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Paper IX (ii) Seals for Passenger Car Wheel Bearing Units
243
Paper IX (ii)
Seals for Passenger Car Wheel Bearing Units J.C.M. Bras
The modern concept of a car wheel bearing is a complete unit that is ready to use, greased and sealed for life. Thus today the lubricant and the seal form an integral part of the bearing. With ongoing improvement in bearing internal geometry and materials, the performance of the seal can even replace rolling contact fatigue as the limiting factor for bearing service life. At the same time, demands for reliability and long, service-free running periods are continuously increasing. Systematic efforts during recent years have resulted in improved seal designs and have provide deeper insight into their functioning and the tribology effects involved. Seals for grease lubricated bearings are defined as active hydrodynamic engineering elements, with a preferential outward pumping effect to prevent penetration of foreign matter into a bearing unit. Seal life is thus governed by the existence of preferential pumping effects. When these have vanished, the seal transforms from an active into a passive engineering element. Bearing life from then depends on environmental factors only. Several mechanisms were identified which influence these effects negatively and thus restrict seal life: of these ageing of the seal lip material and counterface abrasive wear are described. Limiting factors were found and incorporated in a seal life model.
1
INTRODUCTION
Automotive wheel bearings are sealed and greased for life components. The component life is governed by its resistance to rolling contact fatigue, by lubrication and by seal performance. Since seal performance is regarded as critical to the life of a wheel bearing, a development programme was designed to investigate all aspects of this performance. 1.1 A
Notation = Time constant
= Life adjustment factor = ai,,i Life adjustment factors (0-1) C,, = Corrosive wear rate
ai
[hl [CLmhI
DL = Limiting corrosive damage IWI E = Young's modulus [ N.m-21 E, = Activation energy [kJ.K-1.mol-l] WSA = Hardness Shore A
L = Seal life in hours [hl La = Adjusted seal life [hl la^ = Temperature adjusted seal life [hl LB = Basic rating life [hl L, = Expected life at temperature Ti [hl Lj = Expected life at wear process j [hl Pi = Proportion of time at temp. Ti Pj = Proportion of time at wear process j R = Gas constant [kJ.K-1 .mol-l] [K] T = Absolute operating temperature 2
SEALING CONCEPTS AND SEAL LIFE DEFINITION
Sealing is closely related to a pumping effect of the seal lip, orthogonal to the lip's sliding direction. This pumping effect was found [1,2] to be related to speed, viscosity, contact force, contact angle and run-out, but independent of the direction of rotation.
244
2.1 Preferential Dumping for radial seal IiDs, The hydrodynamics of seal lip lubrication is not fully understood and is subject to detailed research programmes [3,4]. From experimental work, however, it is known that this effect is heavily influenced by, amongst others, the detailed design of the lip's contact angle and, if applied, the position of a garter spring with respect to the seal lip contact.
Sealing of a wheel bearing unit against wet contaminant was unambiguously shown to be related to pumping effects. Failure of this mechanism resulted in ingress of wet contaminant and immediate breakdown of the bearing's lubricant film. The latter effect was measured [ S ] . Data obtained in this work can therefore also be utilised as experimental data for a mathematical description of seal life, in accordance with a proper seal life definition. Test data lead to the following: DEFINITION: A bearing seal is a hv- drodvnamic engineering element with a Dreferential ulbnapin~ effect to prevent penetration of foreign matter into a bearing
unit.
In experimental work it has further been found that the function of a seal is degraded in time from "active" to "passive". This transformation process can be categorised as shown in Fig. 2:
Category I ~
Hydrodynamic seal with preferential pumping direction
Nominal contacting seal
Category 111
v
Clearance seal Fig. I : Different contact angles and effects on "pumping". Fig. 2: Seal degradation with time The asymmetric contact angles produce an asymmetric pressure distribution in the seal lip lubricant film, which will produce a side leakage in the direction of the highest pressure gradient, as shown in Fig. 1. (This side leakage will occur in the "full film" condition).
