Shearing Process and Shears
Shearing Process and Shears
Shearing is a simple process used for the cutting of metals. It is basically a metal fabrication process and is being used in rolling mills at several places for hot as well as for cold cutting of the steel material. Shears can be used to cut steel and other materials of any size or shape. In the shearing process, metal is separated by applying a great force enough to cause the material to fail. The most common shearing processes (such as shearing, punching, piercing, slitting, and blanking etc.) are performed by applying a shearing force. When a very large shearing force is applied, the shear stress in the material exceeds the ultimate shear strength and the material fails and separates at the cut location.
The shearing force is applied by two blades, one above and the other below the material (upper blade and lower blade). These blades are positioned at an angle relative to each other. These two blades are forced past each other with the space between them determined by a required offset. Normally, one of the blades remains stationary. The shearing blades used in the shearing process typically have a square edge rather than a knife-edge and are available in different materials, such as high carbon steel, low alloy steel, and tool steel etc.
During the shearing process, shear stress is applied along the thickness of the material being cut. Shearing happens by severe plastic deformation locally followed by fracture which propagates deeper into the thickness of the material. Since shearing of a material involves plastic deformation due to shear stress, the force required for shearing is theoretically equal to the shear strength of the material. Due to friction between shear blade and the material, the actual force required is always greater than the shear strength of the material.
A small clearance is present between the edges of the upper and lower blades, which facilitates the fracture of the material. Blade clearance is the distance between the upper and lower blade of the shear as they pass each other during the shearing process. The clearance between the two blades is an important parameter which decides the shape of the sheared edge. Large clearance leads to rounded edge. The edge has distortion and has burr. The shearing load is also higher for larger clearance. Insufficient clearance leaves sheared pieces with a double cut. Also, a ductile material has burr of larger height. For harder materials and higher thickness, larger clearances are required. Generally, clearance can vary between 2 % and 10 % of the material thickness. The size of the clearance depends upon several factors, such as the specific shearing process, the material, and the material thickness. An optimum blade setting allows the material to fracture cleanly. Most shears are normally equipped with either a manual or powered blade clearance system, however in some cases, there can be an awkward method to set or to have a limited amount of adjustment.
Usually shearing begins with formation of cracks on both sides of the material being sheared, which propagates with application of shear force. A shiny, burnished surface forms at the sheared edge due to rubbing of the material along the shear edge with the blades. Shear zone width depends on the speed of shear blade motion. Larger speed leads to narrow shear zone, with smooth shear surface and vice-versa.
The quality of the cut during the shearing process is directly proportional to the sharpness of the shear blades. Dull blades leave ragged edges. The rake angle of the blade (the angle of the moving blade as it passes the fixed blade) is also important in determining the quality of the cut. Generally, the lower the rake angle, the better is the quality of the cut. Problems with cut quality, such as bow, twist, and camber are seen on shorter pieces (upto 100 mm long) which fall behind the shear after they are cut. Shears with lower rake angles require more power than those which have a higher rake angle.
Geometry of the shearing zone
The effects of shearing on the material change as the cut progresses and are visible on the edge of the sheared material. When the blade impacts the material, the clearance between the blades allows the material to plastically deform and a small projection is formed. This projection is known as rollover. This region corresponds to the small depression made by the shear blade on the material. Below this, the burnished surface is present. The burnished surface is a smooth surface formed by the rubbing of the shear surface against shear blade. The burnished surface is located below the rollover. The burnished region is usually located on the upper side. Below the burnished zone, the fracture zone is located. The burr is formed below the fracture zone. Burr is a sharp edge formed at the end of the process due to the elongation of the material before completely getting severed off. The depth of the deformation zone depends on the ductility of the metal. If ductility is small, the depth of this zone is small. The depth of penetration of the shear blade into the material is the sum of the rollover height and burnishing zone height. The depth of rough zone increases with increase in ductility, material thickness or clearance. There is severe shear deformation in the fracture zone. The stages in the process of shearing are shown in Fig 1.
Fig 1 Stages in the process of shearing
Shears and their types
Shears are used to cut thin sheet, plates, billets, rounds, squares, sections, beams, and bars etc. in the rolling mills. Depending on the application, shears typically employ a fixed lower blade and a moving upper blade to perform the cutting action. The type of shears to be used is determined by many factors, including the material length that it can process and the thickness and type of material which it has to cut.
