Industrial Machinery Repair Part Episode 1 Part 10 docx

Conveyors 209 Configuration Screw conveyors have a variety of configurations. Each is designed for spe- 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 of Stellite (a cobalt alloy produced by Haynes Stellite Co.) or other similarly hard material. Aluminum, bronze, Monel, or stainless steel also may be used to construct the rotor 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 applications with inclines greater than 29 degrees. Variable-Pitch Screw Variable-pitch screws having the short pitch at the feed end automatically control the flow to the conveyor and correctly proportion the load down the 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 do not require a feeder. Cut-Flight Cut-flight conveyors are used for conveying and mixing cereals, grains, and other light material. They are similar to normal flight or screw conveyors, and the only difference is the configuration of the paddles or screw. Notches are cut in the flights to improve the mixing and conveying efficiency when handling light, dry materials. Ribbon Screw Ribbon screws are used for wet and sticky materials, such as molasses, hot tar, and asphalt. This type of screw prevents the materials from building up and altering the natural frequency of the screw. A buildup can cause resonance problems and possibly catastrophic failure of the unit. Paddle Screw The paddle-screw conveyor is used primarily for mixing materials like mortar and paving mixtures. An example of a typical application is churning ashes and water to eliminate dust. Performance Process parameters, such as density, viscosity, and temperature, must be constantly maintained within the conveyor’s design operating envelope. 210 Conveyors Table 10.4 Factor A for self-lubricating bronze bearings Conveyor . 6 9 10 12 14 16 18 20 24 diameter, in Factor A 54 96 114 171 255 336 414 510 690 Slight variations can affect performance and reliability. In intermittent appli- cations, extreme care should be taken to fully evacuate the conveyor prior to shutdown. In addition, caution must be exercised when restarting a conveyor in case an improper shutdown was performed and material was allowed to settle. Power Requirements The horsepower requirement for the conveyor-head shaft, H, for horizontal screw conveyors can be determined from the following equation: H = (ALN + CWLF) × 10 − 6 Where: A = Factor for size of conveyor (see Table 10.4) C = Material volume, ft 3 /h F = Material factor, unitless (see Table 10.5) L = Length of conveyor, feet N = Conveyor rotation speed (rpm) W = Density of material, lb/ft 3 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 are found in Table 10.6. The value for E is usually 90%. Motor hp = HG/E Table 10.5 gives the information needed to estimate the power requirement: percentages of helix loading for five groups of material, maximum material density 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 by the material lb/ft 3 1 45 50 170 110 2 38 50 120 75 331 75 90 60 4 25 100 70 50 512 1 2 30 25 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 H, hp 1 1–2 2–4 4–5 5 G 2 1.5 1.25 1.1 1.0 Volumetric Efficiency Screw-conveyor performance is also determined by the volumetric effi- ciency of the system. This efficiency is determined by the amount of slip or bypass generated by the conveyor. The amount of slip in a screw 212 Conveyors conveyor is primarily determined by three factors: product properties, screw efficiency, and clearance between the screw and the conveyor barrel or housing. Product Properties Not all materials or products have the same flow characteristics. Some have plastic characteristics and flow easily. Others do not self-adhere and tend to separate when pumped or mechanically conveyed. As a result, the volumet- ric efficiency is directly affected by the properties of each product. This also impacts screw performance. Screw Efficiency Each of the common screw configurations (i.e., short pitch, variable pitch, 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 the system. For most medium- to high-density products in a chemical plant, the variable- pitch design normally provides the highest volumetric efficiency and lowest required 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 as molasses, 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, or diameter, of the screw and the conveyor’s barrel, or housing, through- out the operating life of the conveyor. Periodic adjustments to compensate for wear, variations in product, and changes in temperature are essential. While the recommended clearance varies with specific conveyor design and the product to be conveyed, excessive clearance severely impacts conveyor performance as well. Installation Installation requirements vary greatly with screw-conveyor design. The ven- dor’s Operating and Maintenance (O&M) manuals should be consulted and followed to ensure proper installation. However, as with practically all mechanical equipment, there are basic installation requirements common Conveyors 213 to all screw conveyors. Installation requirements presented here should be evaluated in conjunction with the vendor’s O&M manual. If the infor- mation provided here conflicts with the vendor-supplied information, the O&M manual’s recommendations should always be followed. Foundation The conveyor and its support structure must be installed on a rigid foun- 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 be enough lateral (i.e., width) stiffness to prevent flexing during normal operation. Mounting conveyor systems on decking or suspended-concrete flooring