This degradation of "active" hydrodynamic seals is caused by thermal effects and many different tribological processes. The time required for the transformation from Category I to 11, however, can be measured and defined as SEAL LIFE.
245
DEFINITION: Seal life is the time for a correctlv designed seal to jransform from a hvdrodvnamic enpineering element with preferential pumpinp action into an GCement without DR ferential DumDing action,
Results, for 10 points HSA increase, are represented as a straight line in Fig. 3, a modified Arrhenius diagram with the inverse absolute temperature and the log of time on the axes.
With this definition of seal life its actual service performance can be modelled and better understood. THESIS: Seal life is Poverned bv the existence of preferential Dumping effects.
3
PROPOSED GARTER SEAL LIFE MODEL
Seals work because the seal lip exerts a sealing farce against its counterface. These effects can diminish in time because of wear or chemical effects on the elastomer. The physical properties of elastomers change by chain scission, cross-linking or some form of chemical alteration of the polymer chains. This thermal degradation of the elastomer involved leads to hardening, permanent set, stress relaxation and creep, and thus limits seal life. In the case of "Garter spring" loaded seals, stress relaxation, creep and permanent set are compensated by the spring force. Detrimental changes in the mechanical properties of the elastomeric seal under the influence of thermal stress, however, were found to dominate seal life: the seal material loses its elasticity, becomes hard and brittle and its sealing ability diminishes. Rubber elasticity is normally measured with a Shore A durometer and referred to as "hardness" HSA (ASTM D-1415). Correlation to Young's modulus is usually given by an empirical formula, roughly valid in the range 20 > HSA c 90: E=(200+20 HsA)/( 100- HSA) 105
[N.m-2]
An increased hardness from 75 HSA to 85 HSA
is thus shown to nearly double the Young's modulus. This increase was found to limit seal life expectancy and was chosen as the first damage parameter for further modelling.
This damage parameter is a material property, measured on standard 2 mm thick test plates, submitted to air ageing tests. The (apparent) hardness increase is monitored as a function of time in a constant thermal overstress technique.
1000
100
10.000
Time [h] Fig. 3 Acceptance levels for "Garter seal" rubbers The Arrhenius equation, describing the reaction kinetics of the complex underlying processes, will be used as a first garter seal life approximation in:
The effective activation energy E, for different materials can now be derived from the slopes in Fig. 3, to be followed by deriving their time constant. This model describes garter seal life expectancy for only one temperature and is generally applied in standards [6-81 and other publications [9-141. Generalisation of this concept for practical use, however, is not common and will be derived in section 3.1. 3. I TemDerature adiusted garter seal life
For general garter seal life prediction the application of the Palmgren-Miner rule [ 15,161 on cumulative fatigue damage will be applied:
L T I
246
With the introduction of a life adjustment factor,
Li ai = LB
i
LaT
I
=
OTHER TRIBOLOGICAL LIMITATIONS ON GARTER SEAL LIFE
In experimental work, factors other than the temperature cycles encountered, were also found to cause seal failure. Cumulative damage of an individual factor was often found to limit the seal's useful life. The Palmgren-Miner assumption that failure occurs when the total damage done by a single process reaches unity was for those cases correct.
we obtain
or:
4
1
A hypothesis that failure also occurs when the cumulative damage of different tribological processes reaches unity will now be applied to calculate an adjusted garter seal life. LB
Analogous to the previous section, we obtain:
f
Remarks:
l
l
1) The model applies for a temperature range
within: Lower limit : Upper limit :
Tg (glass transition point) Tpek+ 25 OC (at t=100 h in Fig. 3)
or:
2) Contrary to bearing life calculations, this aspect of seal life does not follow a stochastic process.
1
3) Extrapolation of results for long periods of time is allowed for [ 171.
