Material selection criteria for water lubrication

Wear, 25 (1973) 139-153 0 Elsevier Sequoia S.A., Lausanne

MATERIAL SELECTION

139 - Printed

in The Netherlands

CRITERIA FOR WATER LUBRICATION*

WATT V. SMITH U.S. Naval Ship Research and Development Center, Annapolis, Md. 21402 (U.S.A.) (Received

December

18, 1972)

SUMMARY**

The unusual degree of interaction between service performance parameters and material selection for water lubricated contacts is developed. The importance of exploring the full range of variation in operation is stressed. The problems of corrosion, dirt and thin films associated with water lubricants in valves, bearings and seals are discussed. Examples are shown of some benefits gained through the use of water lubrication. The theory of elastohydrodynamic lubrication is shown to be of little use for water lubrication as a result of the negative and small positive (depending on temperature) pressure coefficient of viscosity: The properties of some successful water lubricated contact materials are reviewed with emphasis on water swell, abrasive wear resistance and minimum Hersey Variable values.

INTRODUCTION

Water lubricated surfaces occur in many types of industrial, agricultural, and marine equipment. The primary functional requirement may be either the sealing or the positioning of two relatively movable surfaces. Contact of the surfaces must be expected for at least a part of the operational cycle. The choice of the materials of the opposing surfaces and the conditions under which the contacts are allowed to occur will determine the life of the equipment. Excessive leakage, frictional force, heat and loss of positioning are life limiting conditions for the water lubricated surfaces and often for the equipment of which they are a part. These life limiting conditions occur much sooner if the materials and contact conditions have been poorly chosen. The present paper will examine the operating variables of importance to the selection of materials and the properties of some of the most successful materials used in water lubricated applications. THEORETICAL

ASPECTS

The plot of the coefficient of friction against the Hersey Variable, ZN/P, shown * Paper presented at the First World Conference on Industrial 1972. ** The opinions expressed in this paper are those of the author views of the Navy or of the Naval Service at large.

Tribology,

New Delhi, December,

and do not necessarily

reflect the

140

W. V. SMITH

in Fig. 1, is useful in discussing general lubricating processes. In very general terms, the force between the surfaces is transmitted by solid-solid contact mechanisms in the boundary region. A portion of the farce is transmitted by fiuid pressure in the mixed film region. When full fluid film lubrication is achieved, somewhat to the right of the minimum in the Hersey Diagram, all of the force is transferred by hydrodynamically generated pressure in the film.

I

Fig. 1. Bearing

F

( CP rey/min)

20 I

4a ps’

4.8 x lo-

9.6

friction

w. Hersey

60

80

loo

14.4

19.2

24.0

120 28.8~TO-~

number.

The low viscosity of water, less than l/NJ&h that of oil, implies that water lubricated surfaces wili probably spend a much higher proportion of their lives in the boundary and mixed film regions than wiif similar oil fubricated surfaces, The materials of the contacting surfaces are primarily responsible for performance in the boundary and mixed film regions while design tends to be more important in the full fluid film region. Service applications impose sets of requirements that myst be met by the materials of the rubbing surfaces. The primary objective of the materials selection process is to obtain the most effkient combination of materiaf properties when compared to the real requirements of the service. Material properties, manufacturing methods and labor costs vary so widely that compromise rather than unique selections should be expected.

