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The ultimate vacua of two-stage rotary oil pumps
Vacuum, VoI. I V No. ?
January, 795d
THE ULTIMATE V A C U A OF TWO-STAGE R O T A R Y OIL P U M P S D. A. Hockly, Grad. I.E.E. & C. S. Bull, Ph.D., A.M.I.E.E.*
Research Laboratories, Electronic Tubes Ltd., High Wycombe, Bucks.?
Summary IT HAD BEEN NOTICED THAT rotary p u m p s show an increased ultimate pressure as their t e m p e r a t u r e s rise d u r i n g operation. It is established t h a t this pressure rise is n o t d u e to t h e presence of vapours of t h e oil u s e d in the p u m p itself b u t possibly to the progressive appearance of v a p o u r s f r o m t h e lighter fractions of t h e oil liberated as a result of decomposition. It is c o n c l u d e d that this p h e n o m e n o n resulted f r o m the t h e r m a l effects of the vanes sliding in t h e rotor w h e r e t h e condition o f b o u n d a r y lubrication exists. E x t r e m e p r e s s u r e additives to t h e oil h a d a m a r k e d influence on these pressure variations a n d it m a y be possible to discover an additive w h i c h will completely p r e v e n t the cracking o f t h e oil.
Sommaire ON $'EST APER~U que les p o m p e s /~ palettes pr~sentent u n e d i m i n u t i o n de vide m a x i m u m , p e n d a n t que leur t e m p e r a t u r e a u g m e n t e e n cours d' utilisation. I1 a 6t6 ~tabli que cette a u g m e n t a t i o n de pression n ' e s t pas dfle/~ la pr~senqe de v a p e u r s de l'huile utilis~e dans la p o m p e , m a i s p e u t ~tre bien fi l'apparition progressive de v a p e u r s p r o v e n a n t des plus l~g~res parties de l'huile, lib6r6es par d~composition. O n en conclut que ce p h 6 n o m ~ n e est dfle /t l'effet t h e r m i q u e e n g e n d r 6 p a r les palettes, coulissant dans le rotor, off le p h ~ n o m ~ n e de lubrification limit~e existe. L ' a d d i t i o n ~ l'huile de produits p o u r la lubrification de surfaces coulissant sous de tr~s g r a n d e s pressions m 6 c a n i q u e s , ~ u n effet certain s u r les variations d u vide dans la p o m p e , et il est p e u t ~tre possible de d~couvrir u n p r o d u i t qui e m p ~ c h e r a c o m p l ~ t e m e n t la d6composition de l'huile.
INTRODUCTION
T h e pressure developed by these pumps is well known to consist o f two parts. One is that of permanent gases and is measurable on a M c L e o d gauge. This pressure never exceeds 10-4 mm. Hg in a well assembled two-stage pump, and may be much lower. T h e second part is that which is measurable on Pirani or Alphatron gauges and is called generally 'oil vapour'. This pressure may be anything from 10 3 mm. H g upwards to about 10-2 ram. Hg, depending on conditions. It is best measured with the Alphatron since it is insensitive to an amount of contamination sufficient to cause a Pirani gauge to drift seriously. N o mention o f the nature of this vapour has been found in the literature.
THE FOLLOWINGSETS out the results of experiments aimed at determining the performance of oils in twostage rotary vacuum pumps. Very little information is available about the composition and structure of these oils. It is generally understood that in the main the oils are natural mineral oils selected and blended to give a viscosity which pump manufacturers consider satisfactory and also to have a low water and sulphur content. T h e mean molecular weight of the oils is said to joe about 400. Having this high molecular weight, the fact that they are oils and not waxes indicates that the molecules are not straight aliphatic chains. It is also thought that there are few double bonds in the molecular structure, i.e., they are largely saturated hydrocarbons. M S . received A u g u s t , 1955.
40
* Dr. Bull is n o w at t h e College of T e c h n o l o g y , Birmingh a m , i.
ULTIMATE VACUA OF TWO-STAGE ROTARY PUMPS ~, The experiments to be described were carried out at first with a pump having an Alphatron connected by about a foot of glass tubing. As the experiments proceeded it became apparent that the temperature of the pump varies with time after starting up and that this was accompanied in some pumps by an increase in pressure reading. As a result of this it was decided to heat the pumps artificially by placing heaters under them to determine the relation between pressure and temperature and it was then found that in all cases the pressure increased with increasing temperature. This result is remarkable in that the temperature of the gauge and the tube connecting it to the pump remained at constant temperature, indicating that the pressure reading is not in fact that of a condensable vapour in equilibrium. It is probably due to the products of disintegration of the oil molecules owing to the action of the pump. I f one takes into consideration the volume of the connecting tube and gauge one will realise that a few micrograms of the pump oil disintegrating into ethylene, acetylene or similar fragments could set up the pressures observed. An attempt to measure the vapour pressure of the oil with a thermionic ion gauge in a sealed-off system gave readings well below 10-4 mm. Hg at first, but as the gauge decomposed the oil, the pressures rose. It is concluded therefore that the true oil vapour pressure is very low and can only be measured properly with a Knudsen gauge, which might be made sufficiently sensitive without the heaters reaching a temperature which would cause appreciable decomposition of the oil molecules. The measurements recorded must be viewed only as readings on the Alphatron. Since the nature of the 'vapours' or decomposition products is not known and all ionisation gauges including the Alphatron are sensitive to the molecular weight and structure of the ionised medium, the readings are certainly not true pressure readings.