Shears which are used in rolling mills before the cooling bed are called hot shears while the shears which are used after the cooling bed are cold shears. Hot shears cut desired length and as well as cut front and tail ends of the rolling stock. These shears also cut the bar being rolled in case of cobble in the rolling mill. Hot shears are designed to cut the rolling stock at the rolling temperature. Cold shears are used to cut the rolled product in to desired saleable lengths.
The type of shears normally used in rolling mills are (i) crop and cobble shear, (ii) cooling bed dividing shear, and (iii) rotary shear.
The crop and cobble shear is used in hot rolling mills is to crop front end, tail end, as well as to segment cutting in case of eventualities. These shears are usually of start/stop type and are driven either with flywheel mounted pneumatic clutch/brake or with direct DC (direct current) motor driven. These shears are controlled through PLC (programmable logic control) system and provide very close tolerance of the cut length.
The cooling bed dividing shear is used to cut cooling bed lengths. This shear is usually designed for low surface temperatures. This shear is generally installed before entry to cooling bed. The cooling bed shear is generally stop/start and continuous operating type and is driven by the direct DC motor drive. This shear is also normally controlled through PLC system and hence very close tolerance of cut length is achieved.
The rotary shear is a cost effective shear which is used to cut crop front end, tail end and as well as to scrap the material under rolling during emergency. This shear is of continuous rotating type. Generally this shear is used to trim material in the hot rolling mill at considerably lower speed.
Hot shears are usually flying shears. Flying shears are those shears which cur the material while it is moving in the rolling mill at the rolling speed. Flying shears are used for cutting applications, where endless material to be cut to length, cannot be stopped during the cutting process and the cut is required to be effected ‘on the fly’. The mechanical construction provides a shear system mounted on a carriage, which follows the material with synchronous speed while cutting is in progress, and then returns to a home position to wait for the next cut. The flying shear control is based on a PLC system. The system is generally designed for the special requirements of flying shears under consideration of maximum efficiency and accuracy at minimum stress for all mechanical parts.
Shears can also be categorized into different types by shear design and the drive systems which are used in the design. Two design types are common to power squaring shears. They are (i) the guillotine shear (also known as the slider unit), and (ii) the swing beam shear.
A guillotine shear has a moving blade which runs on straight slides. The moving blade is almost parallel to the fixed blade during the entire stroke. The guillotine design (see Fig 2) uses a drive system to power the moving blade down. The guillotine shear requires a gibbing system to keep the blade beams in the proper position as they pass each other.
The shear with a swing beam design uses one of the drive systems to pivot the moving blade down on roller bearings. This eliminates the need for a gibbing system to keep the blades in proper position as they pass each other.
Fig 2 Types of shears
The drive system of the shears powers the moving blade through the material to make a cut. Drive systems can be categorized into five basic types namely (i) foot or manual, (ii) air, (iii) mechanical, (v) hydro-mechanical, and (v) hydraulic. In rolling mills normally last three types of shear drives are used.
A foot shear is engaged when the operator steps on a treadle to power the blade beam to move down to make a cut. Foot shears are generally used in sheet metal applications. For using an air shear, an operator steps on a pedal which activates air cylinders to make a cut. Shop air system or a freestanding air compressor is used to power an air shear. Air shears have a simple drive design, and they provide overload protection. The direct-drive mechanical shear operates when the operator steps on a pedal to turn on the motor that brings the beam down to make a cut. The motor turns off at the end of the cycle, and the blade beam returns to the top of the stroke. This design is suitable for shears when they are not in constant use because the machine uses power only when it is activated. In case of the flywheel type mechanical shear, the operator steps on a pedal to activate a clutch which engages the flywheel to generate the power to move the blade beam down. Mechanical shears are fast and have a better design for cutting certain types of material. The hydro-mechanical shear has a hydraulic cylinder or cylinders which power a mechanical device such as an arm to move the blade beam down to make a cut. In this type of shear, a smaller hydraulic system can be used since the mechanical device produces the power. Hydraulic shear work with only the hydraulic power and is powered when the operator steps on a pedal to activate the hydraulic cylinders to power the blade beam.
There are a number of shear configurations which have been developed over the years, and which have different potentials for improvement via revamps. These are described below.
Clutch and brake shears – These are of an older design, but can benefit from new automation, though accuracy and repeatability are limited by clutch and brake system performance. The main advantage of this type of the shear is the possibility of fine tuning clutch and brake timing to optimize accuracy and friction material life. Also, a new control system can improve cutting repeatability by minimizing the electrical error.
