The choice of materials used in your final drive motor is extremely important. The engineers behind hydraulic motors don't randomly choose what type of steel or brass to use. So here is a quick introduction to one of the characteristics engineers consider: material hardness.

Here are a few other Shop Talk Blog posts you might find helpful:

• Real-Life Bearing Failure
• Four Horrifying Final Drive Failures
• Why Metal Surface Finish Can Be Important for Final Drives

Why Metal Hardness is Critical 

Metal hardness is crucial for several reasons. First, it determines a material's resistance to deformation, which helps to ensure its structural integrity under varying loads and pressures. Hardness also influences a metal's wear resistance, which is important in components subject to abrasion or friction. Hardness extends a component’s lifespan and reduces its maintenance costs. Harder metals also retain their shape and dimensions better during manufacturing and use. Hardness also correlates with material strength, contributing to a system's overall performance and reliability. 

Testing for Metal Hardness

Several hardness tests exist, with the most common for metals discussed below. The Rockwell Hardness and Brinell tests are most familiar to the author, but are certainly not the only methods used.

Brinell Hardness

In the Brinell hardness test, a known load is applied to the surface through a hardened steel ball with a known diameter. The diameter of the indentation left by the ball is measured, and the BHN (Brinell Hardness Number) is then the load divided by the surface area of the indent. The higher the value, the harder the material.

Rockwell Hardness

The Rockwell hardness test measures a material's resistance to indentation by pressing an indenter (either a diamond cone or a steel ball) into the surface with two specific loads applied sequentially. The depth difference between the initial penetration and the final penetration depth, after removing the higher load, determines the Rockwell hardness value. This value indicates how much the material deformed under pressure, with higher values signifying a harder material.

The Rockwell hardness also has scales for different materials and loads, with the B and C scales the most commonly used ones for steel.

Vickers Hardness

The Vicker’s hardness test works much like the Brinell test with one significant difference: It uses a square-based diamond pyramid with an included point angle of 136°. The Vicker’s hardness number is then the known load in kg divided by the surface area of the impression made by the pyramid. High numbers indicate harder materials, as with the Brinell and Rockwell hardness tests.

Comparing Surface Hardness

The table below compares the surface hardness of iron, stainless steel, and carbon steel. The * is there to remind us that these are just average values that will vary depending on the types of heat treatments performed on the metals.

Achieving Surface Hardness

Metals' surface hardness can be achieved through various processes that can be tailored to a component’s needs. One common method is heat treatment, where metals are heated to high temperatures and then rapidly cooled, altering their microstructure to increase hardness.

Induction hardening is a special type of heat treatment that is often used with gears and bearings. Current is induced in the part to achieve hardness to a certain depth, as shown in the video below.

Another technique is case hardening, in which the metal's surface is enriched with carbon or nitrogen through processes like carburizing or nitriding. This forms a hardened outer layer while preserving the material's overall properties, like toughness and strength. This method is really effective for enhancing surface hardness in applications with critical wear resistance, such as the gears and bearings in your final drive motors.

Surface hardness can be achieved through mechanical processes like shot peening or cold working. Shot peening bombards the metal surface with high-velocity particles that inducing compressive stresses on the surface that make it hard and resistant to fatigue failure. Shot peening works much like abrasive cleaning if the sand particles are hard steel pellets. 

On the other hand, cold working involves deforming the metal at close to room temperature (unlike heat treatment, which occurs above the metal’s recrystallization temperature), causing dislocations and grain refinement that increase hardness.

Conclusion

The hardness of metals in your final drive varies depending on their purpose: gears and bearings require a different hardness level than the hubs and shafts. For example, you want a drive shaft that has a hard, wear-resistant surface but a tough core to handle any loads that try to bend it. If the core is too brittle and rigid, then it will likely break under specific loads and won't be able to handle long-term stresses. Gear teeth must be hard, wear-resistant, and extremely strong while highly resistant to fatigue failure. On the other hand, the exterior hub can handle things like gouges without affecting the drive's performance; the hubs need to handle extremely high internal pressure.

At Texas Final Drive, we ensure that any parts going into your final drive or forming its hub are of the correct type of material for the jobs they have to do. We use high-quality parts that we know will have the correct strength, hardness, dimensions, and tolerances to support the performance of your final drive.