PERFORMANCE
OF R O T A R Y OIL P U M P S
Variations in U l t i m a t e P r e s s u r e All experiments were carried out on pumps which
had been assembled and checked so as to give residual pressures of less than 10-4 mm. Hg on a McLeod gauge. The experiments were carried out in the main on several examples of two types of pump--the Edwards Speedivac 2B and the Pulsometer 2D1. All these pumps had seen many years of service and were cleaned and reassembled many times in the course of the experiments. Occasionally they were cleaned with alumina powder to ensure as far as possible for some experiments that no trace of other oils were left. The ultimate Alphatron reading obtained was usually about 4 × 1 0 -3 mm. Hg. Successive runs on a particular pump and oil often gave considerable differences in ukimate vacua (see Figs. 2 and 10, for example). It came to be regarded as necessary however that a good pump and oil would invariably give less than 10 2 mm. Hg. .Sometimes readings down to 1 z 10 ~ mm. Hg were obtained. It was fortunate that the readings came just within the range covered by the Alphatron.
Stepped Vapour P r e s s u r e Curves Curves of pressure against time for a Pulsometer 2D1 exhibited a sudden rise or step after running for about an hour (see Fig. 1). At the end of the run the temperature of the pump was of the order of 40°C. This rise was thought to be due to the increase of running temperature and it was found that when the pump was heated above the maximum running temperature, a whole series of these steps occurred. The positions of the steps along the time axis were very nearly repeatable (see Fig. 2), although the pressures at which they occurred varied, depending on the lowest ultimate vacuum obtained during the run (see previous section). It was also noticed that the sizes of particular steps were fairly consistent during a series of runs. Speedivac 2B pumps were found to exhibit these steps at a higher temperature than the Pulsometer 291 (Fig. 3). No satisfactory explanation could be found for either the variation in ultimate vacuum, or the occurrence of the stepped temperature/pressure curve. The difference between Speedivac and o
41
D. A. HOCKLY AND C. S. BULL
Fig. I. Pressure/time curve of a Pulsometer pump, Model 2DI, operated
with Dicks No. 2 oil, exhibitin.g a p r e s s u r e rise after about I hour operadon.
I0
20
30
40
50
bO TIME
Pulsometer pumps was particularly curious and suggested that the mechanical design of the pump was responsible. A detailed comparison of the design of these pumps was made, the most significant difference being that the blades of the Pulsometer were o f the same hardness as the chamber, whilst the blades of the Speedivac were very much harder than the chamber. Various experiments such as removing t h e ' h a t leather' sealing off the shaft between the two chambers in the Pulsometer pump, an analysis of the materials of which the pump was made and a study of the mechanical finish gave no clear indication of the reason for the difference between these pumps. It is to be noticed that the more recent design of Speedivac pump, the 2S50 Model, gives a performance similar to that of the Pulsometer pump, even though the materials and finish of the earlier and the later Speedivac pumps are superficially similar.
MODERN
THEORIES
OF
LUBRICATION
A detailed account of the theories o f lubrication is given by Bowden and Tabor 1. It is seen that the
42
70 SO (MINUTES)
~
KX~
I10
120
t30
140
main types of lubrication are : (a) Hydrodynamic Here a continuous oil film separates the sliding surfaces and there is no metallic contact between them.
(b) Boundary In this case the lubricating film is so thin that it is of molecular dimensions and is so closely in contact with the surfaces that it may be considered to be no longer fluid. There is a certain amount o f ' b r e a k - t h r o u g h ' of the lubricating film by surface asperities, so that as the surfaces slide upon each other, metallic wear occurs and high temperatures are developed at the asperities. These temperatures may be as high as the melting point of the sliding material. Special oils containing additives of various types tend to reduce the friction between the surfaces in spite of the extreme pressure.
