Passive resettable rigid damper
The passive resettable stiffness damper (PRSD) addresses inefficiencies in passive dampers by using a toggle valve and gear train mechanism to reset damper forces and enhance energy dissipation, achieving efficient and consistent vibration control with improved energy dissipation and damping force.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- OHIO UNIV
- Filing Date
- 2023-08-08
- Publication Date
- 2026-07-17
AI Technical Summary
Existing passive dampers for structural vibration control lack effective mechanisms for resetting damper forces and efficiently dissipating energy, leading to inefficiencies and potential overheating.
A passive resettable stiffness damper (PRSD) with a mechanically operated toggle valve and gear train mechanism that regulates fluid flow within a cylinder, allowing for controlled damper force reset and enhanced energy dissipation through a closed-loop or open-loop system, utilizing a toggle valve with a reciprocating tip for increased efficiency.
The PRSD effectively resets damper forces and enhances energy dissipation capacity, maintaining consistent damping characteristics and preventing overheating, with closed-loop systems providing twice the damping force of open-loop systems and bilateral designs offering twice the fluid flow rate of unilateral designs.
Smart Images
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Abstract
Description
Technical Field
[0001]
[0001] Exemplary embodiments of the general concept of the present invention are directed to a passive resettable stiffness device that can be used, for example, to enable effective vibration suppression of an object.
Background Art
[0002]
[0002] Vibration control techniques are used to protect structures from dynamic loads by dissipating energy that would otherwise be absorbed by the structures. The characteristics of vibration control techniques are defined by the types of structures and dynamic loads.
[0003]
[0003] In the field of structural engineering, vibration control techniques are utilized to protect building and bridge structures from ground movements caused by earthquakes. Such vibration control techniques are often referred to as dampers and take many different forms depending on their energy dissipation mechanisms and required power. The most reliable type of damper is the passive damper, which does not require external power and generates forces in direct response to the movement of the structure. Passive dampers include, but are not limited to, viscous dampers, viscoelastic dampers, friction dampers, and metal yielding dampers.
Summary of the Invention
Problems to be Solved by the Invention
[0004]
[0004] Exemplary embodiments of the general concept of the present invention present a passive resettable stiffness damper (PRSD) device that includes the aforementioned desirable characteristics.
Means for Solving the Problems
[0005]
[0005] Exemplary embodiments of the PRSD device include, but are not limited to, a cylinder such as a pneumatic or hydraulic cylinder having a reciprocating piston and one or a pair of associated protruding piston rods. A reset mechanism is installed in the cylinder or otherwise associated with the cylinder. The reset mechanism includes a mechanically operated toggle valve with a spring return and a series of discs coupled to a corresponding shaft to form a gear train. The gear train disc and shaft assembly can rotate but translation is limited.
[0006]
[0006] The toggle valve of the reset mechanism is located in a bypass loop that connects the cylinder volumes on both sides of the piston and operates to regulate the fluid flow between them. The toggle valve opens and closes depending on the position of the toggle coupled to the valve.
[0007]
[0007] The reset mechanism is positioned relative to the cylinder such that a first disc of the gear train contacts the piston rod of the cylinder and another disc of the gear train contacts the toggle of the toggle valve. Thus, extension or contraction of the cylinder's piston rod in response to the vibration force causes rotation of the first disc, which in turn causes rotation of the disc that contacts the toggle of the toggle valve. The rotation of the disc that contacts the toggle of the toggle valve results in displacement of the toggle, which in turn moves the toggle valve. This process causes the valve to open and close each time the piston changes direction, thereby causing a reset of the damper force generated by the PRSD.
[0008]
[0008] Other aspects and features of the concept of the present invention will become apparent to those skilled in the art by examining the following detailed description relating to exemplary embodiments together with the accompanying drawings.
