Maintenance Fundamentals Episode 2 part 2 docx

The difficulty is that 1 it is often difficult to mount the transducer close to the individual gears, and 2 the number of vibration sources in a multi-gear drive results in a complex ass

Figure 11.19 Basic shape of bevel gears.

Figure 11.20 Typical set of bevel gears

Figure 11.21 Shaft angle, which can be at any degree.

Figure 11.22 Miter gears, which are shown at 90 degrees

HELICAL

Helical gears are designed for parallel-shaft operation like the pair in Figure 11.25 They are similar to spur gears except that the teeth are cut at an angle to the centerline The principal advantage of this design is the quiet, smooth action that results from the sliding contact of the meshing teeth A disadvantage, however, is the higher friction and wear that accompanies this sliding action The angle at which the gear teeth are cut is called the helix angle and is illustrated in Figure 11.26

It is very important to note that the helix angle may be on either side of the gear’s centerline Or if compared with the helix angle of a thread, it may be either a

‘‘right-hand’’ or a ‘‘left-hand’’ helix The hand of the helix is the same regardless

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of how viewed Figure 11.27 illustrates a helical gear as viewed from opposite sides; changing the position of the gear cannot change the hand of the tooth’s helix angle A pair of helical gears, as illustrated in Figure 11.25, must have the same pitch and helix angle but must be of opposite hands (one right hand and one left hand)

Helical gears may also be used to connect nonparallel shafts When used for this purpose, they are often called ‘‘spiral’’ gears or crossed-axis helical gears This style of helical gearing is shown in Figure 11.28

The worm and worm gear, illustrated in Figure 11.29, are used to transmit motion and power when a high-ratio speed reduction is required They provide

a steady quiet transmission of power between shafts at right angles The worm is Figure 11.23 Typical set of miter gears

Figure 11.24 Miter gears with spiral teeth.

Figure 11.25 Typical set of helical gears

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Figure 11.26 The angle at which teeth are cut.

Figure 11.27 Helix angle of teeth: the same no matter from which side the gear is viewed

Figure 11.28 Typical set of spiral gears

Figure 11.29 Typical set of worm gears.

always the driver and the worm gear the driven member Like helical gears, worms and worm gears have ‘‘hand.’’ The hand is determined by the direction of the angle of the teeth Thus, for a worm and worm gear to mesh correctly, they must be the same hand

The most commonly used worms have either one, two, three, or four separate threads and are called single, double, triple, and quadruple thread worms The number of threads in a worm is determined by counting the number of starts or entrances at the end of the worm The thread of the worm is an important feature

in worm design, as it is a major factor in worm ratios The ratio of a mating worm and worm gear is found by dividing the number of teeth in the worm gear

by the number of threads in the worm

HERRINGBONE

To overcome the disadvantage of the high end thrust present in helical gears, the herringbone gear, illustrated in Figure 11.30, was developed It consists simply of two sets of gear teeth, one right hand and one left hand, on the same gear The gear teeth of both hands cause the thrust of one set to cancel out the thrust of

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the other Thus the advantage of helical gears is obtained, and quiet, smooth operation at higher speeds is possible Obviously they can only be used for transmitting power between parallel shafts

GEARDYNAMICSANDFAILUREMODES

Many machine-trains utilize gear drive assemblies to connect the driver to the primary machine Gears and gearboxes typically have several vibration spectra associated with normal operation Characterization of a gearbox’s vibration signature box is difficult to acquire but is an invaluable tool for diagnosing machine-train problems The difficulty is that (1) it is often difficult to mount the transducer close to the individual gears, and (2) the number of vibration sources in a multi-gear drive results in a complex assortment of gear mesh, modulation, and running frequencies Severe drive-train vibrations (gearbox) are usually due to resonance between a system’s natural frequency and the speed of some shaft The resonant excitation arises from, and is proportional

to, gear inaccuracies that cause small periodic fluctuations in pitch-line velocity Complex machines usually have many resonance zones within their operating speed range because each shaft can excite a system resonance At resonance these cyclic excitations may cause large vibration amplitudes and stresses

Basically, forcing torque arising from gear inaccuracies is small However, under resonant conditions torsional amplitude growth is restrained only by damping in that mode of vibration In typical gearboxes this damping is often small and permits the gear-excited torque to generate large vibration amplitudes under resonant conditions

Figure 11.30 Herringbone gear

One other important fact about gear sets is that all gear sets have a designed preload and create an induced load (thrust) in normal operation The direction, radial or axial, of the thrust load of typical gear sets will provide some insight into the normal preload and induced loads associated with each type of gear

