September 24, 2003

By: Mark Harris
Website: http://www.bearing-n-bearings.com

Space age steel bearings to accept 40% higher loads, run significantly faster & last up to 5 times longer

Cronidur 30, a new steel that has been used successfully in the engines of the space shuttle, exceeds the fatigue life and speeds available from other bearing steels, with the added benefits of superior corrosion and wear resistance. Bearings manufactured from Cronidur 30 are unique to FAG/Barden. They can withstand up to 40% higher dynamic loads, achieve higher constant operating speeds with grease lubrication, and are up to 100 times more corrosion resistant than bearings manufactured from AISI 440C, the standard corrosion resistant bearing steel.

CRONIDUR 30 QUALITIES ADD UP TO IMPROVED PRODUCT RELIABILITY

Cronidur 30 has been developed by FAG in co-operation with Bochum University and VSG Energie. The steel has proved its reliability and toughness both in the extreme high temperature conditions of the main engines of the space shuttle and in the low temperature conditions encountered on the flaps of commercial airlines. Its use in bearings is a natural consequence of the market requirement for a material with performance that exceeds that of standard bearing steel SAE52100 (100Cr6). Notably, by accepting higher loads and higher temperatures, and also in terms of higher speed capability, enhanced corrosion resistance and longer life.

Cronidur 30 is the ideal bearing material to improve the performance and reduce the life cycle costs of today's machine tools. The new steel also provides substantial operational benefits in bearings used in vacuum pumps in the electronics industry, aerospace, canning applications, Formula 1 racing cars and medical applications.

HIGHER CONTACT PRESSURE EQUALS 40% HIGHER LOAD RATING

Proof of the improved fatigue life and higher load capacity of Cronidur 30 has been supplied in comparative tests with standard 100Cr6 and M50 (a steel of considerable high-temperature strength, which is used in engine bearings) under EHD (elasto-hydro dynamic) conditions and with a contact pressure of 2,800 MPa. The calculated fatigue life was exceeded by all the materials. However, the Cronidur 30 bearings had still not failed after 80 times the calculated life. In view of this Cronidur 30 can be assumed to be practically fail-safe at contact pressures below 2,800 MPa.

In a given system this facility for higher permissible contact pressure means a significantly reduced susceptibility to unforeseen operating conditions. It also permits designers to assume higher loads for new designs.

HIGHER SPEED FOR ENHANCED SPINDLE PERFORMANCE

Complementing the superior life and load characteristics of Cronidur 30 spindle bearings is their capability of achieving much higher constant operating speeds. In tests, installed in the spindle of a machining centre, a grease-lubricated (70mm ID) Cronidur 30 bearing ran at a constant speed of 20,000 rpm while exhibiting good even temperature behaviour. This contrasts with the 15,000 rpm achieved in the same test by an SAE52100 (100Cr6) bearing with identical dimensions.

The importance of the higher speed is that it provides machine tool builders, rebuilders and machine users with a relatively simple and low cost method of achieving greater performance from their machine spindles. Added to this, the even temperature operation at higher speeds of the Cronidur 30 bearings provides opportunities for substantial cost savings on spindle systems, simply by changing from air-oil to grease lubrication.

CORROSION RESISTANT SUPERIOR BY A FACTOR OF 100

In addition to its superior rolling bearing properties, Cronidur 30 features an excellent resistance to corrosion. A measure of a materials corrosion resistance is its passive current density value. In this respect Cronidur 30 is better, by about a factor of 100, than AISI 440C, the standard corrosion resistant bearing steel. With this property, and the material's good operating characteristics under starved lubrication conditions, Cronidur 30 can in many cases be used with other lubricants: Liquid Hydrogen, Liquid Oxygen and many others.

Cronidur 30 has been used particularly successfully in pumps with highly inflammable liquids. Tests under conditions that were additionally aggravated for the Cronidur 30 bearings were terminated, after twice the life reached by 440C or 100Cr6 TDC-plated (thin dense chromium plated) bearings, when other pump components failed.

MICRO-FINE STRUCTURE IMPROVES WEAR & REDUCES NOISE & VIBRATION

Cronidur 30 is a martensitic through-hardened steel that can also be induction case hardened. Its extremely fine grain structure minimises the risks of failure under mechanical loading and results in improved noise and vibration behaviour In addition, the heat treatment process used with Cronidur 30 gives the material excellent hardness values, even at higher temperatures. Because of this, bearings manufactured from Cronidur 30 can generally be used at higher temperatures than other bearings steels.

The large carbides and carbide networks (80 - 100m), which are a feature of the structure of AISI 440C, do not exist in Cronidur 30. Instead there is a homogeneous structure of finely dispersed carbonitrides, usually less than 10m in size. As a result, Cronidur 30 does not have the problem of large carbides concentrating at grain boundaries and providing potential "weak spots" which are susceptible to mechanical stressing and corrosion. This is an exceptionally important factor because damage with spindle bearings almost always originates at the surface, whereas classical rolling bearing fatigue usually originates below the surface.

The contrasting micro- fine structure of Cronidur 30 has a positive effect not only on component fatigue strength but also in the micro-range of the Hertzian contact area. The corollary of this is that Cronidur 30 is considerably less susceptible to plastic deformations resulting from indentations. In practice, dirt particles that penetrate into the bearings can leave indentations in the raceways, but these indentations do not propagate premature fatigue damage. Moreover, any bearing damage that does develop spreads more slowly than is the case with spindle bearings manufactured from standard bearing steel.

One of the major benefits of a material structure that minimises damage in this way is that, with few wear particles, there is hardly any contamination of the lubricant. Furthermore, as the few wear particles do not corrode there is no catalytic effect - both with grease and oil lubrication - so the quality of the lubricant remains at a constant level over a longer period.

EXTENDED SERVICE LIFE UNDER MIXED FRICTION CONDITIONS

Another major advantage when specifying Cronidur 30 spindle bearings is their behaviour under conditions of mixed friction. Mixed friction is the result of bearings being subjected to repeated running and stopping (the operation of tool changes in machining centres is a very good example of this). It occurs if set transition speeds are exceeded and there is not a sufficient amount of lubrication in the contact areas. In tests carried out under a defined condition of mixed friction (po = 2500Mpa) spindle bearings manufactured from Cronidur 30 have achieved a tenfold service life, compared to bearings made of the standard material 100Cr6. Added to this, the wear behaviour of the bearing is considerably improved with Cronidur 30. This fact becomes particularly apparent in the case of hybrid bearings, using Cronidur 30 rings and ceramic balls, where the wear rate is considerably below the former usual values.

*CRONIDUR 30 (*Cronidur 30 is the registered trade mark of VSG Essen). Cronidur 30, a new material employed in the manufacture of spindle bearings, was developed by FAG in co-operation with Bochum University and VSG Energie- und Schmiedetechnik Essen. The development of Cronidur 30 achieved recognition in the form of the Steel Innovation Award of the German Stahlinforrnationszentrum (steel information centre).

About The Author:

Mark Harris is a successful author and regular contributor to http://www.bearing-n-bearings.com.  Keep things running smoothly with ball bearings, roller bearings, ball thrust bearings and tapered bearings.