- Interference fit classification
There are three types of interference fit commonly used in machinery manufacturing and equipment maintenance: force fit, thermal expansion fit and shrink fit.
- Interference fit object
- Assemble various hot molds and cold molds
- Assemble various speed change boxes, gears of gearboxes and axles
- Assemble precision machinery elements
- Assemble various motor stator and rotors
- Auto parts and engine parts
- Wide range of application
It is suitable for cryogenic treatment of high speed steel tools, blade, oil pump, nozzle, miniature, motor axle, automobile bearing, golf club head, roller, cryogenic valve, tool, measuring implement, mould, foundry industry, hardware industry, auto parts, textile parts, precision stamping die, nanometre material, precision plastic mould, cutter, hobbing cutter, screw punch, knitting machine parts, precision spring, aluminium alloy material, axis of the motor, screw thread plate, power metallurgy mould, sealed mould, tungsten carbide material, air tool part, the car engine, aluminium alloy, copper alloy, hard alloy, plastic, rubber, asphalt, glass, mine geology drill, spring, gear, bearing, saw blades, the turboshaft of engine, valve, mechanical parts, medicine, bioengineering, aerospace and various metal materials. it is suitable for interference cooling and ultra-low temperature environment simulation of various parts.
2.The craft contrast and benefits of cold assembly
The disadvantage of press fit and thermal expansion fit:
The press fit with higher finish requirement of parts, generally over 8. The parts are easy to deform when assembled and pressed in.
When using thermal expansion, the hardness and strength of the parts may decrease and even produce oxidation skin and deformation in severe cases because of heating. Moreover, the technical requirements for assembly are very strict.
In the assembly work, sometimes we may encounter some mating parts with large outer parts and small inner parts. It’s hard to use a thermal fit because it’s hard to heat the outside. It is much easier to assemble by means of cold matching. Shrinkage coordination is to cool the inner parts to a very low temperature (-80 ℃ or lower), so that it occurs cold shrinkage.
- Compared with hot assembly and pressing assembly, cold assembly has the following advantages:
- Security: cold assembly process is more safe and reliable, the workpiece is not easy to damage. However hot assembly process site temperature is high, easy to cause burns, pressing into the assembly is easy to damage the workpiece.
- Manufacturability: Cold assembly quality is high, workpiece stability is good, strong bearing capacity. However the quality of thermal assembly is not easy to control, the workpiece is easy to deform, and the combination of the parts and the base hole is not firm when pressed into the assembly.
- Operability: Cold assembly operation is simple and convenient, economical and practical. However the thermal assembly process is complex, requiring workers to repeatedly check the aperture size, pressing into the assembly requirements of the hole and the center line of the shaft.
- Benefits:
In the past, the assembly could not be completed without workers using sledgehammers or cranes to lift heavy objects. This kind of operation has certain danger, once had a worker to bump finger to break in bump. However a person can use cold installation to complete small workpiece.Workers simply put the parts into the base hole. In the process of hot installation, the base hole parts need to be heated and then cooled in the air. This process is equivalent to a low-temperature annealing treatment of the parts, which reduces the hardness and strength of the parts. Cold installation is equivalent to a cold treatment of the parts, parts can be obtained after the cold treatment of the following effects:
- The reduction of the austenite can make the product permanent;
- Increase in hardness and its averaging;
- Improve mechanical performance;
- Dimensional stabilization;
- Increasing magnetism;
- Increased wear resistance.
- Thus improved the efficiency, reduced the labor intensity of the workers.
- Cryogenic process analysis
Cryogenic process can improve the performance of metal materials effective and economical technical indicators. During the cryogenic process, the residual austenite is transformed. And the change of the properties of the tiny dispersed carbides in the material can hinder the precipitation of the ultrafine carbides in dislocation motion. Thus, the matrix structure is strengthened, and the grain boundary strengthening effect is given play to, and the following three aspects are improved: impact toughness, wear resistance, size stability.
In industry, the process of further cooling the material to 100℃ ~ 196℃ after normal heat treatment is called cryogenic treatment. It can reduce the residual content, promote the precipitation of fine carbide, reduce the tensile stress between crystals, and improve the following macro properties:
Increase hardness: high speed steel, die steel increase 1~2 Rockwell hardness.
Improved wear resistance :Cr12MoV steel wear resistance increased by 30%, 20Ni3Mo carburize steel wear resistance increased by 89%.
Improved bending strength: the bending strength of 440A cast stainless steel is increased from 1135MPa to 1,355MPa.
Increased toughness :W6Mo5Cr4V2 increased the impact work from 18J to 40J.
Increase red hardness: the hardness of W18Cr4V was increased from 57.9 Rockwell hardness to 63.9 at 625℃.
Increased dimensional stability: 95Crl8 stainless steel retained Austenite reduced from 35% to 10%.
Improve the electrical conductivity copper alloy Cu – 15ni – 8sn resistivity from 2 to 15 86 x l0 Ω.M down to l. 5 l x l0-7 Ω.M.
With the development of cryogenic technology and the perfection of experimental means, the research on cryogenic treatment has been deepened step by step. The scope of research has now extended from iron making to powder metallurgy, copper gold, aluminum alloys and other non-metallic materials. Cryogenic box application industry throughout the aerospace, hardware, tools, molds, friction coupler, precision processing, measuring tools, textiles, automobiles and many other fields. Cryogenic technology is a new and high technology to improve the performance of metal workpiece.
It is a new environmental protection technology, characterized by low consumption of materials and electricity and no environmental pollution.
At present, the technology has been widely used in aerospace, shipping, military, manufacturing, automobile, hardware tools, sports equipment and other industries.