MATERIAL

SELECTION

CRITERIA

141

DESIGN PARAMETERS

The first part of the selection process is the definition of the service factors that are life controlling in the greatest number of cases for a specific application. A truly total definition of significant service factors is probably not attainable for most applications. Efforts expended to determine as complete an operational spectrum as possible will be rewarding. For example, the main propulsion plant bearings of ships are designed to operate over a speed range of about 10: 1, load is independent of speed with the exception of gear and main thrust bearings, operation is above 90% of full power for more than 90% of underway time, and the bearing design is primarily directed at the maximum conditions. All turbine driven ships are also required to operate on jacking gear while warming up and cooling down the turbine to prevent rotor bowing. Jacking speed is customarily one rev. in ten to twelve min. At this speed most if not all the bearings are in the boundary region of operation and undergoing wear. This is especially true of the water lubricated bearing that supports the propeller weight on many ships. Many commercial operators keep the jacking gear operating for the full stay in port. The slow wear is ignored even though replacement could be avoided by securing the jacking gear after the desired turbine temperature is reached. The first service requirement is size. Size is determined by the application and cannot be altered by the material selection process in most cases. Shaft size in rotating machinery is determined by the required torque, by bending stress, by allowable deflections or by the stiffness required to avoid vibration criticals. In valves, the size of the moving surface is determined by the required flow area and by the type of valve being designed. The second design parameter is the space available to the contacting surface. Space requirements for bearings and seals exert a strong influence on the casing or support structure. This space is far more costly than the volume required for the contacts. Space restrictions can dictate a more costly material in order to achieve the minimum required service life. In most cases careful study is required to determine just what space can be devoted to the bearings and seals. Where water is the principal environment, designs based on water lubricated bearings can provide large savings in space and weight. Figure 2 shows a conventional electric motor driven pump supported on grease lubricated bearings. Figure 3 shows the same motor and pump elements converted to water lubricated bearings and encapsulated windings. The length of the unit was reduced from 3G to 18 in. and the weight from 625 to 267 lbs. In addition to the allowable space, a determination must be made of the displacements and rotations to be controlled or accepted. Bearings control displacements and rotations within some prescribed limits necessary to the successful functioning of the machine. Seals on the other hand are expected to accept the displacements that may be imposed. Machines with small clearances cannot tolerate large displacements. This fact precludes the use of very low modulus materials as bearings unless they can be applied as thin layers. Large clearance machines can use the widest possible range of material stiffness. Seals are very sensitive to dynamic displacements. The more rigid the material, the less tolerant it will be of large dynamic displacements. In defining the service displacement requirements,

142

i-i’.V. SMITH

Fig. 2. Conventional

Fig. 3. Submerged

close-coupled

pump.

marine

pump.

MATERIAL

SELECTION

CRITERIA

143

full consideration must be given to the location uncertainties of initial and maintenance assembly. Location uncertainty analysis includes the range of specified tolerances for all components and the effect of these tolerances on the final position of the contacts. The conventional service requirements are speed, load and temperature. The full speed spectrum of a machine will include expected transient speed changes. Modern lubrication analysis has made such great progress in the prediction of performance that there is a tendency to overlook the steady state limitation on these calculations. Water lubrication is less subject to the thermal upsets associated with sudden speed changes than are oil lubricated surfaces. This is probably due to the much higher heat capacity of water and to a lesser extent to the lower viscosity. When materials of high thermal expansion and low thermal conductivity are used, the order of magnitude of transient thermal effects must be examined even for water lubricated contacts. This transient thermal behavior can be of critical importance in establishing minimum design clearance. Journal bearings in submerged electric motors are a good example of this problem. Electric motors reach full speed in a very short time. If insufficient clearance has been provided, the bearing will close in until it acts as a brake. Heat generation rates rise sharply and water flow is drastically reduced. Failure under this condition destroys the bearing surface and often damages the journal. Load spectra should also be determined for all design conditions. Variations in load angle with speed and flow, mass and hydraulic unbalanced loads and the loads imposed by mechanical or thermal misalignment should be included in the load spectra. Load is used in the determination of the minimum film thickness and its angular position in the computer analyses available in refs. 2 and 3. The low viscosity and high heat capacity of water makes the usual viscosity uncertainty in such an analysis small. Load angle is important in determining the position of water supply grooves. In centrifugal pumps with single discharge volutes, the radial load at design flow rates is nearly zero. Reference 4 shows that as flow is varied from shut off to full open, the load vector passes through minimum amplitude at design flow rate. The direction of the load vector varies through nearly 180 degrees. If a water inlet groove or dirt relief slot is located in this arc, the hydrodynamic pressure is relieved and high speed surface contact is very probable. Loads resulting from misalignment or thermal distortion are very troublesome with water lubrication where space or cost considerations prohibit self aligning bearing or seal designs. Loads are concentrated on small areas. Unit loads are limited by the stiffness of the contacting surfaces. The use of easily worn or low modulus materials may alleviate this problem in some cases. Rotating loads of large magnitude may require that the low modulus material be applied to the rotating surface rather than the stationary member. Temperature definition is very important for water lubrication. Changes in water viscosity, clearance and flatness resulting from differential thermal expansion, material strength changes, and water flashing into steam are all significant temperature effects. The range of water viscosity with temperature is shown in Fig. 4. Differential thermal expansion is one of the more troublesome factors in water lubrication. The shape and clearance of the surfaces must be studied to determine the best method of providing the desired shape at operating temperature. In some

