Define the replacement decision
A replacement enquiry should begin with the installed system, not a catalogue resemblance. Record the robot and controller identity, the failed assembly revision, the operating symptom and any changes already made. Then separate mechanical fit, electrical connection, control support and task verification. A supplier may confirm one of those relationships without confirming all the others. This guide explains how to keep those decisions visible. It does not identify available stock, approve a substitute part or claim that a repair has been completed.
Servo drives: match a supported combination
A drive belongs to a combination of motor, feedback device, controller, firmware and parameters. A matching enclosure or connector establishes little about that combination. Keep the original part number and revision exactly as printed, including suffixes, then ask which official document permits the proposed replacement. Distinguish the alarm that triggered the investigation from the component eventually judged faulty. An intermittent feedback or supply problem can be reported at a drive without proving that the drive itself has failed. Restoring a parameter file also requires an applicable device and software version, not merely a file with the right name. [S2]
1. Capture labels and the original alarm timeline before clearing it.
2. Build a motor, feedback, drive and controller revision map.
3. Ask the responsible service party to confirm parameters and restart checks.
Common error: Do not buy by appearance or treat one successful restart as root-cause proof.
Reducers: distinguish clearance from loaded deflection
A reducer comparison should identify the torque condition, the measured end and the definition of the published number. Backlash, hysteresis and torsional stiffness are related to motion behaviour but are not interchangeable labels. A stiffness value describes a load-deflection relationship under specified conditions; it is not a promise of zero displacement. The Harmonic Drive engineering catalogue illustrates terminology for its own product families. Its values must not be assigned to an unknown gearbox. Mounting dimensions, bearing loads, lubrication and the manufacturer service procedure remain separate questions even when a ratio matches. [S1]
1. Keep the drawing revision and joint position with each enquiry.
2. Record the load condition used for every stiffness or lost-motion claim.
3. Confirm the actual series, mounting and approved service instructions.
Common error: An exterior photograph cannot establish remaining life, internal condition or a comparable accuracy rating.
End effectors: document the task and failure behaviour
A tool replacement affects more than the flange. List the complete attached mass, the centre-of-gravity information required by the robot maker, the workpiece contact surfaces and the utility connections. Explain what the tool does if power, air or communication disappears. The tool-close command, the tool status bit and evidence that a workpiece is retained are three different things. A useful trial includes representative objects and failed attempts, not only a carefully positioned sample. When an adapter, finger or protection layer changes, preserve the version so later reviewers know which physical arrangement produced the result.
1. Provide object dimensions, material, contact restrictions and tool identity.
2. Map mechanical, power, air and signal connections separately.
3. Agree how retention, release and unsuccessful picks will be recorded.
Common error: A single retained object does not validate a production cycle, safety behaviour or damage-free handling.
Cables and connectors: follow the complete moving route
A harness is a connection map and a moving assembly. Connector keying does not establish the pin assignment, and wire colour is not an approved pinout. Identify power, feedback, communications, shielding and reference connections from applicable drawings. Include the route through the complete intended travel, the strain relief and the replacement access. A cable can pass a static continuity check yet fail when routed through motion; conversely, a controller symptom does not prove the cable is defective. Functional checks must follow the equipment procedure and use the appropriate personnel, without experimenting with unidentified live pins.
1. Photograph labels, routing and strain relief before removal.
2. Create a pin-by-pin drawing comparison with explicit unknowns.
3. Keep continuity and permitted motion observations as separate records.
Common error: Do not substitute a cable simply because both plugs mate.
A worked example with explicit assumptions
Assume, purely for a comparison exercise, that a motor runs at 3,000 revolutions per minute through an ideal 50:1 reducer. The nominal output speed would be 60 revolutions per minute. If input torque is assumed to be 0.5 N m and efficiency is separately assumed to be 0.8, the estimated output torque is 20 N m. That arithmetic does not establish the reducer rating: allowable loads, temperature, duty and life conditions still govern the actual selection. A revision sheet should preserve the assumed inputs, their units and their sources, then mark each result as calculated rather than measured. An unavailable efficiency must stay unknown rather than being borrowed from another series.
References and scope
[S1] Harmonic Drive Engineering Data — Harmonic Drive LLC
https://www.harmonicdrive.net/_hd/content/documents1/FBB_DifferentialGear.pdf
Supports: Gear terminology including hysteresis and torsional stiffness.
Scope: Specific catalogue families; not unknown gearbox ratings or remaining life.
Checked: 2026-11-04
[S2] UR-Series Service Manual — Universal Robots
https://www.universal-robots.com/manuals/EN/PDF/Shared/service-manual_ur-series/Service_Manual_UR-Series_en.pdf
Supports: Device-specific backup and software restoration restrictions.
Scope: Use the applicable robot and software instructions; not a universal replacement approval.
Checked: 2026-11-04
Original editorial guide. Worked numbers are explicitly stated assumptions, not product specifications or measurements. Source documents retain their own scope; no claim of supplier affiliation or independent expert certification. Corrections: zhilianhai@t-800.cn.