Impact of Feed Force and Initial Chain Tension on Chipper Chain Wear in Gasoline-Powered Chainsaws

Abstract

The aim of the present study was to determine the effects of initial chain tension and feed force on the wear rate of chipper chain cutters in gasoline-powered chainsaws. Wear was assessed by measuring the tip radius as cutter dulling has a considerable influence on chainsaw performance. Tests were carried out for three feed force values: 15 N, 35 N, and 70 N, and for two initial tension settings—a tight chain and a slack chain. The tests also measured cutting efficiency per unit of kerf area. The experiments involved pine wood and were conducted on a specially designed test rig that maintained a constant feed force. The tip radius was determined by means of a microscope and computer image analysis software. Wear intensity over time was evaluated using the dulling coefficient. The results showed that the applied initial chain tension and feed force significantly affected the cutter wear rate, which was significantly higher at lower feed force values. The wear rate was also increased by lower initial chain tension, which can be explained by greater cutter deflection from the kerf plane.

1. Introduction

Despite the increasingly prevalent application of high-performance multifunctional machines [1], gasoline-powered chainsaws are still widely used for timber harvesting and primary processing in many parts of the world. This is mostly due to economic considerations and terrain factors preventing the deployment of large machinery [2,3,4,5]. Unfortunately, chainsaw operation entails risks and hazards [5,6] and involves strenuous work [7,8,9,10], as operators are exposed to excessive noise and vibrations [11,12,13]. The sawdust generated in the process of cutting, whose amount largely depends on cutter geometry, is also detrimental to health [14,15]. While advanced harvesting technologies are gradually becoming more widespread [13], the Scandinavian countries are the only ones where more than 90% of timber is harvested using high-efficiency machines [16]. A very comprehensive review of the research results related to the wood cutting process is included in the review article: Wood Machining with a Focus on French Research in the Last 50 Years [17].

Chainsaws are not rigid tools, and so during operation chain links (cutters) can deflect from the kerf plane. This deflection is especially pronounced for gasoline-powered machines [18].

According to many reports [19,20], initial chain tension and feed force significantly affect cutting efficiency, which decreases with excessive chain slack and low feed force. The moisture content of timber also influences cutting performance by increasing cutting resistance [21]. Lower initial chain tension and feed force values result in a higher likelihood of the chain links deflecting from the kerf plane. Conversely, at greater chain tension and feed force, the links have less freedom to deflect [22].

In research on electric saws, it turned out that the design of the chain saw affects not only the cutting results but also the value of specific energy. Furthermore, working with a loose saw blade reduces cutting efficiency [23].

Cutter wear is associated with changes in cutter geometry [24]. Over time, a chainsaw loses its ability to cut wood due to a reduction in the linear dimensions of the cutting elements [25]. Cutter wear is characterized by changes in the tip radius of the cutting edge, which increases as a result of friction. The greater the area of the executed kerf, the greater the changes in cutter properties. Once the tip radius of the cutting edge reaches a certain limit value, further wood processing is no longer possible. This condition of cutters is referred to as dulling [26].

Cutter dulling significantly affects cutting performance, as established by Górski [27], who found that a change in tip thickness from 10 to 35 μm reduced it by 30%. A previous study on the dulling of chisel-type cutters in the process of pinewood processing [28] reported significant differences in the dulling rate between individual cutters of the studied chain.

The tip radius increases faster when cutting harder wood [28]. The cutter wear rate is affected by initial chain tension: the lower it is, the higher the wear rate [28]. Furthermore, the greater the cumulative kerf area, the greater the discrepancies in the degree of dulling between individual cutters [29]. This variation also depends on chain tension and the species of wood being processed [28]. Operating a chainsaw with a slack chain increases differences between individual cutters, probably due to more pronounced cutter deflection from the kerf plane as compared to a tight chain [19]. The process of cutter wear may also be affected by the initial cutter tip radius [30].