W. V. SMITH

Fig. 4. Water

viscosity

us. temperature

cases the selection of a material that wears in quickly will provide the best solution. Material strength changes are usually a secondary consideration. Surfaces operating hydrodynamically are subjected to a hydrostatic compressive stress. Shear stresses are generated only in those regions where pressure is changing rapidly with linear dimension. Usually at the locations where the rate of change has become very steep, the absolute pressure has fallen too low for the stress to be large enough to cause damage. Flashing could cause damage but other than high levels of vibration the author is not aware of any material damage definitely connected to this cause. A continuous flow of water through the contact region must be maintained for high speed bearings and seals. Water starvation of these surfaces can arise in various ways. Overheating as a result of insufficient flow is a familiar trouble with pump packings and seals. Large face type seals exhibit heat checking, steam emission and rapid wear when pressure changes are encountered after long running at a single pressure. Seals of low stiffness seal tighter than those of higher rigidity and are far more prone to develop symptoms of excessive heat. Bearings with the exposed end of the shaft running in the water supply have separated dissolved air from the water over a period of time to the extent the supply was blocked and the bearing destroyed, Where lubricating grooves surround the shaft the rotating ring of water in the groove generates pressure. The pressure so generated must be overcome by the supply if water starvation is to be prevented. The provision of a sink or discharge is equally necessary if flow of sufficient magnitude is to be maintained. Water lubricated surfaces with flow rates too small to remove frictional heat must become sufficiently hot to conduct the heat through solid surfaces. This is usually difficult and often impossible. CORROSION

EFFECTS

Water lubricated surfaces are corroded by a variety of mechanisms. Externally

MATERIAL

SELECTION

145

CRITERIA

imposed currents, as found in ships propeller shaft systems, flow from the zinc protective anodes through the propeller and thesterntube and strut bearings back to the hull to complete the circuit. The shaft journals are anodic to the bearing surfaces during this process. At least a part of the journal wear must be the result of electrochemical removal of metal from the shaft sleeve. Surfaces at rest in water frequently develop lines of pits adjacent to the contact surface. When the surfaces are at rest, the oxygen between the surfaces becomes depleted and oxygen concentration cells are set up with the liquid of higher oxygen content just outside the contact. The potential gradient results in the line of pits in the journal. The presence of a nonconductor as one of the two surfaces may reduce the effect but will not eliminate it. Periodic operation will restore the oxygen level and will eliminate the concentration gradient and the resultant corrosion. The frequency of operation required will be determined by the rate of oxygen removal from the contact zone. This is a function of the rate of reaction of the contact materials with oxygen, the type of protective films formed and other less well defined parameters. EFFECT

