The advantage of mechanical safety couplings is that in a complete set of mechanical equipment, there are only a few safety protection components per drive shaft. Therefore, only by monitoring the working conditions of these parts and components can ensure the safety of the equipment during work. When the machine is overloaded, these parts ensure that the driveline can be reliably disconnected under all circumstances and that it will not fail itself in the worst case.
Figure 1 Correct design Physical data has important influences on the design of safety coupling components, such as torque, torsional rigidity, rotational speed, spring stiffness, rotational inertia, dynamic balance, and clearance size, etc.
In the past year, the number of components in the drive system of machine tool equipment has increased significantly. Correspondingly, the design and calculation workload of these mechanical components, such as coupling design and calculation workload, have also increased. In the process of designing these parts, physical data has important influences such as torque, torsional rigidity, rotational speed, spring stiffness, rotational inertia, dynamic balance, and gap size (Fig. 1). In order to ensure the high running stability of the drive system, the dynamic balance error of the entire drive system should be as small as possible. The so-called dynamic balance error arises from the asymmetry of the parts manufacturing, that is, the centrifugal force caused by the uneven distribution of the parts' quality causes the transmission parts to vibrate.
Fig. 2 Double-mass model: almost all the components of the drive system can be easily calculated at the same time
Several coupling design methods
There are several different coupling design methods that can effectively improve the dynamic balance of the drive system (Figure 2). The use of processing balance holes to make the mass distribution of the entire part more uniform is one of them. In the drive technology, the accuracy level of the dynamic balance is specified, and the maximum allowable unbalance is limited. Commonly used dynamic balance accuracy grades are G16, G6.3, and G2.5. The smaller the number, the higher the dynamic balance accuracy (the smaller the allowable residual imbalance). Safety couplings that perform safety work in accordance with the ball-to-ratchet principle work in most cases on the basis of the disengagement torque. When the dynamic performance is higher, the influence of the resonant frequency must be additionally considered, and the magnitude of the resonant frequency should be calculated because the torsional stiffness also has an important influence on the dynamic balance. The quick and correct coupling design relies on the determination of the rated torque or the release torque. When the torque transmitted by the transmission system reaches the rated value, the coupling will exert a safety protection function to disconnect the power input and output of the transmission system. Torque-off is the torque normally applied to the input of the drive system. In addition, considering the acceleration torque and other influencing factors, the comprehensive safety factor used in practical applications is 1.5. The safety coupling can be selected according to the following formula.
Where TKN is the rated torque delivered by the coupling in Nm; TAN is the rated torque on the input side of the driveline, and the unit is also Nm.
Figure 3 The safety coupling connects the servo motor with the ball screw of the machine tool
The nominal torque TAN on the drive side is usually known from the equipment nameplate. If this data is not available on the nameplate, it can be calculated from the empirical formula using the transmission power and rotational speed of the drive equipment and converted to Nm using the commonly used number 9550 in the manufacture of machine tools.
Where PAN is the driving side power in kW; n is the speed in r/min.
Another way to determine the torque is to use the acceleration torque (at no-load start-up). Using this method, it is necessary to consider the distribution of mass and the rotational inertia of the drive system. With the help of the correction factor (impact correction factor or load correction factor) determined by the equipment and usage conditions, the value of the acceleration torque can be determined. In most cases, the impact factor and load factor can be determined according to three different operating conditions: SA=1 (harmonic load); SA=2 (periodic load); SA=3~4 (non-periodic load).
The following formula can be used to determine the torque.
Where α is the angular acceleration in 1/s2; JL is the torsional stiffness of the output side and coupling device I in kgm2; JA is the torsional stiffness of the input side and coupling device II in kgm2; TAS is the drive Side of the peak torque; SA is the impact factor or load factor.
If precise torque determination is required, the safety coupling must be considered for acceleration torque and load torque (with load starting). This accurate calculation takes into account conditions that are often at load acceleration and braking. In addition to considering the acceleration torque, consider the load torque TAN. The following formula describes the relationship between them.
