Aspects Regarding the Physical Parameters and Wear in the Work Process of the Disc Openers for Seeding Machines

Abstract

This paper primarily presents statistics on the variation of physical characteristics (dimensions and mass) for the coulter discs of double-disc seeders. This statistic was calculated based on actual measurements of the probability density and cumulative probability for the mass of the discs, their average thickness, and outer diameter. These parameters (m = 4000 g, g = 4 mm, D = 380 mm) are tracked from the design phase to the actual realization phase, being the parameters specified in the purchase phase by users from agricultural machinery distributors. The standard deviation and deviation from the mean for a cumulative normal (Gaussian) distribution were calculated, and an analysis was made for the decrease of the mentioned parameters in discs already used for sowing cereals on an area of 80 hectares, due to the wear that appeared as a trace of the contact with the soil and the abrasive wear caused by it. Testing of disk parameters using both logarithmic and Gaussian distributions was performed, and test results are presented through appropriate graphs. With small deviations (which appeared for known or unknown reasons), it can be stated that the distribution of the mentioned parameters is generally a normal-type distribution. If the new discs are 15″ in diameter, experts recommend that they be replaced when they reach a diameter of less than 14.5 inches. This study can provide a reference for improving the physical characteristics of opener discs of seed drills in no-till conditions and beyond.

1. Introduction

Maintaining the geometry of the working bodies of agricultural tillage machines is an objective of builders and agricultural workers to ensure quality work from an agrotechnical point of view.

The active organs of agricultural tillage machines (such as those for seeding and planting) are, during work, in direct contact with the soil, which leads to their wear as a result of friction and its abrasive action [1].

Between the mass of the soil and a foreign object with which it comes into contact, for example, the coulter of a seeding machine, forces of attraction (adhesion) and, external friction are developing. They also depend on the nature of the working body, but they are of particular interest to the soil factors that influence the external friction, which in turn depends on the relative speed between the elements and the applied load.

Soil type and working depth have a direct effect on the soil disturbance caused by furrow openers, including structural parameters, cutting edge thickness, blade curve, penetration angle, and rake angle [2,3].

To estimate the propagation of stresses in the agricultural soil (when the wheels of the agricultural machinery pass), finite element modelling was used, among other methods. This numerical method allows obtaining approximate solutions, with the agricultural soil being idealized as an elasto-plastic material after the Drucker-Prager criterion [3].

In addition to computer modeling and simulation of the working process of agricultural machines, there is also a need for tests in real operating conditions, in the laboratory or especially in the field. Laboratory testing, however, requires a sufficiently large number of work cycles, with appropriate test programs, so that a comparison can be made with the actual process carried out in the field, both for the whole machine and for the main working organs [4,5].

For each agricultural machine, the specific resistance to contact with the soil (e.g., plowing resistance or cutting resistance) is thus determined, so that the working parts are constructed from suitable materials (usually steel with a high content of manganese—to increase wear resistance, and newer boron-alloyed steels) [6,7], also having an appropriate specific form in correlation with the basic operation it executes.

In addition, for working bodies that work in the soil and are affected by wear, coating treatments with Eutalloy 10,112 (CrNi-based alloy powder with a content of 60% hard particles–Diamax) can be applied to increase the resistance to abrasion and corrosion by either laser melting or flame spraying, with laser surface modification likely to be more effective than flame spraying [8]. Obviously, coatings can also be made with other types of materials, the final goal being the same—increasing the wear resistance of agricultural tillage tools [9].

Steels such as AISI 1010 and AISI C1064 can also be used for the working parts of agricultural machines that perform tillage or trenching for seeding, but researchers have determined that tool wear in AISI 1010 is over 50% higher than in those made of AISI C1064, regardless of operating conditions and soil characteristics [10].

In theoretical analyses, the soil is generally modeled as a viscous elastic medium and the working body of the machine as an absolutely solid/rigid body. The results obtained depend on the geometric parameters of the working body, its movement mode and the mechanical properties of the soil [11].

Thus, seeding machines have the coulter as the main working organ that comes into contact with the soil to open the furrows in order to introduce the seeds into the soil. In row or nest seeders, the coulters can be anchor type, culture type or with disc (single or double). In the case of the coulter with a single disc, it consists of a concave disc, arranged at an angle of 5–8° to the direction of advance, or of a flat disc. The two-disc coulter consists of two flat discs freely mounted on a common axis, inclined to each other at an angle of 9–12°, having a point of tangency in the front part about 7 cm from the bottom of the furrow (Figure 1).

From the variety of coulters, the most used are the double-disc coulters, because they determine lower traction resistance and better quality indicators of the sowing process, but they must be additionally equipped with a seed guidance system and a speed damper, for a uniform distribution along the length of the open furrow and at the seeding depth [12,13].

Equipping seeders (grass plants or creeping plants) with double-disc coulters gives better precision and a constant seed depth. These double-disc coulters do not collect plant debris, helping to break it down. It should be noted that the diameter of the disc coulter is interdependent with the depth of disc penetration into the soil, with the depth of the plant residues layer, and with the compression angle. Also, notched blade disc coulters cut more straw compared to continuous blade disc coulters, being interdependent, however, with straw moisture, which lowers the percentage of straw cut [14].

