Industrial Machinery Repair Part Episode 1 Part 10 docx

infor-Foundation The conveyor and its support structure must be installed on a rigid dation that absorbs the torsional energy generated by the rotating screws.Because of the total overal

Conveyors 209

Configuration

Screw conveyors have a variety of configurations Each is designed for cific applications and/or materials Standard conveyors have a galvanized-steel rotor, or helix, and trough For abrasive and corrosive materials (e.g.,wet ash), both the helix and trough may be hard-faced cast iron For abra-sives, the outer edge of the helix may be faced with a renewable strip ofStellite (a cobalt alloy produced by Haynes Stellite Co.) or other similarlyhard material Aluminum, bronze, Monel, or stainless steel also may be used

spe-to construct the rospe-tor and trough

Short-Pitch Screw

The standard helix used for screw conveyors has a pitch approximately equal

to its outside diameter The short-pitch screw is designed for applicationswith inclines greater than 29 degrees

Variable-Pitch Screw

Variable-pitch screws having the short pitch at the feed end automaticallycontrol the flow to the conveyor and correctly proportion the load downthe screw’s length Screws having what is referred to as a “short section,”which has either a shorter pitch or smaller diameter, are self-loading and donot require a feeder

Cut-Flight

Cut-flight conveyors are used for conveying and mixing cereals, grains, andother light material They are similar to normal flight or screw conveyors,and the only difference is the configuration of the paddles or screw Notchesare cut in the flights to improve the mixing and conveying efficiency whenhandling light, dry materials

Performance

Process parameters, such as density, viscosity, and temperature, must beconstantly maintained within the conveyor’s design operating envelope

Power Requirements

The horsepower requirement for the conveyor-head shaft, H, for horizontalscrew conveyors can be determined from the following equation:

H= (ALN + CWLF) × 10 − 6Where:

A= Factor for size of conveyor (see Table 10.4)

C= Material volume, ft3/h

F= Material factor, unitless (see Table 10.5)

L= Length of conveyor, feet

N= Conveyor rotation speed (rpm)

W= Density of material, lb/ft3

In addition to H, the motor size depends on the drive efficiency (E) and

a unitless allowance factor (G), which is a function of H Values for G arefound in Table 10.6 The value for E is usually 90%

Motor hp= HG/ETable 10.5 gives the information needed to estimate the power requirement:percentages of helix loading for five groups of material, maximum materialdensity or capacity, allowable speeds for 6-inch and 20-inch diameter screws,and the factor F

Conveyors 211

Table 10.5 Power requirements by material group

Material Max cross Max density Max rpm for Max rpm for group section % occupied of material, 6" diameter 20" diameter

Group 1 F factor is 0.5 for light materials such as barley, beans, brewers grains

(dry), coal (pulverized), cornmeal, cottonseed meal, flaxseed, flour, malt, oats, rice, and wheat.

Group 2 Includes fines and granular material The values of F are: alum

(pulverized), 0.6; coal (slack or fines), 0.9; coffee beans, 0.4; sawdust, 0.7; soda ash (light), 0.7; soybeans, 0.5; fly ash, 0.4.

Group 3 Includes materials with small lumps mixed with fines Values of F are:

alum, 1.4; ashes (dry), 4.0; borax, 0.7; brewers grains (wet), 0.6; cottonseed, 0.9; salt, coarse or fine, 1.2; soda ash (heavy), 0.7 Group 4 Includes semiabrasive materials, fines, granular, and small lumps.

Values of F are: acid phosphate (dry), 1.4; bauxite (dry), 1.8; cement (dry), 1.4; clay, 2.0; fuller’s earth, 2.0; lead salts, 1.0; limestone screenings, 2.0; sugar (raw), 1.0; white lead, 1.0; sulfur (lumpy), 0.8; zinc oxide, 1.0.

Group 5 Includes abrasive lumpy materials, which must be kept from contact

with hanger bearings Values of F are: wet ashes, 5.0; flue dirt, 4.0; quartz (pulverized), 2.5; silica sand, 2.0; sewage sludge (wet and sandy), 6.0.

