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.

Opportunities

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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