Industrial Machinery Repair Part Episode 2 Part 1 pptx

For example, keys are used in making the coupling con-nection between the shaft of a driver and a hub or flange on that shaft.Any rotating element whose shaft incorporates such a keyed co

are used to contain the lubricant and seal out the entry of contaminants.The sleeves have lubrication holes, which permit flushing and relubricationwithout disturbing the sleeve gasket or seals.

equip-Mechanical-Flexing Couplings

To maintain coupling reliability, mechanical-flexing couplings require odic inspections on a time- or condition-based frequency established bythe history of the equipment’s coupling life or a schedule established bythe predictive maintenance engineer Items to be included in an inspec-tion are listed below If any of these items or conditions is discovered, thecoupling should be evaluated to determine its remaining operational life orrepaired/replaced

peri-● Inspect lubricant for traces of metal (indicating component wear)

● Visually inspect coupling mechanical components (roller chains and gearteeth, and grid members) for wear and/or fatigue

● Inspect seals to ensure they are pliable and in good condition They must

be installed properly in the sleeve with the lip in good contact with thehub

● Sleeve flange gaskets must be whole, in good condition, clean, and free

of nicks or cracks

● Lubrication plugs must be clean (to prevent the introduction of inants to the lubricant and machine surfaces) before being installed andmust be torqued to the manufacturer’s specifications

contam-● Setscrews and retainers must be in place and tightened to manufacturer’sspecifications.

● Inspect shaft hubs, keyways, and keys for cracks, breaks, and physicaldamage

● Under operating conditions, perform thermographic scans to determinetemperature differences on the coupling (indicates misalignment and/oruneven mechanical forces)

Material-Flexing Couplings

Although designed to be lubrication-free, material-flexing couplings alsorequire periodic inspection and maintenance This is necessary to ensurethat the coupling components are within acceptable specification limits.Periodic inspections for the following conditions are required to main-tain coupling reliability If any of these conditions are found, the cou-pling should be evaluated to determine its remaining operational life orrepaired/replaced

● Inspect flexing element for signs of wear or fatigue (cracks, element dust,

tempera-Combination Couplings

Mechanical components (e.g., grid members) should be visually inspectedfor wear and/or fatigue In addition to the items for mechanical-flexing cou-plings, the grid members on metallic-grid couplings should be replaced ifany signs of wear are observed

Rigid Couplings

The mechanical components of rigid couplings (e.g., hubs, bolts, sion sleeves and halves, keyways, and keys) should be visually inspected

compres-for cracks, breaks, physical damage, wear, and/or fatigue Any componenthaving any of these conditions should be replaced.

Keys, Keyways, and Keyseats

A key is a piece of material, usually metal, placed in machined slots orgrooves cut into two axially oriented parts in order to mechanically lockthem together For example, keys are used in making the coupling con-nection between the shaft of a driver and a hub or flange on that shaft.Any rotating element whose shaft incorporates such a keyed connection isreferred to as a keyed-shaft rotor Keys provide a positive means for trans-mitting torque between the shaft and coupling hub when a key is properlyfitted in the axial groove

The groove into which a key is fitted is referred to as a keyseat when referring

to shafts, and a keyway when referring to hubs Keyseating is the actualmachine operation of producing keyseats Keyways are normally made on akeys eater or by a broach Keyseats are normally made with a rotary or endmill cutter

Figure 11.15 is an example of a keyed shaft that shows the key size versus theshaft diameter Because of standardization and interchangeability, keys aregenerally proportioned with relation to shaft diameter instead of torsionalload

The effective key length, “L” is that portion of key having full bearing onhub and shaft Note that the curved portion of the keyseat made with a

