NZ213624A

NZ213624A - Moulding a radial face seal element from graphite & phenolic resin

Info

Publication number: NZ213624A
Application number: NZ213624A
Authority: NZ
Inventors: E R Bernier
Original assignee: Chesterton A W Co
Priority date: 1982-08-09
Filing date: 1985-09-26
Publication date: 1987-06-30
Also published as: NZ213625A; NZ201535A; NZ213626A

Description

<div class="application article clearfix" id="description"> Priori cy Date(s): Complete Specification Filed: Class: ...e>.-3LqL31.|.X&.; 7 Publication Date: ....?. .Q .^N. J?$7..... P.O. Journal, No: ... ix«p Under the provisions of Regulation 23 (I) the _ CcfafleM 4 Specification has been ante-date<^ to—SL.A^^-s~ 19 * \, -xnA No. : Date: NEW ZEALAND PATENTS ACT 1953 Divided out of New Zealand Patent Application No. 201 535 filed 9 August 1982 COMPLETE SPECIFICATION Mechanical Seal We, A. W. CHESTERTON COMPANY, a Massachussets Corporation, U.S.A., having its principal place of business at Middlesex Industrial Park, Route 93, Stoneham, Massachussets 02180, U.S.A. hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: -1- (foil owed by page 1a) '7 Vc> :|5; 1 • I I! 'v^ 10 o 15 20 25 2 13624 • -la- Field of the Invention This invention relates to mechanical seals for rotating shafts, and a method of making such seals. Background of the Invention Packing seals and mechanical seals are two types of seals used to prevent leakage around a rotating shaft. The prior art packing seal consists of packing material wrapped around the shaft and compressed so as to provide a tight fit against it. As the packing seal is stationary while the shaft rotates, the seal places a great deal of drag on the shaft and will wear the shaft away over a period of time. The worn shaft must then be replaced at considerable cost. The prior art mechanical seal overcomes some of the packing seal drawbacks. These prior art mechanical seals fit over the shaft, and a driver of the seal is held thereto by set screws so that the driver rotates with the shaft. The area between the shaft and the seal is sealed by conventional means, e.g., 0-rings, while the flow path around the outside of the mechanical seal is closed by forcing a thin graphite lip of a seal portion of the mechanical seal against a stationary ceramic collar mounted around but not connected to the shaft. Because of the high speed of rotation of the shaft and the seal, the continual pressure of the seal against the ceramic collar and the high temperature to which the seal is often subjected, the prior art mechanical seal must be very strong. Molaable materials such as plastic or plastic and some graphite do not produce a seal strong enough to be used, and accordingly, the prior art seal is usually made entirely of steel, except for the small graphite lip. As a result, the -2- rl ,/? ^ ^ prior art mechanical seals are very expensive, which expense is increased if the seal must be specially fabricated for a non-standard size shaft. Furthermore, some liquids, e.g., some acids and salt water, will attack steel and other metals, and for those applications, a packing seal must be used instead. Another drawback of the prior art mechanical seals is the sealing lip itself. Because it is made of the graphite, the lip is very brittle and has little dimensional strength. As a result, it is made as small as possible to reduce the chance of breakage. The lip, however, wears as the seal rotates, and when it wears down a relatively short distance, the expensive seal portion must be replaced. A final drawback is that the set screws, which hold the prior art mechanical seal in place, dimple the shaft. If a packing seal is later used on the same shaft, the dimples will tear up the packing material as the shaft rotates. Summary of the Invention It has been discovered that an improved mechanical seal can be made by molding a driver of the seal of a resin compound including-carbon and molding a seal portion including a sealing lip of a graphite with a small percentage of a resin compound, the resulting parts having substantial strength and very similar coefficients of expansion. In addition, it has been discovered that the driver can be held onto a rotating shaft by tightening a removable half-collar of the carbon and resin compound, having a sloped interior wall when unattached, which does not dimple the shaft. In the preferred embodiment, the driver portion