1. Field of the Invention
The invention relates to a device for coupling an electric motor to a load and an electric motor especially constructed therefor and also to a mechanical coupling element to be used in the device and motor.
2. Discussion of Background
In many machines electric motors are used for driving loads that require bearings other than those that are placed within the electric motor itself. In such cases the motor shaft has normally bearings inside the motor and the load has bearings inside the machine. In coaxial arrangements, the motor shaft is then connected to the shaft of the load by some kind of mechanical coupling. The coupling has the purpose of transmitting the torque as provided by the motor to the load shaft with a sufficient elasticity in order to be capable of absorbing normally occurring radial and angular misalignments between the motor shaft and the load shaft. The coupling should perform this with an acceptably low torsional elasticity and with an acceptably low mechanical play.
Both servo motors and conventional electric motors for less precise tasks like driving pumps, fans and conveyor belts are often connected to their loads using couplings that require considerable additional length in the axial direction.
Some devices for reducing the axial lengths of an assembly of two coaxial units each one having radially stiff bearings are known in the prior art. Many of these devices concern small auxiliary units necessary for the operation of a far larger main machine. These auxiliary units, such as tachogenerators, resolvers, encoders and exciters, require very low or almost negligible torques what permits simple coupling arrangements.
U.S. Pat. No. 2,956,187 discloses how an exciter can be connected to a synchronous generator using a annual hole drilled in the end tap of the main shaft of the synchronous generator. Both the exciter and the generator have bearings in both ends of the shafts. The diameter of the generator shaft is reduced in the end in the way commonly used to fit a ball bearing in the rear end of the generator. The torque required to run the exciter is much smaller than the torque required to run the generator. Therefore the coupling used to drive the exciter can be inserted in an annual hole drilled in this bearing tap of the generator shaft without affecting the required exterior diameter of the generator shaft or its bearings.
The Swedish published patent application SE-A 352 491 discloses a generator to be attached to a wheel shaft of a railway vehicle. In order to reduce the axial length, the rotor bearings are located close to each other inside the rotor. A flexible coupling outside one side of the rotor drive a low speed shaft concentric with the rotor shaft. This low speed shaft extends through the full length of the generator rotor and drives a planetary gearbox outside the other side of the rotor. The rotor is driven from this planetary gearbox at an angular speed some seven times higher than that of the driving low speed shaft.
U.S. Pat. No. 3,560,774 discloses a stepping motor having a tube-shaped rotor that does not seem to have any bearings. The output shaft runs through the tube-shaped rotor. The tube-shaped rotor seems to be always in contact with the inner surface of the stator. The inner diameter of the stator is larger that the outer diameter of the rotor. The rotor moves eccentrically relative to the output shaft. Coupling(s) are therefore necessary to transfer the rotor torque to the output shaft. These couplings are located between the two output shaft bearings.
The published Swedish patent application SE-A 369 808 discloses a motor driving a pump. The pump shaft has a full set of bearings, but the motor does only have a bearing in the end thereof which is remote from the pump. In the rotor end facing the pump, the rotor has a flexible end disc in the centre of which an annular device having a key slot is inserted. This key slot fits the driven pump shaft, that has a key which fits the key slot of the rotor. The problem of aligning two shaft with radially fixed bearings is in this case solved by eliminating one of the motor bearings.
The West German patent DE-C2 28 19 765 discloses a tachometer generator. The torque required for driving the generator is supplied through a shaft that enters the rotor and is connected to an internal rotor shaft by a flexible coupling inside the rotor. The shafts as shown in this patent are very thin compared with the normally extremely stiff shafts between a servo motor and its tachogenerator, that is the shafts have diameters of 1/11th of the generator outer diameter and some 1/8 of the rotor diameter. The torsional stiffness required is however very low as the application cited concerns speed measurement of a railway vehicle. As the accelerations of railway vehicles is very low, the torque required for the acceleration of the tachometer rotor is also very low, and the torque required for supplying the electric power taken from a tachogenerator is negligible. It is pointed out in the patent that a tachogenerator according thereto will have a significantly larger diameter than conventional tachogenerators but also have a shorter axial length. It is also pointed out that in the applications intended for the invention, the space occupied by the larger generator diameter earlier has been unused as the device the speed of which is to be measured has a diameter that is far larger than that of the tachometer. It is also pointed out that the increased generator diameter permits a far higher number of poles, thus permitting an output signal with less ripple.
The published British patent application GB-A 2 117 572 discloses an alternative to a flexible shaft coupling when connecting a position sensor to a motor. The sensor and the motor are coaxial units each having radially stiff bearings. Instead of having the frames of the sensor and motor rigidly connected and using a flexible coupling between the shafts, the shafts are joined by a stiff coupling and the two frames are joined by a torsionally rigid but radially flexible diaphragm. The embodiments shown, see e.g. FIG. 4, do not seem to permit shorter axial lengths that of conventional embodiments using bellows couplings, compare FIG. 2.
U.S. Pat. No. 4,577,127 discloses a robot actuator comprising an electric motor inserted inside the cup-shaped casing of a harmonic drive gear box. In the centre of the concentric motor and harmonic drive, there is a coupling 135, see FIG. 3. A shaft, 133 and 131 in FIG. 3 from the coupling drives a resolver 118 and 141 that is located coaxially with the motor and the harmonic drive gear box but outside the motor and harmonic drive.