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In new designs for molded gearing, prototypes are necessary to verify suitability for the intended function. Quite simply, plastic gears do not have a large enough history of usage for the designer to accurately predict funtionality. We can arrive at general indication of suitability, but actual performance under the expected operating conitions is the only way to finally determine suitability. Many of the variables that directly influence the performance of plastic gears cannot be accurately predicted and must be evaluated through testing. The gear designer has three choices in developing prototype gearing: hobbing (cut in the traditional fashion of metal gears), molding (for sampling purposes), and off-the-shelf purchasing (using an approximate gear to test the intended design). The image below shows the variables involved:
There
are advantages and disadvantages to each prototyping approach,
click on the following for more details:
1.
HOBBING (and example)
2. MOLDING
3. OFF-THE-SHELF
PURCHASING
CUT OR HOBBED PROTOTYPE GEARS
Usually the fastest way to make a representative plastic prototype gear is to cut it on a gear hobbing machine.
For an example of a hobbed prototype, click here.
However, the disadvantages of this method include:
- Design limits -- Quite often at Kleiss Gears, we design on gear set for molded manufacture with another slightly de-tuned gear set for cut prototypes. The cut gears usually have a simpler gear blank construction as well. Even if the coring of the eventual molded gear is approximated, any unique molded features are usually too complex for simple cutting machinery. This could affect the overall stiffness and function of the part independent of the performance of the gear itself.
- Material limits -- Cut gears are made from purchased bar stock, which limits the choice of material for the samples. The grain structure of the material will be different as well. This will have a second order effect on unfilled plastics, but could be quite significant for glass-filled gears.
- Quality limits -- The cutting process itself produces a different quality of gear profile that is similar, but not identical, to molded gearing.
MOLDED PROTOTYPE GEARS
Molded prototype gears are usually fabricated from more generic mold tooling that can be modified to produce variable shapes. At Kleiss Gears, we use sub-assembly insert tooling to achieve the highest control with the minimum cost.
Advantages of molded prototypes
- Accuracy -- Molded prototypes will be much more similar physically to the final product and will have the actual gear shape of the final product. This allows a much greater range of testing.
- Material flexibility -- A larger variety of materials can be used since it can be bought in pellet form. But the gear dimensions will usually have to be adjusted for the different shrinkages of different materials. For example, it would be unwise to mold filled nylon in the same mold that had been designed for unfilled acetal.
Disadvantages of molded prototypes
- Compromises to reduce cost -- Even with the best approach, compromises usually must be made for the sake of expense and timing. Special blank features that are not critical are usually skipped at this level.
- Mold cooling may not be optimized -- This is due to the more generic design of these general purpose molds. Sometimes the bores of the gears are made purposely small so they can be reamed to size later.
OFF-THE-SHELF MOLDED PLASTIC GEARS
A few companies sell off-the-shelf molded gears. We do not recommend these gears for prototyping without first inspecting and verifying their geometry, for the reasons listed below.
Advantages of aff-the-shelf gears
- Convenient and usually cost effective
Disadvantages of off-the-shelf gears
- Limits in variety -- Usually only single gears are available in either unfilled acetal or unfilled nylon. The gears are dimensioned as standard gear designs only. If a compound gear is required then two gears must somehow be coupled together.
- High chance of errors -- Our experience is that the manufacture of these gears is highly speculative. We have inspected some off-the-shelf gears with tremendous runouts and material shrinkages, and even deformed teeth. We have encountered customers who determined that plastics were unsuitable based on experiments with these gears, when it was the samples themselves that caused the failure.
A PROTOTYPE GEAR EXAMPLE
Shown is a cut prototype gear for a specific application. We were confident the chosen material, unfilled nylon 6/6, had the strength for the intended application, but we didn't know how it would wear and perform at very high and low temperatures. We decided to cut a few samples without concern for modeling coring or any other features since we were only interested in wear and survivability at temperature extremes. The pictured gears survived continuous loading for 2 million cycles at 260 degrees F, and 1 million cycles at room temperature with less than 30% wear on the cut pinion. This provided an invaluable insight into the suitability of this material for the intended function.
