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Long used for the successful lubrication of worm gears,
polyglycol-based synthetic oils now present a viable option
for large enclosed gear drives.
by Hermann Siebert and Ulrich Mann, Ph.D.
gearsolutionsonline.com • MARCH 2004 • GEAR SOLUTIONS 1
The principal cause of the distinctive behavior of polyglycols,
Introduction however, is the bonded oxygen, which lends the polyglycols
In modern-day gear technology, the lube oil used is an impor- their particularly polar character.
tant element of the gear’s overall design. Increasingly stringent
requirements for large enclosed gear drives—of the kind used Behavior with Water
in the wind power sector, for example—and the base materials By reason of their structure, polyglycols absorb a greater or
industry now necessitate the use of synthetic oils. lesser amount of water. Firstly, this behavior is dependent on
Although the feedback regarding the use of synthetic gear the ratio of ethylene glycol to propylene glycol, and secondly, a
oils based on polyglycol has been positive, there continues to distinction also has to be drawn between absorption due to
be a residual uncertainty concerning their usability in terms of admixture of water, water-solubility, and absorption due to
elastomer seals, and a possible performance restriction due to atmospheric moisture, or hygroscopicity.
their water solubility.
The favorable friction behavior of polyglycols has meant Water Solubility
that these oils have become widely adopted for lubricating The water solubility of polyglycols is determined by their con-
worm gears. This paper intends to show that polyglycols tent of ethylene oxide groups. Whereas pure propylene glycols,
also constitute a fully viable solution for use in large gears, EO/PO 0:1, are not water soluble, but can certainly absorb up
which may even be more advantageous than using the to 3 percent of water, polyglycols can be unrestrictedly mixed
polyalphaolefins. with water as from an EO/PO mixing ratio of 1:1.
This miscibility is limited only by the temperature. At a high-
er temperature, the solubility decreases in dependence on the
The Oils Tested EO/PO ratio. In the case of polyglycols with an EO:PO ratio of
For the tests described below, one mineral oil and two differ- 1:1, this temperature lies at 60 to 65°C.
ent polyglycol gear oils, whose properties are explained in
Table 1, were selected. Hygroscopicity
Polyglycols’ characteristic of forming a hydrate envelope is so
ISO VG Viscosity Index Basis Features
High additiv e content marked that polyglycols absorb moisture from the air, or are
M 320 320 95 Min
CLP FZG A/16,6/90 >13 hygroscopic. This hygroscopicity, too, is influenced significant-
High additiv e content
PG 320 - 1 320 > 230 PG
CLP FZG A/16,6/90 >13 ly by the type of polyglycol involved.
Additive package While pure polypropylene glycols can absorb a max. 3 per-
approved for the food
PG 320 - 2 320 > 220 PG cent of water, the figure for 1:1 types is up to 10 percent at
industry
CLP FZG A/8,3/90 >12
ambient temperature and a relative humidity of 80 percent.
Table 1 — The oils tested However, absorption from the air is substantially influenced
by the relative humidity, the temperature, the surface, and the
Whereas the properties of mineral oil are known, and the time involved. The above-mentioned balancing concentration
behavior of polyalphaolefins in the ranges of relevance for gear is unlikely to be reached in enclosed gears, since the oil surface
construction is comparable to that of mineral oils, polyglycols is very small in relation to the oil volume. In addition, the
exhibit properties that differentiate them from mineral oils standstill times are often very short, so that the amount of
and polyalphaolefins. exchange occurring is small.
By reason of this property, polyglycols are accordingly sup-
Polyglycols plied with water contents of 500 to 2,000 ppm.
The basic raw material used for producing the various synthet-
ic lubricants, which include the polyalphaolefins and polygly- Seal Resistance
cols, is ethylene, which is obtained by cracking crude oil. The The seal materials customarily used in gears are elastomers
polyalphaolefins are produced by polymerization of the olefin based on NBR and FKM. While the use of NBR elastomer seals
1-decen, which in turn is created by oligomerization of ethyl- in combination with mineral oil and polyalphaolefins is
ene. Chain length, branching and positions of the branches in regarded as non-critical, the use of polyglycols always raises the
the molecule determine the viscosity, viscosity-temperature question of compatibility with the elastomer being used.
behavior, and the pour point of the liquid. The static tests conducted in the past for assessing the seal
When ethylene and propylene react with oxygen, we obtain compatibility of various elastomers with the operating liquids
ethylene oxide (EO) and propylene oxide (PO), from which the used are progressively being replaced in the gear construction
polyalkylene glycols are produced by means of polymerization. sector by testing under dynamic conditions.
