CYLINDERS AND PISTONS

CYLINDERS AND PISTONS

The cylinders and pistons should be inspected only AFTER the manufacturer’s technical manual has been consulted. Be careful when removing heads, parti-cularly where metal-to-metal joints are involved, to prevent damage to the joint.
If replacement of piston rings is required because they are worn or broken, take accurate measurements of the cylinder liners. Standard size rings may be used in oversize cylinders if the oversize does not exceed 0.003 inch per inch of cylinder diameter. The liner may also need to be replaced if it is badly worn or out of round. When replacing piston rings, first fit them to the cylinder to check for proper end clearance. You can file the ends, if necessary, to make them fit. The side clearance of the rings should be such that the rings will fall easily into the piston grooves, which should be deep enough for the ring thickness. Ring splits should be staggered. After you assemble the piston, wire the rings tight with a soft copper wire so that they will enter the bore easily. This wire can be removed through the valve ports after the ring has started into the cylinder bore.
When reassembling the air cylinders and heads, be sure they are all drawn down evenly, especially on multistage compressors where the heads contain cylinders for third and fourth stages. Otherwise, the result will be excess wear on the cylinders and pistons. When a compressor piston has been replaced, the piston end clearance must be checked. This is done by inserting a lead wire through a valve port or indicator connection. Jack the compressor over. When the piston has moved to the end of its stroke, the lead will be flattened to the exact amount of clearance. The wire should be long enough to permit a reading near the center of the piston. These readings should be taken after any adjustment or replacement of the main, crank pin, wrist pin, or crosshead bearings. Methods of adjusting the clearances vary according to the compressor design. You should consult the manufacturer’s instructions for suggested adjustment.

MISCELLANEOUS ADJUSTMENTS
From time to time other miscellaneous adjustments are required on compressors, including those pertaining to wrist pins, crosshead shoes, reduction gears, couplings, and V-belt drives. The manufacturer’s technical manual will give you specific information for the care, adjustment, and replacement of all fitted bearings. Refer to the manufacturer’s instructions for detailed information on when and how to make these adjustments.
Wrist pin bushings are replaced when necessary. This is done when they are worn to the point of becoming noisy. In making a replacement, be sure the oil hole in the bushing is properly lined up with the oil hole in the connecting rod. After being pressed into the rod, the new bushing must be reamed.
Crosshead shoes are provided with shim or wedge adjustment. Wear should be slight, but adjustment should be made when the travel of the piston rod causes a movement in the stuffing boxes.
Alignment of reduction gears and pinions should be checked 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 to heat from a turbine.
Flexible couplings require very little maintenance when they are properly lined up. Some types require occasional lubrication to prevent excessive wear of springs and bushings. A noisy coupling is an indication that the bushing is worn and requires replacement. V-belt drives require adjustment for belt tension. Belts generally stretch slightly during the first few months of use. A loose belt will slip on the motor pulley and cause undue heating and wear on the belt. A tight belt will overload the bearings. Belts should be protected against oil and high temperatures. To prevent rapid deterioration, belts should not be used at temperatures above 130°F. V-belts are usually installed in sets of two or three. If a single belt is worn or deteriorated, the complete set should be replaced to ensure that each belt will carry its share of the load.