ENGINE FLY-WHEELS

Hokitika Guardian, 15 December 1921, Page 4

ENGINE FLY-WHEELS

UNSUSPECTED RISKS. i WELLINGTON, December 12. Accidents occurring in connexion with engine fly-wheels led to a brief discussion, of the subject in the annual report of the Inspection of Machinery Department, presented to Parliament to-day. It is not generally realised, tho report states, how dangerous fly-wheels are ami 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 nil increase of speed due to defective governors of valve gears. The main stresses in a fly-wheel are the result o| the centrifugal force of the rim due to the speed at. which the fly-wheel revolves. There is up possible way ol overcoming these stressses. If the sectional area of the rim is increased to make a stmngei: 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 ho run at a greater peripheral speed than a mile a minute. For built-up wheels the speed should he much less, and according to the design of the rimjoints. Some rimjoints have a strength equal to 25 per cent, only of the solid rim. If tho speed of a revolving ily-whool attains double the normal speed, the stresses will he 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 cast iron and require to he thoroughly examined periodically for flaws and defects. Steam users should sec that all governor ami valve guard is kept in first class condition, so that the danger of the engine running away is minimised. A lly-whee! merely averages the speed of an engine during one revolution, and prevents violent changes in the rate of motion in that lime. The numlier of revolutions per minute can only he controlled hy the governor. It prevents any permanent change in the spend. A lly-wheel cannot prevent, an engine running away, hut a governor can, and it is nccessu.ry that it and all the gear and the belting should lie maintained ill first class order and condition.