Experimental results show that disc diameter, working depth, and travel speed have a significant effect on soil cutting resistance, especially in straw-covered (post-harvest) soil conditions, for double-disc openers. The coulter with 450 mm diameter discs has higher straw cutting efficiency compared to 600 mm diameter discs, and 330 mm discs have lower straw cutting efficiency. Therefore, the decrease in diameter of the discs (through wear and tear due to the work process) could decrease the efficiency of the furrow openers and therefore of the whole sowing machine.

The authors of the paper [15] also show that coulters with 450 mm diameter discs offer a straw cutting efficiency of 88.6% at greater working depths (90 mm).

In the case of creeping plants, the double-disc coulters also ensure a good quality of incorporating the seeds into the soil, so they can also be used in the case of a semi-prepared germination bed or with a lot of plant debris [16].

Figure 1. Construction of the coulter with one disc (a) and two discs (b): 1. discs; 2. seed hopper; 3. fixing arm; (c) two discs’ coulter working process [17].

Figure 1. Construction of the coulter with one disc (a) and two discs (b): 1. discs; 2. seed hopper; 3. fixing arm; (c) two discs’ coulter working process [17].

Resistance to advancement increases with increasing working speed and can be affected by coulter pressure, working speed, coulter type (continuous, notched, or crenellated), or even coulter slippage, all of which have a negative effect on the cutting speed of the soil. That is why disc coulters can easily cut above-ground plant debris through their rotational movement, but also through the appropriate choice of placement angles. An understanding of the operation, travel speed, and working strength of coulters is necessary for the design and use of coulters [18].

For good penetration into the soil and the opening of a groove corresponding to the deposit of the seeds through the feeding hopper 2 (Figure 1) reached here through the guiding tubes (flexible, with overlapping hoppers or telescopic) from the distribution devices of the seeding machine, the discs of the coulter have an inclination to both the forward direction (angle of attack) and the vertical position (opening angle).

When the coulters penetrate the compacted soil, it causes a decrease in their life, especially if the attack angle and the opening angle are not chosen properly. For double-disc coulters, working speed has a significant effect on the vertical force component at larger angular disc openings. In addition, it is necessary to increase the pressure on the double disc to maintain the seeding depth [19].

Disc furrow openers generally have less depth variation and soil disturbance compared to other types of coulters, although when seeding with no tillage in stubble conditions, they do have a problem when plant residues remain on the soil surface [20].

The appropriate establishment of the working angles for the coulter discs and their diameter, depending on the degree of coverage with plant residues of the unprocessed soil, is essential for the optimization of the sowing conditions and for the conservation of the soil, but also for productions with high yield [21,22].

There is a wide variety of seeders that include disc coulters, especially for spring crops with a large row spacing (maize, sunflower), with smaller or larger disc diameters, smooth or notched, respectively, with working angles correlated with the type of crop and the sowing depth.

For example, the Gaspardo seeder, model Pinocchio 250/5 is a seeder where the coulter discs are equal and have a diameter of 420 mm and a thickness of 4 mm, which creates a very narrow “V” furrow. In the sections of the seeding machine SPM Mecanica Ceahlau, Romania, with a double-disc coulter, they have a diameter of 370 mm. Coulters with large-diameter discs are said to have smoother operation and can easily cut plant debris in conditions of less tilled land before seeding [23].

So, the diameters of grain seeding discs have values in the range of 340−420 mm, and their thickness is 3.5−4.5 mm.

In general, discs are made of steel with a hardness between 48−60 HRC, lower values at the center of the disc (near the hub) and higher values on the outside (according to John Deere specifications), to ensure flexibility, but also a corresponding wear resistance, 30–35% higher in no-till conditions [24,25].

The discs are made of boron-alloyed steel sheet so that during the heat treatment, variable hardnesses can be obtained in different areas of the disc. Thus, during the process, higher heat is applied to the outer edge of the disc to cause structural changes in the carbon structure of the steel. The hardest area is therefore in the outer section, rated at 55–58 HRC, to provide longer life in this area of the disc.

Naturally, the middle area of the disc also heats up during the heat treatment, but the temperature reached is lower. This area is highlighted in Figure 2 by the orange color and is harder than untreated steel, but still retains a degree of flexibility (50–55 HRC).

The inner area closest to the center of the disc (of the hub), highlighted in blue (or green), does not reach high enough temperatures during treatment, so there are no structural changes to the carbon, and this area remains in its natural state (about 49–50 HRC), ensuring flexibility. This is important because while the cutting edge is exposed to wear due to contact with the ground, the center of the disc is exposed to higher stresses due to the weight of the machine and the strength of the ground, making the area closest to the hub more prone to breakage. Thus, maintaining flexibility in the central area of the disc reduces this risk. Thus blue gives flexibility, yellow gives durability, and red hardness in contact with abrasive and hard soil particles.

From our measurements, carried out at the Metals Study Laboratory of the National University of Science and Technology POLITEHNICA Bucharest, the hardness of the new discs fell between 458–492 HV, for the new discs, with higher values on the outside of the discs, which is equivalent to values of 45–47 HRC, which means a few units less than the literature recommendations mentioned previously.

For discs already used in the seeding process, but with a serrated edge, the hardness of the discs fell within wider limits, from 430 HV to 504 HV (equivalent to 43–48 HRC), but there was not necessarily an increasing or decreasing distribution in radial direction. It should be noted that the determinations were made on two new discs (smooth) and on four used discs (notched), in five points each and in four radial directions at 90°.

