Vacuum pump

A vacuum pump includes a rotor assembly mounted on a driven shaft, and a motor for rotating a drive shaft in forward and reverse directions. A drive member on the drive shaft engages a driven member on the driven shaft. Each member has first and second impact surfaces. The members are configured to permit at least one quarter of a revolution of the drive member relative to the driven member in either the forward or the reverse direction before one of the impact surfaces of the drive member impacts upon a corresponding impact surface of the driven member. This enables the drive member to acquire sufficient angular momentum before it impacts the driven member such that the amount of energy transferred to the driven shaft upon impact can be sufficient to free a pump that has become locked by process deposits.

FIELD OF THE INVENTION

This invention relates to a vacuum pump, and in particular to a vacuum pump having an improved re-start performance following seizure. The invention also relates to an apparatus for coupling a driven shaft of a vacuum pump to a drive shaft.

BACKGROUND OF THE INVENTION

Vacuum pumping arrangements used to pump fluid from semiconductor tools typically employ, as a backing pump, a multi-stage positive displacement pump employing inter-meshing rotors. The rotors may have the same type of profile in each stage or the profile may change from stage to stage.

A number of semiconductor processes can produce a significant amount of by product material in the form of powder or dust, especially if the process gas is condensable and sublimes on lower temperature surfaces. This material can be formed in the process chamber, in the vacuum line, or “foreline”, between the chamber and the pump, and/or in the vacuum pump itself. The material can be in a soft powder form or it can become hard and compacted. Within the pump, such material can accumulate within the vacant running clearances between the rotor and stator elements in the pump, reducing the size of the clearances. While the pump is running continuously, this does not present any problem, but in the event that the pump is switched off (either intentionally for system maintenance or unintentionally in the event of an unexpected power supply interruption) the pump will cool and the size of the running clearances will decrease. Depending on the state of the powder accumulation, this could cause the accumulated material to become compressed between the rotor and stator elements. Due to the relatively large surface area of potential contact that this creates between the rotor and stator elements, such compression of by-product material can significantly increase the frictional forces opposing rotation. When it is then attempted to re-start the pump, the torque available from the pump motor may be inadequate to overcome these frictional forces, resulting in a re-start failure. The current trend towards inverter driven pumps increases the likelihood of re-start failure, as such motors have a lower starting torque than direct-on-line motors conventionally used to drive the rotor elements of vacuum pumps.

It is an aim of at least the preferred embodiment of the present invention to seek to improve the re-start performance of vacuum pumps.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a vacuum pump comprising a pumping chamber, a rotor assembly mounted on a driven shaft for rotation within the pumping chamber, a motor for rotating a drive shaft in forward and reverse directions, a driven member located on or carried by the driven shaft and a drive member located on or carried by the drive shaft for engaging the driven member to couple the driven shaft to the drive shaft, each member having first and second impact surfaces, the members being configured to permit a degree of free angular movement of the drive member relative to the driven member in either the forward or the reverse direction before one of the impact surfaces of the drive member impacts upon a corresponding impact surface of the driven member to transfer angular momentum from the drive member to the driven member.

In a second aspect the present invention provides apparatus for coupling a driven shaft of a vacuum pump to a drive shaft, the driven shaft having mounted thereon a rotor assembly for rotation within a pumping chamber of the pump, the apparatus comprising a drive member locatable on the drive shaft and a driven member locatable on the driven shaft so as to enable free angular movement of the drive member relative to the driven member before an impact surface of the drive member impacts upon a corresponding impact surface of the driven member to transfer angular momentum from the drive member to the driven member, the extent of the free angular movement of the drive member relative to the driven member being at least one quarter of a revolution.

In a third aspect the present invention provides a method of restarting a vacuum pump as aforementioned following a re-start failure, the method comprising the steps of rotating the drive shaft in one direction to cause the first impact surfaces to come into contact, and rotating the drive shaft in the opposite direction to cause the second impact surfaces to come into contact.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a vacuum pump comprising a pumping chamber, a rotor assembly mounted on a driven shaft for rotation within the pumping chamber, a motor for rotating a drive shaft in forward and reverse directions, a driven member located on or carried by the driven shaft and a drive member located on or carried by the drive shaft, each member having first and second impact surfaces, the members being configured to permit a degree of free angular movement of the drive member relative to the driven member in either the forward or the reverse direction before one of the impact surfaces of the drive member impacts upon corresponding impact surface of the driven member to transfer angular momentum from the drive member to the driven member.

