Gearing units for use with mining equipment

A gearing unit comprising:

• (a) an input shaft;
• (b) an output shaft;
• (c) gearing for drivably interconnecting the input shaft to the output shaft;
• (d) a releasable clutch incorporated in the gearing and normally interconnecting said input shaft to said output shaft;
• (e) said clutch including a chamber for receiving pressure fluid for releasing said clutch;
• (f) a hollow, fixed spigot means projecting into one of said shafts adjacent said chamber and including means defining separate fluid flow paths leading to and from said chamber;
• (g) a rapid acting electromagnetic switchable valve device operatively coupled to said spigot means in close vicinity to said chamber and operable to cause pressure fluid to circulate to and from said chamber via said fluid flow paths;
• (h) means responsive to switching of said valve device to a first state for creating an initial pre-determined lower circulating pressure fluid flow to the release chamber when the clutch is operating to couple said input shaft to said output shaft via said pressure fluid flow paths to insure that a pre-determined lower pressure prevails within said chamber;
• (i) a load sensor which provides an electrical signal depending on the load transmitted through the gearing;
• and
• (j) means for actuating the valve device to shift said valve device to a second state in response to said signal in event of excessive loading to cause said circulating pressure fluid flow to increase in pressure and said chamber to be exposed to a higher threshold pressure to release said clutch and disconnect said output shaft from said input shaft and to cause said clutch release to occur in a time not more than 25 milliseconds.