Components for
Mechanical Equipment Plastics equipment components, such as bearings, gears, rollers, guides, and rings are being adopted rapidly by design engineers and equipment manufacturers. The attractive engineering properties of resins developed during the past 15 years and their ability to be molded and extruded economically into useful shapes are stimulating these applications. Plastic bearings and gears can often be operated without lubrication. Low first costs and noise suppression are additional features which make plastics attractive in equipment components. Careful attention to contact pressure and temperature limitations is required. The properties of plastics make them uniquely useful for packings, gaskets, and seals. Many types of packings and gaskets are available for use in stuffing boxes on pumps and valves, for mechanical seals, O-rings, and gaskets. There are many plastics to choose from, used alone or in combination with other materials. Selection depends on service requirements and coat. Several plastics used for this general service are discussed, particularly Teflon tetrafluoroethylene resin which has a combination of properties that makes it of particular value as a packing and gasket material.
J. R. BOYER AND W. R. MYERS E. I . d u Pont de Nemours & Co., Znc., Wilmington, Del.
APID progress has been made since World War I1 in the use of thermoplastic and thermosetting resins for mechanical equipment components and mechanical packings. It would be difficult to make a comprehensive listing and appraisal of all such applications; therefore, the mechanical equipment components discussed deal primarily with mechanical process and auxiliary equipment normally found in chemical plants. The discussion of mechanical packings is limited to applications of gaskets, rod packifigs, piston packings, expansion joints, and mechanical seals used in chemical plants. The data presented are taken almost entirely from Du Pont Co. laboratory results and plant experience. Most commercial plastics have certain desirable properties which can be utilized t o advantage for specific mechanical applications. For example, some plastics, like Zytel nylon resin, can 1324
be operated without lubrication in sliding or rolling contact with metals; hence these make good bearings and gear materials. Other plastics, like Teflon tetrafluoroethylene resin, are slippery and chemically inert. Glass-filled Teflon compositions are well suited for bearings and mechanical seal faces in contact with corrosive fluids. A low yield point and low elastic modulus combined with corrosion resistance make many of the thermoplastic materials useful as gasket materials. Most plastics can be molded into intricate shapes; hence they can be mass produced a t a lower cost than functionally equivalent metal parts. Many plastics possess a combination of high impact and abrasion resistance with high strength-to-weight ratio that makes them attractive for mechanical applications. Where properly applied, these materials are being used t o advantage for mechanical applications in the chemical and process industries.
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
Vol. 47, No. 7
'
-Plastics THERMOSETTING RESINS
Use of thermosetting plastics, particularly laminated phenolic resins, is well established in the mechanical field. Gears and bearings made from resins reinforced with paper, cotton, or linen have been used for at least 25 years. Gears made from these materials, mating with metal gears, are used extensively in mechanical power transmission. The American Gear Manufacturers' Association has published design data for such applications. Ability t o operate with scanty lubrication and a t a lower noise level than metal gears is the principal reason for using plastics. Laminated phenolic bearings have been used for nonprecision applications where water or aqueous solutions must be used for lubrication. The disadvantages of laminated phenolics are that they are relatively expensive and they must be lubricated. Use of phenolic and furan thermosetting resins reinforced with glass fillers is likewise well established for such equipment components as agitators, pump impellers, and volutes, where good corrosion resistance is a prerequisite. These materials, however, are expensive and do not lend themselves readily to mass production. THERMOPLASTIC RESINS
