Meeting Micropitting and Materials Compatibility Challenges

Meeting Micropitting and Materials Compatibility Challenges

Industrial Gear Oils as an Integral Component of Gearbox Design By Tim Cooper, Industrial Product Manager, The Lubrizol Corporation.
From extreme-duty mining to power generation and beyond, gearbox applications throughout the industrialised world are confronting design and construction trends that have led to an increase in micropitting a pheno enon of fatigue stress on gear teeth often attributed to changes in steel quality and surface finishing methods. Additio ally, the drive towards higher power densities, reduced oil volumes and consequently raised temperatures places further stress on the gear teeth, bearings, elastomer seals and other components of modern high-performance industrial gea boxes.

The combination of these factors has created a demand for lubricants that provide improved thermal stability, increased bearing protection, better seal and internal paint compatibility along with exceptional micropitting resistance. Nowhere is this demand more evident than in the stringent Siemens MD specification for helical, bevel and planetary gear units.

The Siemens MD specification
The Siemens MD (formally known as the Flender) specification was designed to ensure trouble-free performance of Flender gearboxes in heavy-duty industrial applications. With its emphasis on materials compatibility, as well as micropitting and scuffing resistance, today´s specification requires gear oil formulators to look at high-performance industrial gear oils as an integral element of gearbox design. In addition to requiring oils to meet DIN 51517-3, Siemens MD requires an FVA 54 micropitting resistance test, dynamic foam testing and several stringent protocols for ensuring internal paint, elastomeric seal and liquid sealant compatibility. The challenge for oil formulators has been that additive chemistries designed to deliver performance in one aspect of the specification can have a detrimental effect on other performance areas. For example, many oils with acceptable seal compatibility have been shown to blister the paints used inside Flender gearboxes, while some oils benign to the paint are incompatible with the elastomerseals. Performance requirements for both paint and elastomer seals have evolved through various revisions to the Siemens MD specification. As a result, formulators are eagerly searching for an additive solution able to ensure the long-term integrity of gearbox materials as well as providing guaranteed resistance to micropitting fatigue.

Ramifications of micropitting
Micropitting is a fatigue mode that begins as tiny pits barely visible to the eye, (see Figure 1). It appears as a grey matte area often described as grey staining or flecking. Over time, this micropitting may attenuate or even polish away and cause no further distress. Alternatively, the micropitting may increase, growing deeper and leading to noise and vibration. It may also prevent smooth gear engagement or even continue on to macropitting, a much more destructive form of gear distress that can result in operational disruptions and costly repairs.

A chemistry breakthrough
With the introduction of a new industrial gear oil technology, Lubrizol has resolved the seals versus paint dilemma, while at the same time delivering excellent micropitting resistance and outstanding bearing protection. Extensive laboratory testing has demonstrated that this new and proprietary chemistry provides improved performance across a range of Group I through IV base stocks in all the following areas:

  • Micropitting resistance.
  • Elastomeric seal compatibility.
  • Dynamic foam control.
  • Bearing protection.
  • Paint durability.
  • Thermal and oxidative stability.

Siemens MD approvals are base stock-specific; accordingly, Lub¡¡rizol provides this exciting breakthrough technology not only as the additive package, but also as a finished lubricant. In addition, Lubrizol provides rebrand approval assistance for oil marketers not having access to the required base oils.

Having successfully vault¡ed over the Siemens MD perform¡ance bar, the new family of industrial gear oil technology from Lubrizol eliminates concerns over-micropitting and material incompatibilities, thereby helping reduce maintenance costs and improving uptime for industrial gearbox operators. This new industrial gear oil technology from Lubrizol ensures:

  • Extended gear and bearing life in enclosed gear drives operating under extreme load, speed, and temperature conditions.
  • Excellent resistance to oil degradation at high temperatures, resulting in extended oil life and longer drain intervals.
  • Smooth operation in both high and low temperature environments for reliable performance year-round.
  • Excellent resistance to corrosion and very good demulsibility for trouble-free operation in applications where water contamination is unavoidable.
  • Reduced filter plugging for fewer filter changes.
  • Proven compatibility with the ferrous and non-ferrous metallurgy, internal paints and both liquid and elastomeric seals used in today´s high-performance gearboxes.

Advantages of Lubrizol technology
There are many factors to be considered in obtaining a Siemens MD approval. The tests are costly, time-consuming and some can only be conducted at a small number of approved external laboratories. When faced with these challenges, oil blenders may prefer to purchase a finished lubricant that has already been tested, proven and certified by the OEM.

Recently, Lubrizol released a family of fully formulated finished lubricants, meaning that the technical work has been completed, the approvals are in place and re-branding those lubricants into an oil marketer´s product line can be initiated. The Lubrizol family of industrial gear oil technology offers the performance demanded by today´s gearbox manufacturers and their industrial users and is available for distribution worldwide.

Reprinted with permission from the November 2012 issue of TLT, the official magazine of the Society of Tribologists and Lubrication Engineers, an international not-for-profit technical society headquartered in Chicago.

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