Abstract.When greater density bales are made, baler manufacturers recommend using twine with greater knot strength. When twine fails, bale integrity is lost, harvesting costs increase, and productivity suffers. Twine failure typically occurs in the knot on the top strand of the twine. A better understanding of twine tension in the top strands could help reduce failures, allow for improved knot strength recommendations, and ultimately lower baling costs. A system was developed to measure the tension of the top strands while baling a variety of crops with a high-density large square baler. Depending on the bale chute design, twine tension was greatest as the bale cantilevered from the chamber but had not yet touched the chute or just as the bale fully exited the bale chamber. In either case, the absolute maximum recorded tensions were typically less than 60% of the twine specified knot strength. Pulses in synchronization with the plunger frequency were superimposed on the nominal twine tension. Tension was usually greatest in the outer left twine and the other right twine because for each, there was only one neighboring twine to share the load. Average twine tension over the first 60 s after the bale rested on the ground was linearly related to bale density. Crop stress relaxation reduced tension up to 20% within 20 min after the bale was placed on the ground. Top strand tension approached 60% of knot strength for only a short duration as the bale exited the chamber and after that, the tension was much less than the specified knot strength. Therefore, design changes or strategies that reduce tension during the critical period when the bale exits the chamber could reduce maximum knot strength requirements and lead to lower baling costs. Keywords: Bales, Density, Tension, Twine.

Abstract. Creating high-density biomass bales would reduce the number of bales handled, stored, and transported to biorefineries, thereby reducing costs. Recompression of large square bales is one approach to increase bale density; however, recompression research on large bales is limited. Recompression of common biomass crops in 80 cm × 90 cm nominal cross-section bales was used to quantify pressure-density and stress relaxation relationships as well as restraining forces required to maintain compressed densities. Linear, power, and exponential models were fit to the pressure-density data, with the linear model providing the best representation of the recompression process. When bales were recompressed soon after bale formation, density increased by an average of 134%, and the target dry-basis density of 205 kg m-3 was exceeded at the end of recompression with all crops. While still compressed, bales were wrapped with four steel cables configured with load cells to measure the restraining force after release of pressure followed by bale re-expansion. Total restraining forces ranged from 18 to 40 kN. Bale density decreased by an average of 17% due to re-expansion after pressure was released. Strain relief induced by allowing the bale to re-expand by approximately 10% before placing the cables around the bales reduced the required restraining force by an average of 35%. Although the forces applied were great (>560 kN), the dry-basis specific energy requirements were comparatively low (0.18 to 0.29 kWh Mg-1) because recompression took place over a relatively long duration of approximately 20 to 25 s. Recompressing large square bales is a low-energy method to achieve bale densities that should ensure weight-limited biomass transport. Keywords: Bales, Biomass, Compression, Energy.

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