In order to improve the load-bearing capacity and service life of low-speed, heavy-duty, large modulus, multi tooth number and large gear, medium hard tooth surface super large displacement is often used in design. The so-called super large modification gear refers to the gear whose modification coefficient exceeds the conventional value (i.e. | x ﹥ π / 4tan α)
(1) the diameter of gear root circle is larger than the diameter of gear indexing circle (positive oversize modification) or the diameter of gear addendum circle is smaller than the diameter of indexing circle (negative oversize modification).
(2) if the tooth thickness of the indexing circle is greater than the pitch of the indexing circle (positive oversize modification) or the tooth thickness of the indexing circle does not exist (negative oversize modification), the larger the modification coefficient of the gear, the more remarkable the anti-wear and anti scuffing performance of the gear.
According to the introduction, the service life of super large modified gear can be increased by 2-5 times, and it is conducive to lubrication; The maximum meshing angle is about 25 ° after using 20 ° large tooth profile angle modification. The 25 ° large tooth profile angle has many advantages over the 20 ° large tooth profile angle. The comprehensive bearing capacity of the 25 ° large tooth profile angle gear is far superior to the 20 ° standard or its modified gear. So why not use the 25 ° large tooth profile angle directly? The main reason is that Gear hobs with 20 ° tooth profile angle are available in stock, cheap and short supply cycle, and most of these gears are single piece, so super large modified gears have certain comparative advantages. Because the range of negative angle modified gear pair is very narrow, most of them are not suitable to be used, so it is mainly positive modified gear in engineering. However, the existing literature is shallow, rarely from the hobbing mechanism and hob wear characteristics and technology, there are many theoretical analysis, some discussions or conclusions are one-sided and not comprehensive and not combined with practice, so there are improper or not rigorous, and even fallacies It is necessary to discuss.
From the above discussion, this paper clarifies the “focus” of the problem caused by hobbing super large modified gear with standard hob, so as to realize that this kind of gear can be processed by all methods of involute gear, because the modification coefficient is particularly large, it will bring some special personality, and pay attention to the difference of segmented Hobbing with standard hob with insufficient cutting edge length, which is the same as hobbing other gears, so it is necessary to save time It reduces the cost of expensive cutting tools, and is easy to operate, but it has low efficiency and certain economy.