Our paper first presents in-depth statistics on the variation of disc sizes of double-disc coulters for a large number of discs, calculating on the basis of the measurements the probability density and the cumulative probability for the mass of the discs, their average thickness, and the outer diameter, parameters whose values are specified when selling/purchasing such coulters by the distributor. The standard deviation and deviation from the mean are calculated for a cumulative normal (Gaussian) distribution, as is normal for a large number of objects (discs). In addition, an analysis is made for reduction of the mentioned parameters in the discs used, due to normal wear as a result of contact with the soil and the abrasive wear caused by it.

Considering that a single disc costs around 80–85 Euro, a seeding machine with only six sections having 12 discs at the furrow openers (the Vaderstad TPT-6 seeder costs around 80,000 Euro including VAT), the change interval being at about two years, it is normal to be interested in the evolution of the wear of these discs. The purpose of the paper is to present the importance of maintaining the agricultural machinery parts at appropriate dimensions (in our case, the coulter discs) for an efficient work process, so that specialists and workers act on their change at the right time.

2. Materials and Methods

The coulter discs of seeders are made by stamping (cutting) from boron (and/or manganese) alloyed steel sheet, with a hardness of 49–50 HRC, to give strength in the process of cutting the soil, following more other processing operations and heat treatment to make them flexible and durable during the seeding process.

The production of the working bodies of agricultural machines, like other machine bodies, is carried out within a range of dimensions and tolerances. In engineering, the concepts of probability theory are best illustrated by experimental determinations, by numerical examples, random events, and sample space being basic terms of this theory [25].

Tests carried out on seeders equipped with a disc coulter showed that the optimum diameter of the coulter disc could be 0.40−0.43 m, taking into account the standard deviation of the seed distribution at the optimal sowing depth, when sowing spring wheat seeds [26].

Probabilistic study allows moving from a sample to a population, with Binomial, Poisson, and Normal probability distributions being the most used in the analysis [27,28].

Additionally, there are works in the specialized literature that appreciate that the deformations and wear of the component parts of some agricultural machines can follow a logarithmic distribution [29,30], as we also try to present in our work. In addition, the Gaussian distribution relationship is presented in this paper in the Results chapter.

The structure of a seeder section is shown in Figure 3a [https://amazone.co.uk/, accessed on 17 May 2024]. In the Figure 3b,c is presented soil movement and furrow opening mechanism by double disc opener, to a greater or lesser opening of the discs, as shown by the white arrows, reproduce after [16].

I had more than 100 discs at my disposal, but the experimental determinations from this paper were carried out on 20 new discs of the cereal seeders, in order to establish the variation of the outside diameter of the discs, their mass, and the distribution of the sheet thickness for the discs used in these seed drills, which work in different agricultural farms.

The 20 discs were weighed, with a precision of 0.01 g, their outer diameter was measured in two perpendicular directions with a precision of 0.01 mm, and the thickness of the disc plate was determined in four radial directions, at distances of 20 mm between measurement points, as stated previously, with an accuracy of 0.01 mm (resulting in five diameters for measurements—280, 300, 320, 340, 360 mm, for smooth discs with an outer diameter of ϕ380 mm), (a total of 23 determinations for each disc). The determined/measured values are centralized in The determined/measured values are centralized in Table 1, presented in the Section 3.1.

Next, it was analyzed whether the studied parameter is a continuous random variable by determining the probability density on the range of values of the respective parameter, for a normal distribution. It was found that the probability density, calculated with the relation:

$$f\left(x\right)=\frac{1}{\sigma \sqrt{2\pi }}exp\left(-\frac{{\left(x-x_c\right)}^2}{2{\sigma }^2}\right)$$

(1)

was always greater than zero, the area under the density curve being equal to one (where σ is the standard deviation, and x_c—the arithmetic mean value of the population of values taken into analysis).

In analysis, the cumulative probability distribution is the element that an analyst needs to measure the probability that the variable x (the parameter) falls within a range of values of the admissible region [28]. Thus, the cumulative probability represents the probability that a value of the parameter is in a certain domain, the probability that the random variable is less than or equal to a given value, and, at the same time, the probability of the occurrence of several independent events in the same type of experiment. To calculate the cumulative probability, the probability of occurrence of each individual event is accumulated. Therefore, the maximum value of the cumulative probability is always less than 1, as also shown in the charts from the Section 3 and Section 4, in which both the probability density curves and the cumulative probability are shown for the disc thickness values for the five diameters mentioned before.

For the standard normal distribution (expressed by Relation (1)), the cumulative probability has the general expression:

$$F\left(k\right)=P\left(x\le k\right)={\int }_{-\mathrm{\infty }}^{k}f\left(x\right)dx$$

(2)

where k is a particular value of x.

Determinations were also made on the coulter discs of a Tempo TPT 6 seeder, series 1912, which has already worked 80 ha, at an average speed of 6.4 km/h, the results being presented in Table 2, presented in the Section 3.2.