The mixing ratio between EO and PO, plus the oxygen The test is run for a duration of 1,000 hours at a test temper-
bonded in the chemical structure, crucially affect the behav- ature of 80°C for mineral products and 110°C for synthetic
ior of polyglycols. products. Usually, the influence on NBR and FKM is examined.
Since the gear industry predominantly uses polyglycols with Assessment criteria for resistance are:
an EO/PO ratio of 50:50 to 60:40, which exhibit very similar
behavior, only these types will be dealt with here. The polygly- • leak integrity
cols featuring this composition are also generally referred to as • carbon buildup
water-soluble polyglycols. • abrasion at the sealing lip
2 Reprinted from GEAR SOLUTIONS • MARCH 2004 • gearsolutionsonline.com
• bubble formation Test procedure M 320 PG 320 – 2
• wear in the shaft Rat ing Rat ing
Stirri ng procedure, 10 % 0 0
wa ter
The tests conducted under the above-mentioned condi-
tions show that the polyglycol gear oils involved have no EM COR Test 2 % w ater 0/0 0/0
adverse effect either on the NBR qualities used or on the EM COR Test 5 % w ater 0/0 0/0
FKM seals employed in terms of their sealing properties. Table 2 — Corrosion Test Results
Polyglycol Properties: The Influence of Water and the lifetime of rolling bearings is adequately known from
various studies and publications in the rolling bearing indus-
For gear oils which are used in large gears, in addition to the try. Fig. 1, from a publication of FAG [03], illustrates the influ-
familiar requirements like viscosity, viscosity index, pour ence of water on the relative bearing lifetime.
point, and aging behavior, there are numerous other test FAG’s diagram shows that even very small quantities of
requirements used nowadays that are designed to assess the water, in the range from 300 to 1,000 ppm, significantly affect
oil’s suitability in terms of its anti-corrosion and anti-wear the bearing’s lifetime. Possible variations in dependence on the
properties, plus its scuffing and micro-pitting resistance for the base oil used, however, are not differentiated.
rolling bearings and toothings used. These tests include: In addition, the Institute for Machine Elements at Aachen
Technical University (IME), in a study dealing with the influ-
• Anti-corrosion tests ence of moisture at grease- and oil-lubricated bearings [04],
• FAG FE 8 Test points to causal connections in terms of dissolved and free
• FZG Scuffing Test water, indicating that more favorable lifetime results can be
• FZG Micro-Pitting Test anticipated with dissolved water contents.
Since water-soluble types are used with polyglycol gear oils, it
The tests listed are met by many of the products currently on is of interest to investigate possible deleterious influences in the
the market, both polyalphaolefin-based and polyglycol-based presence of water for these types.
types. Restrictions on performance due to the presence of water
are uncommon these days. This is dealt with in more detail below. FAG FE 8 Rolling Bearing Wear Test
Water Protection The FE 8 test rig developed by FAG [05] serves not least to
The polyglycols used as gear oils exhibit good water solubility, investigate the anti-wear properties of lubricants under extreme
thanks to their EO/PO ratio of 1:1. The anti-corrosion proper- mixed-friction conditions.
ties, which can be assessed using a variety of test procedures, This test, developed for lube oils, and lasting more than 80
are accordingly of particular interest. hours, is conducted at a test temperature of 80°C. Under these
conditions, the k-value is in the range of 0.04-0.06. The targets
Stirring Procedure to DIN ISO 7120 are rolling element wear of less than 30 mg and cage wear of
The anti-corrosion properties of lube oils are determined with less than 100 mg. The results shown in Fig. 2 compare the per-
the stirring procedure using a round steel rod [01]. 300 ml of formance of a polyglycol of ISO VG 320 without water, and the
the oil under test are mixed by stirring with 30 ml of distilled performance of the same polyglycol after admixture of 1 percent
water, corresponding to 10 percent, at 60°C, for a period of 24 and 5 percent of water.
hours. The corrosion occurring at the immersed round steel Even when 5 percent of water has been admixed, the rolling
rod is graded from zero to three. element wear is still significantly below the limit value of 30
Mineral oils and polyalphaolefin oils achieve the required mg. For cage wear, even lower wear rates were determined.
anti-corrosion properties by means of suitable corrosion
inhibitors. Table 1 also evidences excellent anti-corrosion Rolling Bearing Lifetime
properties for the polyglycol gear oil under test. In order to determine possible influences of the lubricant on the
bearing’s lifetime, life tests are required. In conjunction with the
SKF EMCOR Test ISO 11702 IME, life tests were accordingly conducted on a radial-bearing
The SKF Emcor Test [02] was developed for assessing the anti-cor- test rig. The studies at the IME were designed to discover what
rosion properties of lubricating greases. They are graded on a scale influence water has on lubrication with polyglycols. For compar-
of zero (no corrosion) to five (10 percent of the surface corroded). ison purposes, studies were also carried out with a mineral oil.
