Ball screw straightness across the full travel must be specified independently of lead accuracy because a screw can meet a 300 mm lead criterion while still showing unacceptable full-length alignment error.
The controlling limits are the mounting-surface flatness, screw-shaft cylindricity, accumulated lead error and one-revolution fluctuation, with positioning and transport screws assigned different acceptance criteria under JIS B 1192 and ISO 3408 [S1][S2].
Straightness is not the same as lead accuracy
Straightness describes the geometric departure of the screw axis from a straight reference over the full travel, while lead accuracy describes how rotary input converts into linear displacement. The THK accuracy framework separately defines effective thread length, reference travel distance, actual travel distance, representative travel-distance error and fluctuation [S1].
Lead error can remain controlled over any 300 mm section without proving that the complete screw is straight. For positioning screws, the maximum deviation range over 300 mm, designated ν300, must not accumulate over the complete screw length; transport screws may permit accumulation [S2].
For a direct overview of the component and its associated accuracy concepts, see the ball screw reference. A useful specification therefore states the full straightness requirement separately from ν300 rather than treating both as one tolerance.
Separate four measurable error mechanisms
Full-travel straightness should not be inferred from mounting flatness, lead accuracy or cylindricity because each measures a different physical condition. The table below compares the four principal limits by measured quantity, coverage, consequence and suitable verification method. [S2]
The comparison matters because lead-angle accuracy and mounting-surface flatness are distinct conditions that influence ball-screw performance [S4]. The first is an installation-interface requirement; the second is a component geometry requirement.
Use lead grades as reference limits, not straightness values
JIS B 1192 and ISO 3408 define accuracy grades C0 through C10. Grades C0 to C5 are characterized by linearity and direction, while C7 through C10 are characterized by error relative to a 300 mm travel distance [S1]. The THK table gives per-300 mm fluctuation limits of 3.5 μm for C0, 5 μm for C1, 7 μm for C2, 8 μm for C3 and 18 μm for C5; corresponding one-revolution fluctuation limits are 3, 4, 5, 6 and 8 μm [S1].
These figures constrain lead and travel variation, not axis straightness. The THK table also separates representative travel-distance error from fluctuation, with fluctuation defined by two parallel lines enclosing the actual travel-distance curve [S1]. A screw meeting C5 can therefore still require an independent full-travel straightness limit.
For short strokes, per-revolution fluctuation deserves separate attention because the 300 mm criterion may not reveal pitch-scale irregularity.
Choose the tolerance basis by application
Positioning screws suit machine tools, semiconductor stages and optical assemblies, where a repeatable relationship between motor rotation and carriage position is essential. Transport screws suit material handling, where cost and load transfer matter more than tightly bounded displacement [S2][S4].
Lower grade numbers denote tighter accuracy: P1 is more accurate than P5, while P grades identify positioning screws and T grades identify transport screws under the DIN and ISO convention [S2]. The JIS convention uses C for positioning and Ct for transport, with positioning grades running from 0 to 5 and transport grades commonly assigned 7, 9 or 10 [S2].
This division is not a fit for a simple straightness-tolerance chart. A CNC feed axis may need a C3 or P3 screw plus a tightly controlled mounting interface, whereas a handling axis may accept a C10 or T-class screw with looser lead requirements. In both cases, the straightness tolerance should reflect support stiffness, bearing arrangement, coupling alignment, load direction and the required guide synchronization.
Screw diameter and lead also shape the deflection budget described in the ball screw diameter-to-lead ratio for rigidity, so straightness acceptance must be coordinated with the selected screw size and support arrangement.
Specify mounting and measurement controls
Mounting-surface flatness and axial clearance are important factors in ball-screw performance [S4]. End supports also need controlled perpendicularity to prevent high-speed misalignment [S4].
Preload does not geometrically straighten a bent shaft. It changes contact force and can increase heat, wear and sensitivity to misalignment when support geometry is poor. Excessive axial clearance also reduces reversal accuracy, so axial clearance, preload and mounting alignment must be reviewed together rather than treated as substitutes for straightness [S4].
Verify the complete assembly rather than a cut section
Dynamic measurement can record positional behavior continuously throughout full travel and expose one-revolution irregularities that a short inspection interval may miss [S3]. Cutting a qualifying section from a longer screw does not prove that the delivered segment has suitable full-length behavior, because the segment may not represent the complete lead profile of the parent screw [S3].
Historically, machined screws offered lead variation of about 0.012 mm within 300 mm and cylindricity below 0.008 mm over the total screw length [S5]. These values illustrate the separate geometric and kinematic controls, but they are not a universal straightness specification. For procurement, request the complete tolerance schedule, including accuracy grade, effective thread length, full straightness, cylindricity, mounting flatness, preload and measurement method.
For machine-tool feed axes, combine this geometric schedule with the application-specific guidance in ball screw selection for CNC machine tool feed axes, then validate the assembled axis over its complete intended travel.
Set an acceptance record that cannot be misread
A defensible ball screw straightness specification identifies the datum, full usable travel, measurement equipment, mounting condition and acceptance limit, then keeps ν300, ν2π, cylindricity and mounting flatness as separate entries [S1][S2].
Two trackable checks confirm implementation: a dynamic travel test over the complete stroke and an installed-axis survey from both bearing supports to the nut. Any correction to the reference travel distance, including an intentional value for thermal expansion or contraction, must be declared rather than inferred from nominal lead [S1].