However, Maciak and Kubuśka [28] noted the absence of research on cutter wear under constant feed force. Previous studies known to the authors were conducted under conditions that did not enable precise feed force control (with manually operated chainsaws), which is a factor with a major impact on chainsaw performance. Most of the identified changes in the cutter tip radius occurred in the first and second phases of a typical Lorenz curve characterizing a constant wear rate. To date, the third phase of cutter wear in chipper chains has not been elucidated. It has also been observed that to gain thorough knowledge of the cutter dulling process, one must perform a very large number of repetitions using the same chainsaw [29].

The aim of the presented study was to determine the effects of feed force and initial chain tension on the cutter wear rate. The effect of initial chain tension was evaluated only at a single feed force value. While similar investigations have been carried out under field conditions [28], the purpose here was to corroborate their findings in a laboratory setting. This research constitutes a continuation of a previous study [22], which examined the influence of feed force and initial chain tension on cutter deflection in the process of wood cutting.

2. Methodology

The process of cutter dulling was studied on an experimental rig that ensured maintaining a constant feed force and enabled repeatable wood cutting trials with a gasoline-powered chainsaw. The rig was specially designed and constructed for the purpose of the present study. A schematic diagram of the rig is shown in Figure 1. Feed force was set by applying an appropriate weight.

The use of weights to apply a selected feed force ensured measurement repeatability in terms of feed force. This eliminated the human factor, otherwise introduced by chainsaw operators during experiments.

The chipper chain was mounted on a gasoline-powered chainsaw powered by a 57 cm³ two-stroke engine with a maximum power output of 3.2 kW and a mass of 5.5 kg (without the cutting system and with empty tanks).

The measurements were performed during summer in laboratory conditions in an indoor room. The temperature was constant, 21 °C. The chainsaw’s oil pump was set to maximum lubrication. According to the operating instructions, the oil pump capacity of the chainsaw is from 6.3 mL·min⁻¹ to 13.5 mL·min⁻¹ at a rotational speed of 8500 rpm·min⁻¹.

Tests were carried out for two chain tension settings: tight and slack. Before each measurement series, the initial tension was set using the tensioner with a weight of 20 N suspended at the midpoint of the guide bar (Figure 2). During the adjustment, we measured the distance between the raceway of the guide bar and the cutter toe, referred to as chain sag, Δy. The chain was considered slack at Δy = 8 mm and tight (normally tensioned) at Δy = 5 mm. This method of determining initial chain tension has also been applied in other studies [27,31,32,33].

After setting the desired tension, the chainsaw was started, and the engine ran at high speed for approximately 60 s. Once the chainsaw warmed up, the initial chain tension was rechecked. Tests with appropriately tensioned chains were carried out at three feed force values: 15 N, 35 N, and 70 N. For a feed force of 35 N, tests were also performed with a slack chain. (The effect of initial chain tension on cutter wear is already known; the tests with a slack chain were conducted only to confirm previous findings.) Three repetitions were carried out for each variant, each time using a newly sharpened chain.

The tests involved 200 cm-long pine workpieces with a 14 × 14 cm cross-section. The timber was obtained from the Grójec Forest District. The average ring density was 4.15 cm⁻¹, with a standard deviation of 1.63 cm⁻¹. Wood density was 0.48–0.51 g∙cm⁻³.

The absolute moisture content of the samples, as determined by the oven-dry method, ranged from 49.6% to 57.2%. The initial and final weights of samples were measured with an accuracy of 0.001 g using a RADWAG WPS210S laboratory balance. The samples were dried in a Heraeus UT6120 circulated air oven. The average hardness of the pinewood, as measured using the Brinell method on the cross-sectional surface [34], was 23.7 MPa with a standard deviation of 1.3 MPa.

