednesday, 6 July 2016

ACCIDENTS AND THEIR TYPES

The accidents are the mishaps leading injury to man, machines or tools and equipment and may cause injury and result either death or temporary disablement or permanent disablement of the industrial employees. A survey was conducted in 1952 in America which specified that approximately ten thousands industrial persons were killed in accidents and more than lakhs were injured in a year. The accidents are unwanted events or mishaps that result in some sort of injury to men, material, machines, tools, equipment and semi or finished product hence, a loss to the whole establishment. The total cost of these accidents was more than crores of dollars. An industrial accident may be defined as an event, detrimental to the health of man, suddenly occurring and originating from external sources, and which is associated with the performance of a paid job, accompanied by an injury, followed by disability or even death. An accident may happen to any employee under certain circumstances. The said injury or loss may be minor or major in nature and accordingly the accident is termed as no reportable or reportable kind. It should, however, be understood that no hard and line of demarcation can be laid between these two aspects and their identification varies with the place of application. For example a small burn or cut oft body will be reportable accident in a workshop whereas the same, can be treated by first aid and does not involve any appreciable loss of time, not be considered a reportable accident. Few industries determine the accidents by the extent to which it leads to the disablement of the victim and number of hours or days he is to remain absent from duty on account of the problem. There are others which take into consideration various factors like machine, tools, materials, cost of medicines, loss of production and compensation to be given to the worker who meets with the accident. An accident can be very costly to the injured employee as well as to the employer of the manufacturing concern. Some direct or indirect costs are associated with accidents in industries. The direct costs involve payment of compensation and overheads uncompensated wage losses of the injured employees, cost of medical care and hospitalization. Whereas indirect costs of an accident comprise of costs of damage of machines, materials and plant equipment's, costs of wages paid for time lost by workers not injured, costs of wages paid to the victim, costs of investigating agency involving recording and reporting of accidents and its causes, costs of deputing new employee for replacement of the injured employee, cost of decreased production by the substituting victim, cost of delays in production due to accident, cost of reduced efficiency of the victim when he joins the manufacturing concern after getting recovered and cost of lowered production due to reduced morale of employees. An accident is an unexpected event which is likely to cause, an injury. Proper diagnosis of causes of mishappening and corrective measures of the same always helps in preventing future accidents. Accidents in industries put a heavy burden on society also. All direct and indirect cost to the management will ultimately have to be met by the consumers in terms of increased cost.

Filed Under: really-story on

PLANNING FOR INDUSTRIAL SAFETY

Miss-happening of a large number of fire hazards, accidents, industrial disasters etc., can be reduced to the minimum possible extent through careful safety planning within an industrial organization. All these unwanted events can be prevented by effective planning for safety. Safety consideration includes proper layout of buildings and equipment, such as providing adequate ventilation, sufficient working area to the operator, clear pathways for movement of materials and parts, provision for adequate personnel facilities- viz., canteens, lunch rooms, dispensary, fire fighting services, etc. Careful planning in advance for optimized and safe layout of design and manufacturing activities for industry ensures industrial safety in the manufacturing and inventory areas. Incorporating safety considerations well in time are helpful for the establishment of a new plant as well as an existing plant needing major alterations. Such considerations lead to adequate safety to men, machine and equipment's, reduction in operational time and increase in production. Several codes and standards for industrial safety, health and hygiene, fire prevention, etc. have been prescribed by government and other safety agencies and they should be fully taken care of at the planning and implementation stages of a plant. A number of important features should be considered and suitably incorporated planning the layout of a new plant and its buildings for safety. Hoists and conveyors are commonly used in industries for raising, lowering and transporting loads for limited distances. A high degree of safety is needed while these equipment's are in operation.

