VOL. 9, PP. 341-353 (1975)
J. BIOMED. MATER. RES.
The Development of an Abrasion Testing Machine for Dental Materials ALAN HARRISON, The Welsh National School of Medicine, Dental School and Hospital, Department of Restorative Dentistry, Heath Park, Cardiff, CF4 4 X Y and T. T. LEWIS, Cniversity College, Department of Mechanical Engineering, Newport Road, Cardiff, Wales
Summary Wear testing should be an important part of the investigations into the physical and mechanical properties of some dental materials. I t has, however, largely been ignored because of conflicting and unreproducible results. It was decided, therefore, to review the work done by other researchers and to examine the human masticatory cycle, and then present new parameters to design and construct a new dental abrasion testing machine. This new machine is described in detail and its capabilities briefly illustrated.
INTRODUCTION Dental materials used to replace a part, or the whole, of a natural tooth are subjected to considerable wear during mastication and normal rubbing together of the teeth. This problem is even more pronounced when the entire dentition is replaced because the wear then affects all the teeth and the occlusion of the patient is deranged. Specifications in Britain arid the United States exist for nearly all these materials, except procelain, but, in none of them is the property of abrasion resistance included as a requirement to pass the specification. This is no doubt due to a lack of a reliable testing procedure which can correlate clinical and laboratory wear. For instance, it is possibie, under artificial conditions of wear, to show that procelain abrades faster than acrylic, although we know that clinically this is not the case. Several abrasion testing machines have been designed and used but>,in most cases, the test have in no way simulated the clinical condition arid have produced results at 341 @ 1975 by John Wiley & Sons, Inc.
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variance with one another. For example, the loads, speeds and abrasives used have often been severe by masticatory standards. It was for these reasons that an attempt has been made, using new specifications, to design apparatus more suitable t o the testing of these materials under conditions similar to masticatory function.
Review of Literature
It is of interest to look a t some of the apparatus and specifications used by some earlier workers and to compare the parameters with those proposed in this article. It is useful, in this respect, to consider the work and observations of Cornell, Jordan, Ellis, and Rose,' since they stipulate the basic requirements of a good wear test for plastic teeth. 1. The test should relate as closely as possible t o clinical conditions and should measure comparative wear of tooth material. 2. The test should produce results relating closely to clinical observations of various tooth materials where such data are available. 3. The test should produce a wear pattern on the teeth tested similar t o that seen on teeth present over a long period of time in the human mouth. 4. All artificial teeth tested should be put through a preparatory routine which subjects them to the same conditions encountered in the processing and use of artificial dentures. 5. The test should be conducted either without abrasives or with very mild abrasives, since it is well-known that the human oral mechanism tends to reject gritty particles automatically. 6. The test should be rapid enough to be useful in screening materials, but not so accelerated as to lose its relationship to conditions of use. 7. It should be rapid enough that the development of statistically significant data would be feasible, even though large numbers of materials are being tested. The design of their apparatus, however, did not incorporate many of the features of the human masticatory cycle. No attempt was made to relate stroke length, stroke speed or contact time, and it appears from their account that contact, of the specimens was continouus although run in a water bath or food slurry.
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Boddicker2 used a Kelley grinder (a type of modified dental hinge articulator) to mill opposing complete dentures. He used porcelain teeth on one denture and acrylic, gold or porcelain 011 the other. Forces of 6 to 25 lb were used to maintain occlusal contact and the tests were, therefore, run under continuous contact. He noted that porcelain teet,h cracked and shattered under the loads he was using. Osborne3 used apparatus designed to hold four test pieces under continuous contact with a load of 90 g. The machine completed 300 strokes per min with a stroke length of one in. Slack4maintained it was necessary touse a standard instrument so others could repeat the tests and therefore adapted the Taber abraser (Taber Instrument Corporation, North Tonawanda, N.Y .). He pointed out that the purpose of the study was not to compare abrasion on the abraser with that which actually occurs in the mouth, but t o compare all objects with each other by means of a common abrasive substance, in this case 220 grit carborundum. Taketa, Perdue, O’Rourke, Sievert, and Phillips5 designed wear apparatus consisting of a rotating grinding belt and an aluminium form that holds the test materials a t the desired angle against the belt. No clinical parameters were included in the design and the apparatus was only of use in measuring relative wear of the materials against the grinding belt. NIyersonG used an abrasion testing machine to show the superiority in wear resistance