LUBRICATION TESTS 1. INTRODUCTION - Stellenbosch …appliedmaths.sun.ac.za/~fsmit/Lubrication.pdf · LUBRICATION TESTS 1. INTRODUCTION ... these criteria allow theoretical selection

LUBRICATION TESTS

1. INTRODUCTION

This study concerns the evaluation of possible lubricants that are suitable for the

mechanical part of Safety and Arming Devices (SADs). A series of tests were

conducted with a DIM in a centrifuge to evaluate its performance with different

lubricants within the specified temperature range.

First some background is given on the various lubricant properties that need to be

evaluated. Although the hardware is similar, the working conditions that a SAD is

subjected to are somewhat different than that of small general-purpose mechanical

devices. Therefore the emphasis with regard to lubricant properties was on the

properties that are relevant to SADs, as there are numerous parameters that do not

have a significant influence for this particular application. The importance of various

lubrication aspects are discussed briefly and evaluated with regard to their

applicability to a SAD.

2. BACKGROUND

Numerous oil brands are manufactured around the world, each with its own unique

and specialized characteristics. The majority of oils and greases are suitable for use

between ambient temperature and up to temperatures of approximately 120°C as most

mechanical equipment operate in an ambient environment with some additional

temperature increase due to friction or combustion. A good example is probably the

internal combustion engine found in virtually all land vehicles and sea going vessels.

When dealing with lubrication in the world of aviation, the same basics apply but with

increased specifications as the reliability of mechanical components are much more

critical in the air than on the ground. Therefore almost all lubricants intended for use

in aircraft are of exceptional quality. Aeroshell Fluid 12, as currently used for

lubrication of the mechanical SADs, is a good example of such a lubricant.

Laboratory and field tests are usually performed on lubricants to arrive at the

optimum balance between cost, performance and additives. This performance refers

to the life span of a lubricant evaluated at a given maximum temperature it will be

subjected to in its service life. In aviation applications, another criterion, low

temperature performance, plays a significant role. The main reason being that the

viscosities of virtually all lubricants increase dramatically at temperatures below 0°C.

For Aeroshell Fluid 12 this increase in viscosity is more than 130 fold from +40°C to

-40°C. This results in frictional forces that are drastically higher at subzero

temperatures compared with ambient conditions.

The arming time of a SAD is a critical factor when activated and this is directly

dependent on the accurate repeatable mechanical operation of such a device under all

specified conditions e.g. temperature. This is where the viscosity properties of a

lubricant could play a vital role in the performance of a SAD as it determines the

moving resistance of the mechanical timing components.

3. EVALUATION CRITERIA

In this section the most relevant aspects regarding the selection of a lubricant for use

in devices like mechanical SADs are discussed. It should be noted that even though

these criteria allow theoretical selection of lubricants, the optimum result could only

be obtained through thorough testing under controlled conditions. The following

criteria were used to select commercially available lubricants, with a short discussion

of each on its applicability to SADs.

Oil, grease or solid lubricant: Grease is a base lubricating oil suspended in a

thickener to which some additives are usually added. The consistency range from

solid to soft or semi-fluid. In Table 1 the advantages of each are listed.

2 25

Table 1: Advantages of different types of lubricants

Advantages of oil

1. Heat dissipation is possible when oil is circulated 2. Contaminants can be captured with a filter 3. Higher rotational speeds are possible compared to grease 4. Less heat is generated between frictional surfaces due to lower viscosity

Advantages of grease

1. No circulation system ⇒ reduced design complexity 2. Lifetime lubrication possible ⇒ lower maintenance 3. Simpler seal design and less leakage due to thickener 4. Sealing improved by ‘collaring’ of excess grease 5. High revolutions at low bearing temperatures possible after running-in period with

metered grease quantities

Advantages of solid lubricants

1. Extremely low starting torque (torque ripple) 2. Usually exceed -200°C to 200°C temperature range 3. Lubricant permanently fixed to surfaces but specific design, bearing disassembly

and baking usually necessary 6. No fluid ⇒ no viscosity changes

Table 1 is applicable to most applications of lubricants in the industry and can be

used for deciding between the use of either grease or oil in the design stage.

