Seeking Technology for reduction of friction between oscillating, non-lubricated hair cutters

Dry shavers cut beard hair through systems consisting of outer and inner cutters. In the particular case of oscillatory shavers, the inner cutters oscillate relative to the outer cutters. High cutting efficiencies require close contact between the cutters. For that purpose they are pressed together. The resulting contact pressure in combination with the oscillating motion between the cutters results in friction and wear. Friction results in energy consumption, reduction of operation time and increase of the cutter temperature. Wear may reduce the roughness of the contact surfaces which increases friction further. Friction and wear shall consequently be reduced to a minimum.

The need to reduce friction and wear has increased with the trend to washable shaving heads. In former times friction was uncritical thanks to the lubrication effect of skin sebum that entered the shaving system during shaving. Since shavers are washed, the sebum may be removed and the cutting systems may be operated without lubricant.

• Minimal modification of existing cutting system for maximum reduction of friction and wear. Ideally minor modifications, as e.g. addition of suitable heat treatment, re-design of geometry or micro-structuring of cutter surface.
• Improvement ideally via re-application and leveraging of technologies utilized in other fields of applications as e.g. bearing technologies.
• Minimal costs
• Solution is scientifically well supported and may ideally be described through a corresponding model for friction and wear as basis for further improvement.
• The solutions shall be applied to shavers. Consequently they must be suitable for mass production conditions and must be in compliance with regulations applicable to devices with skin contact, ideally FDA approved, as well as to existing standards for electronic devices.

However, larger changes as e.g. change to new material combinations as well as disruptive solutions for hair cutting are welcome.

The principle of dry shaving systems of the oscillatory type is sketched in Fig. 1. It shows a small section with a hair before being cut. The system is brought into contact with the skin via the outer cutters.

Fig. 1 Sketch of a small section of the cutting system

Those contain openings in form of apertures or slits resulting in net- or comb-like structures so that hair can enter the system. The skin is left within Fig. 1 for clarity. It would cover the hair at the end denoted by “hair root”, be in contact with the outer cutter at the surfaces tagged by “in contact with skin” and bulge into the opening between the bars.

The inner surface of outer cutters is in contact with inner cutters. Those oscillate relative to the outer cutters, so that hairs having entered the system are captured by the inner-cutter, moved to the boundary of the openings and cut there.

Typical dimensions of the section in Fig. 1are shown in Fig. 2. The cutters consist of annealed stainless steel. In few cases, the outer cutters are made from nickel. The inner cutter oscillates relative to the outer cutter with +-1 mm amplitude and a frequency of approx. 175 Hz. Life time shall be 3.10⁷ cycles.

Fig. 2 Typical dimensions (µm) of bars and blades of dry shaving cutting systems

The entire system is formed by approx. 40 segments as shown in Fig. 1 and 2. The segments are repeated along the oscillation direction of the blade. The length of the segments perpendicular to the oscillation direction varies from 4 to 20 mm.

The geometrical description defines a theoretical contact area between outer and inner cutter. The practical contact area is smaller than the theoretical contact area due to gaps between the cutter surfaces and due to the roughness of the surfaces. Typical gaps are of the magnitude of 15µm. They follow from manufacturing inaccuracies and depend on the dynamics and elasticity of the system. They decrease with the usage of the system due to wear: New systems have larger gaps, used ones have much smaller gaps. The roughness declines in the same way. The practical contact area approaches consequently the theoretical contact area. The resulting, with time changing, local contact conditions determine the pressure within the contact area. It depends on location with its integral being equal to the contact force of 0.5 to 2 N between the cutters.

Experiments show the following:

• Operating the system after artificial removal of lubricants results in an excessive increase of friction within few minutes. Microscopic investigations suggest the occurrence of fretting.
• During normal shaving, friction increases continuously during the first six to ten hours shaving time. Microscopic observations indicate that this increase of friction is correlated to the increase of the contact area due to wear. However, the details of the wearing process could not be clarified yet.

No relevant effects with regard to friction and wear are observed since one of the cutters is coated with DLC (Diamond-Like-Carbon).

• Industry our client is in: Small-electrical appliances, part of a leading FMCG organization.
• Annual Revenue: Multi-billion $
• Years in Business: More than 40
• Headquarters Area: EUROPE

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