WO2007119074A1

WO2007119074A1 - A gear

Info

Publication number: WO2007119074A1
Application number: PCT/GB2007/050120
Authority: WO
Inventors: Jack Raymond Tattersall
Original assignee: Edwards Limited
Priority date: 2006-04-13
Filing date: 2007-03-13
Publication date: 2007-10-25
Also published as: JP2009533624A; JP5244784B2; EP2005033A1; GB0607502D0

Abstract

A torsionally compliant gear comprises a gear body (10) having a plurality of teeth (14) at an outer peripheral surface thereof, and a plurality of slots (16, 18) formed through the body. The slots define within the gear body a central hub (20), a rim (24) upon which the teeth are located, a plurality of spokes (22) connecting the rim to the hub, and a plurality of fins (26), extending radially from the hub and located circumferentially between the spokes, for restricting angular displacement of the rim relative to the hub.

Description

A GEAR

This invention relates to the field of torsionally compliant gears.

With reference to Figure 1 , a known vacuum pump includes a pumping chamber through which pass a pair of parallel shafts 1 supported by bearings 2. A rotor 3 is located on each shaft 1 for rotation within the pumping chamber. The rotors 3 have complementary pumping profiles, which may be Roots, Northey (or "claw") or screw. In use, when a motor 4 is driving one of the shafts 1 , the other shaft is rotated synchronously with that shaft by means of meshed timing gears 5 each located on a respective shaft 1. The rotors 3 are so profiled that fluid to be pumped is drawn into an inlet of the pumping chamber and exits from the pumping chamber via an outlet.

Figure 1 illustrates three different pumping configurations. In Figure 1 (a), the rotors 3 are mounted between the bearings 2 and the timing gears 5 are provided at the motor-driven end of the pump. In Figure 1 (b), the timing gears 5 are provided at the other end of the pump, and Figure 1 (c) illustrates a configuration using cantilevered rotors.

Transmission of torque through the meshed gears 5 is affected by gear eccentricity. As a region of smaller radius on the drive gear meshes with a region of larger radius on the driven gear there is a tendency for the latter to be consequently accelerated. However, since the driven gear is connected to a high inertia rotor, any change in speed is effectively prevented in the case of high inertia, lightly loaded, rotating machinery. The resulting mismatch between the gears' teeth can provoke high frequency tooth-to-tooth slapping, a characteristic noise frequently encountered in lightly loaded rotating machinery.

Pump design has been improved to reduce friction and power consumption. Consequently the required steady state drive torque, or torque required to drive the pump at an "ultimate" condition, has reduced and thus rotating machinery has become increasingly sensitive to eccentricity induced torsional vibrations and related noise problems.

The design of timing gears has developed to try to avoid generation of these torsional vibrations and noise. High precision components are therefore often used to avoid the aforementioned eccentricity, as a pair of truly concentric intermeshing gears would not experience the mismatch or the consequential noise generation. Alternatively, complex gear arrangements having a greater number of components are used to inhibit generation or transmission of the associated vibrations. Pumps using these complex devices often have a higher power requirement as the devices typically implement springs which need to be compressed each time a tooth is urged into a cooperating slot on an opposing gear. Variations in geometry can be accommodated by very carefully selecting pairs of gears that display the same level of eccentricity. The gears are subsequently assembled in a way that causes the intermeshing portions of each gear to be consistently of the same radius. This selective assembly technique permits any mismatch in pitch line speed to be avoided or at least minimised.

There is typically a cost and time impact associated with providing such high precision and complex components, or using selective assembly techniques. It is, therefore, desirable to provide a simple, lower cost gear that inhibits the generation and transmission of torsional vibration.

The present invention provides a torsionally compliant gear comprising a gear body having a plurality of teeth at an outer peripheral surface thereof, and a plurality of slots formed through the body to define a central hub, a rim upon which the teeth are located, a plurality of spokes connecting the rim to the hub, and a plurality of fins, extending radially from the hub and located circumferentially between the spokes, for restricting angular displacement of the rim relative to the hub. In high speed machines, typically having gears that experience a pitch line speed in excess of 10ms^"1, high precision and/or highly complex components, and/or sophisticated, selective assembly techniques are typically required to avoid generation of vibration at the interface between the intermeshing teeth. However, by providing the gear with a rim connected to the hub by spokes a circumferential freedom is achieved that permits some geometrical variation in the gear to be accommodated under light loading, whilst the provision of fins between the spokes restricts this circumferential freedom under higher loading so that efficient transmission of torque is effected and rotor timing is maintained.

