Wear pads can often be a headache for engineers. Choosing the right pad material can be very difficult and you may not know which material is right without testing several.
For most sliding wear pad applications, Nylon 6 or UHMW-PE is the right choice. Acetal can be used for applications in chemicals or water. PET and Phenolic can be used at higher loads.
Before we unpack each material, we need to discuss a two topics. They are stiction and wear life. We will also discuss cost briefly too.
Stiction is a made up word that describes a materials likelyhood to be a good or bad bearing material. In physics, we learned that the friction force is related to the normal force in the following equation.
Where f is the friction force, µ is the coefficient of friction and N is the normal force.
As you can imagine, we want a material with a low coefficient of friction, but we are also concerned with another factor of this equation. You may also remember that there is static and dynamic (or kinematic or sliding) coefficients of friction. The static coefficient of friction is what is needed to get an object at rest into motion. The dynamic coefficient of friction is what requires the object to stay in motion. The static coefficient of friction will always be higher than the dynamic. Thus our equation becomes.
If you have even seen a machine lurch and then stop only to repeat the process again, you have witnessed stiction. Stiction can be defined as the difference between the static and dynamic coefficient of friction.
As engineers we want to minimize the effects of stiction in our design. No one wants a machine the lurches or makes lots of noise. This is because the force needed to start the pad in motion is greater than the force needed to keep it in motion.
The easiest way to do this is to minimize the normal force. However, this is not always practical to do. Instead, we want to chose a material where the static and dynamic coefficients are close together so that stiction may be minimized.
Where’s the data?
So when looking at manufacturers data, you will generally find that a range of values. For example, I was able to easily find that nylon (don’t know what type) on nylon has a static coefficient between 0.15 and 0.40. Nylon on steel is 0.4 and nylon on wet snow is 0.4 and 0.3 on dry snow. No dynamic coefficients were given. So what do we do now.
We do a simple experiment to find the coefficients of friction of course. Hey, we are engineers, we can get a little dirty. (I like getting dirty by the way.)
We can mock up a quick experiment using a long thin sample of the material and placing the wear pad on top. We will need to measure the sample’s length, L. We will then pick up one end of the sample material slowly until the wear pad slides. At this point, measure the height, H, it was lifted to. Record these as the static coefficient of friction.
To measure the dynamic coefficient of friction, do the same experiment except this time we will tap the wear pad as we lift the sample material. The tapping action will temporarily get the wear pad in motion by overcoming the static friction. Once the pad is moving, make sure that the pad is sliding at a constant velocity. If it stops or speeds up, the angle isn’t right and the height needs to be increased or decreased.
I’ll save you the math lesson, but from the length and height, we can use the following equations to calculate the coefficient of friction.
This is a pretty easy experiment to do so we will want to do it several times to get a good sample size. We would also like to make changes to the surface of both the sample material and wear pad. The changes should include things like:
- Surface finish – you may want to scratch up the surface
- Rust or corrosion
- Paint – Wet paint, powder coat or no paint will make a difference
- Lubrications – Lubricating the travel surface of the wear pad will decrease the stiction and often eliminate squeaking or moaning noises.
- Effects of contaminates – dirt and grime can have huge adverse effects on the coefficient of friction.
Wear life is an important consideration for your wear pad. The harder the material is, the longer it will last. This is because the pad will be able to smooth out some of the rough spots without prematurely wearing out the pad. Unfortunately, harder materials usually come with higher coefficients of friction.
If the wear pad is properly designed, it should last a long time before replacement. The following three conditions will wear out a pad very fast, avoid them.
- High loads – good slide pads are plastic and don’t have high tensile strengths. Also, don’t assume that the entire pad is going to take the load evenly. Small deflections can make drastic differences in how the pads are loaded. I usually assume the 1/3 to 1/2 of the pad is taking the load.
- High velocity – as the wear pads move, the friction energy converts to heat and this needs to be accounted for. Generally speaking, it is impractical to have external cooling systems for wear pads. Reducing the load helps, but there is an extent where even no load will create too much heat. At this point, it may be better to switch to a live bearing. Finally, limiting the duty cycle can help so that the pad can cool off before the next use.
