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AMAZON multi-meters discounts AMAZON oscilloscope discounts Basic Test Concepts: From time to time over the last 30 or 40 years, the devising of procedures for testing balancing machines, particularly dynamic balancing machines, has occupied many experts and various committees of engineering societies. The chief problem usually has been the interaction of errors in amount indication, angle indication, and plane separation. A requirement for a given accuracy of amount indication becomes meaningless if the machine's indicating system has poor plane separation or lacks accuracy of angle indication; or the best plane separation is useless if the amount and angle indication are inaccurate. As an example of interdependence between amount and angle indication, FIG. 28 illustrates how an angle error of 10° results in an amount indication error of 17.4 percent. The initial unbalance of 100 g was corrected 10° away from where the correction mass should have been attached. The residual unbalance indicated in the next run is 17.4 g at 85°, nearly at a right angle to the initial unbalance. FIG. 28. Residual unbalance due to angle error. TBL. 3 Interdependence of Angle and Amount Indication Angle Error; Amount Error Listed in TBL. 3 are residual unbalances expressed in percent of initial unbalances which result from applying unbalance correction of proper amount but at various incorrect angular positions. Eventually it was recognized that most balancing machine users are really not so much interested in how accurately the individual parameter is indicated, but rather, in the accuracy of the combination of all three. In other words, the user wants to reduce the initial unbalance to the specified permissible residual unbalance in a minimum number of steps. Acceptance of this line of reasoning resulted in the concept of the "Unbalance Reduction Ratio," URR for short (see definition in SECTION 6A). It expresses the percentage of initial unbalance that one correction step will eliminate. For instance, a URR of 95 percent means that an initial unbalance of 100 units may be reduced to a residual unbalance of 5 units in one measuring and correction cycle-provided the correction itself is applied without error. A procedure was then developed to verify whether a machine will meet a specified URR. This test is called the Unbalance Reduction Test, or UR Test. It tests a machine for combined accuracy of amount indication, angle indication, and plane separation, and should be part of every balancing machine acceptance test. Note: On single-plane machines, the UR test only checks combined accuracy of amount and angle indication. Aside from the UR test, acceptance test procedures should also include a check whether the machine can indicate the smallest unbalance specified. For this purpose, a test for "Minimum Achievable Residual Unbalance" was developed, called, "Umar Test" or "Traverse Test," for short. Both Umar and UR tests are described in subsequent SECTIONS. They should be repeated periodically; for instance, once a month if the machine is used daily, to assure that it’s still in proper operating condition. TBL. 4 lists various current standards for testing balancing machines (see also SECTION 6C). === TBL. 4 Standards for Testing Balancing Machines Application General industrial balancing machines Jet engine rotor balancing machines (for two-plane correction) Jet engine rotor balancing machines (for single-plane correction) Gyroscope rotor balancing machines Field balancing equipment --- Title Balancing Machines- Description and Evaluation Balancing Equipment for Jet Engine Components, Compressor and Turbine, Rotating Type, for Measuring Unbalance in One or More Than One Transverse Plane Balancing Equipment for Jet Engine Components Compressor and Turbine, Rotating Type, for Measuring Unbalance in One Transverse Plane Balancing Machine- Gyroscope Rotor Field Balancing Equipment- Description and Evaluation --- Issuer: International Standards Organization (ISO) Society of Automotive Engineers, Inc. (SAE) Society of Automotive Engineers, Inc. (SAE) Defense General Supply Center, Richmond, Va. International Standards Organization (ISO) --- Document no.: DIS 2953 1983* ARP 587 A ARP 588 A FSN 6635- 450-2208 NT ISO 2371 === Inboard Proving Rotors for Horizontal Machines For general purpose machines, and in the absence of a proving rotor supplied by the balancing machine manufacturer, any rigid rotor such as an armature, roll, flywheel, etc, may be made into a proving rotor. Ideally, its weight and shape should approximate the actual rotors to be balanced. Since these usually vary all over the capacity range of a general purpose machine, ISO 2953 suggests one rotor to be near the minimum weight limit, a second rotor near the maximum. Particularly for soft-bearing machines, it’s important to make the Umar test with a small rotor since that is where parasitic mass of the vibratory system (carriages, bridge, springs, etc.) has its maximum effect on the sensitivity of unbalance indication. As a general rule, it would probably be sufficient if the rotor fell within the bottom 20 percent of the machine weight range. For hard-bearing machines, it’s not as important