The food processing industry requires continuous hygienic and sanitary conditions. Gear drive assemblies are used to power some food preparation equipment such as augers and conveyors.
Gearbox housings used with such gear drive assemblies are typically formed using cast iron or cast stainless steel methods. The shape of these gearbox housings is constrained by molten metal flow and cooling rates that determine critical internal and external geometrical features. Subsequent machining is required to finish the product to final shape.
The final shape is typically square or rectangular with flat surfaces and sharp edges. Many designs also include gaps, pockets and/or ledges. The metal also often has cracks that can worsen over time. These designs have a tendency to trap foreign objects such as food and food fluids which can remain even after pressure washing. As a result, bacteria and foodborne pathogens can collect on the gearbox.
In the food processing industry and other industries concerned with microbial growth, it is common to have a cast iron gearbox housing painted to prevent corrosion and rust. The high pressure washdown process used in these industries often damages the paint and causes the paint to chip thereby leaving open cast iron surfaces that eventually rust. In addition, paint chips and/or rust can get in the processed material (e.g., food).
Stainless steel is also used for the housings in the food processing industry and other industries concerned with microbial growth (however at a lower volume due to the higher price compared to painted cast iron). Stainless steel is sometimes chosen due to its mechanical strength, corrosion resistance, longevity and ease of fabrication. However, it has been shown that even with cleaning and sanitation procedures consistent with good manufacturing practices, microorganisms can remain in a viable state on stainless steel equipment surfaces. In addition, stainless steel alloys have been shown to be ineffective at reducing microbial load once they are contaminated.
Antibacterial stainless steel can be produced by adding an antimicrobial element to the alloy. Doping of stainless steel with Ag+-ions can be achieved without loss of its physical properties. However, silver coatings on stainless steel are non-permanent, because the modified surface layer is very thin, and once worn off, the base material loses its antibacterial ability.
Current metal gearboxes are also heavy and subject to a relatively wide range of tolerances in critical areas. These gearboxes can be difficult to install and may require trial and error measurements and adjustments (e.g., shimming).
U.S. Pat. No. 8,584,553 describes a polymer gearbox housing assembly provided in three pieces that are held together with tongue-and-groove mechanisms. The three-piece design can lead to a more expensive and time-consuming fabrication and/or installation. Also, while the '553 patent proposes a contoured outer surface to facilitate cleaning, foreign objects may collect in joints or seams formed between adjacent pieces of the gearbox housing. In addition, required tolerances are difficult to maintain with this design at least because the entire outer edge of the gearbox is the mating point used to combine the pieces. Furthermore, this design is relatively dimensionally unstable due to the multi-piece design and the lack of additional components or housings that can provide structural rigidity during use.