GB2101749A

GB2101749A - An apparatus and a method for applying and measuring tensile force

Info

Publication number: GB2101749A
Application number: GB08202949A
Authority: GB
Inventors: Dennis John Taylor Vaughan
Original assignee: FIRA FURNITURE INDUSTRY RESEAR
Current assignee: FIRA FURNITURE INDUSTRY RESEAR
Priority date: 1981-05-18
Filing date: 1982-02-02
Publication date: 1983-01-19
Also published as: GB2101749B

Abstract

A conventional torque-indicating wrench (not shown in the Figure) is used to rotate a wheel 1 of known diameter to produce a calculable tensile force in an elongate flexible member 5 which extends at a tangent to the circumference of the wheel, towards a sample 8 attached to a sample-retaining means 9. The wheel, which may comprise a plurality of concentric pulleys, rotates about an axis 2 which is in fixed spatial relationship to the sample- retaining means 9. An alternative sample-retaining means 10 is disclosed and is depicted retaining a sample 12. The apparatus is used to measure cross-tensile strength, surface soundness and screw-holding ability of particle boards and fibre building boards. It could also be used to conduct a "peel" test. <IMAGE>

Description

SPECIFICATION An apparatus and a method for applying and measuring tensile force This invention relates to the field of tensile strength testing. In particular, the invention relates to an apparatus and a method for applying and measuring tensile force. A preferred use in the testing of particle board is described.

Common materials in the furniture manufacturing industry include composite boards such as wood chipboard. These comprise small particles of wood bonded with a resin under conditions of elevated temperature and pressure to form boards. The boards may then be veneered with a quality wood or a synthetic material, for example a plastics material. All such boards are generically termed particle boards. The popularity of particle board in the furniture industry is largely due to its cheapness, ease of machining and its stability. In particular, its lack of ordered grain reduces the risk of warp deformation in a finished piece of furniture.

The widespread use of particle board for furniture manufacture has led to the establishment of performance requirements as a basis for selection between the various brands of board now available. In particular, the British Standards Institute issued BS 5669 in 1979.

This standard, entitled "Specification for Wood Chip Board and Methods of Test for Particle Board" sets out a series of tests to be applied to particle board and gives minimum levels for performance of wood chipboard to be used in furniture applications.

Furniture manufacturers often specify particle board quality by density, but density is important only insofar as high density is likely to be associated with high strength properties.

Research and experience has indicated that the relationship between density and strength is not, however, a constant factor.

A more useful indication of the quality of a sample of particle board is its tensile strength perpendicular to the plane of the sample. This property is often called "the cross-tensile strength" and gives an indication of the efficiency of bonding between individual particles used to form a board. In this test a piece of particle board which is, say, 40 mm. square is used. To each face of the sample a support block of the same dimensions is securely bonded. A tensile force is then applied, pulling the two support blocks away from each other until the particle board fails. The sample will fail in the weakest layer in the plane of the board and, with layered boards, this is normally close to the centre of the board.If the cross-tensile strength of the board is low, delamination failures are likely to occur at dowelled or knock-down fitting corner joints between panels forming, for example, a cabinet. British Standard 5669 Appendix C2, recommends a minimum cross-tensile strength of 0.40 MPa for chipboard used for furniture applications. In the case of a test sample 40 mm. square this corresponds to a tensile force of 640 N.

A second important property of particle board for furniture construction is a high resistance to axial withdrawal of wood screws.

This property is often called screw-holding, and is useful for determining the screw-retention characteristics of a particle board. The BS 5669 test involves using a test sample 75 mm. square and inserting a 38.1 mm No. 6 gauge screw 1 3 mm. deep into a face of the sample and two adjacent edges of the sample.

A pilot hole 1.5 mm. in diameter and 6 mm.

deep is used in each case to assist insertion of the screw. A load is then applied axially to each screw in turn and the force required to withdraw the screw is measured. British Standard 5669 Appendix C2 recommends a minimum force of 450 N for withdrawal of a screw inserted into the face of the sample and a minimum of 400 N for a screw inserted into the edge of a sample. A furniture manufacturer attaching hinges and other fittings to a particle board panel will be concerned with the screw-holding characteristics of both the face and the edges. The development of special particle board fittings which distribute the load over a wider area has helped to overcome some of the problems of fixing, but the ability to retain screws is still a desirable property of furniture-grade particle board.

