GB2168498A

GB2168498A - Thrust measuring devices

Info

Publication number: GB2168498A
Application number: GB08530500A
Authority: GB
Inventors: Henry Bryant Boyle
Original assignee: BRITISH MARITIME TECHNOLOGY LI
Current assignee: BRITISH MARITIME TECHNOLOGY LI
Priority date: 1984-12-15
Filing date: 1985-12-11
Publication date: 1986-06-18
Also published as: GB8431731D0; GB8530500D0

Abstract

A device 11 for measuring the thrust in a rotatable shaft 10 comprises annular axially-spaced clamps 12 rigidly attached to the shaft 10. The inner faces of the clamps 12 each have an annular groove receiving a ring 14 of soft material by which the clamp load is transmitted. The device 11 includes a beam 20 comprising two beam portions 21, each secured to one of the clamps 12 via an integral flexure portion 28. The beam portions 21 are connected to each other by stiffening webs 30. A shear plate 31 integral with beam portions 21 extends axially and radially between confronting fiat faces 29 of the beam portions 21, substantially mid-way between webs 30. Axial movement of the shaft 10 between the two clamps is transferred to the shear plate 31 and monitored by strain gauges. <IMAGE>

Description

SPECIFICATION Improvements in or relating to thrust measuring devices BACKGROUND TO THE INVENTION This invention relates to thrust measuring devices and is particularly but not exclusively related to devices for measuring, within acceptable limits, the thrust developed in the propeller shaft of a ship.

Previous thrust measuring devices have not had an accuracy reaching a desired level. Devices such as strain gauges, secured directly to the shaft, rely on the total movement of one part of the shaft only a few millimetres long. Such devices are considerably affected by forces not required to be measured, for example, torque and bending forces which are high in relation to thrust forces.

SUMMARY OF THE INVENTION According to this invention, a device for use in measuring thrust developed in a rotatable shaft comprises means connectable to the shaft at axially spaced locations and including a member, the movement in shear of which, due to thrust forces on the shaft, is substantially the same as axial movement of that portion of the shaft between said locations.

The means connectable to the shaft may comprise annular clamps, a connection between the clamps including said member, and a flexure region permitting angular movement of one clamp relative to the other about the shaft axis.

The connection may comprise a beam having portions defining confronting faces, said member comprising a flat (shear) plate extending axially and radially between said faces.

Strain gauges may be mounted on said faces and connected to form a Wheatstone bridge, the output of which can be employed to indicate thrust developed in the shaft.

The faces may be connected by substantially parallel webs spaced axially from the shear plate member on both axial sides thereof and in planes disposed substantially at right angles to the plate member.

The invention also comprises any novel subject matter or combination including novel subject matter herein disclosed.

BRIEF DESCRIPTION OF THE DRA WINGS An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a plan view of a thrust measuring device in association with a shaft, Figure 2 is a view in the direction A-A of Fig. 1, Figure 3 is an enlarged view of a shear plate unit, Figure 4 is a section taken on the line B-B of Fig. 3, Figure 5 is an isometric view of the device in association with a shaft, and Figure 6 illustrates how the shear plate unit of Fig. 3 deforms under shear.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A rotatable shaft 10, which may be a propeller shaft of a ship, has associated therewith a device 11 for measuring the thrust in the shaft. The device 11 comprises annular axially spaced clamps 12 which are rigidly attached to the shaft 10 by bolts (not shown). The inner faces of the clamps 12 each have an annular groove receiving a ring 14 of soft material and of small (2.0 mm) cross-section by which the clamp load is transmitted. Contact between the shaft 10 and rings 14 ensures that the highest bearing force on the shaft is always at the centre line of a clamp 12. The clamps 12 are adjusted axially of the shaft 10 by means of a third clamp 16 secured by bolts 17 to a clamp 12 and adjustable so that substantially zero strain can be set on strain gauges referred to below. The rings 14 are substantially 200 mm apart.This spacing has provided good results.

The device 11 includes a beam 20 comprising two beam portions 21. Each beam portion 21 has an "L"-shaped member 22 having an axial arm 22a and a radial arm 22b. The arms 22a are secured to the clamps 12 by bolts 25 and the arms 22b are secured to the clamps by bolts 26. The arms 22b are connected to the beam portions 21 by integral flexure portions 28. The beam portions 21 are connected to each other by integral, substantially parallel and radially-disposed stiffening webs 30. A shear plate member 31 extends axially and radially between confronting flat faces 29 of the beam portions 21, substantially midway between webs 30. The shear plate member 31 is integral with beam portions 21.

