Timber harvesters are high powered industrial grade rigs used to fell, delimb, and cut to size large trees. Hydraulic motors power sprockets that drive chains around a guide bar that get extended through logs during cuts. As technological advances allow for higher speeds a phenomenon called “chain shot” has been shown to be a real and potentially injurious or lethal threat to harvester operators. These events have been known about since the early 1990s. Multiple accident reports, hazard warnings, and revised safety standards have been put forth to try to combat this threat. The phenomenon occurs when a primary break on a saw bar allows a chain to whip back and cause a flick strong enough to cause a secondary break on the end of the chain whip potentially launching links of chain back toward operators, depending on bar orientation, with the energy and speed of bullets shot from a gun. The current information on these events is incomplete and at times contradictory. The goal of this project is to design, build, test, and validate a machine that recreates these events and to further the body of knowledge on these sub millisecond, high energy events.
The testing rig build outline and design is explained as well as walking through how tests are conducted and data is processed. This design all works off and expands from a current build used to test in accordance with an ISO standard employed in Sweden to test timber harvester guarding and shielding systems. This build set up the foundation to complete a study on the effects on the characteristics of primary breaks when varying bar length and primary break initiation as well as outline and measure other characteristics of chain shot. Longer bars and resultant chain loops are shown to increase the peak force through break, the impulse, the time of break, and the energy lost in the drive train during break. Stoppage location distance increase is shown to influence the duration of break, and the impulse experienced by the chain. Bar length increase is shown to cause an increase in impulse experienced by the chain, time of break, energy lost in drive train during break, and max force exerted on chain when chain breaks. This machine shows chain shot to usually be a 2 drive link 10.33 gram projectile (.404 pitch, .080” chain) that has speed measurements of up to 342 m/s and rotation up to 154,000 RPM about the center of mass that can penetrate fully through 25.4 mm of High Density Polyethylene (HDPE).