Hand or Rotary Brush - That Is the Question? Part No. 2

 3. How a Fine Rotary Steel Brush Removes Wax

3.1. Specific Design of a Rotary Brush

A rotary brush is functionally similar to a hand brush, but its design and mode of action differ substantially.

For SWIX and TOKO brushes, we will assume:

• a wire length of approximately 10 mm,
• a wire diameter of approximately 0.10 mm,
• an overall brush diameter of approximately 70 mm,
• an operating speed of approximately 800-1,000 rpm.

For the RED CREEK brush:

• a wire length of approximately 6 mm,
• a wire diameter of approximately 0.07 mm,
• an overall brush diameter of approximately 62 mm,
• an operating speed of around 1,000 rpm.

Despite their smaller wire diameter, the rotary-brush wires are approximately eight times stiffer than the reference 25 mm hand-brush wire because of their very short free length.

At 800-1,000 rpm, the tips of the SWIX/TOKO brush wires move at a circumferential speed of approximately 2.9-3.7 m/s.

A rotary-brush wire is therefore:

• significantly shorter,
• stiffer,
• less able to lie along the surface for an extended distance,
• loaded at a many-times higher speed.

3.2. Basic Mechanism

In a rotary brush, each wire moves along a circular path. It enters into contact with the ski base at high speed, bends briefly, shears across the surface and then springs back to its original position.

One working cycle comprises:

1. a brief dynamic contact or micro-impact,
2. rapid bending of the wire,
3. a brief high-speed shear,
4. elastic release,
5. displacement or ejection of a particle.

Each individual contact is very brief, but it is immediately followed by other wires. The surface is therefore worked by a large number of short, overlapping contacts.

3.3. Wax Removal

In the first stage, the rotary brush removes very quickly:

• flakes left after scraping,
• loose wax dust,
• residues of a thicker layer,
• wax from the upper and accessible parts of the structure.

The wire disrupts, fragments or separates the wax by impact and shear. As it springs back, and under the action of centrifugal force, it may eject particles from the contact area.

A short wire can enter wide grooves. In a finer structure, however, it often acts mainly on the peaks and upper parts of the groove walls. It removes wax not in one continuous stroke, but through repeated fragmentation.

3.4. Effect on the Thin Film and UHMWPE

After the macroscopic excess has been removed, the brush continues to work on the thin wax film by the same mechanism.

The short dynamic contacts may:

• shift the film by shear,
• tear it,
• separate it from the surface,
• break it into fine particles,
• remove it progressively.

The brush does not distinguish between excess wax and functional wax.

As the film diminishes, the steel wires come increasingly into direct contact with UHMWPE at peaks, edges and fibrils. They may plastically deform it, micro-plough it, score it or detach microscopic particles from it.

This is unlikely to constitute uniform removal of a continuous layer across the entire surface. Material loss will instead be local and concentrated on the highest and most heavily loaded areas. Even very slight abrasion, however, may reach the thin wax-enriched surface zone.

3.5. Characteristics of Rotary Brushing

A rotary brush is fast, effective and generally more uniform than a hand brush.

Its defining characteristic, however, is the very rapid transition between the individual stages:

1. removal of loose wax,
2. opening of the structure,
3. reduction of the thin functional film,
4. direct contact with UHMWPE,
5. possible micro-abrasion of the wax-enriched polymer layer.

A rotary brush works through a combination of micro-impact, brief shear and a very high number of repeated contacts.

Rotary brushing is faster and more uniform, but it leaves less time to stop the process at the point when the excess has been removed but the functional surface layer has not yet been significantly affected.

4. Hand or Rotary Brush - That Is the Question

Hand-held and rotary steel brushes serve the same basic purpose: to remove excess wax, open the ground structure and prepare the ski base for gliding. The way in which they achieve this result, however, is fundamentally different.

A hand brush works predominantly through slow, continuous shear by long, flexible wires. Its effect develops gradually, stroke by stroke, and the operator can continuously adjust the pressure, speed and number of repetitions. Its disadvantages are the longer working time and lower uniformity of treatment.

