Gear Noise and Vibration Episode 2 Part 7 potx

260 Chapter 16 Fig 16.12 Sketch of vibration responses and paths. In practice, because measuring inside the gearbox is very difficult, it is probably better to rely on T.E. excitation when running or to estimate the internal resonances. The reciprocal theorem is of limited help since, although it may help for cross receptances P 13 and p 2 3, it does not assist access for Pj 2 or the local responses Pn and P 2 2 which need access inside the box in zero space. In some cases the wheel is so massive and its support is so stiff that wheel response may be ignored, simplifying the algebra considerably. 16.7 Coherence Whichever method (b), (c) or (d) is used for measuring a transfer function with a transfer function analyser, it is worthwhile checking coherence if there is any possibility of background noise, whether mechanical or electrical. The idea of coherence is that if we take a single transfer function measurement we can deduce a transfer function. However, we do not know how much of the output is really due to the input and how much is due to random external (or internal) disturbances. Repeating the test many times and getting exactly the same result in both amplitude and phase would suggest that there is little random effect. Any variation would suggest randomness. Coherence analysis routines carry out this check and compare how much of the measured output power (at a Vibration Testing 261 particular frequency) can be attributed to the consistent transfer function. A coherence of 1 suggests that output is firmly connected to input but < 0.5 suggests that random noise is dominating the measurements. Any results with coherence < 0.8 should be viewed with suspicion. Even if there are two vibrations whose coherence is 1 it is not necessarily true to say that the output is "due to" the input since both vibrations may have been generated by another unknown input. In particular a panel vibration may not be caused by the vibration at a bearing housing because both may have been caused by vibration from another bearing or even from a separate slave drive. To carry out a coherence check it is necessary to take multiple tests, typically eight. It is not possible to get a meaningful result from a single test because it is necessary to check whether the result is consistent over time. Extra care should be taken when impact testing because even though the responses may be consistent from test to test there is a greater likelihood of non-linearity. This in turn will lead to false deductions since a high response at one frequency may in fact be due to excitation at, say, one-third of the frequency encountering a non-linearity. [...]... figures are Group Particles per 100 ml 500000 25 0000 130000 64000 320 00 16000 8000 4000 20 00 1000 500 25 0 130 64 32 to to to to to to to to to to to to to to to 1000000 500000 25 0000 130000 64000 320 00 16000 8000 4000 20 00 1000 500 25 0 130 64 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 Bearing manufacturers typically suggest that 18/14 is a "normal" cleanliness for oil and so the above new oil would be "normal"... is for the particle count over 2 urn and is not of interest for gearboxes  27 5 Failures It is of interest to compare the test sizes of 5 and 15 urn with the expected oil film thicknesses, which range from less than 1 um in a rolling bearing to 3 to 4 pun in a medium-sized but lightly loaded gear The particle counts are classified into groups according to ISO 4406 and the figures are Group Particles... gears and rolling bearings, the two figures which are usually quoted (and are of most interest) are for the number of particles above 5 um and the number of particles above 15 um respectively The numbers are not given directly but are classified on an approximately binary scale A brand new clean oil might be naively expected to be particle free but in practice may have a test count of 20 0000 / 70 00 and. .. a further problem for the gearbox user in that there is no direct connection between the specification for the filter and the corresponding particle count in the gearbox Filters are specified in terms of their reduction 27 6 Chapter 18 ratio for particles of a given size so that a (56 ^75 as suggested by FAG would reduce particle count for those above 6 urn by a factor of 75 for a single pass through... debris and noise is unlikely as normal debris is small so gives pulses which are at too high a frequency to hear and which occur intermittently As mentioned previously in Chapter 15, the most sensitive debris detection system yet encountered for small particles is using Smith shocks to detect the particles passing through the mesh but this is too sensitive for use in Failures 27 7 normal commercial gearboxes... and impose severe axial forces to produce failure of motor or gearbox input bearings Gear tooth couplings are compact and light and can take high torques so they are popular in high power drives They are, however, able to impose considerable bending torques on their supporting shafts as mentioned in Chapter 17 Fig 18.3 Sketch of gear tooth coupling Friction forces are controlled by axial velocity and. .. trouble by misaligning an input pinion or sun wheel of an epicyclic and so affecting the gear stressing by increasing the load distribution factors Cm and Km If a gear is tilted by this effect and the load is not evenly distributed along the facewidth the T.E may be increased and so give more noise The other problem that can occur with gear tooth couplings is when alignment is good The coupling then locks... improvements in gear loadings and to the basic scaling laws for gears and rolling bearings As a very rough rule the load on a gear may be increased proportional to size squared whereas the load on a bearing may increase less rapidly If we take figures for the "heavy duty bearing", a spherical roller bearing, then within an O.D of 190 mm we get a C rating of 535 kN and an infinite life rating of 67 kN Doubling... allows a C rating of 173 0 kN and an infinite life of 193 kN This seems to follow a rough rule that doubling the size increases capacity by a factor of three whereas on a gear we would expect an increase by a factor of four The corresponding gear size may be estimated very roughly by using the 100 N/mm/m rule 20 teeth of 20 mm module with a "square" pinion gives a load of 100 x 20 x 400 which is 800... to 1 variation in speed so the ideal oil viscosity for the high speed gears and bearings is totally unsuitable for the low speed gears and bearings It is advisable to bias the choice toward the low speed bearings and increase the viscosity even though this will increase the lubrication losses and thus increase heat generation 18 .7 Debris detection Traditionally debris detection is one of the oldest . ml 500000 25 0000 130000 64000 320 00 16000 8000 4000 20 00 1000 500 25 0 130 64 32 to to to to to to to to to to to to to to to 1000000 500000 25 0000 130000 64000 320 00 16000 8000 4000 20 00 1000 500 25 0 130 64 Group 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 Bearing. loaded gear. The particle counts are classified into groups according to ISO 4406 and the figures are Particles per 100 ml 500000 25 0000 130000 64000 320 00 16000 8000 4000 20 00 1000 500 25 0 130 64 32 to to to to to to to to to to to to to to to 1000000 500000 25 0000 130000 64000 320 00 16000 8000 4000 20 00 1000 500 25 0 130 64 Group 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 Bearing. and p 2 3, it does not assist access for Pj 2 or the local responses Pn and P 2 2 which need access inside the box in zero space. In some cases the wheel is so massive and