Motion System Design

Plastic gear design: doing it right

Improved characteristics of today’s molded plastics offer more choices to gear designers. But, the design process is fraught with pitfalls for the unwary. Here are some tips on how to avoid them.

The convenience of molding, coupled with the high strength and temperature resistance of today’s plastic materials, offers more design flexibility for light-load applications. But, do we have the process of molded gear design and manufacturing well in hand? Far from it! Horror stories about plastic gears abound. The designer’s hope of cost-savings and design flexibility often gives way to frustration, cost overruns, and inadequate performance.

However, all is not gloom and doom. A little knowledge of potential trouble spots, along with the proper tools can move your gear designs from a hit-andmiss proposition to a more efficient and predictable process. Here are the basic tools you need:
• Expertise in gear design and manufacturing.
• Knowledge of material behavior.
• Appropriate design software.


Gears are used in all types of equipment — from bicycles to aircraft, and disk drives to machine tools. And, plastic gear applications are constantly growing. The type of gear and its material depends largely on the application:
• Machined, ground metal gears are typically used when load capacity, reliability, and accuracy requirements are high. If accuracy requirements are less precise, grinding is eliminated. In either case, machined metal gears are the most common type.
• Molded, powder metal gears are used in high-load applications where production volume is high but the cost of parts must be kept low.
• Molded plastic gears, Figure 1, are increasingly being used in light-load applications where production volume is high and the parts cost must be minimized.

Warning: quicksand ahead

You are likely to encounter several problems when designing molded gears, especially those made from plastic. Though powder metal components exhibit some differences from conventional metal parts, plastic gears are the major source of concern.

Many people design gears and select a production process based on “the way we have always done it.” Unfortunately, this approach is often based on two common misconceptions.

Myth 1 — Design it according to the book. Most designers follow the gear design procedures described in machine design books and in gear or machinery handbooks. The common assumption being that if it’s in a book it must be right. But, that isn’t necessarily true. Let’s look at an example:

A spur gear set consists of a 48-tooth plastic (Nylon) gear with 30 diametral pitch and 20-deg pressure angle mating with a 16-tooth stainless-steel gear. A handbook-based design yields the following gear-set parameters:

1.0667-in. center distance.

1.6225 contact ratio, assuming full involute contact is available.

Maximum specific sliding ratio (ratio of slide to roll) of 6.403 for the driver and 1.475 for the driven gear.

54% approach action and 46% recess action.

Tooth contact outside the true involute form (TIF) on the pinion tooth, Figure 2.

So what’s wrong with this design? First, the actual contact ratio is less than the calculated one (which assumes full involute contact) because the tip of the mating gear tooth tries to contact the pinion tooth in the root area where there is no involute surface. Also, such a contact may cause interference in the root area. Second, a high specific sliding ratio causes increased noise and lubrication problems, thereby increasing wear. (As a rule of thumb, a ratio greater than 3 is cause for concern.) And finally, an approach action greater than 50% increases friction and reduces efficiency.

These are major problems which should not be overlooked. This “standard” gear set also helps illustrate the second common misconception.

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Myth 2 — A design is optimum when it’s based on an industry standard. Gear standards provide a basis for design according to procedures commonly accepted within the industry. Designs based on such standards, though generally suitable, are not necessarily optimized to suit a particular application. Also, such standards don’t cover all combinations of materials. Our sample gear set, for example, consists of a plastic gear meshing with a stainless-steel gear. The U.S. has no industry standard, such as those published by the American Gear Manufacturers Association (AGMA), that deals with this combination of materials. So, an industry-standard-based design, in this case, is out of the question.

Now, an appropriate software program is used to optimize this gear-set design, based initially on the assumption that both gears are made from stainless steel. The resulting gear set has the following changes:

1.0763-in. center distance (was 1.0667).

44% approach action (was 54%). 1.518 contact ratio. This is the actual ratio because full involute contact is available.

Maximum specific sliding ratio of 2.692 for the driver and 1.596 for the driven gear.

Here, the contact ratio is well above 1.2 (a rule-of-thumb value that provides a cushion above the 1.0 minimum requirement). Further, the approach is less than 50% and the maximum specific sliding ratio is below the 3.0 rule-of-thumb value discussed earlier. And, there is no contact outside the TIF, therefore, no potential root interference. Not bad for a small change in center distance!

Designing with plastic

The improved design described above still assumes stainless steel gears running against each other. But, changing one of the gears to plastic introduces some additional complications.

First, plastic gearing is more sensitive to temperature and humidity changes. In hot, humid conditions, the gear expands due to the combined effect of temperature and moisture absorption by the plastic. This expansion increases the risk of interference, especially between mating teeth.

After the operating conditions have been determined, the minimum and maximum effective center distances due to temperature and humidity extremes are calculated. Then, the gear set is designed for the minimum effective center distance and zero backlash condition. While doing this, it is necessary to keep several guidelines in mind:
• Trade-off some strength of the metal gear to get maximum strength from the plastic gear.
• Calculate an optimum tip relief so that the gear set operates smoothly, even with the higher tooth deflection experienced by the plastic gear under load.
• Make sure that there is sufficient backlash and root clearance.

It takes sophisticated software, combined with an awareness of the parameters to watch for, to do this type of optimizing.

After designing the gear set for the minimum effective center distance, the geometry is checked for the maximum effective center distance and backlash condition. Some design iteration may be required to obtain optimum conditions. It is also desirable to examine plots of the gear mesh to make sure nothing has been overlooked.

So, now the gear is designed correctly for plastic. Are we home free? Not quite, because other factors can cause trouble during the manufacturing process:
• Inability to duplicate exact geometry in the mold cavity due to plastic shrinkage.
• Involute, pitch, and spacing errors.
• Runout.
• Lead error.

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Shrinkage is a major problem with plastic gearing. The mold cavity must be designed to account for material shrinkage. But, expanding the mold cavity uniformly doesn’t work because the effect of shrinkage is greater in some areas than others, Figure 3.

Problems can also occur when the stainless-steel gear is machined. For example, an improperly designed hob or shaper cutter will produce unsatisfactory gears. Though this seems obvious, the need for proper tooling is often overlooked.

Recipe for success

Recently, a plastic gear in our fax machine broke a tooth, exhibiting the classic symptoms of a design that didn’t properly address the problems discussed earlier. So, how could this have been avoided? By applying these guidelines for a good gear design:
• Properly define specifications.
• Design the total system.
• Select the correct geometry, both for metal and plastic gears.
• Perform a tolerance analysis.
• Perform a load analysis. This may be tough because there is usually not enough load data available for plastic gears. By contrast, AGMA has accumulated much data on steel gears for use in their standards.
• Design the tooling (mold cavity) to allow for shrinkage.
• Use inspection equipment properly to ensure that gears are manufactured according to specifications.

Sam Basile is a project manager, Universal Technical Systems Inc., Rockford, Ill.

Kenneth R. Gitchel, vice president, also contributed to this article.

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