Electric nutrunner

Claim 2

An electric nutrunner for tightening screw joints to a predetermined tightening torque or clamping force, said nutrunner comprising: voltage supply means; motor-starting means coupled to said voltage supply means; drive means, including a DC motor, coupled to said motor-starting means, for tightening a screw joint; control means coupled to said drive means; and means coupling said control means with said voltage supply means; said control means including: signal generating means for measuring the current flowing through said D.C. motor of said drive means, said signal generating means being responsive to a measured D.C. current value corresponding to said predetermined tightening torque or clamping force being reached for providing a terminating signal for causing the stopping of said D.C. motor, thereby causing the tightening of the screw joint to be terminated; and compensating control means coupled to said signal generating means for measuring the growth rate of said current flowing through said D.C. motor and being responsive to said measured growth rate which corresponds to joint stiffness for causing said signal generating means to provide said terminating signal as a function of said measured growth rate, thereby providing compensation for a variation in the terminating torque of the screw joint as the result of varying joint stiffness by varying the time of generation of said terminating signal.

Claim 3

The electric nutrunner of claim 1, wherein said signal generating means and compensating control means comprises an inductance-free resistor connected in series with said motor of said drive means and an inductor connected in series with said resistor; a comparator connected across said resistor and inductor for comparing the voltage across said resistor and inductor with a preset reference voltage; and stopping means coupled to said comparator and to said motor, said comparator delivering an output signal to said stopping means upon the occurrence of coincidence of said reference voltage and said voltage across said resistor and inductor for interrupting the tightening sequence of the nutrunner by stopping said motor.

Claim 4

The electric nutrunner of claim 1 or 2, comprising means coupled to said control means and to said motor for electrically braking said motor responsive to said terminating signal.

Claim 5

The electric nutrunner of claim 3, wherein said braking means includes means for short-circuiting said motor responsive to said terminating signal.

Claim 6

The electric nutrunner of claim 1 or 2, wherein said motor comprises permanently magnetized motor having an iron-free rotor.

Claim 7

The electric nutrunner of claim 5, wherein said rotor comprises magnets of rare earth metal/cobalt type.

Claim 8

The electric nutrunner of claim 1, wherein said motor has a low moment of inertia.

Claim 9

An electric nutrunner for tightening screw joints to a predetermined tightening torque or clamping force, said nutrunner comprising: voltage supply means; motor-starting means coupled to said voltage means; drive means, including a DC motor, coupled to said motor-starting means, for tightening a screw joint; control means coupled to said drive means; and means coupling said control means with said voltage supply means; said control means including; signal generating means for measuring electrical quantities corresponding to the motor voltage and current of said D.C. motor of said drive means, said signal generating means being responsive to a measured electrical quantity value corresponding to said predetermined tightening torque or clamping force being reached for providing a terminating signal for causing the stopping of said D.C. motor, thereby causing the tightening of the screw joint to be terminated; and compensating control means coupled to said signal generating means for measuring the growth rates of said measured quantities corresponding to said motor voltage and current of said D.C. motor and being responsive to said measured growth rates which correspond to joint stiffness for causing said signal generating means to provide said terminating signal as a function of said measured growth rates, thereby providing compensation for a variation in the terminating torque of the screw joint as the result of varying joint stiffness by varying the time of generation of said terminating signal.

Claim 10

An electric nutrunner for tightening screw joints to a predetermined tightening torque or clamping force, said nutrunner comprising: voltage supply means; motor-starting means coupled to said voltage means; drive means, including a DC motor, coupled to said motor-starting means, for tightening a screw joint; control means coupled to said drive means; and means coupling said control means with said voltage supply means; said control means including; signal generating means for measuring electrical quantities corresponding to the motor voltage and current of said D.C. motor of said drive means, said signal generating means being responsive to a measured electrical quantity value corresponding to said predetermined tightening torque or clamping force being reached for providing a terminating signal for causing the stopping of said D.C. motor, thereby causing the tightening of the screw joint to be terminated; and compensating control means coupled to said signal generating means for measuring the growth rates of said measured quantities corresponding to said motor voltage and current of said D.C. motor and being responsive to said measured growth rates which correspond to joint stiffness for causing said signal generating means to provide said terminating signal as a function of said measured growth rates, thereby providing compensation for a variation in the terminating torque of the screw joint as the result of varying joint stiffness by varying the time of generation of said terminating signal.

Claim 11

The electric nutrunner of claim 8, comprising means coupled to said control means and to said motor for electrically braking said motor responsive to said terminating signal.

Claim 12

The electric nutrunner of claim 9, wherein said braking means includes means for short-circuiting said motor responsive to said terminating signal.

Claim 13

The electric nutrunner of claim 8, wherein said motor comprises a permanently magnetized motor having an iron-free rotor.

Claim 14

The electric nutrunner of claim 11, wherein said rotor comprises magnets of rare earth metal/cobalt type.

Claim 15

The electric nutrunner of claim 8, wherein said motor has a low moment of inertia.