Moulded plastic gears: precision and control

28 March 2024

Plastic gears are growing increasingly popular and becoming further used in the field of motion and power transmission: their numbers have now surpassed those of metal gears. They are widely used in the automotive industry, for medical applications and even in the space industry. Their widespread use demonstrates the efficiency and precision that they ensure, as they meet the high quality standards of these professional fields. But what is meant by the quality of a gear and its degree of precision? Let’s find out together.

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What is the degree of precision of a plastic gear?

The degree of precision of a gear can be considered as the combination of all tolerance limits, that is, all allowable deviations. The American Gear Manufacturers Association (AGMA) considers it essential to qualify the range of these tolerance limits so as to facilitate the general definition of the actual precision of these components. It uses progressive numbering from 3 to 15, where 15 represents the most precise gear and 3 the least precise.

Below is a comparative table showing the different interpretations given to this standard by different countries.

Stagnoli designs and manufactures technopolymer gears using injection moulding, taking into account all factors that may influence the degree of precision of gears at every stage of production, from designing the part to the mould.

Thanks to our technical expertise and know-how gained over more than 40 years in the business, we are now fully aware of all the factors that affect the precision of an injection moulded gear, such as:

• Thickness: thickness uniformity plays a key role in achieving a high degree of accuracy.
  
  
• Dimensions: the proportion of module to tooth band width greatly influences the degree of precision.
  
  
• Symmetry: avoids deformation of the part by increasing the degree of precision.
  
  
• Material: choosing materials that have isotropic shrinkage where possible favours the degree of precision.
  
  
• Mould: The correct design, the choice of injection and extraction systems, the choice of centring elements, and the appropriate machining precision of the moulding parts greatly influence the degree of precision of the drive element.
  
  
• Press: choosing a full electric press guarantees greater precision and repeatability in the use of the part.

The quality of plastic gears

The quality of the design and manufacture of a plastic moulded gear can be certified with appropriate instrumentation that determines its degree of precision. But how can the degree of precision be determined and what tools are required to check the fundamental gear parameters?

• Coordinate-measuring machine (CMM): Stagnoli uses this machine to scan the tooth profile with a probe, detecting any profile errors, propeller errors, movement errors and oscillation errors. This allows us to determine the degree of precision in accordance with AGMA390.03, DIN 3962, and ISO 1328, among others.
  
  
• Optical measuring machine: this machine allows us to establish concentricity and form errors for micro gears.
  
  
• Micrometres: we use micrometres to measure the chordal dimension (wildhaber) and its repeatability on all the teeth.
  
  
• Bowers: which allow us to check bore tolerances.
  
  
• Gear-to-gear test bench with master gear: this tool allows us to check the quality of the meshing.

At Stagnoli, we focus on the quality and precision of gears in every single process, because we are aware of the advantages in their application. 

By way of example, here are two scans of the profile quality obtained by the CMM. The comparison shows the result of our intervention on an existing application that showed great weaknesses following a customer's request.

Fig. 1. Propeller and profile prior to our intervention.

Fig. 2. Propeller and profile after our intervention.

We were able to optimise the profile and increase the degree of quality enormously by paying attention to all the individual steps.

Get in touch if you would like to learn more about plastic moulded gears, submit an issue regarding quality, or you simply want to entrust your project to us. We will be happy to examine your request in order to find the most efficient solution together.