# Are Rolling-Element Bearings Practical for Light Loads?

I have a 5" x 4" x 3/8" plastic panel (wall thickness of 1/8", ribbed). I need to slide the panel over a 12" plastic track that is U-shaped (like a skateboard ramp). This motion could be repeated 2 million+ times.

The panel is rigid and maintains 2 contact edges when traversing the curved surfaces.

My question: Should I be worried about wearing-out the plastic components at this scale? What options are there to protect this mechanism?

I'm considering:

Any thoughts on using cheap bearings for this?

• Many questions: when will the 2 million cycles occur? In 1 hour, 1 year? 10 years? Do you have a budget? Can you provide a drawing or sketch of the mechanism? Dec 11 '15 at 1:12
• @GisMofx - The cycles would will occur over 1-2 years. It should take about 1 seconds for the panel to move from one side to the opposite side. I have a reasonable budget, but would prefer a cheaper solution. Sketch added. Dec 11 '15 at 1:53
• Well, a roller bearing/wheel certainly is more robust, and probably within your budget. Also, how were you planning on your panel mitigating that curve with panel? It Seems like your plastic panel will become the "skateboard in the half-pipe" by attaching wheels to the panel. Dec 11 '15 at 2:00
• Just for clarification: you say "skateboard ramp" so presumably "up" in the diagram is away from the ground, is that correct? Also, are there any additional loads on the moving panel, or just its own weight? Dec 11 '15 at 2:09
• @starrise The panel is rigid and maintains 2 contact edges. I will review your answer in detail tomorrow. Thanks! Dec 11 '15 at 3:09

Summary: attempt to minimize wear. Millions of cycles is a lot for even low-load contact surfaces. Use components with high hardness and smooth materials. PTFE or UHMWPE backed by metal for full-contact bearings, and AISI 52100 or AISI 440C for ball bearings. There may be an issue with contact angles at the curves.

# Theory

The primary issue with millions of cycles of amply supported contacting surfaces is almost certainly going to be wear. A simple model for wear is given by the Archard equation - (Wiki), which holds that

$$Q = k\:p\:s$$

where $Q$ is the wear volume, $K$ is a dimensionless constant, $W$ the normal load, $L$ the sliding distance, and $H$ the hardness of the softest contacting material. The Wiki link separates hardness $H$ from $k$. The idea behind the model is that contacting surfaces don't meet uniformly, but instead at randomly distributed, microscopic height differences called asperities. As a result, the actual contact area is much smaller than the apparent macroscopic contact area, and consequently the contact loads are orders of magnitude larger than the apparent macroscopic load. Thus, the size, distribution, ductility, and hardness of the asperities plays a role, as do lubrication, mean height difference (i.e. smoothness), and the presence of external detritus. The constant $k$ depends on all of these parameters, and is typically thus typically empirically determined. However, knowing the proportionality relationships is useful in making first-passes at material selection. Specifically: increasing hardness reduces asperity formation rate, reducing wear; and decreasing surface roughness decreases number of asperities, reducing wear.

Therefore to minimize wear, a smooth, hard material, with an appropriate lubricant should be used. It would also be helpful to minimize external detritus (e.g. dirt, dust) entering the system. You can possibly go either route you describe in the question, depending on the specifics of your setup. I am assuming you require motion along a specific path with minimal deviation, like a train on rails.

# First-pass Material Selection

For plastic-on-plastic with no balls, it would probably help to use parts formed from a hard metal or ceramic and coated with either polyfluorocarbons (PTFE, e.g Teflon) or ultra-high-molecular-weight polyethylene (UHMWPE). The trick here is that your setup requires changing direction smoothly. Getting custom made, non-linear polymer rails and bearings may be more costly than linear alternatives. Additionally, there is the problem of changing contact angles: the bearings would have to be shaped like the inner surface of a toroid, or like a saddle. If using a more typical cylindrical bearing, when the bearing meets the curve, only the leading and trailing edges will be in contact, which will cause damage.

For ball-bearings, there would need to be a special track that solves the issues of leading/trailing edge contact. Ball bearings may be made of metal or ceramic, though ceramics like silicon nitride ($\textrm{Si}_{3} \textrm{N}_{4}$) may be overkill for a low-temperature application as they are more expensive than their metal counterparts. Typically chrome steel (usually near AISI 52100) or stainless steel (usually near AISI 440C) are used as they readily form martensite (a hard, metastable steel phase) during processing and can thus be made quite hard and wear resistant.

If the weight truly is so low, adding any bearings at all will dramatically increase the weight. Instead of having bearings, is it possible to shape the panel to fit into a suitable-width groove dug into the track surface? Then make both out of teflon or UHMWPE, and back the track with brass or steel. A thin layer of dense lubricant could even make the panel float slightly, which would reduce wear to virtually zero.

If the edges are in contact, as noted in the comments, then a track could still work, but would require rounded panel contact edges. The lubricant "float trick" would be messy and probably wouldn't cause flotation when rounding corners.