Lubrication b e t w e e n R o t o r Blades
Pump
Chamber
and
I f we assume that boundary lubrication exists, then it seems likely that the extreme pressure and heat
ULTIMATE
VACUA OF TWO-STAGE
developed during welding and shearing the surface asperities will cause cracking of the lubricating molecules. This will cause the liberation of hydrocarbons of lower molecular weight, which could possibly be gases such as the lowest members of the unsaturated aliphatic series. The liquid oil will be enriched with components of lower molecular weight and also become slightly unsaturated. Thus, not only will the oil acquire components with a higher vapour pressure, but more or less permanent gases will be liberated in the pump. It is tempting to try to explain the steps in the temperature/pressure curves by means of a hypothesis based on the progressive appearance of vapours due to liquid fractions of very low molecular weight. In view of the random character of the action taking place and the complicated nature of the oil however, it appeared unlikely that such an explanation could be substantiated in detail. The fractions yielding the steps in vapour pressure are presumably those of the lighter liquid products formed by breaking down the heavy oil molecules and are not those of the heavy oil fractions themselves. The oil vapours condense in the Alphatron, to the extent that the radioactive source occasionally became contaminated and the gauge required cleaning. In spite of this however, for many runs after such cleaning the gauge would repeat low readings such as those quoted on page 41. The pressure steps observed, then, appear to be due to various fractions of the very light components being successively evaporated due to friction and going over into the Alphatron and dissolving in oils already present. The pressure observed is not that of saturated vapours but of an unsaturated vapour flowing from the pump into the gauge, which was always cooler than the pump. Early in the investigations an oil having a high sulphur content was found to give a consistently lower vacuum than ordinary oils. Since the high sulphur content was outside the specification, the oil was rejected as unsuitable. Further consideration in the light of the ideas on lubrication expressed above suggested that the sulphur was acting as an extreme pressure (E.P.) additive and we were encouraged to investigate the effects of other additives on the
ROTARY
PUMPS
26 q~
o: 22
o/
2o
3b
4'o
sb
66
~o
sb'--
TEMPERATURE °C.
Fig. 2. P r e s s u r e / t e m p e r a t u r e curves of 2 P u l s o m e t e r p u m p s , M o d e l 2 D I , operated o n E d w a r d s No. 8a oil, indicating a series o f p r e s s u r e rises o n h e a t i n g above maximum running temperatures. performance of rotary oil pumps.
Experimental Evidence of Boundary Lubrication It was decided to examine the mode of lubrication existing in the pump chamber by a method similar to that used by Courtney-Pratt 2 for the examination of cylinder lubrication of an internal combustion engine. Thin films of oil exhibit high electrical resistance, whereas when metallic contact exists the resistance is, of course, low. By applying a voltage across the Fig. 3. P r e s s u r e / t e m p e r a t u r e curves o f 2 Speedivac p u m p s , M o d e l 2, operated o n E d w a r d s No. 8a oil indicating t h e s a m e p h e n o m e n a as d e m o n s t r a t e d in Fig. 2 b u t at h i g h e r t e m p e r a t u r e s .
~ o l
2O
,
,
3O
40
,
,
,
,
50
6O °C,
7O
80
TEMPERATURE
48
D. A. HOCKLY AND C. S. BULL
X PLATES
Y PLATES
Fig. 4. Diagram of the experimental arrangement employed to determine the mode of lubrication prevailing in the pump chamber. Note : For calibrating, R is at least io ~: residual circuit resistance.
moving parts and examining the conduction of electricity it is possible to see where a ' b r e a k through' occurs. A time base representing one complete revolution of the pump was used to obtain the horizontal deflection of an oscilloscope, the vertical deflection being obtained from the voltage fluctuations across the rotor and stator via a suitable amplifier. A resistance R was included for calibration purposes (Fig. 4). Contact to the rotor was made via a flexible lead terminating in a mercury pool (Fig. 5). A camera was used to record the oscilloscope trace. Using this apparatus, hydrodynamic lubrication is represented by maximum vertical deflection, and complete breakdown is represented by zero deflection. Fig. 5. Detail of arrangement shown in Fig. 4, showing method of providing electrical contact to pump rotor.
By this method it was possible to establish that the journal bearings, being lightly loaded, were subject only to hydrodynamic lubrication and that the pump chamber was subject to boundary lubrication. Figs. 6, 7 and 8 show typical lubrication conditions occurring in a rotary oil pump using a straight mineral oil. From such curves however it was not possible to predict with certainty any of the steps in the pressure,/ temperature curve. These photographs are the result of single shot exposures, but except for the fine details of the effects due to breakdown of the film, each revolution of the pump produces almost identical oscillograms.
EFFECTS OF EXTREME PRESSURE LUBRICANTS The following E.P. additives were thought by the Regent Oil Company to have a sufficiently low vapour pressure to be of use in vacuum pumps and were mixed by them with a straight mineral oil : - (a) Lead Naphthenate (b) Chlorinated Wax (c) Straight Antoxydant
Lead Naphthenate This additive was found to yield an oil having a very high vapour pressure. Since it was useless as a pump oil, it was not examined in detail, although the lubrication of the chamber appeared to be slightly improved. Fig. 9 shows a pressure/temperature curve for this oil.
Chlorinated Wax
FLEXIBLE DRIVE
PUMP
Fig. 10 shows the pressure/temperature curve for this additive. It is seen to be rather different from the previous graphs. The lowest pressure obtained, 1.5 × 10 -~ ram. Hg, is very good. The steps have disappeared and have been replaced by a series of fluctuations. Figs. 11, 12 and 13 show that the lubrication in the pump chamber has been considerably improved compared with Figs. 6, 7 and 8.