During operations of these equipment's, one should keep in mind the following important safety measures. Material handling and its storage are very common functions in a plant. Material handling when performed manually the chances of injuries are greater. Therefore the following points hold be taken care for carrying out such tasks. All material handling equipment's such as conveyers, automotive guided vehicles, robots, cranes should carry proper guards for its gears and other dangerous moving parts to prevent access from these parts during operation. All hoisting devices must be equipped with limit switches for preventing loads block from over traveling accidentally. Hoisting equipment especially cranes, should only be operated by properly trained personnel for avoiding all sorts of mishaps or accidents. While operating a crane, the operator should be entirely guided by standard signal and both operator and his signaler should be thoroughly trained. Proper protections against fire and explosion hazards are required when gasoline operated carinas are being used. Where manual loading is done on conveyors which run along a vertical path, either partially or totally, safe load sign should be prominently displayed on all loading stations. Sufficient lighting, ventilation, drainage, escape ways and guarding should be provided for conveyors which run in pits, tunnels and similar other enclosures. Riding on a conveyor should always be prohibited. All the persons working on or around the conveyor must wear tight cloths and safety shoes. All rotating, reciprocating and projecting parts of machinery and equipment's such as sprockets, gears, etc., should be adequately protected by proper guarding. An effective lubrication schedule should be worked out and implemented. All inspection should be carried out regularly and worn out parts, if any, should be replaced immediately. The workers should be properly trained to adopt safe working habits and proper supervision should be done while these operations are being carried out manually. Industrial personnel and unskilled workers should be adequately trained for adopting safe working habits in the proper ways of lifting and setting down the objects. They should be told to be careful from pinches and shear points and to grasp the articles firmly when lifting or setting down. Objects which are wet or dirty or have slippery surfaces, such as greasy or oily and wet articles should be completely wiped off dry before handling them. The hands should also be kept free of oil and grease. For preventing hand injuries the handlers should be made to wear protective clothing like leather hand gloves, sleeves, etc. The worker handling materials should always wear foot in order to prevent foot injuries. If an object is to be lifted and carried to some distance it should be ensured that the pathway is not slippery and there are no obstructions on the passage or way. The unskilled industrial workers should be properly trained for keeping correct positions of their feet, positions of back and knees, holding the object close for the body while lifting and carrying, correct and firm grip, position of chin and application of body weight in lifting and setting down by hand. This will help to prevent muscle strains and back injuries. When a gang or team of workers is used to carry a heavy load form one place to another the supervisor should ensure the use of proper tools and direct the work himself to ensure proper synchronization in the lifting, walking and setting down actions of all the workers involved. While transporting material by trucks, the truck should be operated at safe limit speed as specified and special care should be taken at blind corners and doorways. During storing material, it should be ensured that the electrical panels and installations and fire extinguishers and hoses are kept clear and have free accessibility. Also the pathways, entries and exits should always be kept clear for movement. The use of racks and bins enables more storage capacity, easy movement of material from one place to another and ensures better safety in an industrial organization.

Filed Under: really-story on

SAFETY CONCEPTS

In all kinds of industries, each shop supervisor is generally assigned the responsibility of safety in his shop regarding the men, machines and materials. Every supervisor in each shop ensures to the top executives in respect of all kinds of the safety matters. He is supposed to incorporate all new safety measures needed in the shop from time to time. With the growth in the size of the industry and depending upon the hazardousness of industrial processes, a full fledged safety department should be created under the intensive supervision through a safety manager. The safety manager may be given a line position or staff position depending upon the working conditions in the industry. Sometimes the responsibility for safety rests on a safety committee formed by the top executives of the organization. A safety committee may consist of executives, supervisors, and shop floor workers. Thus the lower level employees get a channel of communication on safety matters direct to executive level. It is a matter of fact that those organizations which made safety committees had lower record of accidents than those without safety committees. Safety committees always motivate all the industrial employees for developing safety consciousness. It acts also as a policy making body on safety matters. To enhance the efficiency of the safety committee, some safety problem may be assigned to safety staff for identifying and implementing safety rules and publicizing them. Its members should be asked to go on the shop floor and watch what is being done there till date about the safety measures. It should be asked to report periodically as what improvements have been made and what more can be done for safety aspects in near future for avoiding any mis-happening in the plant. Safety committee often organizes safety programs to make industrial persons sufficiently alert for overall safety within the plant. A safety program tends to discover when, where and why accidents occur. It always aims at reducing accidents and the losses associated with them. It begins with the assumption that more work-connected accidents can be prevented. It does not have an end rather it is a continuous process to achieve adequate safety. It involves providing, safety equipment's and special training to employees. It consists of support by top management, appointing a safety officer, engineering a safe plant, processes and operations, educating all industrial employees to work safely, studying and analyzing the accidents to prevent their occurrence in future, holding safety contests, safety weeks etc., and awarding incentives or special prizes to departments which enforces the safety rules and having least number of accidents. A safety programme should always include engineering safety at the design and equipment installation stage, education of employees in safe practices, concerns the attitude of employees and management. It should motivate all the industrial employees in accident prevention and safety consciousness. It must provide all safety instructions and training essential for the employees to think, act and work safely so that the number of accidents can be minimized. Safety education must give knowledge about safe and unsafe mechanical conditions and personal practices. Safety training must involve induction and orientation of new recruits to safety rules and practices, explaining safety function, during their initial job training through efforts made by the first level supervisors. Formulating employee’s safety committees, holding of employee’s safety meeting, display of charts, posters, film etc. are very much essential in each industry for stressing the need to act safely. It educates employees to develop their safety consciousness. An industrial worker will usually accept the use of a safety measure if he is convinced of its necessity. Therefore, suitable measures must be adopted to increase the awareness of a need for safety in the environment of work. Such measures are required in an industrial organization to develop safety consciousness among workers or other employees. There should be sufficient display of safety posters and films from time to time to remind industrial workers to particular hazards/accidents, providing simple and convenient safety devices, providing time to the worker for setting, removing and replacing any necessary safety devices. All industrial personnel should be asked from the first day to start working to adopt safety measures because an unskilled worker should be familiar fully to work safely. A safety committee should manage regular safety programmes that may hold safety competitions. Award and prizes are also to be given to the winners for imparting due respect and recognition to safe workers and create in employees a feeling of pride in safe work. It should elaborate on the safety theme until all the employees are safety conscious. It must hold regular safety meetings and stimulates the safety ideas in industrial workers for being more safety conscious. It must ask the shop supervisor to display all the safety aspects near the work centre. It should also mail safety information and sufficient literature pertaining to safety for reading at homes of all the industrial employees. It must welcome all safety suggestions. It must mark categorically all accident areas. It must conduct safety training lectures periodically for providing wide publicity to safety aspects for everything including men, machines and material

