STATEMENT OF THE PROBLEM

The final stage in the overhaul of a unit of sophisticated technical objects is engine break-in or breaking in, which facilitates checking the proper operation and repair quality of units. During the break-in process, running-in ensures grinding of friction couplings and preparing them for service loadings, prevents severe wear during operation, and increases the post-repair service life of the units.

One of the complex and critical units, other than the internal combustion engine (ICE), is the group of transmission units: driving axles, transmissions, and industrial gearboxes. According to the normative technical documentation, plants and repair enterprises are obliged to subject these units to breaking-in and running-in procedures and acceptance tests. However, running-in should usually only be performed within 0.5–1.5 h when breaking in, and thus most of the running-in is transferred to the initial operating period. When the parts are incompletely seated, the forced operation of units and assemblies under operational loads can lead to their increased wear; perhaps seizure and scuffing can occur, and eventually the period until an overhaul decreases.

Various studies in this field conducted in many countries have focused on the breaking-in and test modes, processes occurring on friction surfaces during different periods of running-in, and methods of controlling these processes. In particular, there are many studies related to applied running-in, grinding compounds, and running-in oils [1–4]. The bulk of these studies were carried out to break in the ICE, and studies focusing on other components and assemblies are fewer in number [5, 6]. However, the objective of all studies was to intensify the mechanical, physical, and chemical processes on the friction surfaces with a minimum of running-in wear.

In order to accelerate the running-in process, specific breaking-in oils are used or tribotechnical compounds (additives) having various mechanisms of action are added in commercial oils. In our opinion, nanostructured aluminum oxyhydroxide (boehmite) can be used to break in engines and transmission units. Like kaolin used for breaking-in, boehmite as a soft abrasive (Moh’s hardness is about 3.5) can be an advanced material.

The purpose of this study is to assess the effective use of nanostructured boehmite and other materials for breaking in engines and transmission units.

MATERIALS AND METHODS

A nanostructured boehmite powder produced by hydrothermal synthesis from industrial powders of AD-4 aluminum is used [7]. The boehmite obtained using this method is notably characterized by high homogeneity and compositional and structural stability of different batches. The properties of the powder used are as follows: (1) mineralogical hardness 3.5; (2) loss on ignition 15%; (3) bulk density 3.07 g/cm3; (4) crystal size 70 nm; and (5) specific surface area 70 m2/g.

In order to prepare a tribocompound for engine break-in, the boehmite powder was malaxed with the addition of oleic acid and was mixed with lubricating mineral oil; the suspension was ultrasonicated and introduced into the D-243 diesel engine crankcase containing 15 L of M-10G2k engine oil, based on a content of the boehmite powder and the surfactant in the crankcase oil of 0.76 wt % each.

The diesel engine break-in with or without the tribocompound was performed in the same manner according to the standard methods with the KI-3540-GOSNITI advanced roller tester. The diesel engine was assembled from new parts of the cylinder-piston group and new connecting rod pin bushings and connecting rod bearing shells for each break-in procedure.

The surface roughness of the diesel engine bearing shells and piston rings was measured with a Surtronic 3P profilometer (Taylor Hobson, United Kingdom) before and after running-in. The wear of engine couplings at various running-in phases was estimated using the weight percent of solids in the engine oil. Samples of 300 g were taken 3 min after the start of cold running-in, after cold running-in, after hot running-in, and after wearing test. The data collection system allowed the registration of the compression in time, the behavior of the engine shaft speed, the shaft torque, the lube oil pressure, the oil and water temperature, and the flow rate of crankcase gases and fuel.

In order to prepare a tribocompound for running in friction couplings of the drilling rig gearbox built on a GAZ truck chassis, kaolin from the Prosyanovskoe deposit according to technical conditions TU 421-533-2001, TMK-28 talc from the Shabrovskoe deposit, and copper oleate (made by combining oleic acid with copper oxychloride) were used in addition to nanostructured boehmite. The compounding materials were dried. The compound was prepared by grinding in a ball mill for two hours. Next, the powder was stirred with waste mineral oil for diesel engines, which was taken as the base oil (BO), in a Werner mixer and was ultrasonicated at a frequency of 35 kHz for 20–25 min until a homogeneous suspension of the additive was formed. According to the standard breaking-in methods, 10 L of TM-3 transmission oil were poured into the gearbox.

Before the break-in process, the gearbox filled with the tribocompound was introduced into the gearbox housing filled with 10 L of waste mineral diesel engine oil based on the content of kaolin, talc, boehmite, and SAA in the gearbox running-in oil within the following limits, wt %: 0.3–0.6, 0.2–0.4, 0.1–0.3, and 0.06–0.08, respectively. Then, the gearbox breaking-in process was performed according to the standard methods.

