In the field of CNC systems, traditional approaches for handling power failure retraction in gear hobbing machines often rely on additional capacitor modules to supply the necessary energy for safe tool withdrawal after an unexpected power loss. For instance, Siemens and NUM systems incorporate capacitor modules with capacities of 4000 μF and 8250 μF, respectively, typically requiring multiple modules to meet retraction distance demands, alongside UPS systems like Siemens or Phoenix to provide DC24V for system, drive, and PLC operations. Previously, FANUC systems in gear hobbing machines utilized dedicated backup modules such as PFB-C and capacitor modules with a capacity of 25000 μF, which significantly increased costs. This has driven the need for technical improvements aligned with the manufacturing industry’s emphasis on “intelligence, green initiatives, environmental protection, and energy efficiency.” In optimizing gear hobbing machines, we have leveraged the FANUC system’s spindle energy-based power failure retraction function combined with the PFB-24 module to achieve safe radial axis retraction during unexpected power outages, protecting both the tool and workpiece and preventing accidents during gear hobbing processes.
The power failure retraction system in CNC gear hobbing machines is designed to address the critical issue of maintaining synchronization between the tool axis and workpiece axis during sudden power loss. In gear hobbing operations, the FANUC system’s Electronic Gear Box (EGB) function ensures that the tool axis and workpiece axis are engaged. Without proper safeguards, a power interruption can cause rapid stoppage, leading to damage to the gear hobbing machine components. The retraction principle involves using the PFB-24 module to supply DC24V, while the energy required for axis retraction is derived from the spindle’s constant-speed deceleration, which releases kinetic energy. This allows the axes to disengage synchronously even during a power failure. In our improved design for the gear hobbing machine, we have eliminated the need for the PFB-C and capacitor modules, as shown in the conceptual diagram, reducing both complexity and cost while enhancing reliability in gear hobbing applications.
In the initial hardware configuration of the gear hobbing machine’s CNC system, we employed the EGB function with an AIPS series power module and AI series drive modules. Based on the machine design and control parameters, we calculated the required retraction energy. For dry gear hobbing machines, the hob spindle speed typically exceeds 400 r/min, derived from the linear speed of domestic dry hobbing cutters, which satisfies the retraction distance requirements. Through field testing and technical discussions, we confirmed that the spindle energy-based retraction function with the PFB-24 module could maintain synchronization between the tool and workpiece axes during power failure, while enabling the radial axis to retract to a safe distance. This approach not only meets the safety needs of gear hobbing but also aligns with energy-saving goals.
To quantify the energy dynamics, we performed detailed calculations for spindle kinetic energy and retraction energy consumption. The spindle assembly in a gear hobbing machine includes components such as the spindle motor, gearbox, tool holder, and flywheel. The rotational energy of the spindle is given by the formula $$E = \frac{1}{2} I \omega^2$$, where \(I\) is the moment of inertia and \(\omega\) is the angular velocity. The moments of inertia for various components were computed as follows:
| Component | Mass (kg) | Radius (m) | Moment of Inertia (kg·m²) |
|---|---|---|---|
| Spindle Motor | – | – | 0.090 |
| Gearbox | 50 | 0.10 | 0.2500 |
| Flywheel | 40.86 | 0.175 | 0.6257 |
| Tool Holder | 6.4 | 0.032 | 0.0033 |
| Hob | 5.5 | 0.08 | 0.0176 |
The total moment of inertia for the spindle (excluding the motor) is calculated as \(I = I_{\text{gearbox}} + I_{\text{flywheel}} + I_{\text{tool holder}} + I_{\text{hob}} = 0.8966 \, \text{kg·m}^2\). Assuming a minimum spindle speed of 300 r/min in the gear hobbing machine, the angular velocity \(\omega = 2\pi \times \frac{300}{60} = 31.416 \, \text{rad/s}\). The kinetic energy is then:
$$E = \frac{1}{2} \times 0.090 \times (2\pi \times \frac{1200}{60})^2 + \frac{1}{2} \times 0.8966 \times (31.416)^2 = 709.8912 + 442.0059 = 1151.8971 \, \text{J}$$
