Surface rubbings

Franklin defined charge as positive or negative in the way we understand the loss or gain of electrons by mechanically rubbing surfaces together. His definition still stands. 

As a last example we turn to the world of medicine. Osteo-arthritis is an illness that affects many people as they get older. The disease affects the joints between different bones in the body and makes it hard - and painful - to move them. The problem is caused by small lumps of bone which grow on the rubbing surfaces of the joints and which prevent them sliding properly. The problem can only be cured by removing the bad joints and putting artificial joints in their place. The first recorded hip-joint replacement was done as far back as 1897 - when it must have been a pretty hazardous business - but the operation is now a routine piece of orthopaedic surgery. In fact 30,000 hip joints are replaced in the UK every year world-wide the number must approach half a million. 

These pumps require the maintenance of very close clearances between rubbing surfaces for continual volumetric efficiency. Some of the important pumps are discussed. 

Grease is a very important and useful lubricant when used correctly, its main advantage being that it tends to remain where it is applied. It is more likely to stay in contact with rubbing surfaces than oil, and is less affected by the forces of gravity, pressure and centrifugal action. Economical and effective lubrication is the natural result of this property and a reduction in the overall cost of lubrication, particularly in all-loss systems, is made possible. 

Chemical reactions in boundary lubrication are different from static reactions even if the reactive substances involved are the same. The temperature to activate a chemical reaction on rubbing surfaces is usually lower than that required in the static chemical process. Some believe this is because of the naked surfaces and structural defects created by the friction/wear process, which are chemically more active. Kajdas proposed a new concept that accumulations of stress and strain in friction contacts could cause emission of low-... 

Similar results were reported by other investigators, [19,20], but attention was paid to investigating the effect of lubricant additives on the boundary film thickness. It is speculated that there should be no adsorbed layers formed on rubbing surfaces if purified and nonpolar lubricants are applied. The interferometer measurements show that in the case of using base oils, the relation between the film thickness and rolling speed follows the EHL power law pretty well down to 1 nm (Fig. 6(a)), or sometimes the film thickness may deviate from the Hamrock and Dowson s line and turn down quickly (Fig. 6(b)). If there is a small percentage of additives in the lubricant, on the other hand, the deviation from the power law occurs in a different way that the h-V... 

When the polar additive nonylic acid was added into hexade-cane liquid, the contact ratio becomes much smaller than that of pure hexadecane, which is shown in Fig. 39. For hexa-decane liquid, the critical speed to reach zero contact ratio is 50 mm/s, which is much higher than that of mineral oil 13604 because of its much lower viscosity. Flowever, when nonylic acid was added into the hexadecane liquid, the critical speed decreased from more than 50 mm/s to 38 mm/s. The same phenomenon can be seen in Fig. 39(h) which shows the comparison of oil 13604 and that added with 1.8 %wt. nonylic acid. The addition of polar additive reduces the contact ratio, too, but its effect is not as strong as that in hexadecane liquid because the oil 13604 has a much larger viscosity. Therefore, it can be concluded that the addition of polar additives will reduce the contact ratio because the polar additives are easy to form a thick boundary layer, which can separate asperities of the two rubbing surfaces. 

Photoinitiator Cure Dose ( J/cm2 ) MEK Double Rubs Surface... 

Figure 8 Left Schematic graph of the setup for the simulation of rubbing surfaces. Upper and lower walls are separated by a fluid or a boundary lubricant of thickness D. The outermost layers of the walls, represented by a dark color, are often treated as a rigid unit. The bottom most layer is fixed in a laboratory system, and the upper most layer is driven externally, for instance, by a spring of stiffness k. Also shown is a typical, linear velocity profile for a confined fluid with finite velocities at the boundary. The length at which the fluid s drift velocity would extrapolate to the wall s velocity is called the slip length A. Right The top wail atoms in the rigid top layer are set onto their equilibrium sites or coupled elastically to them. The remaining top wall atoms interact through interatomic potentials, which certainly may be chosen to be elastic.

The work function of the rubbing surfaces and the electron affinity of additives are interconnected on the molecular level. This mechanism has been discussed in terms of tribopolymerization models as a general approach to boundary lubrication (Kajdas 1994, 2001). To evaluate the validity of the anion-radical mechanism, two metal systems were investigated, a hard steel ball on a softer steel plate and a hard ball on an aluminum plate. Both metal plates emit electrons under friction, but aluminum produced more exoelectrons than steel. With aluminum, the addition of 1% styrene to the hexadecane lubricating fluid reduced the wear volume of the plate by over 65%. This effect considerably predominates that of steel on steel. Friction initiates polymerization of styrene, and this polymer formation was proven. It was also found that lauryl methacrylate, diallyl phthalate, and vinyl acetate reduced wear in an aluminum pin-on-disc test by 60-80% (Kajdas 1994). 

