GB2469037A

GB2469037A - Gauging probe

Info

Publication number: GB2469037A
Application number: GB0905460A
Authority: GB
Inventors: Roger Pullen; Nick Deadman
Original assignee: SOLATRON METROLOGY Ltd
Current assignee: SOLATRON METROLOGY Ltd
Priority date: 2009-03-30
Filing date: 2009-03-30
Publication date: 2010-10-06
Also published as: GB0905460D0

Abstract

The invention provides a gauging probe assembly comprising a contact gauging probe and a protective element in the form of wire 26. The contact gauging probe comprises a housing 20, a displaceable member (wheel 21 and shaft 22) coupled to the housing and slideable relative to the housing in an axial direction. The contact gauging probe also includes a displacement detection system positioned in the housing for detecting displacement of the displaceable member in the axial direction and providing a voltage output indicative of the amount of displacement. The protective element, wire 26, is coupled to the contact gauging probe and covers at least a portion of the contact end of the displaceable member and extends beyond the contact end of the displaceable member in a direction non parallel to the axial direction. The protective element is shaped to reduced non axial loads on the displaceable member from objects incident on the probe assembly in a direction non parallel to the axial direction. The invention provides a simple but effective solution to the problem of excessive side loading and impact on gauging probes.

Description

GAUGING PROBE

Field of the Invention

The present invention relates to a contact gauging probe for measuring the linear dimensions or position of an object. In particular, the invention relates to a robust contact gauging probe suitable for measurement of the position of sharp, moving objects, such as turbine blades in a turbine engine.

Background to the Invention

Gauging probes of the ball-sleeve type have been used for at least forty years and have proved to be a cost effective and robust solution for contact gauging applications. They remain the most accurate, reliable and compact solution for contact measurement of position or linear dimensions.

is Figure 1 is a schematic illustration of a ball-sleeve gauging probe 10 known in the art. A contact element 11, typically a spherical ball or wheel made of silicon carbide, is mounted on a shaft 12 that extends within the probe housing 13.

The shaft is held in the housing using a ball bearing assembly 14 allowing the shaft to move axially within the housing, with minimal friction, in the direction of arrow 15 (herein referred to as the axial direction). On the opposite end of the shaft to the contact element is a magnetic element 16. Displacement of the contact element in the axial direction results in displacement of the magnetic element. A coil assembly 17 is positioned to surround the magnetic element such that displacement of the magnetic element axially results in a voltage output indicative of the amount of displacement of the contact element. There are several coil arrangements that can be used for measuring displacement.

One of the commonly used arrangements is known as a linear variable differential transducer (LVDT).

The contact element is biased in the axial direction away from the housing using a spring 18 or other suitable biasing means such as a pneumatic assembly. When the contact element is pushed back towards the housing in the axial direction by contacting an object to be measured, the voltage output indicates the amount of displacement of the contact element and hence the dimensions or position of the object being measured.

This type of probe has been used in many applications, such as measuring the profile of machine parts or of glass surfaces.

The life of a gauging probe is dependent on the application it is used for and is thus impossible to define with any degree of certainty. For a probe to be useful it must retain a degree of accuracy and repeatability that is commensurate with the dimensional tolerance of the objects being measured. There will inevitably come a time after repeated use, when a probe is no longer sufficiently accurate io or reliable. At this stage, replacement of the probe is the only solution. This incurs not only the cost of a new probe but also the cost associated with the downtime of the measuring assembly.

Typically, the useful life of a gauging probe of this type is between tens of is millions of duty cycles and tens of thousands of duty cycles, but it may be less if used in very harsh conditions. The least damaging applications are those in which only axial forces are applied to the probe i.e. only forces along the direction of measured displacement are applied to the probe. When side loading occurs it is much more damaging to the probe, and in particular to the contact element and bearing assembly, and significantly reduces the useful lifetime of the probe. It is therefore desirable to minimise the effect of side loading on gauging probes of this type.

There have been several attempts to reduce the effect of side loading, including the use of pneumatic probes. However pneumatic probes are inherently more costly to produce and maintain. In general, the size of the ball or wheel forming the contact element determines how much lift (or displacement) can be applied to the probe from a "side approach" object. As a rule of thumb in this type of gauge, to maximize life the contact point of a side approach object on the contact ball or wheel is limited to be no more than 45° from the axial direction measured from the centre of the ball or wheel. This means a contact ball of 3mm diameter can measure a deflection of up to 0.44mm (3x1/2(1-cos45)) and an 8mm diameter ball can measure a deflection of up to 1.17mm, This means that to measure large deflections requires large contact tips, which are of significant mass, creating significant gravitational effects and high inertia, which limits contact speed. The large tip size also limits the application of the probes in confined spaces.