Start/stop shears – These shears are very similar to clutch and brake shears, but in this case the motor and the shear gear box are permanently connected. This kind of shear needs very accurate blade position control to assure high precision and reliability. In the present applications of these shears, it is usually not necessary to replace the entire system, merely to apply a new motion control system to the existing drive.
Rotating shears – These shears has the leading edge technology and are used when high speed and accuracy are required. This is achieved by an optimized combination of motion control strategies aiming to get the best performance with the minimum effort from the machine. Fast dynamic motion applied to rotating blades and the diverter are necessary to deliver highly versatile and accurate rotating shears, capable of doing head and tail crops, scrapping and cut-to-measure at a speed of upto 100 meters per second. A peculiarity of rotating shears is the synergy between a high inertia system (the shear blades) and a low inertia system (the diverter). The big challenge for upgrades is to use the same motion control system for both parts, optimizing it for the two different tasks.
The common types of shears normally installed in rolling mills are as below.
Snap shears – These shears are normally arranged at the entry side of rolling mill stand 1. They are used for dividing the hot input material conveyed to the rolling mill.
Pendulum shears – These shears consist of cutting systems suspended in an ‘oscillating’ configuration. The cut can be performed for material which is travelling or stopped. These shears are used for cropping head end or tail end or dividing the hot input material conveyed to the rolling mill.
Universal shears – These shears are usually designed for higher product speeds and normally used for head end and tail end cropping as well as for cobble cutting. In these shears the cutting is initiated by automatic pulse. Universal shears are generally of continuous running type.
Dual system shears – These shears are normally used as cooling bed shears. They are equipped with two cutting systems namely (i) crank rotary system, and (ii) crank lever system. The crank lever system is mainly used for cutting sections. The shear is moveable perpendicular to the rolling direction in order to bring the system being used in line of rolling.
Crank shears – These shears can be designed as (i) continuously running shears, (ii) start-stop shears, and (iii) coupling shears with coupling- brake combination. These shears are used for cropping, dividing, or emergency cutting during cobble. Shear start is initiated by pulse generator. Crank shears can be crank lever shear or double crank shears.
Cold shears – These shears are for cutting of cooling bed lengths into saleable lengths. They are installed downstream of the cooling bed exit roller table after the straightener. Cold shears can also be flying shears.
The shears need optimized motion control. These include dedicated motion planning algorithms, drive parameter optimization and fine tuning parameter recording. The shear control system block diagram is shown in Fig 3. All the control technologies are flexible and support different set-ups which are required for different products and different mechanical arrangements, such as shears with a combination of flying and crank arms and optional flywheel.
The start-stop shear cycle can be summarized as (i) acceleration during which the motor is starting from the home position, and accelerate to the speed needed to perform the cut (synchronization speed), (ii) synchronization during which the motor remains at constant speed from the moment the blades impact on the bar until they exit from the synch angle, (iii) deceleration during which period the motor decelerates from the synchronization speed to zero speed, and (iv) repositioning during this phase starting from the stop position the motor is moved to the initial home position ready for the next cut.
The cut cycle is normally performed using an electronic cam. This function controls the position of the slave axis (shear blades) according to the position of the master axis (material position).
Fig 3 Typical start-stop shear configuration and shear control system block diagram
Optimized parameters for different productions are easily selected by an integrated recipe system, and combined with automatically computed motion paths to bring several advantages such as (i) reduced mechanical stress and wear, (ii) reduced operating noise, (iii) reduced electrical stress on both drive and motor, (iv) reduced energy requirements, and (v) cost-effective selection of motors and drives. The main components of the system are given below.
Axis control – It is the heart of the control system and controls the position of the shear blades to assure precision and repeatability of the cut length. For performing this function, it receives, as inputs, the encoder of the stand, the encoder of the shear, the hot metal detector (HMD) and the proximity switch and generates as output the speed or torque request for the shear drive.
Master encoder – It is the incremental encoder connected to the stand motor and is used to detect the material position.
Shear encoder – It is the incremental encoder connected to the shear motor and is used to detect the shear blade position.
Hot metal detector – It is the sensor which is necessary to determine the head and the tail of the bar for the bar position tracking.
Shear proximity switch – It is the sensor which is used to reset the shear position at the moment of the cut.