The drive of the broken-in drilling rig gearbox was provided using an induction motor producing 7.5 kW with a rotational frequency of 750 min–1. In all cases, completion of the running-in procedure was determined from the abatement of the gear train noise and by inspection of the quality of the tooth contact formed upon opening the gearbox side covers.

Some new compounds were tested for comparative studies: the remetallizing compound (RC) Resurs and the mineral friction modifier (MFM) Forsan. They are also recommended by manufacturers for breaking in units.

The laboratory tests of the compounds listed are conducted on a 2070 SMT-1 frictional machine according to the roller–self-aligning brake shoe pattern. The materials are accordingly steel 40X and steel 45 with a hardness of 50–55 HRC, which are ground to Ra = 0.32–0.63 μm. The test mode is as follows. The roller rotational speed is 400 min–1. With a rise in the frictional torque, the loading increases gradually in five-minute intervals during the first fifteen minutes of testing: 1000, 1250, and 1500 N. The load remains constant at 1500 N, and stabilization of frictional torque takes place in the second fifteen minutes. The bottom part of the roller with 1/3 of its volume is immersed in the lubricating compound. During testing, the frictional torque is recorded and the oil temperature is measured. The test length for each compound is 30 min (it is adapted to the technical requirements for breaking-in the transmission units). The experiments are performed in triplicate.

RESULTS AND DISCUSSION

D-243 diesel engine tests show that the addition of boehmite contributes to accelerating the running-in of the liner–ring contacts (Fig. 1) [8]. Thus, compression stabilization takes place at 3.0 MPa after 30 min, but it begins only in the 80th minute without the addition.

Fig. 1.
figure 1

Compression dynamics during the running-in process of the engine [8]: (1) with the addition of boehmite and (2) without the addition of boehmite.

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After 60 min of running-in, the use of boehmite reduces the wear of the first compression ring by half, the flow rate of crankcase gases by a factor of 1.6, and the oil temperature by 15–20°C. The maximum brake power of the diesel engine, which was run using oil with the addition of boehmite for 90 min, is 52.5 kW. The specific fuel consumption is 257 g/hp. h., which is close to the diesel engine performance after 50 and 100-hour operation. Moreover, the breaking-in with commercial oil during the same 90 minutes leads to the fact that the maximum brake power is about 45 kW, and the fuel consumption is about 270 g/hp. h. The wear of the first compression ring is reduced by a factor of 2.5; the flow rate of crankcase gases, by 12.7%; and the oil consumption, by 27% after 120-minute operation of the diesel engine.

In addition, the roughness of the connecting rod bearing shells and main bearing shells increases because of their softness during the running-in process with the use of boehmite, but the roughness of piston rings decreases (Fig. 2). After the running-in process, the roughness obtained in both cases without the addition is larger than that with the addition: 1.87 μm for connecting rod bearing shells with a standard running-in process, but 1.35 μm (i.e., 1.38 times less) with the addition at 0.84 and 0.51 μm (i.e., 1.7 times less) for piston rings, respectively.

Fig. 2.
figure 2

Change in the surface roughness of the parts with different running-in procedures: (1) before running-in; (2) after running-in without the addition; (3) after running-in with the addition of boehmite.

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The roughness during the running-in process of connecting rod-bearing shells and piston rings changes differently because this process not only decreases the roughness but also provides its optimum value in specific contacts [7, 8]. The surfaces are reformed, and the change in their physical and mechanical properties reaches equilibrium values. This leads to a reduction in the wear rate, friction coefficient, and heat liberation.

The addition of boehmite in the D-243 diesel engine oil generally reduces the time it takes to complete of the running-in procedure; increases the wear resistance at the contact areas by 22%; and decreases the running-in wear by 5.8%, the wear of the first compression ring by a factor of 2.5, and the flow rate of crankcase gases by 12.7%.

Furthermore, it should be noted that further comparison tests on the breaking-in materials currently used are still required to break in diesel engines for tractors, especially imported engines, in the repair production activities with a view to making formal recommendations for the use of nanostructured boehmite AlO(OH) produced by GOSNITI.

Taking into account the tribological aspects, we can summarize the role of boehmite in a tribo-environment including standard lubricants as follows: (1) abrasive wear and grinding of raised roughness zones between friction pairs, improvement of surface roughness, and reduction of mechanical friction; (2) removal of deposits, oxide films, and defect structures from friction surfaces, which provides access of the materials in the tribo-environment to the juvenile metal surfaces and accelerates the formation of antifriction coatings; (3) further adsorption of resinous materials on the surfaces of boehmite particles, which leads to the formation of particles separating the parts (“third phase” in the tribo-pair) and reducing friction.