One particular application for gauge probes has highlighted the need for a solution to the problem of side loading. This is the gauging of the run out and diameter of a turbine blade array. The tip of a turbine blade is necessarily sharp, the speed of contact can be high and number of contacts per revolution io can be more than 20. The contact of each blade with the probe tip is thus abrupt and damaging. The repeated impact of the turbine blades quickly reduce the repeatability of the probe measurements and thus necessitate replacement of the probe at frequent intervals.

Summary of the Invention

The invention is defined in the appended independent claims, to which reference should now be made. Preferred features of the invention are set out in the dependent claims.

In a first aspect of the invention, there is provided a probe assembly comprising a contact gauging probe and a protective element, the contact gauging probe comprising: a housing; a displaceable member coupled to the housing and slidable relative to the housing in an axial direction, the displaceable member biased away from the housing in an axial direction and extending beyond the housing in an axial direction and having a contact end furthest from the housing in an axial direction; and a displacement detection system in the housing for detecting displacement of the displaceable member in a axial direction and providing an output indicative of an amount of displacement; the protective element being coupled to the contact gauging probe and covering at least a portion of the contact end of the displaceable member and extending beyond the contact end of the displaceable member in a direction non-parallel to the axial direction, wherein the protective element is shaped to reduce side loads on the displaceable member from objects incident of the probe assembly in a direction non-parallel to the axial direction.

In a second aspect of the invention, there is provided a protective element for a contact gauging probe, the contact gauging probe comprising: a housing; a displaceable member coupled to the housing and slidable relative to the housing in an axial direction, the displaceable member biased away from the housing in an axial direction and extending beyond the housing in an axial io direction and having a contact end furthest from the housing in an axial direction; and a displacement detection system in the housing for detecting displacement of the displaceable member in a axial direction and providing an output indicative of an amount of displacement; is the protective element providing a contact surface, in use, the protective element covering at least a portion the contact end of the displaceable member and extending beyond the contact end of the displaceable member in a direction non-parallel to the axial direction, wherein the contact surface is shaped to reduce side loads on the displaceable member from objects incident of the probe assembly in a direction non-parallel to the axial direction.

Brief Description of the Drawings

An example of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a contact gauging probe; Figure 2a illustrates the contact assembly in a contact gauging probe in accordance with an embodiment of the present invention; Figure 2b illustrates a side view of the contact assembly shown in Figure 2a.

Detailed Description

Figure 1 is a schematic illustration of a ball sleeve gauging probe, as described above.

Figures 2a and 2b illustrate a probe assembly in accordance with an embodiment of the present invention. Specifically, Figures 2a and 2b show the probe tip assembly of a gauging probe of the type shown in Figure 1. Figure 2b is a side view of the probe tip assembly shown in Figure 2a.

The probe tip assembty shown in Figure 2a comprises a probe housing 20 and a non-rotating wheel 21 that laterally locates the wire mounted on a shaft 22.

The wheel 21 and shaft are biased in an axial direction away from the housing 20 but are displaceable in the axial direction back towards the housing. A flexible cover element or gaiter 23 is provided to shield the shaft but allow displacement of the shaft. The wheel 21 is the contact element of the probe and in this example is non-rotatable but may be made rotatable about axis 25 if desired. A ball assembly may used in place of the wheel tip 21 if desired. Axial movement of the wheel and shaft relative to the housing results in a voltage output indicative of the amount of displacement, as described with reference to the probe shown in Figure 1. As described with respect of Figure 1, the shaft is mounted in the housing using a ball bearing assembly (not shown).

The gauging probe shown in Figure 2 additionally includes a protective element in the form of a wire 26, held in place over the wheel by a wire mount 27 and wire fixing nuts 28. The wire substantially eliminates side loading on the wheel tip and bearings within the housing. As a result, relatively high displacement can be achieved with minimal damage to the gauging probe.

As a target object to be measured, such as a turbine blade, approaches the probe from the side i.e. from a direction non-parallel with the axial direction, the wire contact element is initially struck at a relatively shallow angle at some distance from the wheel. This is illustrated in Figure 2a where the contact of a turbine blade 29 with the protective wire is shown. In the case of rotating turbine blades, the blades are rotating such that the axial direction of the probe is the radial direction of the turbine assembly, so that the blades approach the probe tip from a direction substantially normal to the axial direction. As soon as the turbine blade contacts the wire, the wire deflects and pushes the wheel 21 and shaft in the axial direction towards the housing 20. As the turbine blade 29 continues to travel towards the probe tip, the wheel and shaft continue to displace in an axial direction until the turbine blade tip and the wheel tip coincide in the axial direction. As this point, the probe displacement is complete and the measurement peak is recorded.

