Choosing a pair of de-husking rolls for paddy de-husking is difficult task unless we understand the specifications. There are many types of de-husking rolls currently available in the market. It consists of a Metal Drum or Core and Rubber reinforced over it. Bonding between Metal and Rubber must always be intact and must never come out. In case it does, reject whole batch of rubber rolls.
Metal drums may be Cast Iron, Mild Steel, Aluminum, or Aluminum Alloy. Aluminum Alloy Metal drums are light weight, more durable and consumes less electricity
Rubber roll’s history started with Natural Rubber rolls but later shifted to more stable materials like
1. Silica-filled Styrene-butadiene rubber (SBR)
2. Nitrile Butadiene Rubber (NBR)
3. Poly-chloroprene Rubber, and
Rubber Roll material should be uniform and air trap free.
Hardness of Rubber Rolls
Shore A Hardness of a Rubber is measured in IRHD by an instrument called Durometer. Shore Harness of all Rubber Rolls must be checked for its correctness before mounted the rubber rolls on De-Huskers. Depending upon the variety of paddy to be de-husked shore hardness of rubber rolls varies between 82Deg to 95Deg. Although exact Shore A hardness is little difficult to achieve hence the most practical acceptable tolerance norm should be -3 to +2 points e.g. in case you need a Rubber Roll of Shore A Hardness of 87Deg then the acceptable shore A hardness range is 85Deg to 89Deg.
Hardness should be more or less uniform to ensure uniform de-husking with lesser brokens percentage. Please check Shore hardness at approximately 8 points on a Rubber Roll randomly at ambient temperature below 50 Deg centigrade. It must be noted here that while rubber rolls are in operation the temperature of rubber rolls may go up to 83 DegC. Checking shore A hardness at this temperature is of utmost importance. At this increased temperature of 83 DegC the Shore A hardness must not decrease below 5 Deg to 6 Deg. Good Rubber rolls are always designed to work efficiently at working temperatures of rubber rolls that is at 83 DegC.
Significance of Hardness of Rubber Rolls
Rubbers rolls undergo both shear and compressive forces while de-husking paddy, which result in gradual abrasion of the rubber surface and its thermal softening. Soft Rubber rolls will de-husk more but wear fast and similarly hard rubber rolls will de-husk less but last longer. One has to choose optimum hardness of rubber roll based on 85% to 90% shelling rate.
Measuring Hardness of Rubber Rolls