of a combination of acrylic versus procelain over two opposing similar materials. The equipment is not described but the method used was to abrade the teeth against each other for a known period of time, in a liquid bath containing the abrasive, and under a known load. Loads of up t o 40 lb were used on individual teeth. Thompson’ demonstrated the unsuitability of standard wear testing equipment using specimens of acrylic and porcelain and showed in the acrylic resin the signs of unwanted effects of temperature rise, while with porcelain surface cracking and abrasion by accumulated debris occurred. He therefore attempted to design equipment t o simulate mouth conditions. The apparatus was capable of wear testing whole occluding sets of posterior teeth. Loads of 10 t o 20 lb were applied t o the upper arm of the machine and continuous contact of occlusal
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surfaces was maintained during simulated tooth grinding strokes of % in. amplitude. He did point out that the conditions were severe by masticatory standards. HenkelS studied the abrasion of an acrylic tooth against a silverpalladium metal surface. The two surfaces were drawn 1 mm over each other and then lifted 4 mm. At contact, the force was 2 to 3 kg. The tests were carried out in a clear water bath using a slurry of 1% fine quartz in water. He claimed the investigations should be conducted with the simulation of oral conditions but did not explain why he used the above parameters. He showed that a plastic tooth abraded a n opposing metal surface. He used simple equipment; it appeared that it only held one specimen a t a time and his experiments on 14 specimens took two years. Monasky and Taylorgused apparatus which produced intermittent sliding contacts (distance not stated) between specimens immersed in artificial saliva medium. A force of 1lb was used over specimens. No consideration was given to the time of contact or speed of the stroke. Using a commercial abrasion tester, Lugassy and Greenerlo evaluated reinforced and unfilled resins used in dentistry. The machine consisted of grinding wheels which rotated over the surface of flat specimens. The speed of the abrader was 30 cm/sec; this apparatus did not attempt to simulate oral conditions. I n a study of composite resins, amalgam and enamel, Powell" used loads of 1 kg/sq. mm and 2 kg/ sq. mm on the specimens. The specimens were slid across each other a t varying rates from 153 mm/sec to a maximum of 450 mm/sec. The likely normal maximum speed that the mandible attains is 140 mm/sec. Jones, D. W., Jones, P. A., and Wilson12devised a simple abrasion test to evaluate composite and other dental restorative materials. I t s advantages are that it requires no specialized apparatus and is quick and simple to perform. They did not attempt to include masticatory cycle parameters. The specimen is vibrated with a quantity of abrasive in a standard commercial mechanical mixer normally used for mixing dental restorative mat,erials.
The Masticatory Cycle I n a typical chewing stroke the mandible moves from the open position in an upward direction and as the cusps of the teeth approach
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PHASE
CRUSHING PHASE PREPARATORY
I
Fig. 1. Idealized chewing cycle (after Murphy, 1965).
contact the closure rate decreases (Fig. 1). The movement slows even more as the food is crushed and there may be a pause of approximately % sec at the top of the cycle. The mandible then drops rapidly and there is a gradual decrease in movement again as the mouth nears maximum opening. The average time for one complete chew is just less than one sec. A chewing rate of 60 to 80 strokes/min may be regarded, on present evidence, as an acceptable average. The maximum velocity of the mandible, which usually occurs midway in either the opening or the closing portion of the chewing cycle, varies in the reports up to a maximum of 140.46 mm/sec. The speed as the teeth near contact is much less than this since there may be a pause at the top of the cycle. A speed of 5 mm/sec has been taken to be representative of the speed of the mandible during this phase of its cycle.
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The present evidence is strongly in favor of frequent tooth contact during chewing and several researchers have demonstrated the presence of a glide between opposing occlusal surfaces. This distance has ranged from less than 1 mm to 6 nlm, but 1 mm is probably average for European subjects. A narrow range of forces is usually preferred when chewing a t the lower forces. It is generally accepted that the average force produced in chewing on a tooth ranges between 0.2 and 2 kg. Since occlusal pressure is force per unit occlusal area, it is necessary to know the occlusal contact area. An average estimation has shown this to be approximately 16 mm2 for the first molar. The parameters quoted for the design are, therefore, the result of a thorough search of the literature dealing with the masticatory cycle (Bates, Stafford, and H a r r i ~ o n ~ ~The ) . specifications for the design were as follows: Number of specimens Specimen size-pin -plate Specimen load Pin-plate contact frequency Pin-plate contact time Pin-plate contact distance Pin-plate separation Stroke speed Environrnent
10 4.5 mm diam 25 x 5 x 2 mm 50-1000 g 70/min 0.2 sec 1 mm 4 mm 5 mm/sec Liquid, or slurry abrasive
Because of the difficulty of production of some specimens of dental materials, i.e. porcelain, it was necessary to design the apparatus with test beds kept as small as possible.