Table 10.6 Allowance factor

212 Conveyors

conveyor is primarily determined by three factors: product properties, screwefficiency, and clearance between the screw and the conveyor barrel orhousing

Product Properties

Not all materials or products have the same flow characteristics Some haveplastic characteristics and flow easily Others do not self-adhere and tend toseparate when pumped or mechanically conveyed As a result, the volumet-ric efficiency is directly affected by the properties of each product This alsoimpacts screw performance

Screw Efficiency

Each of the common screw configurations (i.e., short pitch, variablepitch, cut flights, ribbon, and paddle) has varying volumetric efficiencies,depending on the type of product that is conveyed Screw designs or con-figurations must be carefully matched to the product to be handled by thesystem

For most medium- to high-density products in a chemical plant, the pitch design normally provides the highest volumetric efficiency and lowestrequired horsepower Cut-flight conveyors are highly efficient for light, non-adhering products, such as cereals, but are inefficient when handling heavy,cohesive products Ribbon conveyors are used to convey heavy liquids, such

variable-as molvariable-asses, but are not very efficient and have a high slip ratio

Clearance

Improper clearance is the source of many volumetric-efficiency problems

It is important to maintain proper clearance between the outer ring, ordiameter, of the screw and the conveyor’s barrel, or housing, through-out the operating life of the conveyor Periodic adjustments to compensatefor wear, variations in product, and changes in temperature are essential.While the recommended clearance varies with specific conveyor design andthe product to be conveyed, excessive clearance severely impacts conveyorperformance as well

Installation

Installation requirements vary greatly with screw-conveyor design The dor’s Operating and Maintenance (O&M) manuals should be consultedand followed to ensure proper installation However, as with practically allmechanical equipment, there are basic installation requirements common

ven-Conveyors 213

to all screw conveyors Installation requirements presented here should

be evaluated in conjunction with the vendor’s O&M manual If the mation provided here conflicts with the vendor-supplied information, theO&M manual’s recommendations should always be followed

infor-Foundation

The conveyor and its support structure must be installed on a rigid dation that absorbs the torsional energy generated by the rotating screws.Because of the total overall length of most screw conveyors, a single founda-tion that supports the entire length and width should be used There must

foun-be enough lateral (i.e., width) stiffness to prevent flexing during normaloperation Mounting conveyor systems on decking or suspended-concreteflooring should provide adequate support

Support Structure

Most screw conveyors are mounted above the foundation level on a supportstructure that generally has a slight downward slope from the feed end to thedischarge end While this improves the operating efficiency of the conveyor,

it also may cause premature wear of the conveyor and its components.The support’s structural members (i.e., I-beams and channels) must beadequately rigid to prevent conveyor flexing or distortion during normaloperation Design, sizing, and installation of the support structure mustguarantee rigid support over the full operating range of the conveyor Whenevaluating the structural requirements, variations in product type, density,and operating temperature must also be considered Since these variablesdirectly affect the torsional energy generated by the conveyor, the worst-casescenario should be used to design the conveyor’s support structure

Product-Feed System

One of the major limiting factors of screw conveyors is their ability to provide

a continuous supply of incoming product While some conveyor designs,such as those having a variable-pitch screw, provide the ability to self-feed,their installation should include a means of ensuring a constant, consistentincoming supply of product

In addition, the product-feed system must prevent entrainment of inants in the incoming product Normally, this requires an enclosure thatseals the product from outside contaminants

contam-214 Conveyors

Operating Methods

As previously discussed, screw conveyors are sensitive to variations inincoming product properties and the operating environment Therefore,the primary operating concern is to maintain a uniform operating envelope

at all times, in particular by controlling variations in incoming product andoperating environment

Incoming-Product Variations

Any measurable change in the properties of the incoming product directlyaffects the performance of a screw conveyor Therefore, the operating prac-tices should limit variations in product density, temperature, and viscosity

If they occur, the conveyor’s speed should be adjusted to compensate forthem

For property changes directly related to product temperature, preheaters

or coolers can be used in the incoming-feed hopper, and heating/coolingtraces can be used on the conveyor’s barrel These systems provide a means

of achieving optimum conveyor performance despite variations in incomingproduct

Operating-Environment Variations

Changes in the ambient conditions surrounding the conveyor systemmay also cause deviations in performance A controlled environment willsubstantially improve the conveyor’s efficiency and overall performance.Therefore, operating practices should include ways to adjust conveyorspeed and output to compensate for variations The conveyor should beprotected from wind chill, radical variations in temperature and humidity,and any other environment-related variables

11 Couplings

Couplings are designed to provide two functions: (1) to transmit torsionalpower between a power source and driven unit and (2) to absorb torsionalvariations in the drive train They arenot designed to correct misalignment

between two shafts While certain types of couplings provide some tion for slight misalignment, reliance on these devices to obtain alignment

correc-is not recommended

Coupling Types

The sections to follow provide overviews of the more common couplingtypes: rigid and flexible Also discussed are couplings used for specialapplications: floating-shaft (spacer) and fluid (hydraulic)

