Relative size
An aluminum-housed reducer is superimposed over an ironhoused reducer of the same center distance. The cast aluminum version takes up approximately one-third less space and is two-thirds lighter.

Aluminum, the light stuff. Properly applied, it provides thousands of devices with some pretty fantastic properties. Yet it is sometimes shunned, being a “non-traditional” material (as an example, the aluminum bat, for all its advantages, is illegal in the Majors) or is discredited as a flimsy alternative to ferrous metals. Still, increasing numbers of cast aluminum machine components are in there doing the job, despite what the skeptics think.

Where worm speed-reducer housings are concerned, the debate is alive and kicking. Some assert that cast iron’s unmatched ruggedness makes it indispensable for power trains. Others, particularly in Europe, favor cast aluminum for its lighter weight and design elegance.

Of course, there are pros and cons to each, and one application might strongly lean toward an iron housing while another cries out for aluminum.

Heated discussion

With worm gears, the housing material, be it cast iron or cast aluminum, will usually not affect the size, style, or grade of the internal working components. In fact, given identical center distance (the distance between worm centerline and output shaft centerline) you’re likely to find similar gears and bearings inside either type of housing.

Different housing designs will, however, tend to have a significant effect on thermal capacity. Worm reducers are valuable for cranking out high torque quietly and reliably, but the sliding engagement of the gearing amounts to intense friction, making these gearboxes among the hottest speed reducers around.

Heat flows from the gears into the moving lubricant and housing, where it dissipates into the surrounding air. The rate of conduction through the housing is expressed in W/m-K. Cast aluminum has a hands-down advantage as a heat conductor, with a thermal conductivity of 160, while that of cast iron is 52.

As it turns out, however, conduction from the inner to the outer housing surface is pretty much a negligible factor when it comes to worm gear design. Both aluminum and cast iron are conductive enough that the reducer housing eventually reaches a steady state where inner and outer surface temperatures are, for practical purposes, about the same.

Open house
Cutaway cast iron (with red trim) and cast aluminum worm reducers have mechanically equivalent gearing and bearings. While the aluminum unit is compact and lightweight, the bigger, heavier cast iron unit has a larger lubricant reservoir and greater surface area for dispersing heat. Cast iron has higher surface hardness and is therefore best for washdown duty and potentially abrasive high-pressure sprays. The iron unit shown here is made for washdown, with epoxy coating, v-ring output shaft seal, o-ring input flange seal, and stainless steel assembly hardware and output shaft.

Rather, the primary concern is convection, the exchange of thermal energy between a fluid and a solid. This form of heat transfer occurs in several areas. Lubricant carries heat from the gearing (the source) to the inner surface of the housing. (Technically, the housing also picks up a little heat by conduction through mountings.) On the exterior, the housing surrenders heat to the air. Both the lubricant and the outside air act as convection fluids. The exposed surface area of the housing figures heavily in the convection process, and therefore in the dispersal of heat from the overall unit.

Convection varies with area, temperature difference, and boundary layer conditions (including fluidic properties) accounted for by the coefficient of convective heat transfer.

The following formula accounts for convection together with the much less significant factor of radiation:

H = C_crA_cΔt

where

H = thermal power dispersed through the housing (W)

C_cr = combined heat transfer coefficient for convection and radiation (W/m²K)

A_c = area of housing exposed to ambient air (m²)

Δt = temperature difference between lubricating oil and ambient air (K) – lubricant temperature is used here instead of housing surface temperature because most applications reach a steady state where the temperatures are almost the same.

So, where worm reducers are concerned, what does the housing material have to do with cooling? It boils down to this: Most cast iron units have larger surface areas and lubricant reservoirs that bring their thermal capacity up to and beyond what is warranted by their mechanical rating – they generally are not “thermally limited.” Cast aluminum worm gear housings, on the other hand, are usually compact, with less surface area and smaller lubricant cavities. Thermal ratings are therefore important when designating these types.

Continue on page 2