1
4) The coefficients derived from Fig. 2 relate to the
minimum material requirements of SKFs seal specification. In practical use, however, the constants of the seal materials must be used and different lives will be calculated, differentiating between suppliers.
5) The maximum operating temperature TB for a basic rating life of LB = 100,OOO h can be derived from the Arrhenius equations. Operating temperatures below TB do not affect the adjusted life.
[4 1
LB
An example of a separate tribological factor will be described in the following section. 4.1
Sea1 lip counterface corrosive wear
In many instances seal life was found to be dominated by crevice corrosion under the seal lip edge. The process was investigated and is described in the following scenario for a bearing seal.
241
- water penetrates under the seal lip.
Remarks:
- the seal actively reverses the inflow of water.
1) Wear rate figures are typically in the range: C,, = 1.4-1.9 ym/h, in a salt water environment.
- at stand-still a (thin) water meniscus
will be trapped in the gap between seal lip and counterface.
- the oxygen dissolved in this water will be quickly consumed in contrary to the surrounding water.
- the pH value in the gap
will decrease and the concentration of aggressive ions will increase.
2) The process describes a severe attack on the seal lip counterface which only starts when contaminated water has entered into the seal lip contact and protective grease additives are "washed out". This applies mainly in countries with relatively high humidity levels (> 65%), in static conditions [18].
- the electrochemical potential will drop so that
protective layers, if present, will dissolve and corrosion takes place in the narrow gap.
The corrosion process described is aggravated by the alloyed steel being used. The corrosive products are removed in the subsequent sliding of the seal lip: this occurs after each stand-still period. The process will repeat itself at each following stand-still period in a wet environment. The end result is a typical pitting corrosion process, as illustrated in Fig. 4, formed on the inboard counterface after only 30 hours total time (at stand still).
5
CONCLUSIONS
Systematic efforts during recent years have resulted in improved seal designs and have provided deeper insight into the functioning and tribology of bearing seals. Defining a seal as a hydrodynamic engineering element makes it possible to describe its life as a transformation from an active to a passive element. Of the mechanisms which influence these effects, ageing of the seal material and seal lip counterface corrosive wear are described and incorporated into a garter seal life model. Application of the garter seal life model has resulted in a new wheel bearing sealing concept, shown in Fig. 5 , today generally applied in the automotive industry.
Fig. 4:Seal lip counterface wear profile Measurements on failed counterfaces showed that a critical wear depth leads to early seal failure. Although more detailed modelling will be required this aspect of seal life can be incorporated as: Fig. 5 : New wheel bearing sealing concept.
248
6
ACKNOWLEDGEMENTS
The author would like to thar, Dr. H.€ Wittmeyer, Managing Director of SKF Engineering & Research Centre B.V., for permission to publish this paper. The author is also indebted to Prof. E. Ioannides and Dr. H. Lankamp for their support and useful discussions and SKF Sealing Systems (RFT SPA and CR Industries) for supplying the test specimens.
REFERENCES [ 11 Kammiiller, M.
"Zur Abdichtwirkung von Radial-Wellendichtringen", Thesis, Univ. of Stuttgart, 1986.
121 Stakenborg, M.J.L. "On the sealing mechanism of radial lip seals", Tribology International, Vol. 21-6, pp.335-340. [3] Gabelli, A. and Poll, G. "Formation of lubricant film in rotary sealing contacts: Part 1 Lubricant film modelling", ASME/STLE paper no. 90- Trib-64. [4] Poll, G. and Gabelli, A. "Formation of lubricant film in rotary sealing contacts: Part 2 A new measuring principle for lubricant film thickness", ASME/STLE paper no. 90-Trib-65.
IS] Bras, J.C.M.