To implement a predictive maintenance program, a great deal of time should be spent understanding the dynamics of gear/gearbox operation and the frequencies typically associated with the gearbox As a minimum, the following should be identified

Gears generate a unique dynamic profile that can be used to evaluate gear condition In addition, this profile can be used as a tool to evaluate the operating dynamics of the gearbox and its related process system

Gear Damage

All gear sets create a frequency component, called gear mesh The fundamental gear mesh frequency is equal to the number of gear teeth times the running speed

of the shaft In addition, all gear sets will create a series of side bands or modulations that will be visible on both sides of the primary gear mesh fre-quency In a normal gear set, each of the side bands will be spaced at exactly the 1X or running speed of the shaft and the profile of the entire gear mesh will be symmetrical

Normal Profile

In a normal gear set, each of the side bands will be spaced at exactly the 1X running speed of the input shaft, and the entire gear mesh will be symmetrical In addition, the side bands will always occur in pairs, one below and one above the gear mesh frequency The amplitude of each of these pairs will be identical For

spaced at exactly input speed and have the same amplitude

If the gear mesh profile were split by drawing a vertical line through the actual mesh (i.e., number of teeth times the input shaft speed), the two halves would be exactly identical Any deviation from a symmetrical gear mesh profile is indica-tive of a gear problem However, care must be exercised to ensure that the problem is internal to the gears and induced by outside influences External misalignment, abnormal induced loads, and a variety of other outside influences will destroy the symmetry of the gear mesh profile For example, the single reduction gearbox used to transmit power to the mold oscillator system on a continuous caster drives two eccentrics The eccentric rotation of these two cams is transmitted directly into the gearbox and will create the appearance of

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eccentric meshing of the gears The spacing and amplitude of the gear mesh profile will be destroyed by this abnormal induced load

Excessive Wear

Figure 11.32 illustrates a typical gear profile with worn gears Note that the spacing between the side bands becomes erratic and they are no longer spaced at the input shaft speed The side bands will tend to vary between the input and output speeds but will not be evenly spaced

FREQUENCY

Figure 11.31 Normal profile is symmetrical

FREQUENCY

Figure 11.32 Wear or excessive clearance changes side band spacing

In addition to gear tooth wear, center-to-center distance between shafts will create

an erratic spacing and amplitude If the shafts are too close together, the spacing will tend to be at input shaft speed, but the amplitude will drop drastically Because the gears are deeply meshed (i.e., below the normal pitch line), the teeth will maintain contact through the entire mesh This loss of clearance will result in lower amplitudes but will exaggerate any tooth profile defect that may be present

If the shafts are too far apart, the teeth will mesh above the pitch line This type

of meshing will increase the clearance between teeth and amplify the energy of the actual gear mesh frequency and all of its side bands In addition, the load-bearing characteristics of the gear teeth will be greatly reduced Since the pres-sure is focused on the tip of each tooth, there is less cross-section and strength in the teeth The potential for tooth failure is increased in direct proportion the amount of excess clearance between shafts

Cracked Or Broken Tooth

Figure 11.33 illustrates the profile of a gear set with a broken tooth As the gear rotates, the space left by the chipped or broken tooth will increase the mechan-ical clearance between the pinion and bull gear The result will be a low ampli-tude side band that will occur to the left of the actual gear mesh frequency When the next, undamaged teeth mesh, the added clearance will result in a higher-energy impact

The resultant side band, to the right of the mesh frequency, will have much higher amplitude The paired side bands will have non-symmetrical amplitude that represents this disproportional clearance and impact energy

FREQUENCY

CRACKED OR BROKEN TOOTH

Figure 11.33 A broken tooth will produce an asymmetrical side band profile

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In Figure 11.34 is typical of a defective gear set Note the asymmetrical relation-ship of the side bands

COMMONCHARACTERISTICS

You should have a clear understanding of the types of gears generally utilized in today’s machinery, how they interact, and the forces they generate on a rotating shaft There are two basic classifications of gear drives: (1) shaft centers parallel, and (2) shaft centers not parallel Within these two classifications are several typical gear types

Shaft Centers Parallel

There are four basic gear types that are typically used in this classification All are mounted on parallel shafts and, unless an idler gear is also used, will have opposite rotation between the drive and driven gear (if the drive gear has a clockwise rotation, then the driven gear will have a counter-clockwise rotation) The gear sets commonly used in machinery include the following

