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Get into Gear: A Practical Guide to Gearbox Selection
By Harry Bate.

To meet the expanding demands of industrial applications, gearbox manufacturers are producing an increasingly wide range of gearbox designs. To further match market needs, these different designs produce an extraordinarily broad range of outputs, ranging from a gentle 2-3 Nm to a leviathan 500,000 Nm and more.

Their functions can vary enormously: from the smoothly silent lifting of the curtain on an operatic performance in Paris, to the continuous powerful driving of conveyors carrying millions of tonnes of products ranging from delicate foods and wine, to bulk grain, coal and steel in Australia. 

The machine that mixes your child’s ice cream; which powers the lifts that take us to work; and which drive the baggage escalators which (sometimes) produce our travel luggage: all require very rugged, very reliable industrial gearboxes. 

But whether almost imperceptibly turning an observatory high in the Andes Mountains, driving agitators in waste water plants, or outloading mountains of cargo with slewing spouts in Esperance, industrial gearbox performance must be tailored to an endless diversity of specifier demands.

It isn’t just as simple as using a gearbox to multiply an engine’s torque to produce the output required. Specifier needs may encompass continuous operation, stop-start operation, compact installation, continuously consistent load and sharply changing load. Operating environments may vary from extremely dirty to hygienically clean, from aggressive marine, to food grade pure. 

So perhaps it is not surprising that the choices, at first glance, can puzzle even professional engineers, and bewilder the plant operator who wants to select the best for his or her installation. 

Naturally, anyone contemplating ideal gearbox selection will consult their gearbox specifying and engineering specialist. But even relative laymen can profit from an understanding of the basic gearbox types available. The range of such gearboxes is expanding considerably, and what was appropriate a few years ago, may not be the optimum now.

Classifications

Gearboxes, or speed reducers, are often classified by the relative position of input and output shafts. This is the concept behind such terms as in-line, parallel shaft or right-angle gear units.

Within the right-angle drive group are worm gear units and bevel helical units. While traditionally identified by the type of gearing rather than by shaft arrangement, both are right-angle drives. 

Different again are gearboxes of planetary construction, which achieve exceptionally compact construction by departing from the traditional arrangement of a pinion driving one large gear on a parallel shaft. Instead, the planetary gearbox surrounds the pinion (called a “sun gear”) with three or more smaller planetary gears mounted in a planet carrier. 

For each specific application, a design engineer may identify two or more product families that meet calculated torque and speed requirements. As a result, a design engineer will need to evaluate different factors in order to establish which among compatible configurations best suits the specific application and which proves best value for money.

Besides a variety of technical considerations, a significant issue can be space availability. The growing demand for more and more compact machines is placing increasing emphasis on space efficiency of motion control systems. This places increased emphasis on the importance of proper specification, installation, ventilation/cooling and maintenance.

Where no limiting factors over-rule technical ideals, design engineers are free to make the most of the features and advantages offered by each product line. A brief outline of such features follows.

Worm gear units

The worm design allows for very high transmission ratios (of up to i = 100 per single stage). This translates into greater cost-effectiveness. Also, worm reducers typically ensure quiet, vibration-free operation. They are inherently a right-angle drive.

Their typically low efficiency (from, say, 90 per cent down to even 35 per cent, depending on ratio, but typically 65-80 percent) suggests their use for low or middle-to-low power demand applications and/or those featuring intermittent duty. They can be ideal for applications that need to resist reversing, such as an inclined conveyor or hoist, but only for ratios of about 70-80 or higher.

In-line helical gear units

This style derives from the traditional form of pinion and gear drive and is characterised by high torque density (ie, transmitted torque per unit of volume) and high efficiency: 97-98 per cent per machine stage. 

These efficient drives offer a natural extension of the electric motor. The load can be driven directly by the parallel output shaft or through an ancillary transmission (belt, chain, or gear type). Advantages generally include wide-span bearing support for the output shaft, which ensures good overhung load capacity and longer-term operational reliability.

Typically available in a wide range of speeds, these reducers generally offer reduction ratios in the range of 3 to 500, with the higher ratios being achieved by use of multiple stages in the gearbox. Ratios outside this range are possible, but less common in ordinary applications.

Major manufacturers offer various options for ease of mounting and/or enhanced space efficiency, including foot or flange mounting configurations, as well as combinations with compact or integrated motors.

Right-angle helical gear units

In this configuration, input and output shafts are arranged at right angles via a gear set with either intersecting (bevel helical) or non-intersecting (hypoid) axes. The right-angle helical design ensures great space efficiency in terms of width, and provides the primary alternative to worm reducers in applications involving right angle drives. 

They once again are characterised by high efficiency and can extend to extremely high reduction ratios (even to 1700:1). The bevel set or hypoid set provide a significant ratio reduction in themselves. The hypoid arrangement used in some designs has an added advantage of being quieter and smoother running. Typically, these drives are a preferred choice where a right-angle drive requires high efficiency. This may be specified for applications involving continuous duty or large kW demand. 