should provide adequate support. Support Structure Most screw conveyors are mounted above the foundation level on a support structure that generally has a slight downward slope from the feed end to the discharge 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 be adequately rigid to prevent conveyor flexing or distortion during normal operation. Design, sizing, and installation of the support structure must guarantee rigid support over the full operating range of the conveyor. When evaluating the structural requirements, variations in product type, density, and operating temperature must also be considered. Since these variables directly affect the torsional energy generated by the conveyor, the worst-case scenario 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, consistent incoming supply of product. In addition, the product-feed system must prevent entrainment of contam- inants in the incoming product. Normally, this requires an enclosure that seals the product from outside contaminants. 214 Conveyors Operating Methods As previously discussed, screw conveyors are sensitive to variations in incoming 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 and operating environment. Incoming-Product Variations Any measurable change in the properties of the incoming product directly affects 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 for them. For property changes directly related to product temperature, preheaters or coolers can be used in the incoming-feed hopper, and heating/cooling traces can be used on the conveyor’s barrel. These systems provide a means of achieving optimum conveyor performance despite variations in incoming product. Operating-Environment Variations Changes in the ambient conditions surrounding the conveyor system may also cause deviations in performance. A controlled environment will substantially improve the conveyor’s efficiency and overall performance. Therefore, operating practices should include ways to adjust conveyor speed and output to compensate for variations. The conveyor should be protected 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 torsional power between a power source and driven unit and (2) to absorb torsional variations in the drive train. They are not designed to correct misalignment between two shafts. While certain types of couplings provide some correc- tion for slight misalignment, reliance on these devices to obtain alignment is not recommended. Coupling Types The sections to follow provide overviews of the more common coupling types: rigid and flexible. Also discussed are couplings used for special applications: floating-shaft (spacer) and fluid (hydraulic). Rigid Couplings A rigid coupling permits neither axial nor radial relative motion between the shafts of the driver and driven unit. When the two shafts are connected solidly and properly, they operate as a single shaft. A rigid coupling is pri- marily used for vertical applications, e.g., vertical pumps. Types of rigid couplings discussed in this section are flanged, split, and compression. Flanged couplings are used where there is free access to both shafts. Split couplings are used where access is limited on one side. Both flanged and split couplings require the use of keys and keyways. Compression couplings are 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 the end of the driver shaft and the other on the end of the driven shaft. These halves are bolted together to form a solid connection. To positively transmit torque, the coupling incorporates axially fitted keys and split circular key rings 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. 216 Couplings Figure 11.1 Typical flanged rigid coupling Split Couplings A split rigid coupling, also referred to as a clamp coupling, is basically a sleeve that is split horizontally along the shaft and held together with bolts. It is clamped over the adjoining ends of the driver and driven shafts, forming a solid connection. Clamp couplings are used primarily on vertical pump shafting. A typical split rigid coupling is illustrated in Figure 11.2. As with the flanged coupling, the split rigid coupling incorporates axially fitted keys and split circular key rings to eliminate frictional dependency in the transmission of torque. Compression Coupling A rigid compression coupling is comprised of three pieces: a compressible core 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 on its external diameter from the center outward to both ends. The coupling halves are finish bored to fit this taper. When the coupling halves are bolted together, the core is compressed down on the shaft by the two halves, and the resulting frictional grip transmits the torque without the use of keys. A typical 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 flex- ing, or combination, allow the coupled shafts to slide or move relative to each other. Although clearances are provided to permit movement within specified tolerance limits, flexible couplings are not designed to compensate for major misalignments. (Shafts must be aligned to less than 0.002 inches for proper operation.) Significant misalignment creates a whipping move- ment of the shaft, adds thrust to the shaft and bearings, causes axial vibrations, and leads to premature wear or failure of equipment. Mechanical Flexing Mechanical-flexing couplings provide a flexible connection by permitting the coupling components to move or slide relative to each other. In order to permit 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 on the coupling components. The most popular of the mechanical-flexing type are the