to test the lower end of the weight range, since parasitic mass has little effect on the readout sensitivity of such machines. Testing both soft- or hard-bearing machines in the upper 20 percent of their weight range will verify their weight carrying and drive capability, but add little additional knowledge concerning the measuring system. On machines with weight ranges larger than 10,000 lbs it may be impractical to call for a test near the upper weight limit before shipment, since a balancing machine manufacturer rarely has such heavy rotors on hand. A final test after installation with an actual rotor may then be the better choice. In any case, it will generally suffice to include one small, or on hard bearing machines, one small to medium size proving rotor, in the purchase of a machine. Rotors weighing several thousand pounds might possibly be furnished temporarily by the balancing machine manufacturer for the acceptance test. For all sizes of proving rotors, a symmetrical shape is preferred to which test masses can be attached at precisely defined positions in 2 transverse planes. Two typical kinds of proving rotors are shown in FIG. 29. ISO 2953 suggests the solid roll-type rotors, with the largest one weighing 1,100 lb. For larger rotors (or even at the 1,100 lb level) a dumbbell type rotor may be more economical. This also depends on available material and manufacturing facilities. Critical are the roundness of the journals, their surface quality, radial runout of the test mass mounting surfaces, and the axial and angular location of the threaded holes which hold the test masses. For guidance in determining machining tolerances, refer to the section on Test Masses. Before using a proving rotor, it will have to be balanced as closely to zero unbalance as possible. This can generally be done on the machine to be tested, even if its calibration is in question. The first test (Umar Test) will reveal if the machine has the capability to reach the specified minimum achievable residual unbalance, the second test (UR Test) will prove (or disprove) its calibration. Whenever the rotor is reused at some future time, it should be checked again for balance. Minor correction can be made by attaching balancing clay or wax, since the rotor will probably change again due to aging, temperature distortion or other factors. The magnitude of such changes generally falls in the range of a few micro-inches displacement of CG, and is not unusual. FIG. 29. Typical proving rotors for horizontal machines. Test Masses Test masses are attached to a balanced proving rotor to provide a known quantity of unbalance at a precisely defined location. The rotor is then run in the balancing machine at a given speed and the unbalance indication is observed. It should equal the unbalance value of the test mass within a permissible plus/minus deviation. Since the rotor with test masses functions as a gage in assessing the accuracy of the machine indication, residual unbalance and location errors in the test masses should be as small as possible. The test procedure makes allowance for the residual unbalance in the proving rotor but not for test mass errors. Therefore, the following parameters must be carefully con trolled to minimize errors. 1. Weight of test mass. 2. Distance of test mass mounting surface to proving rotor shaft axis. 3. Distance of test mass center of gravity (CG) to mounting surface. 4. Angular position of test mass. 5. Axial position of test mass. Since all errors are vector quantities, they should be treated as was done in the error analysis in the section on balancing arbors, i.e., adjusted by the RSS method. The resulting probable maximum error should ideally not use up more than one tenth of the reciprocal of the specified. Unbalance Reduction Ratio factor. For example, if a URR of 95 percent is to be proven, the total test mass error from parameters 1 to 5 should not exceed 0.1 · 5 percent = 0.5 percent of the test mass weight. Often test masses need to be so small that they become difficult to handle. It’s then quite common to work with differential test masses, i.e., two masses 180° opposite each other in the same transverse plane. The effective test mass is the difference between the two masses, called the "differential unbalance." For instance, if one mass weighs 10 grams and the other 9, the difference of 1 gram represents the differential unbalance. When working with differential test masses, the errors of the two comparatively large masses affect the accuracy of the differential unbalance in an exaggerated way. In the example used above, each differential test mass would have to be accurate within approximately 0.025 percent of its own value to keep the maximum possible effect on the differential unbalance to within 2 · 0.25 percent = 0.5 percent. In other words, if the opposed masses are about ten times as large as their difference, each mass must be ten times more accurate than the accuracy required for the difference. Test Procedures To test the performance of a balancing machine, ISO 2953 prescribes two separate tests, the Umar Test and the Unbalance Reduction Test. The origin and philosophy behind these tests and their purpose were explained. Here are the actual test procedures: Umar (or Traverse) Test 1. Perform the mechanical adjustment, calibration and/or setting of the machine for the particular proving rotor being used for the test, ensuring that the unbalance in the rotor is smaller than five times the claimed minimum achievable residual unbalance for the machine. 