The Sticky Stuff
Grease – rarely seen when looking at a gleaming superyacht finish and rightly so. Yet it has a vital job to do, so let us take a closer look at grease with Larry Rumbol from Spectro | Jet-Care
The name comes from the ancient French: Graisse (Latin – fatty, thick, oily). It dates from 1400BC when
Hittite chariot axles were lubricated with a mix of animal fat and limestone. Remaining under load and centrifugal force was pivotal.
Since 1400BC that basic premise of the lubricant staying put and remaining in contact with the surfaces under load still remains. Furthermore, these properties must be retained at low and high speed, and load. Grease cannot squeeze out under centrifugal or centripetal action, by gravity or 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 known as mechanical shear stability… when a grease is worked in contact with mechanical parts it must retain its consistency.
That is a subject in itself called rheology (a branch of physics that deals with the deformation and flow of matter). Shear stability should be related to its intended use onboard and taken from the relative specification sheets provided for each independent piece of equipment.
Just like oil there is not one grease that suits all applications – worse still, yachts will find that grease marketing means not all brand types will be available in both the Med and Caribbean. As such, manufacturers compatibility charts should be consulted (and ideally kept on board).
Greases are a bit of a hybrid in rheology speak, as they have a foot in two important camps. They are viscoelastic (can deform permanently) and are also elastic. As opposed to oil it is possible to ‘grease’ something for life (only under ideal conditions!). Laboratory sampling will identify if that state of grace has been achieved though.
Grease is basically an oil with a thickener plus additives. Application can be simply related to viscosity in a similar way to oils. The thinner the viscosity the lower the load and the higher the speed, conversely, the higher the viscosity the slower the speed and higher the loading. Think high speed light-duty, electric motor at one end of the scale and a slow-moving, heavy-duty crane slew-ring bearing at the other.
There are numerous properties and analyses to consider when choosing and using a grease. These are a few important ones:
- Thickness (adding texture and stickiness) relating to how well the grease will transfer from one part to another. Thickness also affects pumpability – imagine the grease had to reach a bearing some metres from the access point.
- Dropping point – giving an indication of the temperature when the grease will release the oil held within.
- Penetration test – allowing shear stability to be established.
- Moisture content – is the grease required to be waterproof?
- What performance additives are required?Refer to the grease spec sheet.
This is just a snapshot of what should be considered when dealing with grease and note that one grease does not suit all applications which is a common misconception and too much grease can be a bad thing, causing early failure and/or accelerated wear.
For further information on testing and analysis of greases (including oil and hydraulic testing and analysis) contact Larry Rumbol, at Spectro | Jet-Care www.spectro-oil.com
