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636 CHAPTB? 38 device .on. an erpanaon valve or a: capillary tube, winch equalises-pressures at shutdown, the compressor can be started without excessive effort. For; multicylinder 'con>r pressors, an analysis must be made of the number of cylinders .that might possibly be on a compression, stroke at.start and the position of the rods at that moment.; Since the .force needed to push the piston to the top dead center isa function of-how far the rod is away from-the cylinder centerline, ,the worst possible angles these might assume will need,to:be graphically determined by the usual torque effort diagrams. .The torques for some arrangements are.shown below: . 1965 Guide And.QataBook No. (Mini aere .1 -; 33 4 6. Arrangement of Crania Single . Single ' 180*^Apart ` 120* Apart 180" per crank 180" per ' crank Angle ' Between ' Cylinder*' 90" 0" or 180* 60" 120" 90* Approx. Torque from1-: ' Equation 5 .1 7V; " ; ` 1.02SX7V ' ' T. : T. 1.025X7V' ' .60"' ' 1-23X2V Probably as important as starting torque is pull-up torque. Pull-up,torque is, the torque required to accelerate the .com? pressor from rest, overcoming both inertia and gas forces to bring itself to operating speed. It has been found that 'the greatest pull-up torque requirement comes when starting a compressor at a pressure ratio'of-about 2:1. The following empirical equation has been developed experimentally- for multicylinder compressors without unloaders. The equation gives a value' for motor starting torque which provides ade quate starting and pull-up torque to accelerate tire compressor to rated speed at about 2:1 pressure ratio:'1 T. -~-in+2.6); ' (7) Testing Testing;for ratings should be in accordance .with':the ASHRAE 'Standard Methods of Toting'for Rating Refrigerant Compressors (ASHRAE Standard 23-59)';1 '.'hr' ' Testa of a compressor are of two types. The first is for del termination of capacity, efficiency, noise,'motor temperatures; ' etc. The second, equally necessary, is to determine the' prob able life of the machine. Life testing mast be conducted under conditionssimulating thoseunder which thecompressor must operate. A minimum set of conditions would include maximum discharge, maximum suction pressure operation, a medium condition with wet return gas, a minimum suction, high discharge pressure condition with maximum suction gas temperature. A start and .stop test with enough off time to accumulate liquid in thecrankcase will show weaknesses in the lubrication system and resistance to slugging. A reversing test where the motor is reversed every 3 seconds will show weak- neses in mechanical parts, motor and starter.-This test-may be conducted,by removing the suction valves and letting a limited liquid charge drain into the compressor from a con denser mounted higher than the compressor.- Since these tests may easily take:a year; it is important to remember that they can be made-worthless "if significant .changes are made between the start of.testing and the begin ning of( production, in addition to life , toting most manu- . facturere conduct field tests-of new compressor designs.. To establish a uniform industry wide basis for rating com pressors, the Air-Conditioning and Refrigeration -Institute has established standard,.rating conditions noted in ARI Standards 511, 515-60, and 516-60. Presentation of Performance Data An example of a useful presentation of capacity data is given in fig. 2. This is a typical eiet of curves for a 4 cylinder compressor, 2} in. bore, 1} in. 'stroke, 1720 rpm, operating with Refrigerant 22. A set of power curves for the same com pressor is also shown. Fig. 3 shows the heat rejection,curves for the same, compressor. Compressor curves should be labeled with the following information: 1. Degrees gubcooline, or statement that data has been cor rected to sero degrees subcooling. 2. Compressor speed. ,. 3. Type refrigerant. 4. Suction gas superheat. 5. Compressor ambient. 6. External cooling requirements Of required). lf . Crankshafts . FEATURES Higher speeds, the elimination of masive foundations, and more critical user requirements combine to-make'balancing of. crankshafts, necessary- Mathematical treatment .oL this subject can bp found in texts on the subject of vibration.. -, - : Whilethe stressin the shaft is the important factor in shaft life, the deflection of the shaft determines bearing life and plays a significant-role in-noise,'vibration, and efficiency. A shaft with too much deflection will cause excessive edge load ing-resulting in increased clearance, a high 'degree of-noise and vibration;an eccentric and insufficient air gap in hermetic compressors, and eventual bearing failure. ,a ..p Shafts have in general been-made of forged steel' with hardened bearing surfaces to: reduce. wear, - while