However, SADs only need to reliably operate once for a very short duration at the

end of its possible 10-year life span. From Table 1 it is evident that oil, grease or

solid lubricant may be suitable for use in a SAD. One must bear in mind that the

viscosity of greases is usually given for to the base lubrication oil and not the

product as a whole that includes the stiffeners and additives. It is thus not

possible to determine the thickness of grease from a standard product data sheet

at, for example, the -40°C lower temperature limit as required. This might

significantly influence the timing properties of a SAD across the required

temperature range. Running in of the devices can be done to force the majority of

the grease away from the rolling bearings. However, the long storage times and

possible vibration these devices can be subjected to, may cause the grease to

migrate, resulting in a device that need to be ‘run in’ again.

3 25

In the industry, oils are used in small delicate mechanisms such as precision

instruments like watches. On sliding parts, greases are more suitable as their

movements are mainly restricted to one-dimension. Light, low-shear greases are

used effectively for low-power devices such as appliance timer motors. Some of

these greases are semi-fluids, which reduce their noise damping qualities

considerably. Again, in subzero conditions these properties may still change

considerably.

Solid lubricants are well suited for space applications where low out-gassing of

lubricants is crucial. The temperature range at which they are effective is also

extreme. Their main disadvantage is the challenge involved to apply these layers,

as the baking process usually requires temperatures in excess of 180°C, which can

destroy some materials. Therefore, the use of solid lubricants has to be taken into

account at the design stages. However, there are solid lubricants that do not need

any baking or disassembly of bearings and this may give satisfactory results

despite the fact that they offer short wear life.

Grease thickeners: These can be divided into soap and no-soap thickeners. The

soap thickeners are made from fatty acids, e.g. palm, olive and sunflower oil,

where the non-soap thickeners are made up of materials such as clay (bentonite),

silica gel and carbon black. On average, thickeners make up 3% to 15% of grease.

The most useful aspect of thickeners with respect to SADs is the useful lifetime

they are usually associated with. These include thickeners such as sodium

complex, polyurea and plastic (PE, PTFE, FEP) that are used in most long-term

greases. The resulting grease thickness can be measured with a penetrometer

according to ISO 2137 and classified to NGLI classes according to DIN 51 818, as

shown in Table 2:

4 25

Table 2: Classification of grease thickness

NGLI class Structure General application

000 00 0

Fluid Almost fluid Extremely soft

Primarily for gear lubrication

1 21 3

Very soft Soft Moderate

Lubrication for plain and rolling bearings

4 5 6

Stiff Very stiff Extremely stiff

Sealing and barrier greases for labyrinth seals and taps

Additives: Additives are normally used in a lubricant, oil or grease, to enhance

certain properties to make it more suitable for specific operating conditions. The

general rule is to only use an additive if it is necessary the reason being that they

increase production costs. Table 3 gives the basic qualities that can be improved

by additives.

Table 3: Added qualities due to additives 2Additive Purpose

Extreme pressure additive

Increase load carrying capacity, protection against micro-welding

Anti-wear additives Reduction of wear in the mixed friction regime

Friction modifiers Reduction of friction losses, impede stick-slip and noises, neutralization of acid aging products

Corrosion inhibitors Protection against corrosion attack on metallic materials

Oxidation inhibitors Retard oxidation, reduction of surface layer and sludge formation

Solid lubricants Improvement of load carrying capacity, reduction of stick-slip

Tackifiers Improvement of lubricant adhesion to surfaces

1 NGLI grade 2 is most commonly used

2 Information obtained from Klüber Lubrication, P.O. Box 11461, Randhart, Alberton 1457,

Tel: 011 908 2457

5 25

Suitable additives can be selected only once suitable lubricating oil has been

decided upon, as they are not always compatible. A SAD needs to perform only

once which may only necessitate additives that reduce oxidation, corrosion and

stick-slip (low starting friction).

Viscosity: The viscosity of a fluid describes its ability to resist flow. This is one

of the key parameters of an oil specification. Due to the normal decrease in

viscosity with increase in temperature, viscosity improvers are often added to

reduce this effect.

