'Shore a Hardness of a Rubber Cot and Its Restrictions in Measuring the Hardness Value under Mill Conditions'

Abstract

Shore hardness

Generally Shore hardness of a rubber cot is measured by using an instrument called Durometer and the value is expressed in A scale

Cots are available in wide shore hardness ranging from 63 to 90 shore. These ranges are offered to mills based on the following parameters:

• Nature of raw material processed
• Linear density of material.
• Type of application / process / mechanical conditions
• Maintenance aspects.
• Ambient conditions.

A wide spread phenomenon in the industry is that a softer cot having 65 shore A should give relatively better quality yarn as compared to a cot having higher shore A value say,70 shore A. At the same time soft cots have their own drawbacks like wear out rate is faster, damage to the cots surface will be more. If ambient condition in the spinning shed is not well maintained, the soft cot will display greater for lapping.

In terms of maintenance aspect, the grinding machine operator should be highly skilled for achieving good surface characteristics.

Definition of shore hardness

Hardness may be defined as the resistance to indention under conditions that do not puncture the rubber. It is called elastic modulus of rubber compound. These tests are based on the measurement of the penetration of the rigid ball into the rubber test piece under specific conditions. The measured penetration is converted into hardness degrees.

Normally spring loaded pocket size Durometer is commonly used for measuring hardness of the elastomers. Shore A Durometer is used for measuring soft solid rubber compounds. Other scales are also used like Shore D which is used to measure the hardness of very hard rubber compounds including ebonite. The main drawback is in reproducibility of results by different operators. So, a practical tolerance of 5 is acceptable.