GB2132778A - Load measuring apparatus - Google Patents

Abstract

Apparatus for determining the load in an elongated member such as a rod 11 comprises spaced apart clamps 1,2 interconnected by keeper members 3 and adapted to receive the rod 11 therebetween and be fixedly mounted thereto in a releasable manner. A displacement measuring device, for example a linear variable differential transducer 12 extends between the clamps 1,2 and is adapted to sense variations in distance therebetween and provide an output indicative of the corresponding change in load in the rod 11. <IMAGE>

Description

SPECIFICATION Load measuring apparatus This invention relates to apparatus for determining the tensile, compressive or bending load in an elongate member such as a rod, cable, strut or the like. By way of background the invention will be described in the context of its application to suspension slings for power station boilers but as will become apparent its applications extend far beyond this particular field.

In conventional power stations it is not uncommon for the boilers to be suspended over the fuel beds by hundreds of U-shaped steel slings. This suspension, rendered necessary because of problems of thermal expansion, is from beams in the roof of the power station to which the slings, which may be from 2cm to 9cm in diameter, are tensionably attached in rows by nuts or threaded sections at their upper ends.

It is essential that these slings be tensioned correctly so that the boiler weight is spread evenly, since if one sling is overstressed to failure, the tension in others will increase accordingly and cumulative fracture can occur resulting in disastrous collapse of the boiler.

Various methods have bsen proposed in the past to check the tension in individual slings.

One of the quickest but perhaps the most primitive methods is to tap each sling with a hammer and listen to the tone produced. This method depends on the experience and expertise of the tapper, but at its best can be only crudely accurate.

Another method would be to attach strain gauges to each sling to give individual measurements as an output. Whilst the accuracy of such a system would theoretically be highly desirable, as a practical matter it is difficult and expensive to realise and, because of the number of wires and the operating conditions of heat and dirt involved, almost impossible to keep properly maintained.

A third method is to jack up the end of each sling and determine the tension load by reference to the pressure in the hydraulic jack.

This method however is cumbersome and is poor in accuracy.

The object of the invention therefore is to provide apparatus whereby the load in a sling can be measured relatively quickly and conveniently yet to a high degree of accuracy. Such apparatus according to the invention comprises a pair of spaced apart clamps interconnected by a keeper member, each clamp comprising a pair of hingedly connected clamping members adapted to receive said rod, cable, strut or the like therebetween and be fixedly mounted thereto in a readily releasable manner, a displacement sensing device being mounted to a clamping member of one pair of members and cooperating with a corresponding member of the other pair of members to sense variation in distance between the clamps and provide in response thereto an output indicative of the corresponding change in load in the rod, cable, strut or the like.

Thus the invention provides an apparatus which can be portable and can readily be mounted on the member to be tested. In use it will be presented to the member and clamped thereto. Preferably the sensing device output will be adjusted to provide a datum, and then the stress in the member will be relieved. The resulting expansion or contraction of the member will cause a variation in the distance between the clamps which will be sensed by the transducer to give an output indicative of the load previously existing in the test member. This then enables the load adjustment means, i.e. the nut or other threaded connection of the sling, to be adjusted to give the correct tension. The apparatus may then be readily dismounted from the test member and transferred to another.

The displacement sensing device preferably comprises a linear variable differential transducer, although other forms are possible, for example a conductive plastics potentiometer or a linear inductive transducer. Alternatively, the displacement sensing device may involve the use of fibre optics. In one known such device, light is transmitted to the device via a single input fibre optics cable and is split into two beams by means of an arrangement of prisms. The two beams are returned to a suitable detector via separate output cables, and relative variations in the light intensities in the output cables are indicative of relative movements between the input and output cables. Such a device has the advantage of being immune from temperature effects and electrical interference. The remaining description will however refer to the preferred form of the device.

The keeper member is required to connect the two clamps together to form a single unit and also to maintain the spacing between the clamps. Clearly, relative movement of the clamps is required for the apparatus to function, but on the other hand they should be fixable in relative position when the apparatus is being handled, to maintain to an extent the setting of the transducer, and to prevent wild impingements on it which could damage it.

