A vented aluminum adapter connecting a gearhead and servomotor dissipates heat from both components. A fan fits snugly inside the adapter, blowing air around the inside and out through the vents to get rid of even more heat.
Select figure to enlarge.

Geared speed reducers that run continuously at high speeds often generate temperatures that are high enough to breakdown lubricants, leading to premature failure of gears and bearings. This can be a problem even for equipment that runs intermittently (less than 24 hours a day).

Heat causes a lot of wear and tear on gears, often shortening their life considerably. It doesn't matter whether the heat results from continuous or intermittent operation. In general, high temperature is a concern for any gearbox running at least 60% of the time at speeds over 3,000 rpm. So it's best to make sure these devices stay cool.

The breakdown

The biggest problem encountered in a hot-running gearbox is lubricant damage and its effect on various components. Grease, the most common gear lubricant, is fairly thick at room temperature and gradually thins (becomes more viscous) as the temperature rises. At high sustained temperatures, grease starts to separate into its two main ingredients, usually synthetic gear oil and lithium thickener.

As the lubricant breaks down, it loses efficiency. The oil film between rotating components gives way to more severe metal-to-metal contact. Wear occurs more readily, particularly within bearings and gear sets meshing at tight tolerances. Eventually, the increased bearing friction reduces gearbox efficiency.

For the rubber seals, heat takes a toll in two ways. It causes high pressure within the gear housing, which increases stress in the seals. If the lubricant gets too hot, it burns and no longer adequately lubricates the seals. This lets them dry out, eventually leading to embrittlement and microscopic cracks that propagate rapidly in the hot environment. Both conditions contribute to oil leakage.

Heat of the moment

Another form of deterioration consists of hot scoring on the gear tooth contact surfaces. When mating teeth operate at very high temperatures, they form local bonds where they contact. These local bonds increase friction between the teeth and quickly wear away their contact surfaces.

The most critical factor in hot scoring is the flash temperature. This is a momentary peak temperature at the contact surfaces between gear teeth. When it occurs, the mating teeth destroy the oil film between them, and instantaneously weld together. Then, as the gear teeth rotate out of contact, the welded bond breaks and the tooth surface fractures.

Cooling ideas

Reducing the temperature of a gearbox by any means will alleviate lubrication problems and avoid hot scoring. The most effective cooling methods are duty cycle reduction, heat sinks, air cooling, and water cooling.

Various external cooling methods dissipate heat from a servo gearhead running at 3,000 rpm with a 10:1 ratio (torque capacity up to 1,990 lb-in.). Heat sinks and fans offer some relief, especially when combined. However, water cooling does the most good.
Select figure to enlarge.

Normal duty cycles usually lead to moderate temperature rises (up to 80oC) but don't harm gears and bearings. However, long duty cycles induce higher temperatures and maintain them for a longer time, which leads to excessive component wear.

Shortening the duty cycle is a simple way to reduce temperature if the application permits this approach. Try to break a long cycle into two or three shorter ones with intermittent dwell periods during which the gearbox can cool down before starting the next load cycle.

For example, if a conveyor doesn't receive parts steadily, it may be feasible to reduce the speed or even stop the conveyor for a while. This is often the case when transferring a batch of parts between heat treatment operations. The conveyor doesn't need to run while the parts are being treated, and this rest period gives the gearbox, motor, and other conveyor elements time to cool.

Another method uses a structural member as a heat sink to dissipate heat. The best heat sinks usually contain aluminum because of its high coefficient of thermal conductivity (202 compared with steel at 45 and cast iron at 52). Depending on the application, use aluminum supports or mounting tables. A gearbox mounted on an aluminum table typically runs about 10% cooler.

Attaching a metal flange to the gear housing will dissipate additional heat. The larger its surface area, the more heat it conducts. You can mount such flanges in any orientation to fit space constraints.

An aluminum heat sink generally reduces the temperature of a gearbox 5 or 10%. Add a cooling fan, and the temperature drop approaches 30 to 40%. When combining the two, orient them so the fan directs air onto the heat sink.

Sometimes an operation won't let you break long duty cycles into shorter ones, and space limitations preclude using heat sinks. For such applications, consider air or water cooling systems. For air cooling, simply install a small fan (6 to 10- in. diameter) so that it blows air on the gearbox.

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