2. Put 10 to 20 times the claimed minimum achievable residual unbalance on the rotor by adding two unbalance masses (such as balancing clay). These masses shall not be: • in the same transverse plane • in a test plane • at the same angle • displaced by 180° 3. Balance the rotor, following the standard procedure for the machine, by applying corrections in two planes other than test planes or those used for the unbalance masses in a maximum of four runs at the balancing speed selected for the Umar Test. 4. In the case of horizontal machines, after performing the actions described in 1 to 3, change the angular reference system of the machine by 60 or 90°, e.g., turn the end-drive shaft with respect to the rotor, turn black and white markings, etc. 5. For horizontal or vertical two-plane machines, attach in each of the two prepared test planes a test mass equal to ten times the claimed minimum achievable residual unbalance. For example, if the ISO proving rotor No. 5 weighing 110 lbs (50,000 g) is used, the weight of each test mass is calculated as follows: The claimed minimum achievable residual specific unbalance is, say… The claimed minimum achievable residual unbalance per test plane, i.e., for half the rotor weight, is therefore: The desired 10 Umar test mass per plane is therefore equivalent to: If the test mass is attached so that its center of gravity is at a radius of four in. (effective test mass radius), the actual weight of each test mass will be: When two of these test masses are attached to the rotor (one in each test plane as shown in FIG. 30), they create a combined static unbalance in the entire rotor of 10 Umar (or specific unbalance of 10 emar), since each test mass had been calculated for only one half of the rotor weight. FIG. 30. Proving rotor with test masses for "Umar" test. FIG. 31. Log for "Umar" test. FIG. 32. Diagram showing residual unbalance. Note 1: If a proving rotor with asymmetric CG and/or test planes is used, the test masses should be apportioned between the two test planes in such a way that an essentially parallel displacement of the principal inertia axis from the shaft axis results. Note 2: Umar Tests are usually run on inboard rotors only. However, if special requirements exist for balancing outboard rotors, a Umar Test may be advisable which simulates those requirements. 6. Attach the test masses in phase with one another in all 12 equally spaced holes in the test planes, using an arbitrary sequence. Record amount-of-unbalance readings in each plane for each position of the masses in a log shown in FIG. 31. For the older style 8-hole rotors, a log with 45° test mass spacing must be used. 7. Plot the logged results as shown in FIG. 32 in two diagrams, one for the left and one for the right plane (or upper and lower planes on vertical machines). For 8-hole rotors, use a diagram with 45° spacing. Connect the points in each diagram by an averaging curve. It should be of sinusoidal shape and include all test points. If the rotor has been balanced (as in 3) to less than 1/2 Umar, the plotted test readings may scatter closely around the 10 Umar line and not produce a sinusoidal averaging curve. In that case add 1/2 Umar residual unbalance to the appropriate test plane and repeat the test. Draw a horizontal line representing the arithmetic mean of the scale reading into each diagram and add two further lines representing ±12 percent of the arithmetic mean for each curve, which accounts for 1 Umar plus 20 percent for the effects of variation in the position of the masses and scatter of the test data. If all the plotted points are within the range given by those two latter lines for each curve, the claimed minimum achievable residual unbalance has been reached. If the amount-of-unbalance indication is unstable, read and plot the maximum and minimum values for all angular positions of the test mass. Again, all points must be within the range given. Note: If different Umar values are specified for different speeds, the test should be repeated for each. 8. On horizontal and vertical single-plane balancing machines designed to indicate static unbalance only, proceed in the same way as described in 1 and 7 but use only one test mass in the left (or lower) plane of the proving rotor. This test mass must be calculated using the total weight of the proving rotor. 