Professional article: STG 65 - The angular gear without wear
Anyone who wants to survive in today's market has to meet a wide range of requirements. Always according to the motto “higher, faster, further”. This also applies to drive technology and thus also to Dunkermotoren.
In the field of drive technology, the goals are in conflict with each other: speed, torque, efficiency, service life and of course the price. A further important element of many industrial applications is drive solutions with intersecting axes, which in many cases facilitate mounting on machines and are also very space-saving. This is particularly true in the area of conveyor or door drives.
Dunkermotoren covers various customer areas in the field of drive technology. With brush DC motors, customers are offered inexpensive and robust motors. The service life of these motors is limited by the brush technology. Most of the so-called GR motors are combined with the worm gears developed in-house to form the overall drive.
The brushless DC motors, known as BG motors, achieve a significantly longer service life of over 10,000 hours. The worm gears are only suitable to a limited extent for these motors, as they are not designed for the corresponding service life of BG motors.
The STG gearbox with Spirotec gearing is an angular gearbox developed for the long service life of BG motors. Further features of the Spirotec gearbox are a high transmittable torque, a high robustness and a low-noise operation. The Spirotec gearing is a special type of Spiroid gearing in which the gearing parts are designed as a pairing of a helical pinion and a spiral-toothed ring gear. The cylindrical pinion allows free axial positioning, which is the basis for the greatly simplified assembly of the gear parts.
Compared to other angular gear units, the STG offers further advantages. The reduction ratios of the STG gearboxes range from 5:1 to 75:1 for a single-stage gearbox and thus cover the reduction range from bevel gearboxes via hypoid gearboxes to worm gearboxes. In addition, the Spirotec gearing achieves a higher degree of efficiency than worm gear units with the same reduction ratio and similar installation space. The gear efficiency increases with decreasing reduction ratio and increasing speed up to total values up to 90%. This is an outstanding value considering the installed radial shaft seals and bearings.
The special feature of the Spirotec gear teeth are the contact lines running transversely to the rolling direction, which produce an approximately identical lubricating film thickness between pinion and ring gear during rolling. This makes it possible to install hardened steel pinions and ring gears without the risk of seizure. Worm gears, on the other hand, form much smaller lubricating films than Spirotec gears, i.e. they almost always run in the mixed friction area, which is a cause of wear.
In addition, the contact pattern must first be formed with many gearing systems. Abrasion is therefore unavoidable. This is not the case with the Spirotec gear teeth, because the contact pattern is produced by the mathematically exact design. This means that the gearing is almost wear-free in the nominal load range and therefore has a particularly long service life. This also means that the Spirotec gearbox can be lubricated for life without any problems. The great advantage is that the gear unit is maintenance-free despite its long service life.
Only a few suppliers have this Spirotec gearing in their portfolio, as it is not included in the numerous standards for gearing. The basis for the development of this gear type was the close cooperation with gear suppliers and several dissertations. Based on this information, the gearing was designed and both suitable cutters for pinion and ring gear as well as the necessary measurement and test specifications were created.
The elaborate development of this type of gearing has paid off, as the subsequent production of the gearing parts takes place on conventional gearing machines. Only slightly modified hob cutters are used instead of expensive cutter heads such as those used for bevel gears.
The Spirotec gearbox is available in size 65. The STG 65 is the first Dunkermotoren gearbox to be equipped with oil lubrication. New knowledge had to be acquired, particularly in the area of seals.
In Dunkermotoren gearboxes are tested for service life on specially developed test benches. In the test, they are operated permanently at nominal load and the required acceleration torque is also tested each time the motor is started up. The gearboxes are tested both in continuous operation S1 and in alternating intermittent operation S8.
Within the framework of the validation, which was completed in 2015, the end of life of the STG 65 had not yet been reached. At that time, the service life in the validation for the nominal operating point was 10,000 h. However, the continuation of these tests showed that the service life is much longer than expected. These tests were finally completed with a service life of more than 22,000 hours, whereby the gearboxes were still fully functional. The gear parts were evaluated after the dismantling of the tests and no wear was detectable. Only a minimal smoothing can be seen on the tooth flanks (see Figure 2). The advantages of oil lubrication were compared in the STG 65 with grease lubrication in service life tests. In the tests with grease lubrication, significantly shorter service lives were achieved, whereby the lubricating effect of the grease failed completely, and the gear parts were then worn out. The drive combination of BG Motor and STG is therefore suitable for a particularly long service life in applications with intersecting axes.
Another positive aspect is the low heating of the gear unit. In rated operation, the temperatures are in the range of approx. 55°C, at an ambient temperature of 20°C. The test results to date show that the STG 65 can be subjected to even higher loads than previously specified. For this reason, tests have now also been started in which the gearboxes are tested with an increased torque. However, the first interim evaluations of the gearboxes in the test show that even with these increased values, long service lives can be achieved.
It is interesting to note that the gearbox can also be purchased as a single product and is not only available in combination with Dunkermotoren drives. Due to the claw coupling used, it can be mounted on a wide variety of motors.
So far, the STG gearboxes are available with the following reduction ratios: i = 5:1, i = 10:1, i = 25:1. The gearboxes have a standard protection class of IP 54 and are shock and vibration resistant according to DIN EN 61373.