Right angle helical gear reducers come in a wide range of versions. Of particular interest in recent times is the shaft mounting type with hollow output shaft, with or without shrink disc or with tapering lock. In this configuration, the gearmotor is fitted directly onto the shaft of the driven machine, resulting in enhanced space efficiency, ease of mounting and avoidance of alignment issues. 

Shaft-mounted gear units

These can be parallel shaft, helical or right-angle helical, planetary or worm gear units. They are frequently used in conveyor belts. Advantages of the shaft-mount style that make it the ideal selection for many applications are features such as:

• Simple, neat configuration

• Ease of installation

• Avoidance of the complexities of shaft alignment and of costly machining of mating surfaces

• Space saving

• Reduced angular backlash where shrink disc versions are used (because the keyway and its consequent basklash contribution are omitted)

• Torque arms that can be combined with torque-limiting devices, such as load cells

In the configuration featuring a solid input shaft, typically driven by a primary belt-and-pulley transmission, final speed can be adjusted (within limits) simply by changing either of the pulleys so as to modify the transmission ratio. 

Parallel shaft gear units

In all manufacturers’ ranges, these reducers represent generally the heavy duty option, to cater for installed power ranging from a few kW to hundreds of kW and more. 

Comprising helical gear sets, they offer high efficiency. These reducers can feature sturdy bearings, frequently the straight or taper roller type. Such bearings are suitable to withstand the high radial and thrust loading and impact loading typically encountered in many heavy industrial uses. Applications include:

• Wood, stone and ore crushers

• Extruders for plastic materials

• Bucket elevators, conveyor belts

• Dies and winding machines

• Fans and compressors

• Mixers, stirrers

• Many mining applications

Parallel shaft units can have a high number of reduction stages and can range in weight from fewer than 10kg to many tonnes. They generally have a male output shaft. The very large units are fixed in place and form a significant part of the whole machine. 

Small-to-medium parallel shaft units with hollow output shaft are particularly favoured for shaft-mounted screw conveyor drives. Their geometry fits well with the geometry of the screw conveyor, providing a neat and compact drive arrangement.

Parallel shaft units are also frequently used in the shaft-mounted arrangement on belt conveyors. Once again, they provide a neat, compact and efficient arrangement.

Planetary gearboxes

The compact nature of planetary gearboxes is making them increasingly popular in the industry. Their arrangement of several smaller planetary gears around the input pinion (instead of one larger gear running to one side) offers distinct advantages for certain applications: 

• High efficiency. The spur gears used in planetary gearboxes are inherently of high efficiency (97-98 percent per stage)
  

• Space saving. The planetary arrangement facilitates multiple reduction stages in a very compact space. These can be achieved because each reduction stage adds only a small increase in dimension. Compared with a parallel shaft arrangement, a planetary gearbox can often achieve the same ratio with one fewer reduction stage, with cost and dimension savings.
  

• High torque capacity. Because the torque being transmitted at any time is shared between multiple sets of teeth on the primary drive pinion, torque capability is greatly increased.
  

• Increased reliability - and higher radial loads permissible on the gearbox’s output shaft. Reliability and radial load capacity benefit because the shaft itself is not carrying any radial loads induced by the gears themselves (unlike parallel shaft systems). Benefits of reduced radial loads include extended bearing life, a critical component of machinery reliability. Reduced radial loads within the gearbox also permit it to tolerate higher radial loads from the equipment it is driving.
  

• Suitability for shaft mounting. Benefits of shaft mounting, as discussed previously, include elimination of the cost and complexity of couplings and elimination of the time and labour involved in ensuring correct alignment of the gearbox and the plant it is driving. Direct drive through shaft mounting also avoids radial loads imposed by chaindrives. 

Planetary gearboxes are available in a wide range of sizes and can be modular in construction. Heavy duty industrial uses include water treatment agitators, crane slew drives, winches, conveyors, and equipment for the mining, quarrying and steel industries.

Planetary drives are particularly well suited to applications involving high torque and high reduction ratios. On the other hand, their compact size gives rise to reduced surface area. This can result in limitations due to the need for heat dissipation for applications involving high kW continuous operation. Such applications may, for example, require auxiliary cooling arrangements.

They are an excellent option for slew or winch drives, where intermittent high torque is required, or for large agitators, where low kW, high torque is required. 

And now, what size?

Once the gearbox configuration that best suits the application has been identified, the design engineer will move on to selection of the proper size of gear unit and motor.

This is not as simple as it might sound. For example, it is important to select a properly rated gear box with adequate allowance for service factors, to take account of the number of starts, impact loading and the like. Then the specifier must select an electric motor that is powerful enough for the job, but not too powerful. Excessively large motors place unnecessary stress on all drive components (not to mention a large motor’s additional cost) and will require torque limiting, while an under-specified motor is obviously likely to fail prematurely.

Siting, installation, orientation, shaft loading, lubrication, ventilation, commissioning and maintenance are all engineering categories in their own right, which deserve the same attention as gearbox selection to achieve the optimum result.