Choose the physical or data question first
A substitute part should be selected by the relationships it must preserve. A drive, reducer, tool and harness each connects to the robot in a different way, so a single compatibility tick hides important questions. Mechanical fit, electrical connection, supported control behaviour and the observed task are separate layers. The practical choice is to identify which relationship is already established, which one the candidate changes and which original evidence could confirm it. This allows a buyer to distinguish a straightforward exact-part replacement from an alternative that needs additional engineering review.
Drive replacement: the controlled combination matters
A drive is not defined only by its enclosure, power label or connector. The motor, feedback, controller, firmware, parameters and operating mode form the combination that must be supported. A device that can energise a motor is not automatically able to reproduce the installed motion task. Preserve suffixes and revisions, then ask which applicable instruction permits the replacement and parameter handling. A restore file also has an identity and software context; its existence does not establish that it belongs on the candidate device. The UR service documentation gives a device-specific example of restoration restrictions, not universal approval. [S2]
Reducer choice: ratio, lost motion and stiffness answer different questions
Ratio can support a speed calculation, but it does not state every allowed load or the resulting accuracy. Lost motion, hysteresis and torsional stiffness describe different aspects of behaviour under stated conditions. A unit described as having no particular backlash can still deform under load. Compare the measured end, torque range and definition before comparing values. The Harmonic Drive engineering catalogue explains terminology for its specific families; do not transfer a series value to an unknown reducer. Also keep shaft, flange, bearing loads and lubrication as separate requirements. A matching ratio alone cannot settle a replacement. [S1]
End effector: holding the object is a task, not a status bit
A flange that fits does not establish the contact, utility or failure behaviour of a replacement tool. Identify the attached mass and required centre-of-gravity information, the object surfaces, contact restrictions and release conditions. Tool-close acknowledgement, retained-object feedback and a conforming placed object represent different evidence. A successful pick with one carefully arranged sample does not establish performance across the intended range. If a finger, adapter or protective layer changes, it may change the physical contact even when the command is unchanged. The buyer should choose around the object and failure branch, not just an opening dimension.
Moving harness: static connection and motion are separate
A connector shape does not prove pin assignment, signal level, reference or shielding. Use applicable drawings to distinguish power, feedback and communications, and preserve the full route through the intended travel. A static continuity result cannot by itself establish performance during motion. Conversely, an alarm at a controller is not proof that the cable is faulty. The replacement review should therefore separate identity and pin map, permitted electrical connection, routing and strain relief, and the task observation after installation. Unknown live pins are not a suitable way to discover the map. Use the equipment procedure and the appropriate responsible personnel.
Worked example with explicitly hypothetical inputs
Assume an ideal 50:1 reducer and an input speed of 3,000 revolutions per minute. The calculated output speed is 60 revolutions per minute. If input torque is separately assumed to be 0.5 N m and efficiency 0.8, the simple output estimate is 20 N m. Neither result establishes the reducer rating, thermal limit or lifetime. If the candidate has the same ratio but different mounting or loading conditions, the arithmetic does not make it interchangeable. Keep the assumptions visible and return to the actual allowed conditions before any replacement decision. This is a calculation example, not data from an available product.
A replacement decision is not only a dimensional comparison
If a candidate drive fits but its feedback support is unresolved, the next useful purchase may be a compatibility review rather than the drive itself. If the supported combination is known but the fault remains unexplained, preserve the diagnostic boundary. These are different technical blockers and should not be hidden inside one component quotation.
A practical choice sequence
Begin with the original assembly identity and the failed or changed task. If only supply is needed and the exact relationship is documented, review provenance, revision and receipt checks. If the candidate changes an interface or supported combination, obtain that confirmation before treating price as the deciding factor. If the symptom has not identified the faulty assembly, commission a bounded diagnostic step rather than buying several plausible components. After a permitted change, observe the original relevant task and retain failures and the observation window. Mechanical fit cannot substitute for control support; control support cannot substitute for evidence that the actual object and task behaved as required. A useful enquiry names the uncertain relationship and asks for the specific drawing, instruction or observation needed to resolve it.
Primary-source reading
Harmonic Drive Engineering Data
https://www.harmonicdrive.net/_hd/content/documents1/FBB_DifferentialGear.pdf
UR-Series Service Manual
https://www.universal-robots.com/manuals/EN/PDF/Shared/service-manual_ur-series/Service_Manual_UR-Series_en.pdf