Linear guideways and ball screws installed together on a CNC machine axis
Rails and screw on one CNC axis · two-rail parallelism by preload, aligning the screw to the rails, lubricating carriages and nut
A machine-tool feed axis is built from two profile rail guideways that carry the table and a ball screw, held in a fixed and a floating support bearing, that drives it. Buying them as a matched set for one axis means the rails, carriages, screw, nut and supports must fit each other on the machine bed. Three things govern the result: how parallel and level the two rails must be for the chosen preload, how the screw and its bearings are aligned to the rails, and how the carriages and the nut are lubricated.
Matched profile rail guideways and ball screws installed together on one machine-tool or automation axis: two-rail mounting tolerances, aligning the ball screw and its support bearings to the rails, and lubricating carriages and nut. Ball screw selection (critical speed, lead accuracy, life) is covered on the Ball screws for linear axes page; selecting a profile rail guide on the Precision linear guide rail page; guidance assemblies on the Linear motion system page; and complete screw-driven stages on the Linear positioning stage page.
CNFX editorial research · Sources checked October 12, 2026 · 4 sources
Heavier preload leaves less room for rail-to-rail parallelism error
NTN's Linearwizard guide names three mounting tolerances for a pair of rails: parallelism between the rails (e1), height difference between the two rails (e2) and height variation along the rail (e3), all depending on size and preload class. [3] NSK's NH series table shows how fast the parallelism allowance shrinks as preload rises. [1]
The table gives NSK's permissible parallelism e1 between two NH rails. For height between the rails (e2), NSK allows 375 µm per 500 mm for Z0/ZT and 330 µm per 500 mm for the preloaded classes. [1] Editorial calculation: for an NH25 guide, moving from fine clearance (40 µm) to medium preload (20 µm) halves the parallelism allowance; read proportionally, the e2 figure for preloaded classes at 300 mm rail spacing would be 330 × 300 ÷ 500 = 198 µm. Confirm with NSK whether e2 scales with spacing for your layout. [1]
Rockford Linear Motion makes the same point for its RPG guides: the permissible parallelism error depends on the preload selected, and its coplanarity values are given for 500 mm between rails and vary in proportion to that distance. One rail is the master; the auxiliary rail is checked with an indicator and moved to within the limits. [2]
NSK also gives the shoulder that locates each rail: for NH25, a maximum corner radius of 0.5 mm and a shoulder height of 5 mm; for NH45, 0.7 mm and 8 mm. [1] NTN's assembly rules call for fixing screws of property class 10.9 or 12.9 tightened to the specified torque, preservatives removed from mounting surfaces, and no temperature difference between parts during assembly. [3]
Preload class (NSK)
NH15
NH25
NH35
NH45
NH65
Z0, ZT (fine clearance)
22 µm
40 µm
55 µm
65 µm
110 µm
Z1, ZZ (slight preload)
18 µm
25 µm
35 µm
45 µm
70 µm
Z3, ZH (medium preload)
13 µm
20 µm
30 µm
40 µm
60 µm
For a matched axis set, give the maker the axis stroke and rail length, rail spacing, guide size and preload class, carriages per rail, ball screw diameter, lead and accuracy grade, support bearing arrangement, mounting-surface drawings with datum shoulders, motor coupling, and the lubrication system on the machine.
02
Aligning the ball screw and its support bearings to the rails
NTN lists three alignment directions for a screw drive: perpendicularity of the connecting structure to the screw axis, height offset ΔH between the bearings, and lateral offset ΔA between them. The allowance falls with higher preload and depends on screw length. [4]
NTN's acceptance test is practical: the screw must be aligned parallel to the existing guides, and it is correctly aligned if the drive torque stays within the permissible limits over the entire stroke after installation; deviations are corrected by realigning at the end positions, never by forcing the nut along. Nuts supplied without grease must be lubricated before the first movement. [4]
Editorial note: because NSK's rail allowances are tens of micrometres at preload, set the rails first against their datum shoulders, then bring the screw and its fixed and floating bearing housings into line with the carriages, checking torque along the stroke. [1][4] Ask the set supplier to state ΔH and ΔA limits for the exact screw diameter, length and nut preload they deliver.
03
Grease or oil for the carriages and nut, set by speed and stroke
Rockford Linear Motion specifies NLGI grade 2 lithium-complex grease (KP2K-20 to DIN 51825) for its RPG guide blocks and recommends grease unless speeds exceed 60 m/min, when oil should be used, with viscosities from ISO VG 32 to 680 to DIN 51519. [2]
Rockford asks for blocks to be lubricated through the lube port while moving; on vertical axes through the uppermost port; and for short strokes of no more than twice the block length, from both ends so the whole ball track is covered. Blocks with a reservoir should be lubricated 2–5 times at equal intervals over the calculated life, and blocks without a reservoir at shorter intervals. [2]
Editorial note: agree one lubricant that suits both the guide carriages and the ball nut if they share a central lubrication circuit. [2][4]
04
Axis set data for guideways and ball screw
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Axis: [machine type; stroke; orientation; speed; load]
Guides: [size; preload class; accuracy grade; carriages per rail; rail spacing]
Screw: [diameter; lead; accuracy grade; length; nut preload]
Supports: [fixed and floating bearing type; housing heights]
Mounting: [datum shoulders; surface drawings; bolt class and torque]
Lubrication: [grease or oil; central system; port positions]
Luoyang Songju offers linear guideways in P, H and SP grades alongside ground C3 and C5 ball screws and spindle bearings for CNC machine tools.
Luoyang Songju Science and Technology Co., Ltd.
Manufacturing site to be confirmed for the order
Lists linear guideways in P/H/SP precision grades, stated as interchangeable with HIWIN, THK and NSK, with ground C3/C5 ball screws, electric spindles and precision bearings for CNC machine tools.
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
How parallel must two linear guide rails be?
It depends on size and preload. NSK allows 40 µm for an NH25 guide at fine clearance and 20 µm at medium preload. [1]
How do you know the ball screw is aligned to the rails?
NTN states it is aligned when drive torque stays within the permissible limits over the entire stroke after installation. [4]
When should guide carriages be oiled rather than greased?
Rockford recommends grease unless speeds exceed 60 m/min, then oil of ISO VG 32 to 680. [2]
What bolts hold linear guide rails?
NTN calls for property class 10.9 or 12.9 screws tightened to the specified torque. [3]
Sources and what still needs a direct check
Sources checked 2026-10-12. NSK and Rockford Linear Motion publish mounting and lubrication data for their own guides; NTN's Linearwizard explains assembly of linear guides and screw drives.
