Setting Compressor TDC

APPLICATION NOTES

To mark TDC of a compressor cylinder it is necessary to position the piston to outer dead center and compensate for bearing and pin/bushing clearances as well as the relative lack of motion when the piston is 3-4 degrees before and after TDC. A dial indicator (DI) on the crosshead can be used to accomplish this. The goal is to establish when the crankshaft journal is at TDC or outer Dead Center.

Locate an approximate TDC by jacking the machine to outer dead center. Set and zero the DI. Slowly rotate the machine reverse of normal rotation until the dial indicator reads 30 mils or more taking care not to let the DI lose contact with the crosshead. Move the machine in the direction of normal rotation until the DI reads 10 mils. At this point strike a line on the flywheel, coupling or shaft in line with the reference pointer. Slowly rotate the machine in the direction of normal rotation until the piston passes through TDC and begins its backward stroke. Stop rotation when the DI is at 30 or more mils and then reverse rotation in the direction of normal rotation again until the precise reading where the original mark was struck (10 mils) is reached. At this point strike a second line. To locate TDC, measure the distance between the two marks and strike the final mark exactly half way between the two previous marks.

Another method is to block the piston just short of TDC by rotating the piston against a solid object placed in an open suction valve hole. The piston should be snug against the object in order to take up any clearances in the pin/bushing assembly or rod bearing. The first mark is struck in this position. The machine is then rotated opposite normal rotation until the piston comes back into contact with the obstructing object. The same amount of force should be used to snug the piston this time as well. The second mark is struck at this position and the centerline TDC is found in the same manner as above.