To obtain both benefits, in a preferred embodiment of the invention the keeper is selectively manually lockable for transport or releasably extendable for use.

The rods, cables, struts or the like which may be required to be tested may be of varying sizes, for example, as mentioned, from 2cm to 9cm thick. In order to accommodate test members of different sizes the hinged connections of the clamps are preferably extendable to vary the distance between the hinged parts of the clamping members. A preferred way of achieving this is for each hinge to comprise a threaded rod pivotably connected to one clamping member and pass ing through an aperture in the other which is retained on the rod by an adjustable knob handle or the like threadedly engaged there with.

It can happen that because of settlement or assymmetrically supported nuts the rod, strut or the like is subject not only to a tensile or compressive load, but also to a bending load.

Since the point of action of the transducer will necessarily be spaced from the neutral axis of the rod, strut or the like, this can introduce inaccuracy in the reading taken from one side of the test member. In a particularly preferred form of the invention this difficulty is over come by providing a second linear variable differential transducer mounted on the other member of one pair and cooperating with a corresponding member of the other pair to sense variation in distance between the clamps on the other side of the test member and provide in response thereto an output indicative of the corresponding change in load. Thus by combining the readings ob tained from both transducers, the effects of bending can be cancelled out and a true result obtained.

The output from the transducers may be visual, but an electrical output connectible to a remote control unit is preferred. Such a unit may be arranged to provide readings directly in tons and may be arranged to receive out puts not only from the preferably two trans ducers of one apparatus, but from a plurality of apparatuses as well. If for example three slings are tested at the same time, the effect on adjacent slings of tensioning or detension ing a selected sling can be measured.

Above all the apparatus of the invention can be made fully portable. The said control unit can be mounted in a carrying case which is also adapted to receive the measuring apparatus and detachable leads for connecting one to the other.

In order that the invention may be readily understood an embodiment thereof will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is a perspective view of the apparatus in its open configuration, Figure 2 shows the apparatus locked on to a rod to be tested, Figure 3 is a part sectional scrap view showing the point of action of the transducer, and Figure 4 is a block diagram of the electrical circuit.

Referring to Figs. 1 and 2, the apparatus comprises a pair of clamps 1, 2 intercon nected by spacer/keeper members 3. To one clamping member 4 of each clamp is pivotally connected a threaded rod 5 which passes through a registering aperture in the other clamping member 6 which is retained thereon by a knurled nut 7 to form an adjustable hinge.

Another threaded rod 8 is fixedly connected to each clamping member 4 which is adapted on closure of the clamps to be received in slot 9, the clamps being tightenable by means of manual locknuts 10 threadedly engaged on rods 8. It will be seen that nuts 7 enable the general size of the clamps to be adjusted, the final clamping action being achieved by locknuts 10. In this way various different sizes of members to be tested, shown as 11, may be accommodated.

To each of the upper clamping members 4, 6 is mounted a linear variable differential transducer 1 2 which cooperates with a rod 1 3 (only one being visible) which is fixedly mounted to the opposed lower clamping member 4, 6. The nature of this cooperation is seen better in Fig. 3. In a threaded bore 14 formed in the top of rod 1 3 is threadedly engaged an adjusting member 1 6 which contacts the displaceable pin 1 5 of the transducer. In this way the transducer can be zeroed by appropriate rotation of the adjusting member 16.

Reverting to Figs. 1 and 2 the keeper members 3 are sliding received in bores formed in upper clamping members 4, 6 wherein they are screw lockable, for transport of the apparatus, by knurled locking knobs 1 7. The electrical output of the transducers is obtained via sockets 18 which in storage and transport mode are covered by protective caps.

Turning now to Fig. 4, the power supply to the apparatus is provided by a battery 1 9 feeding to the transducers via a stable voltage regulator 20 to maintain a constant voltage.