[0009]
[0009] In the following description of the drawings and exemplary embodiments, similar reference numerals across multiple figures refer to the same or equivalent feature parts. [Brief explanation of the drawing]
[0010] [Figure 1]
[0010] This is an isometric view of an exemplary embodiment of a one-sided passive resettable rigid damper (PRSD) according to the concept of the present invention. [Figure 2]
[0011] Figure 1 is an example of a unilateral PRSD, shown as a top view. [Figure 3]
[0012] Figure 1 is a side view of an example of unilateral PRSD. [Figure 4]
[0013] Figure 1 is an example end view of a one-sided PRSD. [Figure 5]
[0014] Figure 1 is an example of a one-sided PRSD, showing the opposite end view. [Figure 6]
[0015] Figure 1 is an isometric cross-sectional view of an example unilateral PRSD. [Figure 7]
[0016] Figure 1 is an exploded view of an example of unilateral PRSD. [Figure 8]
[0017] Figure 1 is an enlarged view of a portion of an example of unilateral PRSD. [Figure 9A]
[0018] Figure 1 shows the toggle position of the reset mechanism of an example unilateral PRSD. [Figure 9B] Figure 1 shows the toggle position of the reset mechanism of an example unilateral PRSD. [Figure 9C] Figure 1 shows the toggle position of the reset mechanism of an example unilateral PRSD. [Figure 10A]
[0019] Figures 9A to 9C are enlarged views of the toggle mechanism of the reset mechanism. [Figure 10B] Figures 9A to 9C are enlarged views of the toggle mechanism of the reset mechanism. [Figure 11]
[0020] This is an isometric view of an exemplary embodiment of a bilateral passive resettable rigid damper (PRSD) according to the concept of the present invention. [Figure 12]
[0021] It is a top view of an exemplary bilateral PRSD of FIG. 11. [Figure 13]
[0022] It is a side view of an exemplary bilateral PRSD of FIG. 11. [Figure 14]
[0023] It is an enlarged end view of an exemplary bilateral PRSD of FIG. 11. [Figure 15]
[0024] It is an enlarged end view of the opposite side of an exemplary bilateral PRSD of FIG. 11. [Figure 16]
[0025] It is an isometric sectional view of an exemplary bilateral PRSD of FIG. 1,1. [Figure 17]
[0026] It is an exploded view of an exemplary bilateral PRSD of FIG. 11. [Figure 18]
[0027] It is a graph of damper force and piston displacement for an exemplary embodiment of a unilateral closed-loop PRSD according to the concept of the present invention. [Figure 19]
[0028] It is a graph of damper force and piston displacement for an exemplary embodiment of a unilateral open-loop PRSD according to the concept of the present invention. [Figure 20]
[0029] It is a graph of damper force and piston displacement for an exemplary embodiment of a bilateral closed-loop PRSD according to the concept of the present invention. [Figure 21]
[0030] It is a graph of damper force and piston displacement for an exemplary embodiment of a bilateral open-loop PRSD according to the concept of the present invention.
Best Mode for Carrying Out the Invention
[0011]
[0031] An exemplary embodiment of a unilateral passive resetable stiffness damper (PRSD) 5 is shown in FIGS. 1 to 7. As used herein, the term "unilateral" represents that the cylinder component of the PRSD has a single piston rod that only extends or contracts from one end of the cylinder.
[0012]
[0032] As shown, the exemplary PRSD5 includes, but is not limited to, a cylinder 10 containing a piston such as a pneumatic cylinder or a hydraulic cylinder. A reset mechanism 15 is mounted on the cylinder or otherwise associated with the cylinder such that it protrudes over a piston rod 20 of the cylinder. The reset mechanism includes a mechanically operated toggle valve 25 with a spring return and a plurality of discs 30. Each “disc” can be a toothed gear or another form of disc and is connected to a shaft 35 and also in contact (e.g., via toothed engagement, friction, etc.) and configured to give rotational motion relative to each other when it receives a rotational driving force applied to at least one disc. The shaft 35 is preferably supported by a rigid frame 40 so that the discs and shaft are rotatable but translational is limited.
[0013]
[0033] Referring here to Figure 8, the structure and operation of the reset mechanism 15 can be examined in more detail. As shown, a first disk (disk 1) is fixed to a preferably rigid first shaft (shaft 1) between a pair of similar second disks (disks 2), each of the second disks having a larger diameter than the first disk (disk 1). The first disk (disk 1), the second disk (disk 2), and the first shaft (shaft 1) rotate together and therefore have the same angular displacement, rotational speed, and acceleration. The reset mechanism 15 is positioned relative to the cylinder 10 so that the first disk (disk 1) is in contact with the piston rod 20 of the cylinder, causing the first disk (disk 1), the second disk (disk 2), and the first shaft (shaft 1) to rotate together as a unit when the piston rod is extended or retracted.
