Farm Machinery and Equipment-II

LESSON 2. PRINCIPALS AND TYPE OF CUTTING MECHANISM

Generally manual harvesting involves slicing and tearing actions those results in plant structure failure due to compression, tension or shear. The serrated sickle combines a slicing and sawing action. It does not require repeated sharpening as in the case of smooth edge sickle. A single element impact cutting may be either moving or stationary type. An impact cutter has a single high speed cutting element and cuts mainly due to inertia. This cutting method is an economical method widely used in rotary lawn mowers, forage choppers and in some tractor mounted cutter bar. Usually a single element, sharp edged blade requires a velocity of about 10 m/s for impact cutting. A dull edged single element blade requires a velocity of about 45 m/s. In the rotary cutter the knives rotate in a horizontal plane as in the rotary mowers, whereas, in flail shredder the knives rotate in a vertical plane parallel to the direction of travel. In shearing type cutting, cutting takes place due to shear. A system of forces acts upon the material in such a manner as to cause it to fail in shear. Shear failure is invariably accompanied by some deformation in bending and compression, which increases the energy required for cutting. A common way of applying the cutting force is by means of two opposite shearing elements, which meet and pass each other with little or no clearance between them. Both or one of the elements may be moving with a linear uniform, reciprocating or rotary motion. This type of cutting mechanism is most widely used for harvesting agricultural crops. The reciprocating cutter bars that are commonly used for harvesting wheat or paddy crops use this principle. The inclined angle between the cutting edge is about 30 degrees. The serrated blades cannot easily slip between the two cutting edges. Reciprocating cutter bars do an excellent job of harvesting but are characterized by high-energy losses, short dynamic imbalance and limited operating speeds.

 Balancing of Reciprocating Masses

The cutter bar of a reaper is one such component and is major cause for vibrations of the reaper and a limiting factor for the speed of reaper. Mostly slider crank mechanism is employed for converting rotary motion in translating motion. Such components are seldom balanced in the farm machinery and are one of the major factors in the frequent breakdown of such machinery. The inertial force ‘F’ transmitted to the frame and the rotating un-balanced mass ‘m_rot’ are the main causes of vibrations. The periodic vertical reactions F_g may cause early fatigue failure of the knife back if the knife guides are loose. The rotary unbalanced mass can be balanced by providing a counter balancing mass on the flywheel but in opposite direction of crank pin so that

          R_b x m_b        = R x m_rot

      Where, m_b        = the counter balancing mass

               R_b           = distance of counter balancing mass from centre of fly wheel

               R            = distance of un-balancing mass from centre of fly wheel

The full dynamic balancing of reciprocating parts can be obtained by the addition of a second driven reciprocating mass that moves in direct opposition to first mass (cutter bar) and is as closely in line with it as feasible. So the balancing mass induces equal inertial forces but in the opposite direction that of the cutter bar. If the reciprocating counter weight is offset from the line of motion of the knife it will induce a cyclic couple.

The rotating counter weight commonly provided on the flywheel on a pitman type drive does not give complete balancing because its centrifugal force is constant where as the inertia force of the reciprocating parts is a function of the crank angle q. The usual practice is to provide sufficient mass opposite the crank pin to counter balance m_b and half of the reciprocating unbalanced mass m_rot. The vertical vibration component introduced by the counter-weight is then equal to the reduced maximum horizontal component.

Full dynamic balancing minimizes vibration at a given crank speed. It also permits the use of higher crank speeds and, hence, greater forward speeds. Counter balancing is not imperative if the weight of the machine or component supporting the cutter bar is large in comparison with the reciprocating force, as on a self-propelled windrower or combine. But counter balancing is needed much on a field mower because its frame is relatively light.