Rotation of the drive member relative to the driven member allows energy to be stored in the form of angular momentum, which can be instantaneously transferred to the drive shaft upon impact between the impact surfaces. This produces a “hammer effect” which can be many times the locked rotor torque of the motor alone. These impacts will cause the rotor assembly to shake, loosen and re-distribute any debris accumulated within the running clearances between the rotor and the stator elements of the pump. As the drive shaft is rotatable in opposite directions, such impacts can be repeatedly applied in both the forward and reverse directions in quick succession, which can cause the rotor assembly to grind away at any compacted powder located within the running clearances. Consequently, the likelihood of the pump becoming freed from any seizure due to the accumulation of powder within the pump can be significantly improved, thus improving pump reliability and reducing maintenance intervals.

The impact surfaces are preferably spaced about the members so that the extent of the free angular movement of the drive member relative to the driven member is at least one quarter of a revolution, preferably around half a revolution. This can maximise the angular momentum acquired before the drive member impacts the driven member so as to maximise the amount of energy instantaneously transferred to the driven shaft upon impact. This relatively large amount of free play of the drive member relative to the driven member can facilitate independent attachment to the pump gearbox housing, and thereby a very convenient motor coupling to be provided. Thus, in a second aspect the present invention provides apparatus for coupling a driven shaft of a vacuum pump to a drive shaft, the driven shaft having mounted thereon a rotor assembly for rotation within a pumping chamber of the pump, the apparatus comprising a drive member locatable on the drive shaft and a driven member locatable on the driven shaft so as to enable free angular movement of the drive member relative to the driven member before an impact surface of the drive member impacts upon a corresponding impact surface of the driven member to transfer angular momentum from the drive member to the driven member, the extent of the free angular movement of the drive member relative to the driven member being at least one quarter of a revolution.

Features described above in relation to apparatus aspects of the invention are equally applicable to method aspects, and vice versa. Therefore, in a third aspect the present invention provides a method of restarting a vacuum pump as aforementioned following a re-start failure, the method comprising the steps of rotating the drive shaft in one direction to cause the first impact surfaces to come into contact and rotating the drive shaft in the opposite direction to cause the second impact surfaces to come into contact. drive member

With reference toFIG. 1, a vacuum pump10comprises a pumping chamber12through which pass a pair of parallel shafts14(one only shown inFIG. 1) supported by bearings (not shown). A rotor assembly16is mounted on each shaft14for rotation within the pumping chamber12, the rotor assemblies16having complementary pumping profiles, for example Roots, Northey (or “claw”) or screw, such that fluid to be pumped is drawn into an inlet of the pumping chamber12and exits from the pumping chamber12via an outlet. A motor18is provided for rotating one of the shafts14, hereinafter referred to as the driven shaft14, the other shaft14being rotated synchronously with the driven shaft14by means of the meshed timing gears (not shown). Alternatively, the pump10may comprise a single drive shaft14with a rotor assembly mounted on that shaft14. Examples of such pumps include rotary vane pumps and scroll pumps.

A controller20is provided for controlling the operation of the motor18. The controller20is configured to control, inter alia, the starting and stopping of the motor18, the speed of the motor18, and the direction of the motor18, so that a drive shaft22coupled to the motor18can be rotated in either a forward or a reverse direction. The direction of the motor18may be changed using an inverter drive. Alternatively, for a direct-on-line motor, the direction may be changed using two sets of switches50, as illustrated inFIG. 5. These switches may be comprised of contactors or solid-state devices, for example but not limited too: IGBTs, TRIACs or thyristors. These switches may be activated by a voltage or current signal output from a control circuit52.

With reference toFIGS. 1 and 2, a coupling24is provided for-coupling the driven shaft14to the drive shaft22of the motor18. The coupling24comprises a pair of similar halves26,28, hereinafter referred to as the pump coupling half26and the motor coupling half28. The pump coupling half26is mounted on the driven shaft14, and the motor coupling half28is mounted on the drive shaft22. Alternatively, at least one of the two halves of the coupling24may be integral with its respective shaft.