"Specification and seal development for passenger vehicle wheel bearings", SAE, Passenger car meeting, October 19-22, Dearborn
[6] ASTM D-2990 "Tensile, compressive and flexural creep and creep rupture of plastics"
[7] ASTM D-3045 "Heat ageing plastics without load" (81 IEEE 383-1974, 1974 "Standard for type test of Class 1E electric cables, field splitters and connection for nuclear power generating stations"
[9] Robbins, R.D. "Comparative evaluation of square vs round cross-section circular seals using stress relaxation to predict the useful life of a seal." SAE 1979 0148-7 191/79/0226-0351 [ 101 Mendelsohn, M.A. et al.
"Accelerating-ageing tests for predicting radiation degradation of solar collector sealants", 39th ANTEC proceeding Boston, May 4-7 1981, ~~392-39
[ 111 Clough, R.L. et al.
"Accelerated-ageing tests for predicting radiation degradation of organic materials", Nuclear Safety, vol. 35, no.2, March-April 1984, pp 238254
[12] Bumay, S.G. and Hitchon, J.W. "Prediction of service lifetimes of elastomeric seals during radiation ageing". Journal of nuclear materials, vol. 13, 1985, p. 197 [13] Burnay, S.G. and Hitchon, J.W. "The assessment of seal performance and lifetime prediction in a nuclear environment", ASTM Spec. Tech. Publ; (87) p 609-614; vol 956 [14] Parker, B.G. and Raines, C.C. "New life prediction technique tests seals in severe service environments", Elastomerics, May 1989, pp. 20-22
51 Palmgren, A. "Die lebensdauer von Kugellagem. Zeitschrift des Verein deutscher Ingenieure", 1924-04-05 61 Miner, M.A
"Cumulative damage in fatigue", Journal of applied mechanics, 1945-09
[ 171 Moakes R.C.
RAPRA, Part 1 "Programme, test pieces and exposure sites July-Aug. 1975", Part 2 "Rubber properties, (other then compression set) up to 15 years", July-Aug. 1975", Part 3 "Rubber properties, compression set, up to 15 years", Sept.-Oct. 1975
[ 181 Schulze, R.
"Korrosionsbelastung an Personenwagen Karroserien in Europa", ATZ 84, 1982-03
Paper IX (ii)
Seals for Passenger Car Wheel Bearing Units J.C.M. Bras
The modern concept of a car wheel bearing is a complete unit that is ready to use, greased and sealed for life. Thus today the lubricant and the seal form an integral part of the bearing. With ongoing improvement in bearing internal geometry and materials, the performance of the seal can even replace rolling contact fatigue as the limiting factor for bearing service life. At the same time, demands for reliability and long, service-free running periods are continuously increasing. Systematic efforts during recent years have resulted in improved seal designs and have provide deeper insight into their functioning and the tribology effects involved. Seals for grease lubricated bearings are defined as active hydrodynamic engineering elements, with a preferential outward pumping effect to prevent penetration of foreign matter into a bearing unit. Seal life is thus governed by the existence of preferential pumping effects. When these have vanished, the seal transforms from an active into a passive engineering element. Bearing life from then depends on environmental factors only. Several mechanisms were identified which influence these effects negatively and thus restrict seal life: of these ageing of the seal lip material and counterface abrasive wear are described. Limiting factors were found and incorporated in a seal life model.
1
INTRODUCTION
Automotive wheel bearings are sealed and greased for life components. The component life is governed by its resistance to rolling contact fatigue, by lubrication and by seal performance. Since seal performance is regarded as critical to the life of a wheel bearing, a development programme was designed to investigate all aspects of this performance. 1.1 A
Notation = Time constant
= Life adjustment factor = ai,,i Life adjustment factors (0-1) C,, = Corrosive wear rate
ai
[hl [CLmhI
DL = Limiting corrosive damage IWI E = Young's modulus [ N.m-21 E, = Activation energy [kJ.K-1.mol-l] WSA = Hardness Shore A
L = Seal life in hours [hl La = Adjusted seal life [hl la^ = Temperature adjusted seal life [hl LB = Basic rating life [hl L, = Expected life at temperature Ti [hl Lj = Expected life at wear process j [hl Pi = Proportion of time at temp. Ti Pj = Proportion of time at wear process j R = Gas constant [kJ.K-1 .mol-l] [K] T = Absolute operating temperature 2
SEALING CONCEPTS AND SEAL LIFE DEFINITION
Sealing is closely related to a pumping effect of the seal lip, orthogonal to the lip's sliding direction. This pumping effect was found [1,2] to be related to speed, viscosity, contact force, contact angle and run-out, but independent of the direction of rotation.