Straight Antoxydant Fig. 14 shows an unexpected characteristic.
44
The
ULTIMATE
VACUA OF TWO-S'IAGE
ROTARY PUMPS
~0 I
O
0 P =I5.o/* T = 2 5 ' C .
Figs. 6 7 & 8 (above).
P =5.5¢* T =5o-'C.
P = I2.5/* T =77°C.
Oscillographs recording lubrication conditions, typical for rotary p u m p s operated on a straight
mineral oil (Regent 7o3o).
!il
32k
~-- 2 8 <
~26
.I
~22
z
16
I
(
ol
2o
io 20
3o
40
TEMPERATURE
~
i
: / \/\A/°5,At
3'o
,~o ~ go TEMp~,~w.E °c
7'o
~o
P r e s s u r e / t e m p e r a t u r e curves o f a Pulsometer 2 D I p u m p operated on oils with various a d d i t i v e s : - - Fig. 9 (left) : Regent 7o3o + Lead Napthenate. Fig. IO (above) : Regent 7o3 ° + Chlorinated Wax (2 furls on the same pump).
~b
Figs, I I , 12, 13 (below). Oscillographs recording lubrication conditions, obtaining during runs with Regent 703 ° + Chlorinated Wax (see Fig. IO). P = I4/~ T = 2 I ° C .
P =7.5/* T =6o°C.
P = i2.5tz T .-69°c.
a0 i
0
o
,
i 45
D. A. HOCKLY AND C. S. BULL
!J
o0
0
o P-IIF
T = I8~'C.
P--23.5/z T - 5 I
C.
P - 2 7 . 5 / z T - 5 9 . 5 C.
P=25.5/* T : 6 2 C .
P = I 9 . 5 ~ T-=73 C-
0 P -27.5/z T - 6 I ° C .
Fig. I4 (below). Temperature/pressure curves of a Pulsometer zDI pump operated on Regent 7o3 ° -k Antoxydant. Figs. I5, I6, 17, I8, I9, 2o (above). Oscillographs recording lubrication conditions prevailing during runs on Regent 7o3 ° -~ Antoxydant (see Fig. I4). The lubrication is shown to change from boundary to hydrodynamic.
lowest pressure was 10 × 10-3 mm. Hg which although not good is not unduly high (see page 41). T h e pressure rises in the usual manner until a maximum o f 28 x 10 3 mm. H g is reached at 60°C; it then falls (with fluctuations similar to those of the chlorinated wax) to 18× 10 3 ram. H g at 80 C, after which it rises again. T h e most interesting fact here is that the character o f lubrication changes from boundary to hydrodynamic when the m a x i m u m is reached. (,See Figs. 15-20.) CONCLUSIONS
/ ¢
/ J ~ 6
20
46
30
40
50 60 TE MI:~RATU~E
70
80
All the evidence collected indicates that a considerable part o f the pressure obtained from a rotary oil p u m p is due to gaseous and liquid products of decomposition of the oil molecules brought about by the very intense local heating o f the p u m p surfaces resulting from friction and high (mechanical) contact pressure. T h e marked difference between the Speedivac 2B on the one hand and the Pulsometer 2D1 and later Speedivac models is not explained. T h e stepped characteristic o f the pressure reading versus temperature curve is possibly due to the progressive appearance, at measurable pressures, o f vapours originating from the lighter fractions of the
ULTIMATE
VACUA OF TWO-STAGE ROTARY PUMPS
oil resulting from decomposition. The pressures observed are in any case not the saturated vapour pressures of these fractions. The effect of chlorinated wax as an E.P. additive is that the steps are removed and replaced by fluctuations in the temperature/pressure curve; associated with this is a distinct improvement in the number of ' break-throughs '. In the case of the antoxydant the sudden drop in pressure is accompanied by an abrupt change of lubrication mechanism. Fluctuations in pressure seem to be a property of the additives tested and suggest that they are not entirely satisfactory, but the experiments give hope
1 BOWDEN, F. P. and TABOR, D., The Friction am tion of Solids, (Clarendon Press, Oxford, I95C
that an additive may be found which will appreciably reduce the cracking of the oil. In such an event it could be expected that the vapour pressure of the oil would be the true limit of vacuum for these pumps.
The Authors are indebted to Mr. R. G. Gilson and Mr. A. D. Cassell for the valuable assistance given during the experiments. Thanks are due to Mr. N. G. Gullick and the Regent Oil Company for their ready co-operation, and to Mr. B. Wilkinson, Managing Director of Electronic Tubes Limited, for permission to publish this paper.
"NEY-PRATT, J. S. and TUDOR, G. K., Proc. Inst. :h. Engrs., x55, (I946), 293-299. LAnalysis of Lubrication Between the Piston Rings Cylinder Wall of a Running Engine '.