[0014]
[0034] A fourth disk (disk 4) is preferably fixed to a rigid second shaft (shaft 2) between a pair of similar third disks (disks 3), each of the third disks having a smaller diameter than the fourth disk (disk 4). The fourth disk (disk 4), the third disks (disks 3), and the second shaft (shaft 2) rotate together and therefore have the same angular displacement, rotational speed, and acceleration. The engagement of the second disk (disk 2) on the first shaft (shaft 1) and the third disk (disk 3) on the second shaft (shaft 2) causes the fourth disk (disk 4), the third disks (disks 3), and the second shaft (shaft 2) to rotate together when the first disk (disk 1), the second disks (disks 2), and the first shaft (shaft 1) rotate together.
[0015]
[0035] As shown in Figures 1 to 8, the mechanically operated toggle valve 25 is positioned higher than the disk 30 and shaft 35 of the reset mechanism 15. As is best seen in Figure 8, the periphery of the larger fourth disk (disk 4) on the second shaft (shaft 2) is in contact with the free end of the toggle 45 of the mechanically operated toggle valve 25, but not with the piston rod 20 of the cylinder 10. The rotation of the fourth disk (disk 4) causes the displacement of the toggle 45 that moves the toggle valve 25.
[0016]
[0036] It should be noted that the toggles 45 shown in Figures 2, 6, and 7, the toggle shown in Figure 8, and the toggles 75 shown in Figures 12, 16, and 17 are only schematic and general representations. Actual embodiments of the PRSD according to the concept of the present invention are shown in Figures 9A-9C and 10A-10B, and are preferably used and operated in conjunction with toggles having reciprocating tips, as described herein in relation thereto.
[0017]
[0037] In this exemplary embodiment relating to PRSD5, the configuration of the gear train disc 30 amplifies the toggle displacement of the toggle valve 25 relative to the displacement of the cylinder's piston rod 20 (and piston), so that the toggle displacement is greater than the displacement of the piston rod. When further amplification (e.g., for larger movements) is required, additional discs and shafts may be added between the first shaft (shaft 1) and the second shaft (shaft 2) to increase the amplification of the movement while still utilizing relatively small diameter discs.
[0018]
[0038] The toggle valve 25 of the reset mechanism is located within a bypass loop (not shown) that connects the cylinder volumes on both sides of the piston of cylinder 10 and regulates the fluid flow between them. Referring now to Figures 9A to 9C, it is further understood that the toggle valve 25 is closed when the toggle 45 is in the right position (Figure 9A) or the left position (Figure 9C), and the valve is open when the toggle is in the center position (Figure 9B). Note that all of the above positions refer to the diagram of the reset mechanism shown in Figure 8.
[0019]
[0039] For the exemplary PRSD5 shown in Figures 1 to 7, the initial position of the toggle is the right-hand position shown in Figure 9A. If the displacement of the cylinder's piston rod 20 causes the first disc (disc 1) to rotate counterclockwise (with respect to the reset mechanism 15 shown in Figure 8), and through the interaction of the gear train, the fourth disc (disc 4) rotates clockwise, the toggle remains in the right-hand position, the toggle valve 25 remains closed, and the damper force increases.
[0020]
[0040] When the piston rod 20 of cylinder 10 changes direction, the first disc (disc 1) rotates clockwise, which, through the interaction of the gear train, causes the fourth disc (disc 4) to rotate counterclockwise. The counterclockwise rotation of the fourth disc (disc 4) first moves the toggle from the right position in Figure 9A to the central position shown in Figure 9B. During this time, the toggle valve opens and the damper force decreases to zero. As the piston rod 20 continues to move in the same direction, further counterclockwise rotation of the fourth disc (disc 4) moves the toggle from the central position in Figure 9B to the left position shown in Figure 9C, at which point the toggle valve 25 closes and the damper force increases again.
[0021]
[0041] With the end of the toggle in contact with the fourth disc (disc 4) oriented to the left position shown in Figure 9C, the piston and piston rod 20 change direction, and the toggle moves toward the right position shown in Figure 9A, causing the toggle valve 25 to repeatedly open and close. Throughout this process, the damping force is reset each time the piston changes direction.