OF CONTAMINATION

Dirt is a serious problem for water lubricated surfaces that can be alleviated by the proper precautions. Pretilters, either porous or centrifugal, can increase the life and performance of most water lubricated bearings. Bearings designed with straight slots choked at the outlet end provide a combination water supply and dirt relief groove. Such designs tend to prevent wedging of the surfaces by the dirt by providing a preferA red path for dirt discharge rather than requiring that all the dirt exits through the film. Seals usually cannot be given such protection, hence some form of precleaning of the water is even more important. Dirt passage through water lubricated surfaces is a major factor in determining the life of the surfaces. Dirt damage is not independent of the operating zone of the Hersey Curve. Figure 5 from ref. 6 shows the

110-4

-t

b i; lo- s ,o

k 5 g 10-6 ‘

I! t: 10-7 $

AT ii ;

$ 10-e ‘

b

P 10-s @J/px 10’ Fig. 5. Abrasive

wear rate of lignum oitae.

146

W. V. SMITH

strongly negative slope of the rate of wear of lignum oitae with increasing values of the modified Hersey Number (surface velocity instead of r.p.m.). Design may reduce or prevent dirt entry into the lubricating wedge. Once entry has occurred, the properties of the rubbing surfaces determine the nature and extent of damage resulting from its passage. If the dirt is softer than the rubbing surfaces, damage will not result unless the surfaces are wedged together by the dirt. If the dirt is harder than the rubbing surfaces, the softer of the pair will be scored unless it is flexible enough to deflect away without providing enough pressure for scoring. Abrasive and contact wear effects are the primary life limiting mechanisms in water lubricated bearings and seals. The final choice of both design and materials should be made on the basis of required life. The most economical combination of design and material is the objective. A complete cost review includes material, manufacture, total replacement cost, and the full loss potential from premature failure or impaired operation of the component. GENERAL

CONSIDERATIONS

The foregoing paragraphs cover at least a part of the combined design, material, and manufacture selection processes that influence the success of water lubrication and are of general application. materials and manufacture selections should be integrated with and interact on the component design. Virtually all materials have been tried at one time or the other. Continued use implies that the combination of properties and costs provide measurable advantages to both the producer and the user. The properties and applications of at least some of the more successful material combinations for water lubrication will be covered in the balance of the paper. Applications that operate in the boundary lubrication region for their entire lives are characterized by slow speeds, frequently very intermittent, loads may vary widely and the total distance traversed in the entire life of the surface is relatively small. Examples of this type of application include valves, valve packings, bearings for lock gates and simiiar service. When positioning is a very secondary consideration, soft, highly compressible materials are very serviceable. The low cost of vegetable fiber has encouraged its use in low temperature packings. A variety of impregnants, animal fats and petroleum waxes, have been used to provide sealing at low surface pressures thereby obtaining lower friction and wear. In higher temperature applications, asbestos and Tellon fiber products are employed. The porosity of the compressible fiber products precludes signi~cant hydrodynamic reduction of friction and wear. Where positioning is of greater importance as in valve seats, harder materials are required. In general water valve service, bronzes and brasses provide the best combination of properties. Lead is often a part of the composition in the range of one to ten per cent. Lead in bronze or brass alloys remains in the form of dispersed globules. As wear progresses, the lead wipes a thin lubricating film on the surface, providing an easily sheared layer and preventing the development of long grooves. The higher the surface unit loads expected, the higher the lead content required for good service. Tin content is often increased at the same time to offset the softening effect of the lead.

