Engineers and designers can’t view plastic gears as just metal gears cast in thermoplastic. They need to pay attention to special issues and considerations unique to plastic gears. Actually, plastic gear style requires focus on 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 based 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 pass on the load over more teeth. In most applications, load-sharing increases the load-bearing capability of plastic gears. And, consequently, the allowable tension for a specified number-of-cycles-to-failure increases as tooth size deceased to a pitch of about 48. Little increase sometimes appears above a 48 pitch because of size effects and additional issues.
In general, the next step-by-step procedure will generate an excellent thermoplastic gear:
Determine the application’s boundary conditions, such as heat, 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 if 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 house data.
Evaluate the calculated values with allowable strain amounts, then redesign if had a need to greenhouse reducer provide an sufficient safety factor.
Plastic material gears fail for most of the same reasons metal ones 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 subject to stresses at the root of the tooth and at the contact surface area. If the gear is definitely lubricated, the bending tension is the most important parameter. Non-lubricated gears, on the other hand, may degrade before a tooth fails. Therefore, contact stress is the prime aspect in the design of the gears. Plastic gears will often have a complete fillet radius at the tooth root. Thus, they aren’t as prone to stress concentrations as metal gears.
Bending-tension data for engineering thermoplastics is based on fatigue tests work at specific pitch-series velocities. Therefore, a velocity factor ought to be used in the pitch range when velocity exceeds the check speed. Continuous lubrication can increase the allowable tension 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 aspect is used when the pitch-range velocity exceeds the check velocity. Furthermore, a factor is utilized to take into account changes in operating temp, gear components, and pressure position. Stall torque is another factor in the look of thermoplastic gears. Often gears are at the mercy of a stall torque that is considerably higher than the standard loading torque. If plastic material gears are run 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 preferred way is to lessen the peak tension 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 less hysteresis, can also prolong the operational life of plastic-type material gears. To increase a plastic’s stiffness, the crystallinity degrees of crystalline plastics such as acetal and nylon can be increased by processing techniques that boost 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 boosts stiffness by 500% to 1 1,000%. Using fillers does have a drawback, though. Unfilled plastics have fatigue endurances an order of magnitude higher than those of metals; adding fillers decreases this advantage. So engineers who wish to use fillers should take into account the trade-off between fatigue existence and minimal temperature buildup.
Fillers, however, perform provide another advantage in the ability of plastic gears to resist hysteresis failure. Fillers can increase heat conductivity. This can help remove high temperature from the peak stress region at the base of the gear tooth and helps dissipate high temperature. Heat removal is the other controllable general factor that can improve level of 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 oil bath surrounds a gear instead of air, high temperature transfer from the gear to the oils is usually 10 times that of the heat transfer from a plastic material gear to surroundings. Agitating the oil or air also boosts heat transfer by one factor of 10. If the cooling medium-again, air flow or oil-is certainly cooled by a warmth exchanger or through style, heat transfer increases even more.