Along with smooth abrasive friction, the engineered additive provides adsorption reduction of strength and plasticization of sliding surfaces by surfactant (the effect discovered by Academician P.A. Rehbinder).

The results show that the use of nanostructured boehmite in the breaking-in compound for engine running-in increases the load capacity of the friction pair by 16–17% and reduces the friction coefficient by 10–30% and the thermal factor of friction units by 6–15%. Recommendations for running in ICE with nanostructured boehmite have also been developed.

The nanostructured boehmite grinding compound works well in the conditions of running in the ICE parts with a hardness of 52–56 HRC and a roughness level of 10–11. The gear teeth in the transmissions (18KhGT steels) and rotating bearings (ShKh-15 steels) have a hardness of 179–207 MPa or 61–63 HRC with a roughness level of 7–8 with a roughness of Rz 3.2–6.0 µm. The nanodispersity of boehmite particles (crystal size is less than 100 nm, aggregate size is less than 1 μm) leads to their pressing in deep valleys of undulations of the rough gear teeth surfaces, and therefore, the abrasiveness of boehmite powder in the transmission units is weak. Additionally, boehmite can decompose, releasing water and forming anhydrous alumina oxides in the conditions of friction and high temperature at the points of contact. Nevertheless, self-grinding of the virgin crystals and aggregates occurs and brings about a decrease in their size [9]. The temperature relevant to the minimum particle formation is about 500°C.

On this basis, an additive containing nanostructured boehmite, surfactant, talc, and kaolin was developed to run in the transmission units. During preliminary studies, the region of optimum quantity of the compounding material of the complex additive is determined, and the response functions in the tests are as follows: sink rate (stabilization) of frictional torque, temperature, running-in surface area (proportion in the geometric area of contact) under equal conditions of load tests, slip velocity, and test time.

The test results of the experimental compound compared to the test results of the available grinding compounds show the acceleration of running-in, reduction of the lubricant temperature, and the increase in the running-in surface area of the parts (Table 1).

Table 1. Indicators for the running-in process of the test samples.
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The field tests of the engineered running-in additive are conducted using the gearbox of the mobile drilling rig. A design feature is the presence of highly rigid gear wheels and support bearings and the absence of nonferrous metals and soft alloys. The addition of an increased amount (up to two times compared to the additive for the ICE breaking-in) of abrasive materials with moderate hardness (kaolin, talc, and boehmite) and surfactant, which has disaggregating properties, increases the sedimentation stability of the compound and provides the Rehbinder effect, results in the running-in acceleration.

The breaking-in period of the gearbox is reduced by 2.5 times compared to the break-in procedure conducted using the TM-3 standard transmission oil. The additive allows for effective running-in of the parts and the formation of a complete contact patch engaged with the gear teeth, spline shafts, and bearings. Next, the noise and heating of the housing decrease significantly after draining the breaking-in oil and filling with the standard oil, which implies longer-lasting gearbox life.

The beneficial effect of the addition of kaolin and talc is also probably due to their relevant physical and mechanical properties for the running-in of the gears and particle sizes (units of micrometers) comparable with the dimensions of the surface irregularities of the gearbox parts. They are thermally stabler and do not decompose at temperatures of 400–500°C like boehmite with the formation of smaller particles. The talc and kaolin particles wear the microroughnesses existing on sliding surfaces. They increase contact patch area, reduce specific loads, and therefore prevent scuffing. When falling between moving parts and leveling surfaces, they can adsorb resinous and oxygenated products of the environment. Kaolin with larger particles actively participates in the abrasive grinding-in of the gear teeth surfaces, opens their juvenile surfaces, and contributes to a more active action of surfactant according to the Rehbinder effect. Talc as a plastic disperse powder is an intermediate product between the kaolin particles and together with boehmite contributes to the more homogeneous distribution of kaolin particles in the grounding compound and in the running-in oil.

CONCLUSIONS

The addition of boehmite to oils accelerates and improves the quality of the ICE running-in up to two times and contributes to the fast achievement of the rated power and fuel consumption. The developed running-in compound for running in the gearboxes containing kaolin, talc, boehmite, and SAA reduces the breaking-in period by 2.5 times. The running-in efficiency depends not only on the physical and mechanical properties of the additive components but also on the ratio of the compounding material particle size to the surface roughness of the parts that are subjected to grinding.