This invention relates to a test apparatus, and in particular t o a test apparatus intended for use
in deriving data representative of characteristics of a compacted or consolidated powder or
granular material. In particular, the invention relates t o a test apparatus suitable for use in
achieving such compaction or consolidation, and t o methods of operation thereof. The test
apparatus may further be capable of undertaking tests to provide information relating to
characteristics of a material under test.
A number of devices are known for conducting tests on powders or the like. In one such
apparatus, a column of the material under test is compacted within a mould or test chamber.
The mould or test chamber is then removed, and loads are applied to the compacted material
under test to identify how large a load can be applied t o the compacted material under test
before the column of material yields or fails. Such a test is sometimes referred to as an
unconfined yield strength test. It will be appreciated that the accuracy of the data produced by
this test will depend, in part, upon the uniformity of the compaction of the test material. If the
material is non-uniformly compacted so that regions thereof are compacted to a lesser degree
than is desired, then the column of material may yield or fail upon the application of a lower
load than would be the case if the material were uniformly compacted to the desired level, and
as a result the test results may be erroneous or inaccurate.
Whilst this constitutes one example of a test conducted upon a powder or granular material,
other tests are also known, some of which require the powder or granular material under test
to be compacted in a controlled fashion as part of or prior to conducting the test. By way of
example, powder rheology tests may be conducted upon the compressed powder, for example
using the apparatus and methods described in EP0798549B or in WO2003/048743.
Where a compressive load is applied to a quantity of powder or granular material in an attempt
to compact the material, the nature of the material can often result in limited radial movement
of parts of the material occurring as a result of the application of an axial compacting or
compressing load. Where the material is contained within a mould or test chamber, then such
movement is limited, but radial stresses will accumulate and the material will be forced against
the wall of the mould or test chamber. Frictional resistance between the compacted material
and the mould or test chamber can result in parts of the material remote from the surface at
which the compressive or compacting load is applied being exposed to a significantly reduced
compressive or compacting load. Clearly, as mentioned above, the use of a technique such as
this to compact a material for use in a test of the type outlined hereinbefore may lead to
inaccuracies in the test results arising from the non-uniform compaction or consolidation of the
material being tested.
US2013/0086979 describes an apparatus for use in conducting tests on fluids. The apparatus
bears some similarity to the arrangement described above. However, as the characteristics of
fluids are very different to those of powder or granulated materials, it will be appreciated that
the arrangement of US2013/0086979 is only of very restricted applicability to the present
invention. Specifically, US2013/0086979 provides no teaching to suggest that the auger thereof
is suitable for use in applying compressive or compacting loads to a powder or granulated
material under test.
It is an object of the invention to provide a test apparatus whereby the uniformity of compaction
can be enhanced. The invention also relates to methods of conducting a test using the test
apparatus.
According to the present invention there is provided a test apparatus comprising a test chamber,
a powder manipulation device, and a drive arrangement selectively operable to drive the
powder manipulation device for axial movement and for rotary movement relative to the test
chamber, the powder manipulation device comprising a generally helical screw flight.
Preferably, the generally helical screw flight is of at least 360° angular extent. It will be
appreciated that as a result, movement of the powder manipulation device can achieve
controlled compaction of a material under test. The compaction may occur substantially
uniformly over the full cross-sectional area of the test chamber.
In use, where the material under test is to be compacted, the material under test is introduced
into the test chamber. The powder manipulation device (which will be referred to hereinafter
as a compaction device, for convenience) is advanced into the test chamber, being driven for
forward rotation whilst being advanced axially into the test chamber. Once the compaction
device has reached a predetermined position, rotation thereof may cease. Continued axial
movement thereof will serve to compact the material immediately ahead of the compaction
device. Subsequently, the compaction device may be retracted by a predetermined distance
whilst undergoing reverse rotary motion. After retraction, the compaction process of advancing
the compaction device whilst the device is not rotating may be repeated. It will be appreciated
that by repeating this process a number of times, the material within the test chamber may be
compacted, in stages, with a good degree of uniformity.
