SEED Guides Unit Selection - Gearbox < >

Two factors which have an strong influence on the choice of gearbox for a given application are:

• purpose for which the gearbox is required in the context of the total system;
  
  
• quality of the gearbox, which is a function of purpose and which, in turn, affects the commercial context. Quality may be 'general purpose', 'precision', or 'super-precision' affecting the materials used, the manufacturing and assembly techniques employed and the cost. The quality chosen should be only be as high as is necessary to satisfy system requirements.

Other factors, many of which are inter-related, may be classified in groups under the following headings and are summarised in Figure 5.

Commercial factors are both the most significant and the most difficult the establish. The quantity required will strongly relate to price. The commercial life of the product may also affect choice since a product known to be old and likely to be replaced would be an inferior choice for a system requiring a long service life.

Convenience in purchase and manufacturer's reputation are subjective and difficult to quantify but may have a disproportionate effect on the final choice. The former relates to the availability of catalogue information, access to representatives, method of ordering, conditions of purchase etc. The latter involves the experience of the designer and the advice he receives.

Although most gearboxes do not contribute greatly to the overall visual impact of the system, the aesthetics of appearance should not be ignored. In many domestic applications the shape of the box may be a significant factor.

Performance factors relate to what is normally the primary function of the gearbox - the transmission of torque. The inter-related factors of power capacity, torque capacity, and speed limitation will have a strong influence on the choice. These requirements are related to the system output (i.e.: the demand of the driven machine). The nominal torque requirement should be increased by applying a service factor to reflect the duty conditions and the driving and driven machine types. Figure 6 gives some guidance in choosing a suitable service factor.

Speed ratio with output speed (or both input and output speed) must also be influential and are controlled by the 'up-stream' elements of the system.

Gearboxes are (nominally) constant power devices so speed is inversely proportional to torque. Thus high speed and high torque occur at opposite ends of the unit. The duty cycle in terms of torque and speed should be established since it will affect maintenance schedules and long term life.

Secondary functional factors involve the requirement to support loads other than torque. Radial and axial loads on the shafts will be a limiting factor in many gearboxes. These may arise from other system elements such as belt drives and couplings as well as the driven machine itself.

Typical duty categories are:

Light Duty: centrifugal fans, blowers, fans, light conveyors;

Medium Duty: conveyors, mixers, machine tools, positive displacement pumps;

Heavy Duty: reciprocating compressors, hammer mills, punches, presses;

Extra Heavy Duty: crushers, hoists, lifts.

The structural integrity of the casing including its rigidity and strength should be considered. Normally proprietary gearboxes will be adequate for all general purpose applications and data will be difficult to obtain but in special cases it will be necessary to consider this factor.

An extension of the casing is the structure on which it is mounted and this must also be checked to ensure that it will provide the strength and stiffness necessary to resist torque and withstand radial and axial loads applied to the unit.

Thirdly such concerns as reversibility of drive, the torque required for back-driving (or over-running) and inertia should be considered. Sometimes it is important that the system cannot be over-driven by the inertia of the load, while for other systems such driving results in torques which are much higher than the nominal system torque. The inertia of the load will affect both these and the acceleration of the system. In some cases the inertia of the gearbox may be significant in the system.

Finally energy based factors such as efficiency, related to heat generation, noise and backlash should be considered. It must be remembered that single stage gearbox efficiencies may vary between 92% and only 50%. Low efficiencies imply the generation of large quantities of heat which must be dissipated and could be an important consideration. Noise is usually associated with speed, power and the accuracy and form of the gear teeth. Backlash, the torsional flexibility of the shafting and the circumferential clearance between gear teeth, all of which are normally very small, are nevertheless significant for some systems. Some gearsets can be arranged to have zero backlash but all arrangements will exhibit shaft flexibility and tooth clearance.

Installation requirements include a number of important factors. The general ease of installation involves the shape, weight and size of the casing including features which aid lifting, manouvering and securing.

The orientation of shafts, their positions and their directions of rotation are related to the function of the system and it is necessary to ensure that the requirements can be met.

The relationship of the gearbox with its supporting structure is an important consideration. This involves the type of mounting arrangement and its orientation, the accuracy of location or provision for adjustment and the effect of the gearbox on the structure itself (e.g.: the transmission of vibration or heat).

The box may also affect other system elements such as couplings used, the space available and their orientation.

Operation of gearboxes over a long service life with minimum maintenance, especially when duties are light, is not uncommon. However provision for in-service maintenance and/or repair should be considered.

While it is possible to run some non-metallic gears without lubricants all gearsets benefit from lubrication in terms of efficiency, thus generating less heat, longer life and lower wear rate. The lubrication system used will depend on the performance of the unit and its required service life as well as the lubrication requirements of the rest of the system. It is usually contained within the unit but may be served by an external system.

Convenience in performing maintenance and repair tasks, involve accessibility, the need for special tools, schedules for regular maintenance, ease of in-situ reassembly and the consequences of system shut-down.

In applications where continuous running is critical to safety, or where other factors justify the cost, condition monitoring equipment may be considered. By detecting wear, noise or malfunction timely maintenance can be applied thus avoiding a dangerous or costly failure.

Environmental factors involve the entire context of the unit in use, relating to functional, installation and operational requirements.

Firstly the shape and size of the space available for installation should be considered. This relates to the ease of installation and accessibility as detailed above- The load capacity of the support may also be important, particularly when the gearbox is not mounted on a horizontal surface or when the material is weak.

Secondly the ambient air conditions must be considered with regard to both temperature and humidity. Since heat is generated from even the most mechanically efficient gearbox there will be a need for heat dissipation which is a function of both of these. This requirement can be quantified and provision made so that the maximum permitted temperature is not exceeded.

The maximum permitted temperature will be specified in relation to a standard ambient temperature. It should be noted that increasing altitude reduces heat dissipation rates since air density decreases. Significant increases in altitude (> 1000m) should be taken into account.

Humidity could be a factor in heat dissipation but more commonly is associated with the possibility of corrosion, particularly if the gearbox is left unused for long periods of time.

Finally the effect of gearbox failure (which may take a variety of forms) on the system itself, on the immediate environment and on the general environment must be considered.