ENGINE FLY-WHEELS

Waikato Times, Volume 94, Issue 14832, 21 December 1921, Page 5

ENGINE FLY-WHEELS

UNSUSPECTED RISKS. Accidents occurring in connection with engine fly-wheels led lo a brief discussion of ihc subject in the annual report of tlic Inspection of Machinery, presented lo Parliament. It is not generally realised, the report stales, how dangerous fly-wheels are, and 1 a few remarks concerning them may be useful to steam users having no technical knowledge of them. The most common cause of fly-wheels bursting is from an increase of speed due to defective governors of valve gears. The main stresses in a fly : wheel are the result of the centrifugal force of the rim due to the speed at which the fly-wheel revolves. There is no possible way of overcoming these stresses. If the sectional area of the rim is increased lo make a stronger rim, the centrifugal force is increased likewise, because of the added weight of the rim, and the wheel is no stronger than before. There is a definite speed at which any wheel will explode, however sound, and regardless of the amount of material it contains. This theory is a mathematical truth, and has often been verified by practice. Cast iron wheels cast in one piece should not be run at a greater peripheral speed than a mile a minute. For builtup wheels the speed’ should be less, and, according to the design of the rimjoinls. Some rimjoints have a strength equal to 25 per cent only of the solid rim. If the speed of a revolving fly-wheel attains double the normal speed, the stresses will be four times the normal stresses. This, rapid increase of stress due to speed is the reason why wheels amply safe at ordinary running speeds, go to pieces without warning, and cause such destruction. Fly-wheels arc ordinarily made of east iron, and require to be thoroughly examined periodically for flaws and defects. Steam users should see that all governors and valve guards are kept in first-class condition, so that tile danger of Hie engine running away is minimised. A fly-wheel merely averages the speed of an engine during one revolution, and prevents violent changes in the rale of motion in that time. The number of revolutions per minute can only be, controlled toy the governor. It. prevents any permanent change in ttie speed. A fly-wheel cannot prevent an engine running away, but a governor can, and it is necessary that it and all the gear and the belting should bo ..maintained in first-class order and condition.