Feature Drives, controls & transmissions Choosing the right gearbox


With so many gearbox technologies and styles to choose from, selecting the right product for a given speed reduction application might seem a daunting task. Dave Brown of Brevini offers this design guidance


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he gear is one of the oldest power transmission technologies known to man, its fundamental purpose being to transmit uniform motion between two shafts at a constant ratio. Many factors impact on gearbox selection. Consideration must be given to the required ratio, speed, power and torque – not just the continuous torque, but also the repetitive peak torque, acceleration torque and brak- ing torque. But there are many factors which will impact on the most appro- priate gearbox selection, including shaft alignment, duty cycle, backlash, efficiency, mounting arrangements, size, weight, noise, smoothness of transmission, operating lifespan and maintenance requirements.


The simplest type of gear is the spur or straight cut gear, comprising a cylin- der or disk with teeth projecting radi- ally aligned parallel to the axis of rotation. These gears can be meshed together correctly only if they are fitted to parallel axes. They can handle high gear ratios, and offer efficiencies in the range of 94 to 98 per cent. A dis- advantage is the teeth meet suddenly at a line contact across their entire width, causing stress and noise which can be troublesome at higher speeds. A refinement of the spur gear is the helical gear. Here the leading edges of the teeth are not parallel to the axis of rotation, but are set at an angle. Because the teeth are angled, they engage gradually. The result is a far smoother and quieter operation. Due to the geometry and load directions on the gears, they can handle higher


torques than equivalent spur gears. Efficiency is similar, at 94-98 per cent. A disadvantage of helical gears is the axial thrust inherent in the design. This requires appropriate thrust bear- ings to be incorporated, which impacts on lubrication. This can be addressed by using gearboxes built around twin helical stages, with the helix angle of one being the negative of the other. Another solution to the problem of radial thrust is the double helical (or herringbone) gear, in which two sets of helical teeth are set in a V shape with each cancelling out the axial thrust of the other. Like the helical gear, double helical gears transmit power smoothly. However, they are expensive to manu- facture, and only suit heavy machinery. Bevel gears have two shafts which- intersect and the tooth-bearing faces of the gears themselves are shaped like cones. The design offers flexibility in the angle between the shafts, although 90° is most common. Gears have to be precisely mounted, and the axes capa- ble of supporting significant forces. Noise is also an issue at higher speeds. Efficiencies are a little lower than other gear designs at between 93 and 97 per cent. However, they do handle lower gear ratios with high efficiency – better than helical gear designs. The spiral bevel variation offers similar advantages to helical gears in that the teeth engage more gradually, delivering smoother, quieter transmis- sion. They also have high efficiency, in the range 95 to 98 per cent.


The hypoid gear resembles spiral bevel gears except that the shaft axes do


manufacturer of extruded plastic expects to achieve an £80,000 reduction in its annual electricity bill after installing ABB AC motors and low voltage AC drives. Alma Products, supplies extruded plastic products to the food industry. David Green, Alma engineer manager, asked ABB Motor Service Partner, Quantum


Drives cut electricity bill by £80,000 A


Controls, to look at the energy use of its extruders and recommend options for cutting the company’s energy consumption. The extruders were running in a DC drive system which drew a load whether it was required or not, because DC motors draw a current even while producing zero torque. The maintenance cost of the DC motors was also an issue, as it out weighed the AC motor option. As the project was proposed for Carbon Trust funding, a precise energy save reduction had to be obtained for converting


from DC to AC on the extruder lines. Quantum Controls was approached for advice and a hire trial was proposed. A temporary ABB 132kW AC drive and ABB 132kW motor were fitted and monitored. The tests showed the AC drive saved just over 25 per cent of consumed energy compared with the DC drive system. This would give an estimated £80,000 saving when all four main extrusion lines were converted to AC drives. Quantum Controls was awarded the contract to fit motors and drives on all four extruders. The project was funded by a £100,000 loan from the Carbon Trust. ABB


T: 01925 741111 18 www.abb.co.uk/energy Enter 319 Gearbox


technologies and styles vary but with the right advice and guidance you can make the correct choice for your application


Brevini UK T: 01925 636682 www.brevini.co.uk Enter 318


not intersect. They are almost always designed to operate with the shafts at 90 degrees. They are stronger and quieter than spiral bevel gears, and offer high gear ratios in a single stage. Mechanical efficiency is reduced, typically 80 to 85 per cent. These gears are paired and matched for life. Worm gears sets include the worm itself which is meshed with an ordi- nary looking gear wheel. They provide a simple and compact way to achieve a high torque, low speed gear ratio. However they offer lower efficiency, which can be as low as 50 per cent. The planetary gearbox consists of one or more outer gears (planets) revolving around a central (sun) gear. A large outer ring gear (annulus) meshes with all the planets. The axes of all gears are parallel, so input and output shaft are always in line. Planetary gearboxes are highly efficient (96 to 98 per cent per stage) even at low speeds. They offer high gear ratios per stage, and are highly compact, able to transmit three times the torque of a similarly sized, conven- tional spur gearbox. Multi-stage plane- tary gearboxes can deliver extremely high ratios, and efficiency losses are as little as two per cent per stage. Planetary gearboxes can also be combined with bevel and or helical gears to offer the best of both worlds as in Brevini’s Posiplan gearbox which uses a combination of planetary and helical gear technology to achieve a compact, efficient solution.


Regardless of the gearbox technology chosen, efficiency is affected by the load, operating speed, the reduction ratio, the number of stages, ambient conditions and lubricant selection. For optimum efficiency, the gearbox should be carefully matched to load requirements.


Another area impacting on gearbox selection is mounting arrangements, while the materials of construction will impact on factors such as weight, transmission capabilities and longevity in operation. Alloys, cast irons and even plastics are used in the manufac- ture of gears to give different perfor- mance characteristics and advantages. At Brevini we always send out an expert sales engineer to asses each unique application and offer advice and design assistance where required.


MAY 2011 Factory Equipment


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