WO1997031258A1

WO1997031258A1 - Materials testing apparatus

Info

Publication number: WO1997031258A1
Application number: PCT/GB1997/000510
Authority: WO
Inventors: Aidan Verrall
Original assignee: Mecmesin Limited
Priority date: 1996-02-23
Filing date: 1997-02-21
Publication date: 1997-08-28
Also published as: GB2310502A; GB9603908D0; AU1887597A

Abstract

Materials testing apparatus comprises a base (11), an upright column (12) and a vertically moveable member (13), for exerting a load on a test sample (not shown) attached between the base and the moveable member. The column (12) comprises a full length housing (14) which consists of two identical full length aluminium extrusions (14a, 14b) connected at their rear edges. The upper and lower ends of the housing extrusions (14a and 14b) are covered by a top plate (17) and base plate (16) respectively so that four tie rods (15), which are attached to and extend between the base plate (16) and top plate (17), hold the plates (16, 17) and housing (14) together. By tightening nuts (17a), at the top of the rods (15), to a predetermined torque, to compress disc springs (17c) at the top of the rods (15), the housing extrusions (14a, 14b), are pre-loaded with a predetermined compressive force, to increase the rigidity of the entire column.

Description

MATERIALS TESTING APPARATUS

The present invention relates to materials testing apparatus .

Known apparatus for testing the capacity of a sample of material to withstand tensile or compressive forces typically comprises a base, at least one column or tower attached to the base, and a controllably moveable member referred to conventionally as a crosshead member mounted on the column. The sample to be tested is attached both to the base and to the moveable member, wherein the latter is urged to move, thereby placing the sample under tension or compression. Apparatus of th s kind may also, for example, be used to measure the extent to which a sample may be made to flex or stretch prior to failure. The sample may comprise a piece of raw material, or may comprise a manufactured component or product.

The measurements taken may include, for example, the force required to break the sample, the force required to flex or stretch the sample to a predetermined extent or, conversely, the degree to which a sample will flex or stretch under the influence of a given force. Measurements of force are usually taken using a force gauge conveniently located on the moveable member The moveable member is conven ionally limited in its travel by limit-stops which, when contacted by the moveable member, cause it to cease or reverse its motion. Typical test procedures include repeated exercises, wherein the moveable member repeatedly cycles between two limiting positions, and single or "one shot" exercises in which the moveable member makes one journey to a predetermined end point or limit.

For accuracy of test results it is desirable that the column on which the moveable member is mounted should itself flex as little as possible under the bending moments placed on it during testing.

Typical apparatus includes a column comprising a rigid central structure surrounded by a housing or cover to conceal and protect the mechanical components of the apparatus as well as any electrical connections and wires. For increased rigidity, a twin column arrangement has been favoured wherein the moveable member spans two substantially identical columns, each of which has motive means for raising or lowering the moveable member, which motive means operate in a synchronised fashion. However, although the known twin column machines are more rigid than their single column counterparts, such twin column machines are both bulky and expensive to manufacture

According to one aspect of the present invention there is provided materials testing apparatus comprising a base, a column mounted on the base, the column including axially extending guide means within a housing, an a moveable member mounted on the guide means for axial movement on the column for exerting a load on a test sample attached between the moveable member and the base, wherein the housing is a rigid load bearing structure.

Preferably the apparatus further comprises a plurality of tie members attached to and extending between the base and a top plate, which top plate extends across an open end of the housing, wherein at least one of the tie members is tensioned to a predetermined extent so as to place the housing under an axially compressive load to oppose bending moments exerted on the column by a sample under load.

Conveniently the tie members may form the guide means.

According to another aspect of the present invention there is provided a materials testing machine comprising a base, a column secured at one end to the base, a moveable member guided on the column for movement along the column by driving means with a sample to be tested connected between the moveable member and the base, wherein the column comprises a web member extending lengthwise of the column and guide rails for the moveable member extending along the column and having guide surfaces each extending along one side of the respective guide rail, the opposite side of each said guide rail being in supporting contact with the web member, the web member being in contact with at least two said guide rails having their guide surfaces facing in opposite directions.

