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Helical gears differ from spur gears in that their teeth are cut at an angle to the shaft. During operation, more than one tooth is engaged at any given time, which allows helical gears to carry heavier loads with more torque than spur gears. Helical gears also typically offer smoother and quieter operation. Large helical gears are often used in heavy trucks and agricultural equipment due to the rugged demands of these applications.

Why Deep Case Carburizing?

Parts for wear applications are usually case hardened instead of through hardened to preserve a softer, more ductile core. This allows the parts to absorb shock and tolerate the stress of the application without material loss while still providing a strong and wear-resistant surface.

The heavy wear, load-carrying applications for these large helical gears require greater case depth to maximize the strength and useful life of the parts. Helical gears are sometimes sent for finish machining to bring them into tolerance after heat treatment, so extra case depth is specified for thermal processing.

Process Considerations for Deep Case Carburizing

We know that every load of this type of part represents a big investment for our customers. Agricultural and heavy truck helical gears are typically made from higher-end steels such as 8620 and 8822 and weigh anywhere ranging from four to 26 pounds each. 

Prior to processing, proper cleaning of helical gears is critical to ensure that heat treatment yields an even case. Leftover lubricants from the machining process can mask parts of the gear and prevent carbon from diffusing consistently across the entire part surface.

Long cycle times are required to yield a deep case. In fact, the required soak time increases exponentially as the specified case depth grows. Some jobs require cycle times as long as 12 hours or even more.

Quality Control in High Gear

The last thing you want to hear from your heat treater is that they have to scrap a load of these parts due to incorrect processing.

We avoid scrap and rework by combining process controls, measurement, and testing to ensure repeatable results in every process we run, but there are a few key areas we focus on for this application.

Getting Started

First, when evaluating specifications on a given job (or when we’re asked to develop process parameters), we use predictive tools to determine how long the carburizing cycle needs to be based on the desired case depth and the part material. This model was developed by our metallurgy team at Paulo through decades of research and experimentation. 

Figuring out the proper carbon concentration in the atmosphere during processing is important to get right to yield in-spec results. We’ll analyze your specifications to determine whether your requirements call for total case depth or effective case depth. While some heat treaters may attest that there is a sort of black magic to this, we know that’s not the case. We use another predictive model that we developed based on data to ensure the right carbon concentration every time. No magic. Only science.

During Processing

During processing, the Paulo Batch System (PBS) takes your recipe parameters stored in PICS and automatically adjusts the furnace controls to create the perfect atmosphere for processing. If something goes wrong or an error is detected, our alarming system immediately notifies the operator, the parts are quarantined for further inspection, and system safeguards prevent the lot from being shipped out of our facility. This protects you from the risk of part failure or recalls due to improper heat treatment.


Finally, when it comes to testing, we use destructive testing to ensure proper case depth for your parts. Given the complexity of the geometry of these parts, verifying the case depth achieved by heat treatment requires performing testing on an actual part. To ensure proper performance in the application, samples need to be taken from both the tooth and the root of the gears to measure case depth. We work with customers who prefer to measure the part at certain points, and we can perform this destructive testing by taking samples according to your specifications. 

In addition to verifying case depth, we’ll also use microhardness testing to measure the resulting hardness of the gears.

Datagineering Ensures Repeatability and Precision

Our approach to deep case carburizing helical gears reflects Paulo’s overall datagineering approach: combining the best people, systems, and equipment to achieve the ultimate repeatability and precision in thermal processing. If you’re looking for a heat treating partner for helical gears (or any other metal parts), we can help you take your results to the next level. Paulo is CQI-9 and IATF 16949 certified (you can browse a more comprehensive list of our approvals and certifications here) and our St. Louis, Nashville, and Kansas City divisions have capacity available for these types of projects. Request a quote or connect with a Paulo expert today to get started.

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