Resistance spot welder

1. A resistance spot welder comprising:

a motor, having a shaft;

a cylindrical casing;

a hollow guide rod, inserted into the casing, and adapted to be moved in a linear direction along a center axis of the guide rod for moving a first electrode tip facing a second electrode tip provided at a gun arm;

a ball nut, fixed in an inner circumference of the guide rod;

a ball screw, fitted though the hall nut;

a coupling, including a first fastening portion into which the shaft of the motor is inserted and a second fastening portion into which the ball screw is inserted and which is integral with and opposite to the first fastening portion, the first fastening portion defining a hollow cylinder concentric with the center axis and connecting to the second fastening portion via a step, the second fastening portion also defining a hollow cylinder concentric with the center axis; and

a bearing, fitted between an outer circumference of the first fastening portion and an inner circumference of the casing, wherein,

the guide rod is moved in the linear direction, between a first point where the guide rod is retracted toward a motor side and a second point where the guide rod is extended toward an electrode tip side,

the first fastening portion and the second fastening portion are arranged in the linear direction, the first fastening portion being on the motor side, the second fastening portion being on the electrode tip side, and

when the guide rod is positioned at the first point where the guide rod is retracted toward the motorside, the second fastening portion is disposed in the inner circumference of the hollow guide rod,

wherein the inner circumference of the guide rod is no more than the outer circumference of the first fastening portion in diameter, and the inner circumference of the guide rod is no less than an outer circumference of the second fastening portion.

2. The resistance spot welder according to claim 1 , wherein

an inner circumference of the second fastening portion is formed with a step portion on which an end of the ball screw is fixed,

an inner circumference of the bearing is fixed to the outer circumference of the first fastening portion and an outer circumference of the bearing is fixed to the inner circumference of the casing, and

the shaft of the motor is fixed in an inner circumference of the first fastening portion of the coupling.

3. The resistance spot welder according to claim 1 , wherein

a motor bracket is arranged between the motor and the casing, and

the shaft of the motor is fixed in the first fastening portion of the coupling.

4. The resistance spot welder according to claim 1 , wherein

the inner circumference of the guide rod is no more than the outer circumference of the first fastening portion in diameter, and

the inner circumference of the guide rod is no less than an outer circumference of the second fastening portion in diameter.

5. The resistance spot welder according to claim 1 , wherein

the shaft of the motor has one of a cylindrical shape and a tapered shape, and

an adapter is fitted on the shaft of the motor.

6. The resistance spot welder according to claim 1 , wherein

a side wall of the casing is formed with an opening portion extending within a range between the first point and the second point,

a cam groove member is provided on the opening portion and includes at least one guide rail, and

the guide rod includes at least one roller follower guided along the at least one guide rail.

7. The resistance spot welder according to claim 1 , wherein

an oilless bush is disposed in the inner circumference of the casing at the electrode tip side,

a scraper is disposed on the oilless bush at the electrode tip side, and

a cap is mounted on the casing from the electrode tip side.

8. The resistance spot welder according to claim 1 , wherein

the gun arm is formed with a piping groove to be a cooling medium passage,

a cooling tube having a coating cover made of a refractory rubber material or a refractory synthetic resin material is fitted and fixed in the piping groove, and

the cooling tube has one end connected to one of a secondary conductor connected to a welding transformer and a water-cooled manifold and the other end connected to a cooling medium passage leading to an inside of the second electrode tip fixed at the gun arm.

9. The resistance spot welder according to claim 6 , wherein

the cam groove member includes a bottom plate and two guide rails which are parallel with each other and stand on both ends of the bottom plate, and

the at least one roller follower is guided between the two guide rails.

10. The resistance spot welder of claim 8 , wherein

the cooling tube includes an inner tube made of the refractory synthetic resin material under the coating cover.

11. The resistance spot welder according to claim 9 , wherein

a plate cover for closing the at least one roller follower and the cam groove member is provided on the opening portion of the casing.

12. The resistance spot welder according to claim 11 wherein

the bottom plate of the cam groove member is fixed on the plate cover.

13. The resistance spot welder according to claim 10 , wherein

the gun arm is made of a non-ferrous material of an aluminum group and includes a gripping portion having an opened U-shape at a leading end thereof,

the gripping portion communicates with the piping groove, and

a tip base having the second electrode tip is inserted into the opened U-shape of the gripping portion and held by a fastening member applying a force in a direction to close the opened U-shape.

14. The resistance spot welder according to claim 13 , wherein

the tip base has a cooling medium passage connected to the inner tube of the cooling tube.