Water-Soluble Polyglycols for the Lubrication of Large Gear Drives

Published in antriebstechnik 42 (2003) Nr. 9 
by Hermann Siebert and Dr. Ulrich Mann

For large gear drives, synthetic gear oils are indispensable. However, there are still doubts among gear operators regarding the use of polyglycol oils in applications where they come in contact with water. This article describes the special characteristics of polyglycol oils with regard to their being water-soluble.

1 Introduction
In today's gear construction, the choice of the lubricating oil to be used is an important element of the design process. High requirements, as must be met for example in wind power plants and the base materials industry, call for the use of synthetic oils (Fig. 1). While synthetic oils based on polyglycol have been used very successfully in many applications, there is still some uncertainty regarding their behaviour in the presence of water: because they are water-soluble, it is feared that their performance might be impaired. The question of compatibility with elastomers also plays a role in this context, however this subject will not be dealt with in this article. It should be noted, though, that numerous tests, e.g. the "dynamic seal compatibility test" conducted at Freudenberg, have shown that the tested polyglycol gear oils have no negative effects on the sealing capability of NBR and FKM materials. The good friction characteristics of polyglycols have made these oils a preferred choice for worm gears. This article shows why polyglycols offer also high-quality solutions for large gear drives, where they might even prove superior to polyalphaolefins.


2 Oil Test and Comparison
For the tests described in the following, a mineral oil and two different polyglycol gear oils were selected. Their characteristics are shown in table 1. While the characteristics of mineral oils are known, and the behaviour of polyalphaolefins in gears is similar to that of mineral oils, polyglycols are clearly different from those two groups.

Figure 1: synthetic specialty oils offer excellent performance and economy and are therefore highly in demand for the stringent requirements of large gear drives

	ISO VG	Viscosity index	Basis	Features
M 320	320	95	Min	High additive content CLP FZG A/16,6/90 >13
PG 320 - 1	320	> 230	PG	High additive content CLP FZG A/16,6/90 >13
PG 320 - 2	320	> 220	PG	Additive package approved for the food industry CLP FZG A/8,3/90 >12

Table 1: the oils tested
M stands for mineral oil, PG for polyglycol oil

3 Composition and Characteristics of Polyglycols
Synthetic fluids such as polyalphaolefins and polyglycols are all based on ethylene, which is obtained by cracking petroleum. Polyalphaolefins are made by polymerising the olefin 1-decene, which in turn is the product of the oligomerisation of ethylene. Chain length, branching and branching points in the molecule are the factors determining viscosity, viscosity-temperature behaviour and pour point of the fluid. The reaction of ethylene and propylene with oxygen leads to the generation of ethylene oxide (EO) and propylene oxide (PO). In a polymerisation process, these are turned into polyalkylene glycols. The mixing ratio of EO and PO as well as the oxygen contained in the chemical structure have a decisive effect on the behaviour of polyglycols.

Most of the polyglycols used in the gear industry have a EO/PO ratio of 50:50 to 60:40 and are consequently very similar in their behaviour. These types of polyglycols are discussed in this article. Polyglycols of this composition are normally referred to as water-soluble polyglycols. Their particular behaviour is due to the fact that they contain oxygen, which lends these polyglycols their strongly polar character.


4 Water-Miscible and Water-Soluble
Because of their chemical structure, polyglycols absorb a greater or lesser amount of water. This behaviour is dependent on the ethylene glycol to propylene glycol ratio, but a distinction must also be made between absorption due to water being added, water-solubility and hygroscopicity (water absorbed from air humidity).

• Hygroscopicity: The tendency of polyglycols to form a hydrate envelope is strong enough to absorb air humidity, which means that they are hygroscopic substances. Also for the degree of hygroscopicity it is important which type of polyglycol one is dealing with. While pure propylene glycols can absorb 3 % of water at maximum, this figure is 10 % for 1:1-types (both percentages for ambient temperature and an air humidity of 80%). However, water absorption from the air is strongly influenced by relative air humidity, temperature, surface and time. The saturation concentration mentioned will rarely be reached in enclosed gearboxes, since in such applications the oil surface is very small in relation to the oil volume. Moreover, the gears rarely stand still, so there is not much time for such exchange. Because of this characteristic, commercial polyglycols normally have a water content of 500 to 2000 ppm.