In addition, for each of the two sets of coulter discs (smooth), with a diameter of ϕ380 mm (new or used), the average values of the mentioned parameters, the standard deviation, as well as the range of deviations from the average values were determined, by establishing the minimum and maximum value of the parameter deviations. It should be noted that in the two tables presented, each value of the average thickness of the disc is the average of the values obtained in the four radial directions previously mentioned.

For the discs already used (on the Tempo TPT 6 Vaderstad seeder), the thickness of the disc on its outside (at the edge) was also determined, the values presented in Table 2, from the Section 3.2, being also the average of the four determinations in four radial directions at 90 degrees.

3. Results

3.1. Results Regarding Measurements of Parameters of New Discs

From the statistical analysis performed on new discs of double-disc coulters of Vaderstad seeder, based on the values in Table 1, were drawn the probability distribution and cumulative probability charts, shown in Figure 4.

The statistical analysis carried out graphically presents the histograms of the measured values of the disk mass (Figure 5), and of their outer diameter (Figure 6).

Table 1. The parameter values of the new discs, determined in the statistical analysis.

No. Disc	Disc Mass (g)	Average Outer Diameter (mm)	Average Disc Thickness (mm), at Different Diameters, (Measured in 4 Radial Directions)
			D = 360 mm	D = 340 mm	D = 320 mm	D = 300 mm	D = 280 mm
1	4041.60	380.42	4.24	4.20	4.25	4.08	4.17
2	4058.80	380.66	4.22	4.23	4.21	4.00	4.18
3	4045.10	380.35	4.23	4.24	4.21	4.06	4.29
4	4043.80	380.68	4.21	4.25	4.22	4.16	4.27
5	4040.30	380.37	4.24	4.21	4.35	4.17	4.23
6	4036.60	380.18	4.19	4.26	4.45	4.50	4.14
7	4042.20	380.44	4.14	4.18	4.19	4.35	4.47
8	4053.10	380.44	4.26	4.27	4.26	4.22	4.16
9	4055.60	380.52	4.27	4.26	4.28	4.25	4.24
10	4050.90	380.23	4.24	4.21	4.19	4.18	4.10
11	4014.60	380.51	4.17	4.12	4.10	4.08	4.40
12	4053.30	380.63	4.27	4.22	4.21	4.17	4.10
13	4066.00	380.64	4.21	4.19	4.16	4.17	4.12
14	4049.70	380.44	4.29	4.26	4.26	4.24	4.25
15	4016.50	380.38	4.23	4.17	4.23	4.19	4.10
16	4017.50	380.42	4.19	4.19	4.11	4.11	4.20
17	4021.20	380.53	4.17	4.17	4.16	4.13	4.16
18	4028.60	380.43	4.19	4.24	4.19	4.19	4.18
19	4049.20	380.49	4.28	4.23	4.24	4.24	4.34
20	4041.30	380.48	4.17	4.18	4.20	4.19	4.16
Medium values	4041.295	380.462	4.2205	4.214	4.224	4.184	4.213
Standard deviation	14.7498	0.1308	0.0419	0.0391	0.0778	0.1002	0.1009
Deviation from the mean, %	−0.66–+0.61	−0.07–+0.06	−1.91–+1.65	−2.23–+1.33	−2.92–+5.36	−4.40–+7.55	−2.68–+6.10

Table 1. The parameter values of the new discs, determined in the statistical analysis.

No. Disc	Disc Mass (g)	Average Outer Diameter (mm)	Average Disc Thickness (mm), at Different Diameters, (Measured in 4 Radial Directions)
			D = 360 mm	D = 340 mm	D = 320 mm	D = 300 mm	D = 280 mm
1	4041.60	380.42	4.24	4.20	4.25	4.08	4.17
2	4058.80	380.66	4.22	4.23	4.21	4.00	4.18
3	4045.10	380.35	4.23	4.24	4.21	4.06	4.29
4	4043.80	380.68	4.21	4.25	4.22	4.16	4.27
5	4040.30	380.37	4.24	4.21	4.35	4.17	4.23
6	4036.60	380.18	4.19	4.26	4.45	4.50	4.14
7	4042.20	380.44	4.14	4.18	4.19	4.35	4.47
8	4053.10	380.44	4.26	4.27	4.26	4.22	4.16
9	4055.60	380.52	4.27	4.26	4.28	4.25	4.24
10	4050.90	380.23	4.24	4.21	4.19	4.18	4.10
11	4014.60	380.51	4.17	4.12	4.10	4.08	4.40
12	4053.30	380.63	4.27	4.22	4.21	4.17	4.10
13	4066.00	380.64	4.21	4.19	4.16	4.17	4.12
14	4049.70	380.44	4.29	4.26	4.26	4.24	4.25
15	4016.50	380.38	4.23	4.17	4.23	4.19	4.10
16	4017.50	380.42	4.19	4.19	4.11	4.11	4.20
17	4021.20	380.53	4.17	4.17	4.16	4.13	4.16
18	4028.60	380.43	4.19	4.24	4.19	4.19	4.18
19	4049.20	380.49	4.28	4.23	4.24	4.24	4.34
20	4041.30	380.48	4.17	4.18	4.20	4.19	4.16
Medium values	4041.295	380.462	4.2205	4.214	4.224	4.184	4.213
Standard deviation	14.7498	0.1308	0.0419	0.0391	0.0778	0.1002	0.1009
Deviation from the mean, %	−0.66–+0.61	−0.07–+0.06	−1.91–+1.65	−2.23–+1.33	−2.92–+5.36	−4.40–+7.55	−2.68–+6.10

