MEASURING OPERATING PARAMETERS OF AGRICULTURAL MACHINES WORKING IN A HIGHLY POLLUTED ENVIRONMENT

Andrzej Norbert Wieczorek
Institut of Mining Mechanization, Silesian Technical University, Gliwice, Poland

ABSTRACT

The study presents the results of experimental research in the scope of modifying the operating conditions of friction pairs with the use of a metal conditioner. As a part of the study, experimental research at AMSLER test rig was carried out in order to examine the loss of mass in samples lubricated with clean and contaminated gear oil. Apart from the determination of the mass loss, measurements of roughness as well as microscopic examinations of the surfaces of mating samples were performed. A significant improvement in the tribological properties of friction pairs was demonstrated, which was associated with the use of the examined metal conditioner.

Key words: durability, wear, contamination, lubrication.

INTRODUCTION

Durability is one of the main criteria in evaluation of friction pairs [5] and significantly affects the reliability of machines and equipment. This feature is inseparably associated with operating conditions of machines and equipment. For example, the operation of machines used, among other things, in agriculture and forestry is characterized by:

• variable loads with a considerable share of dynamic and percussive excessive loads,
• significant ambient humidity resulting in increased risk of corrosion,
• aggressive action of silica penetrating to the friction pairs of machines during operation.

In order to achieve a high durability of friction pairs working in such an extreme environment, appropriate operating conditions should be ensured – especially in respect of proper lubrication, Proper lubrication at the stage of operation should be ensured first of all by removal of any contamination from the lubricating medium.

During the period of running in and operation of a machine, the lubricant, as a structural element, undergoes quality changes associated with ageing processes, which reduce its lubricating properties. In order to counteract such processes, oil can be filtered, refilled or replaced.

Filtration systems are used in many industrial sectors, but filters used in them usually capture only larger contaminants. A common practice is refilling with clean oil, which leads only to diluting such contaminants. User think that complete oil change will solve this problem, but some dust settles in the drip pan and may again damage surfaces of elements, e.g. gears, bearings and seals. On the other hand, it often happens that an existing damage of seals and the venting system will cause recontamination of the lubrication system. It can even be assumed that normal operation inevitably leads in practice to gradual degradation of the oil [6], and therefore it is very important to reduce the impact of contaminants on the wear of components in machines and equipment.

In the operation process, oils are usually contaminated with solid particles. Contamination of oil with foreign solid particles (solid contaminants) can be divided into [7]:

• Internal contamination: metal particles produced during running-in and normal operation, elastomer particles coming from seals and filter media, oil degradation products.
• External contamination: primarily dust from the operating environment, organic particles, and solid contaminants brought in during oil refill.

The presence of solid contaminants in the operating fluids will intensify destructive processes [6] such as:

• abrasive wear,
• erosion
• fatigue wear.

Figure 1 shows (data based on studies conducted by FAG company) the impact of the contamination of oil with various hard particles on a decrease in the service life L_e/L₁₀ of the 7205 B angular ball bearing. It is easy to notice in this figure that the contamination of oil with sand grains causes a decrease in the nominal service life of the bearing by 98%, and in the case of corundum grains – by 99%.

As it is known from experience in many industrial sectors, including agriculture, the problem of destructive impact of contaminants present in lubricants brings about significant costs resulting from accelerated degradation of machines and equipment. A question should be asked here, whether the lubricants that have been used so far are appropriate for application in the presence of solid pollutants? The development of lubrication engineering indicates that there is a need for using agents that decrease the energy generated during contact of vertices resulting from irregularity of mating surfaces.

Agents that enable implementation of this postulate are currently available and used, however results of tests of their anti-wear properties in the presence of contaminants are not included in the scientific literature concerning this subject [1–10] From among the agents increasing the wear resistance of lubricated friction pairs of machines and equipment, metal conditioners are particularly suitable for this purpose. According to S. Laber [4], metal conditioners are chemical compounds or mixtures of chemical compounds prepared for a specific purpose, e.g. to improve operating conditions of friction pairs by increasing the durability of the boundary layer. Metal conditioners are introduced to friction pairs by oils. As a result of physical adsorption and chemisorption they modify the surface layer to form a modified boundary layer on friction surfaces, which is more resistant to dynamic and temperature loads.

