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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 7, Misalignment

Section 12, Lower Frequency Harmonics

Rolling element bearing with excessive clearance between the inner and outer races usually show several harmonics of 1 x rpm, such as 2 x, 3 x, 4 x, 5 x, 6 x, etc. (Rolling elements have excessive sliding action rather than rolling action.)

Bearings loose and rolling around in the housing. The symptom can also be similar to that for a bearing with excessive clearance.

Pulley or sheave misalignment usually produces higher than usual amplitudes at 1 x rpm and sometimes at 2 x rpm. Shafts may be in proper alignment (such as parallel to each other), but the pulleys or sheave grooves may not be in the same planes relative to each other. Alignment must be made with respect to the sheaves' grooves rather than the sheaves' sides.

Bearing Misalignment. The writer has very little experience with this. It is only suspected that bearing misalignment will produce higher than usual harmonic amplitudes. No papers seem to be available on the subject. For rolling element bearings, an accompanying symptom may be the presence of vibration in the higher frequency range (beyond the range of lower harmonics).

Misaligned rolls such as papermachine rolls. Rolls have to be parallel to each other. Sometimes they are within the proper alignment tolerance but the small 2 x rpm frequency can be magnified by resonance.

Vanepass frequency. Most often there is nothing wrong with the pump itself. The amplitude of this harmonic is usually negligible or not present at all. The vanepass frequency will most likely be in the range of the lower harmonics. It doesn't stimulate other harmonics. Most often this trouble is the result of flow rate that is considerably low as compared to optimum flow (such as below 40 percent of the optimum flow rate). Sometimes the piping system or valve is resonant to the vanepass frequency. Rarely is the vibration due to bad pump design or manufacturing error, such as too little clearance between the impeller outside diameter and the pump case. Another rare possibility is that the outer edges of the vanes and/or volutes require some relief or recontouring. (See section "Flow-Related Problems in a Centrifugal Pump.")

Bladepass frequency. This harmonic amplitude and frequency can be resonated by some part of the fan's base (especially on a flexible steel base), pedestal, shroud or other structural member. It can also be the result of improper damper settings (check with fan manufacturer). Certain fan designs are more responsive to damper settings than others.

Unbalanced reciprocating parts. When a reciprocating machine's piston has to be replaced, the replacement piston must be of equal weight to the piston it is replacing. The same applies to connecting rods. However, two connecting rods may weigh exactly the same but their large ends (that attach to the crankshaft) may not weigh the same. The difference in weight at the big end may be compensated for by an equal difference in the small ends of the rod. This will make the total weight of the rods weigh the same, but the big ends must weigh the same as each other and the small ends must weigh the same as each other. If not, they produce excessive vibration, most often 2 x rpm and sometimes at 4 x rpm. Other harmonics have also been reported, including harmonics that are not a full integer multiple of rpm.

Mechanical looseness. The amplitudes of these harmonics are usually relatively small compared to the vibrations at 1 x rpm and the usual full integer harmonics. The harmonic peaks may be so small that they appear to be almost negligible, but the fact that they do produce definite peaks at definite frequencies is significant since at such frequencies the usual amplitudes are zero or almost zero. The small but definite peaks is then considered "higher than usual." With FFT or similar type analyzer, these small amplitude peaks are easier to discern if the amplitude is on a log scale rather than linear.

The type of looseness described here is between a machine's bottom and its supporting base, or looseness of a bearing inside its housing. When the machine is stopped, the bearing may "feel" tight in the housing but "not tight enough" for the forces that occur while running. The most commonly reported harmonic frequencies are not the usual full integer multiples of rpm. Instead, they produce harmonics of ½x, 1 ½x, 2 ½x, 3 ½x, 4 ½x rpm and so on. Most often only one or two of these "strange" harmonics are produced. For example in one situation, only the ½x rpm harmonic may be produced, in another, the ½x and the 2 ½x rpm but not the 1 ½x rpm.

Rub between the rotor and a stationary part. This can happen for example, between an improperly installed bearing or seal. Frequencies due to rub can be at any frequency, similar to that produced by the squealing of a tire or a squeaking door. However when the rub frequencies are superimposed on other vibration frequencies, harmonics are often produced. Mechanical rub produces similar harmonics at frequencies and amplitudes as are described for vibration due to a looseness. If such harmonics are produced, the writer suspects the possibility of looseness much more than the possibility of a rub. However, what was done to the machine during repair or reassembly will probably guide the analyst more toward one than the other. In still other situations, such as with a very hard rub, all the harmonics may be produced showing their peaks across the whole spectrum.

Cutting teeth impulses. Cutters, such as milling machine cutters or woodworking planer heads, usually have cutting teeth that are equally spaced and, therefore, produce steady vibration at a frequency of number of teeth x rpm. Usually the amplitude is negligible for most machinery operations, and the effect on surface finish is not objectionable. However, if the cutting teeth impulse frequency coincides with a machine part's resonant frequency, then the amplitude can be magnified considerably. This can cause a notably worse surface finish as well as drastically reduced cutter life between sharpenings.

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