Filed Under: really-story on

GEOMETRIC NOMENICLATURE

A. POINTS IN SPACE

A point is an exact location in space or on a drawing surface. Appoint is actually represented on the drawing by a crisscross at its exact location. The exact point in space is where the two lines of the crisscross intersect. When a point is located on an existing line, a light, short dashed line or cross bar is placed on the line at the location of the exact point. Never represent a point on a drawing by a dot; except for sketching locations.

B. LINE

Lines are straight elements that have no width, but are infinite in length (magnitude), and they can be located by two points which are not on the same spot but fall along the line. Lines may be straight lines or curved lines. A straight line is the shortest distance between two points. It can be drawn in any direction. If a line is indefinite, and the ends are not fixed in length, the actual length is a matter of convenience. If the end points of a line are important, they must be marked by means of small, mechanically drawn crossbars, as described by a pint in space.

C. ANGLE

An angle is formed by the intersection of two lines. There are three major kinds of angles: right angels, acute angles and obtuse angles. The right angle is an angle of 900, an acute angle is an angle less than 900, and an obtuse angle is an angle more than 900. A straight line is 1800. The symbol for an angle is < (singular) and <’s (Plural). To draw an angle, use the drafting machine, a triangle, or a protractor.

D. TRIANGLES

A triangle is a closed plane figure with three straight sides and their interior angles sum up exactly 1800. The various kinds of triangles: a right triangle, an equilateral triangle, an isosceles triangle, and an obtuse angled triangle.

E. QUADRIALTERAL

It is a plane figure bounded by four straight sides. When opposite sides are parallel, the quadrilateral is also considered to be a parallelogram.

F. POLYGON

A polygon is a closed plane figure with three or more straight sides. The most important of these polygons as they relate to drafting are probably the triangle with three sides, square with four sides, the hexagon with six sides, and the octagon with eight sides.

G. CIRCLE

A circle is a closed curve with all points on the circle at the same distance from the center point. The major components of a circle are the diameter, the radius and circumference.

♦ The diameter of the circle is the straight distance from one outside curved surface through the center point to the opposite outside curved surface.

♦ The radius of a circle is the distance from the center point to the outside curved surface. The radius is half the diameter, and is used to set the compass when drawing

a diameter.

♦ A central angle: is an angle formed by two radial lines from the center of the circle.

♦ A sector: is the area of a circle lying between two radial lines and the circumference.

♦ A quadrant: is a sector with a central angle of 900 and usually with one of the radial lines oriented horizontally.

♦ A chord: is any straight line whose opposite ends terminate on the circumference of the circle.

♦ A segment: is the smaller portion of a circle separated by a chord.

♦ Concentric circles are two or more circles with a common

center point.

