New HC-KFS73K Mitsubishi HCKFS73K Servo Motor Free Expedited Shipping

Mitsubishi HC-KFS73K | MELSERVO MR-J2S 750W Low Inertia Servo Motor — 2.4 Nm / 3000 rpm / Keyed Shaft / 17-Bit Absolute

Keyed Shaft — The Detail That Determines Compatibility

The HC-KFS73K and HC-KFS73 share identical electrical and performance specifications — same 750W output, same 2.4 Nm rated torque, same 17-bit absolute encoder, same amplifiers, same 80mm flange. The difference is the shaft: the HC-KFS73K has a machined keyway and an included key.

A keyed shaft transmits torque through a positive mechanical interlock — the key bridges matching slots in the motor shaft and coupling hub, creating a joint that does not depend on clamping force. Under sustained high torque, frequent direction reversals, or vibration over long operating periods, a friction-clamped plain-shaft coupling can develop micro-slip that accumulates into a position error or mechanical failure. The keyed interface eliminates slip. The torque capacity is determined by the shear strength of the key, not by clamping force — appropriate for a motor with 7.2 Nm peak torque available at up to 5,175 rpm instantaneous maximum speed.

If the machine was designed for the HC-KFS73K keyed shaft, the coupling hub bore has a matching keyway slot. The HC-KFS73K installs directly. The straight-shaft HC-KFS73 does not fit without modifying the coupling hub.

Key Specifications

17-Bit Absolute Encoder — No Homing at Power-Up

At 131,072 positions per revolution, the encoder retains absolute shaft position through power-off cycles. At machine power-on, the servo controller knows each axis position immediately — no homing traversal required. For machines with multiple axes, complex fixtures, or vertical axes where a homing motion at startup carries risk, eliminating the homing cycle is both a practical and safety benefit.

Compatible Amplifiers

All four MR-J2S-70 variants support the HC-KFS73K — selection is determined by the machine's control bus architecture:

For a motor-only replacement on a machine with a functional amplifier, replace the motor alone. Drive parameters are stored in the amplifier and controller, not in the motor.

FAQ

Q1: Can the HC-KFS73K replace an HC-KFS73 (straight shaft) on the same machine?

Only if the coupling hub is also replaced or modified to include a keyway. A plain-bore coupling hub designed for the straight shaft cannot accept the key — the key would be proud of the shaft surface and prevent hub assembly. If the machine uses a keyed coupling hub, the HC-KFS73K is correct. If it uses a plain-bore clamped hub, the HC-KFS73 straight-shaft variant is the correct replacement.

Q2: How does the HC-KFS73K differ from the HC-MFS73K?

Both are 750W keyed-shaft motors with 17-bit absolute encoders. The difference is the inertia class. HC-KFS (Low Inertia) suits applications with moderate connected loads — conveyors, X-Y tables, gantry axes. HC-MFS (Ultra-Low Inertia) minimises rotor mass for the fastest possible acceleration on very light loads. They are not interchangeable — different rotor dimensions, different flange geometry, different rated current (HC-KFS73K: 5.8A; HC-MFS73K: 5.1A). Replace with the exact nameplate part number.

Q3: Does the fan cooling require external airflow?

No. The internal fan circulates air across the motor's thermal elements independently — no external air supply is needed. The installation must maintain the rated ambient temperature range (0–+40°C) and adequate clearance around the motor body for the fan's heat dissipation. Do not install in a sealed enclosure that traps the heat the internal fan is designed to remove.

Q4: No brake — what holds the axis when the servo is disabled?

Active servo lock holds position while the servo is enabled. When the servo is disabled or power is removed, the shaft is mechanically free. For horizontal axes with balanced loads this presents no issue. For vertical axes or gravity-loaded mechanisms, a counterbalance, external brake, or the HC-KFS73BK variant (keyed shaft with integral electromagnetic brake) is required.

Q5: Can the MR-J2S-70A be upgraded to an MR-J4 amplifier while retaining the HC-KFS73K?

Not as a direct swap — the MR-J4 uses a different encoder communication protocol than the HC-KFS73K's encoder. Mitsubishi's documented migration path uses MR-J4-DU_B-RJ020 amplifiers combined with MR-J2S renewal cabling kits. Confirm specific renewal tool compatibility with Mitsubishi Electric technical support before committing to the hardware purchase.