The "bearings" topic is one of those that passionate skaters: how many balls? What materials? What ABEC grade? What kind of screens? How to clean them? Which lubricant to use? These are just some of the questions that generate long discussions in sports groups and blogs: everyone has their own answers, original or derived from the advice of their friends, acquaintances, teammates and cousins.
I would like to try to make a small personal contribution on this topic, without taking sides too much. The intent is to provide information on the issues underlying the operating mechanics of bearings, so that everyone can deepen the question and possibly think about it a bit.
In particular, I will try to answer a simple question: why lubricate bearings? It is, in reality, a non-trivial question, whose analysis will allow us to introduce some interesting concepts regarding the functioning of these components, which are important for the proper functioning of a roller skate.
Let's start with a consideration that you will probably already be familiar with: the purpose of a ball bearing is to reduce the waste of energy due to the friction that is generated when skating . Friction transforms the kinetic energy, produced by the skater's muscular action, into thermal energy, and makes us slow down. Basically, part of the breakfast we had, skating inevitably becomes heat: the goal is to reduce this waste to the minimum possible. The best strategy available at the moment is to make the moving devices in such a way that, if possible, only rolling friction phenomena are present. This concept will probably be familiar to you: rolling friction is what occurs in case of rolling, while sliding friction occurs in case of sliding . Generally, the resistance produced by rolling friction is much lower than that due to sliding friction (but not always, think of the case of ice skating), and this fact justifies the use of bearings. Note, however, that sliding friction is essential for skating: it is the static sliding friction that allows the wheels and bearings to function properly and the skater to push and bend.
Let's imagine a sphere rolling on a plane, that is a situation quite similar to the one that occurs, inside a bearing, in the contact between one of the spheres present and the so-called tracks, present on the surface of the two rings - internal and external -. If the sphere and the plane were geometrically perfect, made with infinitely rigid and chemically completely inert materials, the contact between the two bodies considered would be limited to a point and most likely the lubrication would be useless, and - indeed - it would worsen the efficiency of the system. . The reality, however, is quite different: n no material is non-deformable and there is always the possibility that the moving parts can form chemical bonds with each other or with other substances, such as gases in the air. For these reasons, a series of dissipative processes are established, mainly linked to two factors:
- the contact between the two bodies is not point-like (and therefore infinitesimal), but characterized by a finite-dimensional imprint, whose area and shape can be defined using the so-called Hertz theory (the scientist who gives the name to 'unit of measurement of frequency, he was a genius in several fields), who tackled the problem as early as 1882, or other later theories, actually quite complex.
- The surfaces of two bodies placed in contact can form chemical bonds with each other and with other substances that may be present (for example oxygen present in the air).
The comparison between the theoretical case and the real case is represented in the following figure, in which the pressure profile that occurs in the sphere contact has also been qualitatively highlighted - floor.
(a) Theoretical case: infinitely rigid bodies. In pink: pressure peak at the point of contact.
(b) Real case: the materials deform, a contact imprint is generated and the pressure peak widens and lowers.
Let's now analyze, in greater detail, the first factor considered. In the particular case of a bearing, the rolling elements (ie the balls), during operation, follow a circular trajectory, rolling on the tracks obtained on the surface of the inner and outer ring. In a skate, when the balls are in the lowest part of their circular trajectory, they undergo the maximum load, given by a fraction of the skater's weight, plus any contributions of an inertial type, that is linked to the accelerations present, deriving for example from irregularities of the road surface, execution of jumps and curves, etc. In a fraction of the path traveled by the sphere in the vicinity of this condition of maximum load, the Hertzian contact between the sphere and the tracks will cause a sliding of a certain intensity of the sphere considered on the track and, therefore, there will be dissipation of energy due to phenomena of sliding friction. The reason is obvious: let's consider a circumference with a maximum diameter placed in correspondence with the rolling plane passing through the center of the sphere and a smaller circumference, placed in coincidence with the extreme point of the contact footprint, as shown in the following figure:
Both circumferences considered are bound to remain in contact with the track during rolling, but they cannot do it without sliding ! In fact, the outermost circumference has a smaller diameter and therefore "will make less road" than the larger one, considering - for example - a complete rotation of the sphere: we therefore speak of differential sliding. Since the contact pressure is maximum at the center of the footprint, it will be the surface of the sphere corresponding to the area surrounding the smaller circumference that will slide against the surface of the track, while the differential slip will be zero at the greater rear circumference.
