Lenze BFK458-25 Electromagnetic Spring Brake – BFK458 Series

Lenze BFK458-25: 400 N·m Fail-Safe Spring-Applied Electromagnetic Brake for High-Inertia Servo Axes

The BFK458-25 occupies the upper end of the Lenze BFK458 spring-applied brake family, delivering a rated static holding torque of 400 N·m at a coil supply of 180 VDC. Within a control loop, this unit functions as the terminal safety element on the mechanical power path: when the drive inverter removes torque command or a safety relay opens the brake circuit, the pre-compressed spring pack closes the friction interface in under 50 ms, arresting shaft rotation independently of any electrical energy source. This architecture satisfies IEC 60204-1 Stop Category 0 and Category 1 requirements and supports functional safety assessments up to SIL 2 / Performance Level d when paired with a certified safety relay.

The module mounts directly to IEC-standard motor flanges and is dimensionally compatible with Lenze MCS and MCA servo motor families, as well as third-party motors sharing equivalent frame geometry. The 180 VDC coil voltage aligns with the brake rectifier outputs integrated into Lenze i700, 8400, and 9400 servo inverter platforms, eliminating the need for an external brake power supply in most OEM configurations.

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Technical Parameters

Hardware Logical Analysis

The BFK458-25 is built around a dual-spring-stack compression architecture. Two concentric coil springs act on a pressure plate that clamps the sintered friction disc against a fixed armature plate. The spring pre-load is factory-calibrated to produce exactly 400 N·m of static friction torque at the disc interface, with a tolerance of ±5% across the rated temperature range. This deterministic spring force eliminates the torque variability seen in single-spring designs under thermal cycling.

The electromagnetic coil is wound on a bobbin seated in a machined steel housing. When 180 VDC is applied, the resulting magnetomotive force (MMF) generates a flux path through the housing, armature plate, and air gap. The flux density at the air gap is sufficient to overcome the combined spring pre-load force, drawing the pressure plate back and releasing the friction disc with a measured air gap of 0.3–0.5 mm. This air gap is the primary wear indicator: as the friction lining erodes, the gap increases, reducing the available electromagnetic force margin. Lenze specifies a maximum permissible air gap of 0.8 mm before lining replacement is required.

EMC Design: The coil circuit incorporates a freewheeling diode across the coil terminals, suppressing the inductive voltage spike (L·di/dt) generated during de-energization. Without this suppression, the spike can reach 5–10× the supply voltage, damaging the brake output transistor in the drive inverter. The diode clamps the spike to approximately 0.7 V above supply, extending both coil insulation life and drive output stage reliability. For installations requiring faster engagement times, a varistor-diode combination (Lenze EBR rectifier option) can be substituted, trading spike amplitude for a 30–40% reduction in engagement time.

Thermal Management: Class F insulation (155 °C rated) provides a 40 °C thermal margin above Class B (110 °C), which is significant in high-cycle applications where coil self-heating accumulates. The steel housing acts as a passive heat sink, conducting coil heat to the motor flange and motor body. In continuous-duty braking applications (e.g., vertical axis holding under load), the steady-state coil temperature rise at 85 W dissipation is approximately 60–70 K above ambient in free-air mounting, well within the Class F limit.

Redundancy Arbitration: In SIL 2 / PLd safety architectures, the BFK458-25 is typically paired with a dual-channel safety relay (e.g., Pilz PNOZ or Siemens 3SK1) that monitors both the brake coil current and the optional microswitch feedback. If the coil current drops below the release threshold without a corresponding microswitch signal change, the safety relay flags a discrepancy and triggers a controlled shutdown. This cross-monitoring eliminates single-point failure modes in the brake release path.

System Integration Benefits

• Zero-power shaft locking: The spring-applied design holds 400 N·m with no electrical energy, eliminating the risk of axis drop during power outages or E-stop events on vertical and inclined axes.
• Direct inverter compatibility: The 180 VDC coil voltage matches the brake output of Lenze i700, 8400, and 9400 series inverters without an intermediate power supply, reducing panel wiring complexity and component count.
• Deterministic engagement timing: Sub-50 ms engagement time is consistent across the rated temperature range, enabling accurate safety function response time calculations in SISTEMA or SISTEMA Lite assessments.
• Diagnostic transparency: The optional microswitch output provides a discrete signal to the PLC or safety relay confirming brake-released state, enabling real-time brake health monitoring without additional sensors.
• Maintenance-free friction interface: Sintered metal lining rated for >1,000,000 switching cycles eliminates scheduled lining replacements in standard duty applications, reducing planned downtime.
• Manual release for commissioning: The integrated mechanical release lever allows axis jogging during drive commissioning and maintenance without energizing the brake circuit, improving technician safety.
• Scalable torque family: The BFK458 series spans multiple torque ratings sharing identical mounting interfaces, allowing engineers to upsize or downsize the brake without redesigning the motor flange or cable routing.
• Broad environmental tolerance: IP54 protection and −20 °C to +40 °C operating range cover the majority of industrial indoor and sheltered outdoor installations without additional enclosure measures.
• Functional safety compliance: CE marking under Machinery Directive 2006/42/EC and UL recognition support global machine builder certification workflows, reducing time-to-market for export machinery.
• Low coil resistance drift: Precision-wound coil with Class F insulation maintains resistance within ±5% of nominal across the operating temperature range, ensuring consistent release force and predictable current draw from the drive brake output.

Quality Assurance & Global Logistics

Every BFK458-25 unit supplied by siemensplc.com is sourced through verified Lenze-authorized distribution channels or authenticated surplus stock with full traceability documentation. Pre-shipment inspection covers: original Lenze labeling and date code verification, coil resistance measurement within ±5% of the published datasheet value, mechanical spring engagement function test, and air gap measurement. Batch photographs and inspection reports are available upon request prior to order confirmation.

Shipments originate from our warehouse in Xiamen, China. Standard export documentation includes commercial invoice, packing list, and Certificate of Conformity. HS code 8505.20 applies for customs classification. Express delivery via DHL or FedEx reaches most destinations in 3–5 business days. Sea freight consolidation is available for bulk orders. All units are shipped in original or equivalent anti-static, foam-lined packaging to prevent transit damage to the friction disc and coil assembly.

A 12-month warranty covers manufacturing defects from the date of shipment. Technical support is provided by our engineering team with direct access to Lenze documentation and dimensional drawings. Replacement or credit is issued for confirmed defective units without requiring return of the original item in most cases.

Contact Information

Email: [email protected]
WhatsApp: +86 18359268345
Web: siemensplc.com
Location: Xiamen, China
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