MATERIAL

SELECTION

147

CRITERIA

The disc area of very large valves will be about ten times the seat area. Even low pressures of 30 to 50 p.s.i. on the disc will produce 300 to 50? p.s.i. on the seat when the valve is fully closed. As the valve is opened the contact area is reduced and the unit load increased. Sliding of the seat under these conditions produces wear and leakage in even the high lead alloys. For most valves, the total distance travelled is so small in a normal operating life that the damage is of small consequence. Valves that are operated several times a day over a twenty to thirty year life span cannot tolerate the damage. Either the seats must be provided with higher viscosity lubricants, much harder and more expensive seat materials provided, or some method of relieving the surface pressure adopted. In one case of a 36 in. diam. wash water valve for back washing filter beds, three small wedges of castable carbides were used to lift the seats apart and limit the distance travelled on each operation to very small values. The valve was operated for more than 5000 cycles without measurable wear on either the carbide wedges or the valve seats. Full boundary lubrication in water is very different from the same region withoilas the lubricant. The development ofelastohydrodynamic theory has identified the increase of oil viscosity with pressure as the key to the prevention of solidsolid force transfer. Most petroleum oils probably become easily sheared solids under sufficiently high pressure and low shear rate. Water viscosity is reduced by pressure at temperatures below 35”-36°C then passes through a minimum before rising very slowly with increased pressure. Figure 6, from ref. 5, shows a +2% change in water viscosity due to pressure up to 1,400 kg/cm’. One must conclude that elastohydrodynamic processes will not exist in rolling or sliding contact processes in water. This does not imply that the selection of materials for water lubrication can be based on the dry sliding behavior modified to account for the heat removal effects of water. Such a conclusion ignores the chemical aspects of water lubrication. To the extent that water is allowed to react with freshly exposed surfaces, the wear resulting from further contacts will be reduced by shearing of the reaction products rather

400

600

800 Pressure

Fig. 6. Relative

viscosity

of water for various

moo

1200

Moo

(kg/cm*)

temperature

and pressures.

148

W. V. SMITH

than the base metal. In metal-nonmetal contacts, the role of water in reducing the wear of carbon and graphite appears to be well established. In the case of metalpolyole~ns contacts, water increases the rate of wear over that of the dry condition. The wear ofpolytetrafluoroethylene increases by orders of magnitude in the presence of water. These effects must be associated with chemical interaction with one or more of the contacting surfaces. Wear and sliding data can be very misleading if dry sliding data is applied to water lubrication. Water lubricated surfaces operating predominantly in the mixed friction region are characterized by low to medium loads, generally below 100 p.s.i., and surface speeds from 100 to 1000 ft/min. The stern tube and strut bearings of ships, radial bearings of large water wheels and pumps, and the seals and packings, for these machines are examples of mixed friction operation. The examples cited are also characterized by very long life requirements in a cool but dirty environment. Water lubricated stern tube and strut bearings of ~~g~~~aitae were used in steam ships from 1854 through the 1960’s. The frictional characteristics of the longitudinally slotted bearing typical of stern tube and strut service is shown in Fig. 7. The very sharp drop in friction with increase in ZN/P is characteristic of this bearing. The absence ofa minimum is also typical. Under some conditions, primarily exceptionally accurate and finely finished surfaces, the curve can show a large reduction in friction and a clear minimum. The longitudinal slot is very effective in reducing the entry of dirt into the contact area. In addition to Signum vitae, laminated phenolic and rubber faced staves have found considerable use in this service. Chukhrin7, reported successful operation of fully cylindrical bearings made of Caprolon “B” (Nylon 6) in bearings of 343 in. diam. in three tugboats operating in dirty water. Where large clearance is acceptable, as in ships bearings, quite long life is obtained often as long as fifteen years in naval service. Full fluid film water lubricated bearings are found in high speed pump bearings and seals. Operation is characteristically well to the right of the minimum in the Hersey curve. In many water lubricated bearings the operation is in the turbulent

Fig. 7. Friction

of stern tube and strut bearings.