From Table 1 it can be seen that the mass of the disks falls between 4014.6–4066.0 g, with deviations from the average of up to 0.66%, plus or minus. The same can be said about the diameter of the new discs, which has minimum values of 380.18 mm and maximum values of 380.68 mm, with deviations from the average of the other values in the range [−0.07–+0.06%]. If at disks mass and diameter, the deviations from the average value are very small, the same cannot be said about their thickness, which turns out to be more non-uniform. Thus, for the diameter of ϕ360 mm (the largest diameter at which measurements were made), the thickness of the new discs has values in the range of 4.14–4.29 mm, with deviations from the average in the range of [−1.91–+1.65%]. At the diameter ϕ340 mm, the thickness of the discs has values in the range 4.12–4.27 mm, with deviations from the average in the range [−2.23–+1.33%]. Near the hub (at ϕ280 mm), the disc thickness had values in the range 4.10−4.47 mm, with deviations from the average in the range [−2.68–+6.10%]. For the other intermediate diameters, the deviation of the disc thickness from the average is even higher, up to 7.55%, as shown in Table 1.

The Fisher test performed in MS Office Excel between the mass of the discs and their diameter shows a variation of 1.86 × 10⁻¹⁸, and between the mass of the discs and their thickness, it shows values between 1.32 × 10⁻² and 4.08 × 10⁻¹⁸.

For the new disks, the value of the Fisher test coefficient is between 9.66 × 10⁻³⁵ for the variation of the mass of the disks with their diameter and 3.83 × 10⁻⁴⁴–4.19 × 10⁻³⁴ for the variation of the mass of the disks with their average thickness, at different distances from the center of the disk.

3.2. Results Regarding Measurements of Disks Parameters after 80 ha Worked

In continuation of the experiments, determinations were made for the discs of a seeder with six sections (12 coulter discs), after they worked an area of about 80 hectares.

The measurement data are expressed in value in Table 2.

Table 2. The values of the parameters for the discs used, determined in the statistical analysis for Tempo TPT 6, series 1912, coulter discs, wear on 80 ha worked, at an average speed of 6.4 km/h.

No. Disc	Disc Mass (g)	Average Outer Diameter (mm)	Average Disc Thickness (mm), at Different Diameters (Measured in 4 Radial Directions)
			D = 360 mm	D = 340 mm	D = 320 mm	D = 300 mm	D = 280 mm	Dext
1	4007.0	380.00	4.15	3.99	3.99	4.57	4.37	1.85
2	3970.8	379.90	3.89	3.86	4.06	4.06	4.06	2.00
3	3966.1	379.90	3.96	3.90	4.06	4.09	4.14	1.76
4	3941.8	379.75	3.63	3.76	3.91	4.04	4.05	1.92
5	3947.0	379.90	3.82	3.82	4.02	4.19	4.02	1.92
6	3949.7	380.00	3.88	3.84	4.08	4.15	4.13	1.84
7	3972.6	379.90	3.89	3.90	4.09	4.23	4.16	1.82
8	3970.8	379.90	3.91	3.85	4.05	4.07	4.17	1.86
9	3940.5	380.00	3.78	3.80	3.97	4.18	4.11	1.93
10	3986.6	379.65	4.05	3.89	4.12	4.16	4.14	1.89
11	3966.9	379.90	3.95	3.91	4.07	4.15	4.11	2.08
12	3977.4	379.78	3.70	4.00	4.11	4.26	4.17	1.76
Medium values	3966.43	379.88	3.88	3.88	4.04	4.18	4.14	1.88
Standard deviation	19.51	0.11	0.14	0.07	0.06	0.14	0.09	0.09
Deviation from the mean, %	−0.65–+1.02	−0.03–+0.03	−6.52–+6.80	−2.94–+3.05	−3.21–+1.80	−3.27–+9.35	−2.73–+5.67	−6.86–+10.39

Table 2. The values of the parameters for the discs used, determined in the statistical analysis for Tempo TPT 6, series 1912, coulter discs, wear on 80 ha worked, at an average speed of 6.4 km/h.

No. Disc	Disc Mass (g)	Average Outer Diameter (mm)	Average Disc Thickness (mm), at Different Diameters (Measured in 4 Radial Directions)
			D = 360 mm	D = 340 mm	D = 320 mm	D = 300 mm	D = 280 mm	Dext
1	4007.0	380.00	4.15	3.99	3.99	4.57	4.37	1.85
2	3970.8	379.90	3.89	3.86	4.06	4.06	4.06	2.00
3	3966.1	379.90	3.96	3.90	4.06	4.09	4.14	1.76
4	3941.8	379.75	3.63	3.76	3.91	4.04	4.05	1.92
5	3947.0	379.90	3.82	3.82	4.02	4.19	4.02	1.92
6	3949.7	380.00	3.88	3.84	4.08	4.15	4.13	1.84
7	3972.6	379.90	3.89	3.90	4.09	4.23	4.16	1.82
8	3970.8	379.90	3.91	3.85	4.05	4.07	4.17	1.86
9	3940.5	380.00	3.78	3.80	3.97	4.18	4.11	1.93
10	3986.6	379.65	4.05	3.89	4.12	4.16	4.14	1.89
11	3966.9	379.90	3.95	3.91	4.07	4.15	4.11	2.08
12	3977.4	379.78	3.70	4.00	4.11	4.26	4.17	1.76
Medium values	3966.43	379.88	3.88	3.88	4.04	4.18	4.14	1.88
Standard deviation	19.51	0.11	0.14	0.07	0.06	0.14	0.09	0.09
Deviation from the mean, %	−0.65–+1.02	−0.03–+0.03	−6.52–+6.80	−2.94–+3.05	−3.21–+1.80	−3.27–+9.35	−2.73–+5.67	−6.86–+10.39

