Getting more performance from a smaller working envelope and saving weight and cost in the process are objectives shared by most design engineers. And certainly by those who design, for example, handling systems, mobile plant and tools, conveyors and steel rolling mills.
Gearboxes account for a significant proportion of the weight, volume, cost and efficiency of cranes and other lifting equipment so there is potential to realise the benefits of alternative gearing technology. Planetary gearboxes are an example of this alternative gearing technology. That is, an alternative to so-called conventional gearboxes, examples of which include basic worm and wheel gear units, where a single steel screw type shaft turns a phosphor bronze single helical or cogged gear wheel, and combinations of helical spur and bevel gears.
In Gear motor handbook, (Springer-Verlag Berlin Heidelberg 1995) editor DW Dudley divides gear types into in-line drives and right angle drives. In-line gears, whether spur or helical, may have input shafts concentric or offset. In some cases, Dudley continues, the concern is to concentrate enough power into a small package which is where epicyclic, or planetary, designs are useful.
Widespread use of planetary drives developed in automotive and mobile plant applications, such as hydrostatic wheel and track hub reducer drives. Italian planetary gearbox manufacturer Brevini, for example, was established in Italy in 1960 essentially to produce planetary gearboxes for these applications. Benefits were identified by the company in the high torque, small size and improved efficiency characteristics of a planetary design for a wider range of applications, including industrial.
For cranes and lifting equipment the planetary design is used in specific areas such as wheel and winch drives and also slewing drives, where they have replaced worm gears, for turning large diameter cogged items that require slow movement at very high loads.
There is a growing market for planetary gearboxes, according to David Makin, director of business development at David Brown Textron which designs and manufactures all types of geared solutions. Brevini also believes the benefits can still be brought to many new applications, particularly in cranes and hoists where more conventional gearboxes are still being used.
Planetary versus conventional
Design suitability is a contentious issue and though the principle of planetary, epicyclic or co-axial gearboxes is not new, for some reason ‘a surprising number’ of designers regard them with suspicion, according to Brevini. There is still some resistance to planetary gears in certain applications, concurs David Makin.
One theory is that prejudice against the use of planetaries could result from the innovative design that makes them so successful. Traditional and, according to Brevini, erroneous thinking is that older, more conventional gearing layouts are more reliable. Also, it is argued that in a failure situation they can be brought back into service more quickly, albeit temporarily, by welding or hand dressing of gears, for example.
Because a planetary gearbox is smaller and lighter, conventional heavy engineering thinking would suggest that it is not as strong. This is an easy assumption to make since it is based on well established ‘conventional wisdom.’ Brevini draws an analogy with Formula 1 motor racing, where carbon fibre, Kevlar, titanium and many other technological developments have proved that ‘light and compact’ can also mean ‘strong’ but it has taken time to filter down to more traditional industry areas.
The tide is changing, however, Brevini says, and many machine builders from automated mobile bridges to high-speed plant machinery are embracing planetary technology and exploiting the benefits. Other industries such as water processing are also using this type of gear reducer to good effect because of its reduction capability and reliability, says Brevini.
Planetary gearboxes are good for low speed applications where only a small space is available and where minimum weight is required, says David Makin. Planetaries can be as much as 60% lighter than conventional types, Brevini claims, which means lighter support structures. They are also chosen for their reduction capability, compact size and, in some cases, Brevini says, because they are weight balanced. Where a conventional gearbox is used, the shaft is not in line with the bulk of the gearbox and its casing, hence there is an overhang and unbalanced weight to deal with. Because planetary gearboxes operate concentrically around a central shaft with a coaxial arrangement of the input and output shafts, they can be used in-line with the motor and the turbine, pump, wheel drive, etc. without obstructing other rotating parts or fluid flow patterns.
However, the planetary does not lend itself to right angle applications, Makin says, and therefore may not be suitable for a gantry or dockside crane’s travel drives, for example. If the gearbox is in line with the wheels it could be intrusive and vulnerable. The width of a helical bevel helical gearbox is only relative to one stage of helical gearing which means it is narrower than it is wide and therefore good for applications that planetaries will never be good for, Makin says.
The number of gears used is also a consideration. With a worm gear only two elements can produce reduction ratios from 7:1 to more than 100:1. Spur gears typically work best in ratios between 1:1 and 9:1, Dudley says, so if a 70:1 ratio, for example, is needed helical gears would have at least four toothed parts in two stages or there might be six toothed parts in three stages.
Resistance to shock loading is a requirement of many applications. A planetary gearbox can resist such loads better than a conventional single helical gear design because it has three (planetary) gears between the pinion and the ring gear and therefore the torque is equally distributed as one third of it through each gear. A worm gear, however, is the best type to resist shock loading, Makin says.
An advantage of planetary gears over the worm type is that they are fully reversible. The design of a planetary gear stage allows it to be equally efficient in either direction, which is an advantage if the machinery regularly runs in clockwise and anticlockwise cycles.