the :core properties giving-good ductility and strength have befimre tained..The finished crankpins and journals are commonly 8 microinch and polished. :n--v Current practices have utilised cast shafts of either >the alloy, or nodular irons, or higher tensile gray irons. These are generally of lower cost- than the forgings, have lower, notch sensitivity and superior damping qualities. The.gray cast irons in the range of-40,Q00.psi tensile can be employed where the lower modulus of elasticity material can, be tolerated, as these are most economical on a first cost, basis and machine- ability. The alloy or nodular irons are slightly-more expensive than -the gray irons and the controls necessary for. good machineability are more expensive and critical. These ma terial^ have excellent wear resistance but most be highly polished to avoid scoring.the bearings when rotating in either direction. To-obtain;.minimum shaft wear and maximum bearing life, a minimum Brinell hardness .of, 220 should, be maintained on all crankpins and main journals. A hardness in this range will give a good pearlitic. structure and insure removal of all .splinters during ,the. grinding, and. polishing operations. The finish requirements for both crankpins and main journals is 16 microinch. Main Bearings It is possible to have one wide outer bearing and to over hang both, motor and crank; some designs are so arranged. The usual practice, however, is to place the cylinders between main bearings and overhang the motor. This arrangement is especially convenient if a forced-feed pump is applied on the end of the shaft. The bearing on one end serves as a conven ience seal and mounting for the pump. -. Main bearings are usually made of steel backed babbitt, aluminum or leaded bronze or cast aluminum which is tut integral part of the crankcase or bearing head. It can generally be g*id that the harder bearing materials require harder shafts, better finishes and more cleanliness in order to operate at higher loadings. Bearing manufacturers publish bearing load ings for limited bearing life (usually 500 hr) and for perfect lubrication. These loadings should never be approached in refrigeration systems because, lubrication can be far from perfect where oil and refrigerants mix readily. Since heatpump and refrigeration service an easily total more than 4000 hr per year, bearings must be chosen to give at least 20,000 hr life. Clearances depend on the speed and materials involved. .For a steel backed babbitt bearing with a cast-iron shaft, the clearance should be 0.00075 to 0.001 in. per inch of diameter for 1800 rpm operation^ Oil grooves are usually required in the main bearings and grooves should always be placed in the unloaded area of the bearing. Grooves in shafts, are used but should be avoided because they have potential oil scraping characteristics. While it is necessary to feed oil to a bearing, it is not the only requirement. Oil must also have a way of escaping from the bearing in the quantities necessary to keep the bearing cooled. This is of particular importance where the bearing also locates the shaft or takes a thrust load. Since shell-iype bearings do not have enough side area to take thrust loads, thrust surfaces with or without thrust washers must be provided. Some type of oil groove should be provided in. the thrust face or washer to allow oil to escape from the bearing. This will also help the lubrication of the thrust surface. The grooves should be placed in the stationary part to avoid the accidental construction of a centrifugal oil pump which may remove oil too fast from the bearing. Depending on the thrust load, the shaft face may bear against the bearing housing face itself, a hardened steel washer; a steel backed babbitt washer or a bronze washer. Connecting Rods Connecting rods are either the eccentric strap (not split) dr blade and cap (split) type. The blade and cap type can be either made of forged steel, cast iron, bronze or aluminum while the eccentric strap is normally made of aluminumor bronze. Where the .connecting rod is made of forged steel or cast iron, a steel backed insert employing babbitt or alumi num is fitted in the crankpin' bore' requiring ho. further machining. The crankpin end is normally narrow, while the diameter is large. This keeps the distance between main bearings to a minimum. The width of wristpinend should preferably be the same as that at the crankpin to facilitate machining. If the rod.is of bronze or aluminum, it is satisfac tory for use as a bearing without the need for inserts or wristpin bushings. In this case, however, the shaft should be hard. Rods should be drilled opposite the point of maximum bearing pressure to admit oil for splash lubricated machines.. Splash machines may have dippers as part of the,rod/'