By reason of the test cycle involved, which features a stand- The grooved ball bearing 6206 used is loaded with a spring
still time of 140 hours, it is also of particular interest for washer component exerting a defined 10 kN. The speed is
assessing polyglycols. 1,500 1/min. Under these EHD conditions, a bearing lifetime
The tests performed with 2 percent and 5 percent water of 673 h was determined, which due to the kappa value of > 4
exhibit no signs of corrosion (Table 2). By comparison: for applies both for the mineral oil and for the polyglycol (Fig. 3).
lubricating greases, the rolling bearing industry accepts a corro- The results (Fig. 4) show that when 2 percent of water is
sion rating of one. admixed (20,000 ppm), there is no measurable influence on the
bearing’s lifetime. Random-sample testing with 5 percent of
Water in Polyglycols in Rolling Bearing Lubrication water in the polyglycol revealed a 70 percent reduction in bear-
The influence of water in the lube oil on the wear behavior ing lifetime if the water is dissolved. If the water is not dissolved,
gearsolutionsonline.com • Reprinted from GEAR SOLUTIONS • MARCH 2004 3
Rel. Service life of the Bearings Springs
F = 10 kN
Tes t Bearing
6206
Support Bearings
Driving Shaft 6304 RS 1
n=1500 min-1
nach FAG
W ater Content: 2% and 5%
W ater Content: 2% and 5% emp erature: 60°C 65°C
TT emp erature: 60∞C - -65∞C
Water Content [%]
Figure 3 — IIME RLP Test
Figure 1 — FAG: the influence of water content on the relative lifetime
of oil-lubricated rolling bearings
extreme scuffing conditions, which meant that scuffing could
P G 320-1 without water P G 320- 1 with 1% water PG 320 - 1 with 5% water already be anticipated at a relatively low load.
The tests revealed that the polyglycol gear oil without water
120
fails in the test by reason of scuffing in load stage seven. By con-
100 trast, the mineral oil reaches the scuffing load stage by reason of
Weight loss [mg]
80 its additive content in load stage 11, as expected.
60 The experiments with water admixture, conversely, show a
changed picture. As in the reference run, the damage occurs in
40
load stage seven for polyglycol with 5 percent of water. The miner-
20 al oil, by contrast, has already failed in load stage 10 (Fig. 5).
0 For the polyglycol, there are no detectable changes after-
Roller Wear Cage Wear
wards. For the mineral oil, the change is slight.
Test Co nditions
Test Cond itio ns :: Polyglycol Friction on the FZG Test Rig
empera tue: 80°C
TT emperature: 80∞C Loa d: 80 kN
Load: 80 kN Speed: 7,5 1/min
Sp eed: 7,5 1/min est Duration 80 h
TT estDurat ion: :80 h
Polyglycols score in terms of their very low friction coefficients.
This favorable behavior is of particular utility for worm gears. In
Figure 2 — FE 8 rolling bearing wear with admixture of water
the case of spur gears, it is frequently argued that these low fric-
tion coefficients produce only slight improvements or none at all.
In the Klüber FZG Efficiency Test, the power loss is deter-
only 8 percent of the mathematical lifetime is reached. The min- mined with Toothing A at the standard circumferential velocity
eral oil with 2 percent reaches 10 percent of the calculated life- and high torque.
time in the random-sample test. The result thus agrees with Analysis of the test run shows a clearly measurable reduc-
those reported by the rolling bearing industry. tion in power loss. For the polyglycol, a 15 percent reduction
in power loss is evidenced. The reduced power loss means,
Scuffing Load Capacity first of all, a lower oil sump temperature, and thus an extend-
From the studies carried out by Bayerdörfer [06] at the FZG ed oil lifetime.
regarding the influence on the tooth profile load-carrying When the change in power loss is converted into an efficien-
strength of operationally entailed changes to the lubricant, it is cy figure, the toothing’s efficiency is improved by approx. 0.2
known that the toothing, too, can be prematurely damaged by percent. In a multi-stage large gear with a power throughput of
water in the oil. Bayerdörfer’s basic studies, too, were conducted several 100 kW, this seemingly modest improvement leads to a
with mineral oil, so that no conclusions can be drawn in regard measurable decrease in power consumption.
to polyglycols.