Cutter wear tests involved four cutting assemblies, each consisting of a new guide bar and three new chains. Prior to measurements, 50 cuts were made using each assembly to achieve a smooth fit of the chains and bars and avoid a reduction in chain tension during the experiments. Finally, before the measurements were properly made, the cutters were sharpened using a dedicated cutter grinder.

The wear of chipper chain cutters was defined in geometric terms by measuring their tip radius. The method for determining tooth sharpness is to measure the radius of the rounding of the cutting edge. The smaller the radius, the sharper the tooth is assumed to be. This method of assessing cutting edge wear is used in many similar measurements. We did not measure the geometric wear of other components of the chain links. We plan to do this in subsequent measurements.

The tip radii were pressed into special plates, three for each of the 26 cutters of a given chain. Impressions were made prior to measurements and after the first 50 and 100 kerfs, and then after each subsequent 100 kerfs. Tests continued until the chain ceased cutting at a given feed force. Then, the final series of impressions was taken after the completed kerf series.

In addition, cutting time was measured with a stopwatch with an accuracy of 0.01 s in order to calculate cutting efficiency.

In the next step, the impressions were photographed under a Nikon ALPHASHOT-2 microscope at a magnification of 400×. The acquired images were analyzed and tip radii were measured using CSS Video Frame Grabber (Figure 3).

Cutting efficiency per unit of kerf area was calculated by dividing the kerf area by cutting time according to the following equation:

$$\eta ={\displaystyle \frac{A}{t}} [{\mathrm{cm}}^2·{\mathrm{s}}^{-1}]$$

(1)

where

• η—cutting efficiency [cm² s⁻¹].
• A—kerf area [cm²].
• t—cutting time [s].

A kerf is a gap created in wood during sawing. Its characteristic feature is the width and height of the kerf. Figure 4 shows how to measure these quantities. The method of calculating the kerf area depends on its shape. In the described tests, the kerf cross-section was square. The formula for the area of a square was used to calculate the kerf area.

The obtained tip radius values and kerf areas were used to calculate the mean cutter wear rate I_m:

$$I_m={\displaystyle \frac{{\rho }_k- {\rho }_0}{A}} [\mu \mathrm{m}·{\mathrm{m}}^{-2}]$$

(2)

where

• ρ₀—mean cutter tip radius after sharpening [μm];
• ρ_k—mean cutter tip radius at the end of the measurement series [μm];
• A—kerf area [m²].

In addition, instantaneous cutter wear rate values were computed from Formula (3):

$$I={\displaystyle \frac{∆\rho }{∆A}} [\mu \mathrm{m}·{\mathrm{m}}^{-2}]$$

(3)

where

• Δρ—increase in the mean tip radius between consecutive measurements [μm];
• ΔA—kerf area added between consecutive tip radius measurements [m²].
• The increase in mean cutter tip radius was calculated from Formula (4):

$${\Delta }_{\rho }={\rho }_{i+1}-{\rho }_i [\mu \mathrm{m}]$$

(4)

where

• ρ_i—mean cutter tip radius for i-th measurement [μm];
• ρ_i+1—mean cutter tip radius for measurement i + 1 [μm].

The area of the kerf made between consecutive tip radius measurements was determined from Formula (5):

ΔA = A_i+1 − A_i [m²]

(5)

where

• A_i—cumulative kerf area up to i-th tip radius measurement [m²];
• A_i+1—cumulative kerf area up to i + 1 tip radius measurement [m²].

For the purpose of this study, we introduced a dimensionless factor called the cutter dulling coefficient Δρ to evaluate and compare the effects of different feed forces and initial chain tensions on the rate of increase in the cutter tip radius. The coefficient is defined as the tangent of the angle of the trend line with respect to the x-axis. The numerical value of this coefficient corresponds to the trend line slope. The greater the coefficient, the higher the rate of increase in the cutter tip radius. The cutter dulling coefficient may be calculated from the following formula:

Δρ = tgρ

(6)

where ρ—slope of the tip radius trend.

