Few words on friction

Everyone knows what friction is, or at least claims to know. Either way, for the sake of argument, let’s dive into the topic for a minute to better understand how friction works and why it does not play in your team. 

Friction is the most common phenomenon in nature. We encounter it at every footstep, both in negative and positive ways. As well in engineering, friction belongs to the most encountered processes, which can be harmful (resistance in motion: in bearings, chain and gear transmissions) or actually help (clutches, brakes). Friction is also the main contributor to the loss of energy needed to overcome the internal drag and to the wear, that causes a necessity of downtimes to replacement of worn components. Therefore, conducting research on tribological processes and practical implementation of their results are important for improving the balance sheets, both - energy and economic.

Friction occurs at the contact of bodies and depends on the following factors:

• Contact area micro-geometry (surface topography)
• Contact area macro-geometry (shape, dimensions)
• Mechanical properties of the bodies (elasticity, plasticity, stiffness, etc.)
• External force (normal, tangent, load type)
• Type and speed of relative movement (as to the value and direction)

In the analysis of friction, the actual area of contact and its properties are essential. It depends on the shape and dimensions of the contacting surface’s micro-irregularities, load, the mechanical properties of materials, and whether elastic or plastic deformation occurs at the contact. 

Let’s take the school example of a friction pair between a box and a floor it stands on. It is obvious, that when you attempt to push it from one side, the friction will try to prevent you from succeeding. Only if you overcome the friction force the box will move.

A zoom on the contact area shows what actually happens. If the magnification is large enough, you will clearly see the shape irregularities on both, the box and the floor surfaces that kind of hook up one another, under the pull of gravity and the applied side force.

For the sake of further argument, imagine you spilled some oil on the floor to make it easier for yourself to move the box. Then we can further analyse the processes that occur:

External (Solid) friction – refers to the relative movement between cooperating solids and occurs between their surface layers without any lubricant separating them or when a lubricant does not provide a complete separation of the highest micro-irregularities on surfaces and they come into direct contact with each other due to broken oil film. 

Internal (Liquid) friction – applies to forced movements of molecules within a solid or fluid. Liquid friction or Hydrodynamic lubrication only occurs within the liquid when it completely separates both cooperating surfaces of solids.

Boundary (technical) friction – occurs between the boundary (absorption) layers of the lubricant, which has been absorbed by the surfaces of solids. These are characterized with different properties than the lubricant in its entire volume and create a quasi-solid, surface coating.

Mixed friction – All of the above types of friction occur at the same time.

 

How does it apply to me?

Now, regardless of the mechanism you pick – car engine, bike sprocket or a chain link, all of these require lubrication in order to preserve under inevitable occurrence of friction. Nevertheless, all of these will have to “wear in” after they are manufactured and assembled, which means that any shape deviations and surface irregularities in the contact area, will get flatten due the solid friction between them, providing a matching friction pair. The process is called the initial “run-in” or “wear in” and plays a crucial part in preparing its lifecycle. 

In microscopic scale a reduction of surfaces’ roughness occurs, due to their intensive exploitation (rot and shear). In macroscopic scale the shape and position deviations are eliminated, which are due to manufacturing or assembly. Both produce a significant amount of metal scrapes, which are abrasive and left in the system can lead to an excessive wear during lifetime. This requires purging the lubricating system with inhibitors, filter replacement and filling it with fresh oil after the run-in process as well as during cyclic maintenance. 

The running-in process does not disappear completely, but is present during the entire life cycle, further known as wear and tear. Its intensity depends largely on the operating conditions of the mechanism – environment contamination, humidity, working speed, load and applied lubricant of course.

For enclosed mechanisms, like a fishing reel or timing chain, the situation looks better, as those are sealed off from external contamination, but are still vulnerable if the lubricant gets contaminated with their own wear products. 

In a case of bike’s chain drive, only manual lubricant application is available. This is the least efficient way because it certainly does not provide the most desirable liquid friction, but a mixed friction at best. This entails frequent cleaning and re-lubrication due to:

• Water or solvent washout.
• Squeezing out the lube through cyclic, heavy loads.
• Accumulation of contamination leading to premature wear by introducing an additional abrasives.
• Operational wear due to post run-in processes.

Having these in mind, the best lubrication for an “exposed” mechanism, in order to prolong its lifespan and reliability, is to achieve as durable boundary layer on cooperating surfaces as possible. Therefore, picking the right lubricant, that can deliver is critical. Otherwise, taking the bike’s example, it leads to chain stretch, roller deformation and sprockets’ wear.

 

How chains elongate?

If you have removed the chain from the bicycle for cleaning, you may compare it with a new chain, as shown in the photo below, by placing the two chains side by side. The example chains are hanging over the top of a recycle bin. At the far side (right), the links are lined up. On the near side, after two feet (0.6 meter) the older chain is about 1/8 inch (3mm) longer, not worth the trouble to clean and reinstall.

