Beyond WD-40: Lubes for the Home Shop

If your shop is anything like mine, you’ve got a large selection of colorful cans claiming to contain the best and absolutely only lubricant you’ll ever need. I’ve been sucked in by the marketing more times than I care to admit, hoping that the next product will really set itself apart from the others and magically unstick all the stuck stuff in my mechanical life. It never happens, though, and in the end I generally find myself reaching for the familiar blue and yellow can of WD-40 for just about every job.

But that’s the easy way out, and it’s not always – or often – the best choice. With that in mind, I wanted to explore what exactly lubricants are and how they work, to enable more informed decisions on which lube to choose.

Keep it Separated

Asperities. Source: USS Bearings
Asperities. Source: USS Bearings

So why do we even need lubricants in the first place? Why can’t two pieces of metal ground and polished to a mirror finish just be rubbed over each other? In a word: asperities. Asperities are the microscopic peaks and troughs that cover even the most finely finished surfaces, and when they drag over each other, they snag and catch. The force needed to overcome the friction this creates can break off the tiny peaks, resulting in both wear of the metal surfaces and heat as the kinetic energy is transformed into thermal energy.

Lubricants are just substances that keep surfaces separated. In the case of our finely polished metals, a lubricant may be a fluid that fills the voids between the asperities, reducing snagging. Friction is reduced, the heat that’s created is conducted away from the surfaces, and any asperities that do touch and break off are flushed away by the lubricant.

Of course not every lubricant is a fluid, and not every fluid lubricant is a liquid. Dry lubricants abound, including graphite, Teflon, graphene, and molybdenum disulfide. For the home shop, though, liquid and semi-liquid lubricants are probably the most convenient, so we’ll concentrate on them.

Thick and thin

Most – but not all – lubricants common in the home shop are based on some form of mineral oil, which is just a highly refined petroleum distillate. Various substances, including viscosity modifiers, detergents, and corrosion inhibitors are added to the base oil to give the resulting lubricant different properties.

One way to classify liquid lubricants and select the right one for the job at hand is to look at viscosity. The thinner the lubricant, the easier it can flow and wick between surfaces, while thicker lubes can typically stand up to higher pressures and temperatures. On the thin end of the viscosity spectrum we find the penetrating oils. Usually the first product we reach for when a rusty bolt resists our attempts with a wrench, penetrating oils are just low-viscosity oils that can seep into the threads and lubricate the surfaces. Common brands include Liquid Wrench, PB Blaster, Kroil, and the ubiquitous WD-40. Each product claims to have just the right mixture of secret ingredients needed to dissolve rust and release that frozen bolt with minimal effort.

All of these products work to some degree, but do they justify their expense? Maybe not. In 2012, a group of engineering students from Drexel University studied alternatives to commercial penetrating oils, which are often expensive or hard to come by in developing countries. In a controlled series of experiments, they determined that a simple mixture of vegetable oil and acetone was far superior to WD-40 in reducing the breakout torque of nuts rusted onto bolts. Similar comparisons show that a 50-50 mix of automatic transmission fluid (ATF) and acetone beats the commercial penetrating oils at a fraction of the cost.