Mill Shaft Exposed to Extreme Environment

Motivation

Untimely failure of the three-roller mill shaft in sugar cane industry is cause of reduced efficiency. This also has a cascading effect as sugar factories run as a seasonal industry.

What is the issue during sugar cane crushing?

Three-roller mill is used to crush sugar cane for juice extraction and is inevitable equipment in sugar manufacturing process globally. Failure due to wear on the top surface of roller shaft in sugar cane mill is a major cause of concern. Initiation of cracks and wear on the surface of the shaft lead to the crack propagation. Consequently, the shaft fails due low cycle fatigue. Untimely failure of the shaft can drastically reduce the productivity and hence the profitability due to extended downtime and unusually high maintenance costs. This failure also affects the reliability of the system. In most cases service life of the roller shaft is reduced by half due this type of failures.

During operation of the mill, the interface is often contaminated with sugar cane juice and dried waste called bagasse. Therefore, it very important to study the most fundamental aspects of wear and friction characteristics of sugar mill roller shaft materials in different sliding media which are present during service conditions. This translational research project our understanding and the science of the initiation of cracks at multi-scales (nanometer and micrometer length scales).

The service conditions of the roller shaft were simulated in the laboratory. We have investigated that coefficient of friction (COF) was reduced but the surface damage was severe when the contaminations were present at the sliding interface of shaft materials. The complex tribochemistry between the sugar cane juice, bagasse, lubricant, and sliding materials caused the initiation of cracks on the surface. This understanding further helped to propose the potential remedy to reduce the surface crack initiation.

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Lubrication: Nano Particles as a Lubrication Additive

Motivation

        The benefit of the addition of nano-sized lubricant additives as a friction modifier is to reduce the use of phosphorus-containing additives. Phosphorus-containing additives are not environmentally friendly. Further conventional friction-reducing compounds such as MoS₂ is not soluble in the lubricant oil or organic media. Dispersion is one way by which these compounds can be used in the form of oil-soluble compounds for example Mo-DDP and Mo-DTC. These compounds are relatively expensive and utilize P2S5 and CS₅ which are hazardous [4]. Potentially the utilization of nano TiO₂ particles as friction modifiers can minimize or eliminate the usage of environmentally unfriendly phosphorous and hazardous P₂S₅ and CS₅.

Why Friction Modifiers?

        Low friction can easily be achieved by the liquid lubrication. Automobiles and most of the engines are facilitated with liquid lubrication to minimize the wear and increase service life and fuel efficiency. The addition of solid lubricant particles to oil to further reduce the friction particularly in the extreme event of boundary lubrication condition is successfully being practiced. The development of new lower friction additives always depends on the existing technologies and materials synthesis processes. Novel materials synthesis methods of nano powders hold potential for additives development for friction reduction and its stabilization.

        In the case of journal bearings, it is recommended that when the length-to-diameter (l/d ratio) is lower than 0.4 a low friction coating should be used as a precaution. When the l/d ratio in these bearing is less than 1 (typically 0.5 or less) the fluid push towards the side can be significant and can cause metal-to-metal contact, especially under severe loading conditions [1]. In this situation, the presence of a thin film of friction modifiers/solid lubricant at the sliding interface can protect the engineering surfaces and prolong its service life.

Nano TiO₂ as Lubricant Additive
        Along with several other engineering applications of TiO₂ such as photovoltaic, electro-chromic, fuel cells, self-cleaning surfaces, hydrogen sensors, photocatalyst for detoxification of pollutants, and hydrogen generation from water, TiO₂ holds advantages with its proven tribological properties. Nano-sized TiO₂ is a potential candidate material as an additive in lubrication.
        There is not a complete understanding of the mechanism of how particle size of TiO₂ provides useful effects. Also, there is a limited understanding of the distinction between the tribological behavior of the rutile and anatase phases of TiO₂. Therefore, this commercially important research inquiry encompasses the detailed and systematic study on friction and wear behavior of nano anatase TiO₂ as an additive in lubricant oil. The outcome of this investigation provided a fundamental and comparative study of the friction and wear performance of P25 (a mixture of rutile and anatase phases) and TiO₂ (100% anatase phase). The fundamental understanding of the mechanism is very essential for its commercial application as a friction modifier. We investigated that the settling behavior (sedimentation), proposed #oleophobicity and mechanical properties (#hardness) of TiO₂ (anatase) provided the protective layer on the sliding surface which further stabilized the friction and modified the wear mechanism of sliding surfaces. This behavior is one of the important factors considered during the selection of lubricant additives (especially friction modifier)

​What is Phosphorus Pentasulfide?
        The chemical formula of phosphorus pentasulfide is P₂S₅. It is a grey-yellow powder molecular weight of 222.27 gram (g)/mol, the melting point of 286­­­^oCelsius (C), and a boiling point of 514^oCelsius. Some of the waste generated during the manufacturing of P₂S₅ are classified as hazardous waste [2].
        In the USA, around 65% of the total production of P₂S₅ feedstock is used for lubricating oil and grease additives (mainly zinc dialkyldithiophosphates ZDDP) and 33% for organophosphorous insecticides (such as acephate, chlorpyrifos, and terbuphos), and rest is used for other uses for example in ore flotation applications [2]. P₂S₅ poses less toxicity but it hydrolyzes instantly with water, moisture in the air and produces hydrogen sulfide (H₂S) gas. The toxicity is primarily governed by the fact of H₂S generation [3], [4]. H₂S is a dangerous chemical that causes irritation to the eyes and respiratory tract. A high concentration of it can stop breathing and cause death [5]. It is very toxic to aquatic organisms [6].

References:

1.     B. Willis, Fighting friction with solid-film coatings, Machine Design 74 (12) (2002) 66.

2.     Phosphorus Pentasulfide Listing Background Document for the Inorganic Chemical Listing Determination. August 2000 ‎9‎/‎12‎/‎2014 1/23/2015]; Available from http://www.epa.gov/wastes/hazard/wastetypes/wasteid/inorchem/pr2000.htm

3.     Immediately Dangerous to Life or Health Concentrations (IDLH): Phosphorus pentasulfide. March 19, 2014 1/23/2015]; Available from: http://www.cdc.gov/niosh/idlh/1314803.html

4.     Association, M.C., Properties and Essential Information for Safe Handling and Use of Phosphorus Pentasulfide. 1958.

5.     Occupational Health Guideline for Phosphorus Pentasulfide, O.S.a.H.A. (OSHA), Editor. September 1978, Occupational Safety and Health Administration (OSHA). p. 5.

6.     Material Safety Data Sheet: Phosphorus Pentasulfide (99%). 26/JUL/2007 11/FEB/2006 [cited 2015 1/23/2015]; 6]. Available from: www.sigma-aldrich.com.