ISO 6336 Gear Rating for Powder Metallurgy Gears: Strength, Fatigue, and What Changes at Less Than Full Density

# ISO 6336 Gear Rating for Powder Metallurgy Gears: Strength, Fatigue, and What Changes at Less Than Full Density

You have a PM gear design. The customer asks for an ISO 6336 rating. You open the standard and immediately hit a problem: every permissible stress number assumes a homogeneous wrought steel at full density. Your PM gear is at 7.0 g/cm³ — 89% of theoretical.

What now?

This guide explains how to adapt the ISO 6336 gear rating methodology for powder metallurgy gears, covering tooth-root bending strength, surface contact (pitting) capacity, and the practical validation steps that turn an analytical rating into a safe production gear. It assumes you have a working knowledge of ISO 6336 Parts 1–3 and the basics of PM gear manufacturing. For PM gear design fundamentals, start with our PM gear design guide.

The Core Problem: Density Deficit

ISO 6336 calculates permissible tooth-root bending stress and permissible contact stress from tabulated material data — σ_Flim (bending) and σ_Hlim (contact) — that represent the stress the material can endure for 3×10^6 to 10^7 load cycles at 99% survival probability.

These tabulated values are based on:

• Wrought, fully dense steel at 7.85 g/cm³
• Specific heat treatment conditions (quenched and tempered, case hardened, nitrided, etc.)
• Surface conditions and hardness measured on non-porous material

A PM gear at 7.0 g/cm³ has roughly 10–15% of its volume occupied by pores. Those pores:

1. Reduce the effective load-bearing area — less metal carries the same bending moment or contact load
2. Act as geometric stress concentrations — sharp pore edges amplify local stress by factors of 1.5–3× depending on pore shape
3. Provide fatigue crack initiation sites — bending cracks often start at interconnected surface pores along the tooth-root fillet

The combined effect is that a PM gear's σ_Flim and σ_Hlim are not simply scaled by density ratio — they degrade more than the proportional loss in cross-sectional area.

Step 1: Confirm Actual Density

The first and most important input to a PM gear rating is density at the critical location — the tooth-root fillet for bending, and the tooth flank subsurface for contact. Not the average part density measured by Archimedes on a whole gear, but the local density in the region that carries the load.

Why local density matters: the tooth tips of a PM gear compact to higher density than the root area because the powder flows more easily into wide-open features than into the tight fillet radius near the die wall. A whole-part density of 7.0 g/cm³ may correspond to 6.9 g/cm³ at the tooth-root fillet — a 0.1 g/cm³ difference that translates to 8–12% in bending fatigue strength.

Practical approach:

• Use image analysis of a sectioned tooth from a process-capability study batch
• Measure the density profile from tip to root across multiple teeth
• Use the minimum density value in the critical zone as the rating input

Step 2: Derate Permissible Stress for Density

Published research from MPIF, Höganäs, Fraunhofer IWM, and other PM organizations provides density-dependent correction factors for PM gear materials. While a full literature review is beyond the scope of this guide, the following approximate framework is widely used in industrial practice for ferrous PM gears with case hardening:

Important caveat: These factors are indicative. The exact derating depends on alloy, heat treatment, pore morphology, and — critically — whether the stress mode is bending or contact. Always verify with your own test data for a new material-process combination.

Bending vs Contact Sensitivity

Contact stress degrades more steeply with density than bending stress. The reason: pitting initiates in a very shallow subsurface layer (0.1–0.3 mm depth), where the probability of a stress-raising pore intersecting the high-shear-stress zone is high. Bending stress distributes through the full tooth section, and the averaging effect across a larger volume partially mitigates the porosity penalty.

If your PM gear program is contact-stress-limited (common in high-torque, low-speed robot or automotive applications), push for maximum achievable density — 7.2+ g/cm³ or surface densification — before accepting the derated rating numbers.