Use of thermoplastic resins, which can be injection- or compression-molded to form finished machine parts, although growing rapidly, is still in an early growth stage, considering the large number of potential applications. Most of the large scale mechanical applications have been made outside the field of chemical process equipment; but, as experience is gained, mechanical applications in allied chemical fields can be and are being made. For example, large quantities of parts molded from Zytel are being used in textile machinery, business machines, home movie cameras, electric shavers, household appliances, and automobiles. Just as metals require stress relief t o obtain dimensional stability, molded and fabricated plastic parts require thermal treatment to relieve residual stresses. Molded or machined Zytel parts can be stress relieved in boiling water (or in a n oil bath if the part is intended for high temperature service). If the parts are to be used with water or an aqueous solution as the lubricant, the moldings should be sized for the saturated condition or be machined to size while the stock is still saturated with water. For air-lubricated applications, the stock should be permitted to resume equilibrium conditions with respect to atmospheric moisture before machining to size. Dimensional stability after such treatment is of the order 0.001 to 0.002 inch per inch of diameter. These dimensional variations are caused by normal humidity changes. Unfilled and glass-filled Teflon parts may also be stress relieved to ensure dimensional stability. Heating for 1 hour at 325' to 350' C . followed by slow cooling will ensure freedom from residual stresses. No allowance is necessary for water absorption since i t is zero for this material. BEARINGS
Both Zytel and glass-filled Teflon have been used successfully as bearing materials. When well lubricated with or immersed in water or a nonabrasive fluid, 6lled Teflon bearings have been used a t PV values up to 50,000 (PV is the product of load in pounds per square inch of projected bearing area times the surface rubbing speed in feet per minute). Lubricated bearings of Zytel approach this same P V value, but of more importance to the designer is the fact that water or nonabrasive aqueous solutions can be used as lubricants instead of mineral oils. Both materials can be operated without lubrication, but under such conditions the maximum P V value should not exceed 10,000. Teflon and glass-filled Teflon compositions have been used July 1955
Construction ivaterials
sucessfully for such diverse applications as bearings for vertical chemical pumps in nitric acid service, blow-back shoe facings for Bird-Young filters, and pump vanes for positive displacement pumps. Operating clearances for bearings made from Zytel or Teflon should be at least 0.003 inch per inch of diameter. This is approximately twice the clearance normally used for metal bearings. The higher values are necessary because of the greater coefficient of expansion of plastics. GEARS
Nonlubricated gears of Zytel mating with metal gears are used on textile, photographic film manufacturing, and similar machinery where lubricant contamination is a problem with conventional gears. Such gears are usually subjected to low stresses in rubbing or sliding contact. T o illustrate the use of Zytel for gear applications, the following problem shows how to determine the required gear face width. For purposes of this discussion, it is assumed that this gear mates with a metal gear in a drive-head gear train operating a t substantially room temperature. Physical properties of Zytel Fatigue strength a t 25' C., Ib./sq. inch Fatigue strength at 100" C., lb./sq. inch Gear data Horsepower Speed, r.p.m. Pitch diam., inches No. of teeth (20'stub tooth form) Diametral pitch Velocity, ft./min. Velocity factor, Kv = 200 X 1300 Tangential load at pitch diam., lb.
+
3000 2000
2 1000 5 15 3 1300 0.25 = 0.35 51
Based on the Lewis formula, the face width of a spur gear should be
JV X DP F W = Kv X S X Y where FW = face width in inches Kv = velocity factor = 0.35 W = tangential load = 51 Ib. S = allowable stress for gear material, lb./sq. inch DP = diametral pitch = 3 Y = outline factor for 15 teeth = 0.34 Following conventional practice, the designer must select an acceptable value for S which is less than the fatigue strength. I n this example, since the gear is to be operated at room temperature, the value for S must be less than 3000 pounds per square inch. Let us assume that he selects 2000 pounds per square inch, then the required face width equals 0.82 inch or say, 1 inch. Zytel gears of such dimensions operating at the assumed speeds are known to have performed satisfactorily when designed up to approximately 3000 pounds per square inch. This stress approximates the endurance limit of the material a t room temperature. Plastic deformation is known to occur under such loading to a n extent which greatly exceeds that which can be tolerated with metal gears. I n intermittent service, recovery takes place, and the original contours are essentially restored during rest periods. This phenomenon indicates that allowable working stresses for continuous service should be lower than for intermittent use. Our experience leads us to believe that the formulas now used for design are not completely applicable for thermoplastic gears since they do not take into account such factors as frictional heat dissipation, equilibrium operating temperature, and relative elastic moduli of mating gears. If modifications of present formulas are found later to be applicable, we are inclined to