244
2.1 Preferential Dumping for radial seal IiDs, The hydrodynamics of seal lip lubrication is not fully understood and is subject to detailed research programmes [3,4]. From experimental work, however, it is known that this effect is heavily influenced by, amongst others, the detailed design of the lip's contact angle and, if applied, the position of a garter spring with respect to the seal lip contact.
Sealing of a wheel bearing unit against wet contaminant was unambiguously shown to be related to pumping effects. Failure of this mechanism resulted in ingress of wet contaminant and immediate breakdown of the bearing's lubricant film. The latter effect was measured [ S ] . Data obtained in this work can therefore also be utilised as experimental data for a mathematical description of seal life, in accordance with a proper seal life definition. Test data lead to the following: DEFINITION: A bearing seal is a hv- drodvnamic engineering element with a Dreferential ulbnapin~ effect to prevent penetration of foreign matter into a bearing
unit.
In experimental work it has further been found that the function of a seal is degraded in time from "active" to "passive". This transformation process can be categorised as shown in Fig. 2:
Category I ~
Hydrodynamic seal with preferential pumping direction
Nominal contacting seal
Category 111
v
Clearance seal Fig. I : Different contact angles and effects on "pumping". Fig. 2: Seal degradation with time The asymmetric contact angles produce an asymmetric pressure distribution in the seal lip lubricant film, which will produce a side leakage in the direction of the highest pressure gradient, as shown in Fig. 1. (This side leakage will occur in the "full film" condition).
This degradation of "active" hydrodynamic seals is caused by thermal effects and many different tribological processes. The time required for the transformation from Category I to 11, however, can be measured and defined as SEAL LIFE.
245
DEFINITION: Seal life is the time for a correctlv designed seal to jransform from a hvdrodvnamic enpineering element with preferential pumpinp action into an GCement without DR ferential DumDing action,
Results, for 10 points HSA increase, are represented as a straight line in Fig. 3, a modified Arrhenius diagram with the inverse absolute temperature and the log of time on the axes.
With this definition of seal life its actual service performance can be modelled and better understood. THESIS: Seal life is Poverned bv the existence of preferential Dumping effects.
3
PROPOSED GARTER SEAL LIFE MODEL
Seals work because the seal lip exerts a sealing farce against its counterface. These effects can diminish in time because of wear or chemical effects on the elastomer. The physical properties of elastomers change by chain scission, cross-linking or some form of chemical alteration of the polymer chains. This thermal degradation of the elastomer involved leads to hardening, permanent set, stress relaxation and creep, and thus limits seal life. In the case of "Garter spring" loaded seals, stress relaxation, creep and permanent set are compensated by the spring force. Detrimental changes in the mechanical properties of the elastomeric seal under the influence of thermal stress, however, were found to dominate seal life: the seal material loses its elasticity, becomes hard and brittle and its sealing ability diminishes. Rubber elasticity is normally measured with a Shore A durometer and referred to as "hardness" HSA (ASTM D-1415). Correlation to Young's modulus is usually given by an empirical formula, roughly valid in the range 20 > HSA c 90: E=(200+20 HsA)/( 100- HSA) 105
[N.m-2]
An increased hardness from 75 HSA to 85 HSA
is thus shown to nearly double the Young's modulus. This increase was found to limit seal life expectancy and was chosen as the first damage parameter for further modelling.
