| | | |

Case study

Slow Speed Diagnostics

Many industrial machines operate at such slow rotational speeds that vibration generated by common machine malfunctions, such as imbalance, misalignment, and bearing faults, falls below the sensor frequency response range. The impulses created by these phenomena are diminished both by their structural path through the machine as well as by the low frequency sensor cutoff. For these reasons, major concerns in selecting condition-based maintenance programs for slow speed machines have focused primarily on the service life of the rolling element bearings being used.

Typical slow speed machines include ball mill mixers, antenna drives, hydraulic electric generators, and rail car turntables where the rotational speed is often less than several hundred rpm and can be as low as a fraction of an rpm. Even though defect impulses occur at extremely low frequencies, the nature of the impulses is such that high frequency components can be analyzed for bearing faults by using acceleration enveloping methods.

Case Study - Rail Car Turntable

In a steel mill, during one phase of operation, scrap is added to a furnace feed hopper from a rail car that is positioned by a turntable under a traversing crane. Recently there were problems associated with the turntable thrust bearing.

The rail car turntable is a critical operation in the steel process, and unscheduled outages incur high production costs. The thrust bearing must sustain the combined rail car and scrap weight as it completes one revolution, taking about 258 seconds. It is therefore critical that the bearing performs reliably. Proactive condition-based monitoring can ensure such reliability. If failure modes are avoided and scheduled maintenance intervals are extended, then large savings can be realized in maintenance costs and process downtime.

An accelerometer was magnetically attached as close to the lower bearing race as possible. It was immediately recognized that conventional vibration measurements would not be fruitful in evaluating the bearing condition, so a portable data collector/analyser was used to measure in the acceleration enveloping mode. To eliminate noise and unwanted signals, the bandpass filter selection was set to a high pass corner frequency greater than 10 times the running speed. The use of a filter separates the defect impulses from the low order vibration components. Higher frequency filters are sometimes preferred when structural resonances amplify the higher harmonic components of vibration. In this case, the 5 to 100 Hz and 50 to 1000 Hz settings were used to make alternate measurements.

The measurement was more difficult to make since the table rotates one revolution in approximately 258 seconds. In order to capture the bearing defect signals, a wide data sampling time window was required. A 96 second data sampling window was obtained by setting the FFT line resolution to 6400, and the maximum frequency range to 4000 pm

Time window = # of lines 96400 lines) divided by f (4000/60) or, T = 6400 x 60 = 96 seconds 4000

Figure 1. Time domain in AE measurement P₁ - P₂ = 28.5 secs. P₃ - P₂ = 27.4 secs

Figure 1 shows a time domain plot using the 5 to 100 Hz filter in the enveloping mode. Repetitive impulse peaks appear at approximately a 28 second interval.

Figure 2 shows an expanded display of the first peak which indicates an impact with its structural response lasting for 0.5 seconds. A similar expansion of the other peaks showed the same rise and fall response curve. The 28 second interval bewteen peaks corresponds to the outer race bearing defect frequency of the model 29452 spherical roller bearing.

Figure 2. Time domain 3 sec expansion about the first peak

A bearing database (Figure 3) identifies the outer ring defect frequency as 9.12 Hz for an inner ring speed of 1 Hz. Since the table rotational interval is 258/9.12=28.2 seconds, the diagnosis of an outer race defect was confirmed.

Figure 3. 29452 Thrust bearing specification

The bearing was replaced and examined for structural faults. A defect was discovered in the outer race which provided further confirmation that acceleration enveloping methods can be successfully applied to very slow speed bearing diagnostics.

Vibration analysis of very slow speed machines is difficult to perform because of sensor low frequency limitations. In addition, the low frequency related vibration responses are characterised by very low amplitude signals, and are usually buried in noise.

However, impulse forces caused by bearing defects and gear faults can often be identified by enveloping techniques. In this case study, the spectrum of the enveloped signal did not provide sufficient information to permit a positive identification; the time domain data however, did provide the diagnostic clues.

The data collectors/analysers and on-line monitoring systems used and supplied by the apt Group include the acceleration enveloping parameter and allow for the optimum frequency to be selected for a particular measurement circumstance. In this case study, a lower frequency bandpass filter provided the best results for positive identification of the bearing flaw.

Top of page  |  Back to Case Studies