3.3. Analysis of Status Parameters for the New Disks

3.3.1. Probability Density and Cumulative Probability for the Thickness of the New Disks

Based on the values presented in Table 1, the charts of probability density and cumulative probability were made for the thickness of the disks, measured from 10 to 10 cm in the radial direction, using Data Analysis from the Microsoft Excel program. In Figure 4 are presented only the charts for the average values of the measurements (because the thickness measurement was carried out in 4 radial directions at 90°). For the normal distribution, the charts show that there are some values (not many, i.e., 1–2 values) that fall outside this type of distribution, which we could define as random errors. If we ignore these values, then the Microcal ORIGIN 7 program presents an extremely high determination coefficient (R² over 0.999, χ² between 4.9 × 10⁻⁶ and 1 × 10⁻⁵ for probability density in Gauss distribution) for the Gaussian distribution function (Equation (3)) applied to the probability density values:

$$y=y_0+\frac{A}{w \sqrt{\pi /2}}{exp}^{-2\frac{{\left(x-x_c\right)}^2}{w^2}}$$

(3)

where: x_c represents the middle of the range of values; y₀—the compensation (the free term of the expression); w—the width of the curve to half its height.

For the Gaussian distribution of the values of the outer diameter of new discs, for example, the Chi square test shows the value of 39.3, while for their mass χ² = 63.1.

For the thickness of new discs at different diameters from the center of the disc, the Chi square test shows values between 13.92 (at 340 mm diameter) and 145.9 (at 320 mm diameter). For the other diameters at which the disc thickness has been determined, the values of χ² fall between these values.

3.3.2. Distribution by Mass and Diameter of New Discs

From the histogram analysis presented in Figure 5, it can be seen that most new disks (about 70%) have masses above the average, weighing over 4.040 kg, but about 15% have masses in the minimum mass range (4.010 ÷ 4.020 kg), below the average of the mass values, which is 4.041 kg.

Also, about 75% of the discs have a diameter over 380.40 mm, with about 15% of the discs having a diameter between 380.30 ÷ 380.40 mm, the average of the disc diameter values being 380.46 mm. It is noteworthy, however, that 40% of the discs have both a mean mass of 4040–4050 g and a diameter of ϕ 380.40–380.50 mm, which represent the means of the ranges of values considered for the two parameters.

3.3.3. The Histograms of Variation of the New Disks Thickness

Proceeding further with the processing of the experimental data and considering the histograms of variation of the disc thickness values at the five diameters mentioned before, the charts in Figure 7 are obtained, in which we find the frequency of the values for the mentioned parameter, but also the cumulative frequency (in percentages) of these values. Carrying out the regression analysis of the cumulative frequency values, using the log-normal function (mentioned on the charts), the theoretical curve of the cumulative frequency is obtained (solid line, in blue on the figures). The relatively high values of the R² determination coefficient (0.900 ÷ 0.978) show the legitimacy of using this function, for the analyzed case. For the Gauss distribution, χ² has values in the range 13.9–145.9.

3.4. Analysis of the State Parameters of the Discs Used for Sowing 80 ha

3.4.1. Thickness Distribution of Discs Used to Use the Lognormal Law

The same is observed for the coulter discs of the Vaderstad Tempo TPT-6 seeder, with six sowing sections and 12 discs, with the same outer diameter (380 mm). Considering the smaller number of discs analyzed and the distribution of disc thickness values for the five diameters (as in the case of new discs) (Figure 8), the values of the determination coefficient R² fall within wider limits, starting from R² = 0.844 to R² = 0.996, when the measured values are within narrower limits. However, as it was mentioned during the paper, if we make abstraction from the values that go far out of the domain (Figure 8a,b,e), then the domain is certainly narrowed and the regression analysis will present a determination coefficient greatly enlarged.

3.4.2. Distribution by Diameter and Mass of the Discs Used

The same mode of variation, with values far outside the plausible range, is also found in the case of the cumulative frequency of disc diameter values for the TPT-6 seeder. Therefore, maybe the disc, with a diameter far outside the range of the other discs, showed either a more pronounced wear on one of the two perpendicular diameters on which the measurement was made, or there was even a measurement error, the problem is that this value is what makes the analysis not give a higher value of the determination coefficient (for the log-normal function R² = 0.301) (Figure 9).

On the other hand, for the distribution of the disc mass values, it is observed that the log-normal function presents a high determination coefficient (R² = 0.932).

This phenomenon happens even for a wider range of values, with more disks occupying this range. For the Gauss distribution, χ² of the disk mass has the value 91.6.