In an alternative mode of use, the compaction device may be advanced axially into the material
under test, whilst also undergoing forward rotation. Once a predetermined position has been
reached, retraction of the compaction device may be undertaken, whilst rotation of the
compaction device continues, the speed of retraction of the compaction device being controlled
relative t o the speed of rotation thereof to ensure that the material is compacted to a desired
extent.
The compaction device conveniently comprises a shaft carrying the screw flight. Preferably, a
single turn of the generally helical screw flight is provided. However, arrangements in which
the screw flight is of greater or lesser angular extent than this, for example including several
turns, are possible.
The compaction device is preferably of metallic, for example steel or aluminium construction.
Such an arrangement is advantageous in that the compaction device may be of good strength.
However, depending upon the application in which the invention is to be used, and the test
materials with which it is intended to be used, other materials may be used. By way of example,
in some applications the compaction device may be of a suitable plastics material.
The invention also relates to methods of using the test apparatus in conducting tests upon a
powder or granular material. In one arrangement, the method comprises the steps of moving
the powder manipulation device to a predetermined position, advancing the powder
manipulation device whilst holding the device against rotation to compact the material ahead
of the powder manipulation device, retracting the powder manipulation device whilst rotating
the powder manipulation device to space the screw flight of the powder manipulation device
from the compacted material, and repeating the step of advancing the powder manipulation
device whilst holding the device against rotation. In this manner, a column of compacted
material may be built up, the column having a substantially uniform level of compaction.
In another approach, the powder manipulation device is advanced into the material under test,
and is subsequently withdrawn therefrom whilst being rotated, the speed of retraction of the
powder manipulation device being controlled relative to the rotary speed thereof to achieve a
desired, substantially uniform, level of compaction in the material under test.
In yet another mode of operation, the powder manipulation device may be advanced into the
material under test and subsequently withdrawn therefrom whilst held against rotation to
remove a quantity of material from the test chamber.
The invention will further be described, by way of example, with reference to the accompanying
drawings, in which:
Figure 1 is a diagrammatic illustration of a test apparatus in accordance with an embodiment of
the invention; and
Figure 2 is a view illustrating part of the test apparatus of Figure 1.
Referring to the accompanying drawings, a test apparatus 10 is illustrated that comprises a test
chamber 12, of generally cylindrical form. In the arrangement illustrated, the test chamber 12
takes the form of a hollow tubular member 14 within a lower end of which is located a table 16,
a securing mechanism 18 being provided t o releasably secure the tubular member 14 to the
table 16. The table 16 closes the end of the tubular member 14 and thus defines a container in
which materials to be tested can be located, in use. It will be appreciated, however, that this
represents merely one design from a wide range of possible designs of test chamber 12.
The apparatus 10 further comprises a powder manipulation device, referred to hereafter, for
convenience, as a compaction device 20 capable of being introduced into the test chamber 12,
movable axially along the length of the test chamber 12 and also capable of being driven for
rotary motion about its axis. A drive mechanism 22 is provided to drive the compaction device
20 for axial movement and for driving the compaction device for rotary movement, the drive
mechanism 22 being capable of controlling the speed and direction of both the axial movement
and the rotary movement of the compaction device 20. Whilst described and illustrated as
having an axially and rotatably moveable compaction device movable relative to a fixed test
chamber, it will be appreciated that the compaction device could be fixed and the test chamber
movable. Indeed, one of the compaction device and the test chamber could be rotatable and
the other axially movable, if desired.
As best shown in Figure 2, the compaction device 20 comprises an elongate shaft 24 connected,
at one end, to the drive mechanism 22, and carrying at the other end thereof a single turn of a
generally helical screw flight 26. Conveniently, the compaction device 20 is of metallic
construction, for example of steel or aluminium form, and as a result is capable of being used in
applying significant compacting loads t o the material under test, in use, and being relatively
resistant to wear. It will be appreciated, however, that depending upon the application in which
the invention is t o be employed, other materials may be used. By way of example, in some
arrangements suitable plastics materials may be used.
The screw flight 26 is conveniently formed on the outer surface of a cylindrical stub 28 which is
secured to the end of the shaft 24. By way of example, the stub 28 and shaft 24 may be coupled
to one another by interengaging screw thread formations. However, other techniques such as
welding may be employed. Alternatively, the flight 26 may be formed directly upon the shaft
24 with no stub 28 being present.