Each half of the coupling24comprises a base30mounted on the respective shaft14,22and a pair of axially-extending protrusions32mounted on or integral with the base30. The protrusions32are located diametrically opposite each other on or towards the external periphery of the base30, such that the protrusions32are eccentric with respect to the longitudinal axis of the respective shaft. When the halves of the coupling24are mounted on the shafts14,22, the protrusions32aof the pump coupling half26are angularly offset relative to the protrusions32bof the motor coupling half28to enable the protrusions to intermesh, as illustrated inFIG. 1.

The sides of each protrusion32provide first and second impact surfaces34,36of the coupling24. With reference toFIG. 3, when the motor18is driven in a forward direction, the first surfaces34of the protrusions32aon the pump coupling half26contact the first surfaces34of the protrusions32bof the motor coupling half28to couple the drive shaft22to the driven shaft14. With reference toFIG. 4, when the motor18is driven in a reverse direction, the second surfaces36of the protrusions32aon the pump coupling half26contact the second surfaces36of the protrusions32bof the motor coupling half28to couple the drive shaft22to the driven shaft14. In order to hold the impact surfaces together, the protrusions of one of the coupling halves26,28may be formed from magnetic material, with the protrusions of the other half being formed from material such as, cast iron to which the magnetic protrusions are attracted. This can reduce noise or vibration when the pump is running that may otherwise occur if the protrusions were freely in contact. Alternatively, both sets of protrusions may be formed from magnetic material, the protrusions of one half having a different polarity to the protrusions of the other half.

As indicated inFIG. 3, the arrangements of the protrusions32allows a certain amount of free angular movement of the motor coupling half28relative to the pump coupling half26before the impact surfaces come into contact to transmit drive from the drive shaft22to the driven shaft14. With the protrusions32being located on opposite sides of the base30of each coupling half, the maximum extent of this angular movement is just under one half of a revolution. The actual extent will be dependent upon the size, shape and number of the protrusions32. In the illustrated example, the extent of the angular movement is around one third of a revolution. Increasing the number of protrusions32will decrease the extent of the angular movement.

This free angular movement of the motor coupling half28relative to the pump coupling half26can provide a number of-advantages. Firstly, it can facilitate independent attachment of the coupling halves to their respective shafts before the motor is offered up and fitted to the pump gearbox housing. Secondly, it can allow acceleration and the storage of energy in the form of angular momentum in the motor rotor and drive shaft22before the impact between, depending on the direction of the motor18, the first or second, impact surfaces34,36. When the impact surfaces come into contact, this energy is instantaneously applied to the driven shaft14, resulting in an impact or hammer effect with many times the locked rotor torque of the motor alone. As the coupling is bidirectional, in that it can couple the driven shaft14to the drive shaft22both when the motor is in the forward or reverse direction, such impacts can be repeatedly applied in both the forward and reverse directions to free a seized pump. The amount of energy transferred to the driven shaft14will depend, inter alia, on the angular distance between the protrusions32a,32b. Thus, in an alternative to the embodiment shown inFIGS. 2 to 4, each coupling half26,28comprises a single protrusion32a,32bin order to maximise this angular distance.

If an inverter is used to change the direction of the motor18, a simple routine may be implemented to free a seized pump. Such a routine may first briefly apply a reverse pulse to prime the coupling24so that the coupling halves26,28take the relative positions shown inFIG. 4, with the protrusions32bof the motor coupling half28located at one extreme of the extent of the free angular movement. Then, a forwards pulse is applied to rotate the protrusions32bof the motor coupling half28to cause the first impact surfaces34of the protrusions to impact to transfer energy to the pump coupling half26and thus to the drive shaft14. If the protrusions of one of the members are magnetic, then this has the advantage of holding the second impact surfaces36together following the application of the reverse pulse, that is, with the maximum angular separation so that when the forwards pulse is-applied the maximum amount of angular momentum can be stored before the first impact surfaces34come into contact. The forwards pulse is applied for a period sufficient to enable the inverter to detect whether the pump has become free, and is starting to run up to working speed.

If the pump fails to re-start, which may be readily detected by the inverter, then a second cycle of a brief reverse pulse followed by a forwards pulse can be applied and repeated any number of times as necessary to free the pump. By repeated application of such a routine, the rotor assembly16can be forced to rotate and clear process deposits to allow re-start of the pump.

While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.