9. On vertical machines, the spindle balance should be checked. Remove the proving rotor and run the machine. The amount of unbalance now indicated should be less than the claimed minimum achievable residual unbalance. Unbalance Reduction Test This test is intended to check the combined accuracy of amount-of unbalance indication, angle indication, and plane separation. Experience gained with running the test in accordance with the procedure described in ISO 2953 (1973 version) showed that the operator could influence the test results because he knew in advance what the next reading should be. For instance, if a reading fluctuated somewhat, he could wait until the indicator showed the desired value and at that moment actuate the readout retention switch. To avoid such operator influence, a somewhat modified procedure has been developed similar to that used in ARP 587 (see SECTION 6C). In the new procedure (ISO 2953-second edition) a stationary mass is attached to the rotor in the same plane in which the test mass is traversed. The unbalance resulting from the combination of two test masses, whose angular relationship changes with every run, is nearly impossible to predict. To have a simultaneous check on plane separation capability of the machine, a stationary and a traversing (or "traveling") test mass are also attached in the other plane. Readings are taken in both planes during each run. Unbalance readings for successive runs are logged on the upper "log" portion of a test sheet, and subsequently plotted on the lower portion containing a series of URR limit circles. All plotted points except one per plane must fall within their respective URR limit circles to have the machine pass the test. A similar procedure has been used by the SAE for more than ten years and has proven itself to be practical and foolproof. The new Unbalance Reduction Test is divided into an inboard and an outboard test. The inboard test should be conducted for all machines; in addition, the outboard test should be conducted for all horizontal two plane machines on which outboard rotors are to be balanced. Each test consists of two sets of 11 runs, called "low level" and "high level" tests. When using the older style proving rotor with eight holes per plane, only seven runs are possible. The low level tests are run with a set of small test masses, the high level tests with a larger set to test the machine at different levels of unbalance. Test mass requirements and procedures are described in detail in FIG. 33. Balance Tolerances Every manufacturer and maintenance person who balances part of his product, be it textile spindles or paper machinery rolls, electric motors or gas turbines, satellites or re-entry vehicles, is interested in a better way to determine an economical yet adequate balance tolerance. As a result, much effort has been spent by individual manufacturers to find the solution to their specific problem, but rarely have their research data and conclusions been made available to others. In the 1950s, a small group of experts, active in the balancing field, started to discuss the problem. A little later they joined the Technical Committee 108 on Shock and Vibration of the International Standards Organization and became Working Group 6, later changed to Subcommittee 1 on Balancing and Balancing Machines (ISO TC-108/Sc1). Interested people from other countries joined, so that the international group now has representatives from most major industrialized nations. National meetings are held in member countries under the auspices of national standards organizations, with balancing machine users, manufacturers and others interested in the field of dynamic balancing participating. The national committees then elect a delegation to represent them at the annual inter national meeting. FIG. 33. Maximum permissible residual specific unbalance corresponding to various balancing quality grades "G," in accordance with ISO 1940. One of the first tasks undertaken by the committee was an evaluation of data collected from all over the world on required balance tolerances for millions of rotors. Several years of study resulted in an ISO Standard No. 1940 on "Balance Quality of Rotating Rigid Bodies" which, in the meantime, has also been adopted as S2.19-1975 by the American National Standards Institute ("ANSI," formerly USASI and ASA). The principal points of this standard are summarized below. Balance tolerance nomograms, developed by the staff of Schenck Trebel Corporation from the composite ISO metric table, have been added to provide a simple-to use guide for ascertaining recommended balance tolerances (see FIGS. 34 and 35). Balance Quality Grades We have already explained the detrimental effects of unbalance and the purpose of balancing. Neither balancing cost considerations, nor various rotor limitations such as journal concentricity, bearing clearances or fit, thermal stability, etc., permit balancing every rotor to as near zero unbalance as might theoretically be thought possible. A tolerance must be set to allow a certain amount of residual unbalance, just as tolerances are set for various other machine shop operations. The question usually is, how much residual unbalance can be permitted while still holding detrimental effects to an insignificant or acceptable level? The recommendations given in ISO 1940 will usually produce satisfactory results. The heart of the Standard is a listing of various rotor types, grouped according to "quality grades" (see TBL. 5). Anyone trying to determine a reasonable balance tolerance can locate his rotor type in the table and next to it find the assigned quality grade number. Then the graph in FIG. 33 or the nomograms in FIGS. 34 and 35 are used to establish the gram· inch value of the applicable balance tolerance (i.e., "permissible residual unbalance" or