The cross-tensile strength test described previously measures the core strength of a board. It is also important to test the strength of a board close to its surface. This test is often known as the surface soundness test. A sample of particle board, 75 mm. square, has bonded centrally to each of its faces a support block 40 mm. square. A force is applied, pulling the two support blocks apart and the force at which the surface of the particle board fails is measured. British Standard 5669 Appendix C2 recommends a minimum value of 1 300 N for surface soundness of wood chipboard. Wood veneers or plastics laminates bonded to boards with low surface strength are likely to separate from the board as a result of differential movement between board and the surfacing material.Another problem with boards of low surface strength is that they are easily damaged, and special care must be taken when handling and machining such boards.

Finally, the modulus of elasticity is, in certain applications, an important property of particle board, as it determines the load-bearing characteristics of shelves or work-surfaces constructed of that board. Excessive deflection of the shelves in a bookcase constructed of particle board with a low modulus of elasticity will give rise to complaint purely on aesthetic grounds. The modulus of elasticity of wood veneers or plastics laminates used as surfacing materials has a substantial stiffening effect on panels. The stiffness of painted, melamine and PVC foil surface panels, however, relies substantially on the stiffness of the board alone. The test set out in British Standard 5669 is to take a sample of particle board 100 mm. wide and 20 times the nominal thickness of a board pius 25 mm. in length.

The board Is then supported at either end ana centrally loaded. The deflection caused by a known load gives a measure of the modulus of elasticity of the board.

The tests described above are all used to provide quality control on the manufacturer's particle boards. In particular, the cross-tensile strength, surface soundness and screw-holding tests are important. In order to perform the tests it is necessary to use a machine capable of providing a substantial force and of indicating the force at which failure occurs.

The existing equipment for such purposes is often expensive and complex to use, requiring skilled labour to produce meaningful test results. For this reason, the testing of particle bards has largely been confined to the manufacturers of particle board, the larger furnitur4e manufacturers and test houses which have the necessary substantial resources.

It has now surprisingly been found possible to provide an apparatus and a method for applying and measuring a tensile force which will provide a simple and cheap means of conducting quality control ir particle board, means which, for example, will enable a small furniture manufacturer to check the quality of incoming supplies of such board.

According to the present invention we provide an apparatus for applying a tensile force to a sample and for measuring the value of the tensile force applied, the apparatus comprising, a sample-retaining means, a wheel whose axis is in fixed spatial relationship to the sample-retaining means, an elongate flexible member having a first end and a second end, the first end being attachable to the sample and the second end being attachable to a point on a circumference of the wheel such that, when so attached, the elongate flexible member extends towards the sample at a tangent to the circumference of the wheel, and a torque-indicating wrench adapted to apply a measurable torque to the wheel.

Accruing to the present invention we also provide a method for applying a tensile force to a sample and measuring the value of the tensile force applied, wherein the sample is retained in a sample-retaining means, a first end of an elongate flexible member is attached to the sample, a second end of the elongate flexible member is attached to a point on a circumference OT a wheel, whose axis is in fixed spatial relationship to the sample-retaining means, such that the elongate flexible member extends towards the sample at a tangent to the circumference of the wheel and wherein a torque-indicating wrench, adapted to co-operate with the wheei, Is used to apply a measurable torque to the wheel, the torque producing a tensile force of calculable magnitude in the elongate flexible member, thus subjecting the sample to a measurable tensile force at a tangent to the circumference of the wheel.

Preferably the elongate flexible member is a steel cable. Alternatively the elongate flexible member may be a flexible strip, for example a strip of steel.

Preferably the wheel comprises two or more concentric pulleys whose diameters are all different. In an especially preferred embodiment, the wheel comprises two pulleys of different diameters. The provision of a wheel having more than one effective diameter allows a wider range of tensile force to be produced using a torque wrench of limited range. The pulleys may, for example, be of diameters 60 and 1 30 mm. The wheel preferably consists of a single casting or mouiciing incorporating a plurality of pulleys.

Preferably the torque-indicating wrench has a range of 5 to 1 50 Nm, more preferably 5 to 90 Nm and most preferably 6 to 80 Nm.

Preferably the torque-indicating wrench is provided with an indicator which registers the maximum torque applied. This allows for a failure torque to be recorded and hence a failure tensile force to be calculated.

In another aspect of the invention we provide a method of testing the strength of a sample which comprises subjecting the sample to the method described above in which a predetermined tensile force is applied to the sample. This predetermined tensile force may, for example, be slightly above a minimum strength value for a particular sample. If failure of the sample occurs at the pre-determined tensile force, then the sample may be deemed to have failed the test.