[0022]
[0042] As described above and as shown in Figures 9A-9C, resetting the damping force of the exemplary PRSD5 requires a toggle 45 on the mechanically operated toggle valve 25 to change its position relative to the fourth disc (disc 4). More specifically, the contact point P between the toggle 45 and the fourth disc (disc 4) moves along the periphery of the fourth disc (disc 4), as shown. Since this requires a change ΔL in the length of the toggle 45, it is preferable that the toggle be configured to include a reciprocating tip or to allow compression of the toggle's length, as shown in Figures 9A-9C and 10A-10B. In the case of the toggle 45 shown in Figures 9A-9C and 10A-10B, the tip of the toggle consists of a hollow shaft having an inner diameter slightly larger than the outer diameter of the toggle shaft and is held in a sliding (reciprocating) relationship on the toggle shaft. A pre-compressed spring is placed on the toggle shaft between the tip of the toggle and the end of the toggle, which is pinned to the toggle valve 25 or otherwise pivotably attached. Therefore, as the toggle 45 moves from one side of the fourth disc (disk 4) to the other, the tip of the toggle undergoes a linear displacement with respect to the length of the toggle shaft, corresponding to the required change in the toggle length ΔL.
[0023]
[0043] It should be noted that the use of a toggle valve having a toggle with a reciprocating tip (or a similar configuration) offers several advantages. These advantages include, but are not limited to, (1) the increased energy dissipation capacity of the PRSD because the toggle valve is only open for a short time during reset due to the toggle's action; (2) the increased energy dissipation capacity of the PRSD due to the toggle valve, which enables high flow rates with a small spring return force; (3) the simplification of the reset mechanism compared to certain reset-type semi-passive rigid dampers (RSPSDs) and reset-type passive rigid dampers (RPSDs) of known designs; and (4) the PRSD becoming more compact due to the simplified reset mechanism.
[0024]
[0044] An exemplary embodiment of a bilateral passive resettable rigid damper (PRSD) 50 is shown in Figures 11 to 17. As used herein, the term “bilateral” refers only to the fact that the cylinder component of the PRSD has two piston rods that extend and retract from both ends of the cylinder.
[0025]
[0045] As shown, the exemplary double-sided PRSD 50 includes, but is not limited to, a cylinder 55 containing a piston such as a pneumatic cylinder or a hydraulic cylinder. The reset mechanism 60 is mounted at both ends of the cylinder, or otherwise associated with both ends of the cylinder, such that each reset mechanism 60 protrudes over the corresponding piston rods 65, 70 of the cylinder 55.
[0026]
[0046] In this exemplary double-sided PRSD embodiment 50, each reset mechanism 60 is of the same design, structure, and operation as the reset mechanism 15 used in the exemplary single-sided PRSD 5. Thus, the toggle 75 of each reset mechanism 60 is also coupled to an associated toggle valve and has a free end that contacts the periphery of the fourth disc (disc 4) on the second shaft (shaft 2) of the gear train, as commonly shown in Figure 8. A further detailed list of the components of the reset mechanism 60 does not need to be repeated here. Similarly, the descriptions made so far regarding the interaction of the components of the reset mechanism (and Figure 8), the movement of the toggle and the reset of the damper (and Figures 9A-9C), and the change in the toggle length of the toggle valve (and Figures 10A-10B) also apply equally to the reset mechanism 60 of the exemplary double-sided PRSD 50. Of course, one difference is that the opening and closing of the valve and the reset of the damper occur as a result of the operation of two reset mechanisms 60 in the exemplary double-sided PRSD 50, in contrast to a single reset mechanism 15 for the exemplary single-sided PRSD 5.
[0027]
[0047] One advantage of a bilateral PRSD over a unilateral PRSD is that by using two toggle valves in a bilateral PRSD, it allows for twice the effective fluid flow rate of a single toggle valve in a unilateral PRSD. This higher fluid flow rate increases the speed at which the damper force drops to zero when the valve opens during reset, thereby improving the energy dissipation capability of the PRSD.
[0028]
[0048] In both the exemplary one-sided and two-sided PRSD embodiments shown and described herein, a toggle valve is located in a bypass loop connecting the cylinder volumes on both sides of the cylinder piston. The movement of the cylinder piston creates pressure on one side of the cylinder and a vacuum on the other. During reset, the toggle valve opens, equalizing the pressure and reducing the damper force to zero. Since the volume of gas inside the cylinder remains constant, such embodiments of PRSD can be described as a closed-loop system.
