Worm Gear - Helical Wheel Pair with Cylindrical Worm and Worm Gear-Helical Wheel Pair with Enveloping Worm - Ghiringhelli - #1

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Page 1 Ghiringhelli S.r.l. I – 20091 Bresso (MI) Via Vittorio Veneto 109 Tel. ++39-02-6108941 Fax.++39-02-6104748 E-mail: info@ghiringhelli-srl.com Web Site: www.ghiringhell-srl.com C.F.-P.I.: 00840170153 C.C.I.A.A. Milano n. 175647 Tribunale di Milano n. 159718/3896/18 Worm Gear - Helical Wheel Pair with Cylindrical Worm and Worm Gear-Helical Wheel Pair with Enveloping Worm. Remarks At times customers ask us why we don’t substitute the normal worm gear with cylindrical worm with the corresponding enveloping worm ( i. e. double envelope). We advise against this solution on the basis of our own experience and we remark that in Europe and Japan the solution of the worm gear with cylindrical screw is the preferred one. Without too many technicalities, we put forward some suggestions. Double Envelope Worms The special shape of the worm gear gives a theoretical contact of 3 - 5 teeth. This should provide a better behaviour when sudden load variations occur: in practice this advantage is not real because of difficulties in manufacture and assembly (fit). To correctly fit an enveloping worm one needs to align correctly both the median axis of the wheel and the axis of the worm but, and this is even more important, the median axis of the worm and the axis of the wheel. One needs specific and costly equipment to correctly fit enveloping worm pairs: in practice the theoretical advantage of mating on more teeth is not attained. A perfect lining up is required because enveloping worms suffer from errors in manufacture and fitting. Misalignments create limited zones of anomalous mating on which the load is concentrated and therefore there is the risk of pitting and abrasions on the wheel. At any rate the mating area between wheel and enveloping worm extends radially for a small portion of the width of the wheel. As a consequence of this elongated shape, the difference between the sliding speeds of the upper and lower areas of the mating zone is high: rubbing increases with reduced output (Fig. 1). Fig. 1