The provision of radially extending spokes provides an enhanced radial stiffness which permits radial concentricity of the gears to be controlled, thus further improving efficient transmission of torque between gears in driving a highly loaded, rapidly rotating mechanism.

The formation of a gear from a single piece of material, namely from the gear body, can enable the gear to be manufactured in a rapid and relatively simple manner, and a relatively low cost. For example, the slots may be formed in the body using wire erosion or other suitable technique.

Elastomehc material, or other similarly compliant material, may be located between the spokes and the fins, and/or between the fins and the rim.

Each fin may be spaced from an adjacent spoke by a distance in the range of 0.02 mm to 2 mm, preferably in the range of 0.05 mm to 1 mm, more preferably in the range of 0.09 mm to 0.2 mm. In the event that the slots cannot be formed economically to the required dimension, the slots may be formed with a larger dimension, and with the excess "space" subsequently filled by bonding or otherwise attaching metallic or other material to the gear body.

The gear may be a timing gear. The gear may be suitable for use in a high speed rotary machine, wherein the gear may be configured to - A -

accommodate pitch line speeds in excess of 10 ms^"1. The gear may be provided a part of a gear assembly, which additionally comprises a second gear and wherein the gears are configured to intermesh so that one gear is driven by the other gear upon rotation thereof. This second gear of the gear assembly may also be a gear of the aforementioned type.

The present invention also provides a vacuum pump comprising a drive shaft, a driven shaft driven by the drive shaft, a rotor located on each shaft, and a gear assembly as aforementioned, each gear of the gear assembly being mounted on a respective shaft. Torque is transmitted from the drive shaft to the gear mounted thereon via the central hub thereof, and is transmitted from that gear to the other gear via the intermeshing teeth of the gears.

The invention is described below in greater detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates schematically three known pump configurations; and

Figure 2 illustrates a plan view of a gear suitable for use in any of the pumps of Figure 1 .

Figure 2 illustrates a torsionally compliant gear which may be used, for example, as one or each of the timing gears 5 for any of the vacuum pumps illustrated in Figure 1 . The gear is a unitary component, formed from a single piece of material. The gear comprises a gear body 10 having a bore 12 for receiving a shaft 1 and a plurality of teeth 14 on its outer peripheral surface for engaging teeth of a similar, adjacent gear to enable the gear to drive, or be driven by, the adjacent gear.

Slots 16, 18 formed through the gear body 10 define within the gear body 10 a central annular hub 20, a plurality of spokes 22, an annular rim 24 and a plurality of fins 26. The hub 20 surrounds the bore 12. The spokes 22 extend radially outwards from the hub 20 and serve to connect the hub 20 to the rim 24, upon which the teeth 14 are located. The fins 26 extend outwardly from the hub 22 between the spokes 22. Radially extending slots 16 separate the spokes 22 from the fins 26, whilst circumferentially extending slots 18, each extending circumferentially between two respective radially extending slots 18, separate the rim 24 from the fins 26. These slots 16, 18 preferably have a narrow width in the range from 0.02 mm to 2 mm, preferably in the range from 0.05 mm to 1 mm, and more preferably in the range of 0.09 mm to 0.2 mm.

The gear illustrated in Figure 1 has six fins 26 and six spokes 22, but the number of fins and spokes may be varied from only one or two up to forty or more depending on the required characteristics of the gear and the complexity of the gear that may be tolerated.

In operation, when mounted on a shaft or otherwise rotatably mounted, the hub 20 of the gear is rotated at a high speed, typically so that the gear experiences pitch line speeds in excess of 10ms^"1. Torque is transmitted to the gear body 10 through the coupling between the hub 20 of the gear and the shaft. The rim 24 experiences a minimal amount of angular displacement, or circumferential freedom, in relation to the hub 20. The extent of this circumferential freedom is determined by the width of the radially extending slots 24, as the extent of angular displacement of the rim 24 relative to the hub 20 is restricted in both angular directions by interaction between the spokes 22 and the fins 26.