is made of a mixture of 60% of a compound based on RYTOK" , a polyphenylene sulfide resin, and 40% carbon filler, which ' mixture is melted, forced into a mold heated to a temperature of 300°F, and then solidified. The seal portion is made of a mixture of 90% graphite powder and 10% phenolic resin, which mixture is also compression molded to form the integral seal portion with a seal lip. Eoth the driver and seal portion are then post-cured at temperatures of 400°F and 350°F respectively for a period of hours so as to cross-link the molecules to |j create a stronger bond between them and give added strength to the parts. Description of the Preferred Embodiment Drawings We now turn to a description of the preferred embodiment, after first briefly describing the drawings. Figure 1 is a perspective view of a mechanical seal of this invention; Figure 2 is a cross-sectional view of the mechanical seal of this invention mounted on a pump shaft; Figure 3 is an enlarged perspective view of a driver of the seal of this invention; Figure 4 is an enlarged perspective view of a clamp of the driver; and Figure 5 is an enlarged perspective view of the seal portion of this invention; and Figure 6 is an enlarged perspective view of a simplified mold for the parts of the mechanical seal of this invention. Structure Referring to Figure 1, a mechanical seal of this invention is shown at 10. Seal 10 generally comprises a driver 20 and a seal portion 60. - 4 - ZL3G24 As shown in Figure 3, the driver 20 comprises a cylindrical sidewall 22 having a semi-circular flange 24 extending around half of its lower end 26. Flange 24 has a pair of flat faces 28, 30 which are 180^ apart, and screw-threaded holes 32, 34 extend through faces 28, 30 respectively. Each hole 32, 34 extends to and opens through (not shown) the side of the flange 24. An annular 0-ring groove 36 is disposed around the inside of the sidewall 22 at its approximate midpoint. A top surface 38 of the driver 20 has a pair of tabs 40, 42 extending upwardly therefrom. The tabs 40, 42 are positioned 180^ apart, and ten spring holes 44 are disposed in the top surface 38. The driver 20 is injection molded in an integral piece of a compound which is primarily carbon and resin. As will hereinafter be explained, the injection molding is done so that all or almost all of the various parts of the driver 20 are integral. This is because any machining of the resin will cut away at least a portion of the resin-rich outer surface, which results in a weaker (more brittle) part. Accordingly, any machining should be kept to a minimum. The base resin used is BR-31 (Ryton) from Phillips Chemical Company of Houston, Texas. The BR-31 is a polyphenylene sulfide resin, but unlike other polyphenylene sulfides, parts from this particular type of polyphenylene sulfide have more durability without internal cracking due to differential shrinkage. Also, thisLjaarticular resin is not corroded by most acids or other rrosion-inducing liquids, e.g., salt water. he base resin is then turned into a compound. Initially, a m \9«0 213624 sheet of the base resin is formed, and carbon fibers, which are chopped strands about 1/4 inch long, are pressed onto the top surface of the sheet. Usually, a resinous, phenolic blend of sizing is first applied to the carbon fibers to aid the fibers compatability with the base resin and to make handling of the fibers and molding easier. As the carbon fibers are distributed evenly over the surface of the sheet, the sheet is then cut into small chips, each of which has the same percentage of carbon and resin. In the preferred embodiment, carbon is about 40% of each chip by weight. The chips of the carbon and resin sheet are fed into an extruder and melted at temperatures between 575&F and 6750f. The total dwell time is five minutes, as any longer will adversely affect the material. The resulting strands from the extruder are chopped into pellets for further compounding, which involves adding a number of other materials. In the preferred embodiment, fiberglass (chopped 1/4 inch strands of E type glass) is added after being treated with a silane coupling agent which aids in obtaining a strong bond between the fiberglass and the rest of the compound. Concentrated pellets of polyphenylene sulfide with a black dye pigment are also added, along with a micro-fine channel black and a coarser bone black. These