In the case of tensioned saws operating with a feed force of 15 N, 2853 cuts and 936 cutting edge radius measurements were made. In the case of loose saws operating with a feed force of 35 N, 3786 cuts and 1170 cutting edge radius measurements were made. In the case of tensioned saws operating with a feed force of 35 N, 4368 cuts and 1326 cutting edge radius measurements were made. In the case of tensioned saws operating with a feed force of 70 N, 17,874 cuts and 4836 cutting edge radius measurements were made. In total, 28,881 cuts and 8268 radius measurements were made during the entire measurements. Such a large number of measurements was very time-consuming. This is the reason why for now we have limited ourselves to only one type of cutting system and one type of wood. We plan to continue measurements using the described method for other cases.

Microsoft Excel was used to process the data, make calculations, and prepare some of the presented diagrams. The results were further analyzed using Statistica 13.3 software to compute means and standard deviations as well as other statistical parameters. The normality of the distribution and homogeneity of variances of the compared measurement series were checked prior to conducting Student’s t-tests for the significance of differences between independent groups. Welch’s test was used when variances were unequal [35,36]. The significance level was set at p = 0.05. Relationships between kerf area, tip radius, and cutting efficiency were analyzed by calculating regression equations and estimating Pearson’s linear correlation coefficients [37].

3. Results

In reporting the obtained results, we use abbreviations to label tables and figures. Numbers denote feed force values in newtons [N], the letter T denotes a tight chain and S denotes a slack chain. For instance, 70T means a tight chain operating at a feed force of 70 N, while 35 S means a slack chain operating at a feed force of 35 N.

Figure 5 shows the relationships between cutting efficiency and kerf area. Cutting efficiency per unit of kerf area was found to decrease with increasing kerf area in all the studied cases.

In all cases, chains ceased cutting after reaching a certain limiting cutter wear value. It was noted that the higher the feed force, the higher the cutting efficiency. Furthermore, at greater feed forces and higher chain tension, the chain could cut wood for a longer time. A tight chain operating at a feed force of 15 N ceased working after reaching a cumulative kerf area of 18.95 m². Slack and tight chains were compared at a feed force of 35 N, with a tight chain being more efficient (29.98 m² vs. 25.48 m² of kerf area). At a feed force of 70 N, a cumulative kerf area of 119.56 m² was obtained. The relationship between cutting efficiency and kerf area may be described with a linear equation:

η = a·A + b;

(7)

where

• η—cutting efficiency [cm²·s⁻¹];
• A—kerf area [m²].

Equation parameters are given in

Table 1. Parameters of the linear equations from Figure 4 and linear correlation coefficients.

Case	a	b	r
15T	−2.3612	71.8255	−0.9389
35T	−1.8908	117.4578	−0.9635
35S	−2.8425	115.1432	−0.9928
70T	−0.8058	117.9323	−0.9793

.

The high correlation coefficients indicate a good fit of the linear equations to the data.

Figure 6 shows relationships between cutter dulling and kerf area for feed forces of 15 N and 35 N. The relationships can be expressed in the form of the following linear equations:

ρ_15T = 1.4016·A + 10.9835  r = 0.98

(8)

ρ_35T = 1.0255·A + 3.377  r = 0.99

(9)

ρ_35S = 1.1253·A + 9.6318  r = 0.99

(10)

where

• ρ_ij—tip radius for cutting at a feed force i and chain tension j [μm];
• A—kerf area [m²];
• r—correlation coefficient.

Figure 6. Relationship between cutter tip radius and kerf area for feed forces of 15 N and 35 N.

Figure 6. Relationship between cutter tip radius and kerf area for feed forces of 15 N and 35 N.

The high correlation coefficients indicate a good fit of the linear equations.

