Blocking machinery is very easy if you use a little common sense. Defined rules will not always work as machine configurations and possibilities of local obstructions are limitless. The following are some considerations that will help develop a “train of thought” in considering block locations that we require.
MOTOR CLOSE-COUPLED PUMPS AND FANS (MCC)
In the case of MCC units, the driven rotating component (impeller or fan wheel) is attached directly to the motor shaft. The pump housing and volute are attached to the motor housing. The shaft and driven components transmit vibration to the machine housing through the motor bearings. For motors under 40 HP, the block location is M2 or the motor driven end.
Sometimes you may encounter a poor mounting surface or obstruction (grease zerk) at the first choice location. If this is the case, move the block onto the pump attachment flange which would be the next best choice. If you need top move the block a few degrees from the vertical plane, you may, but keep it to within a few degrees. If for some reason a block cannot be attached to the preferred locations due to the shape, clearance of nameplate attachment, move it slightly further towards the pump volute or down on the side (horizontal plane) of the motor. However, it is unlikely that you cannot use the first or next best choice of locations.
For MCC pumps larger than 40 HP, we add a second block location to the motor free end (M1). Another rule of thumb – if the span between M1 and M2 is greater then 30 inches, use two blocks on the motor.
On MCC machines, never move the block location from M2 to M1. M1 is physically too far from the driven component. The motor fan cowling is usually made of sheet metal (resonant) and is attached at only a few points, making it a poor test location.
In the case of MCC gland exhaust fans (main, auxiliary, and TG), again we want to see that position M2 is used. The shaft usually has a “heat sling” between the motor bearing and the fan wheel. Care should be used when working around the heat slinger. If there are obstructions that prevent locating the block on the vertical axis, then move the disk to the next best location, which is on the horizontal axis. Again, as in the MCC pumps, position M1 is not acceptable.
“Navy Standard” ventilation fans are also MCC machines. The motor and fan wheels are mounted on the inside of the ducting. Normally, the motor is mounted on the driven end by a stator vane assembly. The free end of the motor is normally attached to the ducting with steel rods. These are easily located as they protrude through the ducting in four locations with nuts attached.
Our rules for MCC machines tell us to attach the block as close to the M2 position. However, it is difficult to locate exactly where the stator vanes meet the outer ducting. For this reason, we attach the block adjacent to the steel rod ends.