carbon blaoks are added in equal amounts. Calcium metasilicate (a high purity grade of Wollastonite) and calcium stearate (molding grade) are also added. In the preferred embodiment, the BR-31 resin amounts to 0f. between 50% and 75%, of the compound by weight, and generally is C"! / * v_. o ' ■ - - : s 7 . - 6 - 213S24 about 60% or somewhat less. The carbon fiber is between .25% . and 50% of the compound by weight, and generally is about 40% or somewhat less. Preferably, the fiberglass is usually about 5% by weight, although it may constitute as much as 50% of the 5 total. The silane coupling agent is .25% to 2% of the fiberglass by weight, and the sizing is between 2% and 5% of the O) carbon fiber by weight. The carbon blacks (both together) and the calcium metasilicate are all .25% to 5% of the resin by weight, while the pigment is between .25% and 4% of the resin by 10 weight, and the calcium stearate is between .25% and 2% of the ~ resin by weight. In addition certain other fiberous and non-fibrous reinforcements may be used in the compound. These include boron, asbestos, polybenzimidazole fiber (all fibrous), and 15 talcs, glass microspheres, clays, calcium carbonate, TEFLONR and other carbon blacks (all non-fibrous). When the additives have been blended together, this carbon and resin compound is kept tumbling and drying in an air-circulating, heated hopper to exclude moisture until the 20 compound is ready for injection molding. The pellets of the carbon and resin compound are then injected into a mold by feeding the pellets into the screw of the -machine through which they are moved through a barrel heated to 500®F to 6000f (nozzle temperature) in steps of 20®F or 50&F, 25 depending upon the number of heating zones. The injection pressure is 150 pounds per square inch when using a Trueblood .0 ton machine, and the mold itself is initially \R F C.V" 213S24 heated to 300®f + lO^F. This mold heating is to assure that the liquified compound will flow through the entire mold and fill it completely. A reduced mold temperature, e.g. under 250®F is not desirable, as the appearance and physical properties of the resulting driver may be adversely affected, and an uneven mold temperature may mean that all the cavities will not fill completely. Accordingly, the mold heating rods or oil passages (not shown) must also be located around the extreme mold areas and any other areas which may fill slowly or last to hold temperature through the mold at 300®F +_10&F for the filling process. The mold itself must be vented, contrary to the usual venting in the prior art thermoplastic molding processes, to eliminate all interior gases, the presence of which might cause the resulting part to have varying densities or blowholes, both of which potential sources of stress failure. As shown in a simplified mold 200 for this invention of Figure 6, mold venting is accomplished in two unconventional ways. First, in the usual molding process, round ejector pins are used to aid in removing the finished product from the mold. In molding a circular ring or a cylinder, four or so pins would usually be located symmetrically around the bottom (bottom edge in the case of a cylinder), and would force the piece up out of the-mold when the piece had hardened. With this invention, ejector pins 202 are flattened, at least where they enter round holes 204 for them in the bottom of the mold 200, so that air and other gases can escape from the mold by the flattened pins. (and many more if the piece is 213624 relatively large, e.g., a 6" diameter cylinder or ring) venting ports 206 spaced symmetrically around the outside of the mold. These ports 206 are located around the top of the mold 200 opposite the ejector pins 202, and the ports 206 are typically 1/4" wide and .004" high. At least one additional port (none shown) is placed in the areas where there may be a particularly significant gas build-up in the mold, e.g., deep grooves, holes and undercuts. The additional ports would be placed on the edges of such areas. Also, to fill the entire mold substantially at once, the mold 200 is filled using a 360^ ring gate 208 (rather than a few individual conduits) fed by a sprue 210, which is round and of highly polished steel with a minimum length, has a taper of at least half an inch per foot. The ring gate 208 is usually about one third the thickness of the part to be