Spur Gears

The shafts are in the same plane and parallel The teeth are cut straight and parallel to the axis of the shaft rotation No more than two sets of teeth are in

1X

FREQUENCY

Figure 11.34 Typical defective gear mesh signature

mesh at one time, so the load is transferred from one tooth to the next tooth rapidly Usually spur gears are used for moderate to low speed applications Rotation of spur gear sets is opposite unless one or more idler gears are included

in the gearbox Typically, spur gear sets will generate a radial load (preload) opposite the mesh on their shaft support bearings and little or no axial load

Backlash is an important factor in proper spur gear installation A certain amount of backlash must be built into the gear drive allowing for tolerances in concentricity and tooth form Insufficient backlash will cause early failure be-cause of overloading

As indicated in Figure 11.11, spur gears by design have a preload opposite the mesh and generate an induced load, or tangential force (TF) in the direction of rotation This force can be calculated as:

calculated as:

where

Helical Gears

The shafts are in the same plane and parallel but the teeth are cut at an angle to the centerline of the shafts Helical teeth have an increased length of contact, run quieter and have a greater strength and capacity than spur gears Normally the angle created by a line through the center of the tooth and a line parallel to the shaft axis is 45 degrees However, other angles may be found in machinery Helical gears also have a preload by design; the critical force to be considered, however, is the thrust load (axial) generated in normal operation; see Figure 11.12

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TTF¼ TF  tan l where

Herringbone Gears

These are commonly called double helical because they have teeth cut with right and left helix angles They are used for heavy loads at medium to high speeds They do not have the inherent thrust forces that are present in helical gear sets Herringbone gears, by design, cancel the axial loads associated with a single helical gear The typical loads associated with herringbone gear sets are the radial side-load created by gear mesh pressure and a tangential force in the direction of rotation

Internal Gears

Internal gears can be run only with an external gear of the same type, pitch, and pressure angle The preload and induced load will depend on the type of gears used Refer to spur or helical for axial and radial forces

TROUBLESHOOTING

One of the primary causes of gear failure is the fact that, with few exceptions, gear sets are designed for operation in one direction only Failure is often caused

by inappropriate bi-directional operation of the gearbox or backward instal-lation of the gear set Unless specifically manufactured for bi-directional oper-ation, the ‘‘non-power’’ side of the gear’s teeth is not finished Therefore, this side

is rougher and does not provide the same tolerance as the finished ‘‘power’’ side

Note that it has become standard practice in some plants to reverse the pinion or bull gear in an effort to extend the gear set’s useful life While this practice permits longer operation times, the torsional power generated by a reversed gear set is not as uniform and consistent as when the gears are properly installed

Table 11.1 Common Failure Modes of Gearboxes and Gear Sets

THE PROBLEM

THE CAUSES

Excessive or Too Little Backlash  

Incorrect Center-to-Center Distance Between Shafts  

Source: Integrated Systems, Inc.

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application In other cases, the overload is intermittent and only occurs when the speed changes or when specific production demands cause a momentary spike in the torsional load requirement of the gearbox

Misalignment, both real and induced, is also a primary root cause of gear failure The only way to ensure that gears are properly aligned is to hard blue the gears immediately following installation After the gears have run for a short time, their wear pattern should be visually inspected If the pattern does not conform

to vendor’s specifications, alignment should be adjusted

Poor maintenance practices are the primary source of real misalignment prob-lems Proper alignment of gear sets, especially large ones, is not an easy task Gearbox manufacturers do not provide an easy, positive means to ensure that shafts are parallel and that the proper center-to-center distance is maintained

Induced misalignment is also a common problem with gear drives Most gear-boxes are used to drive other system components, such as bridle or process rolls

If misalignment is present in the driven members (either real or process induced),

it also will directly affect the gears The change in load zone caused by the misaligned driven component will induce misalignment in the gear set The effect

is identical to real misalignment within the gearbox or between the gearbox and mated (i.e., driver and driven) components

Visual inspection of gears provides a positive means to isolate the potential root cause of gear damage or failures The wear pattern or deformation of gear teeth provides clues as to the most likely forcing function or cause The following sections discuss the clues that can be obtained from visual inspection

NORMALWEAR

Figure 11.35 illustrates a gear that has a normal wear pattern Note that the entire surface of each tooth is uniformly smooth above and below the pitch line

ABNORMALWEAR

Figures 11.36 through 11.39 illustrate common abnormal wear patterns found in gear sets Each of these wear patterns suggests one or more potential failure modes for the gearbox