TechArchive

Original article date: October 1999

With machines running faster than ever, excessive heat generation in speed reducers is becoming an increasing problem. We’ve been talking to the team at BAYSIDE about their ideas on keeping gearbox components suitably cool

When running continuously at high speeds, or even intermittently in some cases, speed reducers can generate temperatures high enough to break down lubricants, leading to premature failure of gears and bearings, shortening their lives considerably and causing costly downtime.

In general, high temperature is a concern for any gearbox running for more than 60% of the time at speeds over 3000rpm. So it’s best to make sure these devices stay cool.

The biggest problem encountered in a hot-running gearbox is of course lubricant damage and its effect on various components. But there are also the rubber seals, where heat takes a toll in two ways: it causes high pressure within the gear housing (which increases stress in the seals), and if the lubricant gets too hot, it burns and no longer adequately lubricates the seals, causing them to dry out. Both conditions contribute to oil leakage. There’s also the problem of of hot scoring on the gear tooth contact surfaces, which can lead to tooth surface fractures.

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.

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 carry or receive loads on a continuous basis, 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.

Another method uses a structural member as a heat sink to dissipate heat. The best heat sinks usually contain aluminium; depending on the application, use aluminium supports or mounting tables. A gearbox mounted on an aluminium 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 aluminium heat sink generally reduces the temperature of a gearbox by 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.

Air or water cooling

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 (15 to 24in diameter) so that it blows air on the gearbox.

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 the gearhead temperature by 20-30%. 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 aluminium 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. 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 6.75 litre/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.

• Bayside Motion Group UK
• Tel: 07071 229743

• Web:www.bmgnet.com

October 1999