DESCRIPTION OF APPARATUS Provision was made to allow ten pairs of pin and plate specimens to wear simultaneously against each other. Between each pair of specimens the normal contact force can be independently maintained with dead aeights and the contact time per cycle for each pair of specimens can also be independently adjusted. The pin specimens have a circular cross section, 4.5 mm diam and are approximately 2 mm thick. These are cemented a t their
DEVELOPING AN ABRASION TESTING MACHINE
Fig. 2.
347
The abrasion testing machine and control unit.
upper faces t o the lower ends of the loading rods, A (Figs. 2 and 3). To the upper ends of these loading rods dead weights are applied t o produce the requisite contact force between the pin and plate specimens. The pin specimens and their loading rods are arranged in two rows of five, each row being symmetrically placed about the longitudinal axis of the machine. Beneath each pin specimen is a plate specimen clamped to a table, B (Figs. 2 and 3). This table reciprocates in the direction of the machine’s longitudinal axis with a constant velocity, in both directions, of 5 mm/sec. Support for the table is provided by a three point kinematic support with two bearings, one a t each end of one side of the table and a roller bearing, bearing on a pillar, situated at the mid point of the other side. Ten plate specimens are clamped to this table, each specimen being 25 mm long in the direction of table movement, 5 mm wide and 2 mm thick. Perspex sides were attached t o the top of the table to allow the specimens to be immersed. The wear cycle was achieved by a geared-down ac motor with an output shaft rotating a t 70 rpm. driving a cam with a constant velocity profile producing a horizontal reciprocating motion of the table. This output shaft also drives a pair of identical cams situated a t each end of the machine, controlling the vertical movement of the loading rods and their attached pin specimens. The cams, C (Fig. 4)) were designed to impart a sinusoidal motion t o the cam followers with an overall lift of 4.1 mm. These cam
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Fig. 3.
The loading rods with pin and plate specimens in contact.
followers slide in bearings machined in the upper and lower bearing plates (in which the loading rods also slide) arid between these plates the cam followers are rigidly attached to horizontal lifting beams, D (Fig. 2), one either side of the top bearing plate, which simultaneously lift the pins from the plate specimens through screwed rods, E (Fig. 3), contacting the under surface of the circular, pin-loading platforms, F (Fig. 3). Rotation of loading rods is prohibited by means of vertical pins attached to the upper bearing plate which slide relative to slots machined in the loading platforms. These also provide an angular register for pin wear measurement as will be described later.
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Fig. 4. The machine with the table hinged through 90" to show the working parts.
With pin specimen contact extending over 1 mm of table displacement arid with a pin diameter of 4.5 mm a pit would be worn in the plate specimen. To ensure that this did not occur, and to even the wear over the length of the plate specimens a slow, constant velocity reciprocating motion of the table was superimposed on the approximate 1 sec period motion. This refinement was achieved by arranging the follower of cam G, (Fig. 4) to be attached to one end of a push rod H, (Fig. 4), the other end of which having another cam follower in contact, with cam J, (Fig.4). Cam J is supported on a cam shaft passing through a bearing block, attached to the plate, the cam being driven through a 1OO:l worm and wheel reduction, K (Fig. 4). The drive to this reduction gear being taken from the longitudinal cam shaft M (Fig. 4) through a pair of identical spur gear wheels N , (Fig. 4), and a spline drive P, (Fig. 4). The cam J, produces a constant velocity reciprocating motion with a 100 sec period and an amplitude of 8 mm, giving a total plate traverse of 16 mm. The lift cams, C, and the reciprocating cam, G, are adjusted to be 90" out of phase so that the bottom of the lift cycle occurs a t the mid point of the forward constant-velocity movement of the table.
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The time during which a pin specimen is in contact with its plate specimen can be adjusted by the screwed rod, E. Each screwed rod passes through a bush insulating it electrically from the horizontal lifting beam. To a screwed, brass sleeve inset into the insulation material a lead is taken to a pin-contact-time measuring instrument. A 1kHz multivibrator oscillator produces 1 msec pulses. These pulses are counted when the screwed rod and the under surface of the pin loading platform separate. Pulse-counting ceases when contact is reestablished. At this instant, the time of separation, and hence the time of pin-plate specimen contact is displayed on the three-figure digital counter, Q (Fig. 2), the last figure reading t,o 1 msec. Contact time is displayed during the subsequent separation of the pin from the plate specimen and also during their next specimen contact period, when another pulse-counting operation is in progress. When specimen contact ceases the new count is substituted for the previous one. A ten-position switch allows each pair of specimens independently to be adjusted for contact time and monitored during a wear test.