47
January, 795d
THE ULTIMATE V A C U A OF TWO-STAGE R O T A R Y OIL P U M P S D. A. Hockly, Grad. I.E.E. & C. S. Bull, Ph.D., A.M.I.E.E.*
Research Laboratories, Electronic Tubes Ltd., High Wycombe, Bucks.?
Summary IT HAD BEEN NOTICED THAT rotary p u m p s show an increased ultimate pressure as their t e m p e r a t u r e s rise d u r i n g operation. It is established t h a t this pressure rise is n o t d u e to t h e presence of vapours of t h e oil u s e d in the p u m p itself b u t possibly to the progressive appearance of v a p o u r s f r o m t h e lighter fractions of t h e oil liberated as a result of decomposition. It is c o n c l u d e d that this p h e n o m e n o n resulted f r o m the t h e r m a l effects of the vanes sliding in t h e rotor w h e r e t h e condition o f b o u n d a r y lubrication exists. E x t r e m e p r e s s u r e additives to t h e oil h a d a m a r k e d influence on these pressure variations a n d it m a y be possible to discover an additive w h i c h will completely p r e v e n t the cracking o f t h e oil.
Sommaire ON $'EST APER~U que les p o m p e s /~ palettes pr~sentent u n e d i m i n u t i o n de vide m a x i m u m , p e n d a n t que leur t e m p e r a t u r e a u g m e n t e e n cours d' utilisation. I1 a 6t6 ~tabli que cette a u g m e n t a t i o n de pression n ' e s t pas dfle/~ la pr~senqe de v a p e u r s de l'huile utilis~e dans la p o m p e , m a i s p e u t ~tre bien fi l'apparition progressive de v a p e u r s p r o v e n a n t des plus l~g~res parties de l'huile, lib6r6es par d~composition. O n en conclut que ce p h 6 n o m ~ n e est dfle /t l'effet t h e r m i q u e e n g e n d r 6 p a r les palettes, coulissant dans le rotor, off le p h ~ n o m ~ n e de lubrification limit~e existe. L ' a d d i t i o n ~ l'huile de produits p o u r la lubrification de surfaces coulissant sous de tr~s g r a n d e s pressions m 6 c a n i q u e s , ~ u n effet certain s u r les variations d u vide dans la p o m p e , et il est p e u t ~tre possible de d~couvrir u n p r o d u i t qui e m p ~ c h e r a c o m p l ~ t e m e n t la d6composition de l'huile.
INTRODUCTION
T h e pressure developed by these pumps is well known to consist o f two parts. One is that of permanent gases and is measurable on a M c L e o d gauge. This pressure never exceeds 10-4 mm. Hg in a well assembled two-stage pump, and may be much lower. T h e second part is that which is measurable on Pirani or Alphatron gauges and is called generally 'oil vapour'. This pressure may be anything from 10 3 mm. H g upwards to about 10-2 ram. Hg, depending on conditions. It is best measured with the Alphatron since it is insensitive to an amount of contamination sufficient to cause a Pirani gauge to drift seriously. N o mention o f the nature of this vapour has been found in the literature.
THE FOLLOWINGSETS out the results of experiments aimed at determining the performance of oils in twostage rotary vacuum pumps. Very little information is available about the composition and structure of these oils. It is generally understood that in the main the oils are natural mineral oils selected and blended to give a viscosity which pump manufacturers consider satisfactory and also to have a low water and sulphur content. T h e mean molecular weight of the oils is said to joe about 400. Having this high molecular weight, the fact that they are oils and not waxes indicates that the molecules are not straight aliphatic chains. It is also thought that there are few double bonds in the molecular structure, i.e., they are largely saturated hydrocarbons. M S . received A u g u s t , 1955.
40
* Dr. Bull is n o w at t h e College of T e c h n o l o g y , Birmingh a m , i.
ULTIMATE VACUA OF TWO-STAGE ROTARY PUMPS ~, The experiments to be described were carried out at first with a pump having an Alphatron connected by about a foot of glass tubing. As the experiments proceeded it became apparent that the temperature of the pump varies with time after starting up and that this was accompanied in some pumps by an increase in pressure reading. As a result of this it was decided to heat the pumps artificially by placing heaters under them to determine the relation between pressure and temperature and it was then found that in all cases the pressure increased with increasing temperature. This result is remarkable in that the temperature of the gauge and the tube connecting it to the pump remained at constant temperature, indicating that the pressure reading is not in fact that of a condensable vapour in equilibrium. It is probably due to the products of disintegration of the oil molecules owing to the action of the pump. I f one takes into consideration the volume of the connecting tube and gauge one will realise that a few micrograms of the pump oil disintegrating into ethylene, acetylene or similar fragments could set up the pressures observed. An attempt to measure the vapour pressure of the oil with a thermionic ion gauge in a sealed-off system gave readings well below 10-4 mm. Hg at first, but as the gauge decomposed the oil, the pressures rose. It is concluded therefore that the true oil vapour pressure is very low and can only be measured properly with a Knudsen gauge, which might be made sufficiently sensitive without the heaters reaching a temperature which would cause appreciable decomposition of the oil molecules. The measurements recorded must be viewed only as readings on the Alphatron. Since the nature of the 'vapours' or decomposition products is not known and all ionisation gauges including the Alphatron are sensitive to the molecular weight and structure of the ionised medium, the readings are certainly not true pressure readings.