Under light loading conditions, for example when the vacuum pump is running at ultimate, noise is typically generated by a conventional gear due to the incomplete intermeshing between the teeth of the gears, which as discussed above results from geometric anomalies between the gears. In contrast, the gear illustrated in Figure 1 may accommodate minimal geometrical variations by the circumferential freedom of the rim 24 relative to the hub 20 so that this noise is at least reduced, if not entirely eliminated. Under a highly loaded condition there must be an efficient transfer of torque from one gear to the adjacent gear through the intermeshing teeth of the gears. The circumferential freedom of the rim 24 relative to the hub 20 is only minimal, and so under a high loading condition, the leading edge of each fin 26 is readily brought into direct contact with the trailing edge of each respective spoke 22 to enable direct and efficient transmission of torque between the two. In other words, under a high load condition the circumferential angular location of the rim 24 relative to the hub 20 is effectively fixed.

The slots 16, 18 may be formed in the gear using an electrostatic discharge machining (EDM) or wire eroding technique. In manufactures, an externally toothed gear body 10 is provided with a central bore 12, and the slots 16, 18 are subsequently formed through the thickness of the gear body 10 to define the hub 20, spokes 22, rim 24 and fins 26. A gear 10 so manufactured is clearly formed as an integral component from a single piece of material.

If so desired, an elastomeric or other similarly compliant material may be provided in one or more of the slots 16 to damp relative movement between the fins 26 and the spokes 22. Similarly, elastomeric material may be provided in one or more of the slots 18 to damp relative movement between the fins 26 and the rim 24. Whilst one or more of the slots 16 may be of enlarged width and filled with compliant material to enhance the damping capacity of the gear, these slots 16 are preferably each retained at a smaller magnitude to limit the angular displacement between the rim 24 and the hub 20.

The torsionally compliant gear provides a simple, yet effective, means for inhibiting the generation and transmission of vibration caused by the mismatch of engagement between teeth on a pair of cooperating gears in a lightly loaded condition of a high inertia rotating mechanism. This inhibition is achieved by enabling the rim of at least one of the gears to fractionally shift in relation to the central hub in order to accommodate slight geometrical variations between the teeth of one gear and the teeth of the other gear. In this way, a continuous synchronous meshing may be achieved without generation of excessive vibration and consequent noise. Under a higher loading condition the limit of torsional compliance is rapidly reached as each fin comes into contact with a respective spoke, resulting in a gear that readily transmits torque to its cooperating gear. The spokes give radial stiffness to the gear such that concentricity between the rim and the central hub is maintained.

In summary, a gear comprises a gear body having a plurality of teeth at an outer peripheral surface thereof, and a plurality of slots formed through the body. The slots define within the gear body a central hub, a rim upon which the teeth are located, a plurality of spokes connecting the rim to the hub, and a plurality of fins, extending radially from the hub and located circumferentially between the spokes, for restricting angular displacement of the rim relative to the hub.

Claims

1. A torsionally compliant gear comprising a gear body having a plurality of teeth at an outer peripheral surface thereof, and a plurality of slots formed through the body to define a central hub, a rim upon which the teeth are located, a plurality of spokes connecting the rim to the hub, and a plurality of fins, extending radially from the hub and located circumferentially between the spokes, for restricting angular displacement of the rim relative to the hub.

2. A gear according to Claim 1 , wherein elastomeric material is located between the spokes and the fins.

3. A gear according to Claim 1 or Claim 2, wherein elastomeric material is located between the fins and the rim.

4. A gear according to any preceding claim, wherein each fin is spaced from an adjacent spoke by a distance in the range of 0.02 mm to 2 mm.

5. A gear according to Claim 4, wherein each fin is spaced from an adjacent spoke by a distance in the range of 0.05 mm to 1 mm.

6. A gear according to Claim 4 or Claim 5, wherein each fin is spaced from an adjacent spoke by a distance in the range of 0.09 mm to 0.2 mm.

7. A gear according to any preceding claim, wherein the gear is a timing gear.

8. A gear assembly comprising a gear according to any preceding claim and a second gear, wherein the gears are configured to intermesh so that one gear is driven by the other gear upon rotation thereof.

9. A gear assembly according to Claim 8, wherein the second gear is a gear according to any of Claims 1 to 7.

10. A vacuum pump comprising a drive shaft, a driven shaft driven by the drive shaft, a rotor located on each shaft, and a gear assembly according to Claim 8 or Claim 9, each gear of the gear assembly being mounted on a respective shaft and wherein torque is transmitted from the drive shaft to the gear mounted on the drive shaft via the hub thereof.