Q: What does FZG mean and why do gear oils have a rating?
FZG stands for “Forschungsstelle für Zahnräder und Getriebebau”, Technische Universität München (Gear Research Centre, Technical University, Munich), Boltzmannstraße 15, D-85748 Garching, Germany.
There are several FZG tests and these vary to establish different things. We will explore the two most common tests and what they mean.
The FZG tests were designed to accurately determine the types of gear failures that were influenced by scuffing, low speed wear, micropitting and pitting1. While there are load other tests for gear oils (such as Timken OK test) these do not accurately identify the actual failure stages that gears experience.
FZG A/8.3/90
One of the most commonly used FZG test is the FZG A/8.3/90 according to DIN ISO 14635-1. This is mainly used for evaluating the scuffing properties of industrial gear oils2. What do the numbers in the test mean?
The “A” represents an A-type gear with Pinion face width = 20mm, center distance = 91.5mm, number of teeth (pinion) = 16, number of teeth (gear) = 24. These are used in the test and are loaded stepwise in 12 load stages between Hertzian stress of pC= 150 to 1800N/mm2.
The “8.3” represents the pitch line velocity of 8.3m/s in which the gears are operated for 15 minutes at each load stage.
The “90” indicates the starting temperature of the oil (90°C) in each load stage under conditions of dip lubrication without cooling.
After each load, the gear flanks are inspected for scuffing marks. However, the fail load stage is determined when the faces of all pinion teeth show a summed total width of damaged areas which is equal or exceeds one tooth width. In the gravimetric test, the gears are dismounted and weighed to determine their weight loss.
FZG A10/16.6R/90
The FZG A10/16.6R/90 on the other hand is used for automotive gear oils (GL4). It is the standard FZG gear rig test but the speed, load, load application and sense of rotation have been slightly altered.
The “A” represents an A-type gear however, these now have a reduced pinion face width to 10mm (from 20mm above).
The “16.6R” represents the increased speed of the pitch line velocity of 16.6m/s in which the gears are operated for 15 minutes at each load stage in a reversed sense of rotation.
The “90” indicates the starting temperature of the oil (90°C) in each load stage under conditions of dip lubrication without cooling.
FZG S-A10/16.6R/90
However, the FZG S-A10/16.6R/90 is the shock level test done for the GL5 oils. In this test the gears are directly loaded in the expected load stage and a PASS or FAIL is issued.
References:
- ISO 14635-1:2000 Gears- FZG test procedures- Part 1: FZG test method A/8,3/90 for relative scuffing load carrying capacity of oils.
- Test methods for Gear lubricants. Bernd-Robert Hoehn, Peter Oster, Thomas Tobie, Klaus Michaelis. ISSN 0350-350X GOMABN 47, 2,129-152 Stručni rad/Professional paper UDK 620.22.05 : 621.892.094 : 620.1.05