Engineers and designers can’t view plastic gears as just metallic gears cast in thermoplastic. They need to focus on special issues and factors unique to plastic Chain material gears. Actually, plastic gear design requires attention to details which have no effect on metal gears, such as heat build-up from hysteresis.
The essential 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. In other words, plastic teeth deflect even more under load and spread the strain over more teeth. Generally in most applications, load-sharing escalates the load-bearing capacity of plastic material gears. And, as a result, the allowable tension for a specified number-of-cycles-to-failure increases as tooth size deceased to a pitch of about 48. Little increase is seen above a 48 pitch because of size effects and additional issues.
In general, the next step-by-step procedure will create a good thermoplastic gear:
Determine the application’s boundary conditions, such as temperatures, load, velocity, space, and environment.
Examine the short-term materials properties to determine if the initial performance levels are adequate for the application.
Review the plastic’s long-term real estate retention in the specified environment to determine if the performance levels will be taken care of for the life of the part.
Calculate the stress amounts caused by the many loads and speeds using the physical property or home data.
Evaluate the calculated values with allowable worry amounts, then redesign if had a need to provide an sufficient safety factor.
Plastic gears fail for most of the same reasons metallic ones do, including wear, scoring, plastic material 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 root of the tooth and at the contact surface. If the gear is normally lubricated, the bending tension is the most important parameter. Non-lubricated gears, however, may degrade before a tooth fails. Therefore, contact stress is the prime factor in the design of the gears. Plastic gears will often have a complete fillet radius at the tooth root. Therefore, they are not as prone to stress concentrations as metallic gears.
Bending-stress data for engineering thermoplastics is based on fatigue tests run at specific pitch-range velocities. As a result, a velocity factor should be found in the pitch series 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 instance, a velocity element is utilized when the pitch-range velocity exceeds the test velocity. Furthermore, a factor is used to take into account changes in operating temperature, gear materials, and pressure position. Stall torque is certainly another factor in the look of thermoplastic gears. Often gears are subject to a stall torque that is considerably 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 methods to reducing this type of heating. The preferred way is to reduce the peak stress by increasing tooth-root area available for the required torque transmission. Another strategy is to reduce stress in the teeth by increasing the gear diameter.
Using stiffer materials, a material that exhibits less hysteresis, can also lengthen the operational existence of plastic gears. To increase a plastic’s stiffness, the crystallinity degrees of crystalline plastics such as for example acetal and nylon can be increased by digesting techniques that raise the plastic’s stiffness by 25 to 50%.
The most effective method of improving stiffness is to apply fillers, especially glass fiber. Adding glass fibers increases stiffness by 500% to at least one 1,000%. Using fillers has a drawback, though. Unfilled plastics have fatigue endurances an order of magnitude greater than those of metals; adding fillers reduces this advantage. So engineers who wish to make use of fillers should take into account the trade-off between fatigue lifestyle and minimal high temperature buildup.
Fillers, however, perform provide another benefit in the power of plastic gears to resist hysteresis failing. Fillers can increase temperature 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 various other controllable general aspect that can improve level of resistance to hysteresis failure.
The surrounding medium, whether air or liquid, has a substantial effect on cooling rates in plastic material gears. If a fluid such as an oil bath surrounds a gear instead of air, high temperature transfer from the gear to the oils is usually 10 occasions that of the heat transfer from a plastic gear to air flow. Agitating the essential oil or air also improves heat transfer by one factor of 10. If the cooling medium-again, atmosphere or oil-is usually cooled by a warmth exchanger or through style, heat transfer increases even more.