Advantageously the web member may be formed by an extrusion and may form part of a housing for the column.

Other features of the invention are set out in the appended claims.

The invention may include any combination of the features or limitations referred to herein. The invention may be carried into practice in various ways, but an embodiment will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which:

Figure 1 is a part-sectional side view of a materials testing apparatus according to an embodiment of the present invention;

Figure 2 is a cross-sectional view of the apparatus of Figure 1 taken along the line A-A' of Figure 1 ;

Figure 3 is a more detailed side view of a roller pre-tensioning means of the apparatus of Figures 1 and 2 ;

Figure 4 is a detailed cross-sectional view of an upper mounting of a ball screw assembly of the apparatus of Figures 1 and 2 ;

Figure 5 is an exploded view of a travel limit rod of the apparatus of Figures 1 and 2 and its attachments; and

Figure 6 is a plan view of part of an adjustable stop, of the rod of Figure 5.

Referring particularly to Figures 1 and 2 there is shown generally at 10 a materials testing apparatus comprising a base 11, an upright column or tower 12 and a vertically moveable member 13, referred to hereinafter as a crosshead 13. The column 12 comprises a full length enclosure wall or housing 14 (shown in Figure 2) which consists of two identical aluminium extrusions 14a. and 14b. interconnected at their rear edges by a full length extruded aluminium connecting member 14c.. The front edges of the extrusions 14a. and 14b_ are spaced apart to define an axial aperture through which the crosshead 13 protrudes. Full length nylon brushes 14e_ are housed m grooves m the inner faces of the extrusions 14a. and 14b. and yieldingly close the front aperture of the housing 14 while permitting movement of the crosshead therealong. In addition, the housing 14 has T-section slots 14f_ around its exterior which slots may be used to affix additional apparatus (not shown) such as guards or measurement scales .

Within the housing 14 are located four upright tie- rods 15 which extend from the base 11 to an end member comprising a top plate 17. The lower end of each rod 15 extends through a steel base plate 16 in the base 11 and carries a nut 16a and a washer 16b.. The upper end of each rod 15 extends through the top plate 17 and carries tensioning means comprising a nut 17a., a washer 17b and a disc spring 17c.. The tie tods are of surface-hardened steel.

The upper and lower ends of the housing extrusions 14a. and 14b_ are covered by the top plate 17 and base plate 16, so that the four tie rods 15 hold the plates 16, 17 and housing 14 together. By tightening the nuts 17a to a predetermined torque, to compress the disc springs l7c_ at the top of the rods 15, the housing extrusions 14a, 14b_ are pre-loaded with a predetermined compressive force, to increase the rigidity of the entire column.

Figure 2 shows the positions of the four tie rods 15 which are supported m grooves 18a. in projecting portions 18 on the inside of the full length aluminium extrusions 14a. and 14b_. The projecting portions 18 on the inside of the extrusions 14a. and 14b. together with the thick portions of the extrusions interconnecting the portions 18 form, for each of the extrusions, a very stiff web. The tie rods 15 are thus rigidly supported in the grooves 18a_..

In addition to holding together the column 12, the tie rods 15 act as guide rails for low friction bearings comprising sets of guide rollers 19 on the crosshead 13. The crosshead 13 has eight rollers 19, with first upper and lower pairs of rollers spaced vertically on the two tie rods 15 at the front of the column m use, and second upper and lower pairs of rollers spaced vertically on the tie rods at the rear of the column in use. The second, rear pairs of rollers are vertically closer than the first, front pairs of rollers.

The crosshead 13 comprises a pair of parallel steel plates 20 on which the four pairs of rollers 19 are mounted, the plates 20 being connected at the front end by a mounting block 21 and button-head screws 22 In use a force gauge (not shown) is attached to the mounting block 21 and a sample (not shown) is attached to the force gauge.

