Rz Surface Roughness |
Surface roughness (Rz) measurements are a useful addition to a pendulum slip test for the purpose of slip risk assessment. Grip Potential conduct surface roughness (Rz) measurement at the site of pendulum testing, giving additional information about the surface's ability to cope with varying contaminants.
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The surface roughness value, specifically Rz, describes the average vertical distance, in microns, between valleys and peaks across a horizontal sample. The measurement is achieved by use of a surface roughness meter of which there are many commercially available. The meter drags a sprung diamond stylus slowly across the floor surface measuring vertical displacement as it goes. Grip Potential use a Surtronic Duo surface roughness meter, in common use, this meter is sometimes affectionately referred to as a "Kenny".
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The micro-roughness of a surface has a significant impact on its slip resistance. This is due to a phenomenon known as hydraulic uplift created by the hydrodynamic squeeze film. When a slip occurs in contaminated conditions the heel slides across the surface compressing the fluid. If the surface is sufficiently rough the fluid disperses into the valleys and the peaks ensure a good sole/floor contact. If the surface is insufficiently rough the sole is unable to make contact with the floor due to the fluid film and friction is significantly reduced.
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The mechanics of a contaminated slip can be explained in a greatly simplified manner by comparing them to that of an aquaplaning tyre. Slick tires offer excellent dry grip but perform very poorly in wet conditions. Grooved tires offer adequate dry grip and far better performance in wet conditions. The profiled tire displaces water and maintains tyre/road contact in a similar way that a profiled floor displaces the fluid film and maintains floor/sole contact.
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As surface roughness measurement gives an indication of the expected behavior in a wet slip rather than measuring slip resistance directly it should be considered as supporting information rather than a slip test in its own right. The UKSRG offer the following classifications for surface roughness to be used in conjunction with pendulum testing;
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The HSE give guidance on the minimum surface roughness levels expected to provide a low risk of slip in various contaminated conditions.
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