Two strain gauges 40 are mounted in a cruciform-like manner on each face 32 of the shear plate 31. The four gauges are connected to form a Wheatstone bridge. Connections (not shown) supply electric input to the Wheatstone bridge and take the output signal from the bridge to a control unit which can be employed to indicate thrust developed in the shaft 10.

The clamp 16 is used to set up the device 11 as follows:- (a) Clamp 12 nearest to adjustment clamp 16 is left slack; clamp 12 furthest from clamp 16 is tightened fully.

(b) Using bolts 25 and 26, the slack clamp 12 is moved relative to the shaft 10 to a position such that the shear plate member 31 is subjected to substantially zero strain.

(c) The clamp 12 nearest the clamp 16 is fully tightened and the bolts 17 removed.

The thrust measuring device 11 makes use of linear or axial movement of the shaft 10 under thrust forces. If L is the distance between tubes 14 and 1 is the radial length of the shear plate member 31, then any shaft axial movement d between the tubes 14 is substantially the same as displacement d of the shear plate member 31. (See Fig. 6).

The device 11 provides very accurate measurement of axial movement of the shaft 10, and hence thrust, particularly when compared with "spot" or localised measurement provided by strain gauges attached directly to the shaft, which can only show movement over a few millimetres.

Compared with "spot" measurement, the present invention provides magnification of strain of 20 times or more. This is accomplished by measuring shaft deformation over a substantial length and transferring that deformation to the shear plate member 31.

Shaft movement due to thrust forces will normally be compressive. However, if the shaft is subjected to reverse thrust forces, the movement will be tensile.

The flexure portions 28 ensure that any movement imparted to the beam assembly due to torque on the shaft is kept to an acceptable level; one clamp 12 can move angularly about the shaft axis relative to the other clamp 12 but the thrust beam remains substantially stiff between its flexure portions 28.

The radial webs 30 act as protectors for the shear plate member 31, in that the effects on the shear plate of (a) shaft torque, (b) bending due to acceleration or deceleration of the shaft, (c) tension due to whirring speeds, are kept to acceptable levels.

The output signals emitted by the strain gauges are collected by a pick-up 41 held in close proximity with a strip 42 encircling the shaft 10. The signals are transmitted to a control unit (not shown) by way of a conductor 43.

Electrical power to the strain gauges is supplied by a power unit 44 mounted on one of the clamps 12 and fed with electrical power by a battery unit (not shown) mounted on the same clamp and disposed diametrically opposite the unit 44.

A charging system for the above-mentioned battery unit is provided, comprising eight substantially equi-spaced elements 45 (only one of which is shown) mounted on the other clamp 12, (i.e. the clamp 12 not carrying the power unit 44). Unit 44 also forms part of the battery charging system. Rotation of the shaft 10 results in the elements 45 producing electrical power.

The output of the control unit can be a digital or analogue output.

More than one shear plate member may be provided. Similarly, more than two flexure portions may be employed. The positions and numbers of shear plate members and flexure portions can be varied.

The device may be used to measure thrust in other rotating shafts, for example, - large diameter shafts used to drive pumps.

Claims

1. A device for use in measuring thrust developed in a rotatable shaft, comprising means connectable to the shaft at axially spaced locations and including a member, the movement in shear of which, due to thrust forces on the shaft, is substantially the same as axial movement of that portion of the shaft between said locations.

2. A device as claimed in Claim 1, wherein the means connectable to the shaft comprises annular clamps, a connection between the clamps including said member, and a flexure region permitting angular movement of one clamp relative to the other about the shaft axis.

3. A device as claimed in Claim 2, wherein the connection comprises a beam having portions defining confronting faces, said member comprising a flat plate extending axially and radially between said faces.

4. A device as claimed in Claim 3, provided with strain gauges mounted on said faces and connected to form a Wheatstone bridge, the output of which can be employed to indicate thrust developed in the shaft.

5. A device as claimed in Claim 3 or 4, wherein the faces are connected by substantially parallel webs spaced axially from the shear plate member on both axial sides thereof and in planes disposed substantially at right angles to the plate member.

6. A device for use in measuring thrust, substantially as hereinbefore described, with reference to the accompanying drawings.

7. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.