 

Chain "stretch", does not mean the side plates of a chain are being pulled out of shape by the repeated stresses of pedalling. This is not actually how chains elongate. The major cause of chain "stretch" is the wearing away of the metal, due to friction, where the link pin rotates inside of the side plate’s bushing as the chain links flex and straighten while going onto and off of the sprockets.

The tension on the chain varies cyclically during the pedal stroke, and with it, the effect of strain and of friction.

 If you take apart an old, worn-out chain, you can easily see the little notches worn into the sides of the link pins by the inside edges of the side plates’ bushings. Add all of these tiny notches across the length of a chain and you will get the actual stretch value.

You can see how the link pin of this unusually badly worn chain has been worn away. Note also how the roller has flopped out of position. The reason the roller flops around is that the "bushing" part of the chain has been eroded away. No doubt the inner surface of the roller has become enlarged as well.

It is important to mention, that if ignored further increase of the bushing’s clearances leads to chain’s weakening and can end up with a chain snap. This is especially dangerous with motorcycles which chains carry high torques at high speeds.

 

Worn chain, worn sprocket

One might say that chains are consumables. Yes, they are. But as they wear out, it starts a sequence of events inevitably leading to excessive wear of other parts of the drive train. Strain (actual elongation of the chain and compression of the rollers and sprocket teeth under load) slightly lengthens the chain links, allowing them to migrate slightly farther outward on the sprocket teeth wearing them out excessively. This will eventually hit your wallet as cassettes, chain rings and derailleurs are more expensive than the chain itself.

Two formerly identical sprockets of a modern, indexed derailleur system viewed from the right side.

As the chain and sprocket wear together, the teeth become sloped at the back, and the rollers ride up on them until the teeth approach a radius that corresponds to the longer pitch of the worn chain. The effective radius (and thus, the effective pitch) of the sprocket has become larger, because the chain is longer and is riding higher.

If there is excess wear on the rollers, they must roll more and press harder because the surface against which they press is not as nearly at a right angle to the direction of tension on the chain. The downward force from the chain at the top of the sprocket is greater and extends farther back around the sprocket with sloped teeth. Further yet around the back of the sprocket, more teeth must push upward to compensate for this downward force. The sprocket also will wear at the height on the teeth which is taking the load. In extreme cases, the chain may lift entirely off the sprocket and skip forward.

A new chain on a worn sprocket with sloped teeth will sit nearer the bottom of the gaps between teeth at the top of the sprocket but will be tensioned farther back and also may slip up off the teeth and jump forward unless the chain's return run is held under significant tension provided by a derailleur, which then wears it out the excessively. 

 

Why are Revolubes the right lubricant for the job?

As mentioned before, the most desired effect from applying a lube is to establish as durable boundary layer on the top surfaces of the friction pair as possible to prevent the excessive wear. Revolubes technology is aimed exactly at this target and to show off, we have conducted a simple experiment that can be carried out on any lubricant or grease to compare their lubrication properties visually and quantitatively.

This test can be considered a derivative of 4-ball apparatus test, as per ASTM D-2266. It also reflects the conditions of a chain roller running up a sprocket tooth as well as the contact between a roller and its bushing or pin to plate connection.

In this test a stationary bearing’s roller of a diameter of 9.5 [mm] is being pressed against a rotating cylinder with increased force (measured on a scale), until it reaches a value at which the lubricating wedge (oil film) is broken and the roller and the cylinder get welded together.

The dynamometer was installed on a quasi - falex lever, with a ratio of 10:1 – Meaning, the actual force pressing the roller against the cylinder was 10 times greater than the force indicated by the scale. The lubrication performance was assessed by the size and depth of the formed groove on the stationary roller. The amperemeter connected to the electric engine clamps, indirectly indicates the resistance the engine must overcome.

At the first instance, test was carried out without using any lubricant. The welding force at the contact surface was 300 [N] ~ 30 [kg].

At the second attempt the pair was lubricated with popular, widely accessible chain lube.  Breaking the lubricating wedge and welding the two parts occurred at a force of 350 [N] ~ 35 [kg].

The third test was conducted using Revolube™ Chain Lube. With a maximum force of 1500 [N] ~ 150 [kg], which could have been applied to the system, the rotation could not be stopped, constituting a preserved lubrication wedge.

Moreover, the value of the friction force has not been substantially increased, even due to significant load differences. The ampere-meter, which indicated the value of current flowing to the electric motor driving the cylinder, indicated the value has increased only slightly (about 1 [A]), against a significant change in force - from 300 [N] to 1500 [N].

The mark of the groove was significantly smaller, as illustrated in table and pictures below. The snapshot video from the test is also available below.

Tested Lubricant	Value of the limit force [N]	Size of the groove in the roller [mm]
No lubrication	300	9x5
Popular Chain Lube	350	8x4
Revolubes Chain Lube	>1500	4x2,5

 

1 - No lubrication groove, 2 - Popular lube groove, 3 - Revolubes Chain Lube groove Note the heat marks on samples 1 and 2. 

Revolubes quasi -falex test bench.