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| | 107 | Originally, lead additives were used in gear oils and were very good at reducing wear in loaded gear sets. Lead though, is not very friendly to the environment and was phased out in favour of a Phosphorus and Sulphur combination. These gear oils used the Phosphorus and the Sulphur to attach to the gears and create a strong sacrificial layer to be worn off over the life of the fluid, thus protecting the gear from abrasive wear, high load and shock loads. Initially, the problem with this type of formulation was that the Sulphur used was active and caused corrosion of yellow and other soft metals used in transmissions and differentials. This happened because active sulphur reacted with some metals and metal alloys, especially those that used copper as the alloying element to form metal sulphides that in turn caused corrosion of the metal. |
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| | 109 | A generation ago deactivated or buffered sulfur was developed that could react with the phosphorous and create a protective and sacrificial layer in conditions created inside gear boxes. This formulation was not corrosive to brass, copper or other metal alloys used in transmissions. Today this is widely used in automotive transmission and gear oils. |
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| | 111 | Although de-activated sulphur may be used in a gear oil and may be safe for yellow metals such as brass and copper, it does not address the question of whether a GL-5 product can be used in all transmissions and especially those with synchromesh. A GL-5 grade oil has a very high rating for EP (Extreme Pressure) protection. EP gear oils contain additives that prevent metal surfaces from cold welding under the extreme pressure conditions found in situations where boundary lubrication prevails. At the high local temperatures associated with metal-to-metal contact, an EP additive combines chemically with the metal to form a surface film that is ductile enough to prevent the welding of opposing surfaces and prevent scuffing or scoring that is destructive to sliding surfaces under high loads. |
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| | 113 | In normal operation, the sulfur/phosphorous additive forms a black sacrificial coating on the gears and anything it touches with a little pressure and temperature. As the gears turn, instead of wearing, the sacrificial coating of additives is peeled off or worn off. This is normal and acceptable in all steel gears. But when one or more of the surfaces is brass or another soft metal, the sacrificial coating is stronger than the base metal, and instead of just peeling off, it takes with it a few microns of the softer metal. |
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| | 115 | An API GL-4 gear oil of any given viscosity has about ½ of the level of sulfur/phosphorous additive that would be in the API GL-5 product, so the bond with the metal surface inside the transmission is not as strong, and therefore can be peeled off without peeling a layer of soft metal. This means that the GL-4 product provides a little less extreme pressure protection than a GL-5 oil but less wear on synchromesh components of a transmission. When a GL-5 oil is used in a transmission with synchromesh it can create up to 4 times the amount of copper in a used oil analysis as that of a GL-4 product. Synchronisers will eventually wear to the point where they become ineffective. |
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| | 117 | When used in a differential, there is a 30% increase in the torque load compared with a transmission, so in this case, the extra EP protection is required, which is where an API GL-5 oil is best suited. |
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