Uper). Except for the upper or lower extremes of the graph in FIG. 33, every grade incorporates 4 bands. For lack of a better delineation, the bands might be considered (from top to bottom in each grade) substandard, fair, good, and precision. Thus, the graph permits some adjustment to individual circumstances within each grade, whereas the nomograms list only the median values (centerline in each grade). The difference in permissible residual unbalance between the bottom and top edge of each grade is a factor of 2.5. For particularly critical applications it is, of course, also possible to select the next better grade. === TBL. 5 Balance Quality Grades for Various Groups of Representative Rigid Rotors in Accordance with ISO 1940 and ANSI S2.19-1 975 Balance Quality Grade G Rotor Types-General Examples G 4000 Crankshaft-drives (2) of rigidly mounted slow marine diesel engines with uneven number of cylinders (3). G 1600 Crankshaft-drives of rigidly mounted large two-cycle engines. G 630 Crankshaft-drives of rigidly mounted large four-cycle engines. Crankshaft drives of elastically mounted marine diesel engines. G 250 Crankshaft-drives of rigidly mounted fast four-cylinder diesel engines (3). G 100 Crankshaft-drives of fast diesel engines with six and more cylinders (3). Complete engines (gasoline or diesel) for cars, trucks and locomotives (4). G 40 Car wheel (5), wheel rims, wheel sets, drive shafts. Crankshaft-drives of elastically mounted fast four-cycle engines (gasoline or diesel) with six and more cylinders (3). Crankshaft-drives for engines of cars, trucks and locomotives. C 16 Drive shafts (propeller shafts, cardan shafts) with special requirements. Parts of crushing machinery. Parts of agricultural machinery. Individual components of engines (gasoline or diesel) for cars, trucks and locomotives. Crank-shaft-drives of engines with six or more cylinders under special requirements. G 6.3 Parts of process plant machines. Marine main turbine gears (merchant service). Centrifuge drums. Fans. Assembled aircraft gas turbine rotors. Flywheels. Pump impellers. Machine-tool and general machinery parts. Medium and large electric armatures (of electric motors having at least 80mm shaft height) without special requirements. Small electric armatures, often mass produced, in vibration insensitive applications and/ or with vibration damping mountings. Individual components of engines under special requirements. G 2.5 Gas and steam turbines, including marine main turbines (merchant service). Rigid turbo-generator rotors. Rotors. Turbo-compressors. Machine-tool drives. Medium and large electrical armatures with special requirements. Small electric armatures not qualifying for one or both of the conditions stated in G6.3 for such. Turbine-driven pumps. G 1 Tape recorder and phonograph drives. Grinding-machine drives. Small electrical armatures with special requirements. G 0.4 Spindles, discs, and armatures of precision grinders. Gyroscopes. NOTES: 1. The quality grade number represents the maximum permissible circular velocity of the center of gravity in mm/sec. 2. A crankshaft drive is an assembly which includes the crankshaft, a flywheel, clutch, pulley, vibration damper, rotating portion of connecting rod, etc. 3. For the purposes of this recommendation, slow diesel engines arc those with a piston velocity of less than 30 ft. per sec., fast diesel engines are those with a piston velocity of greater than 30 ft per sec. 4. In complete engines, the rotor mass comprises the sum of all masses belonging to the crankshaft-drive. 5. G 16 is advisable for off-the-car balancing due to clearance or runout in central pilots or bolt hole circles. === FIG. 34. Balance tolerance nomogram for G-2.5 and G-6.3, small rotors. FIG. 35. Balance tolerance nomogram for G-2.5 and G-6.3, large rotors. CAUTION: The tolerances recommended here apply only to rigid rotors. Recommendations for flexible rotor tolerances are contained in ISO 5343 (see SECTION 6C) or in Reference 2. Special Conditions to Achieve Quality Grades G1 and G0.4 To balance rotors falling into Grades 1 or 0.4 usually requires that the following special conditions be met: For Quality Grade 1: • Rotor mounted in its own service bearings • No end-drive For Quality Grade 0.4: • Rotor mounted in its own housing and bearings • Rotor running under service conditions (bearing preload, tempera ture) • Self-drive Only the highest quality balancing equipment is suitable for this work. Applying Tolerances to Single-Plane Rotors A single-plane rotor is generally disc-shaped and, therefore, has only a single correction plane. This may indeed be sufficient if the distance between bearings is large in comparison to the width of the disc, and pro vided the disc has little axial runout. The entire tolerance determined from such graphs as shown in FIGS. 34 and 6-35 may be allowed for the single plane. To verify that single-plane correction is satisfactory, a representative number of rotors that have been corrected in a single plane should be checked for residual couple unbalance. One component of the largest residual couple (referred to the two-bearing planes) should not be larger than one half the total rotor tolerance. If it’s larger, moving the correction plane to