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Abstract
The fixture design should meet the following basic requirements
(1) the appropriate accuracy and dimensional stability
Folder of the concrete on the important surfaces, such as the installation of the surface of the positioning components, install the surface of the knife and oriented components, as well as the specific folder to install the base surface (connected to the machine surface) should be the appropriate size and shape accuracy, they between should be the appropriate location accuracy.
The folder of the stability of the specific dimensions, casting folder specific to the aging treatment, welding and forging folder specific annealing.
(2) have sufficient strength and stiffness.
Process, the specific folder you want to bear a greater cutting force and clamping force. To ensure that the folder does not produce does not allow deformation and vibration folder should have sufficient specific strength and stiffness, so the folder the specific needs of a certain wall thickness.
(3) the structure of technology is good.
Specific folder should be easy to manufacture, assembly and test. Casting folder the surface of the concrete on the installation of various components should be cast out of 3 ~ 5mm convex in order to reduce the processing area. Casting folder concrete wall thickness should be uniform, corner outside the due R3 ~ R5mm, rounded corners. Specific folder structure should facilitate the loading and unloading of the workpiece.
(4) to have the appropriate chip space and good chip evacuation.
Cutting little jig for cutting can increase the distance between the work surface and fixture positioning components or addition of Chip grooves in order to increase the chip space; for processing a large number of chip fixture, you can set the BTA gap or bevel, bevel desirable.
(5) installed on the machine stable and reliable.
The installation of the fixture on the machine through the folder of the specifics of the installation of the base surface and the corresponding surface on the machine contact or in combination to achieve. When the fixture is installed in the machine table, the center of gravity of the fixture should be as low as possible, the center of gravity the higher the bearing surface shall be the greater; fixture underside of the four sides should be protruding, so that good contact folder of the installation of the base surface and the machine table. Contact edge or the width of the foot should be greater than the width of the machine table, trapezoidal slot, should be a processing and to guarantee a certain plane accuracy; When the fixture is installed on the machine spindle, fixture installation base surface with the corresponding spindle surface should be higher with precision, and to ensure stable and reliable specific folder to install.
(6) have a good look.
Specific folder appearance novel, steel folder to the specific needs of blueing or demagnetization, raw casting parts must be clean and paint.
(7) Stamps play the fixture number to the appropriate location in the fixture tooling management.
fixture manufacturing and process
(1) fixture manufacturing precision
Fixture is usually a single piece production, and manufacturing cycle is very short. In order to ensure that the workpiece processing requirements, many fixtures have a high manufacturing precision. The tool shop has a variety of processing equipment, such as the processing of the holes in the jig borer, processing of complex-shaped surface of the universal milling machines, precision lathes and a variety of grinders, have good processing properties and processing accuracy. Fixture manufacturing, in addition to the different production methods and products in general, in the application of the interchangeability principle under certain restrictions to ensure that the manufacturing precision of the fixture.
(2) guarantee the accuracy of the method of fixture manufacturing
Directly related to the workpiece size and high precision parts, repair method commonly used in the fixture manufacture and adjustment to ensure that the fixture accuracy.
????1) Application of the Method of Repair
Parts need to use the repair method, in its pattern, marked "assembly finishing assembly" or "worthy" words. Assembly of the bearing plate and the supporting nail positioning surface, with the specific folder merge processing to ensure that positioning the face of the folder concrete base surface parallelism.
Lathe fixture errors, and requires greater processing can be used in machine tools, processing positioning surface for concentricity. Such as the measurement process lathe fixture hole round of processing and correction, by excessive disk and use the lathe to connect directly processed, so that these two processing side of the centerline of the lathe spindle center overlap, to obtain more accurate positional accuracy .
Boring jig is often used repair method. For example, boring sets of holes and the use of the actual size of the boring bar with a single gap in between 0.008 ~ 0.01mm can be boring mold high guiding accuracy.
Fixture repair methods are related to a specific folder of the base surface, which does not result in a variety of error accumulation, and to achieve the desired accuracy requirements.
????2) Adjustment Act applications
Adjustment Act and the Method of Repair, usually in the fixture can be set to adjust the washer, adjust the plate, adjust the set of components to control the assembly size. This method is relatively easy to adjust the right choices to other components of the compensation error, in order to improve the manufacturing precision of the fixture.
(3) structural technical
Structural technical performance of the fixture for fixture parts manufacturing, assembly, commissioning, measurement, and use the performance. Structural elements of the general standard of the fixture parts and castings, and so on, can access to the manual design. Fixture components of the processing, maintenance, assembly and measurement process.
????1) Note that the processing and maintenance process
Fixture connection of the main components of positioning screws and pins. In Figure 1.2 (a) of the pin hole is made through-hole, so that maintenance can pin press out; shown in Figure 1.2 (b) pin, the pin hole at the bottom of the horizontal hole demolition; Figure 1.2 (c) is commonly used with threaded taper pins (GB118-2000).
