Considerations When Using Plastic Gears

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

The basic difference in design philosophy between metal and plastic gears is that metal gear design is founded on the strength of an individual tooth, while plastic-gear design recognizes load sharing between teeth. In other words, plastic teeth deflect more under load and spread the load over more teeth. In most applications, load-sharing increases the load-bearing capability of plastic gears. And, as a result, the allowable tension for a specified number-of-cycles-to-failure boosts as tooth size deceased to a pitch of about 48. Little Super Power Lock increase sometimes appears above a 48 pitch due to size effects and additional issues.

In general, the next 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 initial performance levels are adequate for the application.
Review the plastic’s long-term house retention in the specified environment to determine whether the performance amounts will be managed for the life span of the part.
Calculate the stress amounts caused by the many loads and speeds using the physical residence data.
Compare the calculated values with allowable pressure amounts, then redesign if had a need to provide an sufficient safety factor.
Plastic gears fail for most of the same reasons metallic types do, including wear, scoring, plastic flow, pitting, fracture, and fatigue. The reason for these failures can be essentially the same.

The teeth of a loaded rotating gear are at the mercy of stresses at the main of the tooth and at the contact surface. If the gear is definitely lubricated, the bending stress is the most crucial parameter. Non-lubricated gears, on the other hand, may wear out before a tooth fails. Therefore, contact stress is the prime aspect in the design of these gears. Plastic gears usually have a full fillet radius at the tooth root. Therefore, they are not as prone to stress concentrations as steel gears.

Bending-stress data for engineering thermoplastics is founded on fatigue tests work at specific pitch-range velocities. Consequently, a velocity factor ought to be found in the pitch collection when velocity exceeds the check speed. Constant lubrication can boost the allowable stress by a factor of at least 1.5. Much like bending tension the calculation of surface area contact stress requires a number of correction factors.

For example, a velocity element is used when the pitch-line velocity exceeds the check velocity. Furthermore, a factor is utilized to account for changes in operating temperatures, gear components, and pressure angle. Stall torque can be another factor in the look of thermoplastic gears. Often gears are subject to a stall torque that is considerably higher than the standard loading torque. If plastic material gears are run at high speeds, they become vulnerable to hysteresis heating which may get so severe that the gears melt.

There are several methods to reducing this type of heating. The favored way is to lessen the peak stress by increasing tooth-root area available for the mandatory torque transmission. Another strategy is to lessen stress in one’s teeth by increasing the apparatus diameter.

Using stiffer components, a materials that exhibits less hysteresis, can also expand the operational life of plastic material gears. To increase a plastic’s stiffness, the crystallinity degrees of crystalline plastics such as for example acetal and nylon could be increased by digesting 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 boosts stiffness by 500% to at least one 1,000%. Using fillers has a drawback, though. Unfilled plastics have fatigue endurances an purchase of magnitude higher than those of metals; adding fillers decreases this advantage. So engineers who wish to make use of fillers should look at the trade-off between fatigue life and minimal high temperature buildup.

Fillers, however, do provide another advantage in the power of plastic gears to resist hysteresis failing. Fillers can increase warmth conductivity. This can help remove heat from the peak tension region at the bottom of the gear tooth and helps dissipate heat. Heat removal may be the additional controllable general aspect that can improve resistance to hysteresis failure.

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