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  1. burner

    burner Thành viên mới

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  2. burner

    burner Thành viên mới

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    burner Thành viên mới

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  4. burner

    burner Thành viên mới

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    burner Thành viên mới

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    sheet 2
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    burner Thành viên mới

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    sheet 3
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  7. burner

    burner Thành viên mới

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    Updating Shaft Alignment Knowledge
    Defining permissible misalignment tolerances determines a reliability-focused approach.
    By Heinz P. Bloch, P.E.
    In the majority of industrial facilities, worker and technician resources are probably stretched to the limit. They might be looking for ways to simplify some tra***ional work processes and procedures. Or the facilities may have had an experience that reinforces the contention that high-tech tools are not always the answer, and that back-to-basics thinking has considerable merit.
    While no reasonable and experienced reliability professional will take issue with these statements, industry must be cautioned against drawing the wrong conclusions. A recent example of wrong conclusions involves claims that rotating equipment alignment is sufficiently accurate as long as the shaft centerlines in their cold, standstill con***ion are within 0.002 in. (0.05 mm) of each other. Those who believe this and blindly follow this questionable advice may soon find themselves among the repair-focused dinosaurs who are struggling *****rvive.
    On the other hand, those who update their knowledge of shaft alignment and alignment tolerances are on the way to becoming reliability focused. Indications are that only the reliability-focused facilities will be around in a few years.
    Expressing tolerances
    The only correct way to express shaft alignment tolerances is in terms of alignment con***ions at the coupling. This article will describe several ways to do this. It is incorrect to describe alignment tolerances in terms of correction values at the machine feet, and this will be examined also.
    When two machines are directly coupled with a flexible coupling, any misalignment between their centerlines of rotation can result in vibration which, depending on its severity, can produce premature wear or catastrophic failure of bearings, seals, the coupling itself, and other rotating components. Misalignment of the centerlines of rotation has long been recognized as one of the leading causes of machinery damage.
    Decades of well-documented observations attest that misalignment is responsible for huge economic losses. The more misalignment, the greater the rate of wear, likelihood of premature failure, and loss of efficiency of the machine. Misaligned machines absorb more energy and thus consume more power.
    However, even excellent alignment of the shaft centers of rotation does not guarantee absence of vibration because there is still the possibility of imbalance of rotating components. Structural resonance, fluid flow turbulence and cavitation, or vibration from nearby running machines that is transmitted to adjacent machines through either foundation or piping could also exist.
    Foot alignment does not work
    Absolute perfection in the alignment of shafts is not realistically attainable, nor is it needed. The issue is quantification of alignment quality and allowable deviationâ?"the alignment tolerance.
    Misalignment is defined by visualizing the shaft centerlines of rotation as two straight lines in space. The trick is to get them to coincide to form one straight line. If they do not, then there must exist either offset misalignment or angular misalignment (Fig. 1), or a combination of both.
  8. burner

    burner Thành viên mới

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    [​IMG]
    Since the shafts exist in three-dimensional space, these misalignments can exist in any direction. Dividing this space into two planes, the vertical and the horizontal, and describing the specific amount of offset and angularity that exists in each of these planes simultaneously helps define the four specific con***ions of misalignmentâ?"vertical offset (VO), vertical angularity (VA), horizontal offset (HO), and horizontal angularity (HA). These con***ions are described at the location of the coupling because that is where harmful machinery vibration is created whenever misalignment exists.
    The magnitude of an alignment tolerance (the description of desired alignment quality) must be expressed in terms of these offsets and angularities, or the sliding velocities resulting from them. Attempts to describe misalignment in terms of foot corrections alone do not take into account the size, geometry, or operating temperature of a given machine. Accepting the simple foot corrections approach can seriously compromise equipment life and has no place in a reliability-focused facility. An illustration of the fallacy of the foot correction approach will be given later.
    How much vibration and efficiency loss will result from the misalignment of shaft centers depends on shaft speed and coupling type. Acceptable alignment tolerances are functions of shaft speed and coupling geometry. High-quality flexible couplings are designed to tolerate more misalignment than what is good for the machines involved. Bearing load increases with misalignment, and bearing life decreases as the cube of the load increase, i.e., doubling the load will shorten bearing life by a factor of eight.
    Why would high-quality flexible couplings be generally able to accommodate greater misalignment than what is good for the connected machines? A large percentage of machines must be deliberately misalignedâ?"sometimes significantlyâ?"in the cold and stopped con***ion. As they reach operating speeds and temperatures, thermal growth is anticipated to bring the two shafts into alignment.
  9. burner

    burner Thành viên mới

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    Alignment case history
    A refinery has a small foot-mounted steam turbine enveloped in insulating blankets. The operating temperature of the steel casing is 455 F, and the distance from centerline to the bottom of the feet is 18 in. The turbine drives an ANSI pump with a casing temperature of 85 F; its centerline-to-bottom-of-feet distance is also 18 in. Both initially started up at the same ambient temperature. The differential in their growth is (0.0000065 in/in/deg F) Ã- 18 in. Ã- (455 â?" 85) deg F = 0.043 in.
    If these two machines had their shafts aligned center to center, this amount of offset would cast the equipment train into the frequent failure category. Using the 80/20 rule, it would be safe to assume that 20 percent of the machinery population accounts for 80 percent of the maintenance money. This pump train would be in the 20 percent group.
    As mentioned earlier, aligning center to center without paying attention to thermal growth is one of the factors that keeps practitioners in the repair-focused category.
    Permissible tolerances
    There are a number of acceptable ways to describe misalignment at the coupling and to define permissible misalignment tolerances. While many of these are of equal merit, describing alignment tolerances in terms of foot corrections is not acceptable in a reliability-focused environment.
    Offset and angularity at the coupling (for short couplings) is one of the most common ways of correctly defining alignment tolerances. The offset tolerance simply describes the maximum separation that can exist between two machine shafts at a specific location along their shaft axes, usually the coupling center. The angularity describes the rate at which the offset between the shaft centerlines may change as they travel along the axes of the shafts.
    The angularity may be described either directly, as an angle in terms of mils/in. (or milliradians), or as a gap difference at a particular coupling diameter. The latter method is popular because it relates directly to what the mechanic can detect with his feeler gages between the coupling faces. A modern laser shaft alignment system measures the angle between shaft centerlines; such a system can also be set to describe this angle as a gap difference at any desired diameter.
    This approach can have two different interpretations, however. If the permissible offset between the driver and driven shafts is X, does this mean X in any direction, or X individually in both the horizontal and vertical planes? These two alternatives are not the same. The first example, vector tolerance, is more conservative. The second approach, standard tolerance, is the more common approach. If it is not desired to have more than X of offset to exist between the shafts in any direction, then standard tolerances should not be used. Doing so would, in some circumstances, lead to greater-than-intended offsets. Figure 2 illustrates this point.
    [​IMG]
    Fig. 2. Depending on circumstances, using the standard tolerance (left) instead of vector tolerance (right) can lead to greater than intended offsets.
  10. Anhthimeh04

    Anhthimeh04 Thành viên mới

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    Thanks Burner. Mình đang cần tài liệu căn chỉnh đồng tâm. Và căn chỉnh mặt phẳng song song bằng ni vô khung. Bạn có thể cho mình xin tài liệu được không? Nếu đươc bạn gửi vào mail: anhthimeh04@gmail.com

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