Replacing a ball screw: beyond diameter and lead
Compare the screw, nut, machined ends and support bearings as parts of the installed feed axis.
Imagine a machine repair request for a ball screw with a 25 mm shaft diameter and a 5 mm lead. A catalog candidate matches those figures, but arrives with unfinished shaft ends. The old screw has bearing seats and a locknut thread, and its nut preload is unknown. In this hypothetical case, a useful replacement specification connects the catalog size to the mounting interfaces and the performance required in operation.
Define what the replacement includes
Diameter and lead narrow the search. Lead is nominal travel per screw revolution; it cannot always be identified from the spacing of adjacent grooves. Check handedness, required stroke, effective thread length and overall length against the original specification. Changing lead changes feed at a given rotational speed and affects the drive requirements, so a control setting alone does not establish suitability.
State whether the request covers a nut alone, a screw-and-nut assembly, or a package including support bearings and the coupling. The ball screw converts rotation into feed motion. The linear guide rail and blocks guide the table; confirmation of that guidance interface does not establish compatibility of the driven feed assembly.
Connect the end drawing to the installed hardware
THK offers designated products with unfinished shaft ends for further machining. [1] Establish whether the quoted item is unfinished or supplied to a completed end-machining drawing. Relate bearing-seat diameters, shoulder locations, retaining threads and coupling interfaces to the support-unit and mating-part drawings. Set tolerances through an approved drawing rather than a single measurement of the old part.
For the nut, check flange hole pattern, mounting-face position, screw center height, orientation, lubrication access and clearance around the recirculation components. Where markings are unreadable, preserve installed photos and identified measurement locations. Separate worn or damaged areas: their measured dimensions describe the present condition, not necessarily the original nominal size or fit.
- 1Screw shaft and lead
- 2Nut assembly and preload
- 3Machined shaft ends and supports
Schematic only. Numbers identify the threaded shaft, nut and machined end/support interfaces. It does not specify proportions, a preload mechanism, support arrangement or interchangeability.
Separate travel accuracy from reversal behavior
THK treats lead accuracy and axial clearance separately. [2] An accuracy grade alone does not specify the lost motion of the installed axis when direction reverses. Extract the travel-error requirement and the reversal requirement separately from the machine specification. Accuracy figures measured over different lengths or under different methods may not be comparable.
Investigate support bearings, fasteners and the coupling as well as the screw and nut when diagnosing reversal movement. Nut clearance and total axis reversal error are different acceptance quantities. Record measurement position, load, temperature and control conditions for the existing machine, then agree component inspection separately from installed-axis evaluation.
Specify preload for the duty
Ordering “less play” without knowing the original preload can change operating torque and running behavior. THK warns that excessive preload affects heat and life. [2] Identify the original specification; a proposed change needs review against load, speed, operating time and required stiffness by the OEM or responsible machine engineer.
Provide the support arrangement, bearing designations and assembly condition, lubrication, chip or dust protection and ambient temperature. A vertical axis also needs the machine-side load-holding and brake requirements considered. Stroke, acceleration, load and cycle frequency give a supplier more useful information than a request to make the new screw work “as before.”
Compare the work behind the price
In this example, an unfinished assembly and an end-machined assembly with inspection records cover different work. Identify who assesses bearing and locknut reuse, assembles the parts, aligns the machine and performs the installed test. The comparison below illustrates hypothetical supply scopes, not actual offers or availability.
For a nut-only proposal, obtain the manufacturer’s answer on the existing screw identity and condition, permissible combination, and installation or adjustment procedure. Matching envelope and lead do not establish that an arbitrary new nut can work on a worn screw. Keep unresolved compatibility open and compare an assembly replacement or manufacturer assessment as alternatives.
| Candidate | Included scope | Still needed before ordering |
|---|---|---|
| Unfinished-end assembly | Included scopeScrew and nut; shaft ends unfinished. | Still needed before orderingEnd drawing, machining responsibility and final inspection. |
| End-machined assembly | Included scopeScrew and nut with ends to a specified drawing. | Still needed before orderingBearing/retaining-part scope and machine evaluation. |
| Nut-only proposal | Included scopeNew nut, with the existing screw retained. | Still needed before orderingManufacturer confirmation of combination, wear condition and adjustment requirements. |
Order against the finished drawing and test plan
Ask for the finished drawing, full screw-and-nut designations, accuracy and clearance or preload specification, inspection scope, supplied parts and excluded work. Mark deviations from the original and obtain acceptance from the OEM or responsible machine engineer before ordering. A supplier’s ability to manufacture a part and engineering approval to use it in the machine are separate decisions.
Plan receipt inspection against the order drawing and records, followed by an agreed installed evaluation of full-stroke travel, reversal behavior, temperature and unusual noise. Acceptance criteria depend on the machine requirements and should be settled beforehand. HAM can discuss the procurement conditions and help find suitable Japanese companies using the available drawings, original codes and repair concern.
Supplier inquiry to adapt
Open supplier questions
We are considering a replacement ball screw for [machine / axis / original code]. Attached are drawing [number / revision] and installed-condition photos. Diameter, lead and handedness are [specification / unknowns]; duty is [stroke, load, speed, cycle frequency and environment]. Please identify the proposed screw-and-nut combination, accuracy, axial clearance or preload, end machining, nut mounting and support-bearing interfaces, stating confirmed details and unknowns. For a nut-only option, please explain compatibility with the existing screw and the required condition assessment, assembly and adjustment. Please quote with a finished drawing, inspection scope, accessories, exclusions and lead-time assumptions. Identify deviations before manufacture; any change will be adopted only after machine-side approval.
Sources and scope
- THK — Ball Screw with Unfinished Shaft Ends
Official THK unfinished-end product description. Not a statement that all ball screws can be remachined or that a product fits a particular machine.
- THK — Conditions of the Ball Screw
Official THK selection guidance on accuracy, clearance and preload. Full JIS/ISO standards were not read; catalog values are not prescribed as universal acceptance limits.