The outer diameter of the flight 26 is preferably slightly smaller than the inner diameter of the
test chamber 12. By way of example, a clearance of the order of l-2mm may be present
therebetween, to minimise the risk of contact and wear therebetween, in use. However, the
clearance therebetween may be greater or smaller than this without departing from the scope
of the invention.
As shown in Figure 2, the single turn of the flight 26 has an axial pitch length L. The value of the
pitch length L may be selected depending upon the application in which the invention is to be
used and for simplicity of manufacture. In the arrangement illustrated, the pitch length L is
approximately 16mm, but it will be understood that the invention is not restricted in this regard.
The test arrangement 10 may be used in a number of ways to achieve compaction of a material
under test located within the test chamber 12. By way of example, in one mode of operation,
the material under test is introduced into the test chamber 12. The compaction device 20 is
then introduced into the test chamber 12 under the control of the drive arrangement 22. The
compaction device 20 is conveniently advanced axially into the test chamber 12 whilst being
rotated in the forward direction, the speed of rotation of the compaction device 20 being such
that it rotates through a single revolution during the time taken for the compaction device 20
to be advanced by an axial distance equal to the pitch length L. By advancing the compaction
device 20 at this speed, it will be appreciated that the compaction device 20 applies only a
minimal compacting load to the material under test, the flight 26 of the compaction device 20
effectively being 'screwed into' the material under test.
The compaction device 20 continues to be advanced into the material under test in this fashion
until the leading edge of the flight 26 of the compaction device 20 approaches the table 16,
being spaced therefrom by a predetermined distance of, say, in the region of 10mm. Depending
upon the design of the compaction device, the tip of the compaction device may be closer to
the table 16 than this. It will be appreciated that once this position is reached, there will be
approximately 10mm of uncompacted material immediately ahead of the flight of the
compaction device 20. Once this position is reached, rotation of the compaction device 20 is
interrupted, the drive mechanism 22 holding the compaction device 20 in a fixed angular
position, resisting rotation thereof. With the compaction device 20 held against rotary or
angular movement, the drive mechanism applies a load to the compaction device 20 urging the
compaction device 20 to advance axially. It will be appreciated that the axial movement of the
compaction device 20, whilst the device 20 is held against rotation, results in the compaction
device 20 compressing the material immediately ahead thereof, compacting or consolidating
the material.
As the flight 26 extends around at least one complete turn, it will be appreciated that the
material under test located over substantially the entire cross section of the chamber 12 is
subject to the application of a compressive, compacting load, the compacting load being applied
substantially uniformly over the full cross-sectional area.
The drive mechanism 22 conveniently incorporates a sensor 22a operable to measure the
resistance to advancing movement of the compaction device. By way of example, a load cell or
the like may be used as the sensor 22a. However, it will be appreciated that the invention is not
restricted in this regard. By using the output of the sensor 22a in controlling the movement of
the compaction device 20, it will be appreciated that the compaction or consolidation of the
material to a desired level can be accurately achieved, the axial movement of the compaction
device 20 ceasing when a predetermined resistance to axial movement of the compaction
device 20 in the compaction direction is sensed, indicating that a desired level of compaction or
consolidation has been achieved.
After compaction of the material ahead of the compaction device 20 in this manner, the
compaction device 20 is retracted by a predetermined distance, for example by 10mm, whilst
being rotated in the reverse direction at substantially the speed mentioned hereinbefore such
that during the retraction of the compaction device 20, substantially no compacting or
separating load is applied to the material under test. After retraction in this manner, it will be
appreciated that there will be a layer of uncompacted material between the compaction device
20 and the previously compacted layer of material, the thickness of the uncompacted layer of
material substantially equating to the axial distance through which the compaction device 20
has been retracted. The compaction device 20 is then held against rotation whilst being driven
in the forward direction to compact the aforementioned uncompacted layer of material in the
manner set out above.