Viscosity Index: This index is a dimensionless parameter that is commonly used

to give an indication of the tempo at which viscosity increase with temperature for

a specific fluid. The Viscosity Index (V.I.) is defined in the ASTM D 2270

standard and was originally intended to provide a means of comparison between

oils with viscosity improvers and oils without. The V.I. value gives an indication

of the difference in viscosity of a lubricant between 38°C and 99°C, and is

therefore not applicable to temperatures below zero. Thus, for comparison

purposes, the V.I. value only gives a rough indication of the increase in viscosity

in subzero conditions.

Lubricity: The lubricity of a lubricant is the extent to which it reduces the

coefficient of friction between two solids.

Volatility: This describes the evaporation possibility of a lubricant. The more

volatile a liquid, the lower its boiling point and also the higher its flammability.

Flash point: This is the temperature at which a lubricant spontaneously ignites. It

is always safe to keep temperatures well below this value.

High/low temperature capability: Some lubricants are only usable within a very

limited temperature range. When this range is significant, it is necessary to verify

that all critical properties are within acceptable limits, from the lowest to highest

temperature.

Wear-prevention ability: This is a particularly important characteristic that play

a large role with sliding and turning parts at their contact surfaces when no

6 25

hydrodynamic film is present to separate surfaces. Lubricity and extreme pressure

additives help prevent the microscopic roughness of contacting surfaces from

damage. The wear prevention ability of a lubricant can greatly reduce surface

damage caused by vibration such as SADs may be subjected to.

Static friction: The static friction factor is used to determine the force needed to

initiate sliding between two surfaces. In the case of a SAD one would try to keep

this value as constant as possible over the whole temperature range. This friction

is sometimes also referred to as ‘stick-slip motion’, especially when reciprocating

parts are involved.

Migration (spreading): The ability of a lubricant to stay in one place after

application is very important when dealing with lifetime lubrication. Silicone oils

are known for their ‘ability’ to migrate. However, the dipping of SADs in such a

lubricant should be adequate to ensure lubrication for life. A common practice is

to produce light greases of oils that migrate easily but then the temperature

characteristics are also influenced.

Material Compatibility: Compatibility with all materials that a lubricant may be

in contact with should be checked. The lubricant could affect material properties

(e.g. certain plastics) extremely negatively and may also destroy the lubricant.

Aging (gumming): Aging is of particular interest with lifetime lubrication where

virtually no degradation of the lubricant can be allowed for. This is a critical

property when used in SADs and necessitate a chemically stable (preferably inert)

lubricant. Virtually all lubricants used in the aviation industry are very stable and

have a low tendency to gum after prolonged use.

7 25

4. AVAILABLE LUBRICANTS

Two classes of oil lubricants are available for lubrication. The first is natural oils that

are found in nature and include mineral oils, which are widely used in machinery all

over the world. Although mineral oils are the most affordable, they are often

aggressive to materials they come in contact with and also tend to age.

The second class includes all synthetically produced oils. There is quite a large range

of these oils commercially available and the list is constantly growing. The general

advantage of synthetic lubricants is that they are chemically more stable than their

traditional natural counterparts. This make them more suitable for use in applications

where degradation due to reaction between the lubricant and lubricated surfaces is

critical, especially in areas where lifetime lubrication is necessary. In Figure 1 the

main categories of synthetic oils and their associated properties like viscosity and

compatibility, are shown.

The top part of Figure 1 shows the temperature vs. viscosity ranges that the different

types of synthetic oils cover. It is important to note that the axis scales are neither

linear nor logarithmic. The bottom part of Figure 1 shows the most relevant material

compatibility and also the solvents. From Figure 1 it seems that silicone oils are

superior in compatibility, temperature range and viscosity index.

In Figure 2 the temperature ranges for the different types of lubricants are

summarized.

8 25

Figure 1: Synthetic oil viscosity and compatibility chart

9 25

Figure 2: Lubrication temperature ranges (from Nye, www.nyelubricants.com).

5. LUBRICANTS THAT MAY BE SUITABLE FOR USE IN

SADS

The following are the lubricants investigated which may be suitable for use in SADs

sorted according to lubricant oil type.