PERFORMANCE
OF R O T A R Y OIL P U M P S
Variations in U l t i m a t e P r e s s u r e All experiments were carried out on pumps which
had been assembled and checked so as to give residual pressures of less than 10-4 mm. Hg on a McLeod gauge. The experiments were carried out in the main on several examples of two types of pump--the Edwards Speedivac 2B and the Pulsometer 2D1. All these pumps had seen many years of service and were cleaned and reassembled many times in the course of the experiments. Occasionally they were cleaned with alumina powder to ensure as far as possible for some experiments that no trace of other oils were left. The ultimate Alphatron reading obtained was usually about 4 × 1 0 -3 mm. Hg. Successive runs on a particular pump and oil often gave considerable differences in ukimate vacua (see Figs. 2 and 10, for example). It came to be regarded as necessary however that a good pump and oil would invariably give less than 10 2 mm. Hg. .Sometimes readings down to 1 z 10 ~ mm. Hg were obtained. It was fortunate that the readings came just within the range covered by the Alphatron.
Stepped Vapour P r e s s u r e Curves Curves of pressure against time for a Pulsometer 2D1 exhibited a sudden rise or step after running for about an hour (see Fig. 1). At the end of the run the temperature of the pump was of the order of 40°C. This rise was thought to be due to the increase of running temperature and it was found that when the pump was heated above the maximum running temperature, a whole series of these steps occurred. The positions of the steps along the time axis were very nearly repeatable (see Fig. 2), although the pressures at which they occurred varied, depending on the lowest ultimate vacuum obtained during the run (see previous section). It was also noticed that the sizes of particular steps were fairly consistent during a series of runs. Speedivac 2B pumps were found to exhibit these steps at a higher temperature than the Pulsometer 291 (Fig. 3). No satisfactory explanation could be found for either the variation in ultimate vacuum, or the occurrence of the stepped temperature/pressure curve. The difference between Speedivac and o
41
D. A. HOCKLY AND C. S. BULL
Fig. I. Pressure/time curve of a Pulsometer pump, Model 2DI, operated
with Dicks No. 2 oil, exhibitin.g a p r e s s u r e rise after about I hour operadon.
I0
20
30
40
50
bO TIME
Pulsometer pumps was particularly curious and suggested that the mechanical design of the pump was responsible. A detailed comparison of the design of these pumps was made, the most significant difference being that the blades of the Pulsometer were o f the same hardness as the chamber, whilst the blades of the Speedivac were very much harder than the chamber. Various experiments such as removing t h e ' h a t leather' sealing off the shaft between the two chambers in the Pulsometer pump, an analysis of the materials of which the pump was made and a study of the mechanical finish gave no clear indication of the reason for the difference between these pumps. It is to be noticed that the more recent design of Speedivac pump, the 2S50 Model, gives a performance similar to that of the Pulsometer pump, even though the materials and finish of the earlier and the later Speedivac pumps are superficially similar.
MODERN
THEORIES
OF
LUBRICATION
A detailed account of the theories o f lubrication is given by Bowden and Tabor 1. It is seen that the
42
70 SO (MINUTES)
~
KX~
I10
120
t30
140
main types of lubrication are : (a) Hydrodynamic Here a continuous oil film separates the sliding surfaces and there is no metallic contact between them.
(b) Boundary In this case the lubricating film is so thin that it is of molecular dimensions and is so closely in contact with the surfaces that it may be considered to be no longer fluid. There is a certain amount o f ' b r e a k - t h r o u g h ' of the lubricating film by surface asperities, so that as the surfaces slide upon each other, metallic wear occurs and high temperatures are developed at the asperities. These temperatures may be as high as the melting point of the sliding material. Special oils containing additives of various types tend to reduce the friction between the surfaces in spite of the extreme pressure.