made. The ring gate 208 is attached to the mold at the top across from the vents 206. In operation, the liquid compound flows through the sprue 210 and into the ring gate 208, filling it. Once the ring gate 208 is filled, the compound from the ring gate 208 then feeds into the rest of the mold simultaneously around the entire 360^ periphery of the ring gate 208. Gas is forced out of the ports 206 and the portion of the holes 204 not covered by the ejector pins 202. The heating rods near the various cavities in the mold assure that the cavities fill completely. When the mold is filled, water is used externally to reduce the temperature of and the liquid therein," and the Vr-. V- ■ -*r ... - 9 - -ifj . 3&V "S?i-%> 213624 composition solidifies. The driver 20 is then removed from the mold by use of the ejector pins 202 and a reverse taper sprue puller (not shown). The driver is then post-cured for sixteen hours at 400^F, and the post-curing improves the strength of the driver and its mechanical and chemical resistance properties, as it causes the molecules of the driver 20 to become cross-linked thereby forming very strong bonds between them. It also increases the operating temperature at which the driver 20 will melt. 10 O-ring groove 36 and holes 32, 34 and 44 may be molded into the unit, or the driver 20 may be machined after the post-curing step to form them. In the latter case, the machining is done at a non-critical stress point, and therefore the part is not weakened substantially. Also, it is necessary to remove the 15 material solidified in the gate area as well as any material extending from the ports or the ejector pin holes. Post-curing is again performed after the machining operation. The compound made for BR-31 resin is very suitable for this use because it can be made into thick-section parts. 20 Furthermore, unlike with most other polyphenylene sulfide and carbon based compounds, the resulting part here is not brittle, but instead is highly flexible and can stretch. Thus, the part is much tougher than previous PPS-carbon parts. Also, the BR-31 is highly heat resistant, and the part made from the compound 25 can withstand 500^f, which is the desirable upper limit for most "^jsjnechanical seals (i.e. most usual mechanical seal applications n involve flows of 300UF, which means that that is #3 i -10- Z13G24 the minimum temperature applied to the seal, as pump operation will raise the temperature of the seal higher than that of the flow itself). Other known resins could be used in the compound/ but strength, temperature and corrosion characteristics would be somewhat less and unsatisfactory for some applications. In order to complete the driver 20, rubber O-ring 46 is inserted into the O-ring groove 36, and springs 48 are placed in each spring hole 44. The top half of the springs 48, all of which require about thirty-six pounds to compress, extend above the top surface 38. As best shown in Figure 4, a clamp 50 is provided for the lower end 26 of the driver 20. Clamp 50 is generally a half-ring having a pair of screw holes 52, 54 at its ends. Inner wall 56 of clamp 50 is angled slightly, about 1.5°, (the angle shown in Figure 4 is exaggerated), and when the clamp 50 is in place in the driver 20, the narrowest portion of the wall 56 is disposed towards the upper end of the driver 20. Screws 58 hold clamp in place on driver 20. As torque is applied to the screws 58, the clamp material is compressed so that the entire inner surface of the clamp 50 contacts the shaft. Clamp 50 is made of the same material and in the same manner as the driver 20, and as thermal expansion causes the shaft to grow, so will the clamp 50, without loss of sealing area. The seal portion 60 is best shown in Figure 5. Seal portion pS^generally comprises a cylindrical sidewall 62, the inner diiirofeter of which is slightly greater than the outer diameter of # ft m le^idewall 22 of the driver 20, as shown in Figure 2. - • : " \ ~ V"'~ (. >>.. :ir' . ,r.. - \ *>■ ' ■ ■ •> '"r.y t I __ _ ...^•wrr'V o -11- Z15GZ4 Sidewall 62 has an inwardly disposed flange 64 around its top, and a pair of tab slots 66, 68 are disposed in the flange 64, 180® apart. A seal lip 70 having a sealing face 72 extends upwardly from the cylindrical sidewall 62 and the flange 64. 