Synthetic hydrocarbon:

1. Aeroshell Fluid 12 (lubricant currently used)

2. Anderol 401 D

Silicone:

1. Baysilone fluid M 10




PAO (Polyalphaolefin):

1. NYE oil 132B

PFPE (Perfloropolyether):

1. Krytox 143AA

2. Krytox 143AZ

3. Krytox GPL 105 (grease)

4. NYE uniflor 8910

5. NYE uniflor 8920

10 25

Figure 3: Viscosity as a function of Temperature. Logarithmic scale for viscosity axis

1

10

100

1000

10000

-50 -30 -10 10 30 50 70 90

Temperature [deg C]

Vis

cosi

ty [m

m^2

/s]

Aeroshell fluid 12 NYE uniflor 8910 NYE uniflor 8920 NYE synth oil 132 Krytox GPL 105 Krytox 143AAKrytox 143AZ GE M10 GE M20 GE M100 GE M1000

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

-50 -30 -10 10 30 50 70 90

Temperature [Deg C]

Visc

osity

[mm

^2/s

]

Aeroshell fluid 12 NYE uniflor 8910 NYE uniflor 8920 NYE synth oil 132 Krytox GPL 105 Krytox 143AAKrytox 143AZ GE M10 GE M20 GE M100 GE M1000

Figure 4: Viscosity as a function of Temperature. Linear scale for viscosity axis

11 25

In Figures 3 and 4 the viscosity as a function of temperature are shown for the above-

mentioned oils. Anderol 401 D was excluded in these figures as it conforms to the

same specification as Aeroshell Fluid 12. In Figure 3 a logarithmic scale is used

whilst a linear scale is used in Figure 4 for clarity in the higher and lower temperature

ranges respectively.

6. SPECIFIC LUBRICANT PROPERTIES

6.1. AEROSHELL FLUID 12

AeroShell Fluid 12 is currently used in lubricated SADs and conforms to MIL-PRF-

6085D with heading: LUBRICATING OIL: INSTRUMENT, AIRCRAFT, LOW

VOLATILITY. This is a high quality, synthetic hydrocarbon oil developed for use in

the aviation industry.

Table 4: Properties of Aeroshell Fluid 12

Fluid Property Typical value Units

Pour point < -60 °C

Flash Point 220 °C

Density @ 25°C 925 Kg/m3

Viscosity @ 25°C 18 Mm2/s

Distributor:

Shell SA (Pty) Ltd

P.O. Box 2231

Cape Town

8000

Tel: (021) 408 8911

Orders: 0800 021 021

12 25

6.2. ANDEROL 401 D

This is a synthetic oil with a Diester used for the base fluid. It conforms to MIL-L-

6085 specifications. Anderol 401 D is intended for use as lubricant in miniature

bearings that can be sealed for life.

Table 5: Properties of Anderol 401 D


Pour point -65 °C

Flash Point 225 °C

Density @ 15.6 °C 930 kg/m3

Viscosity @ 25°C Not given mm2/s

Distributor:

Tenneco Chemicals

6.3. BAYSILONE FLUID SILICONE OIL

Baysilone Fluid is a pure silicone oil. This product is available in a wide range of

viscosities (3 – 2 000 000 mm2/s) other than that specified at 25 oC. One of the key

advantages of this product is its chemical inertness. It is successfully used in

precision instruments as a lubricant in the industry and also possesses good dielectric

characteristics. The specific properties for the various oils are listed in the respective

tables.

Manufacturer:

GE Silicones

USA

(Silicone manufacturing plant is located in The Netherlands.)

13 25

Distributor:

Silicones & Technical Products CC

P.O. Box 547

Paardeneiland

7420

South Africa

Tel: (021) 511 2151

6.3.1. Baysilone Fluid M 10

Table 6: Properties of Baysilone Fluid M 10


Pour point -90 °C

Flash Point 170 °C

Density @ 25 °C 940 kg/m3

Viscosity @ 25 °C 10 mm2/s




Pour point -70 °C

Flash Point 240 °C



14 25




Pour point -50 °C

Flash Point 300 °C






Pour point -50 °C

Flash Point 320 °C



6.4. NYE OIL 132B

This lubricant conforms to the military specification MIL-DTL-53131A (GRADE 4)

(except that it is not ultra-filtered) with heading:

LUBRICATING OIL, PRESCISION ROLLING ELEMENT BEARING,

POLYALPHAOLEFIN BASED.