The 60/40 mixt was cast-loaded to a density of 1.65 its mp 65-70°, Expln Temp 264° higher Brisance, by Copper Cylinder Crusher Test 100% (PA=100%) Power by Ballistic Mortar 96% (PA=100) Deton Velocity by Dautriche Method 7050m/sec Impact Sensitivity with 15kg Weight 14cm (max wt for no expins) Friction Sensitivity 60kg (max pressure betw two rubbing surfaces for no explns). Used, under the name Type 98, in Navy Bombs,... 

TNAns) Methyl Picrate, or 2,4,6-Trinitro-phenylmethyl Ether, H3C.0.C6H2(N02)3 mw 243.13, N 17.28% yel crysts, d 1.7 1.6 for cast mp 68.4° (pure), 65-67° (coml) Brisance by Cu Cylinder Crusher Test 92% PA or 100% TNT Expln Temp 279° Friction Sensitivity 60 kg (max pressure betw two rubbing surfaces for no expln) Impact Sensitivity with 5-kg wt 19cm Power by Ballistic Mortar 101% PA Rate of Deton 6660m/sec at d 1.59... 

Type 94M (Explosive) (Navy). A cream-yellow expl compn consisting of Trinitroanisole 60 RDX 40%. Its cast d was 1.64 Brisance by Cu Cylinder Crusher Method 107% PA Expln Temp 216° Friction Sensitivity 40—50 kg (max pressure betw two rubbing surfaces for no explns) Impact Sensitivity 13 cm (max for no explns with 5-kg wt) Power by Ballistic Mortar 112% PA Rate of Deton by Dautriche Method 7700 m/sec. Originated as powerful expl for loading Torpedo Warheads but this use was discontinued due to its sensitivity to Sympathetic Detonation. Later uses included Shaped Charge Grenades and as a Booster Surround (Ref 1, p 32 as Type 94 Ref 5, p 379 as Type 94M)... 

In order to eliminate or diminish the friction, so-called lubricants are used. These substances (auch as oils, fats, greases or finely pulverized materials like talcum) possess die property to make the surfaces slippery when interposed between moving parts. Lubrication is the act of making the rubbing surfaces slippery by applyi ng a lubricant... 

Since electric charges may be transferred by rubbing surfaces of nonconductors against each other, the ancients were able to develop a crude triboelectric series based on amber, glass, and other available nonconductors. 

Special cases of solid lubrication arc boundary anil IIP (extreme pressure) lubrication In both cases ihe solid lubricant is formed by chemical reaction of special compounds, usually applied as oil solutions, with the metallic rubbing surfaces Typical boundary lubricants are the fatty acids which react with the metal surface to form metallic soaps which then carry the load. Strongly adsorbed hut nonreacting substances of linear structure, such as long chain tally alcohols, can also act as boundary lubricants but only under very mild conditions... 

Mixed Film Ijibrication. Mixed film lubrication is almost invariably the true state ol affairs when boundary and EP lubrication are encountered, i.c.. an appreciable fraction of the load is carried by the fluid film in the valleys" of the surface w hile the asperities in contact are permitted to carry the halancc of (he load without seizure through ihe beneficent intervention of the boundary or EP lubricant. The very important hreakin process of rubbing surfaces consists in the controlled induction of the number and die size of the surface asperities so that fluid lubrication will prevail for must of ihe lime. 

B4 or Type 2 Explosives. Lt grey powdery mixts of TNAns 60 or 70 w ith A1 40 or 30% used in Japanese incendiary submarine gun shells. The props of the 60/40 mixt were dl.90 (cast), power by ballistic pendulum 64%(PA= 100%), brisance by copper cylinder crusher 82% (PA=100%), explosion point 300 to 505°, impact sensitivity with 5-kg wt I7cm(max for no expins) and friction sensitivity 60kg(max pressure between two rubbing surfaces)... 

The radicals formed are involved in further reactions that result in the formation of polymers and organometallics. Whereas radical reactions within organic additive mixtures lead to polymeric films, organometalhc compounds are understandable as products of the interaction between the metallic surface and the radicals. Both polymeric films and organometallic species can protect the rubbing surface from wear. 

The first step (eq. 4.2) is an adsorption process. ZDDP in solution is adsorbed on the rubbing surfaces. As time goes by, ZDDP is converted into LI-ZDDP (eq. 4.3) which in turn, will be adsorbed on the surface along with ZDDP (eq. 4.4). 

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