Assessment of the quality and performance of tires on agricultural tractors

The agricultural tractor is the main source of power in agriculture, being present in all phases of crop development, such as soil preparation, planting, cultural treatments and harvesting. This self-propelled machine has the ability to perform basic functions, such as: pulling, transporting and activating active parts of agricultural implements that require rotation to carry out the work.

To be able to perform its functions, the tractor uses the three-point lift, the drawbar and the power take-off (TDP), which are called means of harnessing the tractors' power.

Tractors are expected to perform well in order to increase crop productivity, with lower energy demand and lower environmental impact. Regarding performance, one of the ways to quantify it is to evaluate its traction capacity.

Aiming to evaluate the traction capacity of agricultural tractors manufactured and/or sold in Brazil, a survey of 191 models of agricultural tractors with tires was carried out. The survey was carried out by consulting the manufacturers’ technical specifications, previous test reports and measurements.”on-site visit" of some specimens.

The tractors were stratified into type of traction (4x2 and 4x2 TDA) and power ranges, according to Anfavea (2014), named, respectively, as light, medium, heavy and super heavy, as described in Table 1.

Table 1. Power range of agricultural tire tractors sold in Brazil

Banner	Classification	Power Range
		(cv)	(KW)
1	Mild	≤ 49	≤ 36
2	Medium	50 ≤ P ≤ 99	37 ≤ P ≤ 73
3	Heavy	100 ≤ P ≤ 199	74 ≤ P ≤ 146
4	Superheavyweight	200 ≥	147 ≥

Banner

Classification

Power Range

(cv)

(KW)

1

Mild

≤ 49

≤ 36

2

Medium

50 ≤ P ≤ 99

37 ≤ P ≤ 73

3

Heavy

100 ≤ P ≤ 199

74 ≤ P ≤ 146

4

Superheavyweight

200 ≥

147 ≥

Source: Anfavea (2014).

To analyze the traction capacity of agricultural tractors, the following variables were calculated: maximum traction force on BT with and without ballast (FTM); traction force/weight ratio with and without ballast; weight transfer with and without ballast; traction coefficient with and without ballast and traction performance in different soil conditions.

Table 2 presents the average of the parameters that influence the traction capacity of agricultural tractors, with and without ballast, depending on the power range and type of traction.

Table 2. Analyzed parameters of agricultural tractors

	FORWHERE TRACTION (N)
POWER (hp)	with ballast	without ballast
	23868,81	20086,24
50-99	49483,74	39366,67
100-199	104238,02	83288,80
> 200	219805,15	163043,64
TRACTION
4 x 2	46166,28	35349,19
4x2 TDA	81488,03	64654,61
	TRACTION FORCE/WEIGHT RATIO (%)
POWER (hp)
	143, 43	143,12
50-99	145,69	142,74
100-199	149,57	146,15
> 200	176,81	174,91
TRACTION
4 x 2	140,41	137,91
4x2 TDA	151,77	149,02
	WEIGHT TRANSFER (N)
POWER (hp)
	4677,65	3868,86
50-99	9691,91	7709,02
100-199	19922,25	16457,78
> 200	35457,99	27078,78
TRACTION
4 x 2	9364,02	7173,22
4x2 TDA	14465,69	11742,54
	TRACTION COEFFICIENT (###b#<)
POWER (hp)
	153,35	153,76
50-99	159,54	158,84
100-199	164,29	165,06
> 200	199,86	198,29
TRACTION
4 x 2	154,41	152,25
4x2 TDA	167,65	166,34

FORWHERE TRACTION (N)

POWER (hp)

with ballast

without ballast

23868,81

20086,24

50-99

49483,74

39366,67

100-199

104238,02

83288,80

> 200

219805,15

163043,64

TRACTION

4 x 2

46166,28

35349,19

4x2 TDA

81488,03

64654,61

TRACTION FORCE/WEIGHT RATIO (%)

POWER (hp)

