Type of gear material:
The type of gear material plays a crucial role in determining the allowable tooth stress for gear-tooth design. Different materials have different mechanical properties, such as strength, hardness, and fatigue resistance, which directly affect the stress that the gear teeth can withstand. Materials like steel, cast iron, bronze, and plastic can be used for gear manufacturing, and each material has its own allowable tooth stress value. For example, steel gears are generally able to withstand higher stresses compared to plastic gears. Therefore, the type of gear material is an important parameter in determining the allowable tooth stress.

Pitch line velocity:
The pitch line velocity is the linear velocity at which the gear teeth come into contact with each other during operation. It is determined by the rotational speed of the gear and the gear's pitch diameter. The pitch line velocity affects the amount of dynamic load and impact forces experienced by the gear teeth. Higher pitch line velocities result in higher dynamic loads and impact forces, which can increase the stress on the gear teeth. Therefore, the pitch line velocity is another important parameter that influences the allowable tooth stress.

Beam strength of gear teeth:
The beam strength of gear teeth refers to the ability of the gear teeth to resist bending and deformation under load. It depends on factors such as tooth geometry, thickness, and the modulus of elasticity of the gear material. The beam strength of the gear teeth determines their ability to transmit torque and withstand the applied loads without failure. Higher beam strength allows for higher stress levels, while lower beam strength requires lower stress levels to prevent tooth failure. Therefore, the beam strength of the gear teeth is also a significant parameter in determining the allowable tooth stress.

Conclusion:
In conclusion, the allowable tooth stress for gear-tooth design depends on the type of gear material, pitch line velocity, and beam strength of the gear teeth. These parameters directly influence the ability of the gear teeth to withstand the applied loads and prevent tooth failure. Therefore, considering all three parameters is crucial for designing gears that can operate reliably and safely under the specified conditions.