In yet another aspect of the invention, we provide a method of testing the strength of a sample which comprises subjecting the sample to the method described above in which an increasing tensile force is applied to the sample and the tensile force at which failure of the sample occurs is measured. A test of this sort allows for a measurement of the strength of a sample rather than a simple indication of its compliance with a pre-deter- mined minimum strength threshold.

The methods of the present invention may suitably be used for applying a tensile force to other materials, especially particle board fibre building board and other wood based materials. This may form the basis of a quality control assessment.

We now described a specific embodiment of the present invention with reference to the accompanying diagrams in which Figure 1 is a plan view of a preferred embodiment of the invention.

Figure 2 is a front elevation of the embodiment depicted in Fig. 1.

Figure 3 is a side elevation of the embodiment of the invention depicted in Fig. 1.

In this embodiment a pulley wheel 1 is free to rotate about its axis on a bearing spigot or axle 2. The axle 2 is secured to a base-plate 3. The pulley wheel 1 is fitted with a square drive sprocket 4 which will accept the square drive end of a torque-indicating wrench (not shown). One end of a flexible steel cable 5 (shown only in Fig. 1) is attached to a point on the circumference of the pulley wheel 1 at a point indicated by the numeral 6. The other end of the cable 5 is fitted with a linkage 7 which connects it to one end of a test sample 8. The other end of the test sample is connected to either a spigot 9 or a frame 10, depending upon the manner of test to be performed. For example, the spigot 9 is used for securing particle board test samples for cross-tensile strength and surface soundness tests. The frame 10 is used for securing screw-holding particle board samples.

In operation the square drive end of a torque-indicating wrench (not shown) is located in the square drive socket 4. The pulley wheel 1 is rotated in a clockwise direction (as indicated by the arrow in Fig. 1) by the torque wrench which produces a tensile force in the cable which is transmitted to the test sample.

The force is increased until failure of the test sample results. The torque required to produce such failure is indicated by a "lazy finger" pointer on the torque wrench. The torque indicated may be readily converted into a tensile force value since force is directly proportional to torque, in particular torque (t) force(f)= ----------- radius (r) It is apparent from this that the force that can be applied to test samples depends upon the force range of the torque wrench and the size of the pulleys employed. In practice, two pulleys may be used or preferably one casting comprising two integral pulleys. For example, one pulley may have a diameter of 60 mm.

and the other a diameter of 1 30 mm. The torque-indicating wrench allows for the application of torque which is, for example, between 6 and 80 Nm. Thus the overall range of tensile force afforded by this embodiment is in the range of 92 to 2667 N.

In use the base board 3 of the apparatus should be securely screwed or bolted to a rigid bench and the pulley 1 located on the axle 2. The pulley shown in Figs. 1, 2 and 3 is a single pulley. In a preferred embodiment, a casting comprising two pulleys of different diameters may be used. The "double" pulley increases the sensitivity of the apparatus to ensure accurate measurement of test sample failure loads. The large diameter pulley is used to apply tensile force between 92N and 1 230 N and the smaller diameter pulley is used to provide a tensile force in the range of 200 to 2667 N.

The apparatus can be used to measure the strength properties of many materials although it is primarily designed to measure three important stress properties of particle board as indicated above.

Testing for cross-tensile strength.

A 40 mm. square by approximately 20 mm. thick beech block of other dense hardwood block is bonded to each face of a 40 mm. square of the sample to be tested.

The operator must ensure that the side grain of the block is in contact with the test sample.

A suitable adhesive for bonding the blocks to the sample is room temperature curing UF or PVA adhesive. After the adhesive has cured, screw eyes may be inserted into the centre of the beech block. To facilitate this, a pilot hole in each of the blocks may be used. For particle board a pulley of diameter 1 30 mm.

may normally be used in conjunction with the standard torque wrench covering 6 to 80 Nm.

The cable 5 is attached to the pulley 1 and the linkage 7 is connected to a screw eye on the sample. The torque wrench is used to turn the pulley slowly in a clockwise direction to apply a uniformly-increasing load to the test sample. A "lazy finger" on the dial of the torque wrench will record the torque at the point of failure of the test sample. This reading is then converted to a cross-tensile strength using the formula given above.

Cross-tensile strength is defined in terms of a stress, that is to say, a force per unit area and it is therefore necessary to divide the failure force in newtons by the cross-sectional area of the sample in square metres.

Surface soundness.