Step 3: Adjust the Life Factor (Y_NT and Z_NT)

ISO 6336 provides life factors Y_NT (bending) and Z_NT (contact) that adjust the permissible stress for finite life or for very long life beyond 10^7 cycles. These factors assume a typical wrought-steel S-N curve shape.

PM gears often exhibit:

• A steeper S-N slope at long life — the fatigue limit at 10^7 cycles for PM gears can be a lower fraction of the static strength than for wrought steel, meaning the life factor should decrement faster with increasing cycles
• Less pronounced endurance limit — some PM materials show a continued decline in fatigue strength beyond 5×10^6 cycles rather than a true fatigue limit plateau

Practical approach:

• If your rating is at 10^7 or higher cycles, use a more conservative life factor than the standard ISO 6336 default (i.e., assume a smaller Y_NT or Z_NT)
• If your program allows, generate an S-N curve from pulsator specimens of the actual PM material and process, then fit your own Y_NT or Z_NT curve rather than using the standard tabulated values
• For infinite-life designs, assume the PM fatigue strength at 5×10^7 cycles may be 5–10% below the 10^7 cycle value

Step 4: Verify Case Depth on PM Teeth

Case-hardened PM gears develop a hardened surface layer through carburizing or carbonitriding, but the case depth behaves differently from wrought steel:

• Case depth is deeper for the same time and temperature because interconnected surface porosity provides rapid carbon diffusion paths into the tooth
• Effective case depth measurement is less precise — a microhardness indent on a porous surface samples both metal and void, producing scattered readings; use metallographic etching plus microhardness traverses, not single-point readings
• Case-to-core transition is more gradual — the carbon gradient is smeared compared with wrought steel, which can be an advantage (less risk of case-core separation) but makes the standard 550 HV cutoff harder to apply cleanly

Specify case depth as a range (e.g., 0.15–0.35 mm to 550 HV on FN-0205 with case hardening) and verify on a sectioned tooth from a production-representative batch.

Step 5: Validate With Testing

No paper rating replaces test data for a PM gear program. The minimum validation package depends on the application criticality:

Standard industrial (power tools, appliances, non-safety automotive):

• Density measurement on gear teeth (min 30-piece sample)
• Single-tooth bending pulsator test at two stress levels (10 specimens per level)
• Hardness profile on sectioned tooth

Safety-critical or high-reliability (automotive transmission, robot joints, aerospace):

• All of the above, plus:
• Back-to-back gear rig testing at representative torque and speed (min 3 gear sets)
• Pitting endurance test on contact-stressed surface
• Statistical analysis: Weibull or similar for survival probability, not just mean fatigue strength

Full qualification (new material-process combination):

• Full S-N curve from pulsator (4–5 stress levels, 10+ specimens per level)
• Component-level fatigue test with periodic inspection for crack initiation
• Comparison against machined-from-bar baseline of the same alloy and heat treatment

For documentation guidance, see our PM PPAP and sample approval guide.

Summary: The Five-Step PM Gear Rating Methodology

1. Measure the local density at the tooth-root fillet and flank — not just the bulk part average
2. Derate σ_Flim and σ_Hlim using published PM gear data or in-house test correlations for your specific alloy and density
3. Adjust the life factor for the steeper S-N slope typical of PM materials at long life
4. Verify case depth on actual PM tooth sections, accounting for porosity effects on hardness measurement
5. Test — component-level pulsator or rig testing is not optional; it is the evidence that your analytical rating is conservative enough

PM gears are not inherently weaker than machined gears — they are differently strong. A well-designed PM gear at 7.1+ g/cm³ with proper heat treatment can match the performance of a machined gear in many applications, while costing significantly less at volume. The rating methodology just needs to account for the physics honestly.

Related Resources

• PM Gear Design Guide
• PM Material Selection Guide
• How to Read MPIF Material Grades
• PM Sample Approval and PPAP Guide
• Powder Metallurgy Gears — Product Page
• DFM Guide for PM Parts