The Anti-Vibration Coupling with High RigidityMany types of shaft couplings are used with various production equipment, which requires high productivity.
High-torsional stiffness couplings such as Disk type (Pic1) and Bellows type (Pic2) are primarily used in applications that require high positioning accuracy. On the other hand, recently, the speed response frequency of servomotors is increasing (Fig1), the vibration by hunting often occurs, resultantly we cannot use the motor with its maximum performance. Therefore, we need to suppress the performance to reduce the vibration when using fully metal couplings. There are Elastomer type couplings (Pic3) which have elastomers that absorb vibration using rubber and not suppressing motor performance. However, these Elastomer type couplings do have a problem; they have lower torsional stiffness when compared to Disk type or Bellows type. As a result, sometimes the Elastomer type are not good for strong cutting resistance applications such as lead screws of machine tools.
Hence, a big goal for coupling makers is to provide a coupling that can maintain high torsional stiffness and enable vibration absorption in the same coupling. In the past, one would have to choose between high torsional stiffness and vibration absorption functionality.
●Pic4. NBK “XGHW” Couplings
Construction of Anti-Vibration coupling with High RigidityThe dynamic vibration absorber is a mechanism that suppresses the coupling’s vibration when effected by the resonance phenomenon by joining the vibrator (coupling body) and supplementary inertial rotor through the elastic body.
The following Fig2 shows the construction.
●Fig2. Construction of “XGHW”
Effects of “XGHW”
When we increase the gain, hunting may occur; it is one example of a resonance phenomenon of rotational factors when using servomotors. The gain level that hunting occurs is different based on coupling type. The following Table1 shows the result of limit gain (the maximum gain that can be completely controlled without hunting), stabilization time (the time gap between the motor positioning command and actual stopping time) and torsional stiffness for each coupling.
●Table1. Result of each coupling type (Reference)
|Limit Gain *1||Stabilization Time [ms]||Torsional Stiffness [N・ m/rad]|
|Disk with Dynamic Vibration Absorber||32||8||300|
Actuator: KR30H, THK
Servomotor: HG-KR13, Mitsubishi Electric
Load Mass: 3 kg (6.6 lb.)
Servo Adjustment: MRConfigrator 2 Auto tuning mode 2 (each filter off)
*1: Limit value of auto tuning gain that set by auto tuning mode 2.
Elastomer type couplings can suppress hunting’s occurrence via vibration absorption and make the limit gain increase more, resultantly, it can shorten the stabilization time. However, sometimes elastomer type are not good for high torque applications because their torsional stiffness is lower when compared to disk type couplings.
On the other hand, “XGHW” makes it possible to increase gain at almost the same level as elastomer type while maintaining high torsional stiffness similar to the disk type.
“XGHW” has an additional effect that suppresses the lost motion regarding torque conduction because it does not go through an elastic body such as rubber in the rotational factors and thus there is no hysteresis (Fig3) which can be seen in the elastomer type. Lost motion indicates a condition in which an elastic body does not revert to the original condition even if the power is released. This is because a part of the kinetic energy changes to thermal energy when an elastic body of polymers is deformed. In certain applications, having lost motion can result in a loss of positioning accuracy.
●Fig3. Comparison of Static Torsional Stiffness
b) Elastomer type
When using the “XGHW” couplings in conjunction with machine tools, there is a possibility to raise the finishing accuracy by dampening the vibration while keeping machining accuracy when machining high hardness and fragile materials. Recently, using materials like Glass, Ceramic, and Graphite for smartphone casing or production jigs has become more common, and therefore we recommend the “XGHW” couplings for Z-axis of machine tools for processing.
Many users think that XGHW’s outside diameter would be bigger than other disk type because of its inertial rotor but that is incorrect. You can keep a similar outside diameter to your current coupling and maintain the same allowable torque even if it has an inertial rotor because of XGHW’s higher rated torque (almost 1.5 times) due to its construction. It has three fixing bolts for the disks. The common disk type has only two bolts. (Pic5). You can see the actual difference using the following Table2.
●Pic5. Comparison between Common Disk type and “XGHW”
|Type||Disk||Disk with Inartial Rotor|
|Outside Diameter (mm)||26||27|
|Max.Lateral Misalignment (mm)||0.15||0.12|
|Max.Angular Misalignment (°)||1||2|
|Max.Axial Misalignment (mm)||±0.16||±0.2|
NBK has another type coupling with higher vibration absorpotion, our “XG2” couplings (Pic6), the stiffness is slightly lower than “XGHW” but can also enable high gain levels and thereby maximize the motor performance. By installing the “XG2” one can achieve high manufacturing productivity when high stiffness in the X and Y axis are unnececesarry.
NBK offers another White Paper for their “XG2” couplings so please reach out to the NBK team if you are interested in obtaining it.
●Pic6. NBK “XG2” couplings / Highest Vibration Absoption Capability