Screw rotor

Abstract

The present invention is to provide a screw rotor including a resin rotor formed around a metallic shaft without generation of cracks. Spiral chamfers are formed on surfaces of metallic shafts around which resin rotors are formed. Preferably the surfaces of the shafts may be sandblasted, and after the surfaces of the shafts are preliminarily coated with resin and then the rotors may be molded.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a screw rotor according to an embodiment of the present invention;

FIG. 2 is a plan view of a shaft of a male rotor in FIG. 1;

FIG. 3 is a plan view of a shaft of a female rotor in FIG. 1; and

FIG. 4 is a partially enlarged cross sectional view of the shaft of the female rotor in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will be given of an embodiment of the present invention with reference to the drawings.

FIG. 1 shows a cross section of a screw rotor for compressor of an embodiment of the present invention. The screw rotor according to the present embodiment includes a pair of male rotor 1a and a female rotor 1b. Resin rotors 3a and 3b are molded around shafts 2a and 2b which are made of stainless steel SUS420F2 respectively for the male rotor 1a and the female rotor 1b.

The rotors 3a and 3b are molded in such a manner that the shafts 2a and 2b are arranged in molds, a resin such as epoxy resin is poured into the molds, the molds are heated for example to 150° C., and the resin is hardened. Since the resin preferably has a high strength, a high modulus and a dimensional stability, preferable examples of the resin are epoxy resin and urethane resin which include silica fillers or glass fibers as a reinforcing material.

The shaft 2a of the male rotor 1a according to the present embodiment has a diameter of 76 mm, and the rotor 3a having an outer diameter of 154.4 mm and a length of 248.6 mm is a left hand five teeth rotor. Meanwhile, the shaft 2b of the female rotor 1b has a diameter of 54 mm and the rotor 3b having an outer diameter of 132.2 mm and a length of 243.6 mm is a right hand six teeth rotor.

Further, as shown in FIGS. 2 and 3, spiral chamfers 4a and 4b are formed on the shafts 2a and 2b respectively so as to extend directly below tooth root parts of the rotors 3a and 3b. As the female rotor 1b representatively shown in detail in FIG. 4, the chamfers 4a and 4b are formed by flatly cutting the shafts 2a and 2b by a depth of 1.5 mm (2% and 1.1% of the shaft diameters). The chamfer 4a is formed as five streaks and the chamfer 4b is formed as six streaks in correspondence with the number of tooth.

Such chamfers 4a and 4b can be easily formed by placing a plane milling cutter at right angles to the shafts 2a and 2b, and then cutting the shafts 2a and 2b on a multiple lathe for example.

In the male rotor 1a and the female rotor 1b which are formed as above, since the chamfers 4a and 4b play a role of key, a fixing force between the shafts 2a and 2b and the rotors 3a and 3b is strong so as to bear a high torque.

An angle between the chamfers 4a and 4b and outer peripheral surfaces of the shafts 2a and 2b is very obtuse. Therefore, there is no difference in level formed on inner surfaces of the rotors 3a and 3b, stress is only slightly concentrated and cracks are not easily generated in the rotors 3a and 3b.

When the rotors 3a and 3b are formed after surfaces of the shafts 2a and 2b according to the present embodiment axe sandblasted, it is possible to further improve the fixing force between the shafts 2a and 2b and the rotors 3a and 3b.

According to the present embodiment, the surfaces of the shafts 2a and 2b are coated with a resin having good adhesive property to metals such as Araldite, the rotors 3a and 3b are arranged in molds and a resin is poured into so as to form the rotors 3a and 3b. Subsequently, both of the resin (the coated resin and the poured resin) are hardened by heating. The resin coated over the surfaces of the shafts 2a and 2b enhances the fixing force between the shafts 2a and 2b and the rotors 3a and 3b and the rotors 3a and 3b are not easily separated from the shafts 2a and 2b.

The present invention may use an epoxy resin as the coated resin over the surfaces of the shafts since it has a good adhesive property to metals. Examples of preferable epoxy resin include bisphenol A epoxy resin, urethane modified epoxy resin and rubber modified epoxy resin which are thermosetted by hardening agent such as polyamide, polyaminoamide, aliphatic polyamine, alicyclic polyamine, aromatic polyamine and acid anhydride.

It can be thought that the rotors 3a and 3b are molded by urethane resin or the like having less adhesive property to metals than epoxy resin. In this case, it is more effective to mold the rotors 3a and 3b after preliminarily coating the surfaces of the shafts 2a and 2b with the resin.