Top view

End millcutter

TOP VIEWS

Square ends

Square and round

Rounded ends Gib head taper

Plain taperSIDE VIEWS

Figure 11.16 Key shapes

rotary cutter does not provide full key bearing, so “L” does not include thisdistance The use of an end mill cutter results in a square-ended keyseat.Figure 11.16 shows various key shapes: square ends, one square end andone round end, rounded ends, plain taper, and gib head taper The majority

of keys are square in cross section, which are preferred through 412" ter shafts For bores over 412" and thin wall sections of hubs, the rectangular(flat) key is used

diame-The ends are either square, rounded or gib-head diame-The gib-head is usuallyused with taper keys If special considerations dictate the use of a keyway

in the hub shallower than the preferred square key, it is recommended thatthe standard rectangular (flat) key be used

Hub bores are usually straight, although for some special applications taperbores are sometimes specified For smaller diameters, bores are designedfor clearance fits, and a setscrew is used over the key The major advantage

of a clearance fit is that hubs can be easily assembled and disassembled.For larger diameters, the bores are designed for interference fits withoutsetscrews For rapid-reversing applications, interference fits are required.The sections to follow discuss determining keyway depth and width, key-way manufacturing tolerances, key stress calculations, and shaft stresscalculations

Determining Keyway Depth and Width

The formula given below, along with Figure 11.17, Table 11.2 (square keys),and Table 11.3 (flat keys), illustrates how the depth and width of standard

YY

TS

2

Figure 11.17 Shaft and hub dimensions

square and flat keys and keyways for shafts and hubs are determined

Y = D−

D2− W2

2Where:

C= Allowance or clearance for key, inches

D= Nominal shaft or bore diameter, inches

H= Nominal key height, inches

W= Nominal key width, inches

Y = Chordal height, inches

Note: Tables 11.2 and 11.3 shown below are prepared for manufacturinguse Dimensions given are for standard shafts and keyways

Keyway Manufacturing Tolerances

Keyway manufacturing tolerances (illustrated in Figure 11.18) are referred

to as offset (centrality) and lead (cross axis) Offset or centrality is referred

Table 11.2 Standard square keys and keyways (inches)*

Keyways Diameter of holes (inclusive) Width Depth Key stock

Source: The Falk Corporation

*Square keys are normally used through shaft diameter 412"; larger shafts normally use flat keys.

to as dimension “N”; lead or cross axis is referred to as dimension “J.” Bothmust be kept within permissible tolerances, usually 0.002 inches

Key Stress Calculations

Calculations for shear and compressive key stresses are based on thefollowing assumptions:

1 The force acts at the radius of the shaft

2 The force is uniformly distributed along the key length

3 None of the tangential load is carried by the frictional fit between shaftand bore

The shear and compressive stresses in a key are calculated using thefollowing equations (see Figure 11.19):

Ss= 2T

(d)x(w)x(L) Sc= 2T

(d)x(h )x(L)

Table 11.3 Standard flat keys and keyways (inches)

Keyways Diameter of holes (inclusive) Width Depth Key stock

1/2 to 9/16" 1/8 3/64 1/8 × 1/32 5/8 to 7/8" 3/16 1/16 3/16 × 1/8

d= Shaft diameter, inches (use average diameter for taper shafts)

h1= Height of key in the shaft or hub that bears against the keyway,inches Should equalh2for square keys For designs whereunequal portions of the key are in the hub or shaft,h1is theminimum portion

Lead or cross axis

C

CC

C

Figure 11.18 Manufacturing tolerances: offset and lead

hp= Power, horsepower

L= Effective length of key, inches

rpm= Revolutions per minute

Ss= Shear stress, psi

Sc= Compressive stress, psi

T = Shaft torque, lb-in or hp× 63000

rpm

w= Key width, inches

Figure 11.19 Measurements used in calculating shear and compressive key stress

Table 11.4 Allowable stresses for AISI 1018 and AISI 1045

Allowable stresses - psi Heat

Material treatment Shear Compressive

AISI 1018 None 7,500 15,000

AISI 1045 255–300 Bhn 15,000 30,000

Source: The Falk Corporation

Key material is usually AISI 1018 or AISI 1045 Table 11.4 provides theallowable stresses for these materials

Example: Select a key for the following conditions: 300 hp at 600 rpm;

Note: If shaft had been 2-34" diameter (4" hub), the key would be 58"×5

8",

Ss = 9,200 psi, Sc = 18,400 psi, and a heat-treated key of AISI 1045 would

have been required (allowableSs = 15,000, allowable Sc = 30,000).