This damage parameter is a material property, measured on standard 2 mm thick test plates, submitted to air ageing tests. The (apparent) hardness increase is monitored as a function of time in a constant thermal overstress technique.
1000
100
10.000
Time [h] Fig. 3 Acceptance levels for "Garter seal" rubbers The Arrhenius equation, describing the reaction kinetics of the complex underlying processes, will be used as a first garter seal life approximation in:
The effective activation energy E, for different materials can now be derived from the slopes in Fig. 3, to be followed by deriving their time constant. This model describes garter seal life expectancy for only one temperature and is generally applied in standards [6-81 and other publications [9-141. Generalisation of this concept for practical use, however, is not common and will be derived in section 3.1. 3. I TemDerature adiusted garter seal life
For general garter seal life prediction the application of the Palmgren-Miner rule [ 15,161 on cumulative fatigue damage will be applied:
L T I
246
With the introduction of a life adjustment factor,
Li ai = LB
i
LaT
I
=
OTHER TRIBOLOGICAL LIMITATIONS ON GARTER SEAL LIFE
In experimental work, factors other than the temperature cycles encountered, were also found to cause seal failure. Cumulative damage of an individual factor was often found to limit the seal's useful life. The Palmgren-Miner assumption that failure occurs when the total damage done by a single process reaches unity was for those cases correct.
we obtain
or:
4
1
A hypothesis that failure also occurs when the cumulative damage of different tribological processes reaches unity will now be applied to calculate an adjusted garter seal life. LB
Analogous to the previous section, we obtain:
f
Remarks:
l
l
1) The model applies for a temperature range
within: Lower limit : Upper limit :
Tg (glass transition point) Tpek+ 25 OC (at t=100 h in Fig. 3)
or:
2) Contrary to bearing life calculations, this aspect of seal life does not follow a stochastic process.
1
3) Extrapolation of results for long periods of time is allowed for [ 171.
1
4) The coefficients derived from Fig. 2 relate to the
minimum material requirements of SKFs seal specification. In practical use, however, the constants of the seal materials must be used and different lives will be calculated, differentiating between suppliers.
5) The maximum operating temperature TB for a basic rating life of LB = 100,OOO h can be derived from the Arrhenius equations. Operating temperatures below TB do not affect the adjusted life.
[4 1
LB
An example of a separate tribological factor will be described in the following section. 4.1
Sea1 lip counterface corrosive wear
In many instances seal life was found to be dominated by crevice corrosion under the seal lip edge. The process was investigated and is described in the following scenario for a bearing seal.
241
- water penetrates under the seal lip.
Remarks:
- the seal actively reverses the inflow of water.
1) Wear rate figures are typically in the range: C,, = 1.4-1.9 ym/h, in a salt water environment.
- at stand-still a (thin) water meniscus
will be trapped in the gap between seal lip and counterface.
- the oxygen dissolved in this water will be quickly consumed in contrary to the surrounding water.
- the pH value in the gap
will decrease and the concentration of aggressive ions will increase.
2) The process describes a severe attack on the seal lip counterface which only starts when contaminated water has entered into the seal lip contact and protective grease additives are "washed out". This applies mainly in countries with relatively high humidity levels (> 65%), in static conditions [18].
- the electrochemical potential will drop so that
protective layers, if present, will dissolve and corrosion takes place in the narrow gap.
The corrosion process described is aggravated by the alloyed steel being used. The corrosive products are removed in the subsequent sliding of the seal lip: this occurs after each stand-still period. The process will repeat itself at each following stand-still period in a wet environment. The end result is a typical pitting corrosion process, as illustrated in Fig. 4, formed on the inboard counterface after only 30 hours total time (at stand still).
5
CONCLUSIONS
Systematic efforts during recent years have resulted in improved seal designs and have provided deeper insight into the functioning and tribology of bearing seals. Defining a seal as a hydrodynamic engineering element makes it possible to describe its life as a transformation from an active to a passive element. Of the mechanisms which influence these effects, ageing of the seal material and seal lip counterface corrosive wear are described and incorporated into a garter seal life model. Application of the garter seal life model has resulted in a new wheel bearing sealing concept, shown in Fig. 5 , today generally applied in the automotive industry.
