The gear blank is made of 20CrMoH, which belongs to low alloy structural steel. The liquidus temperature reaches 1511 ℃, and the high melting point leads to long solidification time and easy oxidation of steel castings; The liquid metal flow performance of the alloy is also poor, and the inclusions have a great influence on the quality and properties of sand castings; Moreover, the alloy has a solid shrinkage ε It reached 6.18%, and the solidification shrinkage was large. Therefore, the gating system design for this kind of steel castings must make the liquid metal filling mold stable, which is conducive to the sequential solidification of metal in sand castings and the feeding of risers.
The central top casting pouring scheme refers to placing the pouring system in the center of the sand casting, in which the sprue and sprue nest are placed in the geometric center, and the inner sprue cuts into the steel casting horizontally. The pouring scheme can make the liquid metal flow smoothly into the mold cavity of steel castings, and the liquid metal in the riser can effectively feed and shrink the mold of steel castings. However, this kind of pouring scheme is more complex, so it is necessary to accurately calculate the size of each component and riser of the pouring system.
The function of the riser is to store a certain volume of liquid metal, supplement liquid metal to the mold during the forming process of sand castings, introduce shrinkage porosity, shrinkage cavity and other defects into the riser, and guide the sequential solidification of steel castings to improve the forming effect of sand castings. The volume of spiral bevel gear steel casting is about 0.5L. Considering the feeding distance and the position of hot joint, three top risers are selected to be evenly distributed on the upper end of gear blank in the circumferential direction, and the included angle between risers is 120 °; In order to improve the feeding efficiency of riser and the use efficiency of liquid metal, the standard cylindrical atmospheric pressure insulated concealed riser is selected.
There are three main methods for calculating the riser size of steel castings:
- Proportional method: Although the calculation step of this method is the simplest, it needs to be determined by accumulating a lot of production practice experience;
- Feeding liquid method: this method does not consider the influence of sand mold casting shape on the solidification forming of steel castings;
- The heat preservation method of the bevel gear has little influence on the riser volume of the casting, but the heat preservation method of the bevel gear has no influence on the riser. Therefore, this method is selected as the method to calculate the riser volume. Modulus method stipulates that the solidification time of metal castings is determined by the ratio of volume and surface area of sand castings, which is called modulus. Expressed by the following formula:
Where,
M – modulus (CM); V – volume of sand casting (cm3); A – surface area of sand casting (cm2).
The volume of sand casting is 503.216cm3 by constructing the three-dimensional digital model of sand casting in CATIA software and viewing the properties; The surface area is 693.374cm2. By introducing the formula, the modulus m of sand casting can be calculated to be 0.72575cm. The maximum volume of molten metal for steel castings to be fed can be calculated by the following formula:
Where, vs – the maximum volume of sand castings to be fed; ε — Shrinkage of sand castings; VC – volume of sand casting.
By substituting the relevant data into the formula, it is calculated that the liquid volume of steel castings to be fed is 0.03l. According to the production practice experience of the foundry, the module of thermal insulation riser with the same volume is generally 30 ~ 40% higher than that of ordinary riser; The feeding efficiency of insulation riser is 25 ~ 45%; According to the thermal modulus and feeding efficiency of insulation riser, the following formula can be listed:
Where,
Mr – insulation riser modulus; Mr – modulus of common riser; MC – modulus of sand casting;
E – modulus increase coefficient of insulation riser; VF – maximum volume of insulation riser that can be fed;
η — Feeding efficiency of insulation riser; VR – volume of insulation riser.
The feeding effect of insulation riser depends on the material composition and wall thickness of insulation sleeve δ The size of the. The driven spiral bevel gear studied belongs to small steel castings, so the flue ash composite insulation sleeve is selected. The specific material group of insulation sleeve is: flue ash 60%; Silica powder 40%; 12 ~ 14% of water glass with external modulus of 2.2 ~ 2.4. Insulation sleeve thickness δ The calculation formula is:
Substitute the data and round the results approximately to determine the thickness of the insulation sleeve δ = 7mm。 The insulation capacity of this kind of insulation sleeve is strong, and the insulation riser modulus is 50% higher than that of the general riser. Riser feeding efficiency η It is related to the perimeter quotient Q of sand casting (shape coefficient of steel casting). The calculation formula of casting perimeter quotient q is:
Where,
Q – Perimeter quotient of sand casting; VC – volume of sand casting; MC – modulus of sand casting.
Calculate the relevant data and get q = 1427.5. According to the data, check the table to get the feeding efficiency of the selected riser η = 45%。 To sum up, the calculation determines that the insulation riser modulus RM = 6.18mm; According to the casting manual, the riser size is h = 48mm, d = 32mm, and the riser volume RV = 0.0343l. Three such insulation risers are selected, and the maximum volume that can be fed is 3vf = 0.046l.
Check the correctness of riser size design with the maximum volume of steel castings to be fed vs:
Substitute the data to get SV = 0.03l. 3vf is greater than vs, so the size design of the insulation riser is reasonable.