The process described above may be repeated a number of times to form a column of material
of a desired height or length that is substantially uniformly compacted, the degree of
compaction or consolidation being accurately controlled.
Once compacted, a range of tests may be conducted upon the material. By way of example, the
material may be removed from the test chamber, and an unconfined yield strength test
conducted upon the material. Alternatively, with the material contained within the test
chamber, powder rheology tests may be conducted upon the compressed powder, for example
using the apparatus and methods described in EP0798549B. It will be appreciated that the
precise nature of the tests conducted upon the compressed material do not form part of the
present invention and so are not described herein in further detail.
Whilst the description hereinbefore describes one method of use of the test apparatus of this
embodiment of the invention, it will be appreciated that the apparatus may be used in other
operating modes. By way of example, whilst in the arrangement hereinbefore the compaction
of the material is achieved in a series of compaction steps, the arrangement of this embodiment
of the invention may alternatively be operated to apply a substantially continuous compacting
or loosening load to the material to aid the formation of a compacted or loosened column of
material of a substantially uniform level of compaction. In order to operate the test apparatus
in this manner, a quantity of powder t o be compacted is introduced into the test chamber 12,
and the compaction device 20 is introduced into the material and driven to the end of the test
chamber 12 closed by the table 16 in substantially the manner outlined hereinbefore, rotating
the compaction device in the forward direction t o 'screw' it into the material whilst the
compaction device is moved axially towards the table 16. Once the compaction device 20 has
reached this position, retraction of the compaction device 20 commences. During retraction,
the compaction device 20 is rotated in the reverse direction at a rotary speed faster than that
at which it was introduced so that rather than being merely 'screwed out' of the material causing
minimal axial displacement thereof, the rotation of the compaction device 20 drives material
past the screw flight 26 substantially in the manner of an auger, and so causes the material
immediately beneath the flight 26 thereof to be compacted as the compaction device 20 is
withdrawn. The speed of retraction relative to the speed of rotation of the compaction device
20 controls the degree of compaction that occurs. It will be appreciated that this technique
allows a greater degree of uniformity of compaction to be attained. As with the arrangement
described hereinbefore, the output of a sensor, in this case sensitive to, for example, the
resistance to rotation of the compaction device 20, and so providing an indication of the degree
of compaction that has been achieved, may be used in controlling the speeds of rotation and
retraction of the compaction device 20 t o ensure that the desired level of compaction or
consolidation of the material is achieved. By way of example, where the sensor 22a is sensitive
t o the shear stress, the rotational speed of the compaction device 20 may be set at a fixed rate
and the upwards axial movement of the compaction device 20 adjusted t o ensure the sensed
shear stress (or torque) lies within a predetermined range. If the detected shear stress is too
high, then the rate at which the compaction device 20 is lifted will be increased t o bring the
detected shear stress to an acceptable, and vice versa. Of course, alternatively, the axial
movement could be controlled in such a fashion that it occurs at a substantially uniform rate
whilst the speed of rotary movement is varied to achieve the desired level of compaction.
It will be appreciated from the above description that the apparatus described hereinbefore
may be used in a number of ways to achieve compaction or loosening of the material under test
in a controlled manner. When the compaction device 20 is being introduced into the material
under test or withdrawn therefrom, by controlling the speed of rotation of the compaction
device 20 relative to the axial speed of movement thereof, the effective 'angle of attack' of the
flight 26 can be varied despite the flight 26 itself being a rigid structure on the shaft 24 to achieve
consolidation of the material or lifting and loosening thereof. By way of example, if the
compaction device 20 is rotated whilst being moved downwards, the speed of axial movement
relative to that of rotary movement being such that the effective angle of attack is steeper than
the actual helix angle of the flight 26, then some compaction will occur during the downwards
movement. Similarly, if it raised a steeper angle of attack than the helix angle of the flight 26,
then lifting and loosening of the material will occur.
As mentioned above, the consolidation or loosening can be used to prepare the material for
subsequent tests, for example of the type referred to hereinbefore.