5 Performance Hardly Impaired by Water
If gear oils are to be used in large gear drives, it is obvious that they must have a suitable viscosity, viscosity index, pour point and ageing resistance. However, today they are also tested for a number of additional characteristics such as corrosion and wear protection, or scuffing and micro-pitting strength in the rolling bearings and gears to be used. Among these tests are:

• Corrosion protection test
• FAG-FE8 test
• FZG scuffing load test
• FZG micro-pitting test

Many of the products on the market today, polyalphaolefins as well as polyglycols, have undergone these tests with positive results. Performance restrictions due to the presence of water are hardly known. This issue will be dealt with in more detail in the following.

5.1 Effective Corrosion Protection
Because of their EO/PO ratio of 1:1, the polyglycols used as gear oils are fully water-soluble. For this reason, corrosion resistance is of particular interest in this context, and there are several testing methods for this purpose:

• Stirring method according to DIN ISO 7120: The anticorrosive behaviour of lubricating oils is determined through stirring the product with a steel rod [1]. for this purpose, 30 ml of distilled water is added to 300 ml of the oil to be tested (i.e. 10%), and the mixture is stirred for 24 hours at 60 °C. Then the corrosion that has become apparent on the rod is evaluated and graded on a scale from 0 to 3. The required anticorrosive effect is imparted to mineral oils and polyalphaolefins by suitable corrosion inhibitors. Table 2 shows that excellent corrosion protection is also attained by the tested polyglycol gear oil.
• SKF Emcor test according to ISO 11 007: The Emcor test has been developed [2] for the evaluation of the corrosion protection behaviour of lubricating greases, which is graded on a scale from 0 (no corrosion) to 5 (10 % of the surface has suffered from corrosion). As the testing cycle contains also 140 hours of standstill, it is particularly interesting for the assessment of polyglycols. In the tests conducted, for which 2 and 5 % of water was added to the oils, no corrosion became apparent (table 2). The rolling bearing industry, for example, accepts a corrosion degree of 1 for its lubricating greases.

Test procedure	M 320 rating	PG 320 - 2 rating
DIN ISO 7120: stirring procedure, 10 % water	0	0
ISO 11007: EMCOR test, 2 % water	0/0	0/0
ISO 11007: EMCOR test, 5 % water	0/0	0/0

Table 2: corrosion test results

5.1.1 Influence of Water in Rolling Bearing Lubrication
From various tests and publications by the rolling bearings industry, we have sufficient knowledge about the influence water contained in the lubricating oil has on the wear behaviour and service life of rolling bearings. Fig. 2 shows the influence of water on the relative bearing life [3]: even a very low water content of 300 to 1000 ppm may severely affect the service life of the component. However, in this diagram no differentiation is made between different base oils.

The Institute for Machine Elements at the Technical University of Aachen, Germany has examined the influence of moisture on oil- and grease-lubricated bearings [4] and has found that a differentiation must be made between dissolved and free water. The pertinent study says that longer lifetimes can be attained when water is present in dissolved form. Since most polyglycols used as gear oils are of the water-soluble type, it makes sense to examine possible negative effects the presence of water may have when working with these oils.

Figure 2: the influence of water content on the relative lifetime of oil-lubricated rolling bearings (Source: FAG)
Key: X axis - water content in %, Y axis - relative rolling bearing life

5.2 Desired Wear Protection is Attained
The FE8 test rig developed by FAG [5] serves among other things for determining the wear protection offered by a lubricant under extreme mixed-friction conditions. The test developed for lubricating oils runs for over 80 hours and is conducted at a temperature of 80 °C. Under these conditions, the wear index κ is between 0.04 and 0.06. For the rolling elements, a wear quantity of less than 30 mg is set as the target value, and less than 100 mg for the cage.

Fig. 3 shows the results attained with a polyglycol acc. to ISO VG 320 without water against those attained with the same polyglycol after 1 and 5 % water have been added, respectively. Even with a water content of 5 %, wear on the rolling elements remains clearly below the limit value of 30 mg. At the cage, the water has even led to a wear reduction.

Figure 3: FE 8 rolling bearing wear with admixture of water
Test conditions: temperature - 80 °C, load - 80 kN, speed - 7.5 min^-1, test duration - 80 h

5.2.1 Rolling Bearing Life Depending on Water Content
To find out about the effects lubricants have on the service life of a bearing, special tests are conducted. One example are the tests on a radial bearing test rig that is performed in cooperation with the IME in Aachen. These tests were intended to show the influence water has on the lubrication with polyglycols. For comparison, a mineral oil was also tested.

