Engineers and designers can’t view plastic gears as just steel gears cast in thermoplastic. They must pay attention to special issues and factors unique to plastic gears. In fact, plastic gear style 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 based on the strength of a single 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. Generally in most applications, load-sharing escalates the load-bearing capacity of plastic material gears. And, consequently, the allowable gear box for greenhouse 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 because of size effects and various other issues.
In general, the following 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 home retention in the specified environment to determine whether the performance amounts will be maintained for the life span of the part.
Calculate the stress levels caused by the various loads and speeds using the physical property data.
Evaluate the calculated values with allowable worry levels, then redesign if needed to provide an sufficient safety factor.
Plastic gears fail for many of the same reasons metallic types do, including wear, scoring, plastic material flow, pitting, fracture, and fatigue. The cause of these failures is also essentially the same.
One’s 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 certainly 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 may be the prime aspect in the design of the gears. Plastic gears usually have a full fillet radius at the tooth root. Thus, they are not as prone to stress concentrations as metal 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 used in the pitch collection when velocity exceeds the test speed. Constant lubrication can increase the allowable tension by a factor of at least 1.5. As with bending stress the calculation of surface area contact stress requires a number of correction factors.
For example, a velocity aspect is utilized when the pitch-collection velocity exceeds the test velocity. Furthermore, a factor is utilized to take into account changes in operating temperature, gear materials, and pressure position. Stall torque is usually another factor in the design of thermoplastic gears. Often gears are at the mercy of a stall torque that is considerably higher than the normal loading torque. If plastic material gears are run at high speeds, they become susceptible 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 reduce the peak stress by increasing tooth-root region available for the mandatory torque transmission. Another approach is to reduce stress in the teeth by 
increasing the apparatus diameter.
Using stiffer components, a material that exhibits much less hysteresis, can also extend the operational life of plastic-type 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 boost 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 1 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 want to use fillers should look at the trade-off between fatigue existence and minimal warmth buildup.
Fillers, however, perform provide another advantage in the power of plastic material gears to resist hysteresis failing. Fillers can increase warmth conductivity. This can help remove warmth from the peak stress region at the bottom of the gear teeth and helps dissipate heat. Heat removal may be the other controllable general element that can improve resistance to hysteresis failure.
The surrounding medium, whether air or liquid, includes a substantial effect on cooling prices in plastic material gears. If a liquid such as an essential oil bath surrounds a gear instead of air, temperature transfer from the gear to the natural oils is usually 10 situations that of the heat transfer from a plastic gear to atmosphere. Agitating the oil or air also increases heat transfer by a factor of 10. If the cooling medium-again, atmosphere or oil-is definitely cooled by a heat exchanger or through design, heat transfer increases a lot more.
