Brugger test - load capacity of lubricants

Year of publication: 2003

 

The minimum requirements for hydraulic and gear oils are firmly defined in DIN standards. However, they do not ensure the trouble-free operation of certain equipment in all cases. This applies in particular to slow-running, shock-loaded gears, vane pumps, threaded couplings, slideways and steel/bronze pairings in plain bearings. Often the load capacity of the oil used is not sufficient. In order to clearly analyze the load capacity of the lubricant, the practical Brugger test was developed. A Brugger lubricant tester is installed in the OELCHECK laboratory.

The following minimum requirements have so far been defined for hydraulic and gear oils in industrial use. They relate to wear protection and galling behavior.

Table of contents

  1. The Brugger Test – developed by practitioners
  2. Test methods
  3. Significance and reliability
  4. A practical example – AUDI AG monitors large presses with the Brugger test
  5. The power of the Brugger test – an inventory
    1. Pros
    2. Cons

HLP hydraulic oil

  • Standard: DIN 51524/2
  • Previous test methods: FZG test, scuffing load step at least 10 and the vane pump test with a ring wear of maximum 120mg and a vane wear of maximum 30 mg.

 

Gear oil CLP

  • Standard: DIN 51517/3
  • Previous test methods: FZG test, scuffing load step at least 12 and a wear value of maximum 0.3 mg/kWh.

The Brugger Test – developed by practitioners

There was no satisfactory test method for assessing the load-bearing capacity of the lubricant film under mixed or boundary friction. Kurt Brugger and Dr. Eng. Claus-Peter Neumann (Maschinenfabrik Müller Weingarten AG) developed the „Brugger Test“ from years of practical research work A wide range of lubricants was investigated. It soon became obvious that the load-bearing capacity values of lubricants vary extremely. Even though they were all from a particular DIN group, their load-bearing capacity ranged from 25 to 88 N/mm². This is still explainable, since there are no concretely defined requirements.  However, if the equipment manufacturer recommends a lubricant with only a low load capacity as a gear lubricant for highly loaded industrial gears, things can become critical in practice. In their research, Brugger and Neumann also observed the behavior of machines in long-term use and, above all, investigated problem cases. In this way, they were able torelate the test results to the behavior of the lubricants in practical use.

Test methods

At the heart of the tester are two rotationally symmetrical test specimens whose axes are offset by 90° relative to each other. The lower test specimen is a ring with an outer diameter of 25 mm. It is mounted on a shaft driven by a three-phase motor via a cogged belt drive. It rotates at 960 min-1 ±5 % at idle and drops to a maximum of 860 min-1 under load.

The upper test specimen is a cylinder with a diameter of 18 mm. It is firmly clamped in a rotating holder and is pressed by this against the rotating test ring by a weight. The sliding speed is 1.2 m/s.

Both the shaft of the test ring and the pivot points of the levers are mounted on anti-friction bearings. Now the test ring is doused with the lubricant to be tested. The test cylinder is pressed against the test ring with 400 N. The drive of the test ring starts under load. The test duration is 30 seconds.

Depending on the lubricant property, the rotating test ring generates a wear surface of varying size on the stationary test cylinder. The wear surface has the shape of an ellipse. The main axes of the ellipse are measured very precisely using a microscope and a measuring device located inside it. From this, the projection area of the wear surface is calculated (A=a*b*π). The quotient of the pressing force F and the projected wear surface A is given as the load capacity of the lubricant according to Brugger B (N/mm2).

To obtain reproducible results, the initial conditions must always be the same. Since several tests are performed with one test ring, the test ring and test cylinder must each be washed with boiling limit gasoline. The surfaces must not show any traces of residues from the previous test visible to the naked eye. Make sure that the test ring never falls below the diameter of 24.5 mm. Testing in the mixed friction area with the Brugger lubricant tester is specified in DIN 51347-1.

Significance and reliability

The test procedure according to Brugger provides a decision basis for a correct lubricant selection with regard to the lubricant load capacity. A large number of measurements have shown that the reproducibility of the results is good. Scatter is ± 10% of the measured values.

A practical example – AUDI AG monitors large presses with the Brugger test

AUDI AG produces body parts for the Audi A3 and A4 at the pressing plant in Ingolstadt. 7 presses from the manufacturers Müller Weingarten, Schuler and Erfurt are in operation here. The largest press produces up to 10,000 parts in 24 hours. It is filled with approx. 10,000 l of hydraulic oil. Hydraulic oils of types HLP and HLP-D46 are used. Viscosities of ISO VG classes 150 or 220 are mainly used for gears. Gears in large presses are unconventional machine elements due to their low rotational frequency, compact design and high, shock-like load. The bearing points are subjected to increased thermal stress. In addition to the regular lubricant analyses, the equipment manufacturers require the Brugger test. The presses at the Audi plant are regularly subjected to this test once a year. As a result, the oil is safely tested for its load-bearing capacity. At Audi, the Brugger test paid off simply because of the timely detection of wear on a bearing bushing. Major damage was prevented. The production group manager for machine technology was able to optimally fit the repair into the ongoing production process. An unplanned shutdown would have cost several hundred thousand euros. Thus, however, the press continued to operate around the clock, as usual. The repair was carried out during the routine overhaul the following weekend.

The power of the Brugger test – an inventory

Pros

It supports statements about additive changes in used oil analysis, as it shows the load capacity and adhesion of a lubricant under mixed friction, which occurs preferentially with:

  • All stops and restarts of machines or engines
  • Reversing motion sequences such as up and down, back and forth
  • Vibrations and stop loads
  • Low sliding speeds
     

This is shown in real life by:

  • Slip-stick and chattering on hydraulic cylinders
  • Hydraulic pumps, especially vane pumps, with weakened delivery rate
  • Stuttering movements of tool carriers over the slideways
  • Wear of threaded couplings and adjusting spindles
  • Pitting of teeth in slow-running gears
  • Abrasion due to insufficiently wetted steel/bronze pairings, e.g. in plain bearings

 

Cons

As a single test for a lubricant assessment instead of a complete analysis, the test run is not suitable because

  • no single parameter can represent the diversity and complexity of lubricant requirements in practice alone
  • the short test time cannot provide equally meaningful results for all wear mechanisms
  • the transferability of the test results to materials other than the steel materials used for testing must be checked

Hardly any statements can be made on the basis of the test result:

  • regarding the expected running time of a gear, bearing or other machine element
  • to impurities, mixing or viscosity changes in the lubricant
  • on oil service life or oxidation stability