ISO 6281:2020 pdf download Plain bearings — Testing under conditions of hydrodynamic and mixed lubrication in test rigs
6 Test rigs
6.1 General recommendations It is often more practical and efficient to investigate the bearing in a test rig than in an actual application. The design of the bearing test rig should be such as to simulate as far as possible all the relevant characteristic parameters (e.g. geometric, dynamic, hydrodynamic, thermal, and thermodynamic) of the actual application. In addition, the following is recommended for the test rig.
a) A simple mechanical construction.
b) Simple dismantling and assembly procedures for the test objects; with well-defined positioning of the bearing and housing; preferably it should be possible to inspect the test bearing in situ. In addition, the test rig should be equipped with an emergency stop mechanism, both for safety reasons and to allow the inspection of the sliding surface before the onset of catastrophic damage.
c) A well-defined dimensions for the test bearing.
d) A high dimensional stability with little shaft deflection. The test rig should be as rigid as possible, with a high natural frequency. In special cases, however, it can be necessary to vary the dimensional stability or the shaft deflection in order to simulate the operating condition of the actual application.
e) An appropriate lubricant supply condition. When the lubricant flow within the bearing clearance needs to be simulated exactly, the circumferential and axial position of the lubricant supply in the test rig should be the same as in the actual application.
f) Well-defined and experimentally verifiable lubrication conditions.
6.2 Generic types of test rig Generic types of test rig for plain journal bearings are shown in Figure 2 and Figure 3. Figure 2 a) and Figure 2 b) depict the rotational motion of the journal, where a combination of both is also possible. In practice, many more patterns of journal motion other than rotation can occur, such as inclination, bending, axial, conical and their combinations. In addition, the bearing itself can rotate, oscillate or even move in space instead of, or together with, the journal, as with a crank-pin bearing. In any case, the relative motion of the journal to the bearing shall be known (measurable) exactly. However, constant rotational speed of journal and the parallel movement of journal to bearing are the simplest and most preferable for testing. Figure 3 shows patterns of the bearing load. In the case of statically loaded journal bearing [Figure 3 a)], the magnitude, F, and the direction, β, of the bearing load are constant. In a special case of dynamically loaded bearing, F is constant, but β increases or decreases with time [Figure 3 b)]. In the general case of dynamically loaded bearing [Figure 3 c)], both or at least one of F and β change(s) with time, while the remaining variable can be constant.
The change of form of F (also β) is then arbitrary, such as sinusoidal with or without constant offset, curving steeply up and downwards, as, for example, in engine bearing loading. With regard to the loading of the test bearing, it is often more practical to load the test bearing directly supported by the journal [Figure 4 a)], than to load the test bearing indirectly through the journal [Figure 4 b)]. For static loading, a dead weight system, with or without lever, or hydraulic or pneumatic actuation can be used. For dynamic loading, a rotating or vibrating mass system, with or without lever, an electromagnetic exciter, and hydraulic actuation, can be applied. Dynamic loading by means of a mass fixed to the journal seems to be simple, but the amplitude of the bearing load is then determined primarily by the rotational speed of the journal. Therefore, it is not easy to change the load amplitude independently of the rotational speed. Furthermore, the magnitude and direction of the bearing load shall be precisely measured, and it is important to let the journal move freely inside the bearing clearance without hindrance from the loading mechanism. Besides such bearing test rigs operating under hydrodynamic or mixed lubrication, as described above, many other kinds of test apparatus and test methods may be used to investigate the tribological or mechanical properties of bearing materials, including coefficient of friction, mechanical strength, hardness, elasticity, plasticity and bond strength. The study of the tribological properties of boundary films has also led to the development of other test apparatus and methods; these are, however, outside the scope of this document (see ISO 4384-1, ISO 4384-2, ISO 4385, ISO 7148-1, ISO 7148-2, ISO 7905-2, ISO 7905-3, and ISO 7905-4).
NOTE The testing of the resistance to corrosion of bearing materials by the lubricant is the subject of ISO 10129.ISO 6281:2020 pdf download