Mitsubishi HC-SF502B (HCSF502B) — 5kW AC Servo Motor, Straight Shaft + Brake, MELSERVO J2 Series

Product Identification

Part Number: HC-SF502B

Also Searched As: HCSF502B, HC-SF-502B

Series: Mitsubishi MELSERVO HC-SF (J2 Generation)

Motor Type: AC Brushless Servo Motor — Straight Shaft with Electromagnetic Brake, 2000 rpm

Condition: New In Box, Factory Sealed

Overview

The Mitsubishi HC-SF502B is a 5kW medium-inertia AC brushless servo motor from the original MELSERVO J2 platform, fitted with a spring-applied electromagnetic brake on a straight shaft. Rated at 23.9 Nm continuously and 71.6 Nm peak, it is the configuration for heavy-duty axes where the straight-shaft friction-clamp coupling interface is the correct mechanical design choice and where fail-safe mechanical hold on servo-off is not optional — vertical axes, gravity-loaded slides, Z-columns carrying heavy spindle assemblies, and any 5kW drive where losing servo control means the load is free to move in an uncontrolled direction.

The brake is the defining feature here. At 5kW and the load masses that go with axes of this capacity, the consequences of an unbraked axis dropping servo lock are proportional to the size of the machine. The spring-applied brake changes that equation: the shaft is clamped mechanically the instant 24V DC is removed, whether that removal is planned during a normal shutdown sequence or unplanned during a fault, e-stop, or power interruption. The axis holds. The load stays where the last position command placed it.

As a J2-generation motor, the HC-SF502B carries the 14-bit serial absolute encoder at 16,384 ppr and is compatible with both original MR-J2-500 amplifiers and the later MR-J2S-500 platform. For the substantial installed base of production machinery running first-generation MR-J2 hardware, this dual compatibility makes the HC-SF502B the correct and only valid sourcing choice for a like-for-like motor replacement — the HC-SFS502B (17-bit J2S generation) will not operate on MR-J2 amplifiers.

Technical Specifications

Why 5kW, and Why This Axis Needs a Brake

Not every 5kW axis is the same kind of problem. The HC-SF502B addresses a specific combination of demands: a large axis with real torque requirements, a vertical or gravity-affected load profile, and a machine design where axis safety depends on reliable mechanical hold through every possible stop condition.

The torque numbers set the capacity context. At 23.9 Nm continuous, the HC-SF502B can sustain that output indefinitely under rated thermal conditions — shift after shift, cycle after cycle, without approaching overload as long as the effective torque demand stays within that figure. The 71.6 Nm peak is the amplifier's resource during acceleration phases: rapid traverse to the next cut position, deceleration from maximum axis speed into a clamped position, any motion transient where the instantaneous torque demand spikes well above the sustained cutting load. That three-to-one peak-to-continuous ratio gives the axis authority to accelerate and decelerate a high-inertia load quickly without requiring the motor to sustain peak current beyond the brief acceleration interval.

The 7.5 kVA power facility capacity governs the electrical supply infrastructure — cable sizing, fusing, and regenerative energy management all key from this figure. At 25A rated current and the load inertias typical of heavy machine axes, regenerative deceleration energy returned to the DC bus during high-speed stops is a real system design consideration that the panel design must account for.

The brake exists because of what 5kW axes typically carry. A large VMC spindle head can weigh several hundred kilograms. A pallet loaded with a substantial workpiece and vise may approach the same order of magnitude. When an axis of this capacity loses servo current — by design during a shutdown sequence, or abruptly during an alarm trip — the mechanical situation is different from a 200W axis losing servo. The energy stored in the load, and the distance it can travel before friction arrests it, are not negligible. The spring-applied brake on the HC-SF502B holds that load stationary the moment 24V DC is removed, without any dependence on control logic, software state, or amplifier condition.

The Spring-Applied Brake: Engineering Principles at This Scale

The brake mechanism is straightforward in concept and critical in consequence. A mechanical spring applies continuous clamping force to the brake disc against the brake plate — the shaft is held. Energising the brake coil with 24V DC generates a magnetic field that compresses the spring and releases the disc, allowing free shaft rotation. Remove the 24V DC and the spring immediately re-engages. No signal required. No software command. No delay for a control sequence to execute. The shaft is clamped in the time it takes the spring to move.