143, 43

143,12

50-99

145,69

142,74

100-199

149,57

146,15

> 200

176,81

174,91

TRACTION

4 x 2

140,41

137,91

4x2 TDA

151,77

149,02

WEIGHT TRANSFER (N)

POWER (hp)

4677,65

3868,86

50-99

9691,91

7709,02

100-199

19922,25

16457,78

> 200

35457,99

27078,78

TRACTION

4 x 2

9364,02

7173,22

4x2 TDA

14465,69

11742,54

TRACTION COEFFICIENT (###b#<)

POWER (hp)

153,35

153,76

50-99

159,54

158,84

100-199

164,29

165,06

> 200

199,86

198,29

TRACTION

4 x 2

154,41

152,25

4x2 TDA

167,65

166,34

It was possible to prove, according to the results in Table 2, that one of the purposes of ballast is to increase traction force. It is noted, with the addition of ballast, that the FTM values ​​were higher for all power ranges in relation to tractors without ballast. The traction force for tractors with the auxiliary front-wheel drive option is greater when compared to those that do not have front axle drive, and can reach even higher values ​​with the addition of ballasts.

When the tractors went from 4x2 to 4x2 TDA, there was an increase of 56,6% and 54,7% in traction force with and without ballast, respectively

The evaluation of the traction force to weight ratio, with and without ballast, is considered good, as all values ​​are above the reference, which is 85%, according to Cenea (1982). It can be seen that all values ​​were above 100%. This means that tractors in the four power ranges, with and without ballast, can pull more than their own weight. This is justified by the increase in traction force with the increase in the power range.

The traction force to weight ratio is greater on tractors with the option of auxiliary front-wheel drive. This probably occurs because they are tractors with greater power availability and greater maximum traction force.

Weight transfer increased as the power band increased. This happened because weight transfer is related to traction force. This was proven by Schlosser et al. (2004) when they report that the effect of traction caused a greater portion of the weight to be transferred from the front to the rear axle, causing the latter to have a higher dynamic coefficient of traction, therefore, receiving more conditions to improve the traction coefficients. With the addition of ballast, the values ​​were even higher, due to the fact that the placement of the ballast justifies the increase in the amount of front weight that can be transferred to the rear axle.

Tractors that have auxiliary front-wheel drive can transfer more weight to the rear axle. This is due to a more balanced weight distribution between the tractor's axles. With the addition of ballast, this increase is even more pronounced.

In the lower power ranges, tractors without ballast provide a higher coefficient of traction compared to tractors with ballast. Ballasted tractors showed better results in the highest power ranges.

4x2 TDA tractors with ballast have higher traction coefficients when compared to tractors without the front-wheel drive option and without ballast. Ballasted 4x2 TDA tractors have the ability to pull 8% above their rear dynamic weight, when compared to ballasted 4x2 tractors. It is believed that when tractors have front-wheel drive activated, it can help reduce the slipping of the rear drive tires and thus have a greater traction coefficient.

In addition to the parameters shown in Table 2, traction performance is another factor that must be evaluated in the traction capacity, under firm, plowed and loose soil conditions, depending on the power range and type of traction.

The traction performance values ​​present, depending on the soil condition, no or little difference in relation to the increase in the power range. However, within the same power range, there is a decrease in traction performance as the soil becomes smaller or looser aggregates, for example, harrowed soil. This probably happens because when the soil is looser, the adhesion of the rolling contact with the ground is lower and, consequently, the rolling resistance is greater.

Traction performance is higher on tractors without the option of auxiliary front-wheel drive, with the exception of when working on loose soil, as the values ​​were equal to 64%.

Among the parameters that affect traction capacity, it was observed that the power range above 200 hp presents higher values, especially when they are ballasted. Furthermore, when auxiliary front-wheel drive (TDA) is activated, the traction capacity of agricultural tractors increases. However, traction capacity is just one point to be considered in the face of the great diversity of parameters to evaluate the performance of tractors.

This article was published in issue 142 of Cultivar Máquinas magazine. Click here to read the edition.