Contamination Part 2
UNDERSTANDING CONTAMINATION IN METALWORKING FLUIDS
PART II - CAUSES AND EFFECTS OF TRAMP OIL
by Joe Eppert
As outlined in Part I, Introduction and Classification of Contaminants, tramp oil is typically regarded as a primary contaminant present in MWF systems. In this installment, we will discuss some of the published information regarding the causes of tramp oil in MWF systems and the effects this addition has on MWF efficiency.
As defined in Metalworking Fluids (3), tramp oil is the oil content of a particular MWF sample that did not originate from the product concentrate. Similarly, tramp oil can be referred to as extraneous oil, that amount which is external to the original formulation. The most commonly accepted sources of tramp oil are due to the lubrication requirements of the machine tool system and any leaks this system may allow.
In one instance, it was stated that 90% of MWF users reported contamination problems due to tramp oil (2). Additionally, it is typically accepted that tramp oil contamination is one of the major determinants of MWF useful life (3, 4). With this motivation, it is clear that we must attempt to effectively combat the addition of tramp oil, or at least facilitate efficient removal from the MWF system.
Tramp oils originate from the various types of substances used to fulfill the lubrication requirements of the machine tool system. As a MWF is splashed over the metal cutting area and collected for transport back to the sump, it comes in contact with various components of the machine tool system. This includes the hydraulic system and/or slide way system used to orient the workpiece with the cutting tool. Typically, hydraulic oils account for the greatest percentage of tramp oil contamination in MWF systems (1) and the addition of these substances will take place with new or old machine tool systems (1). Other sources of tramp oil include gearboxes and oily workparts from previous operations.
Regardless of how it is introduced to the system, tramp oil generally acts in a characteristic manner once mixed with the MWF. If we assume we are dealing with a MWF that contains an amount of unutilized emulsifier in its formulation, the tramp oil will likely interact with this emulsifier content to form loosely emulsified tramp oil. This is commonly termed "chemically emulsified tramp oil." Additionally, any agitation of the MWF due to pumping or recirculation can cause what is known as "mechanically emulsified tramp oil." Basically, this terminology pays particular attention to how the tramp oil is emulsified in the MWF. In the event that emulsification does not occur, the tramp oil will tend to separate from the water phase of the MWF and is considered free tramp oil. Free tramp oil can be readily removed from the system by skimming.
We now know the primary causes of tramp oil contamination, machine tool leakages (mainly hydraulic oils). We can begin to discuss some of the various effects that tramp oils have on the efficiency of the MWF. These effects can be broadly categorized into chemical effects, interacting effects, machining performance effects, and health and safety effects.
Chemical Effects of Tramp Oil Contamination:
The basic chemical effect of an introduction of tramp oil is the depletion of various MWF additives (1, 6). Any additive that is oil soluble can be stripped from the MWF, leaving the formulation in a deteriorated state compared to the original formulation. Potential oil soluble components include various emulsifiers used to keep the oil/water mix stable, and biocides used to combat microorganism growth. Additionally, tramp oil can cause a degradation of short-chain fatty acids found in the formulation (6).
The effects of chemical depletion are somewhat clear. In example, if the emulsifier content of the fluid is decreased due to tramp oil, we would expect the resultant fluid to become less stable, and separation of the oil and water phases would occur. Tramp oils can cause an alteration of MWF emulsion particle size distribution, which will also lead to a degradation of stability (5).
Interacting Effects of Tramp Oil Contamination:
The primary interacting effect of tramp oil contamination is the support of microorganism growth. If we assume that a layer of free tramp oil has been allowed to accrue on top of the MWF, oxygen diffusion from the surrounding environment to the MWF will be hindered. This may cause the MWF system to experience anaerobic conditions and allow anaerobic bacteria to flourish (1, 3). Anaerobic bacteria have their own effects on MWFs, including a characteristic rotten egg odor (“Monday Morning Stink”).