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perature range -60" to 300" C., plastic gaskets have been used for almost every known process fluid, Listed in Table I1 are representative plastics used by D u Pont as gasket materials and the forms in which they are applied. I n Table I11 the maxiDouble mechanical seal of inverted contracting type using V-rings and gaskets of mum temperatures a t which we Teflon; for some applications, entire stationary seal ring is made of stresshave had successful performance are relieved Teflon; where hack flushing is listed. required, a bushing made of Teflon is often used Rod Packings. Rod packings must seal fluids by sliding or rubbing contact with a shaft or valve stem. Sliding contact plastic packings present the same problems as bearings, except operating clearances are of molecular dimensions instead of the usual metal bearing clearances of 1 or 2 mils per inch of shaft diameter. Solid molded plastic rings or sleeves cannot be used successfully for rod packings with any appreciable rubbing speed because thermal expansion is much higher than the metal parts of the stuffing box, Double mechanical seal of inverted expanding type using wedge rings and seal and the thermal conductivity of plasPaces of stress-relieved Teflon; no gaskets tics is very low compared with metals. are required For these reasons, packing manufacturers have resorted to a number of innovations t o permit "breathing" of plastic rod packings to accommodate frictional and process temperature variations. Table IV gives a partial summary of plastic packing applications within D u Pont for reciprocating rods, rotating shafts, and value stems and an indication of the forms in which they are used. Expansion Joints. Experience is Single outside mechanical seal of inverted type employing bellows made of Teflon; limited with bellows-type plastic exhydraulically balanced seal construction pansion joints, but a few single and multiple convolution expansion joints of Teflon between piping and I 1 1 I process equipment have been used -t successfully. Likewise, single convoFigure 1. Typical applications of plastic components for seals lution bellows-type plasticized polyvinyl chloride expansion joints have been used successfully between vibrating conveyors and hopper sections. Good flex resistance is a believe t h a t working stresses can be permitted to approach the prerequisite for such applications. fatigue limit more closely than is the present practice in design of Mechanical Seal Components. Use of mechanical seals instead metal gears. of conventional stuffing boxes is gaining wide acceptance for sealD u Pont recognizes the lack of data in this field and our laboing rotating shafts. Because so many petroleum derivatives and ratories are now engaged in developing information on our major organic chemicals dissolve or swell natural and synthetic rubbers, plastics when subjected to sliding and rubbing contact. Test molded plastic parts are used for many mechanical seals in work has not progressed far enough to determine if reliable design chemical process service. Figure 1 shows the major types of information can be developed by these means or whether it must mechanical seals used by Du Pont. Plastic parts are identified. be developed by trial-and-error methods in plant tests. Valve Disks and Diaphragms. Molded phenolic resin valve Despite the scarcity of engineering information due to relatively disks for reciprocating pumps have been used widely for many short experience with thermoplastics, a wide variety of mechaniyears. Likewise, hard rubber or molded phenolics have been cal applications has been made within D u Pont on an economiused for globe valve disks. Neither of these materials, however, cally justified or safety basis. A partial list of applications in is particularly well suited for chemical services, other than weak Table I illustrates typical uses. acid or neutral aqueous solutions. Recently, Teflon and molded trifluorochloroethylene resin have gained rather wide acceptance MECHANICAL PACKINGS for stainless steel globe valve disks since they are essentially inert t o all chemicals. Gaskets. The physical properties of most thermoplastics enTeflon, trifluorochloroethylene, and plasticized poly(viny1 courage their use for static seals and gaskets. Within the tem-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 1
Plastics Construction Materials Table I. Part Sliding contact Plain bearings
Application
Plastic
Finish roll shaft bearings of nylon spinning machines; eliminates contamination Guide bearings on vertical submerged pumps in nitric acid service Passenger elevators; improves housekeeping and reduces fire hazard; excellent life Used to rebuild worn spindles on textile equipment
Zytel
Unlubricated passenger elevator guides Spindle sleeves Rolling contact Gears Truck wheels Cams Special applications Transparent cover
Elevator buckets Acid shields
Table 11.