Conclusion 1: in the considered system, during the rolling, there is, inevitably, also sliding, due to the fact that the materials with which the two elements placed in contact are not infinitely rigid. This fact generates a rolling resistance due to the phenomenon described.
Regarding the second factor considered: it is known that two metal surfaces clean and placed in contact, can develop chemical bonds, which manifest themselves in a very evident way in certain circumstances, for example when contact occurs under extreme pressure conditions or in the absence of surface contamination. A typical case is the seizure of two mechanical components; however, even ball-to-race contact in a bearing is not exempt from the problem. In fact, "gluing" phenomena may occur at a microscopic level, between the different components, with consequent dissipation of energy deriving from the formation and breaking of chemical bonds, as well as wear of the affected parts due to localized plasticization and detachment of micro-particles from the surfaces .
Conclusion 2: a bearing can be affected by localized sticking and bending phenomena, which can make a contribution to rolling resistance.
Overall, therefore, it can be said that, in a real situation, a ball bearing will in no case guarantee a resistance linked to rolling friction equal to zero, nevertheless offering a substantial improvement compared to situations in which pure sliding friction phenomena prevail.
Let's see now how the use of a suitable lubricant can help improve the situation.
First of all, a carefully chosen lubricant can reduce the dissipated energy and wear associated with the ball-track sliding shown above. It can do this by interposing between the two components: the friction resulting from the presence of a viscous fluid is always better than the sliding one generated by the direct contact of two metal parts! For the stratagem to work, the lubricant in question must have the right viscosity and ability to adhere to the metal surface: otherwise, due to the high local pressures present in the Hertzian contact, it would be "squeezed"outside the area of interest and its effectiveness would be very poor (this is probably the case, for example, of the famous WD40). There is also no need to unnecessarily exaggerate the viscosity of the lubricant, however, because very viscous fluids dissipate greater amounts of energy by deforming (this is the case with grease). So how should the choice be made? There are several factors at play: rotation speed, rheological characteristics of the lubricant, Stribeck curves ... but we will talk about this another time. To stay on the subject, have you ever noticed that an unlubricated bearing tends to squeak when used? The reason is that the differential sliding present in the track ball contact, under certain conditions, can take place on the basis of a process called stick-slip, i.e. not with uniform motion, but in jerks. This is the same mechanism by which a door can creak at its hinges when it is moved.
Secondly, a lubricant can effectively prevent the formation of chemical bonds between the moving parts: in this way it can reduce the rolling resistance linked to the second factor considered, reducing to at the same time the wear of the bearing. It should also be considered that the steel with which the components of common bearings are made can undergo general or localized oxidation phenomena as a result of contact with atmospheric oxygen and humidity, with consequent loss of material in the form of ... rust. A good lubricant is also designed to protect mechanical parts from this type of process.
It must be said that there are also other good reasons not to operate your bearings "dry": for example, we all know that the balls are kept in the correct position with respect to each other. others by the component called "cage". The latter can be made of steel or plastic material, but in any case it is a component that locally crawls on the surface of the balls during the operation of the bearing: an appropriate lubrication can help reduce the energy dissipated by this ( unavoidable) contact.
General conclusion: lubricating a bearing is a fundamental operation, for the reasons analyzed. The lubricant must be chosen appropriately: some of the selection criteria derive directly from what has been said: you can easily deduce the fact that the use of impromptu lubricants (olive oil, paraffin oil, nautical grease, WD40, etc.) will probably not lead to results you want, or at least won't allow you to get the most out of your expensive bearings, both in terms of performance and durability.
As I told you, there are many other factors that affect the choice of a lubricant suitable for a specific application: I will talk about this on another occasion.
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