FZG Scuffing Test ISO 14635-1: What It Tells You About Gear Oil

ISO 14635-1:2023 (FZG A/8.3/90) measures the load stage at which an oil lets hardened test gears scuff, a relative scuffing ranking, not field life. As of 2026-10-14, ISO 14635-1:2023 is the current edition and ASTM D5182-26 the active US method. The result does not rate micropitting, bearing wear or high-EP oils that pass stage 12.

• DIN ISO 14635-1:2025-04 adopts ISO 14635-1:2023 and replaces DIN ISO 14635-1:2006-05 (DIN, 2025).

• The A/8.3/90 test runs at 8.3 m/s from a 90 °C initial oil temperature, 15 minutes per load stage, up to stage 12 (Michalczewski et al., 2013).

• Stage 12 corresponds to a maximum loading torque of 535 N·m and a maximum Hertzian pressure of 1.8 GPa (Michalczewski et al., 2013).

• ASTM D5182-26 is the active edition of the US method, last updated on 2026-05-13 (ASTM, 2026).

This answer covers industrial gear, hydraulic and transmission oils tested by FZG A/8.3/90. It does not cover high-EP oils that need the ISO 14635-2 step load test, gear greases, which have their own FZG method in ISO 14635-3, or hypoid axle oils. Four-ball tests are a separate method and are not covered here.

What does the FZG A/8.3/90 test in ISO 14635-1 measure, and how?

The FZG A/8.3/90 test in ISO 14635-1 measures the load stage at which a lubricating oil can no longer prevent scuffing on a standard pair of hardened steel test gears (ASTM D5182-26, clause 1.1). ISO states the purpose in its abstract: "This document specifies a test method based on a FZG four-square test machine to determine the relative load-carrying capacity of lubricating oils defined by the gear-surface damage known as scuffing." (ISO 14635-1:2023, abstract). The same abstract explains that "High surface temperatures due to high surface pressures and sliding velocities can initiate the breakdown of the lubricant films."

The method name encodes its conditions: A-type test gears, a circumferential speed of 8.3 m/s and an initial oil temperature of 90 °C (Michalczewski et al., 2013). The gears run dip-lubricated with no cooling, so the oil temperature rises freely during each 15-minute stage (Michalczewski et al., 2013). Load rises stepwise from stage 1 to a maximum of stage 12, where the loading torque reaches 535 N·m and the maximum Hertzian pressure 1.8 GPa (Michalczewski et al., 2013).

From load stage 4, the pinion flanks are inspected after each stage (OELCHECK, accessed 2026). The test stops when the failure criterion is met: the total damaged area on the pinion reaches at least the area of one pinion tooth, about 200 mm2 (Michalczewski et al., 2013). If no stage meets that criterion, the run ends at stage 12 (OELCHECK, accessed 2026).

How do you read FZG A/8.3/90 scuffing test results?

You read an FZG A/8.3/90 result by its failure load stage: the first load stage at which the damaged pinion area meets the failure criterion, reported as a number from 1 to 12, or as ">12" when no stage failed. The failure load stage is "the main measure of the resistance of the test gears to scuffing" (Michalczewski et al., 2013). The useful question is which specification minimum the result clears. Ask for the result as a failure load stage with the standard and edition named, so the band below is clear.

The table below sorts FZG A/8.3/90 results into four bands and shows what each band means against common oil specifications and what to do next.

The key row for gear oil buyers is the third: a failure load stage of 12 meets the DIN 51517-3 CLP minimum of at least stage 12 (OELCHECK, accessed 2026), but it does not show how much margin the oil has above that load.

What does an FZG failure load stage of 12 or more mean on a data sheet?

An FZG failure load stage of 12 or more on a data sheet means the oil carried the A/8.3/90 test to its last load stage, which meets the DIN 51517-3 CLP minimum of at least stage 12 (OELCHECK, accessed 2026).

Oils with strong EP additives can reach that ceiling, so the result stops ranking them. Michalczewski et al. (2013) write that the A/8.3/90 method "makes it impossible to differentiate between gear oils having very good extreme-pressure (EP) properties, from the point of view of the resistance to gear scuffing". ASTM states the same limit for its version of the method: "High EP type oils, for example, those oils meeting the requirements of API GL-4 and GL-5, generally exceed the capacity of the test rig and, therefore, cannot be differentiated with this test method." (ASTM D5182-26, clause 1.1).