In the test, a deep groove ball bearing 6206 was loaded with defined 10 kN by a disk spring assembly. The bearing speed was 1500 min^-1. The bearing life attained under these EHD conditions was 673 hours, which applies to the mineral oil as well as to the polyglycol used due to the κ value being > 4 (Fig. 4).

Fig. 5 shows that a water content of 2 % (20,000 ppm) has no measurable influence on bearing life. When 5 % of water was added to the polyglycol, the service life was reduced by 70 % if the water was dissolved. With undissolved water, only 8 % of the calculated lifetime is reached. Mineral oil containing 2 % of water attained 10 % of the calculated lifetime. All in all, the results obtained here confirm the information given by the rolling bearing industry.

Figure 4: IME RLP Test
The tests were conducted with a water content of 2 % and 5 % and at a temperature (self-adjusting) of approx. 60 to 65 °C

Figure 5: Influence of water content on bearing life according to results obtained on radial bearing test rig
Test conditions: Temperature - 60 °C, load - 10 kN, speed - 1500 min^-1

5.3 Scuffing Load Strength Unchanged
I. Bayerdörfer of the Munich gear research centre FZG conducted a study on how the scuffing load strength of gear flanks is affected when lubricant characteristics change during operation [6]. It revealed that also the gear teeth can suffer premature damage by water contained in the oil. Again, Bayerdörfer carried out his basic tests with mineral oil, so conclusions on polyglycols cannot be made. Therefore, an FZG special test was arranged (A10/16.6R/60) to test the scuffing behaviour of polyglycols with a water content of 5 %. Test conditions were chosen such that a maximum sump temperature of 75 °C was generated with the gear set that is 10 mm wide, with the wheel driving the pinion.

For comparison, a mineral oil with a water content of 5 % was also tested. Both oils were known to have passed the standard FZG test with a scuffing load stage > 12. The smaller gear width of only 10 mm instead of the standard 20 mm was chosen to create extremely tough conditions which would provoke scuffing already at low loads.

In the test, the gear lubricated with a polyglycol gear oil (without water added) suffered scuffing failure at load stage 7. The gear lubricated with mineral oil, on the other hand, did not fail until load stage 11, due to its additives.

However, when water was added, the outcome was different. The polyglycol oil, this time running with 5 % water added, failed again at load stage 7, as in the reference test run. The gear lubricated with mineral oil failed already at load stage 10 (Fig. 6).

5.4 Good Friction Behaviour Reduces Power Loss
Polyglycols are characterised by very low friction coefficients, which makes them particularly suitable for use in worm gears. Regarding their use in spur gears, it is often claimed that a lower friction coefficient does not bring about significant benefits, or no benefits at all, in such applications. In the Klüber FZG efficiency test, power loss is determined with gear pair A running at standard peripheral speed and with a high torque (Fig. 7). The evaluation of the test run showed that with the polyglycol oil the power loss was reduced significantly by 15 %. This means lower oil sump temperature and hence longer oil life. The power loss reduction value obtained can be converted into gear efficiency, resulting in an improvement by approx. 0.2 %. This may seem insignificant, however in large gear drive consisting of several stages and having a power throughput of several 100 kW, this will amount to a measurable reduction of power consumption.

Figure 7: Efficiency loss depending on various lubricating oils in accordance with Klüber FZG efficiency test
Key: X axis - no. of revolution, Y axis - power loss
Test conditions: Gear pair A, load stage 10, peripheral speed - 8.3 m/s

6 Summary
Today synthetic gear oils are indispensable for the operation of large gear drives. As most synthetic gear oils are water-soluble polyglycols, the influence of water on polyglycols was studied. The results obtained show that the tested oils can meet CLP requirements even with a high water content. This does not constitute a general approval of lubrication with polyglycols containing water, but it shows that the performance of polyglycols is not affected more severely by the presence of water than that of mineral oils. When a certain amount of water in the oils has to be tolerated for a period of time, polyglycols may even prove slightly superior. Moreover, the low friction coefficient of polyglycols leads to lower oil sump temperatures and hence longer oil life. Also, power consumption can be reduced. All in all, water-soluble polyglycol oils offer a number of advantages over mineral oils for the lubrication of large gear drives, under aspects of both performance and economy.

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