Engineers and designers can’t view plastic material gears as just metallic gears cast in thermoplastic. They must focus on special issues and factors unique to plastic gears. Actually, plastic gear design requires attention to details that have no effect on steel 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 founded on the strength of a single tooth, while plastic-gear design recognizes load sharing between teeth. Quite simply, plastic teeth deflect even more under load and spread the load over more teeth. In most applications, load-sharing escalates the load-bearing capability of plastic gears. And, because of this, the allowable stress 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 other issues.
In general, the following step-by-step procedure will Gas-cooling Vacuum Pump create a good thermoplastic gear:
Determine the application’s boundary circumstances, such as heat range, 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 home retention in the specified environment to determine whether the performance amounts will be taken care of for the life span of the part.
Calculate the stress levels caused by the various loads and speeds using the physical property or home data.
Evaluate the calculated values with allowable stress and anxiety levels, then redesign if needed to provide an adequate safety factor.
Plastic gears fail for many of the same reasons metallic ones do, including wear, scoring, plastic flow, pitting, fracture, and fatigue. The reason for these failures is also essentially the same.
The 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 important parameter. Non-lubricated gears, however, may degrade before a tooth fails. Therefore, contact stress is the prime factor in the design of these gears. Plastic gears usually have a full fillet radius at the tooth root. Therefore, they aren’t as prone to stress concentrations as metal gears.
Bending-tension data for engineering thermoplastics is founded on fatigue tests run at specific pitch-range velocities. As a result, a velocity factor should be found in the pitch collection when velocity exceeds the check speed. Continuous lubrication can boost 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 factors.
For instance, a velocity aspect can be used when the pitch-series velocity exceeds the test velocity. In addition, a factor can be used to take into account changes in operating heat range, gear materials, and pressure angle. Stall torque can be another factor in the look of thermoplastic gears. Often gears are at the mercy of a stall torque that’s 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 serious that the gears melt.
There are several approaches to reducing this type of heating. The preferred way is to lessen the peak stress by increasing tooth-root area available for the required torque transmission. Another approach is to reduce stress in the teeth by increasing the apparatus diameter.
Using stiffer materials, a materials that exhibits much less hysteresis, can also expand the operational existence of plastic-type material gears. To improve a plastic’s stiffness, the crystallinity levels 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 raises stiffness by 500% to at least one 1,000%. Using fillers does have 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 would like to make use of fillers should take into account the trade-off between fatigue existence and minimal temperature buildup.
Fillers, however, perform provide another advantage in the power of plastic material gears to resist hysteresis failure. Fillers can increase heat conductivity. This helps remove warmth from the peak tension region at the base of the gear teeth and helps dissipate heat. Heat removal is the other controllable general aspect that can improve resistance to hysteresis failure.
The encompassing medium, whether air or liquid, includes a substantial effect on cooling rates in plastic material gears. If a liquid such as an essential oil bath surrounds a equipment instead of air, high temperature transfer from the apparatus to the natural oils is usually 10 instances that of heat transfer from a plastic gear to air flow. Agitating the oil or air 
TB19h3dm9tYBeNjSspa761OOFXaV.png]#also boosts heat transfer by one factor of 10. If the cooling medium-again, surroundings or oil-can be cooled by a heat exchanger or through design, heat transfer increases even more.