VIBRATION SURVEYS & ANALYSIS
This is an area that should be of utmost concern to all roll shops. Tighter tolerances and surface finish requirements are taxing machines to their limits of acceptability. Chatter can be a major contributor leading to loss of production, poor quality, and extensive machine downtime trying to identify the source of the problem. This means that the condition of all rotating elements is critical to a smooth running roll grinder.
RGB Engineering can provide a vibration analysis to investigate the effects of "forced" vibration - unbalances, bearing noise, runouts, belt problems, etc. In addition, arrangements can be made for a complete "modal analysis" of the roll grinder, should it be warranted, which will pinpoint structural weaknesses, stiffness and damping limitations, and general machine instabilities. This proven technique utilizes state of the art FFT computer equipment to model the grinder and monitor the critical points in response to a measured force input. We have been successful in the past using this approach to identify problem areas and apply corrective measures such as vibration absorbers and structural stiffening to eliminate chatter problems.
As is typical with chatter and pattern problems, one can get many different stories concerning the severity of the chatter, what it looks like, at what point it was ground in, if it can be camouflaged, when the problem first occurred, the direction of the helix, spacing of the marks, etc. Having exhausted the normal fundamental mechanical checks and standard troubleshooting procedures, it then becomes necessary to employ vibration analysis techniques to supplement these methods to help identify the problem. The use of a FFT vibration analyzer together with a systematic approach toward problem solving can shorten the time frame considerably. Although not as effective as a comprehensive modal analysis, it is a relatively low cost alternative, especially for the case of "forced" vibration problems. It is certainly more cost effective than the "shotgun" approach, where a multitude of problems are addressed in no particular order, hoping that by chance the right one may get fixed.
Vibration testing in itself is only as effective as the ability of the analyst to interpret the data. This can become very difficult in roll grinder work due to the relatively low levels of vibration compared to other types of equipment. Even problem machines sometimes fail to provide strong enough signals to identify suspect areas. The wheel motors are usually of sleeve bearing design making it difficult to pinpoint unbalances or bearing problems. With a "one-shot" approach, one must rely on comparisons to "normal" operating machines in order to form meaningful conclusions. Having more than 30 years of experience with roll grinder vibration investigations, RGB Engineering has developed an extensive database of comparison surveys. This background of knowledge is what distinguishes a qualified roll grinder consultant from the general pool of vibration analysts.
A CSI UltraSpec 8000 portable spectrum analyzer is used with an accelerometer pickup to record and temporarily store the vibration data in the field, which is then downloaded to a desktop computer back in the office for analysis and plotting. The photo on the right shows a typical measurement being taken. A representative plot is shown on the left. Note that the equipment provides the basic amplitude vs. frequency plots, but it is still necessary to analyze and interpret the data to reach meaningful conclusions. Again, this is an area of expertise that only a person with sufficient roll grinder background experience can possess.
Basically, there are two types of vibration surveys. The first, as described above, is employed primarily to solve a specific chatter or grinding problem. The second is more of a diagnostic tool, used as part of a preventive maintenance program to establish a baseline footprint of the grinder, and then to follow up with scheduled periodic repeat checks, the goal being to minimize excessive machine downtime due to major unscheduled grinder repairs. The background data is used as a comparison for future surveys, and the grinder's condition is monitored by noting discrepancies in the vibration readings from a "normal" pattern.
Please contact RGB Engineering for more detailed information concerning either type of survey. Customer references can be supplied if required.