EP2497954A2

EP2497954A2 - Scroll compressor

Info

Publication number: EP2497954A2
Application number: EP12158522A
Authority: EP
Inventors: Yoshiaki Miyamoto; Yoshiyuki Kimata; Youhei Hotta; Hajime Sato; Toshiyuki Goto
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 2011-03-09
Filing date: 2012-03-08
Publication date: 2012-09-12
Anticipated expiration: 2032-03-08
Also published as: JP5787559B2; EP2497954B1; EP2497954A3; JP2012188964A

Abstract

A scroll compressor including a stationary scroll (41) having a spiral-shaped stationary spiral structure (47) that is vertically provided on one surface of a stationary end plate (45) secured at a stationary-side support position (F) positioned at an outer circumference; and an orbiting scroll (43) having an orbiting end plate (51) that is provided with a spiral-shaped orbiting spiral structure (53) vertically on one surface thereof to be meshed with the stationary end plate (45), and that is supported at a position closer to a center than the stationary-side support position (F) in a manner that allows a revolving motion; wherein, for at least one of a clearance between a tip portion of the stationary spiral structure (47) and the orbiting end plate (51) and a clearance between the orbiting spiral structure (53) and the stationary end plate (45), the clearance (S1) at a side further outward than the orbiting-side support position (C) is made larger than the clearance (S2) at the center side.

Description

Technical Field

The present invention relates to a compressor provided with a scroll compression mechanism.

{Background Art}

A scroll compression mechanism is employed as, for example, a compression mechanism of a scroll compressor, as disclosed in Patent Literature 1, and as a higher-stage compression mechanism of a two-stage compressor, as disclosed in Patent Literature 2.
With the scroll compression mechanism, a stationary scroll and an orbiting scroll are placed with spiral-shaped wall structures thereof combined with each other, and the volume of a compression chamber formed between the wall structures is gradually decreased by causing revolving motion of the orbiting scroll with respect to the stationary scroll, thus compressing a fluid in the compression chamber. The fluid that is introduced to the compression chamber from a peripheral edge portion of the scroll compression mechanism is compressed so as to reach a high-temperature, high-pressure state and is discharged to an external space from a discharge hole provided at a center portion of the stationary scroll.
Therefore, the high temperature and high pressure of the fluid act on the center portion of the scroll compression mechanism and on a rear side, that is, an external portion, of the stationary scroll. On the other hand, a low-temperature, low-pressure fluid is present at a rear side, that is, an external portion, of the orbiting scroll.

{Citation List}

{Patent Literature}

{PTL 1} Japanese Unexamined Patent Application, Publication No. 2003-155984 .
{PTL 2} Japanese Unexamined Patent Application, Publication No. Hei 5-87074 .

{Summary of Invention}

{Technical Problem}

With the scroll compression mechanism, an outer circumferential end portion of the stationary scroll is generally secured to a housing, and the orbiting scroll is supported at a position closer to the center thereof. When the above-described pressure relationship acts on this scroll compression mechanism, the stationary scroll is warped due to thermal expansion and pressure deformation such that the center portion thereof is positioned closer to the orbiting scroll, and the orbiting scroll is also warped in a similar shape. Because the orbiting scroll does not move at the position where it is supported, an outer circumferential portion thereof deforms so as to approach the stationary scroll. Therefore, at a portion closer to the outer circumference than the position where the orbiting scroll is supported, the orbiting scroll and the stationary scroll deform in directions that cause them to approach each other; that is, they undergo pressure deformation.
In recent years, from the viewpoint of global environmental protection, the use of carbon dioxide (CO₂), which is a natural refrigerant, as refrigerant gas has been considered. For example, when a high-pressure refrigerant, such as CO₂, is used as refrigerant gas, because the temperature and pressure thereof, when compressed, become higher than those of ordinary refrigerant, the above-described deformation is increased, and tooth tips and tooth bases of the stationary scroll and orbiting scroll easily come in contact at the outer circumferential portions.
If the tooth tips and tooth bases of the stationary scroll and the orbiting scroll come in contact, the reliability thereof deteriorates because the wall structures may become damaged.
As a countermeasure against this, it is conceivable to increase a clearance (tip clearance) between the tooth tips and the tooth bases of the stationary scroll and the orbiting scroll. However, there is a problem in that, because the clearance is increased at the center portion by doing so, a leakage gap thereof becomes larger, thus causing the efficiency to deteriorate.
In addition, it is also conceivable to decrease the volume of a high-pressure portion to decrease the magnitude of the pressure that acts thereon. However, if the volume of the high-pressure portion is decreased, the efficiency may deteriorate due to an increase in discharge pulsation or an increase in discharge pressure loss.
The present invention has been conceived in light of the above-described circumstances, and an object thereof is to provide a compressor that is capable of ensuring reliability and of enhancing efficiency.

