Industrial Machinery Repair Part 10 pptx

Gear Damage All gear sets create a frequency component, calledgear mesh.. The funda-mental gear mesh frequency is equal to the number of gear teeth times therunning speed of the shaft..

Figure 14.24 Miter gears with spiral teeth

viewed from opposite sides; changing the position of the gear cannot changethe hand of the tooth’s helix angle A pair of helical gears, as illustrated inFigure 14.27, must have the same pitch and helix angle but must be ofopposite hands (one right hand and one left hand)

Helical gears may also be used to connect nonparallel shafts When usedfor this purpose, they are often called “spiral” gears or crossed-axis helicalgears This style of helical gearing is shown in Figure 14.28

Worm

The worm and worm gear, illustrated in Figure 14.29, are used to transmitmotion and power when a high-ratio speed reduction is required Theyprovide a steady quiet transmission of power between shafts at right angles.The worm is always the driver and the worm gear the driven member Likehelical gears, worms and worm gears have “hand.” The hand is determined

by the direction of the angle of the teeth Thus, in order for a worm andworm gear to mesh correctly, they must be the same hand

Figure 14.25 Typical set of helical gears

Helixangle

Figure 14.26 Illustrating the angle at which the teeth are cut

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

Hub on

left side

Hub onright side

Figure 14.27 Helix angle of the teeth the same regardless of side from which the gear is viewed

Figure 14.28 Typical set of spiral gears

Figure 14.29 Typical set of worm gears

worm is an important feature in worm design, as it is a major factor in wormratios The ratio of a mating worm and worm gear is found by dividing thenumber 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 14.30, was developed It consistssimply of two sets of gear teeth, one right-hand and one left-hand, on thesame gear The gear teeth of both hands cause the thrust of one set to cancelout the thrust of the other Thus, the advantage of helical gears is obtained,and quiet, smooth operation at higher speeds is possible Obviously theycan only be used for transmitting power between parallel shafts

Gear Dynamics and Failure Modes

Many machine trains utilize gear drive assemblies to connect the driver tothe primary machine Gears and gearboxes typically have several vibrationspectra associated with normal operation Characterization of a gearbox’s

Figure 14.30 Herringbone gear

vibration signature box is difficult to acquire but is an invaluable tool fordiagnosing machine-train problems The difficulty is that: (1) it is oftendifficult to mount the transducer close to the individual gears and (2) thenumber of vibration sources in a multigear drive results in a complex assort-ment of gear mesh, modulation, and running frequencies Severe drive-trainvibrations (gearbox) are usually due to resonance between a system’s nat-ural frequency and the speed of some shaft The resonant excitation arisesfrom, and is proportional to, gear inaccuracies that cause small periodicfluctuations in pitch-line velocity Complex machines usually have manyresonance zones within their operating speed range because each shaft canexcite a system resonance At resonance these cyclic excitations may causelarge 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 isoften small and permits the gear-excited torque to generate large vibrationamplitudes under resonant conditions

One other important fact about gear sets is that all gear-sets have a designedpreload and create an induced load (thrust) in normal operation The direc-tion, 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 quencies typically associated with the gearbox As a minimum, the followingshould be identified

fre-Gears generate a unique dynamic profile that can be used to evaluate gearcondition In addition, this profile can be used as a tool to evaluate theoperating dynamics of the gearbox and its related process system

Gear Damage

All gear sets create a frequency component, calledgear mesh The

funda-mental gear mesh frequency is equal to the number of gear teeth times therunning speed of the shaft In addition, all gear sets will create a series ofsidebands or modulations that will be visible on both sides of the primarygear mesh frequency In a normal gear set, each of the sidebands will bespaced at exactly the 1X or running speed of the shaft and the profile of theentire gear mesh will be symmetrical

Normal Profile

In addition, the sidebands will always occur in pairs, one below and oneabove the gear mesh frequency The amplitude of each of these pairs will beidentical For example, the sideband pair indicated, as−1 and +1 in Figure14.31, will be spaced at exactly input speed and have the same amplitude