MATERIAL SELECTION CRITERIA

149

flow regime. Turbulence produces high power losses in the machine but has no adverse material effects. Material effects are less critical in water lubricated bearings in the full fluid film region. Those material effects that limit the extent of the pressure film, or that cause local penetration of the film are significant and must be made a part of the selection process. The elastic modulus of the material limits the magnitude of the load that can be supported. Low modulus materials deflect to such an extent that the hydrodynamic pressures are limited by the increased leakage allowed by the deflected surface. Blok’, has called the lubrication process where the maximum film pressure is limited by the deflection of the surface “inverse hydrodynamic lubrication” to distinguish the process from hydrodynamic and elastohydrodynamic lubrication. The low modulus materials do show greater tolerance for misalignment and dirt passage. The selection of a combination of moduius and thickness will provide a spring rate that is most suited to the application. The strength required of water lubricated bearing materials is determined by the compressive yield or creep under the service load. The Hertzian stress produced under stationary conditions is usually the most severe. Elastomers and some plastics exhibit compression set under static load. All or most of the compression set is recovered over a period of time after the load is reduced or removed. The process of set and recovery may alter the film shape adversely. The thermal expansion and conductivity of water lubricated materials are of high significance in high speed bearings and in bearings or seals operating in a high ambient temperature. Differential thermal expansion coefficients between the shaft and the bearing or the bearing support affect the operating clearance. The higher thermal expansion rates of plastics must be offset by the provision of space to accommodate the expansion to prevent a total heat failure. The low thermal conductivity of plastic increases the radial gradient further agravating the effect. Carbons, carbides and ceramics have very low ,thermal expansion rates. These low rates must be considered when selecting initial design clearances. The water swell characteristic of the material is one of the most troublesome factors to be considered in the design of water lubricated contacts. Reliable values for long term equilibrium are di~cult to obtain for the basic materials. Most specification values are determined on the basis of a few hours exposure. The swell process is controlled by the rate of diffusion of water into the material, the concentration gradient of the water in the material, and the saturation concentration. The swell process approaches equilibrium exponentially with time. Service requirements of several years exposure render predictions based on a few hours exposure suspect. Accelerated swelling tests have been used in s~ci~~tions based on boiling the material for 120 hours. Some comparative data on the swell obtained by boiling and by soaking at room temperature are shown in Table I. The 120 hour boil produced at least 70% of the six month room temperature swell. Additional complications are introduced by the higher swell at the intersection of surfaces, by displacement increases induced by the method used to restrain the bearing material, and by anisotropic swelling characteristics of many materials. Preswelling of the material is customary for Zignum vitae but has been used rarely for other materials. Preswelling imposes the requirement that the material be prevented from drying out during the manufacture and installation to avoid warping and cracking. The problem

150

W. V. SMITH

long term swell is illustrated in Fig. 8. Some of the materials appeared to have nearly stopped swelling after four to six months. Others show but little decrease in the rate of swell.

of

TABLE

I

LAMINATED

PHENOLIC

SWELLING

PERPENDICULAR

TO LAMINATIONS

Hours boiling 212°F

Source

48

24 A

0.020 0.021

0.018 0.016

0.022 0.015

0.019 0.012

B

0.022 0.016

C

0.023 0.020

0.014 0.012

D

0.018 0.017

0.017 0.011

E

72

0.018 0.015

0.016

0.020

0.041

0.052

0.033

0.043

F G H * First time removed

0.0001- ’

’

’

12345676

96 --

0.023 0.023 0.020* 0.024 0.018 0.018 0.025 0.023 0.023* 0.024 0.020 0.019 0.022 0.018 0.017* 0.024 0.025* 0.058 0.072* 0.052 0.05 1*

(in.@.)

Six month swell room temperature

120 _ 0.023* 0.023

0.030

0.022

0.023* 0.022

0.028

0.021

0.029 OiO27

0.028

0.026

0.027* 0.026

0.02 1

0.024

0.022* 0.022

0.030

0.020 0.028

0.029

0.045

0.080

0.087

0.05 1

0.058 --

0.059

0.048

from water for measurement

’

Weeks

’

’

’

I

I

3

4

5

6

Months

Fig. 8. Laminated phenolic swelling at room temperature laminations. ------ Swelling parallel to laminations.