In a state that the tensile stress is given to the shafts 2a and 2b according to the present embodiment, the rotors 3a and 3b are formed with resin around the shafts, and the tensile stress to the shafts 2a and 2b is removed after the rotors 3a and 3b are hardened. Consequently, it is possible to give the compressive stress to the rotors 3a and 3b at the normal time by shrinkage of the shafts 2a and 2b.

At the time of driving the screw rotor, the acting tensile stress facilitates the generation of cracks on the inner side of the rotors 3a and 3b. However, by preliminarily giving the compressive stress to the rotors 3a and 3b, it is possible to ease the substantially acting tensile stress so as to suppress the generation of cracks.

Such compressive stress can also be given by heating the shafts 2a and 2b and arranging the shafts in the molds in a state of thermal expansion, charging the resin around the shafts so as to mold the rotors 3a and 3b, and cooling the shafts 2a and 2b after hardening of the rotors 3a and 3b.

On the basis of the above embodiment, the following screw rotors are manufactured as experimental examples and comparative examples, and the strength thereof are tested.

Experimental Example 1

The male rotor 1a and the female rotor 1b are manufactured as an experimental example 1.

Experimental Example 2

An experimental example 2 is formed in such a manner that the rotors 3a and 3b are molded after the surfaces of the shafts 2a and 2b are sandblasted.

Experimental Example 3

An experimental example 3 is formed in such a manner that the rotors 3a and 3b are molded by the surfaces of the shafts 2a and 2b are coated with Araldite resin.

Experimental Example 4

An experimental example 4 is formed in such a manner that the rotors 3a and 3b are molded in a state that the tensile load of about 10 kgf/mm² is given to the shafts 2a and 2b.

Experimental Example 5

An experimental example 5 is formed in such a manner that the rotors 3a and 3b are molded after heating the shafts 2a and 2b to 300° C. and arranging the shafts in the molds. It should be noted that the time required for the hardening of the rotors 3a and 3b is about one hour, and a temperature of the shafts 2a and 2b at the time when the resin of the rotors 3a and 3b is hardened is about 200° C.

Comparative Example 1

A comparative example 1 is formed in such a manner that spiral grooves as described in Japanese Patent Laid-Open No. Hei6-123292 are formed in shafts having the same diameters as the shafts 2a and 2b and the rotors 3a and 3b are molded around the shafts.

Comparative Example 2

A comparative example 2 is formed in such a manner that spiral grooves whose cross sections are connected by a smooth curve as described in Japanese Patent No. 3701378 are formed in shafts having the same diameters as the shafts 2a and 2b and the rotors 3a and 3b are molded around the shafts.

The experimental examples and the comparative examples mentioned above are manufactured. In the comparative example 1, at the stage where the rotors 3a and 3b are hardened, the cracks are already generated on the surfaces of the rotors 3a and 3b.

With regard to the remaining experimental examples 1 to 5 and comparative example 2, when appearance thereof is observed again after the screw rotor is built in the compressor and driven for one mouth, the cracks are generated on an upper part of the difference in level for back clearance of cutters of both ends in the rotors 3a and 3b of the comparative example 2.

Since no damage is observed in the experimental examples 1 to 5, the screw rotors thereof are built in the compressor and driven for a total of six months. Even after that, however, no damage is observed and the performance of the compressor is not lowered.

Therefore, a high torque is given to the screw rotors of the experimental examples 1 to 5 until fractures are generated so as to measure a fracture torque and obtain the following results.

TABLE 1
Sample                 Fracture torque (kgf · m)
Experimental Example 1 256
Experimental Example 2 290
Experimental Example 3 302
Experimental Example 4 277
Experimental Example 5 273

Normally, the torque given to the screw rotors 1a and 1b is about 100 kgf·m at most. Therefore, the above fracture torque shows that each of the experimental examples has a sufficient bearing force.

In the experimental examples 2 to 5, the fracture torque is improved in comparison to the experimental example 1. Therefore, it is confirmed that production processes added to the experimental example 1 contribute to the improvement of the bearing force of the screw rotors 1a and 1b.

Claims

1. A screw rotor comprising a resin rotor formed around a metallic shaft, a spiral chamfer being formed on a surface of said shaft.

2. The screw rotor according to claim 1, wherein the surface of said shaft is sandblasted.

3. The screw rotor according to claim 1, wherein the surface of said shaft is preliminarily coated with resin, and then said rotor is molded.

4. The screw rotor according to claim 1, wherein said chamfer is formed directly below a tooth root part of said rotor.

5. The screw rotor according to claim 1, wherein when forming said rotor, tensile load is given to said shaft in the axial direction, and after hardening of said rotor, said tensile load is removed.

6. The screw rotor according to claim 1, wherein when forming said rotor, said shaft is made to a higher temperature than the resin, and after hardening of said rotor, said shaft is made to a normal temperature again.