The pattern of cutter dulling was markedly different at a feed force of 70 N. While initially it was approximately linear, after reaching a cumulative kerf area of approx. 40 m², the rate of increase in tip radius dulling substantially slowed down (Figure 7). Throughout nearly the entire test, the cutter wear rate at a feed force of 70 N was significantly lower than at the other studied feed forces.

In this case, the cutters reached the third phase of dulling. These changes cannot be described adequately by a linear equation; the best-fitting function was logarithmic:

ρ_70N = 20.3975·log₁₀ (A) + 3.4909

(11)

where

• ρ_70N—cutter tip radius during cutting with a tight chain at a feed force of 70 N [μm];
• A—kerf area [m²].

After sharpening, the mean cutter tip radius in the test chains ranged from 8.24 μm to 8.31 μm, with the differences not being statistically significant.

In the case of cutting at a feed force of 15 N, the mean cutter tip radius increased by 28.01 µm, from 8.25 µm to 36.26 µm, on reaching a cumulative kerf area of 18.95 m². The standard deviation of the cutter tip radius at test conclusion was 0.48 μm. When using a slack chain at a feed force of 35 N, the mean tip angle increased by 29.2 µm, from 8.25 µm to 37.45 µm (with a standard deviation of 0.33 µm) on reaching a kerf area of 25.48 m². In the process of cutting with a tight chain at a feed force of 35 N, the mean radius increased by 30.22 µm, from 8.24 µm to 38.46 µm (with a standard deviation of 0.68 μm), on reaching a kerf area of 28.98 m². The largest kerf area, 119.56 m², was produced with a tight chain at a feed force of 70 N. In that case, the mean tip radius increased by 37.88 µm, from 8.31 µm to 46.19 µm, with a standard deviation of 0.41 μm. When operated at lower feed forces, the chainsaw ceased cutting at lower tip radius values due to the lower pressure exerted on the cutters.

Table 2. Mean cutter tip radii at which the chainsaw ceased cutting and p-values for differences between the variants.

Variants		Mean Radius for Variant I on Test Conclusion [µm]	Mean Radius for Variant II on Test Conclusion [µm]	p-Value
I	II
15T	35T	36.27	38.46	0.00
15T	35S	36.27	37.45	0.00
15T	70T	36.27	46.20	0.00
35T	35S	38.46	37.45	0.00
35T	70T	38.46	46.20	0.00
35S	70T	37.45	46.20	0.00

presents p-values for differences between the mean cutter tip radii at which the chainsaw ceased cutting for all the studied cases. At a feed force of 35 N, the difference between tight and slack chains was small but significant. Compared with a slack chain, a tight chain could continue operation at a cutter tip radius greater by 1.01 µm (p = 0.00). At a feed force of 15 N, the mean cutter tip radius at which the chainsaw ceased cutting was smaller by 2.2 µm than with a tight chain and by 1.19 µm than with a slack chain at 35 N. These differences are significant at p = 0.000.

The p-values for differences between all the studied cases are given in

Table 3. List of p-values for differences in cutter tip radius between all the studied cases.

Kerf Area [m2]	15 N vs. 35 N	15 N vs. 35 L	15 N vs. 70 N	35 N vs. 35 L	35N vs. 70 N	35 L vs. 70 N
0.98	0	0	0	0.015	0.001	0.123
1.96	0	0	0	0.027	0.004	0
3.92	0	0	0	0.096	0	0
5.88	0	0	0	0	0.568	0
7.84	0	0	0	0	0	0.518
9.8	0	0	0	0	0.414	0
11.76	0	0	0	0	0	0.002
13.72	0	0	0	0	0.159	0
15.68	0	0	0	0	0	0
17.64	0	0	0	0	0	0
19.6	-	-	-	0	0	0
21.56	-	-	-	0	0	0
23.52	-	-	-	0	0	0
25.48	-	-	-	0	0	0
27.44	-	-	-	-	0	-

