Considerations When Using Plastic Gears

Engineers and designers can’t view plastic material gears as just metal gears cast in thermoplastic. They must pay attention to special issues and considerations unique to plastic material gears. Actually, plastic gear style requires attention to details which have no effect on metallic gears, such as for example heat build-up from hysteresis.

The basic difference in design philosophy between metal and plastic gears is that metal gear design is based on the strength of an individual tooth, while plastic-gear design recognizes load sharing between teeth. Quite simply, plastic teeth deflect even more under load and pass on the load over more teeth. Generally in most applications, load-sharing escalates the load-bearing capacity of plastic material gears. And, because of this, the allowable stress for a specified number-of-cycles-to-failure increases as tooth size deceased to a pitch around 48. Little increase is seen above a 48 pitch due to size effects and additional issues.

In general, the following step-by-step procedure will create an excellent thermoplastic gear:

Determine the application’s boundary conditions, such as temp, load, velocity, space, and environment.
Examine the short-term material properties to determine if the original performance levels are sufficient for the application.
Review the plastic’s long-term house retention in the specified environment to determine if the performance levels will be preserved for the life of the part.
Calculate the stress amounts caused by the various loads and speeds using the physical property data.
Compare the calculated values with allowable strain levels, then redesign if had a need to provide an sufficient safety factor.
Plastic gears fail for many of the same reasons metal types do, including wear, scoring, plastic flow, pitting, fracture, and fatigue. The reason for these failures is also essentially the same.

One’s teeth of a loaded rotating gear are subject to stresses at the root of the tooth and at the contact surface area. If the gear is certainly lubricated, the bending tension is the most important parameter. Non-lubricated gears, on the other hand, may wear out before a tooth fails. Therefore, contact stress is the prime factor in the design of the gears. Plastic gears usually have a full fillet radius at the tooth root. Therefore, they aren’t as susceptible to stress concentrations as metal gears.

Bending-stress data for engineering thermoplastics is based on fatigue tests work at specific pitch-collection velocities. Therefore, a velocity factor ought to be found in the pitch line when velocity exceeds the check speed. Continuous lubrication can increase the allowable stress by a factor of at least 1.5. As with bending stress the calculation of surface contact stress takes a number of correction elements.

For example, a velocity element can be used when the pitch-collection velocity exceeds the check velocity. In addition, a factor can be used to account for changes in operating temp, gear materials, and pressure angle. Stall torque is another factor in the design of thermoplastic gears. Often gears are at the mercy of a stall torque that is substantially higher than the standard loading torque. If plastic material gears are operate at high speeds, they become vulnerable to hysteresis heating which may get so serious that the gears melt.

There are several methods to reducing this kind of heating. The preferred way is to lessen the peak tension by increasing tooth-root region available for the required torque transmission. Another strategy is to lessen stress in the teeth by increasing the apparatus diameter.

Using stiffer components, a material that exhibits less hysteresis, can also lengthen the operational existence of plastic gears. To increase a plastic’s stiffness, the crystallinity levels of crystalline plastics such as for example acetal and nylon can be increased by processing techniques that increase the plastic’s stiffness by 25 to 50%.

The most effective method of improving stiffness is by using fillers, especially glass fiber. Adding glass fibers increases stiffness by 500% to at least one 1,000%. Using fillers does have a drawback, though. Unfilled plastics have exhaustion endurances an purchase of magnitude higher than those of metals; adding fillers reduces this benefit. So engineers who wish to use fillers should look at the trade-off between fatigue lifestyle and minimal warmth buildup.

Fillers, however, do provide another Worm Gearbox advantage in the power of plastic gears to resist hysteresis failing. Fillers can increase heat conductivity. This helps remove heat from the peak stress region at the base of the gear teeth and helps dissipate temperature. Heat removal is the additional controllable general factor that can improve level of resistance to hysteresis failure.

The encompassing medium, whether air or liquid, has a substantial effect on cooling prices in plastic gears. If a liquid such as an oil bath surrounds a gear instead of air, temperature transfer from the apparatus to the natural oils is usually 10 times that of the heat transfer from a plastic gear to surroundings. Agitating the oil or air also enhances heat transfer by one factor of 10. If the cooling medium-again, air flow or oil-is cooled by a high temperature exchanger or through style, heat transfer increases a lot more.


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