Brugger value

This value is actually only known to die-hard hydraulic engineers, the Brugger value. But what does this value say?

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 gearboxes, vane pumps, threaded couplings, bed ways 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.

Parker Hannifin’s official definition states that mixed friction always occurs in hydraulic components because the sliding speeds between friction partners are often below the limit for the reliable build-up of a hydrodynamic lubricating film. In the case of mixed friction, i.e. when the components are in direct metallic contact, the lubricity is the decisive factor. The lubricity is measured according to DIN/EN 51 347 and given as a permissible specific load in N/mm² at which no wear occurs on the sliding partners. This value is sometimes also referred to as the “Brugger value”.

For general applications, this value must be at least 30 N/mm², measured according to DIN 51 347-2. In dynamically highly loaded systems and machines with rapid load changes (frequent pressure changes between low and high operating pressures, high-speed presses, plastics machines, etc.), fluids with a value not below 50 N/mm², measured according to DIN 51 347-2, should be used.

What can happen in a hydraulic system if the Brugger value of the hydraulic oil used does not reach the recommended minimum value? Due to the insufficient lubricating film, the lubricity between the components moving on each other is no longer sufficient, which leads to damage of the components. The consequences are material abrasion, which in the worst case can damage the entire hydraulic system.