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The Trouble with Tramp Oil

November 01, 2012
John M. Burke
Online Only Articles


Tramp oils are any unwanted oil that enters into a metalworking fluid during the manufacturing process. These oils can enter from a variety of sources such as oil on parts from previous operations, or leaks from hydraulic, way, spindle and gear oils.

So my coolant or cutting already has oil in it, so won’t a little extra oils help? The simple answer to this question is …not likely.

Tramp oil can cause a variety of problems from increased mist and smoke, emulsion destabilization; increase in bacterial growth, dermatitis, staining of certain metals, poor tool life, inconsistent surface finish and even compliance issues when you discharge spent solution to a waste water treatment plant.

Tramp oil reactions with metalworking fluids are highly variable and can be quite complex. Let’s look at two separate fluid types and how tramp oil affects these fluids in separate ways.

Category One: Water diluted fluids, for example emulsified oils (aka soluble oil).  And let’s further assume the tramp oil is from a premium anti-wear, rust and oxidation zinc fortified hydraulic oil. The first problem is sharing the emulsifier. There is only a certain amount of emulsifier present in an emulsified oil. As the tramp hydraulic oil leaks into the fluid, the hydraulic oil and the emulsified oil slowly become one in the same. But now you have two different base stock oils, the emulsified oil is likely naphthenic base stock and the hydraulic oil is likely paraffinic base stock.  Added to that is too much oil and not enough emulsifier and a portion of the metalworking emulsified oil starts to separate. That is not just tramp oil floating on top of your coolant; it could be part of the metalworking fluid as well. If you are using refractive index to measure the coolant, excess tramp oil can easily interfere with the measurement results.

The viscosity of tramp oil and base oil in the emulsion are not likely to be the same. So the first issue is the emulsion becomes more coarse or loose. Tool life and surface finish can suffer since the loose emulsion droplets cannot get between the tool and work piece as efficiently as a fresh emulsion. As cutting tools perform less effectively this results higher tool wear and possibly poor surface finish. This can also result in more smoke and mist at the point of cut.  The zinc from the hydraulic oil can slowly disperse into the water phase of the coolant. Since zinc is a regulated metal, this can lead to compliance issues in some cases if you discharge to a sanitary sewer or state or federal waters.

Here are some basic corrective actions for emulsions, semi-synthetics and synthetic fluids in dealing with tramp oil contamination. Fix the leaks if it is from hydraulic oil gear, spindle, or way oil. Then skimmers and in some severe cases centrifuges may be required. If the oil is coming in from a previous operation, pre-washing, blow off or other forms of oil removal may be management options.

Category Two: If the metalworking oil is straight oil, that is, not intended to be mixed with water, the effects are equally problematic. Hydraulic, way, gear and spindle oils are essentially 100% soluble into straight cutting oils, therefore simple skimmers and centrifuges are not effective separation devices. If the straight oil is a low viscosity oil (such as ISO grade 4), the hydraulic oil will likely be an ISO 32, 46 or 68 grade. So the intermixing of the two oils can seriously affect machinability and productivity due to the straight cutting oil getting more viscous. Straight cutting oils may contain sulfur, chlorine and phosphorus. Intermixing hydraulic oil can affect the stability of these extreme pressure additives. If you are using a filter on your straight oil, and if the stability of the additive is compromised by tramp oil contamination, the filer may deplete these additives even further. There is no effective user-friendly method to separate tramp oils from straight cutting oils.

One type of tramp oil from incoming parts stock is referred to as slushing oil. This is a light coating of oil applied at a rolling mill to provide minimal protection during shipment and short term storage. Some low quality shushing oils can be corrosive when mixed with water mixable metalworking fluids and can lower the pH of that fluid. Also if you change from one mill to another, the shushing oil can change and present an immediate issue when there was not an issue before.

In old style bar machines, the gear oil and the straight cutting oils can be one in the same. In that case a tri-purpose oil can be used to be the cutting oil, gear oil and hydraulic oil. This is one of the few examples that tolerate a continual leak of a contaminant oil into the working straight cutting oil.

You can learn more about the effects of tramp oil and how it interacts with metalworking fluids at the next Metalworking Fluid Management Certificate Course being held in Philadelphia in February 2013.

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