The two rear pairs of rollers, upper and lower, are pre-loaded against guide surfaces (not shown) of the rear tie rods by biasing means comprising roller pre-loading mechanisms 23, which will be described more fully below with reference to Figure 3, and which connect the two plates 20 at the rear of the crosshead 13. The crosshead 13 is driven upwards and downwards, axially along the column on the tie rods 15 by a ball screw assembly which passes between the plates 20 of the crosshead 13 wherein a screw threaded shaft 25 engages a ball nut 24 of the crosshead 13. The shaft 25 extends from the base 11 where it is connected to driving means comprising a motor gear box assembly 26, to the top plate 17 which it engages in a double thrust bearing 27, which will be described below in more detail with reference to Figure 4.

As the motor assembly 26 turns the shaft 25 the crosshead 13 is made to move axially either upwards or downwards by the engagement between the threaded shaft 25 and ball nut assembly 24 of the crosshead 13, dependent upon the direction in which the shaft 25 turns. Thus, if a sample to be tested is attached to the base 11 and via a force gauge to the mounting block 21 of the crosshead 13, a tensile or compressive force may be exerted on the sample by respectively upwards or downwards movement or attempted movement of the crosshead 13.

At the front of the column 12 in the aperture at end 14d. of the housing 14 is located movement limiting means comprising a limiting rod 28 which is arranged in use to place limits on the upward and downward movement of the crosshead 13. The limiting rod 28 is a generally C-shaped aluminium extrusion, which engages correspondingly shaped recesses (not shown) in the top plate 17 and base plate 16, via top and bottom bellows 28a and 28b respectively at opposite ends of the rod 28, so that the rod 28 is able to slide upwards and downwards. At spaced apart locations on the limiting rod 28 are located plastic adjustable stops 29 whose positions on the rod 28 are slidably adjustable and which protrude into the path of the moving crosshead

13.

The limiting rod 28 and adjustable stops 29 are described in more detail below with reference to Figure 5. In use, when the movement of the crosshead 13 causes it to abut one of the stops 29 the rod 28 to which the stops 29 are fixed is momentarily made to slide vertically, which activates a micro-switch (not shown) via a micro-switch actuator 30 at the bottom of the rod 28. When the micro- switch is actuated the motor 26 is either switched off or reversed according to a chosen operating programme.

Turning to Figure 3, this shows in more detail the roller pre-loading mechanism 23, which connects the two plates 20 of the crosshead 13 at its rear (as shown in Figures 1 and 2) .

The roller pre-loading mechanism 23 comprises a bolt 31 the threaded portion of which is engaged in a threaded crossbore in a cylindrical mounting spindle 32 for the two rollers 19. In addition the bolt 31 passes through a plain bore in a pressure block 33. The bolt 31 is thus able to slide axially through the pressure block 33. Between the pressure block 33 and a head 31a of the bolt 31 is located a stack of plain annular washers 34 and disc springs 35 through which the bolt 31 passes. The head 31a of the bolt 31 has a socket (not shown) shaped for engagement with a hexagonal key (not shown) .

The effect of the disc springs 35 is to urge the head 3la of the bolt 31 away from the pressure block 33. From a consideration of Figures 2 and 3 (only part of the roller pre-loading mechanism 23 is shown in Figure 2 for clarity) it may be seen that as the disc springs 35 urge the head 31a of the bolt 31 away from the pressure block 33, so the rollers 19 which are attached via spindle 32 to the bolt 31 are urged to bear against the tie rods 15, and that the pressure which the rollers 19 exert on the rods 15 may be increased or decreased by turning the bolt 31.

An additional advantage of using the mechanisms 23 is that this permits standard stock to be used for the tie rods 15, since any variations in thickness of the rods will be compensated for resiliently by the mechanism 23. Pre-loading mechanisms 23 are provided on the upper and lower rear pairs of rollers 19.

Referring to Figure 4, this shows the uppermost part of the shaft 25, where the shaft 25 is mounted on the top plate 17. In particular, Figure 4 shows, in cross section, the shaft 25, thrust washers 36, thrust bearings 37, a bearing guide 38, the top plate 17 and locking nut 39. As explained above, the shaft 25 which is threaded over most of its length, for engagement with the ball nut 24 of the crosshead 13, is driven by the motor and gear box 26 in the base 11. As the shaft 25 rotates in its bearing in the top plate 17 an electronic encoder (not shown) , with which the shaft is operably engaged, counts the revolutions of the shaft 25. A printed circuit board containing the encoder is shown at 40 in Figure 1. A ribbon cable (not shown) from the printed circuit board 40 conveniently extends along a recess 40a in the rear of the housing 14 (see Figure 2) and out of the base 11 to electronic control and display apparatus (not shown) , with which the testing apparatus 10 is associated. This apparatus may thus display the distance which the crosshead 13 travels along the threaded shaft 25, by electronically processing data from the encoder 40.

