Enclosed gear sets are often lubricated by bath or splash application. For this purpose, the gear oil is much more fluid than that used for open gears and hence not only lends itself to such methods of application but also acts as a coolant. Such systems are cleaner and more economical than many methods of application since only a tight housing is required.
These methods of lubrication are automatic since the oil feed starts with the start of the gears and ceases when they are shut down. These systems are efficient and reliable if the oil is kept at the correct level. Further, a minimum of attention is required.
Experience and location of the gear sets dictate the size of the gear case. Where the cases are small compared to the size of the gears, as is the case in most vehicles, the amount of oil is also restricted with a consequent restriction in cooling effect. In addition, close fitting gear casings can cause excessive oil drag which in turn will increase heating.
Satisfactory lubrication is obtained by bath or splash methods of gearing up to peripheral speeds of 2500 to 3000 feet per minute and occasionally at higher speeds where the operating time is comparatively short. The amount of oil carried to the point of mesh will depend upon the gear size, the speed, and the viscosity of the lubricants and to some extent upon the construction of the gear case. As speeds increase, a point will be reached where centrifugal force will overcome surface tension and viscosity and most of the oil will be thrown off. This may finally reach a point where little lubricants will adhere to gear teeth at the point of mesh. In case an excessive amount of oil does reach the engaging teeth operating at high speeds, the lubricants will be displaced at considerable velocity and cause noise. According to Merritt^37 this condition is likely to occur with double helical gears with continuous teeth operating with the apex trailing.
Such methods of application may include the following:
(1) By bath where the fluid is placed in a sump or bath in the bottom of the enclosed gear case. One of the gears dips into the bath and as this gear rotates it transfers the lubricant to the contacting teeth. Any excess is thrown against the housing and is guided, by means of troughs, either into the bearings or back to the sump. It is important that the gear case should not be too full and that the oil level be determined, if possible, when the gears are idle. Excess lubricant leads to foaming and causes excessive drag and heating. It is suggested that in industrial gearing the oil level should be such that the bottom gear dips into the oil about three times the depth of the tooth spaces. Automotive vehicles generally provide a filler opening and the gear oil should be up to the bottom of this opening; or, if a filler neck is used, the oil should be at the top of this filler neck. It is important that vehicle gear cases be filler when gears are stationary and also that the car or truck is on a level floor.
(2) By splash, using either one of the gears or an auxiliary flinger which dips into the bath of gear oil and throws the lubricant toward the top of the gear case so that it drops back into both gearing and bearings. Merritt^37 states: “ All dip lubricated gears produce different oil levels at different points of running, and the running levels indicated by an oil level gauge may be either higher or lower than the standing level.”
This again shows the necessity of still filling gear cases.
(3) By idler gear. In some instances, particularly on slow moving gear sets, an idler gear dips into a sump and transfers oil to the contacting gears.
While mention is made of bearing lubrication, in bath lubrication the gear oil may or may not also serve the bearings. It is important to recognize this in determining what viscosity or grade of oil to use since bearings normally require lighter oils than do gears. A compromise as to viscosity may therefore be necessary if the lubricant serves more than one function.
The lower teeth of exposed gears are sometimes encased so as to permit bath lubrication. Such application is only practical when speeds are relatively slow, otherwise considerable oil would be thrown out of the gears