Determination of Rod Specimen Wear Loss Measurements of the stainless steel rods, with specimens cemented on their upper faces, were made with a 0 to 100 mm bench micrometer (Herbert Controls and Instruments Ltd., Letchworth, England), (Fig. 5), calibrated t o 0.0002 mm, with a 4 oz. fiducial indicator.
(1) A V block, A, was placed on the table of the bench micrometer and the loading rod, B, located on the V block. The slot machined in the loading platform fitting over the location pin, C, thus ensuring identical orientation of the rod on the V block on each occasion. (2) The length was measured ten times; on each occasion the rod was lifted from the V block and replaced and the V block realigned. The mean of the ten readings was taken as the original length.
(3) The loading rod was then transferred t o the abrasion machine and the wear programme completed. (4) The rod assembly and specimen were then re-measured. The difference in the mean of the original length and the mean of the length after wear was the length of the test material lost during the wear program.
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Fig. 5. 0 mm to 100 mm. Bench Micrometer with a loading rod on the V block.
A statistical analysis of the measurements taken of a “standard pin” showed that differences larger than 0.005 mm are experimental differences rather than measurement differences.
DISCUSSION AND CONCLUSIONS The machine will be used to test dental acrylics in the first instance but later other restorative materials will also be studied in detail. An an example of the performance of the machine, Figure 6 shows the wear in millimeters of specimens of perspex abraded against 600 grit carborundum paper under water and measured at 5 min intervals. Figure 7 illustrates the differential wear rate of five materials which might be used in dentistry.* It also shows that extended tests can be carried out on the machine. It is estimated that the teeth are in contact for only 15 min each day.14 Since the machine “chews” a t 70 strokes per min and contacts for 0.2 sec each time, in 1 hr the total contact time is 14 min. Thus 1 hr wear on the machine is almost equivalent to one day in the mouth, and one day on the machine to 24 days in the mouth.
* Croform is a self-curing acrylic and Kallodent a heat-curing acrylic; both are denture base materials.
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PERSPEX PIN SPECIMENS .08
-
- CARBORUNDUM PAPER ABRASIVE
Pressure on pins 0.11 N / ~ I I I ~
-
600 Grit
m
2 -06 -
c 6. .A
3
'2 5
2
s
'04
-
5
10
15
20
25
30
Time in minutes
Fig. 6. Wear plotted against time for Perspex pin specimens abraded against 600 grit carborundum paper.
0.3
TOTAL WEAR IN 12 DAYS Lactose in Water Abrasive Perspex Plates Pressure on Pins 0.11 N/mm2
v1
h
.-E
0.2
4 I
'2 .5
E
b
0.1
Fig. 7. Bar graph showing the total wear in 12 days of different pin specimen materials against perspex plates, with lactose in water as an abrasive medium.
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The abrasion testing apparatus used by other workers has been reviewed and the important features of the masticatory cycle discussed. These physiological parameters were then incorporated in the design and construction of a new dental abrasion testing machine, which is described in detail. Brief examples of the machine’s performance are given.
References 1. J. A. Cornell, J. S. Jordan, S. Ellis, and E. E. Rose, J . Amer. Dent. Ass., 54, 608 (1957). 2. V. 8. Boddicker, J. Amer. Dent. Ass., 35, 793 (1947). 3. J. Osborne, Rr. Dent. J., 87, 10 (1949). 4. F. A. Slack, J . Amer. Dent. Ass., 39, 47 (1949). 5. F. Taketa, H. S. Perdue, W. F. O’ltourke, H. W. Sievert, and P. H. Phillips, J . Dent. Res., 36, 739 (1957). 6. It. L. Myerson, J . Prosth. Dent., 7, 625 (1957). 7. J. C. Thompson, Dent. Practnr., 15, 233 (1965). 8. G. Henkel, Fogorvosi Szemle, 60, 138 (1967). 9. G. E. Monasky and D. F. Taylor, J . Prosth. Dent., 25,299 (1971). 10. A. A. Lugassy and E. H. Greener, J . Dent. Res., 51, 967 (1972). 11. J. M. Powell, MS Thesis, Indiana University School of Dentistry, Bloomington, Ind. (1972). 12. L). W. Jones, P. A. Jones, and H. J. Wilson, J . Dent., 1 , 2 8 (1972). 13. J. F. Bates, G. D. Stafford, and A. Harrison, J . Oral Rehabilitation, in press. 14. A. A. Brewer, J . Prosth. Dent., 13, 49 (1963).
