In reducing friction and wear within fastening materials: Metal on Metal is better than Plastic on Plastic, right?
Not so fast.
Let’s face it. Humans as a species have a tendency to the “bigger is better” approach. If a single patty is good, a quad-McBurger must be even tastier. If a one-car garage is handy, why not aim for a property with room for seven. And don’t even get us started on the whole collecting fad that sees us using up every nook and cranny in the attic, hoarding everything from stuffed animals to rectangular pieces of paper with posed pictures of long-gone athletes.
Even the engineering, automotive, and building professions aren’t immune to the propensity to overdo. Specifically, when it comes to fasteners such as screws, bolts, clamps, and ties, the common misconception is that only the strongest of metals will do. While the occasional burger patty overload may not cause major injury to anything other than your waistline, opting for a default fastener based on strength without considering other factors can have less than desirable results.
The “problem” lies in a few scientific quirks in the principles of wear and friction. As it turns out, metal versus metal or plastic versus plastic will often wear down faster than a combination of the two. What magical trick of science makes this happen and what does it mean for your next fastener application? Read on to find out!
First, A Few Definitions
To understand why combining like materials in environments prone to movement and sliding could lead to failure issues, we first need to delve into the concept of friction and its impact on the structure of a material.
Friction refers to the natural resistance to sliding motion between two surfaces. Friction is measured by the coefficient of friction. Surfaces with a lower coefficient of friction will more easily slide over each other.
In the fastener world, wear is the counterpart to friction that can cause things to go awry. Wear occurs when the force of two moving pieces in contact gradually removes material from either or both over time. Abrasive wear occurs when a harder material rubs up against a softer one and scrapes the partner surface. Adhesive wear happens when small portions of one or both surfaces break off and adhere to the other.
Friction and wear together
Friction and wear go hand in hand when it comes to securing items with fasteners. The higher the friction the more quickly the fastener will wear. This is turn creates the need for more frequent maintenance or, in worst-case scenarios, mechanical or structural failure. Therefore it only makes sense that when choosing your component pieces you’d want to select the materials that are proven to hold up the longest when coming into contact with one another.
Too Much in Common
How exactly does this equation for friction plus wear play out when it comes to the selection of fastener materials? When two hard metals are placed into contact with each other the abrasion and adhesion between the surfaces will create grooves, ridges, and other imperfections where the material has worn down and then built up. While two brand new metal fasteners made of the same material will hold up well in the short term, over time these ever-deepening, often microscopic, jagged areas will erode the surfaces exponentially causing wear to occur quickly.
Plastic, on the other hand, typically has a lower coefficient of friction and will generally hold up longer when faced with surface to surface wear. On the negative side, plastic isn’t always as strong as metal and plastic can also be a poor material when it comes to conducting heat, the inevitable byproduct of friction, away from the common surfaces.
[In general, which fastener type should you choose? See our article “Plastic Fasteners vs. Metal Fasteners” for more on this topic].
Choosing the Right Materials for Reduced Friction and Wear
If you’ve read through the above and feel a bit of goldilocks-like frustration between metal on metal and plastic on plastic, never fear. It turns out the best solution is often a combination of materials or a buffer of plastic or its close cousin, oil. When a nylon-based plastic rubs up against steel or other metal surfaces it will lose a small amount of its surface via that whole adhesion/abrasion scenario we mentioned earlier. The difference in this equation, however, is that heat interacts with this microscopic coating and creates additional lubrication between the remaining surfaces. The end result is a lower coefficient of friction, a longer lifespan in your fastener, and lower erosion of your metal parts.
If your application calls for the use of metal on metal, a plastic coating may help lubricate and prolong the lifespan of the surfaces. A real-world example of this can be seen in car engines. Most valves, joints, and other moving parts are lubricated by various specialty oils and greases. If you remember that many plastics are derivatives of oil and oil products, the lubricating nature of the material only makes sense.
Consult the Experts
Sure, we may have broken down the science of friction wear between plastics and metal, but if you’re in need of an immediate fastener solution you may not have the time, patience, or technical know-how to know which particular combination best fits your individual needs. This is where a professional fastener and components partner comes into play. Hey, that’s us!
From the basic “nuts and bolts” to specialty circuit board hardware, E&T Fasteners is your metal and plastic fastener partner – and has the product selection and the experience to advise on your specific job requirements. Not only can we ship out individual in-stock items, but we’re also available for consultations with our expert staff. Don’t waste time staring aimlessly at rows of non-specialty solutions or hoping the apron-wearing handyman employed by the big box store actually knows what he’s talking about.