A rotary brush works through brief dynamic contacts by substantially shorter and stiffer wires. At a circumferential speed of several metres per second and with a high number of repeated contacts, it can remove excess wax extraordinarily quickly and uniformly. Once the excess has been removed, however, the same mechanism continues without interruption to work on the thin functional film and subsequently on the highest parts of the UHMWPE itself.

The rotary brush cannot determine whether it is currently removing:

1. undesirable macroscopic excess,
2. a still-desirable thin wax film,
3. or the wax-enriched uppermost polymer layer itself.

The boundary between these stages is not sharp and may be crossed at different times in different areas of the ski base. The high speed of a rotary brush also significantly shortens the time available to the operator to stop the process.

From this perspective, a hand brush may be considered the safer tool. Although it is considerably slower and the result may be less uniform, it offers greater control and a wider time margin between removal of excess wax and the onset of more intensive treatment of the functional surface layer.

A rotary brush has undeniable advantages, especially in professional servicing, where speed, productivity and repeatability are decisive. These advantages, however, come at the cost of greater sensitivity to correct rotational speed, pressure, feed speed and number of passes.

The decisive question therefore cannot be merely which brush removes wax faster. More important is which tool makes it possible to stop brushing more reliably at the point when the processing excess has already been removed while the functional wax and polymer surface zone remains preserved. From this perspective, the hand brush is the more controllable tool and offers a greater tolerance for error.

Effect of Ski Base Material on Structure Quality - Part No. 2

 The Importance of Dimensional Stability in Wet Conditions

Some ski base manufacturers consider dimensional stability to be one of the decisive material properties, particularly for bases intended for warm and wet snow conditions.

In colder conditions, contact occurs predominantly between the ski base and individual ice grains, whose surfaces are covered by a very thin quasi-liquid layer, referred to as the QLL. At the points of actual contact, this layer can act as a highly effective boundary lubricant.

As snow temperature and moisture increase, or as a result of frictional heating, an additional and substantially thicker layer of free water may form in the contact zone. This can be described as a secondary water film.

Under certain conditions, a thin and discontinuous layer of water can reduce friction. Once the secondary water film becomes excessively thick and continuous, however, and is not removed from the contact zone quickly enough, its effect changes. Compared with the very thin QLL on the surface of the ice grains, it may no longer function as an effective lubricant, and other adverse tribological phenomena begin to dominate. These include capillary and suction forces, a substantial increase in the real contact area, viscous resistance during the displacement and shearing of water, non-uniform pressure distribution within the water film, and hydrodynamic losses associated with changes in flow direction and local turbulence.

The ski base structure plays a fundamental role in limiting these adverse effects of the secondary water film. Its purpose is not merely to reduce the contact area mechanically, but also to create a system of grooves and channels that captures, guides and removes water from the contact zone as quickly as possible.

For the structure to perform this function effectively, it must retain the geometry in which it was designed and produced. This remains true when the structure operates in combination with glide waxes or other surface treatments.

If its grooves deform, flatten or partially close under load, their cross-section, depth and water-drainage capacity change. The structure may then cease to function in the way for which it was originally designed.

The requirement for dimensional stability is particularly important in alpine skiing. At high speeds and under substantial dynamic loads, for example in a sharp turn, the ski is subjected to significant loading that is often concentrated on one side.

A soft and elastic ski base may yield locally under this pressure. The geometry of individual grooves can therefore change precisely when they are required to remove the greatest amount of water. A reduction in groove depth or cross-section may restrict water flow and increase the proportion of the secondary water film within the contact zone.

For this reason, some ski bases intended for warm conditions and alpine disciplines have a modulus of elasticity of approximately 950 to 1,050 MPa, equivalent to 0.95 to 1.05 GPa.

A higher E-modulus means that the material undergoes less elastic deformation under the same load. The structure therefore retains its original shape, groove depth and functional cross-section more effectively, even under high local loads.

