Mechanical Chain Wear Indicator

grabcad

This is the kind of thing that could be built in to a hoist and used as an early indication of the chain length increasing and thus as a chain wear early warning. The four gears behave as a differential, the two smaller gears being allowed to move up and down in an arc to indicate a phase difference between the two big gears. The two side by side small gears are connected by a tight friction clutch such that after a new chain is loaded, the pointer can be moved to it's zero position. As the chain length increases, it is the equivalent of the two big gears turning in opposite directions, causing the small gears & swinging arm to move up and thus the pointer move down. In practice, the pointer could be replaced by a switch, activated at the point the chain has lengthened by 2%. A less mechanical solution would be to replace the gears with two rotary encoders, one on each of the chain pulley axle shafts. Then use an electronic counter to detect a phase difference between the two. The counter would be incremented by one encoder and decremented by the other. If the two chain pulleys maintain the same phase relationship, the counter will hover around zero +/- 1 count. If the counter exceeds a value equivalent to 2% increase in length, an alarm is activated. The pointer arm would need to be spring loaded (not shown) towards the uppermost pointer position, this compensates for the backlash in the gears and also the reduction in mesh between the big gear connected to the motor and the small gear. Although the small gear moves out of mesh as the pointer moves down, even with MOD 1 gears, sufficient mesh is maintained for in excess of 2% length increase. Built using rotary encoders, this is obviously not an issue.