Rigid Couplings

A rigid coupling permits neither axial nor radial relative motion betweenthe shafts of the driver and driven unit When the two shafts are connectedsolidly and properly, they operate as a single shaft A rigid coupling is pri-marily used for vertical applications, e.g., vertical pumps Types of rigidcouplings discussed in this section are flanged, split, and compression.Flanged couplings are used where there is free access to both shafts Splitcouplings are used where access is limited on one side Both flanged andsplit couplings require the use of keys and keyways Compression couplingsare used when it is not possible to use keys and keyways

Flanged Couplings

A flanged rigid coupling is comprised of two halves, one located on theend of the driver shaft and the other on the end of the driven shaft Thesehalves are bolted together to form a solid connection To positively transmittorque, the coupling incorporates axially fitted keys and split circular keyrings or dowels, which eliminate frictional dependency for transmission.The use of flanged couplings is restricted primarily to vertical pump shafts

A typical flanged rigid coupling is illustrated in Figure 11.1

It is clamped over the adjoining ends of the driver and driven shafts, forming

a solid connection Clamp couplings are used primarily on vertical pumpshafting A typical split rigid coupling is illustrated in Figure 11.2 As with theflanged coupling, the split rigid coupling incorporates axially fitted keys andsplit circular key rings to eliminate frictional dependency in the transmission

of torque

Compression Coupling

A rigid compression coupling is comprised of three pieces: a compressiblecore and two encompassing coupling halves that apply force to the core.The core is comprised of a slotted bushing that has been machine bored

to fit both ends of the shafts It also has been machined with a taper onits external diameter from the center outward to both ends The couplinghalves are finish bored to fit this taper When the coupling halves are boltedtogether, the core is compressed down on the shaft by the two halves, andthe resulting frictional grip transmits the torque without the use of keys Atypical compression coupling is illustrated in Figure 11.3

Couplings 217

Figure 11.2 Typical split rigid coupling

Figure 11.3 Typical compression rigid coupling

218 Couplings

Flexible Couplings

Flexible couplings, which are classified as mechanical flexing, material ing, or combination, allow the coupled shafts to slide or move relative toeach other Although clearances are provided to permit movement withinspecified tolerance limits, flexible couplings are not designed to compensatefor major misalignments (Shafts must be aligned to less than 0.002 inchesfor proper operation.) Significant misalignment creates a whipping move-ment of the shaft, adds thrust to the shaft and bearings, causes axialvibrations, and leads to premature wear or failure of equipment

flex-Mechanical Flexing

Mechanical-flexing couplings provide a flexible connection by permittingthe coupling components to move or slide relative to each other In order topermit such movement, clearance must be provided within specified limits

It is important to keep cross loading on the connected shafts at a minimum.This is accomplished by providing adequate lubrication to reduce wear onthe coupling components The most popular of the mechanical-flexing typeare the chain and gear couplings

Chain

Chain couplings provide a good means of transmitting proportionately hightorque at low speeds Minor shaft misalignment is compensated for bymeans of clearances between the chain and sprocket teeth and the clearancethat exists within the chain itself

The design consists of two hubs with sprocket teeth connected by a chain ofthe single-roller, double-roller, or silent type A typical example of a chaincoupling is illustrated in Figure 11.4

Special-purpose components may be specified when enhanced flexibilityand reduced wear is required Hardened sprocket teeth, special toothdesign, and barrel-shaped rollers are available for special needs Light-duty drives are sometimes supplied with nonmetallic chains on which nolubrication should be used

Coupling body(s)

1 req’d for each shaft

Figure 11.4 Typical chain coupling

whereas the necessary sliding action and ability for slight adjustments inposition comes from a certain freedom of action provided between the twosets of teeth

Slight shaft misalignment is compensated for by the clearance between thematching gear teeth However, any degree of misalignment decreases theuseful life of the coupling and may cause damage to other machine-traincomponents such as bearings A typical example of a gear-tooth coupling isillustrated in Figure 11.5

is determined primarily by conditions of installation and operation.

Laminated Disk-Ring

The laminated disk-ring coupling consists of shaft hubs connected to a singleflexible disk, or a series of disks, that allows axial movement The laminateddisk-ring coupling also reduces heat and axial vibration that can transmit