The protective wire has a profile that ensures that the impact of the turbine blade is at a shallow angle to the surface of the wire. This reduces the loading on the wire and the lateral load on the probe. The wire mount also absorbs the side load from turbine blade. By coupling the protective wire to the probe io housing (via the wire mount) the side load is transferred to the probe housing rather than the probe tip and bearing assembly.

As shown, the shape of the wire across the wheel tip is a gentle arc. However, the shape of the wire is not limited to this gentle arc. The particular shape chosen will depend on the space available for the probe for a particular application, as well as the direction from which the objects to be measured approach the probe tip and the amount of side load that can be tolerated by the probe, and may be more convex, flat or even concave. Given that the primary purpose of the wire is to limit side loading on the probe, the wire should be arranged such that the objects to be measured strike the wire at a relatively shallow angle, ideally less than 20°, and initially at a point outside the axial extent of the probe. This ensures low side loading on the probe and a gradual displacement of the wheel and shaft until maximum displacement is reached.

This typically means that the protective wire (or the contact surface of some other protective element) has a radius of curvature significantly greater than the radius of curvature of the wheel or ball of the probe tip. The contact surface of the wire is preferably smooth and continuous but need not extend on both sides of the probe if the objects to be measured are always incident on the probe from the same direction.

In this example the wire mount is fixed to the housing so that the wire must flex as it is displaced by the passing turbine blade or other object. Accordingly, the wire is formed from an elastically deformable material such as spring steel.

Alternatively, the wire mount may be slideable in the axial direction relative to the housing so that it does not need to flex, or at least does not need to flex to such an extent, but instead displaces parallel to the wheel tip. A further alternative, or addition, is for the wire mount to include a deformable portion that is elastically deformable to absorb the side loading. For example, springs may be included in the wire mount on either side of the probe, the springs elastically deformable in an axial and/or non-axial direction.

As wear on the wire will inevitably occur after repeated use, the wire mount arrangement shown in Figures 2a and 2b allows for quick and easy replacement of the wire. The wire retaining nuts 28 are simply loosened, the io worn wire moved, a new wire put in place and the nuts tightened again.

As can be seen from Figure 2b, in this embodiment, the wire is simply a thin wire of substantially circular cross-section and is formed from spring steel wire.

This is a simple and cost-effective solution. Alternatively, wires with other Is cross-sectional shapes may be used, such as oval or rectangular cross-section.

The wire may also be ribbon shape or form a substantially circular cap; this may be particularly useful if the wheel or ball contact element has a large contact surface or the object being measured has a large contact area, or if the objects to be measured are incident on the probe from a plurality of lateral directions.

The wire may also be made of different material if desired, provided it has sufficient resilience for repeated use. For example, titanium wire or nylon wire could be used.

Although the invention is described in relation to gauging of turbine blade assemblies, it is useful for other applications as well. Any application that involves measurement of objects having sharp edges, or which will result in significant side loading on the probe, can benefit from a protective wire or sheet in accordance with the present invention. The present invention is also useful for reducing impact on the objects being measured and so is useful for measuring delicate objects.

The present invention provides a simple but effective solution to the problem of excessive side loading and impact on gauging probes. This solution has been tested to hundreds of thousands of turbine rota revolutions without any appreciable degradation in accuracy or repeatability of measurement.

The present invention may be supplied as a complete probe assembly, including the protective element and any mounting required, together with a contact gauging probe. Alternatively, the protective element and any mounting required may be supplied as a kit to fit with existing contact gauging probes.

