All geared up: simple steps to gearbox longevity

Gearboxes are so common in so many applications that it is easy to overlook this vital link in production and processing. But maintaining a speed reducer in top condition depends on correct installation, proper use and regular checking.

Providing a speed reducer or gear motor is correctly selected, installed and regularly maintained, it should operate satisfactorily for years, but there are key steps to prevent malfunctioning. When selecting a gearbox, ensure the characteristics of the driven load are considered, so adequate 'service factors' are applied to the design, thereby ensuring an adequate safety factor for the application.

At installation, the usual needs of rigid, flat mounting surfaces and accurate shaft alignment must be also observed. Where possible, install flexible couplings, because these will help compensate for any shaft misalignment. If a shaft-mounted configuration is compatible with the application being considered, choose this because, by its nature, it is not affected by misalignment (providing the correct length of torque arm and a resilient bush are used.)

Overloading

For applications where overloading is possible, or when an overhung load could not be estimated accurately, consider installing a torque limiting device, such as a clutch, fluid coupling, or shear pin. Except for fluid couplings, such devices are most effective when installed between the speed reducer and driven machine.

It is important to select a properly rated electric motor, one 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.

Shaft loading

In some applications, input or output shafts may be exposed to radial loadings generated by the application or by an external power transmission component, such as a belt, chain or gear. Such an overhung load should be minimised to preserve the life of the bearings.

Accordingly, the transmission pulley, sprocket or pinion should be of the largest diameter compatible with the transmission ratio. It should also be located as close as possible to the shaft shoulder, to minimise the load offset. This point is simple, but often overlooked.

Orientation

Except for very small sizes, gearboxes are often supplied for a specific orientation (or mounting position). This will govern oil level, position of sight glass, etc, and may require special arrangements (such as sealed bearings) for points which will be operating above the normal oil level.

It is most important that the gearbox is installed in the orientation advised to the manufacturer at the time of order. If this cannot be complied with, then the supplier must be consulted for guidance.

Lubrication

Before start-up, make sure lubrication positions match the configuration specified on the order - ie, that the fill, drain and level plugs are located in the proper positions. Obvious as this seems, it can save enormous time wasting and difficulty further down the track. Except for life-lubricated gear units, it is normally advisable to double check lubricant levels.

When filling oil, adhere to the manufacturer's specifications as to oil type, quantity, viscosity and any additives. Quite apart from warranty considerations (which are very important in themselves), the specifications have been set for reasons that may not be obvious at first glance. Remember that the manufacturer has tested his units to the limit, and specified them so normal users will never have to approach their extremes.

Do not forget that both ambient temperature and duty cycle play a role in the selection of optimal lubricant viscosity. Synthetic oils are generally preferred to mineral oils. Although more expensive initially, synthetic oils allow for extended change intervals, lasting up to three times as long as mineral oils.

In addition, their higher viscosity rating maintains their properties unaltered across a broader range of ambient temperatures. (Note: Mineral and synthetic oils must not be mixed under any circumstances whatsoever.) In many instances, manufacturers recommend oil should be changed after an initial period of running-in.

Ventilation systems

A simple rule is to keep reducers away from heat to minimise thermal radiation. If necessary, install suitable guards or a forced ventilation system. The fan cowl of the electric motor should never be obstructed and should be kept clean, avoiding the accumulation of dust over time, which inhibits cooler running.

When installing a gear unit, allow enough free space all around to ensure effective natural airflow, as well as to facilitate routine maintenance according to the manufacturer's instructions.

Commissioning and disassembly

Sometimes speed reducers are painted to customer requirements before commissioning. When this is the case, be sure not to spill solvents on rubber oil seals. Protect them with rubber tape or grease. Do the same with drain or level plugs, as well as the nameplate, which will contain information that may be important later in the gearbox's life.

Whenever the reducer has been partially or fully disassembled for maintenance, replace gaskets and oil seals with brand new ones, because the used ones are likely to have been damaged in the process. Be sure to have the required spare parts available, and use only original spares.

Finally, strictly follow all applicable safety regulations in force during maintenance or installation. Only qualified personnel must perform such operations.