the other side of the disc (or to some optimal location between the disc faces) may help. If it does not, a second correction plane will have to be provided and a two-plane balancing operation performed. Applying Tolerances to Two-Plane Rotors In general, one half of the permissible residual unbalance is applied to each of the two correction planes, provided the distance between (inboard) rotor CG and either bearing is not less than 1/3 of the total bearing distance, and provided the correction planes are approximately equidistant from the CG, having a ratio no greater than 3 : 2. If this ratio is exceeded, the total permissible residual unbalance (Uper) should be apportioned to the ratio of the plane distances to the CG. In other words, the larger portion of the tolerance is allotted to the correction plane closest to the CG; however, the ratio of the two tolerance portions should never exceed 7:3, even though the plane distance ratio may be higher. For rotors with correction plane distance (b) larger than the bearing span (d), the total tolerance should be reduced by the factor d/b before any apportioning takes place. For rotors with correction plane distance smaller than 1/3 of the bearing span and for rotors with two correction planes outboard of one bearing, it’s often advisable to measure unbalance and state the tolerance in terms of (quasi-) static and couple unbalance. Satisfactory results can generally be expected if the static residual unbalance is held within the limits of and the couple residual unbalance within (where d = bearing span) If separate indication of static and couple unbalance is not desired or possible, the distribution of the permissible residual unbalance must be specially investigated, taking into account, for instance, the permissible bearing loads 4. It may also be necessary to state a family of tolerances, depending on the angular relationship between the residual unbalances in the two correction planes. For all rotors with narrowly spaced (inboard or outboard) correction planes, the following balancing procedure may prove advantageous if Uper is specified in terms of residual unbalance per correction plane. 1. Calibrate respectively the balancing machine to indicate unbalance in the two chosen correction planes I and II (see FIG. 36). 2. Measure and correct unbalance in plane I only. 3. Recalibrate or set the balancing machine to indicate unbalance near bearing plane A and in plane II. 4. Measure and correct unbalance in plane II only. 5. Check residual unbalance with machine calibrated or set as in 3. Allow residual unbalance portions for the inboard rotor as discussed above (inversely proportional to the correction plane distances from the CG), considering A and II as the correction planes; for the out board rotor allow no more than 70 percent of Uper in plane II, and no less than 30 percent in plane A. FIG. 36. Inboard and outboard rotors with narrowly spaced correction planes. Experimental Determination of Tolerances For reasons of rotor type, economy, service life, environment or others, the recommended tolerances may not apply. A suitable tolerance may then be determined by experimental methods. For instance, a sample rotor is balanced to the smallest achievable residual unbalance. Test masses of increasing magnitude are then successively applied, with the rotor under going a test run under service conditions before each test mass is applied. The procedure is repeated until the test mass has a noticeable influence on the vibration, noise level, or performance of the machine. In the case of a two-plane rotor, the effects of applying test masses as static or couple unbalance must also be investigated. From the observations made, a permissible residual unbalance can then be specified, making sure it allows for differences between rotors of the same type, and for changes that may come about during sustained service. Applying Tolerances to Rotor Assembly Components If individual components of a rotor assembly are to be pre-balanced (on arbors for instance), the tolerance for the entire assembly is usually distributed among the components on the basis of the weight that each component contributes to the total assembly weight. However, allowance must be made for additional unbalance being caused by fit tolerances and mounting surface run-outs. To take all these into account, an error analysis should be made. Testing a Rotor for Tolerance Compliance If the characteristics of the available balancing equipment don’t permit an unbalance equivalent to the specified balance tolerance to be measured with sufficient accuracy (ideally within ± 10 percent of value), the Umar test described earlier may be used to determine whether the specified tolerance has been reached. The test should be carried out separately for each correction plane, and a test mass equivalent to 10 times the tolerance should be used for each plane. Balance Errors Due to Drive Elements During balancing in general, and during the check on tolerance compliance in particular, significant errors can be caused by the driving elements ( For example, driving adapter and universal-joint drive shaft). In FIG. 37 seven sources of balance errors are illustrated: 1. Unbalance from universal-joint shaft. 