Abstract
Typical particle rotary dampers generate constant torque throughout the cycle of rotation. However, this research is focused on generating angle dependent torque i.e. different torque at different angles. The damper is designed to generate an elevated torque at a specific angle. This can be achieved by employing ferromagnetic particles and by introducing a permanent magnet in the damper. This study is carried out using two major parameters I) base torque which is the measure of damping capacity and II) percentage increase in torque which is the measure of angle dependency. From the experimental investigations, it has been found that I) base torque increases with packing fraction and rotational speed and decreases conditionally with the amount of carbonyl iron present in the particle while II) percentage increase in torque decreases with packing fraction, remains constant with rotational speed and increases conditionally with the amount of carbonyl iron present in the particle.
Introduction
A damper is a mechanical device used to attenuate kinetic energy from a system. General applications of dampers are in buildings and machinery such as seismic dampers and shock absorbers. Types of dampers include viscous, viscoelastic, friction and impact dampers. Widely used hydraulic dampers employ liquid in it for damping. But a particle damper uses solid particles instead of liquid [1,2]. Major advantages of a particle damper are elimination of oil leakage problem, simple construction, light weight, easy maintenance and high temperature range of use. Previous researches were concentrated on investigating damping force properties of linear particle dampers and torque properties of rotary particle dampers [1-8]. The damping capacity of a rotary damper is measured in damping torque. Generally, rotary dampers generate constant torque throughout the cycle of rotation. However, this research is focused on achieving angle dependency i.e. varying the torque depending on the angle of rotation of the rotor. The target of this research is application for a door closer which makes the door to close at a quite lower angular velocity just before latching to eliminate slam. This study shows how various parameters such as particle composition, packing fraction and rotational speed influence the angle dependency.
Experimental method
Elastomer
The particles used in this study are silicone rubber particles called elastomers which are viscoelastic in nature. They are spherical in shape with a diameter of 3 mm. A certain fraction of carbonyl iron powder is added to the silicone rubber before molding to make the particles ferromagnetic. The carbonyl iron powder is used because of its high magnetic permeability and it has a particle size of 4.2 μm and density of 7.86 g/cm3.
Experimental setup
Figure 1 shows the schematic of the experimental setup. The motor produces the rotational motion which is transferred to the rotor shaft via the couplings. The rotational speed of motor is controlled using the control unit. The drive motor unit acts as a link between the motor, control unit and the computer. The torque meter is used to measure the output torque generated by the damper. The interface fetches the torque readings from the torque meter and feeds it to the computer.
Schematic diagram of the experimental setup.
Figure 2 shows the schematic of the damper with elastomer particle assemblage. The rotational motion is transferred to the rotor via the rotor shaft. The rotor is of rectangular cross section which is surrounded by elastomer particles. The outer cylinder covers the whole arrangement and the flange which holds the entire damper is mounted to the setup. A neodymium permanent magnet of square cross section is inserted into a part of the outer cylinder. Ends of the rotor are supported both axially and radially by tapered roller bearings. All the parts of the damper except the bearings are made of SAE304 grade stainless steel which is non-magnetic in nature.
Schematic diagram of the prototype rotary damper with particles.
When the rotor is rotated, it pushes the particles. The particles provide a resistance to the motion of the rotor. The three causes for this resistance are elastic repulsion, friction and magnetic attraction. When the rotor rotates, it may deform some particles and thus it has to apply an elastic force. Also, to move the particles, the rotor has to overcome the friction between adjacent particles as well as the friction between particles and the parts of damper. Moreover, if the particles are influenced by a magnetic field, a force is needed to overpower the magnetic force of attraction. These factors cause resistance to the rotor motion thus producing torque [6,8]. Further, the resistance will increase in the proximity of the magnet due to two factors, namely increase in magnetic attraction and increase in local packing fraction. The magnetic force of attraction increases as the distance between the particle and magnet decreases. Also, the particles get attracted to the magnet and the local packing fraction of the region near the magnet will be higher. These factors cause increased resistance which leads to increase in torque at a particular angle thus angle dependency is achieved.
