At the maximum axial load, a ball screw can lose useful stiffness through elastic deflection even when its static load rating is not exceeded.
Rigidity depends on screw size, lead, nut construction, preload, support arrangement, free length, and the stiffness of the associated support bearings.
Maximum load is a limit, not a stiffness operating point
The basic dynamic load rating Ca is the axial load that allows 90% of a group of the same ball screws [S6]. It is a life-based capacity, not a continuous-load recommendation and not a stiffness value.
The basic static load rating, Coa, covers permanent deformation of the ball and raceway contact.
The permissible axial load must not be exceeded [S2]. At that limit, however, the screw may still deflect elastically, the nut and mounting bolts may be highly stressed, or the shaft may buckle. Those checks can govern before Coa does.
Rigidity must be checked at the actual load
Ball screw rigidity defines the elastic deformation in the axial direction under a given load [S7]. The useful engineering result is axial displacement under a defined force, normally expressed in micrometres, and the corresponding inverse stiffness is expressed in N/µm or kN/µm.
Rigidity falls as the axial load rises because the balls, screw, nut, and support system all deform. Preload improves rolling-contact stiffness and removes much of the unloaded axial play, but it does not make the complete support structure infinitely stiff. Double-nut construction is also used to increase system rigidity [S5].
A catalog rigidity value should not be applied without checking its test condition. A value stated at 30% Ca cannot automatically be treated as the deflection at 50%, 80%, or 100% Ca.
For predictable deflection, specify the applied thrust, mounting condition, free length, lead, nut arrangement, preload class, and required displacement. The force must include machining force, friction force, inertial force, and gravitational force where applicable [S3].
Compare ball screws by capacity, life, and stiffness
The main selection trade-off runs between load capacity, achievable accuracy, and axial compliance. A larger screw can support greater load and usually deflects less, but its mass, inertia, cost, and required support components increase. A smaller screw may satisfy life requirements yet fail a maximum-load deflection limit. [S1]
For low-speed force actuation, load can be much higher than the normal machine-tool band. Such an application demands a separate maximum-deflection and transient-load calculation. A comparable overview of ball screws explains the relationship between shaft, nut, rolling elements, and recirculation system.
Buckling and mounting can dominate near maximum load
Compressive shaft buckling is a separate failure mode controlled by axial load, elastic modulus, second moment of inertia, mounting support factor, and the distance between mounting positions [S3].
A long screw at maximum permitted thrust can therefore buckle before reaching the catalog static rating. Shorter mounting spans, greater second moment of area, and fixed-fixed support can improve buckling capacity, but the arrangement also changes preload and thermal-growth requirements.
Support bearings and alignment affect maximum-load deflection
Total axis deflection includes the screw-and-nut span plus the shaft deflection outside the nut. The bearing arrangement used to support the screw must be included in the system calculation, rather than treating the ball screw as an isolated component. [S1]
For a screw supported at one end, axial force transferred into the shaft can create a bending reaction at the support. The related comparison of angular contact bearings explains how contact angle and preload affect axial-load capability.
Misalignment introduces radial force. Steinmeyer states that a radial load below 5% of the minimum axial load causes no problem under normal conditions, but the catalog axial capacities apply to pure axial loading and radial effects must be minimized [S1]. At high thrust, support-bearing alignment, housing stiffness, and the ball bearing preload should be verified as one assembly.
Static strength, nut fracture, and bolt strength remain separate checks
Exceeding static capacity can cause permanent brinelling of the balls and races, while excessive cycling can produce fatigue [S1]. Fatigue risk grows when a screw turns too many revolutions under a load beyond its dynamic capacity [S1].
Nut-body, flange, and connecting-bolt strength cannot be inferred from Coa alone. Failure of the nut or bolts may occur before the screw reaches its static capacity [S1]. At maximum load, calculate the axial stress in the nut flange and bolts, include preload and any external reaction, and apply an appropriate design margin.
Permissible shaft stress must also be checked under combined axial and torsional loading. One precision-design example uses an allowable stress of 147 MPa and calculates equivalent von Mises stress from axial and torsional components [S3]. Such a limit belongs to the cited calculation example and should not be transplanted into a different material, diameter, heat treatment, or duty without validation.
Use the manufacturer load-rating definition in the purchase specification
The procurement specification should state which rating is being referenced: Ca for basic dynamic load rating, Coa for basic static load rating, or Fp for permissible axial load. Mixing these ratings can produce an invalid stiffness and life selection. [S3]
NSK identifies standard axial play classes of 0.020 mm or less and 0.050 mm or less in its high-load ball screw catalogue [S6]. An axial play requirement belongs to the accuracy and preload specification, while stiffness belongs to the loaded deflection calculation. High precision grades from C0 to C10 and diameters from 4 mm to 80 mm appear in one supplier range, with dynamic linear force up to 7,240 kgf [S4].
The governing purchase data should include screw diameter, lead, stroke, Ca, Coa, nut model, preload or axial play class, accuracy grade, allowable axial load, mounting arrangement, support-bearing model, lubrication, and a target maximum displacement under the specified peak load.
A trackable verification is the calculated displacement at the specified maximum thrust and the margin between Coa and the loaded axial force. If the screw passes both checks but the support system, nut flange, or bolts do not, the selected assembly still lacks the required maximum-load rigidity.