2. Unbalance-like effect from excessive looseness or tightness in universal joints. 3. Loose fit of adapter in universal-joint flange. 4. Offset between adapter pilot (on left) and adapter bore (on right). 5. Unbalance of adapter. 6. Loose fit of adapter on rotor shaft. 7. Eccentricity of shaft extension (on which adapter is mounted) in reference to journals. FIG. 37. Error sources in end-drive elements. The effects of errors 1, 3, 4, and 5 may be demonstrated by indexing the rotor against the adapter. These errors can then be jointly compensated by an alternating index-balancing procedure described below. Error 2 will generally cause reading fluctuation in case of excessive tightness, nonrepeating readings in case of excessive looseness. Error 6 may be handled like 1, 3, 4, and 5 if the looseness is eliminated by a set screw (or similar) in the same direction after each indexing and retightening cycle. If not, it will cause nonrepeating readings. Error 7 won’t be discovered until the rotor is checked without the end-drive adapter, presumably under service conditions with field balancing equipment. The only (partial) remedy is to reduce the runout in the shaft extension and the weight of the end-drive elements to a minimum. Balance errors from belt-drive pulleys attached to the rotor are considerably fewer in number than those caused by end-drive adapters. Only the pulley unbalance, its fit on the shaft, and the shaft runout at the pulley mounting surface must be considered. Such errors are avoided altogether if the belt runs directly over the part. Certain belt-drive criteria should be followed. Air- and self-drive generally introduce minimal errors if the cautionary notes mentioned previously are observed. Balance Errors Due to Rotor Support Elements Various methods of supporting a rotor in a balancing machine may cause balance errors unless certain precautions are taken. For instance, when supporting a rotor journal on roller carriages, the roller diameter should differ from the journal diameter by at least 10 percent, and the roller speed should never differ less than 60 rpm from the journal speed. If this margin is not maintained, unbalance indication becomes erratic. A rotor with mounted rolling element bearings should be supported in V-roller carriages (see Nomenclature, SECTION 6B). Their inclined rollers permit the bearing outer races to align themselves to the inner races and shaft axis, letting the rolling elements run in their normal tracks. Rotors with rolling element bearings may also be supported in sleeve or saddle bearings; however, the carriages or carriage suspension systems must then have "vertical axis freedom" (see Terminology, SECTION 6A). Without this feature, the machine's plane separation capability will be severely impaired because the support bridges (being connected via the rotor) can only move in unison toward the front and rear of the machine; thus only static unbalance will be measured. Vertical axis freedom is also required when the support bridges or carriages are connected by tiebars, cradles, or stators. Only then can couple unbalance be measured without misaligning the bearings in each (out-of phase) back-and-forth movement of the support bridges. This also holds true for hard-bearing machines, even though bridge movement is microscopically small. Figure 6.38. 180° indexing plot. Index-Balancing Procedure A procedure of repetitively balancing and indexing (by 180°) one component against another leads to diminishing residual unbalance in both, until eventually one component can be indexed against the other without a significant change in residual unbalance. Index-balancing may be used to eliminate the unbalance errors in an end-drive adapter, for biasing an arbor or for improving the residual unbalance in a rotor mounted on an arbor. If the procedure is used for a single-plane application (e.g., an end-drive adapter), one half of the residual unbalance (after the first indexing) is corrected in the adapter, the other half in the rotor. The cycle may have to be repeated once or twice until a satisfactory residual unbalance is reached. Care should be taken that after each indexing step, set screws are tightened with the same torque and in the same sequence. The procedure does not work well unless the position of the indexed component is precisely repeatable. If the above iterative process becomes too tedious, a graphic solution may be used. It’s described below for a two-plane rotor mounted on an arbor, with FIG. 38 showing a typical plot for one plane. 1. Balance arbor by itself to minimum achievable residual unbalance. 2. Mount rotor on arbor, observing prior cautionary notes concerning keyways, set screws, and fits. 3. Take unbalance readings for both planes and plot points P on separate graphs for each plane. (Only one plot for one plane is shown in FIG. 38. The reading for this, say the left plane, is assumed to be 35 units at an angular position of 60°.) 4. Index rotor on arbor by 180°. 5. Take unbalance readings for both planes and plot them as points P¢. ( Reading for left plane assumed to be 31 units at an angle of 225°.) 6. Find midpoint R on line connecting points P and P¢. 