Gears keep their COOL
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.
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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If a motor and gearhead are attached, such as in a servo application, be sure to cool them separately and not with the same fan. Position the motor fan so it doesn't blow heat from the motor onto the gearhead. Air cooling generally removes enough heat through convection to lower gearhead temperature by 20 to 30%. Consider this method for any system with heavy loading or long duty cycles (over 60%).
In many servo applications, a motor and gearhead connect through an adapter. This device is typically an aluminum block that houses a floating bearing and input shaft seal. As the motor and gearhead both generate heat, this block absorbs and transfers heat between the two devices.
Here, there are two ways to dissipate the combined heat: thin and vent the adapter, and install an internal fan. The adapter is usually thick and it provides just enough clearance to accommodate the mounting. Reducing its mass by enlarging the opening (thinning the walls), and cutting venting slots through the sides will reduce the temperature. However, the vents may expose the shaft seal to more environmental contamination.
Besides thinning and venting, a fan can be mounted inside the adapter on the gear pinion where it couples to the motor. The fan, operating at the motor output speed, forces air across the inner walls of the adapter and the seal and out through the vents. This air flow greatly reduces the operating temperature. These changes have one big advantage: they all take place within the adapter, which means they don't take up more space than the basic motor-gearhead assembly.
Water cooling is the most effective method. Consider it for harsh loading or long duty cycles, especially in large systems where cooling system cost is nominal relative to the consequences of downtime.
Typically a water jacket is installed around the gear housing. Room-temperature water flowing through the jacket and around the housing at about 1.5 gal/min will reduce the temperature 35 to 45%. However, this approach takes more space.
As an option to these cooling methods, consider switching the lubricant from grease to a high-temperature gear oil and changing the oil at regular intervals (usually every 10,000 operating hours). Steady state temperatures will typically be 10% cooler than with grease.
When is it too hot?
To determine if a gearbox needs cooling, measure the surface temperature at the input section of the housing and compare it to the following guidelines. Normally, internal temperatures are 10 to 20% higher.
Safe continuous operation. Below 80oC, continuous operation shouldn't damage the lubricant or cause hot scoring. If the duty cycles exceed 75%, however, you should still consider cooling to reduce oil separation.
Cooling recommended. From 80 to 100oC, add some type of cooling. A lubricant operating continuously in this range will deteriorate, causing oil separation and possible leakage. Flash temperatures may get high enough to cause hot scoring. Also, the housing surface is hot enough to burn an operator.
Cooling required. Any gearbox operating above 100oC, even for a brief time, will have a shortened service life. Lubricating grease deteriorates rapidly, causing leakage, and gear teeth wear prematurely. Hot scoring is a major concern. Also the housing surface is hot enough to cause major burns to an operator.
Designing for less heat
External cooling methods dissipate heat that's already present in a gearbox. However, a look inside the unit may uncover ways to reduce the amount of heat that it generates.
Gear tooth design. Most gear teeth have an involute profile, which produces a combination of rolling and sliding between meshing teeth. Some designers go a step further, removing small amounts of material from the tooth tip and root surfaces to minimize impact and sliding friction, and therefore heat.
Tooth hardening and finishing. Gear teeth often undergo heat treatment to obtain hard, wear-resistant surfaces. The resultant distortion requires additional machining to obtain a smooth surface. However, this process closes the pores of the metal, making it more difficult to retain lubricant. Another hardening method called plasma nitriding achieves a hard, smooth surface without distortion, thereby eliminating re-machining. The resultant surface has low friction, and is well suited to retaining grease.
Bearings. The choice of bearings in a gearbox depends on the amount and type of load it needs to transmit. However, some bearing types generate more friction, which adds heat and reduces gear efficiency. For example, a tapered roller bearing provides excellent radial and axial load capacity, but generates more friction and heat than other bearings. On the other hand, a deep groove ball bearing usually provides adequate radial and axial load capacity in combination with low frictional drag.
Alan Feinstein is director of R&D, and Paul Ferrari is a design engineer, Bayside Motion Group, Port Washington, N.Y.