[0029]
[0049] One advantage of a closed-loop system over an open-loop system is that the gas inside the damper cylinder can be compressed to increase the damping force. Another advantage of a closed-loop system over an open-loop system is that the damping force is generated by both the pressure on one side of the cylinder piston and the vacuum on the other side of the cylinder piston, so a closed-loop design can produce about twice the damping force of an open-loop design. In embodiments of PRSDs where a pressurized cylinder is used, it is preferable that the cylinder pressure be monitored to detect leaks that may cause a pressure drop on one or both sides of the cylinder and the resulting change in damping characteristics.
[0030]
[0050] In an alternative embodiment of the PRSD, one of the toggle valve ports remains open to the atmosphere, resulting in an open-loop PRSD. In an open-loop PRSD, the movement of the cylinder piston pressurizes the cylinder volume on one side of the piston, while leaving the cylinder volume on the other side open to atmospheric pressure. When the piston changes direction, a reset occurs, releasing the previously pressurized cylinder volume to the atmosphere and pressurizing the previously open cylinder volume.
[0031]
[0051] The advantage of the open-loop system is that a new volume of air is pressurized each time the cylinder piston changes direction. As a result, there is no concern that the PRSD will overheat during use, and the damping characteristics remain constant.
[0032]
[0052] To verify the concept of the present invention, prototype PRSDs using atmospheric pressure air were prepared and tested. A total of four PRSDs were tested: (1) a one-sided closed-loop PRSD, (2) a one-sided open-loop PRSD, (3) a two-sided closed-loop PRSD, and (4) a two-sided open-loop PRSD.
[0033]
[0053] During testing, each prototype PRSD was subjected to a sinusoidal cylinder piston displacement with an amplitude of 30 mm and a frequency of 0.25 Hz for 40 cycles. For each prototype PRSD, the output force was plotted against the piston displacement.
[0034]
[0054] The results of the aforementioned tests are shown in Figures 18 to 21. As can be seen, the force-displacement loop shows that all prototype PRSDs behaved as intended, with the damper force increasing with increasing piston displacement and decreasing to zero each time the piston direction changed. Figures 18 to 21 further show that the force-displacement loop was stable for all four prototype PRSDs, with little variation in force-displacement characteristics over 40 cycles of motion.
[0035]
[0055] A comparison of Figures 18 and 19, and Figures 20 and 21, shows that the closed-loop PRSD embodiment has higher effective rigidity than the open-loop PRSD embodiment. This may be because the closed-loop embodiment utilizes the cylinder volume on both sides of the piston cylinder during operation (one volume is pressurized, the other is vacuumed), whereas the open-loop embodiment utilizes only the cylinder volume on one side of the cylinder piston (pressurized).
[0036]
[0056] Similarly, comparisons of Figures 18 and 20, and Figures 19 and 21, show that the double-sided PRSD has a higher peak damper force than the single-sided PRSD. This may be due to the double-sided PRSD having cylinder pistons extending from both sides of the cylinder, thereby approximately doubling the amount of friction between the piston rod and the seal.
[0037]
[0057] While specific exemplary embodiments of the concept of the present invention have been described in detail above, the scope of the general concept of the present invention is not to be considered limited by such disclosure and can be modified without departing from the spirit of the general concept of the present invention as expressed by the following claims.
Claims
1. A single-sided passive resettable rigid damper (PRSD), A cylinder having a reciprocating piston with an associated piston rod, wherein the piston rod protrudes from a first end of the cylinder and A reset mechanism associated with the cylinder and located near its first end, A mechanically operated toggle valve having a spring return mechanism and positioned within a fluid passage with the cylinder, A gear train having a plurality of rotatable discs, and having a configuration and arrangement adapted such that the linear displacement of the piston rod causes the discs of the gear train to rotate, and A toggle valve having one end pivotably coupled to the toggle valve and the other free end in contact with the gear train. A reset mechanism including, Equipped with, A PRSD, wherein the change in the rotational direction of the gear train disc, caused by the reciprocating motion of the piston rod in response to a vibration force, is adapted to the toggle to change the fluid flow path through the toggle valve, thereby resetting the PRSD.
