The choice of worm
gear housing – cast
iron or cast aluminum
– influences factors
such as size, heat
resistance to harsh
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.
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.
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 = CcrAcΔt
H = thermal power dispersed through the housing (W)
Ccr = combined heat transfer coefficient for convection and radiation (W/m2K)
Ac = 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.
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There are exceptions though. Some cast iron designs can be compact and thermally limited; likewise, there are spacious, thermally unconstrained cast aluminum versions. Nevertheless, standard configurations tend towards larger cast iron and smaller cast aluminum housings. The general idea is for aluminum housings to be lightweight and small, while the “heavy duty” cast iron designs usually are not intended to conserve space.
Iron has nearly three times the density of aluminum. It follows that, given identical center distances, worm reducers with cast iron housings, along with a more generous size, weigh approximately three times more than those encased in cast aluminum. This usually holds true even after factoring in the equivalent parts – gearing, shafts, and bearings.
Weight and size may not be a concern when the reducer is mounted to large industrial machinery, but smaller equipment can often benefit from less-cumbersome aluminum-housed reducers.
Cast iron and cast aluminum both provide excellent durability. Either version will hold up under most applications. Since cast aluminum won’t rust, it can often forego a protective finish, while cast iron housings are always painted. Nevertheless, aluminumhoused reducers are not recommended for washdown applications. The caustic detergents can eat away at bare aluminum, and while it is easily painted, the metal may succumb to abrasive high-pressure sprays. Cast iron is harder, and is a better choice for food plants and other sanitary environments. It is in fact the only metal approved by the Baking Industry Sanitation Standards Committee (BISSC) for washdown-duty speed reducers.
Although cast iron has a widely held reputation of being stronger than cast aluminum, this is not necessarily the case. Most aluminum alloys have a higher tensile strength. Aluminum can also resist impact better than iron, which is notoriously brittle. This could theoretically give aluminum housings the edge in withstanding shock loading. But in practice, the typically larger housings let cast iron reducers accommodate shock as well or better.
Rigidity, stability, and damping are nearly always more important than strength in a speed reducer housing. A rigid housing ensures alignment of shafting and bearings under stringent use. Cast iron housings are superior in rigidity and therefore preferable for applications subject to unpredictable operating conditions. Plus, with a coefficient of thermal expansion less than half that of aluminum, iron maintains its size and shape across a wide range of temperatures; this is especially important at bearing bores, seal surfaces, and other critical-fit areas. And, as no material is completely rigid, a one-piece housing – whether cast aluminum or iron – is favorable for preventing lubricant leaks.
No matter how wonderful the housing, the parts inside are the heart of the gearbox. The worm and input shaft are usually an integral part made of alloy steel. The worm teeth should be hardened and highly finished (ground and polished) to withstand the constant rubbing and heat. Worm teeth having higher lead angles give quieter, more efficient operation but require more driving torque. The gear, or wheel, is usually made of bronze for lubricity. Casting the bronze gear onto an iron hub adds strength.
A reducer’s mechanical rating accounts for the maximum continuous output torque or power. This rating is based around load analysis. The most critical factor is the wear load calculation, the gear components’ resistance to wear and pitting. Max output torque also relates to center distance, the distance from the worm centerline to the output shaft centerline. With an equal reduction ratio, a unit with a larger center distance (given by larger gearing components) can transfer greater torque. Whether the housing is aluminum or iron generally has little to do with the mechanical rating.
Bearings that are positively retained by the housing and bearing cover, rather than held by snap rings, provide higher shock capabilities, an especially important consideration in cyclic and reversing applications. Output bearings should have enough capacity to carry overhung loads, such as may come from belts or chains attached to the output shaft. Tapered roller bearings are the superior choice for high overhung loads. Supporting the output shaft often means handling thrust loads as well, which are usually present when the shaft is coupled directly to the driven machine.
Mark Baake is Gearing Specialist and Dave Brick is Director of Gear Engineering at Leeson Electric Corp., Grafton, Wis.