The use of boards with low surface soundness can result as described above in the delamination of decorative surface materials such as plastics, laminate or wood veneer. In testing the surface soundness, 40 mm. square by approximately 20 mm. thick beech or other dense hardwood blocks are bonded centrally to each face of 75 mm. square sample of particle board. As above, a room temperature curing UF adhesive such as Cascamite (Trade Mark) may be used. The force required to induce failure of a test sample in a surface soundness test is considerably greater than that for a cross-tensile strength test and a smaller pulley, for example a 60 mm pulley, may be used. Otherwise the test procedure follows the procedure described above for cross-tensile strength testing.

Screw-holding.

Clearly, if a material is to be useful in the furniture industry, satisfactory screw-holding strength is a positive requirement. In testing for screw-holding ability, a 75 mm. square of particle board is prepared by drilling pilot holes 1.5 mm. in diameter by 6 mm. deep into the mid-points of one face and two adja- cent edges of the sample. No. 6 gauge 38 mm. long slotted, counter-sunk mild steel wood screws are then inserted into these pilot holes to a depth of 1 3 mm.This sample is mounted in the frame 10 as shown in Fig. 1 which depicts the sample 1 2 abutting the frame 10 with the screw to be tested 1 3 protruding through an aperture 1 4 in the frame 1 0. The cable 5 is then attached to the screw 1 3 and the force applied as previously described. The remaining screws in the sample are tested in a similar way. Again the tensile force required to withdraw the screw axially is recorded.

The above tests correspond to those laid down in British Standard 5669, but in principle the apparatus described herein may be used for any form of testing in which a tensile force is to be measured. For example, the apparatus may be used to carry out tests on other materials by adapting the sample-retaining means. It is envisaged that a "peel" test could be suitably conducted using this apparatus.

Claims

1. An apparatus for applying a tensile force to a sample and for measuring the value of the tensile force applied, the apparatus comprising; a sample-retaining means, a wheel whose axis is in fixed spatial relationship to the sample-retaining means, an elongate flexible member having a first end and a second end, the first end being attachable to the sample and the second end being attachable to a point on a circumference of the wheel such that when so attached the elongate flexible member extends towards the sample at a tangent to the circumference of the wheel, and a torque-indicating wrench adapted to supply a measurable torque to the wheel.

2. An apparatus according to claim 1 wherein the elongate flexible member is a steel cable.

3. An apparatus according to claim 1 wherein the elongate flexible member is a flexible strip.

4. An apparatus according to claim 3 wherein the flexible strip is a strip of steel.

5. An apparatus according to any one of the preceding claims wherein the wheel comprises two or more concentric pulleys whose diameters are all different.

6. An apparatus according to claim 5 wherein the wheel comprises two pulleys of different diameters.

7. An apparatus according to claim 5 or 6 wherein the wheel consists of a single casting or moulding incorporating a plurality of pulleys.

8. An apparatus according to any one ot the preceding claims wherein the torque-indicating wrench has a range of 5 to 1 50 IMm.

9 An apparatus according to claim 8 wherein the torque-indicating wrench has a range of 5 to 90 Nm.

1 0 An apparatus according to claim 8 or 9 wherein the torque-indicating wrench has a range of 6 to 80 Nm.

11. An apparatus according to any one of the preceding claims wherein the torque-indicating wrench is provided with an indicator which registers the maximum torque applied.

1 2. A method for applying a tensile force to a sample and measuring the value of the tensile force applied, using the apparatus according to any of the preceding claims wherein the sample is retained in a sampleretaining means, a first end of an elongate flexible member is attached to the sample, a second end of the elongate flexible member is attached to a point on a circumference of a wheel whose axis is in fixed spatial relationship to the sample-retaining means, such that the elongate flexible member extends towards the sample at a tangent to the circumference of the wheel and wherein a torque-indicating wrench, adapted to co-operate with the wheel, is used to apply a measurable torque to the wheel, the torque producing a tensile force of calculable magnitude in the elongate flexible member, thus subjecting the sample to a measurable tensile force at a tangent to the circumference of the wheel.

1 3. The method according to claim 1 2 when used for applying a tensile force to samples of wood.

1 4. The method according to claim 1 3 when the wood sample is a sample comprising at least one portion of particle board.

1 5. A method of testing the strength of a sample which comprises subjecting the sample to a method according to any one of claims 1 2-1 4 in which a pre-determined tensile force is applied to the sample.

1 6. A method of testing the strength of a sample which comprises subjecting the sample to a method according to any one of claims 12-14 in which an increasing tensile force is applied to the sample and the tensile force at which failure of the sample occurs is measured.

1 7. An apparatus according to claim 1-11 as hereinbefore described and with reference to the accompanying drawings.

18. A method according to claims 12-15 as hereinbefore described and with reference to the accompanying drawings.