If the gear mesh profile were split by drawing a vertical line through theactual mesh, i.e., the number of teeth times the input shaft speed, the twohalves would be exactly identical Any deviation from a symmetrical gear

mesh profile is indicative of a gear problem However, care must be cised to ensure that the problem is internal to the gears and induced byoutside influences External misalignment, abnormal induced loads and avariety of other outside influences will destroy the symmetry of the gearmesh profile For example, the single reduction gearbox used to trans-mit power to the mold oscillator system on a continuous caster drives twoeccentrics The eccentric rotation of these two cams is transmitted directlyinto the gearbox and will create the appearance of eccentric meshing of thegears The spacing and amplitude of the gear mesh profile will be destroyed

exer-by this abnormal induced load

Excessive Wear

Figure 14.32 illustrates a typical gear profile with worn gears Note that thespacing between the sidebands becomes erratic and is no longer spaced atthe input shaft speed The sidebands will tend to vary between the inputand output speeds but will not be evenly spaced

In addition to gear tooth wear, center-to-center distance between shafts willcreate 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 dropdrastically Because the gears are deeply meshed, i.e., below the normalpitch line, the teeth will maintain contact through the entire mesh Thisloss of clearance will result in lower amplitudes but will exaggerate anytooth profile defect that may be present

If the shafts are too far apart, the teeth will mesh above the pitch line Thistype of meshing will increase the clearance between teeth and amplify the

Figure 14.33 A broken tooth will produce an asymmetrical sideband profile

energy of the actual gear mesh frequency and all of its sidebands In addition,the load bearing characteristics of the gear teeth will be greatly reduced.Since the pressure 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 to the amount of excess clearance between shafts

Cracked or Broken Tooth

Figure 14.33 illustrates the profile of a gear set with a broken tooth As thegear rotates, the space left by the chipped or broken tooth will increase themechanical clearance between the pinion and bullgear The result will be alow amplitude sideband that will occur to the left of the actual gear meshfrequency When the next, undamaged teeth mesh, the added clearance willresult in a higher energy impact

The resultant sideband, to the right of the mesh frequency, will havemuch higher amplitude The paired sidebands will have nonsymmetricalamplitude that represents this disproportional clearance and impact energy

If the gear set develops problems, the amplitude of the gear mesh frequencywill increase, and the symmetry of the sidebands will change The pat-tern illustrated in Figure 14.34 is typical of a defective gear set Note theasymmetrical relationship of the sidebands

Common Characteristics

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

LOW SPEED SHAFT

SIDE BANDS

TWICE GEAR MESH THREE TIMES GEAR MESH GEAR MESH FREQUENCY

Figure 14.34 Typical defective gear mesh signature

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 in also used, willhave opposite rotation between the drive and driven gear (if the drive gearhas a clockwise rotation, then the driven gear will have a counterclockwiserotation) 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 andparallel to the axis of the shaft rotation No more than two sets of teethare in mesh at one time, so the load is transferred from one tooth to thenext tooth rapidly Usually spur gears are used for moderate- to low-speedapplications Rotation of spur gear sets is opposite unless one or more idlergears are included in the gearbox Typically, spur gear sets will generate aradial load (preload) opposite the mesh on their shaft support bearings andlittle or no axial load

Backlash is an important factor in proper spur gear installation A certainamount of backlash must be built into the gear drive allowing for tolerances

in concentricity and tooth form Insufficient backlash will cause early failuredue to overloading

As indicated in Figure 14.11, spur gears by design have a preload oppositethe mesh and generate an induced load, or tangential force, TF, in the

direction of rotation This force can be calculated as:

hp= Input horsepower to pinion or gear

Dp= Pitch diameter of pinion or gear

rpm= Speed of pinion or gear

φ = Pinion or gear tooth pressure angle

in normal operation; see Figure 14.12

TF= 126,000∗ hp

Dp∗ RPM

STF= TF∗ tan φ

cosλ TTF = TF ∗ tan λ

TF= tangential force

STF= separating force

TTF= thrust force

hp= input horsepower to pinion or gear

Dp= pitch diameter of pinion or gear

rpm= speed of pinion or gear

φ = pinion or gear tooth pressure angle

λ = pinion or gear helix angle

Herringbone Gears

Commonly called “double helical” because they have teeth cut with right andleft 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 gearsets Herringbone gears, by design, cancel the axial loads associated with asingle helical gear The typical loads associated with herringbone gear setsare the radial side-load created by gear mesh pressure and a tangential force

in the direction of rotation

“power” side

Note that it has become standard practice in some plants to reverse thepinion or bullgear in an effort to extend the gear set’s useful life While this