"Failure load stage 12" means damage reached the criterion in stage 12. "Greater than 12" or ">12" means stage 12 was completed without reaching it (OELCHECK, accessed 2026). Some data sheets show values above 12, such as ">14" for FZG A/8.3/90 to DIN ISO 14635-1 (FUCHS, accessed 2026), from runs past the standard's 12 stages; read these as ">12" under the standard unless the laboratory states the extended procedure, and ask which convention the sheet uses before comparing. When two high-EP oils both show 12 or ">12", ISO 14635-2:2023 is the FZG step load test designed for "relative scuffing load-carrying capacity of high EP oils" (ISO 14635-2:2023, title).

How does ASTM D5182 relate to ISO 14635-1?

ASTM D5182 is the US standard for the same FZG A/8.3/90 scuffing test, and ISO 14635-1:2023 states that its method "is technically equivalent to ASTM D 5182-19 and CEC L-07-A-95" (ISO 14635-1:2023, abstract). The ISO edition was published in July 2023 as edition 2 and replaced ISO 14635-1:2000 (ISO, 2023).

ASTM has revised D5182 since that equivalence note. The ASTM store lists D5182-26 as the active edition, last updated on 2026-05-13, with D5182-25A, D5182-25 and D5182-19 as earlier versions (ASTM, 2026). The 2026 title is "Standard Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG A/8.3/90 Method)", while the scope still calls it the FZG Visual Method (ASTM D5182-26, clause 1.1). ISO has not restated equivalence with D5182-26 on its page as of 2026-10-14, so treat the two as the same test family and report which standard and edition the laboratory used.

ASTM D5182 also names scoring as a failure mode: "Scoring, a form of abrasive wear, is also included as a failure criteria in this test method." (ASTM D5182-26, clause 1.1). Specifications often cite older editions. ANSI/AGMA 9005-F16 lists ISO 14635-1:2000 with ASTM D5182-97(2014) as its normative reference for scuffing (AGMA, 2026). In Germany, DIN ISO 14635-1:2025-04 adopts the 2023 ISO text (DIN, 2025).

How well does the FZG scuffing test predict gearbox scuffing in the field?

The FZG scuffing test predicts field scuffing only indirectly: it ranks oils under one fixed set of gear, speed and temperature conditions, and it does not give a field life or a pass load for a different gearbox. ISO calls the result the "relative load-carrying capacity" of the oil (ISO 14635-1:2023, abstract), and ASTM describes its use as screening oils for spur and helical gear units (ASTM D5182-26, clause 5.1).

ASTM names the transfer limit directly: "the performance observed may not directly relate to scuffing performance observed with spiral bevel on hypoid gearing" (ASTM D5182-26, clause 5.2). Gear size matters too. Amendola et al. (2020) note that allowable flash temperature limits were established with FZG rigs, but these small gears "heat up quite differently than gears of larger dimensions that are ultimately employed in service".

The oil is also one of several causes of field scuffing. Amendola et al. (2020) list five typical root causes: gear geometry that ignores the scuffing threshold, a lubricant not in line with the original specification, uneven load distribution along the flank, excessive surface roughness and excessive oil inlet temperature. The FZG stage is most useful as an input to a scuffing risk calculation; AGMA 925-A03 uses the FZG oil load stage as an input parameter (Amendola et al., 2020). This suggests a field engineer should read a high stage as a margin on one factor, not as proof the gearbox will not scuff.

Why does a high FZG stage not prove micropitting or bearing protection?

A high FZG stage does not prove micropitting or bearing protection because micropitting and rolling bearing wear are different damage modes with their own tests, and the A/8.3/90 test inspects the test pinion for scuffing. Scuffing is an adhesive failure that can occur instantly, while micropitting is a fatigue phenomenon (Amendola et al., 2020). The same authors add: "High FZG-rated lubricants provide scuffing resistance, but over time fatigue of the surface asperities may result in micropitting."

Micropitting has its own gear test. FVA 54/7, published as DIN 3990-16, measures a lubricant's capacity to resist micropitting of gear teeth, and it has been an issue in wind turbine gearsets in particular (STLE, 2024). Rolling bearings have their own test as well: the FE8 rolling bearing test, DIN 51819-3 for oils, checks wear protection of roller bearings in heavily loaded gearbox conditions (STLE, 2024).