That mechanical simplicity is exactly what makes the spring-applied design fail-safe. The failure mode of the brake is always toward the safe state — if the coil circuit fails, if the relay drops out, if the 24V supply is interrupted for any reason, the spring engages and the axis holds. A power-applied brake (one that requires current to hold) has the opposite failure characteristic: any coil circuit failure releases the axis. On a 5kW gravity-loaded drive, the difference between these two failure modes is not academic.

Correct brake system integration in the machine requires three elements:

The MBR signal from the MR-J2S or MR-J2 amplifier must control the brake relay. The MBR output is the amplifier's brake interlock signal — it delays brake engagement until after the amplifier has completed its deceleration sequence and confirmed the motor has come to rest. Bypassing the MBR interlock and wiring the brake relay directly from an e-stop contact causes the spring to engage against a rotating 5kW shaft. The resulting mechanical shock is severe enough to cause immediate brake damage and generates impact loads in the drive train that can affect the ballscrew, coupling, and motor bearings as well as the brake itself.

A surge absorber wired directly across the brake coil terminals is not optional. The brake coil is a significant inductive load. Switching off 24V DC through a relay with no arc suppression generates a voltage transient that can damage relay contacts, the relay driver output in the amplifier, and other components sharing the same 24V supply bus. The absorber must be positioned at the coil — at the motor connection point, not at the relay — to be fully effective.

For vertical axis applications, Mitsubishi's published specification guidance places the recommended maximum static unbalanced torque at 70% or less of the motor's rated torque — approximately 16.7 Nm at the shaft for this motor. Axis designs with higher gravitational load imbalance should use supplemental mechanical counterbalancing alongside the servo and brake system. The brake is designed to hold a load within its rated capacity, not to substitute for a missing counterbalance on an overloaded vertical drive.

Straight Shaft: Coupling Selection and Installation at 5kW

The straight shaft on the HC-SF502B accepts friction-clamp couplings — disc couplings, bellows couplings, and split-clamp jaw coupling hubs where the clamping force between hub bore and shaft OD transmits torque. This is the standard and well-proven interface for high-performance CNC ballscrew axis drives, and it works correctly when the coupling is properly specified and installed.

Proper specification means the coupling is rated for the peak torque, not the continuous torque. The 71.6 Nm peak is the figure that governs coupling selection. A coupling selected at the 23.9 Nm continuous rating but marginal at 71.6 Nm will eventually slip under the rapid-traverse and deceleration transients that are normal operating conditions on a CNC axis. Slip on a 5kW axis with a heavy load is a production-stopping event, not a recoverable nuisance.

The coupling manufacturer's service factor for reversing servo duty should be applied to the peak torque when making the final selection. Conservative coupling specification at this capacity level costs little and prevents the diagnostically challenging scenario of intermittent position errors that only appear during specific motion profiles — the fingerprint of a marginally slipping coupling hub.

Hub fitting follows the same guidance that applies across the HC-SF family: use the shaft-end threaded hole and a drawbolt to seat the hub axially onto the shaft. Hammering or press-driving the hub on at this motor's frame size transmits impact energy through the shaft to the encoder disc and bearing assembly at the rear. The encoder damage this produces is rarely immediate — it tends to manifest months later as intermittent position alarms under vibration, which are genuinely difficult to trace back to a hub installation event that occurred before the machine was commissioned.

For applications where the driven mechanism requires a key-and-hub positive torque connection rather than a friction interface — timing belt pulleys, gear hubs, sprocket drives — the correct motor is the HC-SF502BK (keyed shaft with brake). The HC-SF502B is the specification when the coupling design is friction-clamp and the mechanical interface does not require a keyway.

J2 Generation Encoder and the Dual-Compatibility Advantage

The HC-SF502B uses the J2 platform's 14-bit serial absolute encoder at 16,384 positions per revolution. Serial absolute means the encoder transmits a digital position word to the amplifier at every sample interval and maintains a multi-turn absolute counter through power-off via battery backup. On any power-on following any type of interruption — planned, unplanned, brief, or extended — the amplifier reads the current absolute position and the axis comes up in its correct location without a reference return cycle.