Shaft Stress Calculations

Torsional stresses are developed when power is transmitted through shafts

In addition, the tooth loads of gears mounted on shafts create bending sses Shaft design, therefore, is based on safe limits of torsion and bending

stre-To determine minimum shaft diameter in inches:

Minimum shaft diameter= 3

From Figure 11.20 at 225 Brinell, allowable torsion = 8,000 psi

Minimum shaft diameter= 3

From Table 11.5, note that the cube of 714" is 381, which is too small (i.e.,

<402) for this example The cube of 712" is 422, which is large enough

To determine shaft stress, psi:

Torsion

Figure 11.20 Allowable stress as a function of Brinell hardness

Table 11.5 Shaft diameters (inches) and their cubes (cubic inches)

The basic operations performed by dust-collection devices are: (1) ing particles from the gas stream by deposition on a collection surface,(2) retaining the deposited particles on the surface until removal, and(3) removing the deposit from the surface for recovery or disposal.The separation step requires: (1) application of a force that produces adifferential motion of the particles relative to the gas, and (2) sufficient gas-retention time for the particles to migrate to the collecting surface Mostdust-collections systems are comprised of a pneumatic-conveying systemand some device that separates suspended particulate matter from the con-veyed air stream The more common systems use either filter media (e.g.,fabric bags) or cyclonic separators to separate the particulate matter fromair.

separat-Baghouses

Fabric-filter systems, commonly calledbag-filter or baghouse systems, are

dust-collection systems in which dust-laden air is passed through a bag-typefilter The bag collects the dust in layers on its surface, and the dust layeritself effectively becomes the filter medium Because the bag’s pores areusually much larger than those of the dust-particle layer that forms, theinitial efficiency is very low However, it improves once an adequate dustlayer forms Therefore, the potential for dust penetration of the filter media

is extremely low except during the initial period after startup, bag change,

or during the fabric-cleaning, or blow-down, cycle

The principle mechanisms of disposition in dust collectors are: (1) itational deposition, (2) flow-line interception, (3) inertial deposition,(4) diffusion deposition, and (5) electrostatic deposition During the ini-tial operating period, particle deposition takes place mainly by inertialand flow-line interception, diffusion, and gravity Once the dust layerhas been fully established, sieving is probably the dominant depositionmechanism

A baghouse system consists of the following: a pneumatic-conveyor system,filter media, a back-flush cleaning system, and a fan or blower to provideairflow

Pneumatic Conveyor

The primary mechanism for conveying dust-laden air to a central collectionpoint is a system of pipes or ductwork that functions as a pneumatic con-veyor This system gathers dust-laden air from various sources within theplant and conveys it to the dust-collection system

Dust-Collection System

Design and configuration of the dust-collection system varies with the dor and the specific application Generally, a system consists of either asingle large hopper-like vessel or a series of hoppers with a fan or bloweraffixed to the discharge manifold Inside the vessel is an inlet manifold thatdirects the incoming air or gas to the dirty side of the filter media or bag Aplenum, or divider plate, separates the dirty and clean sides of the vessel.Filter media, usually long cylindrical tubes or bags, are attached to theplenum Depending on the design, the dust-laden air or gas may flow intothe cylindrical filter bag and exit to the clean side, or it may flow throughthe bag from its outside and exit through the tube’s opening Figure 12.1illustrates a typical baghouse configuration

ven-Fabric-filter designs fall into three types, depending on the method ofcleaning used: (1) shaker-cleaned, (2) reverse-flow-cleaned, and (3) reverse-pulse-cleaned