{Solution to Problem}

In order to solve the above-described problems, the present invention employs the following solutions.
Specifically, an aspect of the present invention is a compressor having a scroll compression mechanism including a stationary scroll having a spiral-shaped stationary-side wall structure that is vertically provided on one surface of a stationary-side end plate secured at a stationary-side support position positioned at an outer circumference; and an orbiting scroll having an orbiting-side end plate that is provided with a spiral-shaped orbiting wall structure vertically on one surface thereof to be meshed with the stationary-side wall structure, and that is supported at a position closer to a center than the stationary-side support position in a manner that allows a revolving motion; wherein, for at least one of a clearance between a tip portion of the stationary-side wall structure and the orbiting-side end plate and a clearance between the orbiting-side wall structure and the stationary-side end plate, the clearance at a side further outward than the orbiting-side support position is made larger than the clearance at the center side.
With this aspect, the stationary-side wall structure of the stationary scroll and the orbiting-side wall structure of the orbiting scroll are placed so as to be meshed with each other, thus forming the scroll compression mechanism.
When the orbiting scroll performs a revolving motion in this state, because the volume of a compression chamber formed between the meshed stationary-side wall structure and orbiting-side wall structure gradually decreases toward a center portion thereof, a medium introduced into the compression chamber from a peripheral edge portion of the scroll compression mechanism is gradually compressed to reach a high-temperature, high-pressure state, and it is discharged to an external space from a discharge hole provided at a center portion of the stationary-side end plate of the stationary scroll.
Because the stationary-side end plate of the stationary scroll is pressed by this compressed high-temperature, high-pressure medium, it undergoes thermal expansion and pressure deformation. Because the stationary-side end plate is secured at a stationary-side support position positioned closer to an outer circumference, it warps so that a center portion thereof is positioned on the orbiting scroll side.
On the other hand, the high temperature and high pressure of the compressed medium at the center portion sandwiched by the stationary scroll and the orbiting scroll act on the orbiting-side end plate of the orbiting scroll. Because low-temperature, low-pressure medium is present at a surface of the orbiting-side end plate on an opposite side with respect to the stationary scroll, the center portion of the orbiting-side end plate warps so as to be separated from the stationary scroll. Specifically, during the compression operation, the stationary-side end plate and the orbiting-side end plate both warp so that the center portions thereof protrude toward the orbiting scroll.
At this time, because the orbiting-side end plate is supported at the orbiting-side support position positioned closer to the center than the stationary-side support position, and because the orbiting-side end plate does not move at this position, a portion of the orbiting-side end plate closer to the outer circumference than the orbiting-side support position deforms so as to approach the stationary scroll.
Therefore, at the portion of the orbiting scroll closer to the outer circumference than the orbiting-side support position, the orbiting scroll and the stationary scroll deform in the directions that cause them to approach each other.
With this aspect, for at least one of the clearance between the tip portion of the stationary-side wall structure and the orbiting-side end plate and the clearance between the orbiting-side wall structure and the stationary-side end plate, the clearance at a side further outward than the orbiting-side support position is made larger than the clearance on the center side; therefore, even though the orbiting scroll and the stationary scroll deform in the directions that cause them to approach each other, it is possible to prevent them from coming into contact with each other. Because it is possible to prevent the orbiting scroll and the stationary scroll from coming into contact with each other in this way, it is possible to prevent damage to the stationary-side wall structure or the orbiting-side wall structure caused by the contact, and thus, the reliability can be ensured.
In addition, because the clearance between the tip portion of the stationary-side wall structure and the orbiting-side end plate and the clearance between the orbiting-side wall structure and the stationary-side end plate at the center portion do not become large, a leakage gap does not become large, and it is possible to prevent deterioration of the efficiency. The efficiency can be enhanced as compared with the case in which the overall clearance is increased to prevent a collision.
Note that, for the clearance between the tip portion of the stationary-side wall structure and the orbiting-side end plate and the clearance between the orbiting-side wall structure and the stationary-side end plate, it is desirable that the smaller clearance of the two be expanded, or both clearances may be expanded.
With this aspect, the clearances may be changed by adjusting height of the stationary-side wall structure or the orbiting-side wall structure.
Because the height of the stationary-side wall structure or that of the orbiting-side wall structure can be processed more easily than cutting the surface of the stationary-side end plate or that of the orbiting-side end plate, manufacturing thereof can be facilitated.
In this case, it is preferable that the clearances be changed so as to be 0.25 to 2.0 x 10^-3 times the thickness of the stationary-side end plate or the orbiting-side end plate.

