On the arc length from the cotton comb to the working point of the ginning rib, the saw teeth and the cotton rolls are in contact with each other. Due to changes in working conditions such as cotton properties, cotton lap density, gin productivity, saw blade drum speed, etc., the interaction between the saw teeth and cotton laps is very complex.
So far, no mathematical model has been established to describe the relationship between saw teeth and sub-rolls.
(1) The effect of sawtooth on cotton
Sawtooth ginning relies on the sawtooth to pierce the fiber layer of cotton, so that the fibers are embedded in the tooth recesses and then try to be pulled out of the cotton. Sawtooth gin must meet two conditions.
(1) The cotton roll rotates;
(2) The linear speed on the surface of the cotton roll lags far behind the linear speed on the surface of the saw blade drum.
On the arc section from the cotton comb to the working point of the ginning rib row, as the linear speed direction of the saw tooth surface and the linear speed direction of the sub-cotton roll surface change, the effect of the saw teeth on the sub-cotton roll and sub-cotton also changes. . The saw teeth have three functions on the cotton roll and cotton.
(1) The saw teeth hook the fibers on the cotton, causing the fibers to enter the saw tooth notches; (2) the saw teeth pull the cotton roll, causing the cotton roll to rotate; (3) the saw teeth pull the fibers off the cotton , to separate fibers from cotton seeds.
Research shows that: in D area, saw teeth mainly have the first effect on seed cotton, and the second and third effects are very little; in Ji area, saw teeth mainly have the third effect on seed cotton, and the second effect Secondly, there is almost no first effect; in area E, sawtooth has the second and third effects on cotton, and the first effect is smaller.
(2) Analysis of saw teeth causing fibers to enter the tooth recess
From Figure 8-19, we can see the movement direction of the surface of the sub-roll. In area D, the movement direction of the sub-roll is inconsistent with the movement direction of the sawtooth. Assumptions:
(1) The speed of the sub-roll in area D is vβ, and the speed direction forms an angle β with the radial line of the saw blade; (2) The sawtooth speed in area D is v.
Then, the angle between v and vβ is (90°-β). The relative speed W of the saw tooth and the cotton roll can be obtained according to the following vector formula:
In this way, once the single particle cotton in the sub-cotton roll comes into contact with the sawtooth, the fibers on the sub-cotton enter the sawtooth notch at a speed W. Now, it is easy to prove the first effect of saw teeth on cotton. In area D, the value of vβ is larger and the β angle is less than 90°. The value of v is a constant, so the direction of W must point to the sawtooth notch, that is, in area D, the sawtooth has a stronger effect on making fibers enter the tooth recess. The sawtooth has a strong ability to pick up cotton in this area, and the number of cotton picks up is large. In the E area, although the value of vβ is the largest, the β angle is approximately 90°, making the γ angle very small, or even zero, so the direction of W is difficult to point to the sawtooth notch. That is, in the E area, the sawtooth causes the fiber to enter the tooth recess. It has little effect, and the sawtooth’s hooking ability in this area is very weak. In the self-area, because the β angle is greater than 90°, the direction of W must deviate from the sawtooth notch, that is, in the self-area, the sawtooth cannot allow the fiber to enter the tooth recess.
In areas E and J, the β angle of the single particle cotton on the surface of the cotton roll is sometimes less than 90°, so that the first effect of the saw teeth on the cotton is compensated in areas E and J. Further research found that the above rules may not be accurate in very few cases, mainly due to changes in sub-lap density, operating conditions and other factors.
(3) The saw teeth separate the fibers from the cotton seeds
Most of the fiber bundles hooked by the saw teeth in the D area reach the working point, enter the gap between the ginned rib rows, and then separate from the cotton seeds.
The cotton that moves to the working point with the saw teeth immediately changes its direction and speed after encountering the embossed rib row, and moves upward along the surface of the rib row. The speed of the cotton roll is the slowest here, 1.1~1.5m/s, but the density is the highest at this time. At the working point of the embossing rib, the ability of the saw teeth to pull fibers off the cotton has a great relationship with the relative position between the embossing rib and the saw blade. The relative positional relationship between the embossed rib and the saw blade is generally expressed by the pressure angle. The pressure angle is the angle between the tangent line of the arc at the working point of the embossed rib and the tangent line of the saw blade circumference, represented by the letter α, as shown in Figure 8-20.