A tendency of thinning of the discs towards the outer diameter is observed, where, in fact, the discs are sharp (the thickness at the outer limit being much smaller than in the other areas where measurements were made). Evidently, this thinning is due to the fact that it is the outside of the discs that work in the soil being in constant contact with it during work, and it is normal for there to be some wear on this area.

3.4.3. Distribution by Diameter and Mass of the Discs Used

If we further analyze the last rows of Table 1 and Table 2 (with the average values of the parameters studied), it is even more noticeable that the discs that worked (even if only 80 hectares or 380 ha) present both a lower mass, as well as a smaller outer diameter due to wear in the work process (

Table 3. The values of the parameters for the discs used, determined in the statistical analysis for Tempo TPT 6, series 1912, coulter discs, wear on 80 ha and 380 ha worked.

New Discs
	Mass, g	Dext, mm	t360, mm	t340, mm	t320, mm	t300, mm	t280, mm
Medium values	4041.295	380.462	4.2205	4.214	4.2545	4.204	4.229
Discs used at 80 hectares sown
Medium values	3966.43	379.83	3.85	3.88	4.04	4.22	4.20
Δm	74.865	0.632	0.3705	0.334	0.2145	0.016	0.029
Discs used at 380 hectares sown
Medium values	3843.17	379.11	3.28	3.53	3.79	3.77	3.80
Δ′m	198.12	1.35	0.94	0.68	0.46	0.43	0.42

Note: t₃₆₀ (mm)—disc thickness at 360 mm diameter; Δ_m, Δ′_m—deviations from the new disc.

). The same phenomenon is also shown by the values of the average thicknesses of the discs used for sowing, compared to the values of the average thicknesses of the new discs (which were also presented graphically in Figure 10).

We have to note that there are disks that show values far away from the determined average values (one at most two disks), both for used disks and for new disks, which lead to a lower R² coefficient. If we were to ignore these deviations that we can consider random, a very high value of R² would be obtained, but this is not the purpose of our presentation. We would like to draw attention to the fact that there can be deviations even in the manufacturing process that lead to a size distribution that is not normal, and these deviations can go up to 6–7%.

4. Discussion

4.1. Generalities for Disc Coulters

In the working process of the seeders, the disc coulters have the role of opening furrows in order to introduce the seeds into the soil. In contact with the soil, they also acquire a rotational movement around their axis of symmetry, which makes abrasion wear less pronounced than in other types of coulters.

The physical characteristics of the coulter discs (diameter, plate thickness, disc mass) depend on the type of crop sown, the degree of soil settlement, and the amount of plant debris on the soil at the time of seeding.

Disc coulters are generally used for sowing crops in uncultivated land, but they can also work in previously prepared land.

At these coulters, it is important to adapt the functional parameters to the soil conditions–degree of weediness, soil moisture, degree of settlement, but also to the type of culture that is established. Specialists in the field say that it is necessary to adjust the position angles of the discs in good time, as well as the working depth, in order to obtain a rich harvest.

Moreover, in the area of the furrow opened by the disc coulter, between the two discs or on the side (for the single disc coulter), helpful elements have been added that improve the work process, including the complete cutting of vegetable remains, as specified in the paper [31].

Research shows that disc geometry and working depth have a significant impact on the furrow opening process, taking into account resisting forces, straw cutting efficiency, soil disturbance width, and soil penetration depth. Thus, a vertical disc has a higher average straw cutting efficiency and lower tillage forces, but also a smaller soil disturbance width than the disc coulter. Furthermore, a rippled disc has higher straw cutting efficiency, moderate tillage force, and adequate soil disturbance width among five tools investigated by the authors of the papers [32,33].

4.2. Discussion for New Disc Parameters

Through simulation tests, the results show that when the relative height is 82 mm, the diameter of the double-disc coulter is 297 mm, and the angle between the two discs is 14°, the stability performance of the tillage depth reaches 91.64%. With a working strength of only 93.93 N, the trencher achieves optimal operational performance in these conditions. Field validation tests show a tillage depth stability coefficient of 92.37% and a working strength of 104.2 N. These values deviate by 0.73% and 10.93%, respectively, from to the simulation results, confirming the reliability of the simulation model [33,34].

The authors of paper [35] show that in order to ensure good quality when using flat double-disc coulters, the diameter of the discs should be at least 21.4 cm and the sharpening angle is 25°, which ensures a furrow width at the bottom of a little 15 mm [36]. In addition, they say that the height of the meeting point of the coulter discs should be at least 8.6 cm.

Specialists at John Deere specify that the coulter discs that were initially 380 mm in diameter should be replaced when the wear has reached more than 12–14 mm, especially if the wear on the two coulter discs is not equal [37].

Excessive disc wear causes incorrect planting depth and improper seed placement in the open furrow.

Our research on new, flat, smooth-edged discs used in seed drills in Romania shows that their average thickness is about 4.204 ÷ 4.254 mm, with standard deviations of 0.039–0.163 mm and deviations from the mean between 0.06–11.41% for the normal distribution of the considered size.

For the mass of new discs, values in the range 4014.6–4066.0 g and mean value 4041.295 g, standard deviation 14.750 g and deviations from the mean −0.66 ÷ +0.61% were also found. On the other hand, for the disc diameter, the size range was ϕ380.18 ÷ ϕ380.68 mm (mean value ϕ380.462 mm) with a standard deviation of 0.131 mm, the deviations from the mean being −0.07 ÷ +0.06%.