Typical Applications of Plastics
Teflon (25% glass-filled) Zytel Zytel
Zytel Zytel Zytel
Spinning pump drives Pallet trucks Numerous installations
Luci? aerylic resin
A transparent cover was fabricated
for air-purged slip ring housing of synchronous motor to permit ready inspection of brushes without air purge Replaced stainless steel on cost basis Mplded spljt covers for pipe flanges in corrosive service
Zytel Polyet hylene
Typical Applications of Plastic Gaskets
Gasket Type Plain solid ring
Plastic
Plasticized po!y(vinyl chloride) Teflon
Teflon fabric laminates Spiral-wound with Teflon filler
Teflon
Teflon envelope
Teflon
Molded T-gasket
Polasticieed poly(vinyl chloride) Plasticized poly(vinyl chloride)
Irregular shapes cut from sheet stock
Application Hot oxidizing acids: concd. HzOz, Clz, organic chemicals; sizes up to 84-inch diameter Cold oxidizing acids; weak HZOP Hot oxidizing acids: concd. HzOz, Clz, organic chemicals Stainless steel pipe flange gaskets for organic chemicals in batch or continuous operations; diameter from 1 to 60 inches Manhole and large diameter flanges of distillation towers for organic chemicals Glass column; HC1 and organics Stainless steel exhaust ducts
Table 111. Temperature Limit Experience of Plastic Gaskets and Paclcings Upper Service Temperatures, C. Packing8 (slow moving Gaskets applications)
*
Teflon fnonreinforced) Teflon (asbestos reiniorced) Poly(viny1 chloride) (plasticized) Trifluorochloroethylene Polyethylene Zytel (molded nylon) Phenolics (paper or linen reinforced) P henolics (asbestos reinforced)
Table IV.
chloride) have been used successfully as diaphragms for Saunderstype valves in chemical services. A wide variety of special diaphragms for pumps, control instruments, and rupture disks has been made from Teflon, Zytel, trifluorochloroethylene, and others, where chemical inertness is a prerequisite. MAJOR FACTORS WHICH AFFECT PLASTIC PACKING PERFORMANCE
Thermoplastic resins have certain properties which limit their use as construction materials and for mechanical packings. Three major properties to consider are: 1. High creep rates at elevated temperatures 2. LONthermal conductivity and high coefficient of thermal
expansion 3. High retraction modulus
Creep a t Elevated Temperatures. When a thermoplastic material like plasticized polyvinyl chloride or Teflon is used as a gasket between flange faces, it must be plastically deformed to make an effective seal. It will remain tight as long as the residual
290 300
200
250 fin _. 150 35 125
.. ..
125 100 200
.. ..
Typical Applications of Plastic Paclcings
Packing Type
Application
Shredded Teflon die-formed to size
Pumps and agitator shafts
Solid braided Teflon
Pumps, agitator shafts, and valve stems (maintenance requirements)
Braided Teflon yarn filled wkth Teflon dispersion
Pumps and agitator shafts
Mixed Teflon-asbestos braids
Pumps and agitator shafts
Asbestos braid filled with Teflon dispersion; braided over a n elastic core Teflon lantern rings
Pumps, agitator stems, etc.
shafts,
valve
Pumps and agitator stuffing boxes
Teflon (principally) V-rings U-rings Wedge-rings
Valve stems
Phenolic Piston rings Full-floating segmental ring packings
Gas compressors, hot oil
July 1955
Figure 2. Three-inch Teflon gasket after 5 cycles of alternate 150 lb./sq. inch steam and cold water, when installed between thick stainless steel flanges of special design
Figure 3. Three-inch woven asbestos gasket (40% Teflonfilled) after 50 cycles of alternate 150 lb./sq. inch steam and cold water, when installed between thick stainless steel flanges of special design
INDUSTRIAL AND ENGINEERING CHEMISTRY
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contact pressure between flange faces is a t least equal to or greater than the internal pressure. Like rubber, compressed asbestos, and other gasket materials, plastics tend to lose residual contact pressure by extrusion of material between the Range faces, This phenomenon has variously been referred to in the literature as “creep,” “cold flow,” and “stress decay.” It is well established that the tendency for plastics to creep is accelerated rapidly by increased temperature. Only meager data are available to predict rate of creep of plastics. However, if the total elastic deformation of a 2-inch, 300-pound flanged joint resulting from bolt stresses of 30,000 pound per square inch is of the order 0.003 inch, one might reasonably expect leakage if the creep rate of the gasket material exceeds 0.001 inch per inch per month. Because all plastics have a high rate of creep, they have not been very successful as O-ring static seals. The tendency for thermoplastics to creep or extrude when used as gaskets can be overcome for all practical purposes by using adequate reinforcement or filler materials. For example, a woven asbestos gasket, treated with 40% by weight of Teflon dispersion and subsequently baked, can withstand alternate heating with 150 pounds of steam and quenching with cold water without appreciable creep or cold flow (Figures 2 and 3). Another method for confining the plastic is to use it as a filler in a spiral-wound or metal-jacketed gasket construction. Both methods have been used very successfully. Influence of High Coefficient of Expansion and Low Thermal Conductivity. If thermoplastic gaskets are used under cyclic temperature conditions, they should be installed in a tongueand-groove-type joint or the mechanical equivalent. It was pointed out earlier in this paper that solid plastic rings could not