♦ Eccentric circles are two or more circles with out a common center  point.

Filed Under: really-story on Tuesday, 5 July 2016

Failure Theories

Section 5–1 illustrated some ways that loss of function is manifested. Events such as distortion, permanent set, cracking, and rupturing are among the ways that a machine element fails. Testing machines appeared in the 1700s, and specimens were pulled, bent, and twisted in simple loading processes. If the failure mechanism is simple, then simple tests can give clues. Just what is simple? The tension test is uniaxial (that’s simple) and elongations are largest in the axial direction, so strains can be measured and stresses inferred up to “failure.” Just what is important: a critical stress, a critical strain, a critical energy? In the next several sections, we shall show failure theories that have helped answer some of these questions. Unfortunately, there is no universal theory of failure for the general case of material properties and stress state. Instead, over the years several hypotheses have been formulated and tested, leading to today’s accepted practices. Being accepted, we will characterize these “practices” as theories as most designers do. Structural metal behavior is typically classified as being ductile or brittle, although under special situations, a material normally considered ductile can fail in a brittle manner (see Sec. 5–12). Ductile materials are normally classified such that εf ≥ 0.05 and have an identifiable yield strength that is often the same in compression as in tension (Syt = Syc = Sy ). Brittle materials, εf < 0.05, do not exhibit an identifiable yield strength, and are typically classified by ultimate tensile and compressive strengths, Sut and Suc, respectively (where Suc is given as a positive quantity). The generally accepted theories are:

Ductile materials (yield criteria)

• Maximum shear stress (MSS), Sec. 5–4

• Distortion energy (DE), Sec. 5–5

• Ductile Coulomb-Mohr (DCM), Sec. 5–6

Brittle materials (fracture criteria)

• Maximum normal stress (MNS), Sec. 5–8

• Brittle Coulomb-Mohr (BCM), Sec. 5–9

• Modified Mohr (MM), Sec. 5–9

It would be inviting if we had one universally accepted theory for each material type, but for one reason or another, they are all used. Later, we will provide rationales for selecting a particular theory. First, we will describe the bases of these theories and apply them to some examples.

Distortion-Energy Theory for Ductile Materials

The distortion-energy theory predicts that yielding occurs when the distortion strain energy per unit volume reaches or exceeds the distortion strain energy per unit volume for yield in simple tension or compression of the same material.

The distortion-energy (DE) theory originated from the observation that ductile materials stressed hydrostatically (equal principal stresses) exhibited yield strengths reatly in excess of the values given by the simple tension test. Therefore it was postulated that yielding was not a simple tensile or compressive phenomenon at all, but, rather, that it was related somehow to the angular distortion of the stressed element. To develop the theory, note, in Fig. 5–8a, the unit volume subjected to any three dimensional stress state designated by the stresses σ1, σ2, and σ3. The stress state shown in Fig. 5–8b is one of hydrostatic normal stresses due to the stresses σav acting in each of the same principal directions as in Fig. 5–8a. The formula for σav is simply σav = σ1 + σ2 + σ33

(a)Thus the element in Fig. 5–8b undergoes pure volume change, that is, no angular distortion. If we regard σav as a component of σ1, σ2, and σ3, then this component can be subtracted from them, resulting in the stress state shown in Fig. 5–8c. This element is subjected to pure angular distortion, that is, no volume change.

Filed Under: really-story on Sunday, 3 July 2016

Static Strength (Topic 2)