Balancing Machine Elements with Rotary & Translatory Motion

The balancing of members that do not move with plane rotation is much more difficult. A typical four-bar mechanism is the most common of all mechanisms (representing rocking mechanism or mechanism for transferring rotary motion to oscillating motion, oscillating sieves or oscillating blade). Crank 2 rotates with a constant angular velocity, rocker 4 oscillates, and coupler 3 moves with a combination of rotation and translation. The system can be balanced by introducing mechanisms that produce the opposite effects. A mechanism that is the mirror image of the original but moves in the opposite sense can be introduced to balance the vertical shaking forces and moments due to angular accelerations of links 3 and 4. The motion of the combination of linkages would give unbalanced horizontal shaking forces, which can be balanced by a mechanism that is a mirror image of this combination but moves in the opposite sense. This method of balancing is usually impractical.

Partial balance of the four-bar mechanism can easily be obtained. First, it is necessary to consider equivalent links. Two members are dynamically equivalent if they have the same total weight, the same center of gravity, and the same moment of inertia. The equivalent coupler replaces the original coupler. Link 2 and weight W₁ can be balanced by W₃ to bring the centre of gravity to centre of rotation. Similarly, link 4 and W₂ can be balanced by W₄. This will eliminate all vertical and horizontal shaking forces, but there will remain the unbalanced and variable torque on the frame owing to the angular acceleration of links 2.3 and 4.

The sieves of thresher sieve rocking mechanism and oscillating blades of oscillating implements can be partially balanced by this technique.

Shearing - type cutting mechanism

The cutting mechanism depending entirely on shearing utilizes the principle of scissor action which causes cutting by bringing two edges to bear on one plane across the material to be cut. As such there is negligible bending or crushing and only resistance to be overcome is the shearing strength of the material. The well-known mechanism in this category is the pitman and reciprocating knife-bar type mechanism. The invention of this mechanism can truly be regarded as a "mechanical break-through" for harvesting. The cutter bar has triangular knife sections riveted to a bar which reciprocates in slots of the stationary guards to cut the crop against the ledger plates of the guards. This type of cutting mechanism is used in the mowers, reapers, binders, forage harvesters and combines. The greatest efficiency in this mechanism is affected when there is minimum of crushing and bending of the material and when there is no bunching of material at the rear of the knife sections. Generally, a 3" (75 mm) stroke is used. However, double knife bars with 6" (150 mm) stroke with opposite motions are also used now. The reciprocating motion is imparted with the help of a crank wheel and a pitman.

Cutting Speeds required for reapers:

The speed requirement of the reciprocating knife-bar at peak cutting velocity seldom exceeds 100 m/min for the reapers, binders and combines. But when such a mechanism is used in a grass mower/forage harvester, the speed ranges from 120 to 170 m/min. The speed of the knives is essentially dictated by the "feed rate" of the material to be cut and its shearing resistance. The shearing resistance of the grain crop depends upon the moisture content of the stalks. As such the stage of maturity at the time of harvesting is an important parameter in determining the power requirements for cutting. For the grasses and fodder crops, the feed rate is usually higher than the grain crops. As such the shearing resistance is also considerably higher, depending upon the succulence and fiber-content of the stems and leaves. The reciprocating speed of the knife bar should be 20 to 25 higher than the forward speed of travel for efficient shearing of grain crops but about 40 to 45 higher than the forward speed for grass mowers to overcome the problem of bunching and over-feeding. In the reapers, binders and combines, it is not essential to use knife-clips to minimize clearance at shear and ensure maximum bearing of the shearing edges. In fact, in many grain harvesting machines now a days, there are not separate ledger plates, instead the case-hardened guard edges only serve as stationary shear plates. Even in such machines, the impact has little to do with cutting, since the cutter bar speed is too low. It is unwise to attempt to gain affect of impact-shear in cutter bar-type mechanism because 800 to 1600 strokes per minute with a back and forth stroke of 75 mm is far too short of the 4000 strokes which is the minimum requirement to effect the shear by impact alone. It is not possible to use higher speeds in the pitman and reciprocating cutter-bar type mechanisms since it is difficult to balance the unbalanced forces. A part of the unbalance is caused by uneven load and a part is the function of the constantly changing crank angle. If higher cutting bar speeds are to be used in reapers, binders and combines, it would lead to rapid wear and even mechanical failure of components.