The additives that improve scuffing results can work against bearings. Bock and Heinemann (2013) report that older gear oils with highly active phosphorus-sulfur compounds generated FE8 roller wear of 200 mg to 300 mg, while by 2013 industrial gear oils were formulated with milder compounds to meet roller wear specifications of less than 30 mg. ANSI/AGMA 9005-F16 added the FE8 test to its antiscuff oil table (AGMA, 2026), so an antiscuff claim against F16 rests on both tests, not on FZG alone.

How should two gear oils' FZG results be compared?

Two gear oils' FZG results should be compared only when both come from the same method, edition and reporting convention, and only below the stage 12 ceiling of the A/8.3/90 test. Above that ceiling, two results of ">12" are a tie, not a ranking (Michalczewski et al., 2013).

Check these points before you compare:

1. Method and edition. ISO 14635-1:2023 and ASTM D5182-19 are technically equivalent (ISO 14635-1:2023, abstract); a report against ASTM D5182-26 or ISO 14635-1:2000 needs the edition stated.

2. Reporting convention. Confirm that both sheets give a failure load stage under ISO 14635-1 or ASTM D5182, and treat values above 12 from extended runs as ">12" unless both used the same extended procedure.

3. Test report or typical value. A data sheet value is often a typical value; ask for the report for the batch or grade you buy.

4. Viscosity grade. Compare the same ISO viscosity grade where possible, because a sheet may quote the result for one grade of a product line.

5. Ceiling. If both oils reach 12 or ">12", compare them in ISO 14635-2:2023, which uses a higher speed and a higher temperature than Part 1 (OELCHECK, accessed 2026).

Treat a one-stage difference below 12, such as 10 against 11, as a screening difference: check each result against your specification minimum and judge the oils on the full test set, including oxidation, demulsibility, FE8 and micropitting results.

Common mistakes with FZG scuffing test results

The most common mistake with FZG A/8.3/90 scuffing test results is reading a failure load stage of 12 or ">12" as a ranking between high-EP oils, when the test ceiling makes those results a tie (Michalczewski et al., 2013). Other mistakes recur in specifications and data sheet reviews:

• Treating FZG as a micropitting or bearing test. The A/8.3/90 result is a scuffing result; micropitting needs FVA 54/7 (DIN 3990-16) and bearing wear needs FE8 (STLE, 2024).

• Using FZG A/8.3/90 to rank API GL-4 or GL-5 axle oils, which generally exceed the capacity of the rig (ASTM D5182-26, clause 1.1).

• Mixing editions without saying so. AGMA 9005-F16 cites ISO 14635-1:2000 (AGMA, 2026), while ISO 14635-1:2023 is current (ISO, 2023).

• Calling every flank mark scuffing. ASTM D5182 counts scoring, a form of abrasive wear, as a failure criterion (ASTM D5182-26, clause 1.1), while marks only within 1 mm of the tooth addendum and light scratches are neglected when the damaged area is calculated (Michalczewski et al., 2013).

• Reading the stage as a field load limit. The test uses small gears at one speed and temperature, and small test gears heat differently from service gears (Amendola et al., 2020).

• Copying a CLP minimum into a hydraulic oil specification. DIN 51524-2 HLP asks for stage 10, not 12 (OELCHECK, accessed 2026).

What to use alongside the FZG scuffing test

Use the FZG A/8.3/90 scuffing test alongside a micropitting test, a bearing wear test and, for high-EP oils, the ISO 14635-2 step load test, because each covers a damage mode or load range that the A/8.3/90 result does not. For gear oils, the usual set looks like this:

• ISO 14635-2:2023, the FZG step load test A10/16,6R/120, for high-EP oils used with highly stressed cylindrical gearing; it is not suitable for hypoid bevel gear oils (ISO 14635-2:2023, abstract).

• FVA 54/7 micropitting test, DIN 3990-16, when the gearbox has a micropitting history or runs in a wind turbine (STLE, 2024).

• FE8 rolling bearing test, DIN 51819-3, for gearboxes with heavily loaded rolling bearings; ANSI/AGMA 9005-F16 includes it for antiscuff oils (AGMA, 2026).

• A scuffing risk calculation such as AGMA 925-A03, which takes the FZG oil load stage as an input along with geometry, load and temperature (Amendola et al., 2020).

Four-ball tests measure a sliding contact between steel balls rather than a gear mesh (STLE, 2024), so they answer a different question. Lubinpla applies this kind of reading to a customer's own TDS, SDS, CoA and site history and returns one cited document for review.