Ideally, in designing any machine element, the engineer should have available the results of a great many strength tests of the particular material chosen. These tests should be made on specimens having the same heat treatment, surface finish, and size as the element the engineer proposes to design; and the tests should be made under exactly the same loading conditions as the part will experience in service. This means that if the part is to experience a bending load, it should be tested with a bending load. If it is to be subjected to combined bending and torsion, it should be tested under combined bending and torsion. If it is made of heat-treated AISI 1040 steel drawn at 500°C with a ground finish, the specimens tested should be of the same material prepared in the same manner. Such tests will provide very useful and precise information. Whenever such data are available for design purposes, the engineer can be assured of doing the best possible job of engineering. The cost of gathering such extensive data prior to design is justified if failure of the part may endanger human life or if the part is manufactured in sufficiently large quantities. Refrigerators and other appliances, for example, have very good reliabilities because the parts are made in such large quantities that they can be thoroughly tested in advance of manufacture. The cost of making these tests is very low when it is divided by the total number of parts manufactured. You can now appreciate the following four design categories: 1 Failure of the part would endanger human life, or the part is made in extremely large quantities; consequently, an elaborate testing program is justified during design. 2 The part is made in large enough quantities that a moderate series of tests is feasible. 3 The part is made in such small quantities that testing is not justified at all; or the design must be completed so rapidly that there is not enough time for testing. 4 The art has already been designed, manufactured, and tested and found to be unsatisfactory. Analysis is required to understand why the part is unsatisfactory and what to do to improve it. More often than not it is necessary to design using only published values of yield strength, ultimate strength, percentage reduction in area, and percentage elongation, such as those listed in Appendix A. How can one use such meager data to design against both static and dynamic loads, two- and three-dimensional stress states, high and low temperatures, and very large and very small parts? These and similar questions will be addressed in this chapter and those to follow, but think how much better it would be to have data available that duplicate the actual design situation. Failure” is the first word in the chapter title. Failure can mean a part has separated into two or more pieces; has become permanently distorted, thus ruining its geometry; has had its reliability downgraded; or has had its function compromised, whatever the reason. A designer speaking of failure can mean any or all of these possibilities. In this chapter our attention is focused on the predictability of permanent distortion or separation. In strength-sensitive situations the designer must separate mean stress and mean strength at the critical location sufficiently to accomplish his or her purposes.

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Pressure-Fed Bearings

The load-carrying capacity of self-contained natural-circulating journal bearings is quite restricted. The factor limiting better performance is the heat-dissipation capability of the bearing. A first thought of a way to increase heat dissipation is to cool the sump with an external fluid such as water. The high-temperature problem is in the film where the heat is generated but cooling is not possible in the film until later. This does not protect against exceeding the maximum allowable temperature of the lubricant. A second alternative is to reduce the temperature rise in the film by dramatically increasing the rate of lubricant flow. The lubricant itself is reducing the temperature rise. A water cooled jump may still be in the picture. To increase lubricant flow, an external pump must be used with lubricant supplied at pressures of tens of pounds per square inch gage. Because the lubricant is supplied to the bearing under pressure, such bearings are called pressure-fed bearings.To force a greater flow through the bearing and thus obtain an increased cooling effect, a common practice is to use a circumferential groove at the center of the bearing, with an oil-supply hole located opposite the load-bearing zone. Such a bearing is shown in Fig. 12–27. The effect of the groove is to create two half-bearings, each having a smaller l/d ratio than the original. The groove divides the pressure-distribution curve into two lobes and reduces the minimum film thickness, but it has wide acceptance among lubrication engineers because such bearings carry more load without overheating. To set up a method of solution for oil flow, we shall assume a groove ample enough that the pressure drop in the groove itself is small. Initially we will neglect eccentricity and then apply a correction factor for this condition. The oil flow, then, is the amount that flows out of the two halves of the bearing in the direction of the concentric shaft. If we neglect the rotation of the shaft, the flow of the lubricant is caused by the supply. The length-diameter ratio l/d of a bearing depends upon whether it is expected to run under thin-film-lubrication conditions. A long bearing (large l/d ratio) reduces the coefficient of friction and the side flow of oil and therefore is desirable where thin-film or boundary-value lubrication is present. On the other hand, where forced-feed or positive lubrication is present, the l/d ratio should be relatively small. The short bearing length results in a greater flow of oil out of the ends, thus keeping the bearing cooler. Current practice is to use an l/d ratio of about unity, in general, and then to increase this ratio if thin-film lubrication is likely to occur and to decrease it for thick-film lubrication or high temperatures. If shaft deflection is likely to be severe, a short bearing should be used to prevent metal-to-metal contact at the ends of the bearings. You should always consider the use of a partial bearing if high temperatures are a problem, because relieving the non-load-bearing area of a bearing can very substantially reduce the heat generated. The two conflicting requirements of a good bearing material are that it must have a satisfactory compressive and fatigue strength to resist the externally applied loads and that it must be soft and have a low melting point and a low modulus of elasticity. The second set of requirements is necessary to permit the material to wear or break in, since the material can then conform to slight irregularities and absorb and release foreign particles. The resistance to wear and the coefficient of friction are also important because all bearings must operate, at least for part of the time, with thin-film or boundary lubrication. Additional considerations in the selection of a good bearing material are its ability to resist corrosion and, of course, the cost of producing the bearing. Some of the commonly used materials are listed in Table 12–6, together with their composition and characteristics.

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