Regarding efficiency, a planetary gearbox is likely to be more efficient than a worm type but it could be less efficient than a helical type. A 60:1 reduction ratio helical bevel helical gearbox has an efficiency of more than 90% which is really as good as any type of gearbox. There is a perception that worm gears are inefficient, David Makin says, but at a 5:1 reduction ratio efficiency will be basically the same as the bevel helical at more than 90%. At a ratio of 70:1, however, the efficiency of a worm gear is around 65% – much less than a planetary unit. So smaller worm gears are relatively efficient and the very low efficiency figures of 50% for some transmissions are related to the larger worm gear units with high reduction ratios, an area where planetary units excel with their 90%-plus efficiency. This means a smaller motor gearbox combination and lower running costs.
Efficiency depends on friction so to increase efficiency friction must be reduced. A planetary gearbox is driven by a centre shaft that is connected to a carrier plate carrying three smaller gears. The gears run inside a toothed outer ring and drive a central gear cut around the central output shaft. This means that the load is spread over several contact points around each gear. Many, lighter contact points mean less friction, hence increased efficiency. The vital difference between a worm gear unit and a planetary, Brevini says, is that a worm gear unit relies on a turning surface across the driven wheel’s teeth to achieve motion which inevitably causes wear and higher friction. Planet gears run on two rows of needle bearings and so have a very low coefficient of friction. Less energy is required to turn them and so more of the driving force is converted into useful turning force after being geared down.
Heat capacity varies between the different types of gearbox. Conventional parallel shaft and intersecting shaft gearboxes have the highest thermal efficiency because they have the largest housing for their power capacity and reduction ratio, and are therefore better at dissipating heat. Planetary gearboxes have a relatively small volume and exchange surface, Dudley says, and therefore a lower heat capacity. Heat generation is based on duty cycle and load and there are no real differences worth highlighting between the gearbox types in crane applications, Brevini says. Thermal efficiency could be an issue though, Makin says, for example, on a planetary gearbox driving a winch in continuous operation. Either a larger size of gearbox or a cooling system could be needed which could cancel out the benefits of a planetary.
In terms of manufacturing, to assemble a worm gear, for example, both screw and gear components have to be matched so a certain amount of tolerance is built in to suit the application, meaning that each unit has to be manufactured to match the application and is subject to the vagaries of the engineer who is making them, Brevini says. A planetary gearbox is modular and so all components are manufactured to closely controlled and consistent tolerances so there is less room for error.
It also means that lead times are very much lower on planetary units – hours rather than weeks, Brevini claims. The design flexibility of the modular construction means standard modules from stock can be combined to customise a gear unit for non-standard applications, reducing the requirement for specialised components, and allowing a wide range of different units to be compiled virtually off the shelf. Even larger units (output torques of more than 100kNm can be supplied within 12 weeks, Brevini claims. And volume production of the standard planetary stages means economies of scale and competitive pricing.
Size for life
Regardless of the type of gearbox, it must be sufficiently large to give an adequate service life. Planetary gearboxes are smaller and more ‘power dense’ than conventional gearing layouts – they can transmit more torque through a smaller space, David Makin says. They can be as little as half the size of conventional gear units and fit where other types will not, Brevini claims, which means easier installation and neater design solutions.
To ensure that a planetary gearbox will last the lifetime of the equipment engineers at Brevini use a computerised selection programme to specify the correct gearbox for an application. It is used to effectively build up a picture of what the gearbox is doing, the demands on it and the expected performance. Duty cycle is entered in first and can be any combination of torque, speed and time. Environmental conditions are then considered; type of oil, ambient operating temperature, type of lubrication system, radial and axial loads, etc. The prime mover is also considered, i.e. is it an air motor, hydraulic motor, etc. The level of reduction is then entered, for example, 100:1. The programme then matches the smallest Brevini unit available that fits the application criteria; the engineer can then scroll upwards in size based on the service life in number of hours of operation. An intermittent task such as a winch on a crane might be 10,000 hours for example, or a constant duty application, say on a water pump, could be ten years, or 100,000 hours.
The information provided by the computer selection is then added to the knowledge and experience of an engineer. Elements such as application experience of extreme thermal conditions, difficult lubrication or intermittent shock load can be crucial to making the right choice. The specification is applied to ISO DP6336 standard calculations for life expectancy. In technical terms: ‘the load capacity of the planetary gear sets is calculated with a hertzian pressure and breaking strain in accordance with ISO DP6336 in relation to theoretical life.’
Through life costing, considered as part of the selection process, includes the initial cost of the gearbox, its durability and its maintainability. Other than routine oil changes, no maintenance is required on Brevini planetary gearboxes throughout their design life, not even replacement bearings, the manufacturer claims, so no costly stocks of spare parts are needed. Brevini says that maintenance of planetaries is comparable to conventional gearboxes in terms of lubrication in that it is required at regular intervals.
Makin says that planetaries are less easy to service in the field than conventional gearboxes and if repairs are necessary the gearbox might have to be removed and returned to a workshop. A correctly sized and maintained planetary gearbox should last for the designed life of the machine, Brevini says.
