Engineers and designers can’t view plastic gears as just metallic gears cast in thermoplastic. They need to pay attention to special issues and factors unique to plastic gears. Actually, plastic gear design requires focus on details that have no effect on metallic gears, such as heat build-up from hysteresis.

The basic 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. Put simply, plastic teeth deflect even more under load and pass on the strain over more teeth. Generally in most applications, load-sharing increases the load-bearing capability of plastic material gears. And, as a result, the allowable stress for a specified number-of-cycles-to-failure raises as tooth size deceased to a pitch around 48. Little increase sometimes appears above a 48 pitch due to size effects and various other issues.

In general, the following step-by-step procedure will generate 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 sufficient for the application.
Review the plastic’s long-term real estate retention in the specified environment to determine whether the performance levels 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 residence data.
Compare the calculated greenhouse gearbox values with allowable pressure amounts, then redesign if needed to provide an sufficient safety factor.
Plastic gears fail for most of the same reasons metal types do, including wear, scoring, plastic material flow, pitting, fracture, and fatigue. The reason for these failures is also essentially the same.

One’s teeth of a loaded rotating gear are subject to stresses at the main of the tooth and at the contact surface. If the gear is usually lubricated, the bending stress is the most important parameter. Non-lubricated gears, however, may wear out before a tooth fails. Therefore, contact stress is the prime factor in the design of these gears. Plastic gears will often have a complete fillet radius at the tooth root. Thus, they are not as susceptible to stress concentrations as metal gears.

Bending-stress data for engineering thermoplastics is founded on fatigue tests run at specific pitch-line velocities. As a result, a velocity factor ought to be found in the pitch series when velocity exceeds the check speed. Constant lubrication can raise the allowable stress by one factor of at least 1.5. Much like bending stress the calculation of surface area contact stress takes a number of correction elements.

For example, a velocity aspect is utilized when the pitch-collection velocity exceeds the test velocity. Furthermore, a factor is used to account for changes in operating heat range, gear materials, and pressure angle. Stall torque is normally another factor in the look of thermoplastic gears. Frequently gears are at the mercy of a stall torque that is substantially higher than the standard loading torque. If plastic gears are operate at high speeds, they become vulnerable to hysteresis heating which might get so severe that the gears melt.

There are several approaches to reducing this type of heating. The favored way is to lessen the peak stress by increasing tooth-root area available for the mandatory torque transmission. Another approach is to reduce stress in one’s teeth by increasing the apparatus diameter.

Using stiffer components, a material that exhibits less hysteresis, can also lengthen the operational existence of plastic-type gears. To increase a plastic’s stiffness, the crystallinity degrees of crystalline plastics such as acetal and nylon could be increased by processing techniques that boost the plastic’s stiffness by 25 to 50%.

The most effective approach to 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 greater than those of metals; adding fillers reduces this benefit. So engineers who want to make use of fillers should take into account the trade-off between fatigue lifestyle and minimal heat buildup.

Fillers, however, do provide another advantage in the ability of plastic material gears to resist hysteresis failure. Fillers can increase high temperature conductivity. This helps remove warmth from the peak stress region at the bottom of the gear teeth and helps dissipate heat. Heat removal is the additional controllable general factor 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 fluid such as an essential oil bath surrounds a gear instead of air, high temperature transfer from the gear to the natural oils is usually 10 occasions 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, surroundings or oil-is normally cooled by a high temperature exchanger or through design, heat transfer increases even more.