Relation between Stump Diameter and Length of Shear - Caused Splits in Planed

When logs of three 4iaineter classes and two specific gravity classes were sheared with a 3/8-inch-thick knife travelling at 2 inches per minute, shearing force and work averaged greatest for dense 13.6-inch logs cut with a knife having a 45" sharpness angle (73,517 pounds; 49,838 footpounds). Force and work averaged least for 5.1-inch bolts of low density when cut with a knife having 22-1/2" sharpness angle (9,975 pounds; 2,885 foot-pounds). Values for 9.7-inch bolts were intermediate. Shear force reached a maximum about three-fourths the way through the log; it then dropped rapidly as the knife travelled the remaining distance. Momentary peaks of force commonly occurred near the three-quarter point. The greatest observed force to shear was 92,000 pounds required for a 13.6-inch log of 0.51 specific gravity (ovendry weight and green volume) when cut with a knife having 45" sharpness angle. When sheared logs were viewed in radial section. each annual ring showed a check at the earlywood-latewood boundary. Checks were least severe in small logs sheared with the 22-1/2" knife, where they averaged 0.8 inch deep; they were most severe in large logs of low density sheared with the 45" knife, where they averaged 1.4 inches deep. Each sheared log generally also had one to several rather lengthy checks that formed just prior to emergence of the knife. Regression expressions were developed to predict force and work to shear as well as average and maximum check depth-all in terms of sharpness angle, wood specific gravity, and log diameter. THE WORK REPORTED HEllE is an extension of a genela.l study of orthogonal cutting of southern pine wood (12). When a knife is arranged to cut across the grain against an anvil or opposing knife, and the chip and the workpiece are of about equal thickness (visualize rose stems cut with pruning shears), the process is described as shearing (Fig. 1). Shears to fell trees or reduce long logs to pulpwood lengths are in common use. Published research is chiefly on species other than southern pine, but it defines principles that probably are widely applicable (1, 3, 4, 5, 6, 7, 8, 10, 11). In general, shear forces are less in warm than in frozen wood, less in clear than in knotty wood, less in heartwood than in sapwood, less in low-density than in dense wood, and less where the shearing direction is perpendicular rather than parallel to the annual rings. Above the fiber saturation point, moisture content a.pparently makes little difference in the force required to shear. The friction coefficient between a steel knife and green wood is approximately 0.2. Greasing the knife does not reduce shearing forces greatly; Teflon surfaces on the cutter are more effective. Axial loads (simulating the weight of a standing tree) do not appreciably increase shearing forces. Lateral vibration of the cutter reduces shear forces required, as does tapering the cutter plate (Fig. 2A) to give clearance between the plate and the wood. In a review of Russian work, Kubler (9) reported that vibration in the feed direction also reduces shear forces required. The author Is Chief Wood Scientist, Southern Forest Experiment Station, USDA Forest Service, Pineville, La. This paper was presented at Session 10 Mechanical Conversion of the 24th Annual Meeting of the Forest Products Research Society, June 30, 1970, in Miami Beach, Fla. It was received for publication in September 1970. FOREST PRODUCTS JOURNAL VoL 21, No.3 21 Table 1 RELATION BETWEEN STUMP DIAMETER AND LENGTH OF SHEAR-CAUSED SPLITS IN PLANED BOARDS. SpIlt yngth Stvmp Diameter Boards Measured Maximum Average Logs Cut -Inch..14 8.1 25 10.3 31 11.6 22 8.7 50 20.6 Inch.. 12 14 16 18 20 . . .Number6 36 12 70 8 44 2 21 2 21 For parallel-sided cutters (Fig. 2B), thin blades shear with less force than thick blades. Blades tapered so that the plate near the cutting edge is thin and the root thick (Fig. 2C) require forces intermediate to thin and thick plates without taper. Sharp blades shear with less force than dull blades. In comparison with a straight edge, an edge in the shape of an open V does not appreciably lessen forces. Shear force is least when the specimen is cut between opposing knives; if cutting is by a single knife against an anvil, a narrow anvil requites less force than a wide one. The quality of sheared ends is impaired-i.e., knifeinduced splits tend to be deep-if the wood is frozen, the knife dull, or the blade thick. Mdntosh and Kerbes (10) found that lumber losses from splitting were less than 1 percent when lodgepole pine (Pinll! con/oriA Dougl.) and white spruce (Picea gJalica var. gJallca) trees less than 14 inches in diameter were sheared at 4~.F. with a knife 1¥2 inches thick. In trials (unpublished) of industrial shears with two ¥2-inch thick blades (45. sharpness angle) converging at the center of the tree, lumber loss from splitting in southern pine butt logs was more severe than indicated by McIntosh and Kerbe!. In the test, 202 sheared butt logs scaling 7,280 board feet (Scribner standard rule) yielded 8,064 board feet of lumber when sawed. From production records, the mill estimated that, if the trees had been felled with a chainsaw, the butt logs would have yielded 8,820 board feet of lumber; i.e., the sheared logs produced 10.7 percent overrun as compared to the usual overrun of 20.1 percent. The same mill (located in South Carolina) observed what appeared to be even more severe splitting when 30 longleaf pines were sheared with a single blade closing against a fixed anvil. The sheared butt logs were conventionally converted to lumber, dried, and planedexcept that the butt ends of the boards were not trimmed. Length of shear-caused splits in the planed boards was positively correlated with stump diameter (Table 1). Mill mana~ent concluded that butt logs from sheared trees 16 inches and less in diameter would have to be cut back 24 inches before conversion to lumber; larger butt logs would require a 48-inch trim to eliminate most splits in the lumber. Such a trimming practice, although resulting in lumber loss, has some virtue; Hallock (2) has reported that the portion of loblolly pine trees immediately adjacent to the ground yields lumber that frequently warps excessively when dried. As the fundamentals of shearing appear well established, and since southern pine is seldom cut at temperatures below free%ing, the study here reported was confined to measurement of force and work to shear and of magnitude of knife-induced splits as affected by log diameter, wood specific gravity, and knife sharpness angle. Because of equipment limitations, cutting speed was 2 inches per minute. The literature indicates that cutting velocity apparently has little effect on shear. ing force, but there are no published comparisons of fast and very slow cutting speeds.