7. Draw line SS¢ parallel to PP¢ and passing through O. 8. Determine angle of OS (52° for left plane) and distance RP (32.5 units for left plane). Add correction mass of 32.5 units at 52° to rotor in left correction plane. 9. Steps 6-8 must also be performed for the right plane. The rotor is now balanced and the residual unbalance (OR in FIG. 38) remaining in the arbor/rotor assembly is due to arbor unbalance and run-out. If this residual unbalance is corrected by adding a correction mass to the arbor equal hut opposite to OR, the arbor is corrected and biased for subsequent rotors of the same weight and configuration. Additional indexing and unbalance measurement may uncover a much reduced residual unbalance, which can again be plotted and pursued through steps 4-9. This will probably refine both the balance of the rotor and the bias of the arbor. If additional indexing produces inconsistent data, the minimum level of repeatability in locating the rotor on the arbor has been reached. Further improvement in rotor balance is not possible with this arbor. === TBL. 6 Recommended Margins Between Balance and Inspection Tolerances Adjustment to Recommended Tolerances Quality Grade for Balancing For Inspection === Recommended Margins Between Balance and Inspection Tolerances Quite often the residual unbalance changes (or appears to have done so) between the time when: 1. The rotor was balanced originally. 2. It’s checked for balance by an inspector. The following factors may contribute to these changes: • Calibration differences between balancing and inspection machines • Tooling and/or drive errors • Bearing or journal changes • Environmental differences (heat, humidity) • Shipping or handling damage • Aging or stress relieving of components To provide a margin of safety for such changes, it’s recommended that the tolerance allowed the balancing machine operator be set below, and the tolerance allowed at time of inspection be set above the values given in graphs such as FIGS. 33 and 6-34. Percentages for these margins vary between quality grades as shown in TBL. 6. FIG. 39. Hard-bearing balancing machine controlled by a combination of desk top computer and vectometer instrumentation. Computer-Aided Balancing In recent years, the practice of balancing has entered into a new stage: computerization. While analog computers have been in use on hard bearing machines ever since such machines came on the market, it’s the application of digital computers to balancing that is relatively new. At first, desk top digital computers were used in large, high-speed balancing and overspeed spin test facilities for multi-plane balancing of flexible rotors. As computer hardware prices dropped, their application to more common balancing tasks became feasible. The constant demand by industry for a simpler balancing operation performed under precisely controlled conditions with complete documentation led to the marriage of the small, dedicated, table-top computer to the hard-bearing balancing machine as shown in FIG. 39. And a happy marriage it’s indeed, because it proved cost effective right away in many production applications. Features The advantages that a computerized balancing system provides versus the customary manual system are the many standard and not-so-standard functions a computer performs and records with the greatest of ease and speed. Here is a list of basic program features and optional subroutines: • Simplification of setup and operation • Reduction of operator errors through programmed procedures with prompting • More precise definition of required unbalance corrections in terms of different practical correction units • Direct indication of unbalance in drill depth for selectable drill diameters and materials • Averaging of ten successive readings for increased accuracy • Automatic storage of readings taken in several runs with subsequent calculation of the mean reading (used to average the effect of blade scatter on turbine rotors with loose blades) • Readout in any desired components (in certain workpieces, polar correction at an exact angular location is not possible. Instead, correction may only be applied at specific intervals, e.g., 30°, 45°, 90°, etc. The computer then calculates the exact correction mass to be applied at two adjacent components for any desired angle between components) • Optimized distribution of fixed-weight correction masses to available locations • Automatic comparison of initial unbalance with maximum permissible correction, and machine shutdown if the initial unbalance is too large • Automatic comparison of residual unbalance with predetermined tolerances or with angle-dependent family of tolerances • Translation of unbalance readings from one plane to another without requiring a new run (for crankshafts, for instance, where correction may only be possible at certain angular locations in certain planes) • Permanent record of the balancing operation, including initial and residual unbalance, rotor identification, etc. (for quality control) • Machine operation as a sorter • Statistical analysis of unbalance • Connection to an X-Y plotter for graphic display of unbalance • Operator identification • Inspector identification • Monitoring of the number of runs and correction steps • Time and/or date record Next>> |