The above design method is related to the power input and output components given by the manufacturer. In addition to considering the torque value, consider the mass moment of inertia and the possible acceleration. Determining the release torque according to the feed force is another possible method of calculation. This method can be used not only for transmission systems of transmission shafts, but also for transmission systems with toothed belt drives.
In the drive shaft drive system, the screw pitch and efficiency of the drive shaft also have an important effect on the drive torque. The input drive torque can be calculated as follows.
Where s is the pitch of the lead screw and the unit is mm; FV is the feed force and the unit is N; η is the efficiency of the drive shaft.
If the power input and power output are not transmitted by the same propeller shaft but are transmitted using a toothed belt, the input torque can be calculated as follows.
Where d0 is the diameter of the small toothed pulley and its unit is mm.
If the working range of the coupling exceeds the range of the resonant frequency, detailed knowledge of each parameter is required. Each component of the transmission system has its own natural frequency. The drive system should not operate within the so-called resonant frequency range. The following formula can calculate the resonant frequency of the coupling and the entire drive system. However, the premise of using this formula is to integrate the moments of inertia of the various components of the drivetrain into a total moment of inertia. The torque stiffness of the total moment of inertia also has an important influence on the vibration.
The unit of natural frequency is 1/min, which can be calculated as follows:
In the formula, e is the natural vibration frequency of the transmission system; CT is the torsional stiffness in Nm/rad; ne is the natural vibration frequency of the system.
The torsional stiffness of the entire device plays an important role in the design and calculation of the coupling. Whether the stiffness and damping performance of a machine tool is designed to be higher or lower is closely related to the use of machine tools. In principle, the rigidity of all parts should be fully considered.
The object is distorted under a certain force (torque) condition, and the torsional angle is expressed by the rigidity of the object (resistance to torsion). The following formula clearly shows the relationship between them.
Where φ is the twist angle and the unit is degree.
When the safety coupling is accurately designed and calculated in accordance with the torque, resonance frequency or torsional rigidity, it is necessary to conduct appropriate and repeated dialogue and technical consultation with the coupling manufacturer. Their experience in this field ensures that they can accurately determine the size of the coupling.
Fig. 4 The safety coupling developed and produced by R+W Company has a very compact structure. According to the size of the safety coupling and the transmitted torque, the stroke distance of the switch ring should be between 0.7~3mm so that the machine tool and The equipment is safe.
Quick release and only small residual friction
Today's couplings in machine tool equipment must have two major functions, namely quick release and disengagement of small residual friction. When overloaded, the coupling must disconnect the power input and output within a few microseconds to protect the entire drive system. Its main purpose is to disengage the drive system as quickly as possible to reduce the maintenance costs of the damaged drive system. After the safety coupling is disengaged, the residual friction cannot be so great that the connected parts will not continue to accelerate due to an excessive moment of inertia. This is because the smaller the residual friction force after the safety coupling is disengaged, the smaller the load on the power input side and the output side in the disengaged state.
Using a precision disc spring can achieve the above two functions. The X-axis in the graph 5 indicates the disengagement and opening times of the safety coupling and the Y-axis indicates the input torque. The red area in the figure indicates the area formed by the disengagement speed curve and the input torque of the transmission system.
With the use of the disc spring, the reflection process when the coupling is overloaded is faster, and the disengagement of the coupling or the disengagement of the power input end from the output end is also faster. In addition, the disengagement speed along the X-axis direction is also affected by the quality of the coupling components, such as the switch ring. Its weight should be as light as possible in order to achieve rapid axial movement. The area of ​​the red zone represents the work done during the system's disengagement. The larger the red area, the more heat is emitted from the drive train. This type of safety coupling is known for its excellent release properties and relatively small red areas in the performance curve. A specially developed disc spring with special spring characteristics bears the task of stabilizing the adjustment range of the safety coupling and reducing the residual frictional moment at the power input and output ends when it is disengaged. Its operating characteristics are not based on ordinary characteristics, but rather have their own unique operating characteristics. As with all other safety couplings that require the use of inductive proximity switches or end point switches, the switch travel of such switch rings is also monitored. In order to ensure reliable operation even when contaminated by dirt, the control loop of the switch ring should be as large as possible.