Turning to Figure 5, this is an exploded view of the limit rod 28 and stops 29. The rod 28 which is a generally C-shaped aluminium extrusion is mounted via plastic bellows 28a and 28b on top plate 17 and base plate 16 respectively The bellows 28a and 28b. are themselves press-riveted nto the respective plates and the rod 28 is press-riveted to the bellows 28a and 28b.. The stops 29 comprise a plastics mam body 29a, a plastics-coated nut 29b_ and a plastics-coated screw 29c., such that, once assembled, the clamps 29 may be securely fixed in position on the rod 28 by manually tightening the screws 29c., and may, conversely, be moved along the rod 28a by slackening the screws 29c. As explained above, when the crosshead 13 abuts either of the stops 29 continued movement of the crosshead moves the rod 28 either up or down a short distance as permitted by the bellows 28 _/ 28b_ This has the effect of moving the micro-switch actuator 30 to cause the motor 26 to cease operation or operate m reverse according to a predetermined program.

Figure 6 shows, in plan view, the mam body 29a of one of the stops 29 The mam body 29a includes a C- shaped aperture 29d. for engagement with the rod 28 When the screw 29c. (shown Figure 5) is tightened in the nut 29b. (also shown in Figure 5) two portions 29≤. and 29f. of the plastics body 29a are urged together to grip the rod 28 in the aperture 29d. As an initial safeguard against the crosshead 13 travelling too far downwards particularly when used for the first time, there is provided an additional permanent limit stop 41 which similarly comprises a main body 41a, nut 41b and socket-headed screw 41c., which screw 41c requires an a key to slacken or tighten it.

When the moveable member exerts a force on a sample, the tie rods 15 on which the crosshead 13 is mounted, experience a moment about the front lower pair of rollers in the case of a tensile force applied to the sample, or a moment about the front upper pair of rollers in the case of a compressive force applied to the sample. The pre¬ loading of the rear pairs of rollers opposes pivoting of the crosshead 13. The extent of pre-loading of the rear pairs of rollers is such as to permit them to withstand a shared force of, for example, up to 5,500 Newtons which may be exerted by the crosshead 13 on the sample.

Similarly, the pre-loading of the tie rods 15 is to maintain rigidity of the column 12 which might otherwise bend under the forces exerted on the crosshead 13. The degree of pre-loading of the tie rods 15 is a compromise between providing sufficient rigidity, and avoiding excessive pre-loading which might result in bowing of the housing 14. In practice, a pre-loading of 3,000 Newtons for each tie rod has been found to be suitable for the working range of forces exerted on the crosshead 13 in the example mentioned above.

An example of an operation of the apparatus 10 will now be described. To test the capacity of a sample (not shown) to withstand a tensile force, a force gauge (not shown) is attached to the mounting block 21, and the sample to be tested is attached between the force gauge and the base 11 of the apparatus, via an anchor (not shown) on the base 11. The force gauge may be either electronic or mechanical. If the force gauge is electronic, which is preferred, appropriate electronic connections may be made to the control and display apparatus (not shown) with which the apparatus is associated. The motor 26 is then activated to turn the drive shaft 25 in such a way that the crosshead 13 moves upwards. A tensile force is thus exerted on the sample, which force may be measured directly from the force gauge. By continuing to urge the crosshead 13 upwards the force exerted on the sample may be increased, until an end-point of the test is reached, either by breaking the sample, or by reaching a pre¬ determined maximum force. Conversely a test for the capacity of a sample to withstand compressive force may be performed by lowering the crosshead 13.