Fig. 4:Seal lip counterface wear profile Measurements on failed counterfaces showed that a critical wear depth leads to early seal failure. Although more detailed modelling will be required this aspect of seal life can be incorporated as: Fig. 5 : New wheel bearing sealing concept.
248
6
ACKNOWLEDGEMENTS
The author would like to thar, Dr. H.€ Wittmeyer, Managing Director of SKF Engineering & Research Centre B.V., for permission to publish this paper. The author is also indebted to Prof. E. Ioannides and Dr. H. Lankamp for their support and useful discussions and SKF Sealing Systems (RFT SPA and CR Industries) for supplying the test specimens.
REFERENCES [ 11 Kammiiller, M.
"Zur Abdichtwirkung von Radial-Wellendichtringen", Thesis, Univ. of Stuttgart, 1986.
121 Stakenborg, M.J.L. "On the sealing mechanism of radial lip seals", Tribology International, Vol. 21-6, pp.335-340. [3] Gabelli, A. and Poll, G. "Formation of lubricant film in rotary sealing contacts: Part 1 Lubricant film modelling", ASME/STLE paper no. 90- Trib-64. [4] Poll, G. and Gabelli, A. "Formation of lubricant film in rotary sealing contacts: Part 2 A new measuring principle for lubricant film thickness", ASME/STLE paper no. 90-Trib-65.
IS] Bras, J.C.M.
"Specification and seal development for passenger vehicle wheel bearings", SAE, Passenger car meeting, October 19-22, Dearborn
[6] ASTM D-2990 "Tensile, compressive and flexural creep and creep rupture of plastics"
[7] ASTM D-3045 "Heat ageing plastics without load" (81 IEEE 383-1974, 1974 "Standard for type test of Class 1E electric cables, field splitters and connection for nuclear power generating stations"
[9] Robbins, R.D. "Comparative evaluation of square vs round cross-section circular seals using stress relaxation to predict the useful life of a seal." SAE 1979 0148-7 191/79/0226-0351 [ 101 Mendelsohn, M.A. et al.
"Accelerating-ageing tests for predicting radiation degradation of solar collector sealants", 39th ANTEC proceeding Boston, May 4-7 1981, ~~392-39
[ 111 Clough, R.L. et al.
"Accelerated-ageing tests for predicting radiation degradation of organic materials", Nuclear Safety, vol. 35, no.2, March-April 1984, pp 238254
[12] Bumay, S.G. and Hitchon, J.W. "Prediction of service lifetimes of elastomeric seals during radiation ageing". Journal of nuclear materials, vol. 13, 1985, p. 197 [13] Burnay, S.G. and Hitchon, J.W. "The assessment of seal performance and lifetime prediction in a nuclear environment", ASTM Spec. Tech. Publ; (87) p 609-614; vol 956 [14] Parker, B.G. and Raines, C.C. "New life prediction technique tests seals in severe service environments", Elastomerics, May 1989, pp. 20-22
51 Palmgren, A. "Die lebensdauer von Kugellagem. Zeitschrift des Verein deutscher Ingenieure", 1924-04-05 61 Miner, M.A
"Cumulative damage in fatigue", Journal of applied mechanics, 1945-09
[ 171 Moakes R.C.
RAPRA, Part 1 "Programme, test pieces and exposure sites July-Aug. 1975", Part 2 "Rubber properties, (other then compression set) up to 15 years", July-Aug. 1975", Part 3 "Rubber properties, compression set, up to 15 years", Sept.-Oct. 1975
[ 181 Schulze, R.
"Korrosionsbelastung an Personenwagen Karroserien in Europa", ATZ 84, 1982-03