New Plastic Cylinder for Servo Unit
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S"EWHAT unfortunately, the
exigencies of war have called for servos with a force in excess of 230 lb. at a time when rubber supplies have become very short, and before synthetics are available in sufficient qnantity to meet the probable demand in this field.
The mechanical properties of the rubber mix, used for bellows on the servo of conventional design, are such that thick sections must necessarily be used. Thus, the belloivs on one 230-1b. pulltype weigh 54 lb., and it is to be presumed that models with greater loading capacity would consume proportionately, still .larger quantities of rubber.
. It will be seen, then, that two difficulties had to be encountered. First, shortage of raw material; secondly.
these problems, Feeny and Johnson, Ltd., Ealing Road, Wembley, decided to carry out experiments on the replacement of the rubber bellows by. plastic cylinders.
Laminated Phenolic Cylinder Preliminary trials proved highly successful and production is now under way. The material selected for the cylinder is a heavy-dnty laminated phenolic, which, besides possessing the high strength to resist implosion or bursting, has also a high resistance to impact.. This is clearly necessary, as, in service, the unit might be subjected to shock loading.
During actual bench trials, two incidents occurred to demonstrate this last-named property. While a 64-in. cylinder was being tested to 16 cwt. lift under pressure, and while the JO-in, cylinder was lifting (vacuum) a deadweight of 4 ton, the supporting chain tackle broke after peak load had been maintained for some few minutes. The cylinder units were shot with considerable violence on to a stone floor, but subsequent examination, and retest showed that no cracking or other damage had occurred. •
There are being manufactured, at the moment, cylinders of 8-in. and 10-in. bores. These give under vacuunt pulls of approximately 330, 500 and 750 lb., respectively.. The 64-in, cylinder is also in production as a pressure unit. It will be appreciated that, under operating conditions, the sliding properties of the bore are of importance. In no circumstances must this wear or develop roughness, occurrences which would, in either case, result in loss in efficiency and rapid destruction of the cup piston.
In practice, as has been demonstrated before, the phenolic resins provide an extremely hard, wear-resistant, smoothrunning surface; they are indeed, as is well known, used for bearings.
Perhaps the most striking advantage of the replacement of rubber by plastics is the great reduction in weight which is achieved. Thus, as already noted, a 230-1b. pull-type rubber bellows weighs 54 lb.; the 330 lb. pull 64-in. plastic cylinder weighs 1 lb. 1 oz.; the 8-in. cylinder 1 lb. 8 oz., and the 10-in. cylinder 2 lb. 8 oz. The pulling capacity of this last is more than three times that of the 54). rubber bellows, the weight of the unit being less than half.
As to the lasting properties in service of a thin-walled phenolic-resin cylinder, this will depend partly op the mechanical properties already cited and partly on certain chemical and physical properties. Chemically, unlike rubber, the phenolic resins do not undergo deterioration on ageing. They are' unaffected by exposure to petrol of any type, to oils of all types and to sunlight, all of which can cause rapid breakdown of natural rubber.
Physically, the phenolic resins are, again, superior to rubber. Their surface is non-absorbent, both to oil and water. They do not soften or alter in mechanical properties under normal . exposure to any ordinary working temperatures. Thus, any heating up which may occur in service, although to some extent contributed to by the poor thermal conductivity of these materials (rubber and plastics alike), will give rise to no ill-effects with the plastic. ,