Aerospace Helical Gears

Renold Gears, a division of Renold Power Transmission is harnessing the extremely high (DIN 2) accuracy capabilities of the new GTG2 gear grinder from Group Company Holroyd Machine Tools in order to provide a bespoke service for high precision helical/spur gears. The new service meets the market need for ever increasing accuracy of small volume, high precision gears in aerospace and high performance automotive applications, industrial optics and custom designed industrial products. Advice and expertise in developing gear tooth profiles to suit specific applications is part of the service, which offers gear sizes in diameters upto 350mm and gear face widths up to 160mm.

This sort of accuracy means that their service is ideal for providing items such as master gears for prototyping, timing gears for use in aerospace applications and helical gears for use in high performance automotive gearboxes and oil pumps.

One of the major problems in helical gear manufacture is predicting actual stresses. This is important because the more pressure on the gear teeth, the more risk there is of pitting (macro or micro) and eventual gear failure. Stress prediction software eliminates this by optimising gear design, with profile and lead modifications to achieve the optimum contact conditions for low noise and high strength.

The software, however, is only one part of the package. Crucially, we must be able to replicate the profile derived from the software in the finished product. On conventional machines this can be difficult , but the GTG2 is designed with just this capability. It's on-board scanning probe and dresser compensation enables any lead and profile modification from prediction software to minimise contact stress and pitting, with reduced risk of thermal damage.

There is also a unique method of compensating for 'helical twist', an unwelcome condition that occurs when helical gears are 'lead crowned' to reduce transmission errors which often occur when the gears experience misalignments or deflections under load. Lead crowning varies the amount of material removed from the flank of a tooth across the face width, by causing the tool motion to deviate from a true helix. Besides giving the desirable 'relief' to the helix, this gives an undesirable 'lean' to the profile where the crowning is greatest. In many applications this deviation is not of concern, but in high precision, low noise applications it affects gear wheel performance by concentrating loads on particular areas of the teeth during meshing.

Correction of this problem is achieved by calculating additional motions of the grinding wheel and controlling them during the grinding operation using Holroyd's unique software. This modifies the angle of inclination of the grinding wheel during the grinding operation, either alone or in combination with other axis deviations. The result is better tooth contact during meshing, giving improvements in load bearing and transmission accuracy, i.e. smoother, more durable gears.

Renold Gears has over 100 years' accumulative experience in the manufacture of precision gears and is one of the very few British-based gear manufacturing companies still in existence.