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???(A) The pin hole is made through-hole (b) A transverse hole (c) with threaded
Figure 1.2 pin hole connection process
Fig. 1.2 pin hole connection technology
????2) Note that the measurement process of assembly
The fixture assembly measurement is an important part of the fixture manufacturing. Assembly of the repair method or the method of adjustment assembly, or submit a test fixture accuracy, should deal with the problem of a good benchmark.
In order to fixture assembly, measurement process, we should follow the benchmark uniform principles, to clip the specific surface is uniform benchmarks, in order to facilitate the assembly measurement fixture manufacturing to ensure accuracy. When the basal plane of the fixture can not meet the above requirements, you can set the process holes or craft Boss.
BEARING LIFE ANALYSIS
1 .WHY BEARINGS FAIL
An individual bearing may fail for several reasons; however, the results of an endurance test series are only meaningful when the test bearings fail by fatigue-related mechanisms. The experimenter must control the test process to ensure that this occurs. The following paragraphs deal with a few specific failure types that can affect the conduct of a life test sequence.
The influence of lubrication on contact fatigue life is discussed from the standpoint of EHL film generation. There are also other lubrication-related effects that can affect the outcome of the test series. The first is particulate contaminants in the lubricant. Depending on bearing size, operating speed, and lubricant rheology, the overall thickness of the lubricant film developed at the rolling element-raceway contacts may fall between 0.05 and 0.5 m . Solid particles and damage the raceway and rolling element surfaces, leading to substantially shortened endurances. This has been amply demonstrated by Sayles and MacPherson [19.6] and others.
Therefore, filtration of the lubricant to the desired level is necessary to ensure meaningful test result. The desired level is determined by the application which the testing purports to approximate. If this degree of filtration is not provided, effects of contamination must be considered when evaluating test results. Chapter 23 discusses the effect of various degrees of particulate contamination, and hence filtration, on bearing fatigue life.
The moisture content in the lubricant is another important consideration. It has long been apparent that quantities of free water in the oil cause corrosion of the rolling contact surfaces and thus have a detrimental effect on bearing life. It has been further shown by Fitch [19.7] and others, however, that water levels as low as 50-100 parts per million(ppm) may also have a detrimental effect, even with no evidence of corrosion. This is due to hydrogen embrittlement of the rolling element and raceway material. See also Chapter 23. Moisture control in test lubrication systems is thus a major concern, and the effect of moisture needs to be considered during the evaluation of life test results. A maximum of 40 ppm is considered necessary to minimize life reduction effects.
The chemical composition of the test lubricant also requires consideration. Most commercial lubricants contain a number of proprietary additives developed for specific purposes; for example, to provide antiwear properties, to achieve extreme pressure and/or thermal stability, and to provide boundary lubrication in case of marginal lubricant films. These additives can also affect the endurance of rolling bearings, either immediately or after experiencing time-related degradation. Care must be taken to ensure that the additives included in the test lubricant will not suffer excessive deterioration as a result of accelerated life test conditions. Also for consistency of results and comparing life test groups, it is good practice to utilize one standard test lubricant from a particular producer for the conduct of all general life tests.
The statistical nature of rolling contact fatigue requires many test samples to obtain a reasonable estimate of life. A bearing life test sequence thus needs a long time. A major job of the experimentalist is to ensure the consistency of the applied test conditions throughout the entire test period. This process is not simple because subtle changes can occur during the test period. Such changes might be overlooked until their effects become major. At that time it is often too late to salvage the collected data, and the test must be redone under better controls.
For example, the stability of the additive packages in a test lubricant can be a source of changing test conditions. Some lubricants have been known to suffer additive depletion after an extended period of operation. The degradation of the additive package can alter the EHL conditions in the rolling content, altering bearing life. Generally, the normal chemical tests used to evaluate lubricants do not determine the conditions of the additive content. Therefore if a lubricant is used for endurance testing over a long time, a sample of the fluid should be returned to the producer at regular intervals, say annually, for a detailed evaluation of its condition.
Adequate temperature controls must also be employed during the test. The thickness of the EHL film is sensitive to the contact temperature. Most test machines are located in standard industrial environments where rather wide fluctuations in ambient temperature are experienced over a period of a year. In addition, the heat generation rates of individual bearings can vary as a result of the combined effects of normal manufacturing tolerances. Both of these conditions produce variations in operating temperature levels in a lot of bearings and affect the validity of the life data. A means must be provided to monitor and control the operating temperature level of each bearing to achieve a degree of consistency. A tolerance level of3C is normally considered adequate for the endurance test process.
The deterioration of the condition of the mounting hardware used with the bearings is another area requiring constant monitoring. The heavy loads used for life testing require heavy interference fits between the bearing inner rings and shafts. Repeated mounting and dismounting of bearings can produce damage to the shaft surface, which in turn can alter the geometry of a mounted ring. The shaft surface and the bore of the housing are also subject to deterioration from fretting corrosion. Fretting corrosion results from the oxidation of the fine wear particles generated by the vibratory abrasion of the surface, which is accelerated by the heavy endurance test loading. This mechanism can also produce significant variations in the geometry of the mounting surfaces, which can alter the internal bearing geometry. Such changes can have a major effect in reducing bearing test life.