At the working point of the embossing rib, the moment the saw teeth pull the fibers off the cotton, the operating force P of the saw blade can be decomposed into two mutually perpendicular components R and T. The relationship between the three is as follows:
R=Psinα
T=Pcosα
Among them, the direction of P is consistent with the movement direction of the sawtooth at the working point; R is perpendicular to the embossed rib and is the direction of movement of the sawtooth at the working point. The force that pulls off the fiber on the cotton; T is parallel to the ginning rib, and is the force of the saw teeth at the working point to push the cotton lap to run. Under the action of the component force R perpendicular to the embossed ribs, a friction force F is generated between the cotton and the rib surface. The direction of F is opposite to the direction of T, and its value is:
The above calculation shows that: with the increase of pressure angle , the force of the saw teeth at the working point to pull fibers off the cotton gradually increases, but the force to promote the operation of the cotton roll gradually weakens; when the pressure angle increases to 74°, the operating force P of the saw teeth at the working point is almost all It is used to pull off the cotton fiber, and the pushing force on the cotton roll is almost zero. However, the sub-roller relies on the push of all the saw teeth extending into the working box to operate, so in this case, the sub-roller may not stop rotating.
Whether all the fibers in the sawtooth notch can enter the gap between the embossed rib rows depends to a large extent on the relative position between the sawtooth working edge and the embossed rib. If the tooth tip precedes the tooth root,When the embossed ribs enter the rib row gap, the effect of the embossed ribs on the fibers is to press them toward the tooth roots, and the fibers in the tooth recesses can smoothly enter the rib row gaps; conversely, if the tooth roots enter the rib row gaps before the tooth tips, the rolling The flower ribs push the fibers in the tooth recesses to the tooth tips, and the fibers in the tooth recesses cannot enter the rib row gaps. The relative positional relationship between the sawtooth working edge and the embossed rib is generally expressed by the advance angle. The advance angle is the angle between the sawtooth working edge located at the working point and the arc tangent of the embossed rib working point, represented by the letter λ, as shown in Figure 8-21. The advance angle refers to the angle at which the tooth tip enters the working point gap ahead of the tooth root.
That is, the tooth tip enters the embossed rib row gap 10° behind the tooth root, and most of the fibers in the sawtooth notch enter the embossed rib row When there is a gap, slide out of the tooth recess. Therefore, the advance angle cannot be less than or equal to zero. However, the advance angle should not be too large, otherwise, the fibers in the tooth recess will be embedded in the tooth root under the action of the embossed ribs, which will cause difficulty in brushing the rear box.
It can be seen that the pressure angle is a very important working parameter. When determining the pressure angle parameters, factors such as cotton grade and sawtooth sharpness should be comprehensively considered. When processing high-grade cotton, when the saw teeth are blunt, the pressure angle should be set to a large value to ensure the quality of lint; when processing low-grade cotton, when the saw teeth are sharp, the pressure angle should be set to a small value to increase ginning output. The pressure angle ranges from 55° to 65°.
(4) Other factors affecting the sawtooth hooking (pulling off) cotton fibers
The ability of the sawtooth to hook and pull off cotton fibers directly affects ginning productivity. First of all, the maturity and moisture content of the cotton have a great impact on the hooking ability of the saw teeth. Seed cotton with good maturity and low moisture content has a fluffy surface and greater elasticity, which is conducive to sawtooth puncture and has a high hooking ability. Seed cotton with poor maturity and high moisture content contains more stiff cotton. Its surface is not fluffy and has low elasticity, which is not conducive to sawtooth puncture and the hooking ability of the sawtooth is reduced. Therefore, when processing low-grade cotton with high moisture content, the hooking efficiency of the saw teeth is low, and the ginning yield is significantly reduced. From a process perspective, in addition to factors such as the pressure angle, the shape of the work box, and the linear speed of the sawtooth surface that affect the sawtooth hooking of fibers, there are also the following factors.
