'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.
