Engineers and designers can’t view plastic gears as just metal gears cast in thermoplastic. They need to focus on special issues and factors unique to plastic material gears. In fact, plastic gear design requires focus on details which have no effect on metal gears, such as for example heat build-up from hysteresis.
The essential 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. Basically, plastic teeth deflect even more under load and spread the strain over more teeth. In most applications, load-sharing increases the load-bearing capability of plastic material 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 increase is seen above a 48 pitch due to size effects and other issues.
In general, the following step-by-step procedure will create a good thermoplastic gear:
Determine the application’s boundary conditions, such as temp, load, velocity, space, and environment.
Examine the short-term materials properties to determine if the initial 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 span of the part.
Calculate the stress amounts caused by the various loads and speeds using the physical residence data.
Compare the calculated values with allowable stress levels, then redesign if needed to provide an adequate safety factor.
Plastic gears fail for most of the same reasons metal ones do, including wear, scoring, plastic flow, pitting, fracture, and fatigue. The cause of these failures can be 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. If the gear is lubricated, the bending tension is the most crucial parameter. Non-lubricated gears, on the other hand, may wear out before a tooth fails. Therefore, contact stress may be the prime factor in the design of the gears. Plastic gears usually have a complete fillet radius at the tooth root. Hence, they are not as prone to stress concentrations as steel gears.
Bending-tension data for engineering thermoplastics is based on fatigue tests work at specific pitch-line velocities. As a result, a velocity factor should be found in the pitch range when velocity exceeds the test speed. Continuous lubrication can raise the allowable stress by a factor of at least 1.5. Much like bending stress the calculation of surface contact stress requires a number of correction factors.
For instance, a velocity aspect is utilized when the pitch-range velocity exceeds the check velocity. In addition, a factor is utilized to account for changes in operating temperatures, gear materials, and pressure angle. Stall torque is another factor in the design of thermoplastic gears. Frequently gears are subject to a stall torque that is substantially higher than the normal loading torque. If plastic material gears are operate at high speeds, they become susceptible to hysteresis heating which might get so severe that the gears melt.
There are several approaches to reducing this kind of heating. The favored way is to lessen the peak stress by increasing tooth-root region available for the required torque transmission. Another strategy is to reduce stress in the teeth by increasing the apparatus Super Power Lock diameter.
Using stiffer components, a materials that exhibits less hysteresis, can also lengthen the operational life of plastic material gears. To increase a plastic’s stiffness, the crystallinity levels of crystalline plastics such as for example acetal and nylon can be increased by digesting techniques that increase the plastic’s stiffness by 25 to 50%.
The most effective approach to improving stiffness is by using fillers, especially glass fiber. Adding glass fibers increases stiffness by 500% to 1 1,000%. Using fillers does have a drawback, though. Unfilled plastics have exhaustion endurances an order of magnitude higher than those of metals; adding fillers decreases this advantage. So engineers who want to make use of fillers should take into account the trade-off between fatigue life and minimal temperature buildup.
Fillers, however, perform provide another advantage in the ability of plastic gears to resist hysteresis failing. Fillers can increase warmth conductivity. This can help remove heat from the peak stress region at the bottom of the gear teeth and helps dissipate heat. Heat removal is the additional controllable general element that can improve resistance to hysteresis failure.
The surrounding medium, whether air or liquid, includes a substantial influence on cooling prices in plastic gears. If a liquid such as an essential oil bath surrounds a equipment instead of air, heat transfer from the gear to the oils is usually 10 situations that of heat transfer from a plastic gear to air flow. Agitating the oil or air also boosts heat transfer by a factor of 10. If the cooling medium-again, air flow or oil-is certainly cooled by a heat exchanger or through design, heat transfer increases a lot more.