2. The PRSD according to claim 1, wherein the cylinder is a pneumatic cylinder.
3. The PRSD according to claim 1, wherein the cylinder is a hydraulic cylinder.
4. The PRSD according to claim 1, wherein the reset mechanism is installed at the first end of the cylinder.
5. The first of the plurality of disks contacts the piston rod, The second disk of the plurality of disks comes into contact with the toggle, As a result, the reciprocating motion of the piston rod is adapted to cause rotation of the first disk, which is adapted to cause rotation of the second disk, which is adapted to cause displacement of the toggle, which is adapted to actuate the toggle valve and reset the PRSD, according to claim 1.
6. The PRSD according to claim 5, wherein the piston rod is adapted to change direction in response to a vibration force, and as a result, the PRSD is reset each time the piston rod changes direction.
7. The PRSD according to claim 5, wherein the first disc and the second disc are engaged by teeth.
8. The PRSD according to claim 5, wherein the first disk and the second disk are engaged by friction.
9. The PRSD according to claim 5, wherein the first disk has a smaller diameter than the second disk.
10. The first disk is fixed to the first shaft between the first pair of disks of the plurality of disks, and each of the first pair of disks has a larger diameter than the first disk, so that the first disk and the first pair of disks are adapted to rotate as a single unit. The second disk is fixed to a second shaft between the second pair of disks of the plurality of disks engaged by the first pair of disks, and each of the second pair of disks has a smaller diameter than the second disk, so that the second disk and the second pair of disks are adapted to rotate as a single unit. The PRSD according to claim 9, wherein the second disk and the second pair of disks are adapted to rotate together when the first disk and the first pair of disks rotate together.
11. It has a double-sided passive resettable rigid damper (PRSD), A cylinder having a reciprocating piston equipped with a pair of piston rods, wherein the pair of piston rods protrude from both ends of the cylinder and the cylinder, Individual reset mechanisms are positioned near each of the ends of the cylinder and associated with a corresponding one of the piston rods, each of which reset mechanisms is A mechanically operated toggle valve having a spring return mechanism and positioned within a fluid passage with the cylinder, A gear train having a plurality of rotatable discs, wherein the configuration and arrangement are adapted such that the corresponding linear displacement of one of the piston rods causes the discs of the gear train to rotate, and A toggle valve having one end pivotably coupled to the toggle valve and the other free end in contact with the gear train. A reset mechanism including, Equipped with, A PRSD, wherein the change in the rotational direction of the gear train disc of the reset mechanism, caused by the reciprocating motion of the piston rod in response to a vibration force, is adapted to the toggle to change the fluid flow path through the toggle valve, thereby resetting the PRSD.
12. The PRSD according to claim 11, wherein the cylinder is a pneumatic cylinder.
13. The PRSD according to claim 11, wherein the cylinder is a hydraulic cylinder.
14. Each reset mechanism is installed at the respective end of the cylinder, the PRSD according to claim 11.
15. Regarding each reset mechanism, The first disk of the plurality of disks contacts the corresponding piston rod, The second disk of the plurality of disks comes into contact with the toggle, As a result, the reciprocating motion of the corresponding piston rod is adapted to cause rotation of the first disk, which is adapted to cause rotation of the second disk, which is adapted to cause displacement of the toggle, which is adapted to actuate the toggle valve and reset the PRSD, according to claim 11.
16. The PRSD according to claim 15, wherein for each reset mechanism, the corresponding piston rod is adapted to change direction in response to a vibration force, and as a result, the PRSD is reset each time the piston rod changes direction.
17. The PRSD according to claim 15, wherein, for each reset mechanism, the first disk and the second disk are engaged by teeth.
18. The PRSD according to claim 15, wherein, for each reset mechanism, the first disk and the second disk are engaged by friction.
19. The PRSD according to claim 15, wherein for each reset mechanism, the first disk has a smaller diameter than the second disk.
20. Regarding each reset mechanism, The first disk is fixed to the first shaft between the first pair of disks of the plurality of disks, and each of the first pair of disks has a larger diameter than the first disk, so that the first disk and the first pair of disks are adapted to rotate as a single unit. The second disk is fixed to a second shaft between the second pair of disks of the plurality of disks engaged by the first pair of disks, and each of the second pair of disks has a smaller diameter than the second disk, so that the second disk and the second pair of disks are adapted to rotate as a single unit. The PRSD according to claim 19, wherein the second disk and the second pair of disks are adapted to rotate together when the first disk and the first pair of disks rotate together.