Microorganisms require a wide array of nutrient sources to flourish at maximum capacity. If the MWF system does not provide one or more of the essential substrates, microorganism growth will be limited by the absence of the missing component(s). However, with the addition of tramp oil, a new set of nutrients may be provided for the microorganisms, including those that the organisms may have been lacking. Sulfurized materials found in tramp oils have the potential to act as a substrate for microorganism growth (1).
As mentioned previously, tramp oils may cause a chemical depletion of biocides in the MWF formulation. If not corrected for, this depletion will make the fluid less able to effectively combat microbial growth. Results from both industrial and laboratory experiments indicate that the efficiency of particular biocides is affected by tramp oil contamination (1).
Machining Performance Effects of Tramp Oil Contamination:
If we assume that an optimal MWF has been formulated for a particular operation, any alterations in the MWF will result in machining performance less than optimal. With the introduction of tramp oil to this ideal MWF, the oil content of the fluid will be increased. Since oil is not as efficient a coolant when compared to water, the overall cooling ability of the MWF will be decreased (1, 4, 7, 8). This has an effect on all thermal related performance parameters, such as tool life, surface finish, and dimensional accuracy (7, 8).
The chemical depletion caused by tramp oils also acts to decrease the machining efficiency of the MWF (as discussed previously).
Health and Safety Effects of Tramp Oil Contamination:
Introducing tramp oil to a clean MWF results in degradation of overall MWF cleanliness, leading to the formation of residues on workparts and machine components (3, 4, 7). Any particulate found in the fluid may be suspended by, or coated with, the tramp oil, causing a decline in the filterability of the fluid (1, 3, 4, 7).
Health problems associated with tramp oil contamination include the formation of smoke at tramp oil levels greater than 0.5 to 1% by volume (8), the formation of mist with an increased concentration of oily content (1, 4, 7), and various forms of skin irritation (1).
As discussed by Turchin and Byers (9), tramp oil has a severe effect on the amount of mist formation for a grinding operation. Specific conclusions from their study include the observation that tramp oil contamination increases misting for all types of water soluble MWFs and that the quantity of tramp oil in the MWF has a greater effect on misting than does a variation in MWF type (9).
Tramp oil contamination -
is a prime determinant of MWF useful life.
is primarily due to machine tool hydraulic system leakage.
may remove functional components of the MWF, leaving the fluid in a deteriorated state.
promotes microbial growth.
decreases the cooling ability (and therefore machining performance) of the MWF.
leads to various health and safety issues, most notably the formation of oil containing mist.
Abanto, M., J. Byers, and H. Noble, 1994, “The Effect of Tramp Oil on Biocide Performance in Standard Metalworking Fluids,” Lubrication Engineering, Vol. 50, No. 9, pp. 732-737.
Archibald, L.C., and C. Bowes, “Who Knows the Real Costs of Poor Fluid Management?,” PERA, UK.
Byers, J.P. ed., 1994, Metalworking Fluids, J.P.B., ed., Marcel Dekker, Inc, New York.
Gedlinske, B., 1997, “Coolant Care,” Cutting Tool Engineering, Vol. 49, No. 6.
Nachtman, E.S., and S. Kalpakjian, 1985, Lubricants and Lubrication in Metalworking Fluids, Marcel Dekker, Inc, New York.
Organization Resources Counselors, Inc., Metal Removal Fluids – A Guide to their Management and Control.
Sluhan, C.A., 1986, “Considerations in the Selection of Coolants Used in Flexible Machining Cells,” SME Technical Paper, MS86-124.
Threadgill, J., 1993, “Animal, Vegetable, or Mineral,” Cutting Tool Engineering, Vol. 45, No. 6.
Turchin, H., and J.P. Byers, 2000, “Effect of Oil Contamination on Metalworking Fluid Mist”, Lubrication Engineering, Vol. 56, No. 7.