The manufacture of rubber goods : a practical handbook for the use of manufacturers, chemists, and others . ecially observed, if the rolls are to workperfectly, and accuracy in running the sheet of rubber is to besecured: the rolls should be of sufficient strength not to spring,and must in every instance be ground true. It is most convenientto have cast-iron rolls 1250 mm. long and 500 mm. in diameter,which have been ground on a Pooles grinding-machine. Three-and four-roll calenders may be driven by a special motor to ensurea uniform speed to the calenders, and to avoid the irregularities 48 R
The manufacture of rubber goods : a practical handbook for the use of manufacturers, chemists, and others . ecially observed, if the rolls are to workperfectly, and accuracy in running the sheet of rubber is to besecured: the rolls should be of sufficient strength not to spring,and must in every instance be ground true. It is most convenientto have cast-iron rolls 1250 mm. long and 500 mm. in diameter,which have been ground on a Pooles grinding-machine. Three-and four-roll calenders may be driven by a special motor to ensurea uniform speed to the calenders, and to avoid the irregularities 48 RUBBER MANUFACTURE. brought about by driving the mixing rolls from the same mainshaftinof. Fig. 21a shows a modern set of three-roll calenders,electrically driven, and provided with a friction-clutch. In laying down new plant all the gear-wheels should havedouble-helical, or at any rate helical teeth, the preference beingalways given to the former, however, for when they are used themotion of the calender rolls is absolutely uniform, as is mostmarkedly noticeable in calendering very thin sheet. Ordinary. Fig 21a. teeth always give rise to a certain amount of back-lash, whichmakes itself evident in the form of striped markings right acrossthe calendered sheet, where the rubber runs thin. The arrangement for adjusting the top and bottom rolls shouldbe fixed in a convenient position about half-way up the machine,and it should be possible to adjust either top or bottom roll bymanipulating a single wheel. The bearings of the middle roll arefixed. The gearing, as also the worm-wheels for the spindles, andthe spindles themselves, cannot be chosen large enough to allow ofa certain and easy raising and lowering of the rolls. Chain-driveshould be altogether avoided, and the vertical shaft with bevel- THE RAW MATERIAL. 49 wheel gearing should be selected as the best arrangement. Thenormal speed of the rolls is 4 revs, per minute, but for friction-ing the lower rolls should be geared up so
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