Experimental results and discussions
A typical angle dependent torque characteristic for one complete rotation of rotor is similar to a bell curve. As the rotor rotates through the particles, the damping torque increases from a base value (T min ) to a peak value (T max ) and then returns back.
The damping capacity of the damper is measured in the magnitude of base torque (T min ). The degree of angle dependency is measured by the percentage increase in torque, i.e., 100[(T max − T min )∕T min ] [%]. Packing fraction (PF) of a particle damper is the ratio of volume occupied by the particles to the actual volume of the damper [7].
Effect of particle composition
Effect of particle composition on base torque
Figure 3 shows the influence of volume fraction of carbonyl iron when rotational speed is 1 rpm and PF 50%. Figure 3(a) shows the influence on base torque (T min ). This characteristic shows decrease in base torque in the low iron region of 0 – 20 vol.% which can be clearly understood from Fig. 3(c). However, the base torque has a slight increase in the high iron region of 30 vol.%. Mixing of fine particles of low friction co-efficient into particles is supposed to decrease the overall friction coefficient of the particle. Thus, the frictional force needed to move the particle decreases with increase in carbonyl iron content. Similarly, the elastic modulus of the particle increases with increase in the amount of carbonyl iron. Hence, the deformation of the particle decreases and so the area of contact of the particles decreases. This in turn further decreases the friction involved [7]. These are considered to be the reasons for the fall of base torque in the low iron region. The increase in carbonyl iron content increases the number of magnetic domains in a particle thus increasing the magnetic force of attraction under the influence of a magnetic field. This increase in magnetic force in the high iron region eclipses the decrease in frictional force and provides a slight increase of base torque.
(a) Effect of particle composition on base torque; (b) Effect of particle composition on angle dependency; (c) Angle dependent torque characteristic showing the influence of particle composition.
Figure 3(b) shows the influence of volume fraction of carbonyl iron on angle dependency (% Increase in Torque). The angle dependency increases with increase in volume fraction of carbonyl iron up to 12 vol.% which can be clearly understood from Fig. 3(c). There is a sharp drop at 15 vol.% and again the angle dependency increases anew.
This can be summed up as increase in angle dependency with increase in carbonyl iron content with a sharp drop at 15 vol.%. The increase in carbonyl iron content increases the magnetic force of attraction between magnet and the particles near the magnet where the local packing fraction is high. This is the reason behind the increase in angle dependency. The sharp drop at 15 vol.% is due to a phenomenon called ‘Two-peak phenomenon’.
Figure 4 shows the angle dependent torque characteristics when the rotational speed is 1 rpm and PF 50%. When the volume fraction of carbonyl iron is 10 vol.%, the characteristic shows a single peak. When carbonyl iron content was increased to 20 vol.%, it was expected to give a similar single peak with a higher peak torque. But rather it resulted in the formation of two peaks. This phenomenon of formation of two peaks at higher carbonyl iron content is called two-peak phenomenon.
Angle dependent torque characteristics when volume fraction of carbonyl iron is (a) 10 vol.% (b) 20 vol.%.
The cause of two-peak phenomenon is explained using Fig. 5. At stage (i), the rotor starts to rotate in a clockwise direction. At stage (ii), the rotor is about to reach 90° of its rotation and a void gap can be seen adjacent to the rotor. The rotor further rotates and reaches stage (iii). Now the void gap increases and still it is not getting filled by the particles. This happens because the magnetic lines of attraction hold the particles firmly and it forms a wall of particles. The particles on the other side of the wall does not have sufficient force to break this wall and to enter the void gap. The rotor had to apply an additional to break the wall of particles and it finally reaches stage (iv). Now, the gap gets filled with particles. This increased force given to the break the wall appears as peak. In the case of lower carbonyl iron, the wall of particles is broken only once in one cycle and so a single peak. Whereas in the case of higher carbonyl iron, the magnetic force of attraction between the magnet and particles increases thus leading to formation of an extra wall at an earlier stage. Hence in total, two walls are broken leading to the formation of two peaks. It should not be misunderstood that an extra peak is formed in additional to the original peak. Rather, the original peak is split into two peaks which is evident by the decreased peak torque.
