A long-travel ball screw must keep its maximum expected compressive load below the calculated buckling limit, with a manufacturer-recommended safety factor of at least 2 against the maximum applied load [S3][S4].
Buckling load depends on the screw's root diameter, unsupported length, and end-bearing configuration [S3].
Buckling Limit Definition and Governing Inputs
Ball screw buckling is a shaft instability under axial compression, and its limit is governed by the screw root diameter, elastic modulus, second moment of inertia, unsupported length, and end-bearing arrangement [S3][S4].
For a horizontal axis, axial force generated through friction can be much smaller than the translated load, but pressing and injection applications can add a direct axial force that makes buckling a controlling selection criterion [S3]. In a vertical axis, gravity acts directly along the screw, so the full suspended load must be included in the compressive calculation [S3].
The ball screw must also be checked for tension-compression load in addition to its buckling limit [S8]. Dynamic rating, static rating, nut-body strength, connecting-bolt strength, critical speed, and life are separate checks: a screw can remain below dynamic capacity and still be unsuitable because of shaft instability or nut failure [S2].
Compare Support Arrangements by Buckling Capacity
The end-bearing factor depends on the screw's end fixity, and some manufacturers incorporate a safety factor of at least 2 into their published factors [S3]. Published factors are not interchangeable, so the manufacturer calculation method must be used consistently [S3].
For equal material, diameter, and length, increasing end fixity improves buckling resistance. Adding an intermediate support reduces the unsupported span and therefore increases allowable compressive load, while also increasing critical speed [S4].
Calculate the Maximum Compressive Load
Applied thrust is the sum of machining force, friction force, inertial force, and gravitational force [S1]. These values must be converted into force and duty-cycle inputs rather than treated directly as a permissible screw load.
Ball screw manufacturers recommend a safety factor of at least 2 for the maximum applied buckling load [S3].
Preload is a separate issue. TBI Motion recommends that preload applied on CNC machine tools be no heavier than 8% of the dynamic load [S6]. Preload increases axial contact forces, but it does not increase the shaft’s Euler buckling capacity.
Choose Diameter, Length, and Support for the Duty
The most effective geometric change is to increase root diameter, because the second moment of inertia rises rapidly with diameter and directly increases Euler buckling load [S3][S4]. Reducing unsupported length is equally decisive because Pb varies inversely with the square of L [S4]. For long axes, shortening the span with an intermediate support can outperform a modest diameter increase while also reducing deflection and raising critical speed [S4].
A long screw with a small root diameter reaches its buckling point under a lower axial load than a shorter, larger-diameter screw [S3].
Set Load, Life, and Speed Limits Together
These values do not replace the buckling calculation. A low-load screw can still buckle if its unsupported length and root diameter are inadequate.
Static safety factors commonly range from 1.0 to 2.0 for normal operation and from 2.0 to 3.0 where impacts and vibration are present [S1]. The stated preload, load direction, and support arrangement should be included in the final calculation because each changes either the applied compressive force or the effective buckling length.
Failure Modes and Procurement Checks
Five documented overload modes are excessive dynamic loading, exceeding static capacity, shaft buckling, nut-body or connecting-bolt failure, and unaccounted radial loading [S2]. Published axial capacities apply to pure axial loading; alignment tolerances can introduce radial force, and a radial load below 5% of the minimum axial load is treated as unproblematic under normal conditions [S2].
For procurement, request the root diameter, unsupported span, end-bearing arrangement, calculated Pb, maximum applied compressive load, safety factor, dynamic load rating, static load rating, allowable speed, critical speed, preload method, and tension-compression check [S3][S5][S8]. Checking ball-spline is unnecessary for a pure screw axis, but support-bearing selection still affects alignment and radial loading.
Use ball bearings or the specified fixed and floating support arrangement to achieve the assumed end fixity, since actual support stiffness changes the published factor. If the calculated margin is below 2, increase root diameter, shorten unsupported length, add an intermediate support, or move to a more rigid end arrangement before reducing the safety factor.
A practical release criterion is Pb ≥ 2 × Fmax,compressive, with Fmax including gravity, process force, friction, acceleration, and preload effects, and with Pb calculated using the selected manufacturer's end-support convention [S3][S4]. Track the selected root diameter, free length, support condition, and duty cycle at design release because each is a direct input to the buckling margin.