Lubrication b e t w e e n R o t o r Blades
Pump
Chamber
and
I f we assume that boundary lubrication exists, then it seems likely that the extreme pressure and heat
ULTIMATE
VACUA OF TWO-STAGE
developed during welding and shearing the surface asperities will cause cracking of the lubricating molecules. This will cause the liberation of hydrocarbons of lower molecular weight, which could possibly be gases such as the lowest members of the unsaturated aliphatic series. The liquid oil will be enriched with components of lower molecular weight and also become slightly unsaturated. Thus, not only will the oil acquire components with a higher vapour pressure, but more or less permanent gases will be liberated in the pump. It is tempting to try to explain the steps in the temperature/pressure curves by means of a hypothesis based on the progressive appearance of vapours due to liquid fractions of very low molecular weight. In view of the random character of the action taking place and the complicated nature of the oil however, it appeared unlikely that such an explanation could be substantiated in detail. The fractions yielding the steps in vapour pressure are presumably those of the lighter liquid products formed by breaking down the heavy oil molecules and are not those of the heavy oil fractions themselves. The oil vapours condense in the Alphatron, to the extent that the radioactive source occasionally became contaminated and the gauge required cleaning. In spite of this however, for many runs after such cleaning the gauge would repeat low readings such as those quoted on page 41. The pressure steps observed, then, appear to be due to various fractions of the very light components being successively evaporated due to friction and going over into the Alphatron and dissolving in oils already present. The pressure observed is not that of saturated vapours but of an unsaturated vapour flowing from the pump into the gauge, which was always cooler than the pump. Early in the investigations an oil having a high sulphur content was found to give a consistently lower vacuum than ordinary oils. Since the high sulphur content was outside the specification, the oil was rejected as unsuitable. Further consideration in the light of the ideas on lubrication expressed above suggested that the sulphur was acting as an extreme pressure (E.P.) additive and we were encouraged to investigate the effects of other additives on the
ROTARY
PUMPS
26 q~
o: 22
o/
2o
3b
4'o
sb
66
~o
sb'--
TEMPERATURE °C.
Fig. 2. P r e s s u r e / t e m p e r a t u r e curves of 2 P u l s o m e t e r p u m p s , M o d e l 2 D I , operated o n E d w a r d s No. 8a oil, indicating a series o f p r e s s u r e rises o n h e a t i n g above maximum running temperatures. performance of rotary oil pumps.
Experimental Evidence of Boundary Lubrication It was decided to examine the mode of lubrication existing in the pump chamber by a method similar to that used by Courtney-Pratt 2 for the examination of cylinder lubrication of an internal combustion engine. Thin films of oil exhibit high electrical resistance, whereas when metallic contact exists the resistance is, of course, low. By applying a voltage across the Fig. 3. P r e s s u r e / t e m p e r a t u r e curves o f 2 Speedivac p u m p s , M o d e l 2, operated o n E d w a r d s No. 8a oil indicating t h e s a m e p h e n o m e n a as d e m o n s t r a t e d in Fig. 2 b u t at h i g h e r t e m p e r a t u r e s .
~ o l
2O
,
,
3O
40
,
,
,
,
50
6O °C,
7O
80
TEMPERATURE
48
D. A. HOCKLY AND C. S. BULL
X PLATES
Y PLATES
Fig. 4. Diagram of the experimental arrangement employed to determine the mode of lubrication prevailing in the pump chamber. Note : For calibrating, R is at least io ~: residual circuit resistance.
moving parts and examining the conduction of electricity it is possible to see where a ' b r e a k through' occurs. A time base representing one complete revolution of the pump was used to obtain the horizontal deflection of an oscilloscope, the vertical deflection being obtained from the voltage fluctuations across the rotor and stator via a suitable amplifier. A resistance R was included for calibration purposes (Fig. 4). Contact to the rotor was made via a flexible lead terminating in a mercury pool (Fig. 5). A camera was used to record the oscilloscope trace. Using this apparatus, hydrodynamic lubrication is represented by maximum vertical deflection, and complete breakdown is represented by zero deflection. Fig. 5. Detail of arrangement shown in Fig. 4, showing method of providing electrical contact to pump rotor.
By this method it was possible to establish that the journal bearings, being lightly loaded, were subject only to hydrodynamic lubrication and that the pump chamber was subject to boundary lubrication. Figs. 6, 7 and 8 show typical lubrication conditions occurring in a rotary oil pump using a straight mineral oil. From such curves however it was not possible to predict with certainty any of the steps in the pressure,/ temperature curve. These photographs are the result of single shot exposures, but except for the fine details of the effects due to breakdown of the film, each revolution of the pump produces almost identical oscillograms.
EFFECTS OF EXTREME PRESSURE LUBRICANTS The following E.P. additives were thought by the Regent Oil Company to have a sufficiently low vapour pressure to be of use in vacuum pumps and were mixed by them with a straight mineral oil : - (a) Lead Naphthenate (b) Chlorinated Wax (c) Straight Antoxydant
Lead Naphthenate This additive was found to yield an oil having a very high vapour pressure. Since it was useless as a pump oil, it was not examined in detail, although the lubrication of the chamber appeared to be slightly improved. Fig. 9 shows a pressure/temperature curve for this oil.
Chlorinated Wax
FLEXIBLE DRIVE
PUMP
Fig. 10 shows the pressure/temperature curve for this additive. It is seen to be rather different from the previous graphs. The lowest pressure obtained, 1.5 × 10 -~ ram. Hg, is very good. The steps have disappeared and have been replaced by a series of fluctuations. Figs. 11, 12 and 13 show that the lubrication in the pump chamber has been considerably improved compared with Figs. 6, 7 and 8.