Illustration of two peak phenomenon.
Effect of packing fraction on base torque
Figure 6 shows the effect of packing fraction when the rotational speed is 1rpm and the volume fraction of carbonyl iron is 12 vol.%. Figure 6(a) shows the influence on the base torque (T min ). It is evident that the base torque increases with increase in PF which can be clearly understood from Fig. 6(c). Also, the rate of increase of base torque with respect to PF is higher in the higher PF region. When the PF is increased, the particles will be more densely packed in the damper. Thus, higher elastic force is exerted by the rotor [8]. Also, when there is higher compression, the area of contact of the particles increases which in turn leads to increase in frictional force. The higher elastic and frictional force altogether result in higher torque. The elastic force needed to compress the particles increases exponentially with increase in the amount of compression. This is considered to be the reason for increased slope in the higher PF region.
(a) Effect of packing fraction on base torque; (b) Effect of packing fraction on angle dependency; (c) Angle dependent torque characteristic showing the influence of PF.
Figure 6(b) shows the effect of PF on angle dependency (increase in torque). This characteristic is almost the mirror image of Fig. 6(a). The angle dependency decreases with increase in PF which can be clearly understood from Fig. 6(c). As discussed earlier, one of the sources of angle dependency is increase in local packing fraction in the proximity of magnet. In case of lower overall packing fraction, the quantity of particles near the magnet will be comparatively higher than other regions of the damper. This gives rise to increase in local packing fraction near the magnet. However, when the overall packing fraction is higher, the local packing fraction of the damper at any region tends to be the same. Since there is no increase in local packing fraction near the magnet, there is a decrease in angle dependency.
Effect of rotational speed
Effect of rotational speed on base torque
Figure 7 shows the effect of rotational speed when the PF is 50% and the volume fraction of carbonyl iron is 10 vol.%. Figure 7(a) shows the influence on the base torque (T min ). It is evident that the base torque increases with increase in rotational speed which can be clearly understood from Fig. 7(c). The elastic force needed to compress the particles increases with increase in the speed of compression. When the rotational speed of rotor is increased, compression of particles occurs at an increased speed. Hence the elastic force increases which in turn increases the base torque.
(a) Effect of rotational speed on base torque; (b) Effect of rotational speed on angle dependency; (c) Angle dependent torque characteristic showing the influence of PF.
Figure 7(b) shows the effect of rotational speed on angle dependency (increase in torque). The research target is application for a door closer and so the rotational speed was set to a maximum of 15 rpm. From the Fig. 7(b), it is evident that the rotational speed has no appreciable effect on the angle dependency which can be clearly understood from Fig. 7(c). This is because the sources of angle dependency have no influence from the change in rotational speed. From the above results, it has been found that the damper is capable of producing high angle dependency which makes it suitable for door closers. By including a spring mechanism, this prototype damper can be applied for general uses.
Conclusion
A new particle rotary damper with angle dependent torque characteristics has been designed, developed and studied. From the experimental studies, it is understood that the effect of increase in local packing fraction has higher influence on angle dependency than the increase in magnetic force of attraction. Also, it is evident that there is not even a single parameter with which both the base torque and the angular dependency can be increased simultaneously. In other words, whenever there is an increase in base torque, the angle dependency either decreases or remains constant and vice versa. The parameters for maximum angle dependency that can be achieved with this damper has been found. The future research will be focused on increasing the base torque while retaining the maximum angle dependency.