The detection of bearing failure is also a major consideration in a life test series. The fatigue theory considers failure as the initiation of the first crack in the bulk material. Obviously there is no way to detect this occurrence in practice. To be detectable the crack must propagate to the surface and produce a spall of sufficient magnitude to produce a marked effect on an operating parameter of the bearing: for example, noise, vibration, and/or temperature. Techniques exit for detecting failures in application systems. The ability of these systems to detect early signs of failure varies with the complexity of the test system, the type of bearing under evaluation, and other test conditions. Currently no single system exists that can consistently provide the failure discrimination necessary for all types of bearing life tests. It is then necessary to select a system that will repeatedly terminate machine operation with a consistent minimal degree of damage.
The rate of failure propagation is therefore important. If the degree of damage at test termination is consistent among test elements, the only variation between the experimental and theoretical lives is the lag in failure detection. In standard through-hardened bearing steels the failure propagation rate is quite rapid under endurance test conditions, and this is not a major factor, considering the typical dispersion of endurance test data and the degree of confidence obtained from statistical analysis. This may not, however, be the case with other experimental materials or with surface-hardened steels or steels produced by experimental techniques. Care must be used when evaluating these latter results and particularly when comparing the experimental lives with those obtained from standard steel lots.
The ultimate means of ensuring that an endurance test series was adequately controlled is the conduct of a post-test analysis. This detailed examination of all the tested bearings uses high-magnification optical inspection, higher-magnification scanning electron microscopy, metallurgical and dimensional examinations, and chemical evaluations as required. The characteristics of the failures are examined to establish their origins and the residual surface conditions are evaluated for indications of extraneous effects that may have influenced the bearing life. This technique allows the experimenter to ensure that the data are indeed valid. The “Damage Atlas” compiled by Tallian et al. [19.8] containing numerous black and white photographs of the various bearing failure modes can provide guidance for these types of determinations. This work was subsequently updated by Tallian [19.9], now including color photographs as well.
The post-test analysis is, by definition, after the fact. To provide control throughout the test series and to eliminate all questionable areas, the experimenter should conduct a preliminary study whenever a bearing is removed from the test machine. In this portion of the investigation each bearing is examined optically at magnifications up to 30 for indications of improper or out-of-control test parameters. Examples of the types of indications that can be observed are given in Figs. 19.2-19.6.
Figure 19.2 illustrates the appearance of a typical fatigue-originated spall on a ball bearing raceway. Figure 19.3 contains a spalling failure on the raceway of a roller bearing that resulted from bearing misalignment, and Fig. 19.4 contains a spalling failure on the outer ring of a ball bearing produced by fretting corrosion on the outer diameter. Figure 19.5 illustrates a more subtle form of test alteration, `where the spalling failure originated from the presence of a debris dent on the surface. Figure 19.6 gives an example of a totally different failure mode produced by the loss of internal bearing clearance due to thermal unbalance of the system.
The last four failures are not valid fatigue spalls and indicate the need to correct the test methods. Furthermore, these data points would need to be eliminated from the failure data to obtain a valid estimate of the experimental bearing life.
2 .AVOIDING FAILURES
The best way to handle bearing failures is to avoid them．This can be done in the selection process by recognizing critical performance characteristics．These include noise，starting and running torque，stiffness，non-repetitive run out，and radial and axial play．In some applications, these items are so critical that specifying an ABEC level alone is not sufficient．
Torque requirements are determined by the lubricant，retainer，raceway quality(roundness cross curvature and surface finish)，and whether seals or shields are used．Lubricant viscosity must be selected carefully because inappropriate lubricant，especially in miniature bearings，causes excessive torque．Also，different lubricants have varying noise characteristics that should be matched to the application. For example，greases produce more noise than oil．
Non-repetitive run out(NRR)occurs during rotation as a random eccentricity between the inner and outer races，much like a cam action．NRR can be caused by retainer tolerance or eccentricities of the raceways and balls．Unlike repetitive run out, no compensation can be made for NRR.
NRR is reflected in the cost of the bearing．It is common in the industry to provide different bearing types and grades for specific applications．For example，a bearing with an NRR of less than 0.3um is used when minimal run out is needed，such as in disk—drive spindle motors．Similarly，machine—tool spindles tolerate only minimal deflections to maintain precision cuts．Consequently, bearings are manufactured with low NRR just for machine-tool applications．
Contamination is unavoidable in many industrial products，and shields and seals are commonly used to protect bearings from dust and dirt．However，a perfect bearing seal is not possible because of the movement between inner and outer races．Consequently，lubrication migration and contamination are always problems．
Once a bearing is contaminated, its lubricant deteriorates and operation becomes noisier．If it overheats，the bearing can seize．At the very least，contamination causes wear as it works between balls and the raceway，becoming imbedded in the races and acting as an abrasive between metal surfaces．Fending off dirt with seals and shields illustrates some methods for controlling contamination．
Noise is as an indicator of bearing quality．Various noise grades have been developed to classify bearing performance capabilities．
Noise analysis is done with an Ander-on-meter, which is used for quality control