Received September 19, 1974
J. BIOMED. MATER. RES.
The Development of an Abrasion Testing Machine for Dental Materials ALAN HARRISON, The Welsh National School of Medicine, Dental School and Hospital, Department of Restorative Dentistry, Heath Park, Cardiff, CF4 4 X Y and T. T. LEWIS, Cniversity College, Department of Mechanical Engineering, Newport Road, Cardiff, Wales
Summary Wear testing should be an important part of the investigations into the physical and mechanical properties of some dental materials. I t has, however, largely been ignored because of conflicting and unreproducible results. It was decided, therefore, to review the work done by other researchers and to examine the human masticatory cycle, and then present new parameters to design and construct a new dental abrasion testing machine. This new machine is described in detail and its capabilities briefly illustrated.
INTRODUCTION Dental materials used to replace a part, or the whole, of a natural tooth are subjected to considerable wear during mastication and normal rubbing together of the teeth. This problem is even more pronounced when the entire dentition is replaced because the wear then affects all the teeth and the occlusion of the patient is deranged. Specifications in Britain arid the United States exist for nearly all these materials, except procelain, but, in none of them is the property of abrasion resistance included as a requirement to pass the specification. This is no doubt due to a lack of a reliable testing procedure which can correlate clinical and laboratory wear. For instance, it is possibie, under artificial conditions of wear, to show that procelain abrades faster than acrylic, although we know that clinically this is not the case. Several abrasion testing machines have been designed and used but>,in most cases, the test have in no way simulated the clinical condition arid have produced results at 341 @ 1975 by John Wiley & Sons, Inc.
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variance with one another. For example, the loads, speeds and abrasives used have often been severe by masticatory standards. It was for these reasons that an attempt has been made, using new specifications, to design apparatus more suitable t o the testing of these materials under conditions similar to masticatory function.
Review of Literature
It is of interest to look a t some of the apparatus and specifications used by some earlier workers and to compare the parameters with those proposed in this article. It is useful, in this respect, to consider the work and observations of Cornell, Jordan, Ellis, and Rose,' since they stipulate the basic requirements of a good wear test for plastic teeth. 1. The test should relate as closely as possible t o clinical conditions and should measure comparative wear of tooth material. 2. The test should produce results relating closely to clinical observations of various tooth materials where such data are available. 3. The test should produce a wear pattern on the teeth tested similar t o that seen on teeth present over a long period of time in the human mouth. 4. All artificial teeth tested should be put through a preparatory routine which subjects them to the same conditions encountered in the processing and use of artificial dentures. 5. The test should be conducted either without abrasives or with very mild abrasives, since it is well-known that the human oral mechanism tends to reject gritty particles automatically. 6. The test should be rapid enough to be useful in screening materials, but not so accelerated as to lose its relationship to conditions of use. 7. It should be rapid enough that the development of statistically significant data would be feasible, even though large numbers of materials are being tested. The design of their apparatus, however, did not incorporate many of the features of the human masticatory cycle. No attempt was made to relate stroke length, stroke speed or contact time, and it appears from their account that contact, of the specimens was continouus although run in a water bath or food slurry.
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Boddicker2 used a Kelley grinder (a type of modified dental hinge articulator) to mill opposing complete dentures. He used porcelain teeth on one denture and acrylic, gold or porcelain 011 the other. Forces of 6 to 25 lb were used to maintain occlusal contact and the tests were, therefore, run under continuous contact. He noted that porcelain teet,h cracked and shattered under the loads he was using. Osborne3 used apparatus designed to hold four test pieces under continuous contact with a load of 90 g. The machine completed 300 strokes per min with a stroke length of one in. Slack4maintained it was necessary touse a standard instrument so others could repeat the tests and therefore adapted the Taber abraser (Taber Instrument Corporation, North Tonawanda, N.Y .). He pointed out that the purpose of the study was not to compare abrasion on the abraser with that which actually occurs in the mouth, but t o compare all objects with each other by means of a common abrasive substance, in this case 220 grit carborundum. Taketa, Perdue, O’Rourke, Sievert, and Phillips5 designed wear apparatus consisting of a rotating grinding belt and an aluminium form that holds the test materials a t the desired angle against the belt. No clinical parameters were included in the design and the apparatus was only of use in measuring relative wear of the materials against the grinding belt. NIyersonG used an abrasion testing machine to show the superiority in wear resistance