(one inch cubes). ~

Swelling perpendicular

to

MATERIAL SELECTION CRITERIA

151

The abrasion resistance of the material determines the life of the contact in service where dirt is a major conside~tion. That the wear is strongly affected by operating conditions was shown in Fig. 5. Presumably the observed decreases were the result of increased film thickness with respect to the particle size entering the rubbing contact. A number of studies have shown, for oil lubricated bearings, that wear will not result unless the particles are larger than the film thickness. In water lubrication, the small film thickness from hydrodynamic forces would seem to offer little hope of a wear free surface. The extremely rapid decrease of wear rate with increase in 2X/P would suggest that an asymptotic limit may exist where wear will be vanishingly small. The Signum vitae data were obtained using high concentrations of abrasive to accelerate the wear process. Lower concentrations produced orders of magnitude less wear. In applications where a wide range of abrasive materials and concentrations are to be expected, material selection can be guided by the relative resistance of a group of otherwise usable materials to an arbitrary abrasion test procedure. Reference 8 describes the basis of a test procedure that has been found to be useful. Briefly, a test journal, 2 in. diam. by 0.5 in. wide of nickelsopper-aluminium alloy (KMonel) ground to 8-14 r.m.s. roughness, is rotated at 97 r.p.m. on the apparatus shown in Fig. 9. The bearing material specimen is provided with a flat face 1 in. long by 0.44 in. wide and is loaded against the journal with 4 lb./in. of width. Aluminum oxide abrasive grains, passing a number 50 standard sieve and held on a number 60 standard sieve, are deposited on the journal at a uniform rate of 75 g/h. A stream of water (75 ml/min) containing 0.04 ~01.~4of sulphonated oil (wetting agent) washes the used abrasive and wear product off the journal surface. The depth of wear after one hour is used as a measure of the abrasive wear resistance. Table II shows the values obtained for a number of non-metallic materials. The superior abrasive wear resistance of the elastomers as compared to the more rigid plastics and wood is evident. Thejournal wear in these tests was approximately 0.017 x tom6 in./in. In clean fully hydrodynamic applications, the ability of a bearing to operate at or near the minimum of the &V/P curve has considerable meaning. The ability of

Journal specimen

(Not to scale)

Fig. 9. Kommers abrasive wear apparatus.

152 TABLE

W. V. SMITH II

ABRASIVE

WEAR RESISTANCE

OF NON-METALLIC

MATERIALS

Murerial

Shore hardness

Wear rate (in./in. oj tracel)

Buna N rubber Polyurethane Acetat + teflon fiber Acetat + moIybdenum disulfide Polycarbonate

70-85 A 55-57 D 82 D

0.2-0.8 x

Polycarbonate Chlorinated polyether Polyethylene Laminated phenolic Lignum tiitae

85 D

IO--h

05 1.3 3.4 2.8

85 D

1.5 1.5 2.9 3.8 3.5-5.5 4-9

LE III PERTIES on

OF SOME

ZNjP minimum

3 5 6 6 7 15 18 20 25 30 30 50

COMMERCIAL

CARBONS Water absorbed

Hardness Rockwell “S’*

Scleroscope**

21 20 27 20 26 23 24 25 20 14 0 4

94 62 97 65 88 80 80 81 75 57 42 48

Ash

T,i consumed

(“‘0)

Specific volume (g/ml)

(“’

1200°F

0.12 1.96 1.32 0.00 0.32 0.52 2.95 1.11 5.48 2.76 1.83 8.01

1.815 1.687 1.739 3.071 1.707 2.918 1.740 1.800 1.730 1.586 1.816 1.618

1.98 96 9.71 60.11 3.70 91.46 Fused*** 0.46 90.71 Fused’ 1.96 93.89 9.88 66.36 2.38 86.25 6.22 89.12 0.01 9.42 0.046 72.00 - - ---~-

1400’1 -_

-~

._

95.41 89.64 94.00 89.80 96.60 87.92 96.80 92.68 84.54 99.20

Rockwell “s” Scale 4” ball, 100 kg load. Diamond Striker. Lead 37%. Tin 14%. “opper 300/,, Lead 14%.