The battery backup for the absolute counter uses the A6BAT lithium cell installed in the servo amplifier. It is serviced at the amplifier during planned maintenance. Replace it when the amplifier displays its low-battery warning alarm, before the cell is fully depleted. A fully depleted A6BAT causes the multi-turn counter to reset, and the machine cannot resume production without a reference return cycle on that axis.

The practical value of the J2-generation encoder at this point in the HC-SF product timeline is the amplifier compatibility it enables. The 14-bit serial protocol is readable by both amplifier generations without adaptation:

• MR-J2-500A / MR-J2-500B — Original J2 amplifiers. Full compatibility, no restrictions.
• MR-J2S-500A / MR-J2S-500B / MR-J2S-500CP — J2-Super amplifiers. Backward-compatible with the J2 encoder.

The HC-SFS502B with its 17-bit J2S encoder runs on MR-J2S-500 hardware only. Connecting a 17-bit J2S motor to a first-generation MR-J2 amplifier produces an encoder protocol fault; the axis will not operate. The HC-SF502B has no such restriction. For every machine in service with original MR-J2-500 amplifiers, the HC-SF502B is the exact sourcing target — a motor replacement that brings the drive back to original specification without any amplifier change, parameter re-engineering, or additional commissioning work.

HC-SF502B vs HC-SFS502B: One Decision Point

Both motors are 5kW, 23.9 Nm, straight shaft with electromagnetic brake, on a 176 × 176 mm flange. The mechanical output and physical mounting are identical. The encoder generation and amplifier requirement are not.

The sourcing decision reduces to one verification: check the amplifier nameplate. MR-J2-500 (without S) means the HC-SF502B is the only correct motor for that drive. MR-J2S-500 means both motors are compatible — the HC-SFS502B offers higher encoder resolution, but the HC-SF502B is a fully valid alternative for machines where 14-bit feedback has always been sufficient.

HC-SF 2000 rpm Range — Where the 502B Sits

The HC-SF502B shares the 176 × 176 mm flange with all HC-SF motors from 2kW to 7kW. Within the 502 capacity group, four variants cover the full shaft-and-brake matrix: no suffix (straight, no brake), B (straight with brake), K (keyed, no brake), BK (keyed with brake). All four share the same flange dimensions, encoder specification, and amplifier compatibility. Shaft type and brake presence have no effect on electrical or amplifier selection.

Typical Applications

VMC Z-axis drives on large vertical machining centres. The gravity-loaded Z-column is the definitive application for any braked 5kW servo motor — heavy spindle head, direct vertical travel, mandatory mechanical hold on servo-off. The straight-shaft HC-SF502B suits the disc or bellows coupling interfaces used on high-stiffness VMC ballscrew Z-axis drives, and the brake holds the column at the parked position through every tool change, program pause, and shift-end shutdown.

HMC spindle quill and boring bar drives. Horizontal machining centre quill traverse axes that move heavy spindle and boring tool assemblies along the spindle axis under sustained boring loads need the 5kW capacity to maintain constant cutting velocity. The quill is not a free-falling axis in all configurations, but on designs where the quill carries significant unsupported overhang, the brake provides the mechanical assurance that servo lock alone cannot.

Large CNC lathe cross-slide X-axis. Heavy-duty turning centre X-axis cross-slides carrying large turret and toolpost assemblies need sustained torque at CNC feedrates and reliable mechanical hold at the machining position between cuts. The 23.9 Nm continuous provides the feed authority, and the brake holds the cross-slide position during e-stops and machine resets on inclined bed lathes where the cross-slide is not fully self-locking against gravity.

Transfer machine lift and lower stations. Industrial transfer line lift stations that raise and lower workpiece fixtures between conveyor levels use servo drives with fail-safe brakes as a standard design requirement. The HC-SF502B's combination of 5kW output capacity, spring-applied brake, and straight-shaft coupling interface suits the geared or belt-driven actuator designs used on medium-format transfer lift mechanisms.