Straight Antoxydant Fig. 14 shows an unexpected characteristic.
44
The
ULTIMATE
VACUA OF TWO-S'IAGE
ROTARY PUMPS
~0 I
O
0 P =I5.o/* T = 2 5 ' C .
Figs. 6 7 & 8 (above).
P =5.5¢* T =5o-'C.
P = I2.5/* T =77°C.
Oscillographs recording lubrication conditions, typical for rotary p u m p s operated on a straight
mineral oil (Regent 7o3o).
!il
32k
~-- 2 8 <
~26
.I
~22
z
16
I
(
ol
2o
io 20
3o
40
TEMPERATURE
~
i
: / \/\A/°5,At
3'o
,~o ~ go TEMp~,~w.E °c
7'o
~o
P r e s s u r e / t e m p e r a t u r e curves o f a Pulsometer 2 D I p u m p operated on oils with various a d d i t i v e s : - - Fig. 9 (left) : Regent 7o3o + Lead Napthenate. Fig. IO (above) : Regent 7o3 ° + Chlorinated Wax (2 furls on the same pump).
~b
Figs, I I , 12, 13 (below). Oscillographs recording lubrication conditions, obtaining during runs with Regent 703 ° + Chlorinated Wax (see Fig. IO). P = I4/~ T = 2 I ° C .
P =7.5/* T =6o°C.
P = i2.5tz T .-69°c.
a0 i
0
o
,
i 45
D. A. HOCKLY AND C. S. BULL
!J
o0
0
o P-IIF
T = I8~'C.
P--23.5/z T - 5 I
C.
P - 2 7 . 5 / z T - 5 9 . 5 C.
P=25.5/* T : 6 2 C .
P = I 9 . 5 ~ T-=73 C-
0 P -27.5/z T - 6 I ° C .
Fig. I4 (below). Temperature/pressure curves of a Pulsometer zDI pump operated on Regent 7o3 ° -k Antoxydant. Figs. I5, I6, 17, I8, I9, 2o (above). Oscillographs recording lubrication conditions prevailing during runs on Regent 7o3 ° -~ Antoxydant (see Fig. I4). The lubrication is shown to change from boundary to hydrodynamic.
lowest pressure was 10 × 10-3 mm. Hg which although not good is not unduly high (see page 41). T h e pressure rises in the usual manner until a maximum o f 28 x 10 3 mm. H g is reached at 60°C; it then falls (with fluctuations similar to those of the chlorinated wax) to 18× 10 3 ram. H g at 80 C, after which it rises again. T h e most interesting fact here is that the character o f lubrication changes from boundary to hydrodynamic when the m a x i m u m is reached. (,See Figs. 15-20.) CONCLUSIONS
/ ¢
/ J ~ 6
20
46
30
40
50 60 TE MI:~RATU~E
70
80
All the evidence collected indicates that a considerable part o f the pressure obtained from a rotary oil p u m p is due to gaseous and liquid products of decomposition of the oil molecules brought about by the very intense local heating o f the p u m p surfaces resulting from friction and high (mechanical) contact pressure. T h e marked difference between the Speedivac 2B on the one hand and the Pulsometer 2D1 and later Speedivac models is not explained. T h e stepped characteristic o f the pressure reading versus temperature curve is possibly due to the progressive appearance, at measurable pressures, o f vapours originating from the lighter fractions of the
ULTIMATE
VACUA OF TWO-STAGE ROTARY PUMPS
oil resulting from decomposition. The pressures observed are in any case not the saturated vapour pressures of these fractions. The effect of chlorinated wax as an E.P. additive is that the steps are removed and replaced by fluctuations in the temperature/pressure curve; associated with this is a distinct improvement in the number of ' break-throughs '. In the case of the antoxydant the sudden drop in pressure is accompanied by an abrupt change of lubrication mechanism. Fluctuations in pressure seem to be a property of the additives tested and suggest that they are not entirely satisfactory, but the experiments give hope
1 BOWDEN, F. P. and TABOR, D., The Friction am tion of Solids, (Clarendon Press, Oxford, I95C
that an additive may be found which will appreciably reduce the cracking of the oil. In such an event it could be expected that the vapour pressure of the oil would be the true limit of vacuum for these pumps.
The Authors are indebted to Mr. R. G. Gilson and Mr. A. D. Cassell for the valuable assistance given during the experiments. Thanks are due to Mr. N. G. Gullick and the Regent Oil Company for their ready co-operation, and to Mr. B. Wilkinson, Managing Director of Electronic Tubes Limited, for permission to publish this paper.
"NEY-PRATT, J. S. and TUDOR, G. K., Proc. Inst. :h. Engrs., x55, (I946), 293-299. LAnalysis of Lubrication Between the Piston Rings Cylinder Wall of a Running Engine '.
47