. The results revealed significant differences (p = 0.000) for all comparisons between a tight chain operating at a feed force of 15 N and chains operating at a feed force of 35 N (whether tight or slack) and 70 N. On completing a kerf area of 3.92 m², no statistically significant differences were observed between a slack and tight chain operating at a feed force of 35 N (p = 0.096). A comparison of tight chains operating at feed forces of 35 N and 70 N did not indicate significant differences on reaching kerf areas of 5.88 m² (p = 0.568), 9.8 m² (p = 0.414), or 13.72 m² (p = 0.159). In the other cases, the differences between these chains were statistically significant. When comparing a slack chain operating at a feed force of 35 N and a tight chain operating at a feed force of 70 N, no significant differences were noted in two cases, i.e., on completing a kerf area of 0.98 m² (p = 0.123) and 7.84 m² (p = 0.518). In the remaining cases, differences in the performance of these chains were statistically significant. On completing a kerf area of 29.98 m², only the chain operating at a feed force of 70 N could continue cutting. Between that time point and reaching a kerf area of 119.56 m², the mean cutter tip radius increased from 33.13 μm to 46.19 μm.

The effects of the studied factors on the cutter wear rate were evaluated by means of the cutter dulling coefficient reflecting the rate of increase in the cutter tip radius. The coefficient was the highest (1.4016) for chains operating at a feed force of 15 N. In the case of the feed force of 35 N, the cutter dulling coefficient amounted to 1.0255 for a tight chain and 1.1253 for a slack chain. This means that a decreased initial chain tension led to a faster cutter wear rate. In the case of a tight chain operating at a feed force of 70 N, the relationship between the tip radius and kerf area was no longer linear as the wear rate per unit of kerf area diminished with the cumulative kerf area. Nevertheless, one can distinguish four characteristic phases in the cutter wear profile: Δρ = 1.8239 for a kerf area between 0 m² and 10 m², Δρ = 0.4850 between 10 m² and 20 m², Δρ = 0.2613 between 20 m² and 40 m², and Δρ = 0.906 above 40 m². This pattern is shown in Figure 8.

After sharpening, standard deviations (SDs) of the cutter tip radius were very similar for all the studied chains. The SD for a chain operating at a feed force of 15 N ranged from 0.48 µm (at a kerf area of 18.95 m²) to 1.25 µm (at 5.88 m²). When operating a tight chain at 35 N, the SD ranged from 0.38 µm (at a kerf area of 21.56 m²) to 1.31 µm (at 13.72 m²). In turn, when operating a slack chain at 35 N, the SD ranged from 0.33 µm (at a kerf area of 25.48 m²) to 0.81 µm (at 11.76 m²). The SD of the cutter tip radius for a feed force of 70 N ranged from 0.26 µm (at a kerf area of 88.2 m²) to 1.17 µm (at 3.92 m²). In none of the studied cases was SD dependent on the kerf area.

Figure 9 shows a comparison of the relationship between instantaneous cutter wear values and the area of the completed kerf.

The curves presented in Figure 8 may be described with the following general equation:

I = a·log₁₀(A)·b [μm]

(12)

The values of coefficients a and b for the various cases are given in

Table 4. The values of coefficients for equations describing changes in the relationship between instantaneous wear intensity and cumulative kerf area.

Feed Force and Initial Tension	a	b
15 N tight	−2.048	3.3629
35 N tight	−0.7251	1.5377
35 N slack	−1.0796	2.3526
70 N tight	−1.2191	2.3863

.

Instantaneous values of cutter wear ranged between 0.19 µm·m⁻² and 4.65 µm·m⁻² for a tight chain at a feed force of 15 N, between 0.44 µm·m⁻² and 2.24 µm·m⁻² for a tight chain at 35 N, between 0.79 µm·m⁻² and 2.65 µm·m⁻² for a slack chain at 35 N, and between 0.02 and 3.02 µm·m⁻² for a tight chain at 70 N.