of a combination of acrylic versus procelain over two opposing similar materials. The equipment is not described but the method used was to abrade the teeth against each other for a known period of time, in a liquid bath containing the abrasive, and under a known load. Loads of up t o 40 lb were used on individual teeth. Thompson’ demonstrated the unsuitability of standard wear testing equipment using specimens of acrylic and porcelain and showed in the acrylic resin the signs of unwanted effects of temperature rise, while with porcelain surface cracking and abrasion by accumulated debris occurred. He therefore attempted to design equipment t o simulate mouth conditions. The apparatus was capable of wear testing whole occluding sets of posterior teeth. Loads of 10 t o 20 lb were applied t o the upper arm of the machine and continuous contact of occlusal
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surfaces was maintained during simulated tooth grinding strokes of % in. amplitude. He did point out that the conditions were severe by masticatory standards. HenkelS studied the abrasion of an acrylic tooth against a silverpalladium metal surface. The two surfaces were drawn 1 mm over each other and then lifted 4 mm. At contact, the force was 2 to 3 kg. The tests were carried out in a clear water bath using a slurry of 1% fine quartz in water. He claimed the investigations should be conducted with the simulation of oral conditions but did not explain why he used the above parameters. He showed that a plastic tooth abraded a n opposing metal surface. He used simple equipment; it appeared that it only held one specimen a t a time and his experiments on 14 specimens took two years. Monasky and Taylorgused apparatus which produced intermittent sliding contacts (distance not stated) between specimens immersed in artificial saliva medium. A force of 1lb was used over specimens. No consideration was given to the time of contact or speed of the stroke. Using a commercial abrasion tester, Lugassy and Greenerlo evaluated reinforced and unfilled resins used in dentistry. The machine consisted of grinding wheels which rotated over the surface of flat specimens. The speed of the abrader was 30 cm/sec; this apparatus did not attempt to simulate oral conditions. I n a study of composite resins, amalgam and enamel, Powell" used loads of 1 kg/sq. mm and 2 kg/ sq. mm on the specimens. The specimens were slid across each other a t varying rates from 153 mm/sec to a maximum of 450 mm/sec. The likely normal maximum speed that the mandible attains is 140 mm/sec. Jones, D. W., Jones, P. A., and Wilson12devised a simple abrasion test to evaluate composite and other dental restorative materials. I t s advantages are that it requires no specialized apparatus and is quick and simple to perform. They did not attempt to include masticatory cycle parameters. The specimen is vibrated with a quantity of abrasive in a standard commercial mechanical mixer normally used for mixing dental restorative mat,erials.
The Masticatory Cycle I n a typical chewing stroke the mandible moves from the open position in an upward direction and as the cusps of the teeth approach
DEVELOPING AN ABRASION TESTING MACHINE
~/-GRJNDWG
345
PHASE
CRUSHING PHASE PREPARATORY
I
Fig. 1. Idealized chewing cycle (after Murphy, 1965).
contact the closure rate decreases (Fig. 1). The movement slows even more as the food is crushed and there may be a pause of approximately % sec at the top of the cycle. The mandible then drops rapidly and there is a gradual decrease in movement again as the mouth nears maximum opening. The average time for one complete chew is just less than one sec. A chewing rate of 60 to 80 strokes/min may be regarded, on present evidence, as an acceptable average. The maximum velocity of the mandible, which usually occurs midway in either the opening or the closing portion of the chewing cycle, varies in the reports up to a maximum of 140.46 mm/sec. The speed as the teeth near contact is much less than this since there may be a pause at the top of the cycle. A speed of 5 mm/sec has been taken to be representative of the speed of the mandible during this phase of its cycle.
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The present evidence is strongly in favor of frequent tooth contact during chewing and several researchers have demonstrated the presence of a glide between opposing occlusal surfaces. This distance has ranged from less than 1 mm to 6 nlm, but 1 mm is probably average for European subjects. A narrow range of forces is usually preferred when chewing a t the lower forces. It is generally accepted that the average force produced in chewing on a tooth ranges between 0.2 and 2 kg. Since occlusal pressure is force per unit occlusal area, it is necessary to know the occlusal contact area. An average estimation has shown this to be approximately 16 mm2 for the first molar. The parameters quoted for the design are, therefore, the result of a thorough search of the literature dealing with the masticatory cycle (Bates, Stafford, and H a r r i ~ o n ~ ~The ) . specifications for the design were as follows: Number of specimens Specimen size-pin -plate Specimen load Pin-plate contact frequency Pin-plate contact time Pin-plate contact distance Pin-plate separation Stroke speed Environrnent
10 4.5 mm diam 25 x 5 x 2 mm 50-1000 g 70/min 0.2 sec 1 mm 4 mm 5 mm/sec Liquid, or slurry abrasive
Because of the difficulty of production of some specimens of dental materials, i.e. porcelain, it was necessary to design the apparatus with test beds kept as small as possible.
