Page 64 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

rotational speed [1 × rpm] of the rotating element, and has an
amplitude proportional to the amount of unbalance. The vibration
will be largest in the radial direction, and only a single-phase
mark will be present.
Equipment with overhung rotors may exhibit high axial vi-
brations due to unbalance. See Figure 4-11.

SIMPLY
SUPPORTED OVERHUNG

Figure 4-11. Types of Support for Rotating Elements

Example 4.4
An electric motor driven pump operates at 1,750 rpm and a
velocity reading taken on the inboard motor bearing was .106
inches per second. The motor’s rotor weighs 24 pounds. If the
vibration was due to a missing piece of key material in the cou-
pling, what was the unbalance force? If the keyway was 1/2" × 1/
2,” how long was the missing key if the motor shaft was 4 inches
in diameter?

Step 1. The velocity reading needs to be converted into accelera-
tion and then into the force. Using Equation (2.33), G = F*V/
3,689.848201. Since the vibration was due to unbalance, it was
occurring at 1 × rpm or at a frequency of 1,750 cycles per minute.
Thus, F = 1,750 cpm. Now, G = 1750 * .106/3689.848201 = .0527,
and A = 386.4 * G = 386.4 * .0527 = 19.42 feet per second per sec-
ond.

Step 2. The mass of the rotor is W/g = 24/32.2 = .745 pounds
mass. F = MA = .745* 19.42 = 14.47 pounds force. Now the force