{Advantageous Effects of Invention}

With the present invention, for at least one of a clearance between a tip portion of a stationary-side wall structure and an orbiting-side end plate and a clearance between a orbiting-side wall structure and the stationary-side end plate, the clearance at a side further outward than an orbiting-side support position is made larger than the clearance at a center side; therefore, it is possible to prevent damage to the stationary-side wall structure or the orbiting-side wall structure caused by contact between them, and thus, the reliability can be ensured. In addition, it is possible to prevent deterioration of the efficiency.

{Brief Description of Drawings}

Fig. 1 is a longitudinal cross-sectional view showing a compressor according to an embodiment of the present invention.
Fig. 2 is a cross-sectional view showing a scroll compression mechanism in Fig. 1.
Fig. 3 is a partial longitudinal cross-sectional view showing a portion of the scroll compression mechanism in Fig. 1.

{Description of Embodiment}

An embodiment of the present invention will be described below by using Figs. 1 to 3.
Fig. 1 is a longitudinal cross-sectional view of a compressor according to an embodiment of the present invention. Fig. 2 is a cross-sectional view showing a scroll compression mechanism in Fig. 1. Fig. 3 is a partial longitudinal cross-sectional view showing a portion of the scroll compression mechanism in Fig. 1.
The compressor 1 is provided with a cylindrical sealed housing 3, a lower-stage rotary compression mechanism 5 provided at a lower portion of the sealed housing 3, a higher-stage scroll compression mechanism 7, and an electric motor 9 that is provided between the rotary compression mechanism 5 and the scroll compression mechanism 7 and that drives the rotary compression mechanism 5 and the scroll compression mechanism 7.
The sealed housing 3 has a hollow cylindrical shape in which a cylindrical portion 11 is closed at the top and bottom by welding a bottom portion 13 and a lid portion 15 thereto.
The electric motor 9 is provided with a driving shaft 17 that is disposed so as to extend in the axial direction of the sealed housing 3, a rotating element 19 that is secured in the area surrounding the driving shaft 17, and a stationary element 21 that is secured to the sealed housing 3 so as to cover the area surrounding the rotating element 19.
The rotary compression mechanism 5 is provided with an eccentric portion 23 provided below the driving shaft 17; a rotor 25 rotatably fitted to the eccentric portion 23; a cylinder member 29 that has a space which forms a cylinder chamber 27, which has circular cross-sectional shape and linearly comes in sliding contact with an outer circumferential surface of the rotor 25 at one location thereof, and that is joined to the sealed housing 3, for example, by means of welding, in a state in which the rotor 25 is disposed in this space; a top bearing 31 that is secured to a top-end surface of the cylinder member 29 to support the driving shaft 17 in a freely rotatable manner above the rotor 25; a bottom bearing 33 that is secured to a bottom-end surface of the cylinder member 29 to support the driving shaft 17 in a freely rotatable manner below the rotor 25; a blade (not shown) that partitions the interior of the cylinder chamber 27 into an intake side and a discharge side; a blade-pressing spring; and so on.
The rotary compression mechanism 5 is not limited to this configuration, and a known structure may be employed.
With the lower-stage rotary compression mechanism 5, low-pressure refrigerant gas (medium) is taken into the cylinder chamber 27 via an accumulator 35 and an intake pipe 37 and this refrigerant gas is discharged into the sealed housing 3 via a discharge chamber 39 after being compressed to intermediate pressure by rotation of the rotor 25. The intermediate-pressure refrigerant gas discharged into the sealed housing 3 flows to a space above the electric motor 9 via a gas channel, etc. provided in the rotating element 19 of the electric motor 9, from where it is taken into the higher-stage scroll compression mechanism 7 to be compressed in two stages.
The scroll compression mechanism 7 is provided with a stationary scroll 41 and an orbiting scroll 43.
The stationary scroll 41 is provided with a stationary end plate (stationary-side end plate) 45 and a spiral-shaped stationary spiral structure (stationary-side wall structure) 47 that is vertically provided on a bottom surface of the stationary end plate 45.
The stationary end plate 45 is mounted on an annular base provided inside the cylindrical portion 11 and is secured to the sealed housing 3 by being connected at multiple locations with bolts. Specifically, the stationary end plate 45 is secured at stationary-side support positions F (see Fig. 2) on an outer circumference side where the bolts are positioned.
A discharge port 49 for the compressed refrigerant gas is formed at a substantially center portion of the stationary end plate 45 so as to pass therethrough.
The orbiting scroll 43 is provided with an orbiting end plate (orbiting-side end plate) 51 and a spiral-shaped orbiting spiral structure (orbiting-side wall structure) 53 that is vertically provided on a top surface of the orbiting end plate 51.
The orbiting scroll 43 is provided so that the orbiting spiral structure 53 meshes with the stationary spiral structure 47. The stationary scroll 41 and the orbiting scroll 43 are meshed so as to have a 180° phase difference in a state in which they are decentered from each other by a predetermined distance, thereby forming sealed spaces, which serve as compression chambers P, at multiple locations having point-symmetrical positional relationships with respect to the centers of the stationary spiral structure 47 and the orbiting spiral structure 53.
