Forced feed application of gear oils may take the form of circulating systems which deliver the lubricant as a stream running over the gearing or to jets or sprays of some type. Such methods not only provide lubrication but also utilize the gear oil as a cooling medium. Brewer^5 mentions three types of circulating systems, namely:
(1) Wet sump, in which the oil is pumped under pressure directly from the sump to the lubricating points. Failure of the pump on such a system will cause almost immediate starved condition of lubrication.
(2) Dry sump, where a scavenging pump draws the excess oil from a sump and delivers it to a reservoir located above the gearing. A circulating pump then draws oil from this reservoir and delivers it to the point requiring lubrication. Since the scavenging pump has greater capacity than the circulating pump, the sump remains practically dry. A pump failure on such a system does not result in as star rapid vation as in the wet system since some oil can feed by gravity from the reservoir through an idle pump.
(3) Gravity feed which the sump pump delivers the oil, dripping from the gearing, to a reservoir set high enough to give the desired pressure. From this tank which may be thirty feet above the gear sets, gravity feed through regulating valves and perhaps sight glasses provides the lubrication.
Any of the above systems may also include oil filters, coolers, and even alarm devices to warm against malfunction of pumps. Gear oils used in paper mills, steel mills, and other locations are frequently contaminated with water, mill scale etc. Such lubricants may have to be reclaimed by settling, filtering and centrifuging before reuse. The oils used in forced feed applications are generally those which are quite fluid at ambient temperature and not of the residual type. However, if warmed, the latter type of lubricant can be handled in circulating systems.
Barring a pump or line failure, circulating oil systems provide a reliable flow of clean oil to gears. If necessary, this oil can serve additional functions, such as a hydraulic fluid in addition to lubricating practically any mechanism necessary. Such methods flush contaminants continually out of the lubricating areas and in most cases sufficient oil is in the system so that a portion can be withdrawn for a purification step during the cycle. Most of the heat absorbed by the oil passing through the operating area is released in the storage or by coolers before the return cycle.
The lower the viscosity of a gear oil, the more satisfactory it will be for removal of heat from gear sets. In circulating systems, Dudley^16 considers that about a gallon of oil per minute will remove the heat developed when 400 hp are transmitted through a gear set. With larger installations considerably less than this ratio circulating oil to power is required.
