CYLINDERS AND PISTONSThe cylinders on pistons should be inspected onlyAFTER the manufacturer’s technical manual has beenconsulted. Be careful when removing heads, parti-cularly where metal-to-metal joints are involved, toprevent damage to the joint.If replacement of piston rings is required becausethey are worn or broken, take accurate measurements ofthe cylinder liners. Standard size rings may be used inoversize cylinders if the oversize does not exceed 0.003inch per inch of cylinder diameter. The liner may alsoneed to be replaced if it is badly worn or out of round.When replacing piston rings, first fit them to the cylinderto check for proper end clearance. You can file the ends,if necessary, to make them fit. The side clearance of therings should be such that the rings will fall easily intothe piston grooves, which should be deep enough for thering thickness. Ring splits should be staggered. Afteryou assemble the piston, wire the rings tight with a softcopper wire so that they will enter the bore easily. Thiswire can be removed through the valve ports after thering has started into the cylinder bore.When reassembling the air cylinders and heads, besure they are all drawn down evenly, especially onmultistage compressors where the heads containcylinders for third and fourth stages. Otherwise, theresult will be excess wear on the cylinders and pistons.When a compressor piston has been replaced, thepiston end clearance must be checked. This is done byinserting a lead wire through a valve port or indicatorconnection. Jack the compressor over. When the pistonhas moved to the end of its stroke, the lead will beflattened to the exact amount of clearance. The wireshould be long enough to permit a reading near thecenter of the piston. These readings should be taken afterany adjustment or replacement of the main, crank pin,wrist pin, or crosshead bearings. Methods of adjustingthe clearances vary according to the compressor design.You should consult the manufacturer’s instructions forsuggested adjustment.MISCELLANEOUS ADJUSTMENTSFrom time to time other miscellaneous adjustmentsare required on compressors, including those pertainingto wrist pins, crosshead shoes, reduction gears,couplings, and V-belt drives. The manufacturer’stechnical manual will give you specific information forthe care, adjustment, and replacement of all fittedbearings. Refer to the manufacturer’s instructions fordetailed information on when and how to make theseadjustments.Wrist pin bushings are replaced when necessary.This is done when they are worn to the point ofbecoming noisy. In making a replacement, be sure theoil hole in the bushing is properly lined up with the oilhole in the connecting rod. After being pressed into therod, the new bushing must be reamed.Crosshead shoes are provided with shim or wedgeadjustment. Wear should be slight, but adjustmentshould be made when the travel of the piston rod causesa movement in the stuffing boxes.Alignment of reduction gears and pinions should bechecked periodically, especially on a new compressor.Misalignment may be caused later by settling, straining,or springing of foundations; pipe strains on turbine-driven compressors; bearing wear; or springing due toheat from a turbine.Flexible couplings require very little maintenancewhen they are properly lined up. Some types requireoccasional lubrication to prevent excessive wear ofsprings and bushings. A noisy coupling is an indicationthat the bushing is worn and requires replacement.V-belt drives require adjustment for belt tension.Belts generally stretch slightly during the first fewmonths of use. A loose belt will slip on the motor pulleyand cause undue heating and wear on the belt. A tightbelt will overload the bearings. Belts should be protectedagainst oil and high temperatures. To prevent rapiddeterioration, belts should not be used at temperaturesabove 130°F. V-belts are usually installed in sets of twoor three. If a single belt is worn or deteriorated, thecomplete set should be replaced to ensure that each beltwill carry its share of the load.LUBRICATION SYSTEMProper care of a compressor lubrication systemincludes the following:-Keep the oil at a normal level in the reservoirat all times to maintain proper oil temperature.- Change crankcase oil periodically, and at thesame time clean and flush the crankcase and clean theoil filter.-Maintain proper lube-oil pressure by keepingthe oil pump in good working order and adjusting thebypass relief valve.6-9
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