1. Density of the sub-cotton roll
If the sub-cotton roll is appropriately tight, it can increase the pressure on the saw teeth, force the fibers into the tooth recesses, and create sufficient friction between the fibers in the saw tooth recesses and the saw teeth. The strength makes it easier for the saw teeth to pull out fibers from cotton. On the contrary, the cotton roll is loose, the cohesion force between the cotton roll hooked by the saw teeth and the cotton roll is small, the relative speed between the saw teeth and the hooked cotton roll is small, and it is difficult for the saw teeth to pull off the fibers on it.
However, the cotton roll is too tight, or the fibers in the tooth recess are rubbed by the compressed cotton on both sides and leave the tooth recess, forming free fibers in the cotton roll; or the fibers in the tooth recess are tightened. The sawtooth roots make it difficult to brush the cotton; it may even cause the cotton roll to stop and prevent production.
2. Saw blade tooth shape parameters
The value of the tooth inclination angle in the saw blade tooth shape parameters directly affects the ability of the saw teeth to pull fibers. It can be seen from λ=α+β-90° that the value of the tooth inclination angle also affects the advance angle, thereby affecting the fiber entering the gap between the embossed rib rows. The tooth inclination angle is large, the inclination of the working side of the saw tooth is large, the force of the saw tooth piercing the cotton fibers is strong, and the hooked fibers are not easy to slip out of the tooth concave; the tooth inclination angle is small, the inclination of the working side of the saw tooth is small, that is, the saw teeth become upright, and the saw teeth The effect of puncturing the cotton fibers is weak, and when the pressure angle is constant, the advance angle decreases. However, if the tooth inclination angle is too large, it will also cause difficulty in brushing cotton. Therefore, the choice of tooth inclination angle is very important. The tooth inclination angle of the embossing saw blade is specified as 40°.
The pitch of the saw teeth is large, and the volume of the saw tooth recess is also large. There are more fibers that can be filled in the tooth recess, so the saw teeth draw more fibers in the D area. However, if the tooth pitch is too large, the cotton seeds will be pinched and even the cotton seeds will be embedded in the tooth recesses, making it difficult for the saw teeth to hook the fibers. Therefore, the maximum size of the tooth pitch cannot exceed the waist diameter of the cotton.
The tooth pitch of embossing saw blades is 3.3mm and 4.5mm. The former is used on embossing saw blades with a diameter of 320mm, and the latter is used on embossing saw blades with a diameter of 406mm.
In addition to tooth inclination and tooth pitch, saw blade tooth shape parameters also include tooth height, tooth root angle (or tooth tip angle), etc. When the diameter of the saw blade is determined, these parameters are mutually restricted. Generally speaking, when selecting these parameters, the mechanical requirements such as strength, stiffness, and stability of the sawtooth itself should be met, and the requirements for embossing quality should also be considered.
3. Sawtooth status
Sawtooth status refers to the smoothness, sharpness and defectiveness of the sawtooth. The state of the saw teeth has a great influence on the ability of the saw teeth to hook and pull off the cotton fibers. The saw teeth are sharp and the surface smoothness is high. The saw teeth have a strong puncture effect on the cotton fibers, making it easy to brush the cotton. The saw blade has few missing teeth, which ensures the pulling of fibers from the number of teeth. Therefore, the smoother the saw teeth, the better, and the fewer defects, the better.
There is a limit to the sharpness of the saw teeth, because the saw teeth are too sharp and can easily pierce the cotton seeds, increasing the fibrous lint and lint, and the edges of the saw teeth can easily chew off the short lint on the cotton seeds. Generally, for cotton with high processing grade, good maturity and low moisture content, the saw teeth should not be sharp. The solution to overly sharp saw teeth is to sand the saw blade drum in a sandbox. The newly punched saw blade has many edges and corners on the tooth surface, has cracks, and has a very poor finish. After being installed into a saw barrel, it needs to be sanded before it can be used for processing. Sanding the saw barrel can not only improve the smoothness of the saw teeth, but also solve the problem of the saw teeth being too sharp. Therefore, sanding the saw barrel is an indispensable part of the saw barrel installation and maintenance process.
The technological requirements for the state of the saw teeth are that they are sharp and smooth, without crooked or inverted teeth, and the number of missing teeth on each saw blade does not exceed the specified value.
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The technological requirements for the state of the saw teeth are that they are sharp and smooth, without crooked or inverted teeth, and the number of missing teeth on each saw blade does not exceed the specified value.
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