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.)
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.
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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