chain and gear couplings. Chain Chain couplings provide a good means of transmitting proportionately high torque at low speeds. Minor shaft misalignment is compensated for by means of clearances between the chain and sprocket teeth and the clearance that exists within the chain itself. The design consists of two hubs with sprocket teeth connected by a chain of the single-roller, double-roller, or silent type. A typical example of a chain coupling is illustrated in Figure 11.4. Special-purpose components may be specified when enhanced flexibility and reduced wear is required. Hardened sprocket teeth, special tooth design, and barrel-shaped rollers are available for special needs. Light- duty drives are sometimes supplied with nonmetallic chains on which no lubrication should be used. Gear Gear couplings are capable of transmitting proportionately high torque at both high and low speeds. The most common type of gear coupling consists of two identical hubs with external gear teeth and a sleeve, or cover, with matching internal gear teeth. Torque is transmitted through the gear teeth, [...]... manufacturer’s specifications Typical elastomeric couplings are illustrated in Figure 11 .10 Combination (Metallic-Grid) The metallic-grid coupling is an example of a combination of mechanicalflexing and material-flexing type couplings Typical metallic-grid couplings are illustrated in Figure 11 .11 224 Couplings Figure 11 .10 Typical elastomeric couplings The metallic-grid coupling is a compact unit capable... expansion of the equipment components Figure 11 .7 illustrates a typical bellows coupling Flexible Shaft or Spring Flexible shaft or spring couplings are generally used in small equipment applications that do not experience high torque loads Figure 11 .8 illustrates a typical flexible shaft coupling 222 Couplings Figure 11 .7 Typical bellows coupling Figure 11 .8 Typical flexible shaft coupling Diaphragm...Couplings 219 Roller-chain coupling Coupling cover (1/ 2 shown) (optional) Roller chain 1 req’d to join couplers 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 in position comes from a certain... application is shown in Figure 11 .13 The floating-shaft coupling consists of two support elements connected by a shaft Manufacturers use various approaches in their designs for these couplings For example, each of the two support elements may be of the 226 Couplings Laminated disk-ring coupling, spacer type Gear coupling, spindle type Gear coupling, high speed spacer type Figure 11 .12 Typical floating-shaft... coupling, spacer type Gear coupling, spindle type Gear coupling, high speed spacer type Figure 11 .12 Typical floating-shaft or spacer couplings Figure 11 .13 Typical floating-shaft or spacer couplings for high-temperature applications Couplings 227 Figure 11 .14 Typical hydraulic coupling single-engagement type, may consist of a flexible half-coupling on one end and a rigid half-coupling on the other end,... disk-ring coupling also reduces heat and axial vibration that can transmit Couplings 2 21 Morflex couplings Laminated disk-ring coupling (standard double-engagement) Dropout style Laminated disk-ring coupling (high speed spacer type) Figure 11 .6 Typical laminated disk-ring couplings between the driver and driven unit Figure 11 .6 illustrates some typical laminated disk-ring couplings Bellows Bellows couplings... The flexibility of this grid provides torsional resilience Special Application Couplings Two special application couplings are discussed in this section: (1) floatingshaft or spacer coupling and (2) hydraulic or fluid coupling Couplings 225 Figure 11 .11 Typical metallic-grid couplings Floating-Shaft or Spacer Coupling Regular flexible couplings connect the driver and driven shafts with relatively close... misalignment decreases the useful life of the coupling and may cause damage to other machine-train components such as bearings A typical example of a gear-tooth coupling is illustrated in Figure 11 .5 220 Couplings Figure 11 .5 Typical gear-tooth coupling Material-Flexing Material-flexing couplings incorporate elements that accommodate a certain amount of bending or flexing The material-flexing group includes laminated... of the diaphragms to complete the mechanical connection A typical diaphragm coupling is illustrated in Figure 11 .9 Elastomeric Elastomeric couplings consist of two hubs connected by an elastomeric element The couplings fall into two basic categories, one with the element Couplings 223 Figure 11 .9 Typical diaphragm coupling placed in shear and the other with the element placed in compression The coupling... torsional shock from sudden changes in equipment loads (e.g., compressors) Figure 11 .14 is an illustration of a typical hydraulic coupling Coupling Selection Periodically, worn or broken couplings must be replaced One of the most important steps in performing this maintenance procedure is to ensure that the correct replacement parts are used After having determined the cause of failure, it is crucial to . operating envelope. 210 Conveyors Table 10 .4 Factor A for self-lubricating bronze bearings Conveyor . 6 9 10 12 14 16 18 20 24 diameter, in Factor A 54 96 11 4 17 1 255 336 414 510 690 Slight variations. 6" diameter 20" diameter by the material lb/ft 3 1 45 50 17 0 11 0 2 38 50 12 0 75 3 31 75 90 60 4 25 10 0 70 50 512 1 2 30 25 Group 1 F factor is 0.5 for light materials such as barley, beans,. 2.5; silica sand, 2.0; sewage sludge (wet and sandy), 6.0. Table 10 .6 Allowance factor H, hp 1 1–2 2–4 4–5 5 G 2 1. 5 1. 25 1. 1 1. 0 Volumetric Efficiency Screw-conveyor performance is also determined