Harvesting System Design and Performance - Engineering and Science of Biomass Feedstock Production and Provision - page 86

Fig. 5.2 Cropstemcuttingmode:unsupported,singleshear,anddoubleshear.Adaptedfrom[ 19 ]

peak cutting forces. Stems can be supported in three different ways while cutting:
upper shear, lower shear, and double shear (see Fig. 5.2 ). Impact cutting typically
requires more energy but does not require sharp blades or ideal crop conditions [ 17 ].
Forexample,shearcuttingofgrassstems(about2.5 mmdiameter)required30mJ
perstem,whereasimpactcuttingenergyrequired100-1,000 mJperstem[ 18 ].
Higher energy in impact cutting is attributed to increased blade-stem friction and
increased acceleration of the plant stem. High speed unsupported cutting may result
in greater plant compression and deformation leading to elevated power usage [ 15 ].

5.2.3.2 Cutting Energy

Cuttingstandingcropisthemostimportantfunctionaloperationperformedbya
harvesting machine. A cutting system should be able to maintain a uniform height
of cut, harvest lodged crop, leave minimum stubble, promote regrowth or emer-
gence of the subsequent crop, and consume minimum cutting energy. For a specific
crop, the cutting energy depends on stem diameter, cutting speed, blade type, blade
geometry, and height of cut. For example, Fig. 5.3a shows that energy required to
cut sugar cane stems was proportional to the stem diameter [ 20 ]. Figure 5.3b shows
that the cutting force for the flat blade was higher than the serrated blade for cutting
Miscanthus stems [ 9 ].

5.2.3.3 Critical Cutting Speed

Unsupported and partially supported cutting requires that the cutting force is sup-
ported by the plant's structural rigidity or inertia [ 15 ]. Hence, cutting can only occur
when the resistive forces of the plant exceed the required cutting force. Since cutting
forces generally decrease with decreasing cutting speed in grasslike stems, it is pos-
sible to define a critical cutting speed in which cutting forces exactly equal the reac-
tive forces of the plant. A clean cut requires the stem to be severed above the critical
speed. It also ensures significantly less stem deflection, which results into lower and