Variable eccentric rheometer system

Details Variable eccentric rheometer system Variable eccentric rheometer system

2003-05-14

2004-01-27

Williams, Hezron (Department: 2856)

Measuring and testing

Liquid analysis or analysis of the suspension of solids in a...

Viscosity

Reexamination Certificate

active

06681617

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally resides in the art of rheometer systems for testing polymers. More particularly, the present invention relates to a variable eccentric cam that is incorporated into to a rheometer system for converting rotary motion at a motor-driven drive shaft into oscillatory motion at a polymer sample die.
Polymers are most often tested according to one of five ASTM methods, namely, ASTM D1646, D2084, D5289, D6204 and D6601. Method D1646 describes the use of a shearing disk viscometer to measure the viscosity and scorch characteristics of a polymer. In this method a rotor continuously rotates, as opposed to oscillating. The second method, D2084, describes a curemeter utilizing an oscillating rotor. The degree of oscillation is fixed and determines the percent strain, such that the device employed is a constant strain instrument. Method D 5289 describes three rotorless curemeter systems wherein one die is oscillated at a fixed amplitude. As in the methods previously discussed, this amplitude is fixed, and the instrument is, therefore, a fixed strain instrument. The present invention is concerned with oscillating instruments, and may be applied to oscillating test instruments currently known or to oscillating test instruments developed in the future, when the test instrument is suitable for the incorporation of the teachings herein.
Several patents describe instruments operating in accordance with ASTM D2084 and D5289. U.S. Pat. No. 3,681,980 illustrates the application of a fixed eccentric cam to facilitate oscillation of a rotor. This amplitude of oscillation is determined by the position of the pin on the eccentric. U.S. Pat. No. 4,794,788 also describes the use of an eccentric to facilitate oscillatory motion. The amplitude of oscillation for these instruments has been fixed for any given test, although the amplitude of oscillation can be changed between tests by changing the position of the pin on the eccentric or by changing the eccentric to one with a different off-set. The structure of such an eccentric cam is shown in
FIGS. 1 and 2
.
In
FIG. 1
, it can be seen how the operation of a rotating eccentric cam
100
can convert rotary motion (represented by arrow R) at eccentric cam
100
into oscillatory motion (represented by arrow O) at a polymer sample die
102
. A pin
104
extends from eccentric cam
100
to connect to a first end of an eccentric arm
106
. The second end of eccentric arm
106
attaches to the first end of the drive plate
108
, which has its second end rigidly fixed to die shaft
110
. Pin
104
is placed off center on eccentric cam
100
such that, as eccentric cam
100
is rotated by a drive shaft and motor (not shown) pin
104
rotates about the central axis of rotation of eccentric cam
100
. It will be appreciated that this causes eccentric arm
106
to be alternatively pushed and pulled by the rotation of eccentric cam
100
, such that drive plate
108
is also pushed in and pulled to create oscillation at die shaft
110
and die
102
.
In order to achieve different amplitudes of oscillation, eccentric cam
100
might be designed as shown in
FIG. 2
, wherein eccentric cam
100
provides for three pin positions
104
A,
104
B and
104
C. Each pin position
104
A, B, C represents a different amplitude of oscillation, and, thus, the amplitude of oscillation may be changed between tests by changing the position of eccentric arm
106
on the eccentric cam
100
(i.e., by moving the connection of eccentric arm
106
to a new position on eccentric cam
100
). While
FIG. 2
shows one eccentric cam with three pin positions
104
A, B, C, it should be appreciated that, rather than providing multiple pin positions on one eccentric cam
100
, multiple eccentric cams
100
might be provided, each with a single pin position
104
placed at a different distance from the center axis of rotation of the eccentric cam
100
. In such a case, the eccentric cam would need to be changed between tests in order to provide for differing amplitudes of oscillation.
ASTM D6204 describes the use of a constant strain test, and also discloses the capability of performing a variable strain test. ASTM D6601 fully describes the conditions for evaluating a specimen at more than one strain amplitude during a single test. In this method, a specimen is subjected to strains of 1, 2, 5, 10 and 20% during the same test. In order to use the previous apparatus as shown in
FIG. 1
, the test must be momentarily stopped while the strain amplitude is manually changed by either (1) changing the position of the eccentric arm on one eccentric cam (when the eccentric provides multiple pin position, as in FIG.
2
), or (2) changing the eccentric cam itself (when multiple eccentrics are available having only one pin position).
In the case of (1) above, it would be difficult, if not impossible to have one eccentric with the eccentric positions necessary to achieve 1, 2, 5, 10 and 20% strain. In order to obtain these strain amplitudes for a typical rheometer geometry, the pin positions on the eccentric cam would need to be located at approximately 0.004, 0.009, 0.021, 0.043 and 0.086 inches from the center axis of rotation of the eccentric cam. Even using an eccentric pin as small as 0.125 inches, it would be nearly impossible to accurately machine these positions on one eccentric cam, along a single radius (as in FIG.
2
). Even if the positions were machined on the eccentric such that they did not align along a single radius, they would have to be very accurately placed, and some means would need to be provided for distinguishing between each position.
In example (2) above, exchanging eccentric cams to obtain different strains also makes it difficult to achieve all of the strains described in ASTM D6601. In this situation, the test would have to be momentarily stopped while the eccentric cam has changed. However, the procedures require that individual eccentric cams be calibrated, and such calibration would be tedious and time consuming, since five different eccentric cams would be needed. Calibration is critical since the position of the eccentric pin determines where the amplitude crosses through zero amplitude.
To avoid the problems associated with either changing eccentric pin locations or changing eccentric cams, the current apparatus to test according to ASTM D6204 and D6601 are direct drive instruments as described in U.S. Pat. No. 5,079,956. The direct drive system is illustrated in FIG.
3
. Therein, a motor
200
is programmed to oscillate, and it is directly fixed to polymer sample die
202
through a die shaft
204
, such that the oscillatory motion of motor
200
is directly transferred to die
202
. While these direct drive systems are easily calibrated, and allow for fast and automated changing of the oscillation amplitude, they suffer from the fact that continuously testing at small amplitudes can cause excessive wear on the motor and premature failure. In addition, the motor must be very precise in order to provide control at small amplitudes.
Thus, while the prior art has provided some means for converting rotary motion into differing amplitudes of oscillation, there exists a need in the art for a variable eccentric cam that can provide for virtually any desired amplitude of oscillation without necessitating that eccentric cams be changed or requiring that discrete positions for the attachment of the eccentric arm to the eccentric cam be chosen.
SUMMARY OF THE INVENTION
In general, the present invention provides an eccentric cam for use in oscillating rheometer systems wherein the eccentric cam is mounted on a drive shaft and attaches to an eccentric arm for converting rotary motion of the drive shaft into oscillatory motion at a rubber specimen die, which is operatively connected to the eccentric arm through a drive arm. The eccentric cam includes a slide housing that is operatively connected to the drive shaft to rotate therewith such that the slide housing has a center axis of rotation. A slide screw is mounted in the slide ho

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