- Surface roughness – Rough surfaces will destroy a wear pad very quickly. If the sliding surface was welded on, be sure to remove the weld bebes. If possible, sand blast the surface.
- Interference – I once designed a wear pad system where the wear pad transitioned from one surface to another. When welded, the surfaces were off from each other by nearly 1/16″. As a result, each time the wear pad transitioned, it shaved 1/16″ off the surface of the pad. It lasted 3 cycles. The moral of the story is to make sure that the surfaces are flat and one piece.
UHMW is an acronym for ultra high molecular weight polyethelene. I didn’t understand the name until a few years ago when it was explained to me. It is a plastic with incredibly long molecules. If you remember your hydrocarbons, the natural gas is defined as CH4. The carbon molecule is in the center and there are four hydrogen molecules branching out at each 90°. For propane (C3H8), there are 3 carbon atoms in a row and the hydrogen atoms attach at each end, top and bottom.
Anyway, UHMW is a ridiculously long version (like millions) of this chain. This long chain gives the material its strength and toughness as there are fewer grains when compared to other materials. UHMW is white in color and easy to machine.
Its coefficient of friction is very low (0.15 -0.20 static; 0.12-0.20 kinetic) when used on steel. It also is the best material for minimizing stiction. Unfortunately, the tensile strength is low (4300 psi / 26.7 MPa) so you will need more material to carry the same load.
UHMW is my go to material for wear pads, especially when the surface appearance of the other material is delicate or just needs to look good.
Nylon is a tougher material than UHMW, so it is my next choice for a wear pad. Being tougher, it has a longer wear life and higher tensile strength. (It can be more than double UHMW). It also has an increased coefficient of friction.
Nylon is the middle of the road in cost, easy to machine and available in a variety of shapes, sizes, and blends. Nylon 6, 6/6, 6/10 and 6/12 are the most popular. See the manufacturers specifications for more information.
Nylon can also be impregnated with UV inhibitors, oils and other things. There are also proprietary blends as well like Nylatron® GSM. These additives can increase material life by preventing degradation and reducing friction.
Acetal or Delrin® is the standard on which other plastics are compared for machining. It cuts fast and leaves a great finish. The cost is also about the same as nylon. Acetal provides great chemical resistance including use in water. It also resists expansion when heated or used in humid or moist conditions. It is available in black and white. The white color acetal has been approved for use with food by the FDA.
Acetal is stronger than UHMW but not quite as strong as nylon. Acetal is a good choice because it has a lower coefficient of friction than nylon.
PET or PETE or Polyester
PET is a wonderful material that is roughly the same as nylon; they have equivalent machining properties, impact strength and tensile loads. Polyester cost about 30% more than nylon.
PET is only available in sheets so your application must be designed that that in mind.
Phenolic is a thermosetting plastic that is hard, strong and tough. It can also be used at higher temperatures than other plastics. It also boasts of a low coefficient of friction.
Teflon® or PTFE
You may have noticed this one missing from the list above. PFTE is very slippery and has the lowest coefficient of friction of almost any solid material in existence. Unfortunately, it is very weak, wears very fast and costs a lot too.
PTFE’s greatest strength is also its greatest weakness: it doesn’t stick to anything. With the other materials mentioned, some of the plastic material will transfer to the component it contacts with. This reduces the friction a little more as well. PTFE won’t do this. The shaved off material will be blown away with the wind or the next cycle of the machine.
One thing that can be tried is to have a nylon wear pad drilled out and PTFE discs pressed into the holes. I have tried this one time and it was super expensive and didn’t solve my problem with noise.
Because of the strength, wear rate and cost, I do not recommend PTFE for use in sliding wear pad applications.
Typical Dynamic Coefficients of Friction
|UHMW-PE||0.10 – 0.22|
|Nylon 6||0.15 – 0.40|
|Acetal||0.15 – 0.35|
|PTFE||0.04 – 0.25|
There are many plastics to choose from for your sliding wear pad application. UHMW-PE and Nylon are the most practical choices for general use. These materials are strong and slide easily.