These items are housed in a portable carry and control unit (not shown) and connected to the transducers 1 2 by detachable electrical leads 21 whose distal ends carry plugs engageable in sockets 1 8. The leads also incorporate return wires which connect into the control unit.

The return circuit passes through a rod diameter selector 22 which enables the control unit to be adjusted by the user for use with discrete sizes of test member from say 2cm to 9cm in diameter, by appropriate rotation of a selector knob. The apparatus can also deal with test members of different metals by a similar selector 23 for youngs modulus values varying between say mild steel and light aluminium alloy. This is not essential however, as the device can be factory set for use with steel rods only. This setting is obtained through the calibration attenuator 24.

The readout 25 of the apparatus can take any suitable form, but for ease of use, an LED display is preferred, which reads out directly in tons.

In use, the upper and lower clamps 1, 2 are clamped to the test member 11 after suitable adjustment of the hinges. The keeper members 3 are then released by the release knobs 1 7 and the readout is zeroed by use of the adjusting member 1 6. The stress in the test member is then relieved and the average of the two readings obtained from the respective transducers is taken to eliminate the effects of bending stresses as explained earlier. This then gives the initial stress present in the test member, and the stress can now be reimposed until a zero reading is obtained, if it was the desired stress, or it can be corrected to the required stress.

The control unit may have as many readouts as desired. In the above example two would be preferred so that instantaneous readings of both transducers can be seen. Six readouts however would accommodate three such apparatuses if the monitoring of adjacent slings were required. More could of course be provided. The control unit may of course also incorporate memory facilities, for example using a microprocessor and memory store, to give an added range to its capabilities. Thus the tension in all slings may be recorded for subsequent calculations and analogue displays.

As mentioned above, the invention has been described with particular reference to suspension slings for boilers. Other structural members however could be tested by apparatus in accordance with the invention, for example in bridges, oil rings or steel scaffolding either around buildings or formwork, or indeed any load carrying prop or tie such as a coal mine shaft support. To measure the load in the legs of say a silo enables by suitable calculation the weight of the contents thereof to be determined.

In addition to its principal function described above, the apparatus could also be used for secondary uses. If say it was desired to accurately determine the extension of two telescopically engaged members, this could be done by mounting each clamp to a respective member and from a suitable datum, monitoring the amount of extension or retraction from the readout of the device.

Claims (9)

1. Load measuring apparatus comprising a pair of spaced apart clamps interconnected by a keeper member, each clamp comprising a pair of hingedly connected clamping members adapted to receive said rod, cable, strut or the like therebetween and be fixedly mounted thereto in a readily releasable manner, a displacement sensing device being mounted to a clamping member of one pair of members and cooperating with a corresponding member of the other pair of members to sense variation in the distance between the clamps and provide in response thereto an output indicative of the corresponding change in load in the rod, cable, strut or the like.

2. Apparatus as claimed in claim 1 further comprising means for adjusting the sensing device output to provide a datum.

3. Apparatus as claimed in claim 1 or 2 wherein the sensing device comprises a fibre optics displacement measuring device.

4. Apparatus as claimed in claim 1 or 2 wherein the sensing device comprises a linear variable differential transducer.

5. Apparatus as claimed in any preceding claim wherein the keeper is selectively manually lockable for transport or releasably extendable for use.

6. Apparatus as claimed in any preceding claim wherein the clamps are extendable to vary the distance between the hinged parts of the clamping members.

7. Apparatus as claimed in claim 6 wherein each hinge comprises a threaded rod pivotably connected to one clamping member and passing through an aperture in the other which is retained on the rod by an adjustable knob handle or the like threadedly engaged therewith.

8. Apparatus as claimed in any preceding claim further comprising a second displacement sensing device mounted to the other member of one pair and cooperating with the corresponding member of the other pair to sense variation in distance between the clamps on the other side of the test member and provide in response thereto an output indicative of the corresponding change in load.

9. Apparatus as claimed in any preceding claim wherein the or each displacement sensing device is connected to a control unit adapted to provide an indication of load changes.

1 0. Load measuring apparatus substantially as herein described with reference to the accompanying drawings.