Shaft motion takes one of three forms: rotation, reciprocation, or oscillation. For rotating shaft applications (see Figure 131), the direction of rotation is very important to seal design; a shaft may rotate only clockwise (CW), only counterclockwise (CCW), or be bi-rotational (variously rotating both CW and CCW). It’s also important to note whether the rotation is continuous or intermittent.

The speed at which a shaft rotates is noted in revolutions per minute, or RPM. When designing a shaft seal, it’s important to know the shaft’s normal RPM (the speed at which it rotates most of the time), as well as any temporary acceleration it may undergo (up to its maximum RPM).

The surface speed of the shaft is dependent on RPM and the shaft diameter. Surface speed is typically expressed in feet per minute (FPM) or meters per minute (MPM). Table 42 can assist you in determining the relationships between shaft diameter, MPM, FPM, and RPM. To find either the MPM or FPM for a given shaft diameter and RPM, lay a straight edge over the known values on the Shaft Diameter and RPM lines. The straight edge crosses the MPM/FPM line at the corresponding speed. In the example shown, a line drawn from 1" shaft diameter to 400 RPM crosses the MPM/FPM line at about 110 FPM, thus indicating the corresponding surface speed of the shaft.

The motion of a reciprocating shaft (one that moves back and forth, see Figure 132) is chiefly defined by two variables: 1) the length of each stroke (defined as the total movement in one direction and expressed in inches or millimeters), and 2) the number of cycles per minute (the number of times the shaft makes a “round trip,” i.e. one stroke in and one stroke out).

An oscillating shaft rotates, but rather than going round and round endlessly (as with a true rotary shaft), an oscillating shaft rotates back and forth within an arc defined by the degrees of rotation (see Figure 133). An oscillating shaft is defined by 1) the degrees of this arc, and 2) the cycles per minute (the number of times the shaft moves across this arc and returns to its beginning position).

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