Reduction of Vibration Transmission in String Trimmers

It has been well documented that over long periods of time, people who regularly operate hand tools powered by small internal-combustion engines can become affected by a debilitating set of clinically irreversible effects, collectively referred to as hand-arm vibration syndrome (HAVS). Although HAVS cannot be cured, the onset of the disorder can be delayed or, in fact, prevented, by restricting either the duration of the exposure, and/or the magnitude of the vibration transmitted from the tool to an operator\u27s hands and arms (per ANSI and similar standards). In this paper, we’ve evaluated an approach for reducing the magnitude of transmitted vibration at the engine-side grip of one commercial- and one consumer-grade string trimmer. The ultimate goal of this work is to develop a set of simple, passive retrofits for these tools that will reduce the likelihood of HAVS in full-time grounds-maintenance workers. The idea for this work stems from a stated need of Goodwill Industries of San Antonio, our NPA (non-profit agency) collaborator over the past several years. Based on this preliminary work, we’ve found that grips incorporating half-inch rubber studs placed at areal densities of either 25/in2 or 12/in2 significantly reduce the RMS vibration transmitted to the hand and arm at all frequencies (including the low frequency components known to be most harmful)

Three-dimensional Acceleration Measurement Using Videogrammetry Tracking Data

In order to evaluate the feasibility of multi-point, non-contact, acceleration measurement, a high-speed, precision videogrammetry system has been assembled from commercially-available components and software. Consisting of three synchronized 640 X 480 pixel monochrome progressive scan CCD cameras each operated at 200 frames per second, this system has the capability to provide surface-wide position-versus-time data that are filtered and twice-differentiated to yield the desired acceleration tracking at multiple points on a moving body. The oscillating motion of targets mounted on the shaft of a modal shaker were tracked, and the accelerations calculated using the videogrammetry data were compared directly to conventional accelerometer measurements taken concurrently. Although differentiation is an inherently noisy operation, the results indicate that simple mathematical filters based on the well-known Savitzky and Golay algorithms, implemented using spreadsheet software, remove a significant component of the noise, resulting in videogrammetry-based acceleration measurements that are comparable to those obtained using the accelerometers