At a center of the bottom surface of the orbiting end plate 51, a hollow-cylindrical orbiting boss 55 is provided so as to protrude downward.
A bearing member 59 provided with a bearing portion 57 that supports the driving shaft 17 is fixed in the sealed housing 3. The orbiting scroll 43 is supported by the bearing member 59 at orbiting-side support positions C (see Fig. 2) on an outer circumference side of the orbiting boss 55. The orbiting-side support positions C are located at inner positions closer to the axial center than the outer circumferential end of the orbiting end plate 51. Therefore, the orbiting-side support positions C are located at positions closer to the axial center than the stationary-side support positions F that are positioned at the outer circumference side of the stationary end plate 45.
Height (tooth size) H1 of the stationary spiral structure 47 at positions closer to the outer circumference than the orbiting-side support positions C is smaller than height H2 of the stationary spiral structure 47 at positions closer to the inner circumference than the orbiting-side support positions C. Therefore, a clearance S1 between the tip (tooth tip) of the stationary spiral structure 47 and the orbiting end plate 51 at locations closer to the outer circumference than the orbiting-side support positions C is larger than a clearance S2 between a tip of the stationary spiral structure 47 and the orbiting end plate 51 at locations closer to the inner circumference than the orbiting-side support positions C.
Although the difference between the height H2 and the height H1, in other words, the difference between the clearance S2 and the clearance S1, is determined by various conditions, it is preferable that the difference be, for example, 0.25 to 2.0 x 10^-3 times the thickness of the orbiting-side end plate 51.
The height of the stationary spiral structure 47 in this embodiment is adjusted to have the clearances described above. This is because the stationary end plate 45 structurally deforms more easily, and also because the clearance between the tip of the stationary spiral structure 47 and the orbiting end plate 51 is smaller than the clearance between the tip of the orbiting spiral structure 53 and the stationary end plate 45.
For example, in the case in which the clearance between the tip of the orbiting spiral structure 53 and the stationary end plate 45 is smaller than the clearance between the tip of the stationary spiral structure 47 and the orbiting end plate 51, the height of the orbiting spiral structure 53 may be adjusted. In addition, the height of the stationary spiral structure 47 and the height of the orbiting spiral structure 53 may both be adjusted.
Furthermore, instead of adjusting the height of the stationary spiral structure 47 and the height of the orbiting spiral structure 53, the clearances may be adjusted by cutting surfaces of the stationary end plate 45 and/or the orbiting end plate 51.
A rotation prevention mechanism 61 that causes the orbiting scroll 43 to revolve while preventing it from rotating is provided between the orbiting scroll 43 and the bearing member 59.
An eccentric pin 63 whose axial center is decentered is provided at a tip portion of the driving shaft 17 positioned in a hollow portion of the orbiting boss 55. An orbiting bearing 65 that engages with the orbiting boss 55 is provided at the outer circumference of the eccentric pin 63. The rotation of the driving shaft 17 is transmitted to the orbiting boss 55 via the eccentric pin 63 and the orbiting bearing 65, thereby driving the orbiting scroll 43 and causing it to revolve.
A discharge valve 67 that opens/closes the discharged port 49 and a discharge cover 71 that forms a discharge chamber 69 between the stationary end plate 45 and itself are secured at the top surface of the stationary end plate 45.
An end portion of a discharge pipe 73 that is connected to the exterior, that is, a refrigerating cycle, by passing through the lid portion 15 of the sealed housing 3 is attached to the discharge cover 71 so as to pass therethrough.
The scroll compression mechanism 7 takes the intermediate-pressure refrigerant gas, which has been compressed at the rotary compression mechanism 5 and discharged into the sealed housing 3, into the compression chambers P and discharges it into the discharge chamber 69 via the discharge valve 67 after compressing the intermediate-pressure refrigerant gas into a high-temperature, high-pressure state by means of the orbiting scroll 43 that is driven to revolve. This high-temperature, high-pressure refrigerant gas is expelled outside the compressor 1, that is, to the refrigerating cycle, from the discharge chamber 69 via the discharge pipe 73.
In addition, a known displacement-type oil supply pump 75 is mounted between the bottom end of the driving shaft 17 and the bottom bearing 33 of the rotary compression mechanism 5. The oil supply pump 75 pumps out lubricant loaded in an oil sump 77 formed at the bottom portion of the sealed housing 3 to forcedly supply the lubricant 13, via an oil supply hole 79 provided in the driving shaft 17, to sites requiring lubrication, such as the bearing portions, etc. in the lower-stage rotary compression mechanism 5 and the higher-stage scroll compression mechanism 7.
The operation of the compressor 1 of this embodiment, configured as above, will now be described.
Upon activating the electric motor 9, the driving shaft 17 is rotated, thus starting the operation of the compressor 1.
