pounds per square inch p s i to be maintained in the airbags 22 the amount of fluid p s i maintained in the airbags 22 will determine the lifting capacity of the hoist 10 an eye bolt 36 extends through apertures formed in the hoist load hook guide plate 24 and the hoist hook mounting plate 26 the eye bolt 36 receives the attachment means 38 which is attached to a raising lowering device 40 the threaded end of the eye bolt 36 is adapted to extend through an aperture in the plate 26 and receives a nut 42 attached thereto the nut 42 contacts a lower surface of the hoist hook mounting plate 26 and limits downward movement of the hoist hook mounting plate 26 a limit switch assembly 44 is mounted on the backing plate 16 and includes a pivotally mounted arm that is biased to contact the hoist hook mounting plate 26 normally the pivotally mounted arm of the limit switch assembly is in contact with the lower surface of the hoist hook mounting plate 26 the limit switch 44 is connected to a pilot valve 46 shown in fig3 the pilot valve 46 permits the passage of pressure fluid therethrough while the pivotal arm of the limit switch 44 is in a first position if the arm is moved into a second position by upward movement of the mounting plate 26 the limit switch 44 causes the pilot valve 46 to move into a second position to prevent the flow of pressure fluid therethrough the pilot valve 46 is in fluid communication with a control valve 48 which in turn is in fluid communication.