DESCRIPTION OF APPARATUS Provision was made to allow ten pairs of pin and plate specimens to wear simultaneously against each other. Between each pair of specimens the normal contact force can be independently maintained with dead aeights and the contact time per cycle for each pair of specimens can also be independently adjusted. The pin specimens have a circular cross section, 4.5 mm diam and are approximately 2 mm thick. These are cemented a t their
DEVELOPING AN ABRASION TESTING MACHINE
Fig. 2.
347
The abrasion testing machine and control unit.
upper faces t o the lower ends of the loading rods, A (Figs. 2 and 3). To the upper ends of these loading rods dead weights are applied t o produce the requisite contact force between the pin and plate specimens. The pin specimens and their loading rods are arranged in two rows of five, each row being symmetrically placed about the longitudinal axis of the machine. Beneath each pin specimen is a plate specimen clamped to a table, B (Figs. 2 and 3). This table reciprocates in the direction of the machine’s longitudinal axis with a constant velocity, in both directions, of 5 mm/sec. Support for the table is provided by a three point kinematic support with two bearings, one a t each end of one side of the table and a roller bearing, bearing on a pillar, situated at the mid point of the other side. Ten plate specimens are clamped to this table, each specimen being 25 mm long in the direction of table movement, 5 mm wide and 2 mm thick. Perspex sides were attached t o the top of the table to allow the specimens to be immersed. The wear cycle was achieved by a geared-down ac motor with an output shaft rotating a t 70 rpm. driving a cam with a constant velocity profile producing a horizontal reciprocating motion of the table. This output shaft also drives a pair of identical cams situated a t each end of the machine, controlling the vertical movement of the loading rods and their attached pin specimens. The cams, C (Fig. 4)) were designed to impart a sinusoidal motion t o the cam followers with an overall lift of 4.1 mm. These cam
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Fig. 3.
The loading rods with pin and plate specimens in contact.
followers slide in bearings machined in the upper and lower bearing plates (in which the loading rods also slide) arid between these plates the cam followers are rigidly attached to horizontal lifting beams, D (Fig. 2), one either side of the top bearing plate, which simultaneously lift the pins from the plate specimens through screwed rods, E (Fig. 3), contacting the under surface of the circular, pin-loading platforms, F (Fig. 3). Rotation of loading rods is prohibited by means of vertical pins attached to the upper bearing plate which slide relative to slots machined in the loading platforms. These also provide an angular register for pin wear measurement as will be described later.
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Fig. 4. The machine with the table hinged through 90" to show the working parts.
With pin specimen contact extending over 1 mm of table displacement arid with a pin diameter of 4.5 mm a pit would be worn in the plate specimen. To ensure that this did not occur, and to even the wear over the length of the plate specimens a slow, constant velocity reciprocating motion of the table was superimposed on the approximate 1 sec period motion. This refinement was achieved by arranging the follower of cam G, (Fig. 4) to be attached to one end of a push rod H, (Fig. 4), the other end of which having another cam follower in contact, with cam J, (Fig.4). Cam J is supported on a cam shaft passing through a bearing block, attached to the plate, the cam being driven through a 1OO:l worm and wheel reduction, K (Fig. 4). The drive to this reduction gear being taken from the longitudinal cam shaft M (Fig. 4) through a pair of identical spur gear wheels N , (Fig. 4), and a spline drive P, (Fig. 4). The cam J, produces a constant velocity reciprocating motion with a 100 sec period and an amplitude of 8 mm, giving a total plate traverse of 16 mm. The lift cams, C, and the reciprocating cam, G, are adjusted to be 90" out of phase so that the bottom of the lift cycle occurs a t the mid point of the forward constant-velocity movement of the table.
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The time during which a pin specimen is in contact with its plate specimen can be adjusted by the screwed rod, E. Each screwed rod passes through a bush insulating it electrically from the horizontal lifting beam. To a screwed, brass sleeve inset into the insulation material a lead is taken to a pin-contact-time measuring instrument. A 1kHz multivibrator oscillator produces 1 msec pulses. These pulses are counted when the screwed rod and the under surface of the pin loading platform separate. Pulse-counting ceases when contact is reestablished. At this instant, the time of separation, and hence the time of pin-plate specimen contact is displayed on the three-figure digital counter, Q (Fig. 2), the last figure reading t,o 1 msec. Contact time is displayed during the subsequent separation of the pin from the plate specimen and also during their next specimen contact period, when another pulse-counting operation is in progress. When specimen contact ceases the new count is substituted for the previous one. A ten-position switch allows each pair of specimens independently to be adjusted for contact time and monitored during a wear test.