be used successfully’as rod packings because of high rates of thermal expansion combined with poor thermal conductivity. These same factors influence thermoplastic gaskets and O-ring seal applications adversely because the plastic tends t o be extruded or forced out between flange or sealing faces when subjected to cyclic temperature variations. Elastic Retraction Modulus. Despite the fact that unoriented thermoplastics show an apparent low elastic modulus when subjected to a compressive stress, the retraction modulus-Le., the slope of the stress-strain curve when the stress is removed-is quite high. For example, Teflon shows an apparent elastic modulus as determined by ASTM Method D 638467 of 58,000 pounds per square inch, whereas the retraction modulus is about 500,000 pounds per square inch as measured by subjecting a gasket to a compressive load and then releasing it. This means that only a slight amount of creep of the bolts or flanged structure will result in a rapid decrease in gasket contact pressure. CONCLUSION
The use of plastics, particularly the thermoplastic type, for components or mechanical equipment is in its infancg. Experience, though limited, indicates clearly that plastics will play an increasingly important role in the mechanical equipment component field. More data and experience verification are needed by the engineer in arriving a t optimum design. I n the meantime, a progressive attitude toward the use of plastics, seasoned with a searching engineering approach, will pay substantial dividends both to the manufacturer and to the users of his equipment. RECEIVED for review September 17, 1954.
ACCEPTEDApril 27, 1955.
Vessels T h e selection of suitable plastic materials for vessel construction is determined by several factors which include operating temperature, environment, mechanical and electrical properties. Thermosetting materials in common use include phenolic resins, furan resins, polyesters, epoxy resins, and hard rubber. Commonly used thermoplastics are polyethylene, poly(viny1 chloride), saran, and poly(methy1 met hacrylate).
!i
f
J. A. NEUMANN
F. J. BOCKHOFF
American Agile Corp., Bedford, Ohio
Fenn College, Cleveland, Ohio
I
N SELECTING a suitable plastic material for vessel construction, several factors must be considered. These factors include operating temperature, mechanical property requirements, electrical property requirements, chemical environment, and ease of fabrication. Initial cost of fabrication, as well as maintenance cost, is an important factor in making a choice among different plastic materials. One must not allow initial cost to become an obstacle to the selection of the proper corrosion-resistant material. The decreased maintenance costs for suitable materials will more than compensate for Q higher initial investment THERMOSETTING MATERIALS USED IN VESSEL CONSTRUCTION
The commercial use of thermosetting resins as materials of chemical construction preceded that of thermoplastic resins b y a few decades. Small scale production of phenolic resins was already under way in 1907, under the auspices of the Bakelite Co.,
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founded b y Leo H. Baekeland. Phenolic materials a t that time were primarily used as insulating varnishes for the electrical industry. Much the same type of casting resin is used extensively today in the production of large, chemically resistant selfsupporting structures. The filled phenolic resins used today are generally condensation polymers of phenol and formaldehyde, having a specific gravity in the cured state of about 1.7 and a maximum recommended operating temperature of 265 O F. These resins are applied with either asbestos or graphite fillers. The asbestos-filled phenolic material exhibits fair shock resistance and excellent resistance t o inorganic salts and acids, including hot hydrochloric acid. It is not, however, recommended for oxidizing agents or strong caustics, which would include strong alkaline salts. Because of the incorporation of asbestos as a filler, this resin cannot be recommended for use in contact with hydrofluoric acid or fluorides, since these chemicals attack asbestos. Graphite-filled phenolic resins are specifically designed to overcome this lack of resistance toward fluorides and hydrofluoric
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Vol. 47, No. 7