The E-modulus value cannot, however, be assessed in isolation. It also depends on the measurement temperature, the test method used, the loading rate, specimen orientation, and whether the manufacturer reports a tensile, compressive or flexural modulus. Direct comparison of different ski bases therefore requires values obtained using the same test method and under comparable conditions.

Stiffness and Wear Resistance

Higher stiffness does not automatically imply a longer ski base service life.

The exceptional abrasive wear resistance of pure UHMWPE is related, among other factors, to its high toughness, elasticity and extremely long, entangled polymer chains. When exposed to an abrasive particle, an ice grain or another surface irregularity, the material can partially yield elastically without a fragment being immediately detached.

Adding a rigid reinforcing component limits deformation of the matrix and increases its dimensional stability. At the same time, however, local stresses may become concentrated around the particles.

If the amount, type or dispersion of the filler is unsuitable, the polymer matrix may be damaged around the particles, which can gradually loosen and be pulled out, causing the surface to crumble. Increased dimensional stability may therefore come at the cost of higher abrasive wear.

In simplified terms, pure UHMWPE absorbs part of the load through elastic deformation, whereas a stiffer composite material deforms less but may be more susceptible to local damage and particle separation.

There is therefore a trade-off between dimensional stability, machining quality, toughness and wear resistance.

The optimum ski base material need not be either the softest or the hardest. Its composition and internal structure must allow it to deform as little as possible during grinding, permit clean material separation, produce a minimum of unwanted fibres, retain the geometry of the structure under load, and at the same time preserve sufficient toughness and wear resistance.

The quality of the resulting structure is therefore determined not only by the grinding stone and machine settings. It is also significantly influenced by the design of the ski base material itself: the properties of the UHMWPE matrix, the type and quantity of added particles, their size and structure, the quality of their dispersion, and the course of the sintering process.

The same structure produced with the same grinding stone may therefore not have the same geometry, cleanliness or function on different materials. The ski base material is not merely a passive carrier of the structure. It actively determines how accurately the structure can be produced, how clean its surface will be, and to what extent it will retain its geometry under actual loading during skiing.

 

What is crucial to achieve a reliable kick? Part III.: correctly defined kick area of the ski

In the article "What is crucial to achieve a reliable kick? Part I.: right stiffness and length of the skis” it was explained that skis base for classic cross-country skiing style consists of three parts / areas, two gliding areas (one in tip and one in tail of the ski) and one kicking area (in the centre of the ski).

If the skier is standing on both skis, in other words skier’s load is transferred to both skis equally, the middle area (kicking area) should not touch the snow surface and the skis should glide on the snow surface in the tip and tail areas (gliding areas).

In the opposite if the skier concetrates his/her load on one of both skis, the middle area is pressed down and get in touch with snow surface enabling the kick.

For a reliable kick is thus responsible stiffness and length of the skis (they must correspond with the weight and height of the skier) but also the length and position of the kicking area.

If the kicking area - where kick waxes are applied - is A. wrongly positioned along the ski or B. too short or too long no reliable kick can be provided.

If the kicking zone is correctly positioned, but too short the consequence is missing kick but still good gliding properties.

If the kicking zone is correctly positioned, but too long the cosequence is A. a reliable kick but bad gliding properties or B. freezing wax with no kick and horrible gliding properties or C. skis do not enable any movement.

If the kicking zone is incorectly positioned, the result is normaly too long kicking zone with consequences A / B / C depending on other factors as snow conditions, wax type etc.

Conclusions

Define the kicking area where kick waxes are applied carefully before you start to ski. If you do not know the kicking area, apply the kick waxes in a short window which can be increased with insufficient kick. In direction to ski tail the kicking zone ends latest where the ski boots end. If you need to improve kick properties by enlarging the kicking zone, go direction to tail first and stop at ski boot end. After you have reached the ski boots end, enlarge the kicking zone in direction to tip.