Table 14.1 Common failure modes of gearboxes and gear sets

Gear set not suitable for

application

Incorrect center-to-center distance

between shafts

• •

practice permits longer operation times, the torsional power generated by

a reversed gear set is not as uniform and consistent as when the gears areproperly installed

Gear overload is another leading cause of failure In some instances, theoverload is constant, which is an indication that the gearbox is not suitablefor the application In other cases, the overload is intermittent and onlyoccurs 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 gearfailure The only way to assure that gears are properly aligned is to “hardblue” the gears immediately following installation After the gears have runfor a short time, their wear pattern should be visually inspected If thepattern does not conform to vendor’s specifications, alignment should beadjusted

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

is maintained

Induced misalignment is also a common problem with gear drives Mostgearboxes are used to drive other system components, such as bridle orprocess rolls If misalignment is present in the driven members (either real

or process induced), it also will directly affect the gears The change inload zone caused by the misaligned driven component will induce mis-alignment in the gear set The effect is identical to real misalignment withinthe gearbox or between the gearbox and mated (i.e., driver and driven)components

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

Normal Wear

Figure 14.35 illustrates a gear that has a normal wear pattern Note that theentire surface of each tooth is uniformly smooth above and below the pitchline

Figure 14.35 Normal wear pattern

Figure 14.36 Wear pattern caused by abrasives in lubricating oil

Abnormal Wear

Figures 14.36 through 14.39 illustrate common abnormal wear patternsfound in gear sets Each of these wear patterns suggests one or morepotential failure modes for the gearbox

Abrasion

Abrasion creates unique wear patterns on the teeth The pattern varies,depending on the type of abrasion and its specific forcing function.Figure 14.36 illustrates severe abrasive wear caused by particulates in thelubricating oil Note the score marks that run from the root to the tip of thegear teeth

Figure 14.37 Pattern caused by corrosive attack on gear teeth

Figure 14.38 Pitting caused by gear overloading

Chemical Attack or Corrosion

Water and other foreign substances in the lubricating oil supply also causegear degradation and premature failure Figure 14.37 illustrates a typicalwear pattern on gears caused by this failure mode

Overloading

The wear patterns generated by excessive gear loading vary, but all sharesimilar components Figure 14.38 illustrates pitting caused by excessive tor-sional loading The pits are created by the implosion of lubricating oil Otherwear patterns, such as spalling and burning, can also help to identify specificforcing functions or root causes of gear failure

“Only Permanent Repairs Made Here”

Hydraulic Knowledge

People say knowledge is power This is true in hydraulic maintenance Manymaintenance organizations do not know what their maintenance personnelshould know We believe in an industrial maintenance organization where

we should divide the hydraulic skill necessary into two groups One isthe hydraulic troubleshooter; they must be your experts in maintenance,and this should be as a rule of thumb 10% or less of your maintenanceworkforce The other 90% plus would be your general hydraulic main-tenance personnel They are the personnel that provide the preventivemaintenance expertise The percentages we give you are based on a com-pany developing a true preventive/proactive maintenance approach toits hydraulic systems Let’s talk about what the hydraulic troubleshooterknowledge and skills should be

Hydraulic Troubleshooter

Knowledge:

● Mechanical principles (force, work, rate, simple machines)

● Math (basic math, complex math equations)

● Hydraulic components (application and function of all hydraulic systemcomponents)

● Hydraulic schematic symbols (understanding all symbols and their tionship to a hydraulic system)

rela-● Calculating flow, pressure, and speed

● Calculating the system filtration necessary to achieve the system’s properISO particulate code

● Trace a hydraulic circuit to 100% proficiency

● Set the pressure on a pressure compensated pump

● Tune the voltage on an amplifier card

● Null a servo valve

● Troubleshoot a hydraulic system and utilize “Root Cause Failure Analysis”

● Replace any system component to manufacturer’s specification

● Develop a PM program for a hydraulic system

● Flush a hydraulic system after a major component failure

General Maintenance Person

Knowledge:

● Filters (function, application, installation techniques)

● Reservoirs (function, application)

● Basic hydraulic system operation

● Cleaning of hydraulic systems

● Hydraulic lubrication principles

● Proper PM techniques for hydraulics

Skill:

● Change a hydraulic filter and other system components

● Clean a hydraulic reservoir

● Perform PM on a hydraulic system

● Change a strainer on a hydraulic pump

● Add filtered fluid to a hydraulic system

● Identify potential problems on a hydraulic system

● Change a hydraulic hose, fitting, or tubing