Machinery Matters: by Larry D. Rumbol
Iconic singer songwriters are not known for their engineering wisdom, yet James Taylor put it rather succinctly: “I think it’s true what they say about the squeaky wheel always getting the grease.”
He is absolutely right, that is a common practice and in its way a maintenance procedure of sorts (albeit too late). We can do better.
Grease is a sticky glutinous lubrication substance, so called from the ancient French graisse, which in Latin means fatty, thick, oily. In its most basic form, it dates back to 1400BC when Hittite chariot axles were lubricated with a mixture of animal fat and limestone. The idea was sound: the lubrication needed to stay put under load. If a chariot were built today, it would have roller bearings, and about 90% of all roller bearings are grease lubricated as that lubricant just has to stay there despite extreme centrifugal force that would quickly spin out a thin oil.
We have moved on quite a lot since 1400BC (in most areas) but the basic premise remains: the lubricant has to remain in contact with the moving surfaces. It cannot squeeze out under centrifugal or centripetal action, by gravity, or through dynamic force or extremes of temperature. Yet it must flow slowly from one area to another and not impede or noticeably slow the moving parts it is lubricating and protecting.
Grease must also withstand what is called Mechanical Shear Stability. When a grease is “worked” in contact with mechanical parts it must not change its consistency. That’s a subject in itself called rheology, a branch of physics that deals with the deformation and flow of matter. A complex subject not needed here, but the shear stability of grease is important to note and relevant to its intended use onboard. It will be stated in the grease spec sheet. Just as with oil, there is not one grease that suits all applications (sorry about that). As yachts are migratory, there are always differences in product names and indeed availability — even within one manufacturer — and this can cause problems with grease incompatibility. Most manufacturers produce compatibility charts and this is a useful document to keep onboard.
Greases are a bit of a hybrid in rheology-speak as they have a foot in two important camps. They are viscoelastic; they can deform permanently and are also elastic. One rather special property of grease is that under ideal conditions — I’ll repeat that: under ideal conditions — grease can perform a “for life” lubrication. There is a caveat to that, of course. One needs to sample it and analyze said sample in a laboratory to be certain that one is enjoying such a state of grace.
Let us look at the differences in greases, their applications and the properties that are important to use on board in a yacht-based marine environment. Grease is basically an oil with a thickener plus additives. As we know, there is not one oil that fits all applications so what kind of oil is used? That depends on the application and is related to viscosity in a similar way that an oil grade is selected. The thinner the viscosity with a low load, the higher the speed. Conversely, the thicker the viscosity with the same load, the slower the speed. Think high-speed, light-duty electric motor at one end of the scale and slow-moving heavy-duty crane slew ring bearing at the other.
Thickeners are present in grease to affect the grease property, trapping the lubricating oil rather like a sponge might soak up a liquid, and they also add what is known as texture or stickiness. This property relates to how well the grease will transfer from one part to another. Dropping point is another quality to note (all of these should be on the grease spec sheet) and is an indication of the temperature when the grease will release the oil held within. Thickeners also have a direct effect on the previously mentioned shear stability and also flow characteristics (pumpability). Imagine grease had to be pumped down a line that was 10 feet (3m) long as opposed to directly into a bearing housing.
Last but by no means least and particularly relevant to the marine environment is a grease’s ability to withstand water (especially sea water) without negative effects.
For major onboard equipment, it is unlikely that a decision as to which grease to use will be required as the manufacturer of the equipment will have done that work, and it is important that their recommendation is followed. If that grease fails to perform, it is important to know the basics about grease to allow protection of the asset as the original operating parameters may have changed. Should analysis prove that there is a breakdown of the grease, accelerated wear or contamination, a discussion with the manufacturer or service team should ensue. In addition to that, an investigative maintenance intervention should be undertaken. Did the grease fail or was it pushed into failure by a machinery fatigue incident?
It is useful to know that not all of the qualities that the different thickeners can give correlate into desirable qualities of equal benefit. For example, the thickener with the best shear stability is a complex lithium, yet that only has moderate water resistance. Sodium grease has no water resistance at all. Product specification sheets will be your friend here.
Is my grease still OK?
There are numerous condition monitoring tests that can be performed in a laboratory to decide if used grease is still performing satisfactorily or should be purged and replaced. The most well-known dynamic test relevant to yacht applications is often confused with the dropping point, for obvious reasons. It is the penetration test, in which a given volume of grease is placed in a cup-like container and a cone of given shape and weight is dropped into the grease. After 5 seconds, the amount the cone has penetrated the grease is measured.
This test is performed on fresh new grease as a reference and also on grease that has been worked in a special lab device (a grease worker) in accordance with an ASTM method to simulate an amount of shearing. The comparison allows the shear stability of the grease to be measured. If this is plotted and trended, the grease life and potentially its suitability for the task in hand can be identified. Again, the product spec sheet will help here as grease consistencies are quantified by an NLGI Number in accordance with the ASTM method for working (basically 60 strokes of the grease worker) and cone penetration at 25C in tenths of a mm. The lowest grade NLGI number is 000, essentially a fluid with the consistency of olive oil up to NLGI 6 which is a hard grease rather like a hard cheese. NLGI 2 is the most common grease with
a consistency like a butter spread. Extremes of temperature in marine applications should be considered as these will have an effect on grease properties. There are other specific tests that can be performed on greases that are more specific to mass applications, new grease selections and storage conditions.
On yachts, however, we are more concerned with used grease condition monitoring to ensure the grease is still fit for purpose, is not contaminated and that no accelerated wear of the machinery has occurred.
In addition to the penetration test, microscopic examination, moisture content, wear particles, acid build up and spectrographic elemental analysis will give a precise picture and a clear indication of what action to take next. Interpretation of this analysis is an important conversation to have with the laboratory.
Extreme Pressure (EP) additives. These enable grease to withstand extremes of pressure and temperature. Most often used for gear applications.
AntiOxidants. These provide corrosion protection from either water or high temperatures.
Anti-wear agents. These typically are used for bearing applications when wear cannot be tolerated.
Viscosity modifiers. As we know, heat affects viscosity, and these additives have a direct effect on thinning at higher temperatures
Anti-rust additives. These reduce the reactivity to water and oxygen, preventing corrosion.
These are the basic qualities of general greases and are briefly explained here for the purposes of an overview. There are always speciality greases that display enhanced properties or certain environmental or performance characteristics and it is important to match the required performance with a specific grease. There are few applications where a suitable grease cannot be selected for optimum performance.
Grease guns – a deadly weapon to machinery in the wrong hands. Greasing practice is commonly thought to be: 1. Load grease gun. 2. Apply grease until excess shows. This is incorrect. A grease gun is capable of applying a 10-15,000 psi pressure and this can readily blow out a bearing seal. Excess grease will expand as it heats up and can actually draw in contaminants to the bearing, causing imminent bearing failure.
Knowing the right amount of grease to put in is as vital as is knowing the amount a grease gun pumps at each stroke. An engine is never randomly filled with an unknown quantity of oil and just as too much oil in an engine can have unpleasant consequences, so it can be for grease in a bearing. Over greasing of bearings vies with bearing misalignment during installation as by far the two main causes of failure.
The marine environment is one of the most harsh there is; saltwater and metals are not a good marriage. Grease, like oil, is a protector but its condition needs to be monitored.
Larry D. Rumbol has 40 years of expertise in marine condition monitoring and is marine business development manager with Spectro | Jet-Care in the United Kingdom, United States and Switzerland. Comments are welcome below.