The greatest instantaneous cutter wear values were found for chains operating at a feed force of 15 N, with the wear rate being the highest at the beginning of chainsaw operation in all tested cases. These results are consistent with the Lorenz wear curve, according to which the highest wear occurs at the initiation of tool operation.

Furthermore, we calculated mean cutter wear rates for the entire time of chainsaw operation. The highest mean cutter wear rate, 1.48 µm·m⁻², was found for the chain operating at a feed force of 15 N. The mean cutter wear rates for slack and tight chains operating at a feed force of 35 N were 1.15 µm·m⁻² and 1.04 µm·m⁻², respectively, with the difference being rather small. By far the lowest mean cutter wear rate (0.32 µm·m⁻²) was obtained for the tight chain operating at a feed force of 70 N. The results show that at a given feed force, an increased initial tension leads to a lower cutter wear rate. Similarly, the cutter wear rate was found to be reduced with the application of higher feed forces.

4. Discussion

The tests presented in this study were conducted under diverse operational conditions, that is, different feed forces and different initial chain tensions. Typically, both of these parameters depend on the chainsaw operator [28]. Cutter wear was described by means of the tip radius.

The lowest cutting efficiency was recorded for a feed force of 15 N. Significantly greater efficiency was obtained at higher feed forces. Other researchers have also reported that the cutting efficiency per unit of kerf area increases with feed force [32,38]. In case of petrol and electric chainsaws, working with a loose chainsaw worsens the cutting results [23].

The current results have also corroborated the findings of Górski [27] and Maciak and Kubuśka [28] to the effect that a slack chain reduces cutting efficiency as compared to a properly tensioned one. During our long-lasting experiments, cutting efficiency over time decreased at the fastest rate for a slack chain. At the other extreme, the chainsaw operating at a feed force of 70 maintained high cutting efficiency for the longest time.

Previous research revealed that a lower feed force leads to greater cutter deflection from the kerf plane [22], which is also the case for lower initial chain tensions. Therefore, it could be hypothesized that a higher cutter wear rate is attributable to the greater possibility of cutter deflection in the process of cutting.

In 2013, Maciak reported the maximum cutting efficiency to be 120 cm²·s⁻¹. In this study, the maximum cutting efficiency for a chain with newly sharpened cutters was similar, at 123 cm²·s⁻¹ (at a feed force of 70 N). The difference may result from the wood samples used in the studies.

Cutter wear considerably affects chainsaw cutting efficiency. The greater the cutter tip radius, the lower the cutting efficiency per unit of kerf area. This relationship was described by Bieńkowski [31], Maciak [19], and Górski [27]. In 2005, Górski [39] reported that cutter wear leads to a situation in which wood stops being cut and starts being ripped, which is less efficient. Furthermore, he found that an increase in cutter tip radius from 10 µm to 30–35 µm reduced cutting efficiency by 30%.

In the present study, an increase in the tip radius from 10 μm to 35 μm caused a reduction in cutting efficiency by 21–57%, depending on the applied feed force and initial chain tension. The greatest reduction was observed for a feed force of 15 N and amounted to 42.5% (from 74.82 cm²·s⁻¹ to 43.04 cm²·s⁻¹). The reduction for a feed force 35 N was similar, amounting to 43% (from 115.1 cm²·s⁻¹ to 65.6 cm²·s⁻¹). The lowest percentage reductions were found for tight chains operating at feed forces of 35 N and 70 N, i.e., 21.15% (from 118.21 cm²·s⁻¹ to 93.21 cm²·s⁻¹) and 23% (from 116.49 cm²·s⁻¹ to 89.71 cm²·s⁻¹), respectively.

Taking into account the increasing cutter tip radius, the reduction in cutting efficiency proceeds faster for slack chains. This is consistent with Maciak and Kubuśka [28], who found that at higher feed forces, cutting efficiency decreases more slowly despite an increasing tip radius. We also confirmed the findings of Lewandowski [40], who reported that inappropriate chain tension affects the cutter wear rate in chainsaws.

