Engineers had been entranced with the physics of full-fluid-film lubrication of the type that makes the plain bearings in trendy engines workable. They weren’t so pleased after they tried to use the identical physics to the lubrication of energy gearing. Their tried-and-true equations indicated that such gearing, working at very excessive tooth-to-tooth pressures, couldn’t probably work. Steel-to-metal contact ought to happen, inflicting speedy put on adopted by early failure.
However gears do work. Properly-made, surface-hardened metal energy gearing survives over lengthy durations of operation at tooth-to-tooth pressures as excessive as 100,000 psi. In any trendy bike engine, energy passes by means of a pair of main gears from crankshaft to gearbox enter shaft, after which by means of any of the six pairs of gears within the ordinary 6-speed gearbox to the output sprocket or shaft. I’ve by no means even seen a main gear failure (apart from one attributable to lack of lubricant).
A 1949 paper by Russian scientist A.N. Grubin described what he referred to as an “elastohydrodynamic” impact (EHD), with a really skinny layer of lubricant someway trapped between the elastically deforming surfaces of onerous gear enamel. What may probably trigger a liquid to stay in such a spot, reasonably than be immediately squirted out?
The colours we see when oil varieties a movie on the floor of water come from interference, because the various thickness of the movie trigger some gentle frequencies to intrude with one another as they’re mirrored from the highest and backside of the oil film. The identical impact, however attributable to clear floor oxide layers, produces the colour bands seen on welded titanium exhaust pipes. This identical impact was used at Imperial Faculty, London, to measure the elastohydrodynamic oil movie between surfaces closely loaded towards one another. Researchers discovered movie thicknesses of 0.1 to 1.0 micron.
How thick is a mono-molecular layer of oil? A traditional experiment attributed to the ever-curious Benjamin Franklin gives the reply. Dusting a non-wettable powder over the water floor permits the experimenter to simply see the clear space coated by a spreading oil movie after putting a drop on the water’s floor. As soon as it has unfold so far as it could actually and has a uniform look it’s simply arithmetic to match the amount of the unique oil drop (measure its diameter simply earlier than placing it on the water) with the amount of the movie, which is simply its space multiplied instances what we wish to know, which is its thickness. In a single case, the reply was .017 micron.
Evaluating that with the EHD movie thickness vary measured at Imperial Faculty, we discover this to be between 6 and 60 molecular thicknesses.
Subsequent query: what can probably enable that oil to stay between two surfaces which are exerting such super stress to squeeze it out? A method to take a look at this is able to be to say that the viscosity of the liquid is someway massively elevated by such nice stress. The calculation confirmed that the oil’s viscosity had elevated by roughly 5 orders of magnitude (that’s, by an element of 100,000, or ten to the fifth energy).
We all know that hydrocarbon molecules take the type of lengthy chains of carbon atoms, each bonded to hydrogen atoms. For hydrocarbons within the lubricating oil vary, there are roughly between 20 and 70 carbon atoms, making the chains quite long. What has been found is that underneath nice stress, oil molecules self-align to turn into parallel with one another. As this occurs in oil trapped between onerous surfaces, their interplay with one another more and more assumes the character of a crystalline strong. In a crystal, the atoms kind extremely common ranks and rows, every bonded by electrical forces to its neighbors. One thing comparable occurs when rubber is enormously deformed – its lengthy chain molecules align with each other and could also be attracted to one another by short-range forces: the rubber now assumes the properties of a crystalline strong.
Pc simulations of molecular-level oil habits in such unusual circumstances point out that the one approach any particular person molecule can escape the zone of excessive stress is end-wise – a tough course of. A witty researcher has given this the title “reptation” (snaking).
This molecular alignment and confinement appears to be the mechanism that enables closely loaded gears to outlive by means of lengthy and productive lives.
On the 2 events once I’ve ridden in helicopters (It was a John Wayne/LAX commuter service) I’ve discovered myself fascinated by the roughly 100-to-one rpm discount from the quick spinning generators to the primary rotor, beating its approach by means of the air at 300 or so rpm. Helicopter gearboxes are extremely refined items of equipment.
Good factor oils simply occur to strategy a strong crystalline state as they get pinched between the massive variety of gear enamel that serve the human challenge.