The low-temperature, low-pressure refrigerant gas that is taken into the cylinder chamber 27 of the lower-stage rotary compression mechanism 5 via the intake pipe 37 is discharged into the discharge chamber 39 after being compressed to intermediate pressure by means of the rotation of the rotor 25. This intermediate-pressure refrigerant gas flows to the space above the electric motor 9 via a gas channel, etc. provided in the rotating element 19 of the electric motor 9, after being discharged from the discharge chamber 39 into a space below the electric motor 9.
At the higher-stage scroll compression mechanism 7, the action of the eccentric pin 63 associate with the rotation of the driving shaft 17 drives the orbiting scroll 43, causing it to revolve with respect to the stationary scroll 41.
The intermediate-pressure refrigerant that has flowed to the space above the electric motor 9 is guided to an inlet of the scroll compression mechanism 7 provided in the stationary scroll 41 by passing through gaps, etc. between the bearing member 31 and the sealed housing 3, which constitute the higher-stage scroll compression mechanism 7, and is then taken into the compression chambers P.
Because the volumes of the compression chambers P gradually decrease toward the center, the intermediate-pressure refrigerant gas that has been taken in is gradually compressed to reach the high-temperature, high-pressure state.
The refrigerant gas that has undergone the two-stage compression to the high-temperature, high-pressure state at the scroll compression mechanism 7 in this way passes through the discharge port 49, pushes up the discharge valve 67, and is discharged into the discharge chamber 69. The high-temperature, high-pressure refrigerant gas in the discharge chamber 69 is expelled outside the compressor 1, that is, to the refrigerating cycle, via the discharge pipe 73.
At the scroll mechanism 7, high-temperature, high-pressure refrigerant gas 81 is present in the discharge chamber 69 and high-temperature, high-pressure refrigerant gas 83 is present at a center portion of a space sandwiched by the stationary end plate 45 and the orbiting end plate 51.
Because the stationary end plate 45 of the stationary scroll 41 is pressed by this refrigerant gas 81, thermal expansion and pressure deformation thereof occur. Because the stationary-side end plate 45 is secured at stationary-side support positions F positioned on the outer circumferential side, it warps so that the center portion thereof is positioned closer to the orbiting scroll 43, as shown in Fig. 3.
Because the high temperature and high pressure of the refrigerant gas 83 act on the orbiting end plate 51 of the orbiting scroll 43, the orbiting end plate 51 warps so that the center portion thereof is separated from the stationary scroll 41, as shown in Fig. 3. Specifically, during the compression operation, both the stationary end plate 45 and the orbiting end plate 51 warp so that the center portions thereof protrude downward.
Because the orbiting end plate 51 is supported at the orbiting-side support positions C such that the orbiting end plate does not move at these positions in the top-bottom direction, the orbiting end plate 51 deforms at a portion closer to the outer circumference than the orbiting-side support positions C so as to approach the stationary scroll 41.
Therefore, at the portion closer to the outer circumference than the orbiting-side support positions F, the orbiting scroll 41 and the stationary scroll 43 deform in the directions that cause them to approach each other.
In this embodiment, because the clearance S1 between the tip of the stationary spiral structure 47 and the orbiting end plate 51 further toward the outer circumferential side than the orbiting-side support positions C is made larger than the clearance S2 between the tip of the stationary spiral structure 47 and the orbiting end plate 51 at further toward the inner circumferential side than the orbiting-side support positions C, even though the stationary scroll 41 and the orbiting scroll 43 deform in the directions that cause them to approach each other at the portions closer to the outer circumference than the orbiting-side support positions C, as described above, it is possible to prevent them from coming into contact with each other.
Because the stationary scroll 41 and the orbiting scroll 43 can be prevented from coming into contact with each other in this way, it is possible to prevent damage to the stationary spiral structure 47 or the orbiting spiral structure 53 caused by the contact, and thus, the reliability thereof can be ensured.
In addition, because the clearance between the tip portion of the stationary spiral structure 47 and the orbiting end plate 51 and the clearance between the orbiting spiral structure 53 and the stationary end plate 45 at the portion closer to the center than the orbiting-side support positions C do not become large, a leakage gap does not become large, and it is possible to prevent deterioration of the efficiency. Therefore, the efficiency can be enhanced as compared with the case in which the overall clearance is increased to prevent a collision.
Note that the present invention is not limited to the above-described embodiment, and appropriate modifications are possible within a range that does not depart from the spirit thereof.
For example, although the above-described embodiment is described in terms of a sealed two-stage compressor as an example, in which the rotary compression mechanism 5 serves as a lower-stage compression mechanism and the scroll compression mechanism 7 serves as a higher-stage compression mechanism, the compressor is not limited to the two-stage compressor, and it may, of course, be a single-stage compressor (scroll compressor) provided only with a scroll compression mechanism.