The loss of ability to cut wood with a chainsaw results from a reduction in the linear dimensions of the cutting elements, which, in this case, are cutters. During chainsaw operation, the cutter tip radius increases [25]. The greater the cumulative area of the executed kerf, the more the cutter properties are affected. After the cutter tip radius reaches a certain limit value, further cutting becomes impossible. In the current study, the limit tip radius for chains operating at 15 N was 36.26 µm, which was reached at a kerf area of 18.95 m². The limit tip radius preventing further cutting for tight chains operating at a feed force of 35 N was 38.46 µm (reached at 28.98 m²), while that for slack chains operating at the same feed force was 37.45 µm. Tight chains operating at a feed force of 70 N maintained their cutting ability up to a tip radius of 46.19 µm, which was reached at a kerf area of 119.56 m².

Maciak, Górska, and Zach [29] found that variation in the wear of individual chainsaw cutters increased with the area of the completed kerf, which is not consistent with the present findings. In the present study, differences between cutters in terms of wear depended on operating conditions. Out of all the studied cases, the lowest wear variation was noted in chains with low initial tension subjected to a feed force of 35 N. In line with the proposition of Maciak and Kubuśka [28], it was confirmed that wear variation is dependent on the initial chain tension; it is also affected by the applied feed force. The largest differences between individual cutters were observed for the chain operating at a feed force of 15 N. Previous studies on the process of cutter dulling were conducted up to the second phase of the Lorenz wear curve [28,29], and so the third phase had not been elucidated.

Cutter dulling may proceed differently in electric chainsaws, which operate quite differently from gasoline-powered ones [18]. However, to investigate that process it would be necessary to conduct similar studies involving an electric chainsaw. The practical application of the obtained results is the recommendation for operators to frequently check the saw pre-tension and work with the highest possible feed force.

5. Conclusions

1.

The cutter wear rate is inversely proportional to the applied initial chain tension and feed force. In the reported study, the highest mean cutter wear rate (1.48 µm·m⁻²) was found for the tight chain operating at a feed force of 15 N. An increase in feed force to 70 N reduced this value to 0.32 µm·m⁻². Different chain tensions were tested at a feed force of 35 N. In that case, a reduction in initial chain tension increased the mean wear rate from 1.04 µm·m⁻² to 1.15 µm·m⁻².

2.

For the purpose of the present study, we also defined a dimensionless cutter dulling coefficient. Its value was the highest (1.4016) in the case of the chainsaw operating at the lowest investigated feed force and decreased with the applied feed force. At 35 N, it ranged from 1.0255 to 1.1253, depending on the initial chain tension (with the higher value corresponding to a slack chain). At a feed force of 70 N, the cutter dulling coefficient ranged from 1.8239 to 0.0906, as it decreased with the area of the completed kerf.

3.

Cutting efficiency per unit of kerf area depended significantly on the applied initial chain tension and feed force. After sharpening, the highest cutting efficiency, 119.88 cm²·s⁻¹, was obtained for a tight chain operating at a feed force of 70 N. A decrease in the feed force to 15 N reduced cutting efficiency to 72.8 cm²·s⁻¹. In the case of a feed force of 35 N, the application of a lower initial tension led to a reduction in cutting efficiency from 116.4 cm²·s⁻¹ to 113.47 cm²·s⁻¹.

4.

The higher the applied feed force, the greater the area of the kerf that can be made with a given chainsaw. For a feed force of 15 N, the chainsaw ceased cutting after completing a kerf area of 18.95 m², with the cutter tip radius reaching 36.26 μm. At a feed force of 70 N, the chainsaw was able to complete a kerf area of 119.56 m² with the cutter tip radius reaching 46.19 μm. However, it should be noted that a higher feed force entails greater effort on the part of the chainsaw operator.