In initial trials, the scuffing behavior of polyglycols with a
water content of 5 percent was accordingly determined in an Summary
FZG Special Test, A10/16,6R/60. The test conditions were select- In large gears, the use of synthetic gear oils cannot be dispensed
ed so as to ensure that when the 10-mm-wide wheel set is used with. When polyglycols are used, there are uncertainties regard-
the sump temperature adjusts itself to max. 75°C when the ing how to factor in their effects on elastomers and their behav-
wheel is driving. ior in the presence of water. The paper describes the special fea-
By comparison, a mineral oil with likewise a 5 percent water tures of polyglycols in regard to their water solubility.
content was also tested. For both oils, it was known that they pass The dynamic seal compatibility tests presented show good com-
the standard FZG Scuffing Test with a load stage of > 12. The small- patibility with NBR and FKM elastomers for the polyglycol tested
er gearwheel width of 10 mm instead of the customary 20 mm as well. The tests conducted over a period of 1,000 hours produce
gearwheel width thus made it possible to run the test under results comparable to those for mineral oil.
4 Reprinted from GEAR SOLUTIONS • MARCH 2004 • gearsolutionsonline.com
L50na Polyglykol without water
Polyglykol with 2% water Polyglykol with 5% water dissolved P G 320-2 P G 320-2 w ith 5% Water M 320 M 320 w ith 5% w ater
Polyglykol with 5% water not dissolved M 320 with 2% water
700 12
600 10
running time [h]
Load Stage
500 8
400
6
300
4
200
2
100
0
0 Water influence on Scuffing Load Capacity
Water Influence on Lifetime
Te t Condition s:
TesstCondi tion s:
Tem perature: 60°C
Tem per atur e: 60∞C Load: 10 kN
Loa d: 10 kN Speed: 1500 1/min
Speed: 1500 1/min Te st Conditions : :
Te st Conditions
Te mperatur e: 60°C
Te mperatur e: 60∞C ear Type A
GGearTy pe :: A eriph er al S peed: 16,6 m/s
PP eriph er alSpeed: 16,6 m/s
Figure 4 — IME RLP Test: water’s influence on the relative bearing
lifetime Figure 5 — Results of the FZG Scuffing Test
Since in gear construction it is predominantly water-solu-
ble polyglycols which are used, the influence of water on 2,00 120
Küber FZG
polyglycols was examined. The results obtained show that 1,90
110
Efficiency Test
the oils tested are still able to meet CLP requirements, even
Ölsumpf Temperatur [°C]
1,80
100
Leistungsverlust [kW]
with a high water content. 1,70
Polyalphaolefin
90
This does not mean that lubrication with water-content 1,60
polyglycols should be permitted in all cases. The results merely 1,50
80
demonstrate that the effects for polyglycols in conjunction with 1,40
PG 320 - 2
70
water need not be regarded as more critical in terms of function 1,30
60
than for mineral oils. If water penetrates into the system unin- 1,20 50
0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000
tentionally, polyglycol might even offer slight advantages. Anzahl Umdrehungen
In addition, the low friction coefficient of polyglycols
TT estCC onditions:
est onditions: Ge ar Ty pe
Ge ar Ty pe AA oad stag 10
LL oadstag ee10 Peripheral Speed 8, 33 m/
Peripheral Speed 8, m/s s
offers an option for achieving extended oil lifetimes thanks
to lower gear sump temperatures. The power consumption Figure 6 — Efficiency losses in dependence on various lube oils
involved can be measurably reduced.
Bibliography
[01] DIN ISO 7120
Mineralöle und andere Flüssigkeiten.
Bestimmung der
About the authors: Korrosionsschutzeigenschaften in
Gegenwart von Wasser
[02] ISO 11007
Hermann Siebert. P.E. is department Rust-prevention characteristics of
lubricating greases.(EMCOR)
head for Technical Elements and OEM [03] FAG,Pall
Business at Klüber Lubrication München Niedrige Wassergehalte und
Verfügbarkeit non-stop Publikation
in Munich, Germany. He can be reached FAG, FAG-Publ.-Nr. WL 40207DA
[04] van de Sandt, N., Gold, P. W., Loos,
at 49 89 7876-713, or via e-mail at J., Assmann, C.
hermann.siebert@klueber.com. Ulrich Feuchtigkeitseinfluss bei fett- und
ölgeschmierten Wälzlagern, VDI
Mann, Ph.D., is director of Marketing Tagung 2002
[05] DIN 51819
& Application Technology and can be Mechanisch-dynamische Prüfung auf
reached at 49 89 7876-253 or dem, Wälzlagerschmierstoff
Prüfgerät FE 8.
ulrich.mann@klueber.com). The company’s [06] Bayerdörfer, I.
Einfluß von betriebsbedingten
Web site is [www.klueber.com]. Schmierstoffveränderungen auf die
Flankentragfähigkeit einsatzge-
härteter Stirnräder, Dissertation FZG
München, 2000
gearsolutionsonline.com • Reprinted from GEAR SOLUTIONS • MARCH 2004 5
