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Aviation History
1920
1920 - 1148.PDF
NOVEMBER 4, 1920 THE PISTON-RING PROBLEM The Allen Solution BY ARTIFEX THE efficiency of any engine which embodies pistons depends in great measure on its piston-rings. If these fail to make a leak - proof packing, or make one that from some defect in its function acts as an oil-pump, or over-act so that their pressure on the cylinder - walls is excessive, the best design otherwise is beaten. Now one, some or all of these possible defects of function are to be anticipated in any ring with a single division, or what is known as the Ramsbottom type, as leakage occurs at the cut, no matter how it may be stepped, or no matter how the later style of concentrically-cut—and therefore higher-pressure—ring may be duplicated or built-up in series. Moreover, the fluid-pressure medium will get behind either single ring or segment and drive it out in excessive pressure upon the very surface where such excess immediately mounts-up the cost, both in lubricant to ameliorate the friction, and developed horse-power— referring directly to fuel-cost—to overcome it. The foregoing considerations lead up—as they have done, one by one during the past twenty-five years, as they came to be realised—to the "Allen" segmentary and gapless—or rather sealed-gap—piston rings. These were invented twenty-five years ago by Mr. Robert Allen, to cure a defective tug-boat engine. Further developed by him jver since, and manufactured for some years past by Messrs. Allen and Simmonds, of Reading, they have been put through all experi- ments with steam at all known pressures, temperatures and /7777///7T7 Allen patent piston (automobile type), and "A" type segmental ring, with gaps sealed by the Allen patent choke-pieces : The section of the latter shows how the in- ternally-angled segment-ends are pushed out by the slotted sleeve portion of the choke and sealed by an arrow-head, within the slots, pushed out independently by the inner spring. Note the ready in- setting of the choke-unit thimbles into the vacant part of the piston interior, and the slight depth of the few rings required. Another type of Allen piston intended for high-speed vertical or Vee-type internal- combustion engines, and fitted with choke- units with obliquely set-in thimbles, A, so that the spring pressure may be mainly upward, to afford a locking ring-grip on the groove-entry face B, thus preventing gas-pressure entry behind the ring to create pressure-packing, and therefore ring- pressure on the cylinder-walls in excess of the 3 lb. normal. Note that the thimbles are not only clear of the heat, but kept cool by the air beneath the piston head, and that the underside of the ring has plenty of space for flushing-out deposits.! velocities, until latterly they have been adapted for the most exacting requirements of internal-combustion engines, the highest speeded aero-tvpes and even the high-pressure Diesels. Briefly each ring consists of three or more segments cut to the same centre, and having their inwardly-angled ends pressed outward by a spring-loaded sleeve-piece slotted to take the arrow-like head of a spring-loaded tongue-piece within the sleeve, so that the arrow-head faces may bear upon the segment ends, and thus seal the gap between them, independ- ently of the sleeve-end pressure that moves them out, the whole being contained in a thimble pressed on to or formed in the piston-wall, and styled a " choke." Now it follows that this series of segments will continue to make a circular contact, no matter how far pressed-out by the sleeve-ends ; and that they will still remain gapless in effect, as the gaps continue to be sealed by the independent pressure of the arrow-heads within the sleeves. Thus—apart The Allen " once-cut " Ramsbottom type ring; sealed at the gap for use in general applications in unworn engines, but not claimed to prevent or cure pressure or block-packing in the manner of the Beg- in en tal type. For fine groove clearances only. II5O
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