Determination of Rod Specimen Wear Loss Measurements of the stainless steel rods, with specimens cemented on their upper faces, were made with a 0 to 100 mm bench micrometer (Herbert Controls and Instruments Ltd., Letchworth, England), (Fig. 5), calibrated t o 0.0002 mm, with a 4 oz. fiducial indicator.
(1) A V block, A, was placed on the table of the bench micrometer and the loading rod, B, located on the V block. The slot machined in the loading platform fitting over the location pin, C, thus ensuring identical orientation of the rod on the V block on each occasion. (2) The length was measured ten times; on each occasion the rod was lifted from the V block and replaced and the V block realigned. The mean of the ten readings was taken as the original length.
(3) The loading rod was then transferred t o the abrasion machine and the wear programme completed. (4) The rod assembly and specimen were then re-measured. The difference in the mean of the original length and the mean of the length after wear was the length of the test material lost during the wear program.
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Fig. 5. 0 mm to 100 mm. Bench Micrometer with a loading rod on the V block.
A statistical analysis of the measurements taken of a “standard pin” showed that differences larger than 0.005 mm are experimental differences rather than measurement differences.
DISCUSSION AND CONCLUSIONS The machine will be used to test dental acrylics in the first instance but later other restorative materials will also be studied in detail. An an example of the performance of the machine, Figure 6 shows the wear in millimeters of specimens of perspex abraded against 600 grit carborundum paper under water and measured at 5 min intervals. Figure 7 illustrates the differential wear rate of five materials which might be used in dentistry.* It also shows that extended tests can be carried out on the machine. It is estimated that the teeth are in contact for only 15 min each day.14 Since the machine “chews” a t 70 strokes per min and contacts for 0.2 sec each time, in 1 hr the total contact time is 14 min. Thus 1 hr wear on the machine is almost equivalent to one day in the mouth, and one day on the machine to 24 days in the mouth.
* Croform is a self-curing acrylic and Kallodent a heat-curing acrylic; both are denture base materials.
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PERSPEX PIN SPECIMENS .08
-
- CARBORUNDUM PAPER ABRASIVE
Pressure on pins 0.11 N / ~ I I I ~
-
600 Grit
m
2 -06 -
c 6. .A
3
'2 5
2
s
'04
-
5
10
15
20
25
30
Time in minutes
Fig. 6. Wear plotted against time for Perspex pin specimens abraded against 600 grit carborundum paper.
0.3
TOTAL WEAR IN 12 DAYS Lactose in Water Abrasive Perspex Plates Pressure on Pins 0.11 N/mm2
v1
h
.-E
0.2
4 I
'2 .5
E
b
0.1
Fig. 7. Bar graph showing the total wear in 12 days of different pin specimen materials against perspex plates, with lactose in water as an abrasive medium.
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The abrasion testing apparatus used by other workers has been reviewed and the important features of the masticatory cycle discussed. These physiological parameters were then incorporated in the design and construction of a new dental abrasion testing machine, which is described in detail. Brief examples of the machine’s performance are given.
References 1. J. A. Cornell, J. S. Jordan, S. Ellis, and E. E. Rose, J . Amer. Dent. Ass., 54, 608 (1957). 2. V. 8. Boddicker, J. Amer. Dent. Ass., 35, 793 (1947). 3. J. Osborne, Rr. Dent. J., 87, 10 (1949). 4. F. A. Slack, J . Amer. Dent. Ass., 39, 47 (1949). 5. F. Taketa, H. S. Perdue, W. F. O’ltourke, H. W. Sievert, and P. H. Phillips, J . Dent. Res., 36, 739 (1957). 6. It. L. Myerson, J . Prosth. Dent., 7, 625 (1957). 7. J. C. Thompson, Dent. Practnr., 15, 233 (1965). 8. G. Henkel, Fogorvosi Szemle, 60, 138 (1967). 9. G. E. Monasky and D. F. Taylor, J . Prosth. Dent., 25,299 (1971). 10. A. A. Lugassy and E. H. Greener, J . Dent. Res., 51, 967 (1972). 11. J. M. Powell, MS Thesis, Indiana University School of Dentistry, Bloomington, Ind. (1972). 12. L). W. Jones, P. A. Jones, and H. J. Wilson, J . Dent., 1 , 2 8 (1972). 13. J. F. Bates, G. D. Stafford, and A. Harrison, J . Oral Rehabilitation, in press. 14. A. A. Brewer, J . Prosth. Dent., 13, 49 (1963).
Received September 19, 1974