Claims

Claims 1 A probe assembly comprising a contact gauging probe and a protective element, the contact gauging probe comprising: a housing; a displaceable member coupled to the housing and slidable relative to the housing in an axial direction, the displaceable member biased away from the housing in an axial direction and extending beyond the housing in an axial direction and having a contact end furthest from the housing in an axial direction; and a displacement detection system positioned in the housing for detecting displacement of the displaceable member in an axial direction and providing an output indicative of an amount of displacement; is the protective element being coupled to the contact gauging probe and covering at least a portion of the contact end of the displaceable member and extending beyond the contact end of the displaceable member in a direction non-parallel to the axial direction, wherein the protective element is shaped to reduce non-axial load on the displaceable member from objects incident on the probe assembly in a direction non-parallel to the axial direction.
2. A probe assembly according to claim 1, wherein the protective element is coupled to the housing.
3. A probe assembly according to claim I or 2, wherein the protective element is a length of wire.
4. A probe assembly according to any preceding claim, wherein the protective element is elastically deformable.
5. A probe assembly according to any preceding claim, further comprising a protective element mount, coupled to the housing and to the protective element, to hold the protective element in position.
6. A probe assembly according to claim 5, wherein the protective element mount is slideable relative to the housing in the axial direction.
7. A probe assembly according to claim 5 or 6, wherein the protective element mount includes an elastically deformable portion.
8. A probe assembly according to any preceding claim, wherein the protective element has a contact surface having a radius of curvature significantly greater than the radius of curvature of the contact end of the contact gauging probe.
9. A probe assembly according to any preceding claim, wherein the protective element is formed from spring steel.
10. A protective element for a contact gauging probe, the contact gauging probe comprising: a housing; a displaceable member coupled to the housing and slidable relative to the housing in an axial direction, the displaceable member biased away from the housing in an axial direction and extending beyond the housing in an axial direction and having a contact end furthest from the housing in an axial direction; and a displacement detection system positioned in the housing for detecting displacement of the displaceable member in an axial direction and providing an output indicative of an amount of displacement; the protective element providing a contact surface, in use, the protective element covering at least a portion of the contact end of the displaceable member and extending beyond the contact end of the displaceable member in a direction non-parallel to the axial direction, wherein the contact surface is shaped to reduce non-axial load on the displaceable member from objects incident on the probe assembly in a direction non-parallel to the axial direction.
11. A protective element according to claim I 0, wherein the protective element comprises a length of wire. -11 -
12. A protective element according to claim 10 or 11, wherein the contact surface is elastically deformable.
13. A protective element according to any one of claims 10 to 12, further comprising a mount assembly for holding the contact surface in position relative to the probe housing.
14. A protective element according to claim 12, wherein the mount assembly is configured to be slideable relative to the housing in the axial io direction.
15. A protective element according to claim 13 or 14, wherein the mount assembly includes an elastically deformable portion.
16. A protective element according to any one of claims 10 to 15, wherein the contact surface has a radius of curvature significantly greater than the radius of curvature of the contact end of the contact gauging probe to which it is coupled in use.
17. A protective element according to any one of claims 10 to 16, wherein the contact surface is formed from spring steel.
18. A probe assembly comprising a contact gauging probe and a protective element, the contact gauging probe comprising: a housing; a displaceable member coupled to the housing and slidable relative to the housing in an axial direction, the displaceable member biased away from the housing in an axial direction and extending beyond the housing in an axial direction and having a contact end furthest from the housing in an axial direction; and a displacement detection system positioned in the housing for detecting displacement of the displaceable member in an axial direction and providing an output indicative of an amount of displacement; the protective element covering at least a portion of the contact end of the displaceable member and extending beyond the contact end of the displaceable member in a direction non-parallel to the axial direction, wherein the protective element is coupled to the housing.
19. A probe assembly according to claim 18, further comprising a protective element mount, wherein the protective element is coupled to the housing via the protective element mount.
20. A probe assembly according to claim 19, wherein the protective element mount is slideable relative to the housing in the axial direction.
21. A probe assembly according to claim 18 or 19, wherein the protective element mount includes an elastically deformable portion.
22. A probe assembly according to any one of claims 18 to 21, wherein the protective element is shaped to reduce non-axial load on the displaceable member from objects incident on the probe assembly in a direction non-parallel to the axial direction.
23. A probe assembly according to any one of claims 18 to 22, wherein the protective element is a length of wire.
24. A probe assembly according to any one of claims 18 to 23 wherein the protective element is elastically deformable.
25. A probe assembly according to any one of claims 18 to 24, wherein the protective element has a contact surface having a radius of curvature significantly greater than the radius of curvature of the contact end of the contact gauging probe.
26. A protective element for a contact gauging probe, the contact gauging probe comprising: a housing; -13-a displaceable member coupled to the housing and slidable relative to the housing in an axial direction, the displaceable member biased away from the housing in an axial direction and extending beyond the housing in an axial direction and having a contact end furthest from the housing in an axial direction; and a displacement detection system positioned in the housing for detecting displacement of the displaceable member in an axial direction and providing an output indicative of an amount of displacement; the protective element being configured for coupling to the housing, in use, the protective element covering at least a portion of the contact end of the displaceable member and extending beyond the contact end of the displaceable member in a direction non-parallel to the axial direction.
27. A protective element according to claim 26, wherein the protective is element comprises a length of wire,
28. A protective element according to claim 26 or 27, wherein at least a portion of the protective element is elastically deformable.
29. A protective element according to any one of claims 26 to 28, further comprising a mount assembly for holding the protective element in position relative to the probe housing, wherein, in use, the protective element is connected to the mount assembly and the mount assembly is connected to the housing.
30. A protective element according to claim 29, wherein the mount assembly is configured to be slideable relative to the housing in the axial direction.
31. A protective element according to claim 29 or 30, wherein the mount assembly includes an elastically deformable portion.
32. A protective element according to any one of claims 26 to 31, wherein a contact surface of the protective element has a radius of curvature significantly greater than the radius of curvature of the contact end of the contact gauging probe to which it is coupled in use.
33. A probe assembly substantially as described herein with reference to the accompany drawings.
34. A protective element substantially as described herein with reference to the accompany drawings.