15 25

Table 10: Properties of NYE oil 132B


Pour point <-62 °C

Flash Point 227 °C

Density @ 25 °C Not supplied kg/m3


Manufacturer:

NYE

12 Howland Road

Fairhaven

MA 02719

USA

Distributor:

Corruflex

Pomona

South Africa

Tel: 083 279 8007 - David Kuhn

6.5. KRYTOX FULLY SYNTHETIC OILS

Krytox fully synthetic oils and greases have excellent lubrication properties, which

make them useful in a wide range of critical applications. Krytox is a

perflouropolyether (PFPE) and Storage and Shelf life is claimed to be indefinite if

container as supplied is unopened and stored in clean dry location. The specific

properties are listed in the respective tables,

Manufacturer:

Du Pont Performance Lubricants

Deepwater

NJ 08023

16 25

Distributor:

Chempro

P.O. Box 336

Milnerton

7435

Tel: (021) 550 8181

6.5.1. Krytox 143 AA

Table 11: Properties of Krytox 143 AA


Pour point -50 °C

Flash Point Not given °C



6.5.2. Krytox 143 AZ

Table 12: Properties of Krytox 143 AZ


Pour point -55 °C




6.5.3. Krytox GPL 105

17 25

Table 13: Properties of Krytox GPL 105


Pour point -36 °C



Viscosity @ 25 °C ±450 mm2/s

6.6. NYE UNIFLOR

This is a Perfloropolyether oil that is chemically inert and will not age, even at high

temperatures.

Manufacturer:

NYE

12 Howland Road

Fairhaven

MA 02719

USA

Distributor:

Corruflex

Pomona

South Africa

Tel: 083 279 8007 - David Kuhn

18 25

6.6.1. NYE Uniflor 8910

Table 14: Properties of NYE Uniflor 8910


Pour point <-70 °C

Flash Point None °C



6.6.2. NYE Uniflor 8920

Table 15: Properties of NYE Uniflor 8920


Pour point <-70 °C

Flash Point None °C



19 25

7. LUBRICATION TESTS

7.1. TEST EQUIPMENT AND PROCEDURE

A Delay Inertia Mechanism (DIM) is a mechanisms used in SADs as a timing device.

An inertia, or eccentric mass is driven by an acceleration force and retarded by an

oscillating mass through an escapement wheel and gearbox.

A DIM was used for the lubrication tests with a gearbox of which the escapement

mass was increased. The increased mass results in the mechanism moving slower

under the same force. The purpose is to have an amplifying effect of the influence the

lubricants will have at extreme temperatures due to viscosity. The DIM also has an

inertia mass that locks the eccentric wheels in the inactivated state. The inertia mass

did not form part of the set-up and had no influence on the results.

The DIM, shown in Figure 5, was mounted on a base plate together with a small

pneumatic cylinder. An adjustable screw-in type pin on the cylinder ram interfaced

with a pin on one of the eccentric wheels, which locked the DIM in the inactivated

position. This allowed for the centrifuge to stabilize at a predetermined rotating speed

to ensure the acceleration force exerted on the DIM is accurate and constant

throughout the test.

An electrical contact plate was mounted on the base plate to coincide with the pin on

one of the eccentric wheels at the end of its travel to provide an electrical signal at the

end of the wheel’s travel at exactly the same point for every test.

The “Arming” time for of the DIM was taken from the time of the electrical pulse

used to operate the pneumatic cylinder solenoid valve to the point of receiving the

contact plate signal. The pins and contact plate were not adjusted during the test

series in order to make comparisons between the various lubricants. The complete

DIM/angle bracket was mounted in an electrical box to seal the unit off from the

environment. This not only prevented the unit from frosting up during low

20 25

temperature tests, but also prevented temperature loss during set-up and testing. The

sealed electrical box with DIM is shown in Figure 6.