5 Lip 70 is offset from the edge of sidewall 62. Around the inside of the seal portion 60 at its opposite end is an annular O-ring groove 74. O-ring seal 76 is placed in the groove 74 after the seal portion is made. The seal portion 60 is made of graphite powder and phenolic 10 resin. The graphite powder, which is 99% pure carbon, is mixed with the phenolic resin. The resin makes up about 10% of the total mixture, although up to 20% resin is also satisfactory. The mixture is then poured into a mold and compression molded into the shape of the seal portion 60. The loading used may 15 vary between 5 and 10 0 tons. When the seal portion 60 has been formed, it is then post-cured at 3500f for four hours. As with the driver 20, this cross-links the molecules, making the part stronger and improving its temperature resistance. As shown in Figures 1 and 2, the seal portion 60 is placed 20 over the top of the driver 20. The driver tabs 40, 42 slide into the tab slots 66, 68 which assures that the seal portion 60 will rotate with the driver 20, and the top of the springs 48 contact the underside of the seal portion flange 64. Operation 25 A portion of a pump 100 is shown generally in Figure 2. The pump 100 has a rotating shaft 102 and a ceramic collar 104 around but spaced apart from the shaft 102. 'i Assembled seal 10 is slipped over the shaft 102. The dimensions of the seal are such that the inner diameter of the driver 20 and seal portion flange 64 are only slightly larger than the diameter of the shaft 102. As these seal parts are molded, exact-fitting seals for odd-sized shafts can be easily and inexpensively made. The seal 10 is sealed to the shaft 102 by O-ring seal 46, and O-ring seal 76 prevents any leakage between the driver 20 and the seal portion 60. The seal 10 is held in place on the shaft 102 by clamp 50. Tightening the clamp screws 58 forces the angled wall 56 against the shaft from the position shown in Figure 2, and the seal 10 is held firmly in place without dimpling the shaft 102. This attachment is made close enough to the ceramic collar 104 so that the sealing surface 72 of the seal lip 70 is forced against the collar 104. As the shaft turns, the lip 70 rotates while the collar remains stationary^ but as the lip 70 is mostly graphite, this interface is virtually frictionless. The lip 70 will eventually wear down with use, but because the lip 70, as well as the rest of the seal portion 60, has substantial strength, and because the lip is longer than the effective length of the brittle graphite lips of conventional seals, the seal portion of this invention has a much longer useful life. Also, the driver and seal position have substantial resistance to corrosion and high temperature. The driver 20 is subject to more stress than the seal portion 60, and it is somewhat stronger. Nevertheless, the coefficient of expansion of the 'driver 20 and seal portion 60 -13- ^ o OK/ 4i are almost the same so no leakage results when the parts expand due to use in a high temperature liquid. Other variations will occur to those skilled in the art. 5 © O </div>

Claims

<div class="application article clearfix printTableText" id="claims"> ."V j / ' •"*» 'M yrv>.<-n -llf- tj •_> ^ -i WHAT WE CLAIM IS: <43^

1. A method of making a seal portion of a mechanical seal comprising: mixing a compound of substantially pure carbon graphite powder and .gl-ienofcic resin said resin amounting to 10-20% of the resulting compound; filling a mould for the seal portion of said seal with said compound, and solidifying said compound; removing said solidified seal portion from said mould; and post-curing said solidified seal portion at a temperature sufficient to effect cross-linking of the molecules comprising said compound.

2. A method as claimed in Claim 1 wherein said post-curing is at a temperature of 350^F for four hours.

3. A method as claimed in Claim 2 wherein said solidifying said compound in said mould comprises applying loading to said compound, said loading being between 5 and 100 tons.

4. A method as claimed in any one of the preceding Claims further comprising machining said solidified seal portion after post-curing.

5. A method as claimed in Claim 4 wherein said seal portion is post-cured for a second time after said machining.

6. A method of making a seal portion of a mechanical seal substantially as hereinbefore described with reference to the accompanying drawings.ji-jjs DAY OF V "A A. J. PARK & SON PER \ X • \ AGENTS FOR THE APPLICANT </div>