VOLATILITY OF LUBRICATING GREASES IN DIFFERENT MEDIA UDC 621.892:536.423.1

N. N. Sukharina, V. A. Nasonov, S. I. Krakhmalev, O. V. Ataeva, and L. I. Nazarova

In order to ensure extended life in microcryogenlcsystems (MCS), rolling-contact bearings with lubricating greases are used. A necessary condition for reliable operation of an MCS is a high degree of purity of the working gas circulating in the system, since the presence of variousimpurities will have an adverse effect upon the stability of the operating parameters [i]. The grease currently used in MCS machinery is W~II NP-207. When this grease is used for 100-250 h of operation, the working gas of the machines becomes polluted with products of grease vaporization. The present work has been aimed at the selection of lubricating greases wlth minimum volatility so as to ensure a high degree of purity of the working gas. Data were presented in [2, 3] on the volatility of certain greases in various media and in v~cuum. It was shown that for a given temperature, the replacement of an air medium by an inert medium leads to a decrease in vaporization, but the use of vacuum leads to an increase in vaporization. It has been noted [3] that the volatility of lubricating greases is determined mainly by the volaitlity of the dispersion medium. The type of thickening agent has some effect on the volatility of greases wlth a given dispersion medium. The research on greases in inert media and in vacuum is still inadequate, and it is virtually impossible to select greases for an MCS on the basis of the currently available data. Here we are presenting data on the volatility of lubricating greases in helium and under a vacuum of 0.13 Pa at 130~ the maximum temperature to which the greases are heated in the components of the machines. The objects of investigation were the greases VNII N P - 2 0 7 , 274N, 256, 271, 233, and 503, wlth dispersion media and thickening agents of different chemical TABLE i

k

VNII I ~ , ,~,,ea~e Pararncter

2'56

Dropping point. *C Yield ~'ess at 50"C, Pa Apparent viscosity.(in AKV-2), Pa 9see D = 1000see-* +50 ~ D = 10 sec-l--30~ D = I0 s e c - l - ~ 0 ~ Volatility in air (PIM-2), wt. % .

.

.

.

.

'

150 ~ 200 ~ 250 ~

27'4H

503

271

207

233

190 250

170 50--200

250

8O*

170 110

260

100-.250 0,9--1,2

1,45

7,9T

t,59

I,I

2t

200

40--70

37,4

27 8 I,'56

Wear ,car diameter (fot~-ball, 400 N, 120"C), mm Exlxeme-pressure properties (120"C),

1309 --

"50--70 10,5

150

Solidifies

6 41

6

0,35

1,21

0,37

0,44

75O

--

1260

5O00'$

2O--3O

3,8

25,6

4,3

13

33

m r n

Colloidal stability (KSA, 0.3 kg), % oil pressed out

3--5

5,2

*At 80~ TAt D = I00 set-*. % A t 200~ Scientific-Industrial Association Mikrokriogenmash. nologiya Topliv i Masel, No. 4, pp. 30-31, April, 1981.

204

0009-3092/81/0304-0204507.50

Translated from Khimiya i Tekh-

9 1981 Plenum Publishing Corporation

.o~

0

.I0

ZO

30

110

Time, h '

Fig. i. Grease vaporization as a function of test time in helium at 130~ for following VNII NP greases: I) 256; 2) 274N; 3) 233; 4) 503; 5) 207; 6) 271.

/

f

f H I"

~J

~o,2 > 0

I0

zo

Time, h

Fig. 2

3o

4.o

0

I0

2o 30 Time, h

Fig. 3

Fig. 2. Grease vaporization as a function of test time under vacuum at 130~ following VNII NP greases: i) 274N; 2) 233; 3) 207; 4) 503. Fig. 3. Vaporization rate as function of test time under vacuum at 130~ lowing VNII NP greases; i) 274N; 2) 233.

for

for fol-

types. The main physicochemical properties of these greases are listed in Table i; with respect to their tribological properties, they do provide extended operating life for the machines. The determination of volatility by the generally accepted procedure [4] (in a l-h test) is not an adequate basis for the selection of greases, since the vaporization rate of greases is not constant over an extended heating period [2]. We determined the volatility of greases when exposed for long periods to elevated temperatures (thermostated), in the interest of reducing the amount of products released into the gas medium of the machines. The greases, in a layer 2-3 nun thick, were thermostated at 1300C; the quantity of grease used in the bearings of microcryogenic equipment is 10-20 g. The volatility was evaluated by laboratory methods with a helium flow rate of i0 liters per h, and also under vacuum in specially designed chambers, using evaporation dishes of the type specified in the procedure of [4]. For most of the greases, the vaporization rate in the first 10-20 h of thermostating was higher than in the subsequent heating (Figs. 1 and 2), both in the inert gas and under vacuum. The oil losses were greater when the grease was heated under vacuum.

205

The most volatile are the greases VNII NP-271 and 503, formulated with an ester of sebacic acid, and the least volatile are the greases VNII NP-256 and 274N, formulated respectively with an alkylaromatic oil and with a polychlorosiloxane fluid. For these last two greases, after 10-20 h of heating in helium, the rate of evolution of volatile material into the gas medium decreases, amounting to about 0.003%/h (by weight). The volatility of the VNII NP-233grease within the interval of time investigated , in helium or in vacuum, after release of most of the volatile material became constant after 4-8 h and 30-35 h, respectively. The volatility of the VNII NP-207 grease in either medium, after a certain increase during the initial heating period, was proportional to the thermostating time. For the greases that we investigated, we typically find higher vaporization rates in the initial heating period, apparently due to removal of the readily volatile components from the dispersion medium; then the vaporization rate becomes constant. This means that holding the greases at a high temperature before their use in the friction components of the machines will lead to a decrease in fouling of the gas medium in the machines. The length of the heating period can be determined from the curves shown in Fig. 3. The vaporization rate of the VNII NP-233 and 274N greases in vacuum is stabilized in 30-35 h and 10-15 h, respectively; the corresonding oil losses are 2.7% and 0.9% by weight. Since the VNII NP-207 grease contaminates the gas medium of the machine with vaporized products after a certain time, the volatility of greases recommended for MCS machines must be lower. Of the greases investigated, this condition is met by the VNII NP-233, 256, and 274N. LITERATURE CITED i. 2. 3.

A . K . Grezin and V. S. Zinov'ev, Microcryogenic Equipment [in Russian], Mashinostroenie, Moscow (1977), pp. 50-58. V . M . Martynov, I. D. Kuchinskaya, and L. N. Boyutikova, Tr. Vses. Nauchno-Issled. Inst. Pererab. Nefti, No. ii, 24-46 (1969). K . S . Klimov and V. P. Skryabin, Tr. Vses. Nauchno-Issled. Inst. Pererab. Nefti, No. 16,

3-7 (1976). 4.

206

GOST [All-Union State Standard] 9566-74, Lubricating Greases, Method for Determination of Volatility.