a material to operate successfully in this region implies a very smooth surface, good wear-in characteristics and low porosity. Table III gives the performance and properties of 12 commercially available carbons from four manufacturers. Performance was determined by a fixed land tapered land thrust bearing 22 in. outside diam. by 1: in. inside diam. by $ in. thick. Six pads were provided with 3 of the pad having a taper of 20 x 10W6in. taper. The remaining 3 of the pad was flat to better than three light bands. The minimum ZN/P was determ.ined at 430

MATERIAL SELECTION CRITERIA

153

t.p.m. by increasing the load in 10 lb, increments until the increase in friction indicated that the ~nimum had been passed. The water absorption values. shown in column 5, were deter~ned by placing a specimen in water in a vacuum desiccator, reducing pressure above the water and holding until bubbles of air stop rising, then raising pressure to atmospheric. The specimen is allowed to stand for a period, then removed, blotted dry, and the weight gain is measured. This method shows the porosity rather than the dimensional change associated with long term swell. The carbon consumed at 1200°F is a measure of the carbon present as graphite while the carbon consumed at 1400°F measures total carbon. The noncombustible material was’primarily silica, The ZiVjP minimum does not correIate well with the other properties measured. The actual application data available does appear to validate the ZN/P minimum as the best indicator of service behavior. CONCLUSION

In summary, water lubricated sliding surfaces foIIow the classic hydrodynamic theory were fairly rigid continuous surfaces form the contacting pairs. The elastohydrodynamic lubrication analysis is not useful since water has a negative coefficient of viscosity with respect to pressure, The inverse hydrodynamic lubrication analysis appears to offer the best approach to the use of soft materials in water lubricated sliding contacts. Water lubricated sliding contacts have very thin lubricated films as a result of the low viscosity of water. The thin films and the prevalence of dirt in water lubricated applications requires that a very careful analysis of service factors be made to provide the basis of material selection. The high heat capacity of water will permit the use of lower thermal conductivity contact materials in water lubricated applications than are used in oil lubrication. ~igni~~a~t savings in space and weight can be achieved by designing for water lubrication where water is the operating environment. REFERENCES 1 W. V. Smith and L. G. Schneider, Lubr~t~on in a sea water environment, Navel Ew* J;, (1963) 841-854. 2 P. Pincus and B. Sternlicht, Theory of Hydrodynumic Lirbricdtion, McGraw-HiII, New York, 1961. 3 J. J. O’Connor and J. Boyd (eds.), Standard Handbook of Lubrication Engineering, McGraw-Hill, New York, 1968. 4 A. Agostinelh, D. Nobles and C. R. Mockridge, An experimental investigation of radial thrust in centrimgal pumps, J. Errg. Power, ( 1960) 12(r126. 5 E. M. Stanley and R. C. Batten, Viscosity of water at high pressures and moderate temperatures, J. Phys. C&m., 73 (1969) 1187-1191. 6 Properties of Lignum Vitae and its use as a bearing material, Sot. Nan Arch. Marine Engrs.: 7 & R BuK, No. 3-21, 1967. 7 L. A. Chukhrin, Ispytaniya deydvudnykh podshipnikov iz kaprolona “B” (Tests of stern tube bearings made of captolon ‘B’), Sudostroenie fS@buiIdhg), 6 (f968) 53-56. 8 W. V. Smith, Wear properties of plstics as ships bearings, In J. V. Smitx and W. E. Brown (eds.), Testing of Po~ymers~Yoi. 3, Interscience, New York, 1967, pp. 231-244.

9 H. Blok, Inverse probIems in hydrodynamic lubrication and design directives for lubricated flexibIe surfaces. In D. Muster and B. Sternlicht (eds.), Proc. Intern. Symp. on Lubrication and Wear, University of Houston, 1965, pp. l-151.