Brevini's Dave Brown provides design guidance on selecting the right gearbox for a given speed reduction application. The fundamental purpose of a gear drive is to transmit uniform motion between two shafts at a constant ratio. Many factors affect gearbox selection in a given application; 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 braking torque.
But there are many factors that will affect the most appropriate gearbox selection, including shaft alignment, duty cycle, backlash, efficiency, mounting arrangements, size, weight, noise, smoothness of transmission, operating lifespan and maintenance requirements. Brown assesses the various common gearbox arrangements and examines their features and benefits as well as their advantages and disadvantages. The simplest type of gear is the spur or straight-cut gear, which comprises a cylinder or disk with teeth projecting radially 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 94-98 per cent. A disadvantage of spur gears is that the teeth meet suddenly at a line contact across their entire width, causing stress and noise. This noise is said to be particularly troublesome at higher speeds. Therefore, spur gears tend to be used most often in low-speed applications or where noise control is not an issue. 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.
The gear is curved, with the tooth shape describing a segment of a helix. Helical gears can be meshed in parallel or crossed orientations (usually 90deg for skew gears) but the load capacity of crossed helical gears is much reduced. As the teeth are angled, they engage gradually. Each pair of teeth first make contact at a single point at one side of the gear wheel and then a moving curve of contact grows gradually across the tooth face to a maximum, before receding until the teeth break contact at a single point. The result is far smoother and quieter operation than with spur gearboxes. Owing to the geometry and load directions on the gears, they are also able to handle higher torques than equivalent-sized spur gears.
Efficiency is similar, in the range of 94-98 per cent. A disadvantage of helical gears is the axial thrust inherent in the design; this requires appropriate thrust bearings to be incorporated, which has an impact on lubrication requirements. This disadvantage must either be accommodated or 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 very expensive to manufacture and are really only suitable for heavy machinery. Bevel gears provide another option: gears where the two shafts intersect and the tooth-bearing faces of the gears themselves are shaped like cones. The design offers a high level of flexibility in the angle between the shafts, although 90deg is most common. Considerations in specifying bevel gears are that the gears have to be precisely mounted and the axes capable of supporting significant forces. Noise is also an issue at higher speeds.
Efficiencies are a little lower than the gear designs discussed so far - typically 93-97 per cent. However, they do handle lower gear ratios with high efficiency - better than helical gear designs. These gears are generally only used at lower speeds. A variation on the bevel gear is the spiral bevel design, which offers similar advantages to helical gears in that the teeth engage more gradually, delivering smoother, quieter transmission. They also have very high efficiency, around 95-98 per cent. A further variation is the hypoid gear, which resembles spiral bevel gears. Hypoid gears are almost always designed to operate with the shafts at 90deg.
They are stronger and quieter than spiral bevel gears and offer high gear ratios in a single stage. However, mechanical efficiency is reduced - typically 80-85 per cent. In addition, these gears are paired and matched for life. Worm gear sets include the worm itself, which is meshed with an ordinary-looking gear wheel. The sets provide a simple and compact way to achieve a high-torque, low-speed gear ratio. However, the downside is 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 will always be in line. Planetary gearboxes are highly efficient (96-98 per cent per stage of reduction), even at low speeds. They offer high gear ratios per stage and are highly compact and able to transmit three times the torque of a similarly sized, conventional spur gearbox, because the load being transmitted is shared across multiple planets. Multi-stage planetary 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 in terms of performance characteristics, while also allowing the input and output shafts to be turned through 90deg. An example of such a hybrid design can be found in Brevini's Posiplan gearbox. The Posiplan range uses a combination of planetary and helical gear technology to achieve an extremely compact and efficient solution in many applications. It is worth noting that, regardless of the gearbox technology chosen, efficiency is affected among other things by the load, the operating speed, the reduction ratio, the number of stages, the ambient conditions and the lubricant selection.
For optimum efficiency, the gearbox should be carefully matched to load requirements. Optimum gearbox efficiency is obtained at maximum load, but efficiency decreases at light loads. Other areas affecting gearbox selection include mounting arrangements. The customer needs to ask whether the gearbox should be flange mounted, shaft mounted or foot mounted. Not every technology will offer every option. He or she also needs to consider the materials of construction, which will affect factors such as weight, transmission capabilities and longevity in operation. Numerous alloys, cast irons and even plastics are used in the manufacture of gears to provide different performance characteristics and advantages.
Steels are the most commonly used because of their high strength-to-weight ratio, but where weight is a prime concern a properly designed plastic gear can replace steel in some applications, offering higher resistance to dirt and better low-speed meshing. Brevini sends out a sales engineer to assess each unique application and to offer advice and design assistance where required. The company can then advise and specify the correct gearbox for any task, meeting all the specific requirements for performance, size, mounting arrangements, reliability and longevity.