With some (minor) abstractions, it can be said that the three parameters follow a Gaussian normal distribution, as shown in Figure 5, Figure 6 and Figure 7.

From our observations on the variation of the frequency of new disc thickness values and the cumulative frequency (in percentage) of these values, a log-normal variation of the cumulative frequency was found, the regression analysis performed showing high values of the determination coefficient R² (0.838 ÷ 0.943) for this function.

4.3. Discussion for Used Discs Parameters

This mode of variation is also observed for the parameters of the Tempo TPT-6 seeder discs, which were used to sow 80 ha, for all three parameters analyzed in the work: disc thickness, diameter, and mass. It is important to remember that among all the analyzed values there are some values that fall outside the normal distribution and, therefore, also outside the log-normal distribution of the cumulative frequency of the values for these parameters (maximum 1 value for the thickness of the disks in the radial direction and only one value for the outer diameter of the disks), but which lead to lower values of the determination coefficient R² (see Figure 9 and Figure 10).

The discs already in use (80 ha) showed a general thinning tendency from the center of the disc outwards, due to the contact with the soil and the abrasive wear exerted by it, both for the disc on the left and for the disc on the right of the coulter. Excluding the last point (the one at the outer edge of the disc), the regression analysis with the linear law shows a slope of the regression lines between −0.00 ÷ −0.02, with average values of −0.0080. From the analysis of the thickness values at the diameter of ϕ360 mm, an average value of the disc thickness of 3.865 mm results, much smaller compared to the thickness of the disc at the inner diameter, i.e., ϕ280 mm, where the thickness is 4.114 mm.

It should be noted that a pronounced wear of the discs means that they no longer open appropriate channels, the width of the channel at the bottom changes, plant residues (in no-till conditions) penetrate more easily into the furrow channel, and the seeds are no longer deposited at the depth required by the regulations. This is also because the disc holders cannot be adjusted for a convenient approach of the discs and obtaining a proper channel. In addition, adjustments should be made differently from one section to another of the sowing machine.

The thickness difference between the two diameters is 0.249 mm. If the discs were used to sow only 80 hectares, it would mean that they thinned by at least ¼ mm, arriving by arithmetic logic at 1 mm after 320 hectares sown, from which we deduce that the discs should be changed after about 500 hectares of seeding.

If the data from Table 3 is analyzed, it can be see that the change in the geometric parameters of the discs during the work process, their values decreasing due to wear. Thus, if the mass of the discs decreases by 1.85% after 80 hectares sown, after 380 hectares their mass decreases by 4.90%. The same thing is noted for the disc diameter, which decreases by 0.17% after 80 hectares worked and by 0.35% after 380 hectares. If we take into account the remark from before, that after a reduction in diameter by 10–12 mm the discs should be changed, we can say that the discs can be used for sowing only about 3000 hectares. Moreover, the discs become thinner due to the abrasive wear produced in contact with the ground, obviously more on the outside and less in the center of the disc, as shown by the data in Table 3. Thus, at the level of the diameter of 360 mm (for discs with the exterior diameter of 380 mm) wear per thickness is very high, i.e., about 22.3%, and at the 340 mm diameter level, this wear reaches 16.14%, which means that the discs will never be able to work the 3000 hectares mentioned previously, because they will break due to wear on the thickness of the disc.

5. Conclusions

Specialists in agriculture and agricultural machinery believe that more attention should be paid to the possibilities of using disc harrows and machines adapted to particular soil conditions, possibly with quick adjustments of disc angles and machine speed, to increase sowing precision, especially in no-tillage conditions.

The discs of grain sowing machines, regardless of the manufacturing company, are made in a range of values of the basic parameters (diameter, mass, disc thickness), variety that can follow a normal distribution or another type of distribution, without taking into account that they can be smooth or wavy, with a serrated edge.

The analysis made in the paper shows the deviations from the design values of the coulter discs, normally having the diameter ϕ${380.46}_{-0.28}^{+0.22}$ mm (with a measurement error of ${}_{-0.07}{}^{+0.06}\%$), mass of 4041^±27 g (with a measurement error of ${}_{-0.66}{}^{+0.61}\%$) and a sheet thickness between 4.18^±0.18 mm and ${4.22}_{-0.08}^{+0.07}$ mm (with a measurement error also contained between ${}_{-1.91}{}^{+1.65}\%$ and ${}_{-4.40}{}^{+7.55}\%$), from the outer to the inner diameter, as well as the probabilities of differences from the average, both for new coulters and for coulters who have already worked about 80 hectares at sowing.

It was established that there is a relationship between the wear of the discs on their thickness and their outer diameter and the area of land sown, the discs thinning from the outside to the inside by about 0.25 mm per 100 hectares sown (about 22% for 380 hectares sown), and decreasing their diameter by 0.17–0.35% for land surfaces sown from 80 to 380 hectares. It is obvious that new research is needed, i.e., the continuation of research to establish the physical parameters of coulter discs for seeders of other companies, of other types of seeders, as well as research to establish the dependence of coulter disc wear on the surface of land sown with cereals.

It would be interesting if research could be carried out on the dependence of the parameters mentioned above with the soil category in which the sowing takes place.