In order to determine possibilities of reducing the negative impacts of lubricant contamination, this study presents the results of research on properties of the selected metal conditioner in combination with the TRANSOL VG 220 mineral gear oil. The selected metal conditioner consists of compounds from a group of primary and secondary zinc alkyl dithiophosphates, aryl and alkylphenol aromatic amines, organic sulphides, superalkaline magnesium sulphonates, alkene-succinic acids, sulphurized fatty acids, polymethylsiloxanes, alkylmethacrylates, ethylene-propylene copolymers, mixtures of synthetic polyol esters derived from polyhydric alcohols, corrosion inhibitors, oxidation inhibitors (the brand name of the metal conditioner is MOTOR-LIFE PROFESSIONAL). It is characterized by a high molecular weight, high chemical and thermal stability, with the kinematic viscosity of 5.63 mm²/s at 100°C.

MATERIAL AND METHODS

The experiment enabling a quantitative determination of the wear of mating parts has been carried out on the AMSLER oil properties testing rig. The tested friction pair consisted of two cylindrical rings contacting with other on cylindrical surfaces, loaded with a constant radial force and rotating in such a way that a backward motion of the samples was generated. The roughness of sample surfaces prior to the wear tests, described by the R_a parameter, was 0.9962 µm (this parameter was determined in accordance with PN-EN ISO 4287 and PN-ISO 4288 with the use the Taylor Hobson FORM TALYSURF 120 device).

The test conditions on the friction machine are shown in Table 1.

Measurements of the loss of mass were performed after: 5, 20, 50 and 80 minutes of test samples mating. Before stating the wear test and after each friction path cycle (after thorough cleaning and drying), the mass of the sample was determined five times using an analytical balance with the measurement accuracy of ± 0.5 mg. Student's t-test was used for determining the uncertainty of the measurement. The determined test parameter adopted as the measure of wear was the loss of mass in both samples.

Before starting the wear tests, an adequate mixture was prepared basing on the variant of the tested combination of oil, metal conditioner and contaminant. This mixture was prepared in a brand new plastic container with a capacity of 5 litres. For this purpose, the assumed amount of oil, i.e. 2 litres, was dispensed with the use of a laboratory graduated cylinder with a capacity of 0.5 dm³. The accuracy of the capacity determination was ±1 mm³.

The impact of four variants of lubrication on the loss of mass was determined under this study. These variants are shown in Table 2.

TEST RESULTS. DISCUSSION OF THE RESULTS

Figure 2 shows the results of the wear tests in the form of courses of the mass losses in samples after 80 minutes as a function of the load – for all lubrication variants in question. When comparing Variant 1 with Variant 2 and Variant 3 with Variant 4 it can be easily seen that the addition of the metal conditioner both to the clean and contaminated oil led to decreasing the mass loss, which is used as a measure of wear of the test samples.

After the wear tests had been completed, macroscopic observation of the surface subjected to the tests was performed, which allowed noticing a significant difference in the appearance of these surfaces. In the case of lubrication with oil with addition of the metal conditioner, the surface was glossy, did not have any cavities, and had the appearance typical of a sample properly run in. In the second case, numerous cavities in the form of long, narrow and relatively deep hollows were observed. Observation of the surface lubricated with the contaminated oil brought similar results.

These findings were confirmed by microscopic observations. The surface of the sample lubricated with oil with addition of the metal conditioner is typical in the case where only abrasive wear occurs; in many places, traces of previous polishing were left. On the surface of the samples that were lubricated with oil only, there are visible, apart from abrasive wear, surface deformation zones, which are typical in the case of slip of mating parts, which in many cases turned into a more severe form: deep chipping. The shape of such chipping has a characteristic scaly form that suggests occurrence of pitting. The chipping was more intense in the case of the lubrication variant No. 4.

Results of surface roughness measurements show the differences between both lubrication variants in a quantitative form. A reduction in the surface roughness is clearly visible in the case of lubrication with oil with addition of the metal conditioner, both when using the clean oil (see Fig. 3) and the oil containing contaminants (see Fig. 4).

SUMMARY

The study presents the results of experimental research in the scope of modifying the operating conditions of friction pairs with the use of a metal conditioner. As a part of the study, experimental research at AMSLER test rig was carried out in order to examine the loss of mass in samples lubricated with clean and contaminated gear oil. Apart from the determination of the mass loss, measurements of roughness as well as microscopic examinations of the surfaces of mating samples were performed.

The studies clearly demonstrated a significant improvement in the tribological properties of friction pairs lubricated wit contaminated oil, which was associated with the use of the examined metal conditioner.

The studies not only confirmed the beneficial properties of metal conditioners – the results also allow stating that there is a need to use these conditioners in running-in processes. This will reduce the destructive impact of the contact of mating surface irregularities.

Accepted for print: 4.08.2014

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.