This energy is available for retraction during power failure. The energy consumption during retraction consists of several components. First, energy consumed before retraction initiation, assuming the tool axis motor (AII 15/8000-B, 15 kW) and workpiece axis motor (AIF 40/3000, 6 kW) operate at rated power for 0.2 seconds: $$E_1 = (15 + 6) \times 0.2 = 4.2 \, \text{J}$$. Second, energy required for retraction axis movement, including acceleration and friction. For the retraction axis motor (AIF 22/3000), with motor inertia \(J_m = 0.012 \, \text{kg·m}^2\) and load inertia \(J_l = 0.0145 \, \text{kg·m}^2\), and a retraction speed of F5000 (motor speed 2500 r/min), the energy is:
$$E_2 = 5.48 \times 10^{-3} \times (J_m + J_l) \times V_m^2 + 6.28 \times T_l \times d_l = 5.48 \times 10^{-3} \times (0.012 + 0.0145) \times 2500^2 + 6.28 \times 5.4 \times 30 = 907.625 + 101.736 = 1009.361 \, \text{J}$$
Third, energy to maintain the system’s 24V supply, assuming a current of 8A and retraction time of 0.5 seconds: $$E_3 = I_S \times 24 \times 1.2 \times T = 8 \times 24 \times 1.2 \times 0.5 = 115.2 \, \text{J}$$. The total energy consumption for a retraction distance of 30 mm is \(E_{\text{total}} = E_1 + E_2 + E_3 = 1128.761 \, \text{J}\). Since the spindle provides approximately 1152 J during deceleration, the energy balance supports reliable retraction in the gear hobbing machine. For dry gear hobbing with a maximum module of 6 mm, the radial feed is 13.5 mm, and with a safety factor of 1.5, the required retraction distance is at least 20.25 mm. Our tests set a safe distance of 30 mm, ensuring that the tool and workpiece disengage synchronously.
In experimental validation, we configured the FANUC system parameters for power failure retraction. Key parameters included setting PFBFEN (parameter 4353#4) to 1 to enable the function, PFBTYP (parameter 4542#6) to 1 for controlled deceleration, retraction speed (parameter 7740) to 5000, and retraction distance (parameter 7741) to 30. Using SERVO GUIDE software, we monitored variables such as DC link voltage (VDC), X-axis position (POSF), synchronization error (ERR), spindle speed (SPEED), and power failure retraction signal (PMC X4.7). Tests involved simulating power failure by disconnecting the main switch, during which the radial axis retracted at the set speed and distance while maintaining EGB synchronization.
The results from both no-load and load tests (under cutting conditions) are summarized in the table below, demonstrating the relationship between spindle speed and retraction distance in the gear hobbing machine:
| Spindle Speed (r/min) | Test Condition | Retraction Distance (mm) | Energy Balance (J) |
|---|---|---|---|
| 250 | No-load | 24 | 1017 (available) vs 1129 (consumed) |
| 300 | No-load | 34 | 1152 vs 1129 |
| 350 | No-load | 45 | Increased with speed |
| 400 | Load (cutting) | 50 | Sufficient |
| 500 | Load (cutting) | 50 | Sufficient |
At 250 r/min, the calculated spindle energy was about 1017 J, slightly below the consumption, but practical tests showed a retraction of 24 mm, indicating minor discrepancies between theory and experiment. From 300 r/min onward, retraction distances exceeded the safe requirement of 30 mm, with distances increasing proportionally to spindle speed. At speeds of 400 r/min and above, the retraction reached the parameter limit of 50 mm, fully ensuring safety in gear hobbing operations. This reliability was confirmed in multiple tests on production-grade dry gear hobbing machines, with all 28 units sold performing satisfactorily in field use.
In conclusion, the spindle energy-based power failure retraction function in FANUC systems provides a cost-effective and efficient solution for gear hobbing machines. By eliminating the need for additional capacitor modules and leveraging the PFB-24 module, we have reduced design and maintenance costs while enhancing safety. The energy calculations and experimental data confirm that at spindle speeds of 300 r/min and above, retraction distances meet or exceed safety requirements, with performance improving at higher speeds. This innovation not only prevents damage to tools and workpieces in gear hobbing applications but also supports the industry’s move toward green and energy-efficient manufacturing. The successful deployment in multiple gear hobbing machines underscores the practicality and benefits of this approach, contributing to lower costs and higher reliability in modern gear production.