Shaker-Cleaned Filter

The open lower ends of shaker-cleaned filter bags are fastened over ings in the tube sheet that separate the lower, dirty-gas inlet chamber fromthe upper clean-gas chamber The bags are suspended from supports, whichare connected to a shaking device

open-The dirty gas flows upward into the filter bag, and the dust collects on theinside surface When the pressure drop rises to a predetermined upperlimit due to dust accumulation, the gas flow is stopped and the shaker isoperated This process dislodges the dust, which falls into a hopper locatedbelow the tube sheet

Bag support

and shaking

mechanism

Clean gas side

Dirty gas side

Dust discharge

Figure 12.1 A typical baghouse

For continuous operation, the filter must be constructed with multiplecompartments This is necessary so that individual compartments can

be sequentially taken offline for cleaning while the other compartmentscontinue to operate

Ordinary shaker-cleaned filters may be cleaned every fifteen minutes to eighthours, depending on the service conditions A manometer connected acrossthe filter is used to determine the pressure drop, which indicates whenthe filter should be shaken Fully automatic filters may be shaken everytwo minutes, but bag maintenance is greatly reduced if the time betweenshakings can be increased to 15 to 20 minutes

The determining factor in the frequency of cleaning is the pressure drop

A differential-pressure switch can serve as the actuator in automatic cleaningapplications Cyclone precleaners are sometimes used to reduce the dustload on the filter or to remove large particles before they enter the bag

It is essential to stop the gas flow through the filter during shaking in orderfor the dust to fall off With very fine dust, it may be necessary to equalize

the pressure across the cloth In practice, this can be accomplished withoutinterrupting continuous operation by removing one section from service at

a time With automatic filters, this operation involves closing the dirty-gasinlet dampers, shaking the filter units either pneumatically or mechanically,and reopening the dampers In some cases, a reverse flow of clean gasthrough the filter is used to augment the shaker-cleaning process

The gas entering the filter must be kept above its dew point to avoid vapor condensation on the bags, which will cause plugging However,fabric filters have been used successfully in steam atmospheres, such asthose encountered in vacuum dryers In these applications, the housing isgenerally steam-cased

water-Reverse-Flow-Cleaned Filter

Reverse-flow-cleaned filters are similar to the shaker-cleaned design, exceptthe shaker mechanism is eliminated As with shaker-cleaned filters, com-partments are taken offline sequentially for cleaning The primary use ofreverse-flow cleaning is in units using fiberglass-fabric bags at temperaturesabove 150◦C (300◦F).

After the dirty-gas flow is stopped, a fan forces clean gas through the bagsfrom the clean-gas side The superficial velocity of the gas through the bag isgenerally 1.5 to 2.0 feet per minute, or about the same velocity as the dirty-gas inlet flow This flow of clean gas partially collapses the bag and dislodgesthe collected dust, which falls to the hopper Rings are usually sewn intothe bags at intervals along their length to prevent complete collapse, whichwould obstruct the fall of the dislodged dust

Reverse-Pulse-Cleaned Filter

In the reverse-pulse-cleaned filter, the bag forms a sleeve drawn over a drical wire cage, which supports the fabric on the clean-gas side (i.e., inside)

cylin-of the bag The dust collects on the outside cylin-of the bag

A venturi nozzle is located in the clean-gas outlet from each bag, which isused for cleaning A jet of high-velocity air is directed through the venturinozzle and into the bag, which induces clean gas to pass through the fabric

to the dirty side The high-velocity jet is released in a short pulse, usuallyabout 100 milliseconds, from a compressed air line by a solenoid-controlledvalve The pulse of air and clean gas expand the bag and dislodge the col-lected dust Rows of bags are cleaned in a timed sequence by programmedoperation of the solenoid valves The pressure of the pulse must be sufficient

to dislodge the dust without cessation of gas flow through the baghouse