(4)

(3

(2)(1)

Figure 5: The DIM mounted on a base plate (1), with small pneumatic cylinder (2), adjustable screw-in type pin (3) and pin (4) on one of the eccentric wheels

Figure 6: The sealed electrical box with DIM.

The locking pin, used for locking the eccentric wheel of the DIM in the armed

position, was removed in order to allow the DIM to return to the safe or inactivated

position when the acceleration force was removed. This allowed for tests to be

21 25

repeated without opening the box or disturbing the set-up from ambient to low/high

temperature tests.

Initially the complete DIM was ultrasonically cleaned in Methelene Chloride. After a

clean and dry test, the gearbox was removed, lubricated by submerging it in the

lubricant and then spinning the excess lubricant off on a centrifuge at approximately

25G. The gearbox was then fitted to the DIM and the unit sealed in the electrical box.

An acceleration of 20G was used for each test. The ambient temperature test was

followed by the low and high temperature tests, respectively. After these tests, only

the gearbox was removed from the DIM, ultrasonically cleaned and then lubricated

with the next lubricant, using the same procedure.

7.2. TEST RESULTS

Table 16: Relative arming times for ambient tests. No Clean Old

Aeroshell 12 New

Aeroshell 12M20 M100 M1000 Uniflor 8190 NYE Synthetic

oil 132B

1 -1.17 -19.1 -18.68 -16.18 -16.18 -15.76 -18.68 -20.77

2 -0.75 -19.52 -19.1 -16.6 -16.6 -16.6 -19.52 -20.77

3 -0.75 -19.93 -19.52 -17.01 -16.6 -17.01 -20.35 -21.18

4 -0.33 -19.93 -19.93 -17.01 -17.01 -17.01 -20.35 -20.77

5 -0.33 -19.93 -19.52 -17.01 -17.01 -17.43 -20.35 -21.18

6 0.08 -20.35 -19.52 -17.01 -17.01

7 0.5 -20.35 -19.1

8 0.5 -20.35 -19.52

9 0.92 -20.35

10 1.33 -20.35

22 25

Table 17: Relative arming times for cold tests. No Clean Old

Aeroshell 12 New


oil 132B

1 -7.42 35.53 19.27 20.93 23.85

2 Not Not -12.43 -12.43 -15.35 No -9.51 -2.42

3 tested tested -17.43 -15.35 -17.01 arming -14.93 -11.18

4 -19.52 -16.6 -17.85 occurred -17.01 -16.6

5 -19.93 -18.27 -19.1 -19.52

Table 18: Relative arming times for hot tests. No Clean Old

Aeroshell 12 New


oil 132B

1 -19.52 -16.18 -16.18 -16.18 -19.93 -21.18

2 Not Not -19.93 -16.6 -16.6 -16.6 -20.77 -22.44

3 tested tested -19.52 -16.18 -16.6 -17.01 -21.18 -22.02

4 -19.93 -16.18 -16.6 -17.43 -21.18 -22.44

5 -16.18 -17.43 -21.18 -22.44

-30

-20

-10

0

10

20

30

40

0 2 4 6 8 10 12

Measurement number

Perc

anta

ge (%

)

Clean Old Aeroshell 12 New Aeroshell 12M20 M100 M1000Uniflor 8190 NYE Synthetic oil 132B

Figure 7: Relative arming times for ambient tests

23 25

-30

-20

-10

0

10

20

30

40

0 1 2 3 4 5 6

Measurement number

Perc

anta

ge (%

)


Figure 8: Relative arming times for cold tests

-30

-20

-10

0

10

20

30

40

0 1 2 3 4 5 6

Measurement number

Perc

anta

ge (%

)


Figure 9: Relative arming times for hot tests

24 25

25 25

Table 19: DIM temperature as function of time delay during cold test

Measurement No.

Time delay (s)

DIM temperature (oC)

1 35 -50

2 70 -49

3 105 -47

4 140 -45

5 175 -43

6 210 -41

7 245 -40

Documents

LUBRICATION TESTS 1. INTRODUCTION - Stellenbosch …appliedmaths.sun.ac.za/~fsmit/Lubrication.pdf · LUBRICATION TESTS 1. INTRODUCTION ... these criteria allow theoretical selection