{Reference Signs List}

1 compressor (sealed compressor)
7 scroll compression mechanism
41 stationary scroll
43 orbiting scroll
45 stationary end plate
47 stationary spiral structure
51 orbiting end plate
53 orbiting spiral structure
C orbiting-side support position
F stationary-side support position
S1, S2 clearance

Claims

A compressor comprising a scroll compression mechanism (7) including:
a stationary scroll (41) having a spiral-shaped stationary-side wall structure (47) that is vertically provided on one surface of a stationary-side end plate (45) secured at a stationary-side support position positioned closer to an outer circumference; and

an orbiting scroll (43) having an orbiting-side end plate (51) that is provided with a spiral-shaped orbiting wall structure (53) vertically on one surface thereof to be meshed with the stationary-side wall structure (47), and that is supported at a position closer to a center than the stationary-side support position in a manner that allows a revolving motion;
wherein, for at least one of a clearance between a tip portion of the stationary-side wall structure (47) and the orbiting-side end plate (51) and a clearance between the orbiting-side wall structure (53) and the stationary-side end plate (45), the clearance (S1)at a side further outward than the orbiting-side support position is made larger than the clearance (S2) at the center side.
A compressor according to Claim 1, wherein the clearances (S1,S2) are changed by adjusting height of the stationary-side wall structure (47) or the orbiting-side wall structure (53).
A compressor according to Claim 1 or 2, wherein the clearances (S1, S2) are changed so as to be 0.25 to 2.0 x 10^-3 times the thickness of the stationary-side end plate (45) or the orbiting-side end plate (51).