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PRACTICAL # 8 DETERMINATION OF GREEN & DRY STRENGTH OF DIFFERENT SAND MIXTURES
Overview: Strength or bond strength is the property of sand whereby it offers resistance to deformation and enables the sand to hold together under pressure. Green strength is referred to the strength of moist or tempered sand whereas dry strength is strength of sand dried at 110oC. The green strength of sand is usually lower than its dry strength. Bond in moulding sand is due to the presence of binders such as clay, core gum, molasses, linseed oil etc.
Universal Sand Strength Testing Machine: The Universal Sand Strength Testing Machine consists of three major parts; frame, pendulum weight and pusher arm. The pusher arm is motivated by means of a small handwheel, which, through a gearbox, rotates a pinion engaged in a rack on the quadrant. The pendulum weight swings on a ball bearing and can be moved by the pusher arm, via test specimen, from a vertical position, with a consequent increase of a load on the test specimen. A magnetic bar is moved up the calibrated scale by the pendulum weight and indicates the point at which the specimen collapses. The machine can be calibrated in g / cm3.
Figure: Universal sand testing machine
WWW.07MET.TK Casting & Working of Metals Lab Manuals
A. Green Compression Strength:
Test Procedure: 1. Place the compression heads in the lower position as shown in the figure. 2. Raise the weight arm slightly and insert an A.F.S. standard 50 mm X 50 mm test specimen between the compression heads so that the face that was uppermost in the ramming operation is facing the righthand compression head. Care should be taken not to damage the specimen. 3. Ensure that the magnetic rider is resting against the pusher plate and that there is at least 6mm clearance between the rubber bumper and the lug on the weight arm. If this clearance is insufficient, it means that the specimen is smaller than the permitted tolerance and should be discarded. 4. Apply a load to the test specimen by turning the hand wheel at uniform rate (approximately 30 g / cm3 / sec) until the specimen collapses. 5. Record the reading shown on the lower edge of the magnetic rider, reading the scale designated “Green Compression Strength”. 6. Return the weight to zero by gently reversing the rotation of the hand wheel. Remove the sand from the compression heads.
B. Green Shear Strength: Test Procedure: 1. Place the shear test heads in the lower position in the machine, with the head having the half round holder attached to it in the pusher arm. 2. Raise the weight arm slightly and insert an A.F.S. standard 50 mm X 50 mm test specimens between the heads. 3. Ensure that the magnetic rider is resting against the pusher arm and that there is 6mm clearance between the rubber bumper and lug on the weight arm. 4. Apply the load uniformly until the specimen shears. 5. Read the lower edge of the magnetic rider on the scale designated “Green Shear Strength”. 6. Return the weight to zero by reversing the rotation of the hand wheel. Remove the sand from the shear heads.
WWW.07MET.TK Casting & Working of Metals Lab Manuals
C. Dry Compression Strength: Test Procedure: 1. Place the compression heads in the top position of the machine. This position increases the load applied by factor of 5. 2. Prepare A.F.S. standard 50 mm X 50 mm test specimen in the usual way and dry in an oven at 110oC for few hours. 3. When cool, place in position between test heads and adjust clearance between rubber bumper and the lug on weight arm to approximately 13 mm using the adjusting screws in the pusher arm. 4. Apply the load as for “Dry Compression” until the specimen collapses. 5. Red the scale designated “Dry Compression Strength” according to the test heads being used. 6. Return the weight to zero by reversing the rotation of the hand wheel. Remove the sand from the compression heads.
D. Dry Shear Strength: Test Procedure: 1. Place the shear heads in the top position of the machine. This position increases the load applied by factor of 5. 2. Prepare A.F.S. standard 50 mm X 50 mm. test specimen in the usual way and dry in an oven at 110oC for few hours. 3. When cool, place in position between test heads and adjust clearance between rubber bumper and the lug on weight arm to approximately 13 mm using the adjusting screws in the pusher arm. 4. Apply the load as for “Dry Shear” until the specimen shears. 5. Red the scale designated “Dry Shear Strength” according to the test heads being used. 6. Return the weight to zero by reversing the rotation of the hand wheel. Remove the sand from the shear heads.
WWW.07MET.TK Casting & Working of Metals Lab Manuals