In addition to these tests it is possible to test for the extent to which a sample will flex, or will stretch when pulled or pressed by the moving crosshead 13. The electronic encoder in the printed circuit 40 in association with control and display circuitry (not shown) enables the extent of movement of the crosshead 13 to be measured accurately.

The control and display apparatus may perform either single "one shot" tests, or alternatively may perform repeated cycles of a particular test according to a predetermined program. The shape of the housing extrusions 14a, 14b. combines a high structural strength, a functional, simple construction and an aesthetically appealing appearance. Conveniently the connecting member 14c. may comprise the same extrusion as the limiting rod 28.

Preloading the housing 14, by means of the tie rods 15 and nuts 16ar 1 a/ results in a stiff but lightweight column. The increased rigidity permits a higher degree of accuracy when testing a sample.

Use of the tie rods 15 as guide rails supported by the massive webs 18 for the rollers 19 simplifies the construction of the apparatus, and the roller take up mechanisms 23 on the rear pairs of rollers ensure that the rollers 19 are held tightly against the tie rods, thereby preventing rocking or tilting of the crosshead even under load, and permitting the use of standard stock for the tie rods 15.

The invention may also be applied to a twin column machine, for example by turning two of the columns 12 inwardly so that the open ends 14d of their housings 14 face each other, and by providing a common base 11 and top plate 17.

Claims

CLAI S:

1. Materials testing apparatus comprising a base, a column mounted on the base, the column including axially extending guide means within a housing, and a moveable member mounted on the guide means for axial movement on the column for exerting a load on a test sample attached between the moveable member and the base, wherein the housing is a rigid, load bearing structure.

2. Apparatus according to Claim 1, further comprising a plurality of tie members attached to and extending between the base and a top plate, which top plate extends across an open end of the housing, wherein at least one of the tie members is tensioned to a predetermined extent so as to place the housing under an axially compressive load to oppose bending moments exerted on the column by a sample under load.

3. Apparatus according to Claim 2, wherein the tie members form the guide means.

4. Apparatus according to any of Claims 1 to 3, wherein the moveable member is guided with pre-load on the guide means .

5. Apparatus according to any of Claims 1 to 4, wherein the housing defines a cavity therein, in which is located the guide means, the housing comprising two portions wherein each portion extends axially for substantially the full length of the housing, the portions being joined at respective axially extending first edges thereof and spaced apart at respective axially extending second edges thereof to define an axially extending aperture through which the moveable member projects.

6. Apparatus according to Claim 5, further comprising movement limiting means arranged to limit the movement of the moveable member, the movement limiting means being located in the axially extending aperture and comprising a rod, slidably mounted in the aperture, and a number of stop members adjustably mounted on the rod, the or each stop member being arranged to project into the path of the moveable member as it moves axially on the column m use, thereby to define one limit of axial movement of the moveable member, wherein sliding movement of the rod is arranged in use to activate a switch which controls the operation of a motor which drives the moveable member.

7. Materials testing apparatus substantially as herein described, with reference to the accompanying drawings .

8. A materials testing machine comprising a base, a column secured at one end to the base, a moveable member guided on the column for movement along the column by driving means with a sample to be tested connected between the moveable member and the base, wherein the column comprises a web member extending lengthwise of the column and guide rails for the moveable member extending along the column and having guide surfaces each extending along one side of the respective guide rail, the opposite side of each said guide rail being in supporting contact with the web member, the web member being in contact with at least two said guide rails having their guide surfaces facing in opposite directions.

9. A materials testing machine according to Claim 8, wherein the web member is formed by an extrusion.

10. A materials testing machine according to

Claim 8 or 9, wherein the web member forms part of a housing for the column.

11. A materials testing machine according to any of the Claims 8 to 10, wherein the rails extend into the base and an end member at the opposite end of the column to the base and are pretensioned to place the web member under compression.

12. A materials testing machine according to any of the Claims 8 to 11, wherein the moveable member has two longitudinally spaced guide rollers in guiding contact with each rail.

13. A materials testing machine according to Claim 12, wherein the guide rollers are pre-loaded against the guide rails.

14. A materials testing machine substantially as herein described with reference to the accompanying drawings .