Key takeaways

• ISO 14635-1:2023 (FZG A/8.3/90) ranks oils by the load stage at which test gears scuff; DIN ISO 14635-1:2025-04 is the German adoption.

• A failure load stage of 12 meets the DIN 51517-3 CLP minimum; HLP hydraulic oils need stage 10.

• Results of 12 and ">12" do not rank high-EP oils; use ISO 14635-2 for that.

• ASTM D5182 is the same test family; state which standard and edition the report used.

• FZG does not test micropitting or bearing wear; ask for FVA 54/7 and FE8 results.

Lubinpla applies this kind of reasoning to your own documents and site history and drafts the document for your review before it goes out.

Sources checked 2026-10-14.

Sources

1. ISO. ISO 14635-1:2023 Gears: FZG test procedures: Part 1: FZG test method A/8,3/90 for relative scuffing load-carrying capacity of oils. Edition 2, 2023. https://committee.iso.org/standard/86086.html

2. DIN. DIN ISO 14635-1:2025-04 Gears: FZG test procedures: Part 1 (ISO 14635-1:2023), record with change summary. 2025. https://www.din.de/en/wdc-beuth:din21:387063861

3. ASTM International. ASTM D5182-26 Standard Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG A/8.3/90 Method). 2026. https://store.astm.org/d5182-26.html

4. ISO. ISO 14635-2:2023 Gears: FZG test procedures: Part 2: FZG step load test A10/16, 6R/120 for relative scuffing load-carrying capacity of high EP oils. Edition 2, 2023. https://committee.iso.org/standard/86337.html

5. Michalczewski, R., Kalbarczyk, M., Michalak, M., Piekoszewski, W., Szczerek, M., Tuszynski, W. and Wulczynski, J. New Scuffing Test Methods for the Determination of the Scuffing Resistance of Coated Gears. In Tribology: Fundamentals and Advancements, InTech, 2013. doi:10.5772/54569. https://pdfs.semanticscholar.org/da26/31dafc4f9c2a3b7f0eb1080c70b3a297ea7a.pdf

6. OELCHECK. FZG test, oils (test method description). Accessed 2026-10-14. https://en.oelcheck.com/analyses/test-methods/fzg-test/

7. DIN Media. DIN 51517-3:2018-09 Lubricants: Lubricating oils: Part 3: Lubricating oils CLP, Minimum requirements. 2018, current. https://www.dinmedia.de/en/standard/din-51517-3/290902488

8. DIN. DIN 51524-2:2017-06 Pressure fluids: Hydraulic oils: Part 2: HLP hydraulic oils, Minimum requirements. 2017, current. https://www.din.de/en/wdc-beuth:din21:272262662

9. American Gear Manufacturers Association (AGMA). ANSI/AGMA 9005-F16 Industrial Gear Lubrication, store page with foreword and normative references. 2016, reaffirmed May 2026. https://members.agma.org/MyAGMA/MyAGMA/Store/Item_Detail.aspx?iProductCode=9005_F16&Category=STANDARDS

10. Holdmeyer, D. Industrial and automotive gear oil tests and specifications. STLE Tribology and Lubrication Technology (TLT), July 2024. https://www.stle.org/files/TLTArchives/2024/07_July/Lubrication_Fundamentals.aspx

11. Amendola, J., Amendola III, J. and Errichello, R. Calculated Scuffing Risk: Correlating AGMA 925-A03, AGMA 6011-J14 and Original MAAG Gear Predictions. Gear Technology, March/April 2020. https://ik.imagekit.io/agmamedia/gt/issues/0320x/scuffing-risk.pdf

12. Bock, W. and Heinemann, H. (Fuchs Europe Schmierstoffe). Lubricants for Wind Turbines, Part 2: Gear Oils, Demands and Characteristics. Lube-Tech No. 86, LUBE Magazine No. 113, February 2013. https://www.lube-media.ambrit.com/wp-content/uploads/2017/11/Lube-Tech086-LubricantsforWindTurbinesGearOils-DemandsandCharacteristics-Part2.pdf

13. FUCHS. RENOLIN CLP industrial gear oils, technical data sheet (copy hosted by a distributor). Undated, accessed 2026-10-14. https://catalog.lukoil-masla.ru/u/product_document_file/3368/tds_renolin_clp_en.pdf

Written by the Lubinpla team from the sources listed and checked against them before publication.