Greases are suitable for use on slow-moving machinery where the pressure is not great. Even where the speed is comparatively high, but the pressure is light, a grease will often give excellent results, if the proper grade or consistency be selected. As a usual thing, however, if grease is used indiscriminately on a large scale, especially on textile machinery, a noticeable increase in the friction load results.
Greases may be divided into two classes, the lime and potash soaps, or high melting-point greases; and the tallow base, or low melting-point greases. The first are made by changing a small amount of fatty oil into a soap by means of lime water, caustic potash, or other alkali, and mixing it with a large amount of petroleum oil, such as engine oil. Such greases have a melting point of 140° to 180° F. The tallow base greases are composed of a large percentage of tallow combined with an alkali, and are brought to the desired density by means of vaseline, petroleum, or petroleum oils. Such greases, owing to their large content of tallow, have a low melting point, usually about 116° to 120° F.
The high melting-point greases usually require forcing down between the journal surfaces by means of compression grease cups. The low melting-point greases can often be packed in the journal box or directly on the bearings, as a low frictional heat causes them to melt, change to an oil, and lubricate the bearings.