In the arrangements described hereinbefore, the compaction device 20 is used to consolidate
or loosen substantially the full cross sectional area of the material located within the test
chamber. It will be appreciated that this need not always be the case. If desired, the outer
diameter of the flight 26 could be considerably smaller than the internal diameter of the test
chamber 12, for example around half the diameter thereof. With such an arrangement, in one
mode of operation the compaction device 20 could be rotated in the forward direction whilst
being axially driven towards the table 16. Once located at a desired depth, which may be
adjacent the table 16, rotation of the compaction device 20 may cease and the com paction
device 20 withdrawn axially to shear a central column of material from the test material located
within the test chamber, leaving an annular column of material in position upon which tests can
be conducted. Conveniently, where the compaction device 20 is to be used in this manner, the
flight 26 of the compaction device 20 is of multi-turn form, extending over substantially the full
height of the material in the test chamber 12.
Whilst one embodiment of a test apparatus 10 in accordance with the invention, and a couple
of methods of use thereof, are set out hereinbefore, it will be appreciated that a wide range of
modifications and alterations may be made, both to the test apparatus itself and to the methods
of use thereof, without departing from the scope of the invention as defined by the appended
claims.
CLAIMS:
1. A powder or granulated material test apparatus comprising a test chamber, a powder
manipulation device, and a drive arrangement selectively operable to drive the powder
manipulation device for axial movement and for rotary movement relative to the test chamber,
the powder manipulation device comprising a generally helical screw flight.
2. An apparatus according to Claim 1, wherein the generally helical screw flight is of at
least 360° angular extent
3. An apparatus according to Claim 1 or Claim 2, wherein the powder manipulation device
comprises a shaft carrying the flight.
4. An apparatus according to any of the preceding claims, wherein the powder
manipulation device comprises a single turn of the generally helical flight.
5. An apparatus according to any of the preceding claims, wherein the powder
manipulation device is of metallic construction.
6. An apparatus according to any of the preceding claims, further comprising a sensor
sensitive to the resistance to axial movement of the powder manipulation device.
7. An apparatus according to any of the preceding claims, further comprising a sensor
sensitive to the resistance to rotary movement of the powder manipulation device.
8. An apparatus according to any of the preceding claims and adapted for use in
conducting unconfined yield strength tests.
9. An apparatus according to any of the preceding claims and adapted for use in
conducting powder rheology tests.
10. A method of using the test apparatus of any of the preceding claims, comprising the
steps of:
moving the powder manipulation device to a predetermined position relative to the test
chamber;
advancing the powder manipulation device relative to the test chamber whilst holding
the powder manipulation device against rotation relative to the test chamber to compact the
material ahead of the powder manipulation device;
retracting the powder manipulation device relative to the test chamber whilst rotating
the powder manipulation device relative to the test chamber to space the flight of the powder
manipulation device from the compacted material;
and repeating the step of advancing the powder manipulation device relative to the test
chamber whilst holding the device against rotation relative to the test chamber.
11. A method as claimed in Claim 10, wherein during the retracting step the powder
manipulation device is retracted at a speed such that it is retracted by a distance substantially
equal to a pitch length of the flight per revolution of the powder manipulation device.
12. A method as claimed in Claim 10, wherein during the advancing step a resistance to
movement of the powder manipulation device is monitored.
13. A method of using the test apparatus of any of Claims 1 to 9, comprising the steps of:
advancing the powder manipulation device into the material under test; and
subsequently withdrawing the powder manipulation device from the material whilst
rotating the powder manipulation device relative to the material, the speed of retraction of the
powder manipulation device being controlled relative to the rotary speed thereof to achieve a
desired level of compaction in the material under test.
14. A method according to Claim 13, wherein during the withdrawal step the powder
manipulation device is retracted at a speed such that it is retracted by a distance greater than a
pitch length of the flight per revolution of the powder manipulation device.
15. A method according to Claim 13, wherein during the withdrawal step a resistance to
rotation of the powder manipulation device is monitored.
16. A method of using the test apparatus of any of Claims 1 to 9, comprising the steps of:
advancing the powder manipulation device into the material under test whilst rotating
the powder manipulation device; and
subsequently withdrawing the powder manipulation device therefrom whilst holding
the powder manipulation device against rotation to remove a quantity of material from the test
chamber.