Servo press feed and straightener axes. Servo-driven press feed units for coil stock straightening and feeding operations use high-torque servo drives on the feed roll and straightening roll axes. These axes run under continuous high torque demand, reverse direction repeatedly with every press stroke, and must hold the strip in position between strokes. The brake holds the strip while the press operates; the J2-generation encoder provides the absolute position reference needed for consistent feed length across production runs.

New In Box, Factory Sealed

Factory sealed means original Mitsubishi packaging with everything in place — outer carton intact, inner foam support undisturbed, shaft-end cap installed, all connector ports covered, oil seal in as-manufactured condition. The motor and integrated brake assembly have never been powered, never installed, and carry no thermal or mechanical history. The brake friction surfaces are factory-fresh; there has been no disc cycling from prior service.

For a production machine stopped while waiting on this motor, in-stock new-in-box eliminates repair turnaround time and delivers a unit in fully known condition. No questions about previous installation quality, no uncertainty about prior fault events or overload history. For planned spare parts inventory on fleets of J2-era machines where this capacity appears on critical axes, factory-sealed stock provides consistently commissionable units that go directly from storage to installation.

Stored under stable temperature and low-humidity conditions away from vibration, factory-sealed HC-SF502B stock maintains full specification over several years. Beyond five years, a slow pre-commissioning shaft rotation as part of the installation check redistributes bearing grease before the motor first powers up.

Frequently Asked Questions

Q1: Which amplifiers are compatible with the HC-SF502B?

The HC-SF502B is compatible with both J2-generation and J2-Super amplifiers at the 500 class. Confirmed compatible models are MR-J2-500A and MR-J2-500B (original J2 generation), and MR-J2S-500A, MR-J2S-500B, and MR-J2S-500CP (J2-Super generation). The 14-bit J2 encoder is readable by both platforms without modification. The HC-SF502B is not compatible with MR-J3 or MR-J4 amplifiers.

Q2: What is the difference between the HC-SF502B and the HC-SFS502B?

Both motors are 5kW, 23.9 Nm, straight shaft with electromagnetic brake, on a 176 × 176 mm flange — physically interchangeable at the mount. The difference is the encoder generation: the HC-SF502B uses a 14-bit encoder (16,384 ppr) and works with both MR-J2 and MR-J2S amplifiers. The HC-SFS502B uses a 17-bit encoder (131,072 ppr) and requires MR-J2S amplifiers only. If the machine runs original MR-J2-500 hardware, source the HC-SF502B. If it runs MR-J2S-500, either motor is compatible.

Q3: How should the electromagnetic brake be sequenced to avoid premature wear?

The brake must only engage after the motor has fully decelerated to rest. Always control the brake relay through the MR-J2 or MR-J2S amplifier's MBR (electromagnetic brake interlock) output, which times engagement after confirmed motor stop. Wiring the brake relay directly from an e-stop contact without the MBR interlock will cause the spring to engage against a rotating 5kW shaft, causing immediate brake damage. Additionally, always fit a surge absorber directly across the brake coil terminals to suppress the inductive voltage spike at switch-off, protecting the relay and amplifier output circuitry.

Q4: Where is the absolute encoder battery located, and when should it be replaced?

The Mitsubishi A6BAT lithium battery backing the absolute encoder is inside the servo amplifier, not the motor. It maintains the multi-turn position counter through any power interruption, eliminating homing cycles on restart. Replace it when the amplifier displays its low-battery warning — before full depletion. A fully discharged A6BAT causes the absolute position counter to reset, requiring a reference return cycle before production can resume. The motor itself requires no battery maintenance.

Q5: Can the HC-SF502B replace an HC-SF502BK if the keyed-shaft variant is unavailable?

The HC-SF502B and HC-SF502BK are identical except for the shaft. The HC-SF502B has a plain straight shaft; the HC-SF502BK has a machined keyway. If the driven-side coupling hub has a keyway, the HC-SF502B cannot be directly substituted without modifying the hub — the key has nowhere to engage on a plain shaft. Substituting a straight-shaft motor for a keyed-shaft design requires either replacing the hub with one designed for a plain shaft friction clamp, or sourcing the correct keyed-shaft variant. Do not attempt to run a keyed hub on a plain shaft relying solely on friction; at 5kW, the torque demands make this unreliable.