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ASME B5.48-1977 (2013) — Ball Screws
⚙️ ASME B5.48-1977 (2013) is the American National Standard for ball screws, approved on April 7, 1977, and reaffirmed in 2013. It covers definitions, classes of ball screws, recommended combinations of screw diameters and leads, a recommended drawing format, and the performance characteristics of ball screw and nut assemblies as applied to machine tools. It is a key reference for ball screw design, selection, and acceptance worldwide. The values stated in U.S. customary units are to be regarded as the standard; metric values are converted per the recommendations of ASME Guide SI-1.
💡 A ball screw converts sliding friction into rolling friction by circulating balls between the nut and screw helical races, achieving drive efficiency above 90 percent, along with high stiffness, high positioning accuracy, and backdriving ability — which makes it the core feed-drive element of CNC machine tools.
1. Purpose, Scope, and Core Definitions 📌
Section 2 of the Standard provides a complete, unified terminology for the ball screw field. Key definitions include:
| Term | Definition |
|---|---|
| Ball Screw | The shaft member containing the inner helical ball track. |
| Ball Nut | The member containing the outer helical ball track. |
| Ball Screw Assembly | A device consisting of a ball nut, ball screw, and balls. |
| Ball Track | The specially designed helical groove that transmits the load reaction between the ball nut or screw and the balls. |
| Circuit | A continuous closed path of recirculating balls. |
| Lead | Axial distance the screw or nut travels in one revolution. |
| Lead Error (Deviation) | The measured lead minus the specified lead. |
| Backlash | Axial free motion between the nut and screw. |
| Backdriving | The ability of the screw or nut to rotate when thrust load is applied to the other member of the assembly. |
Other important definitions include: Basic Load Rating (the constant axial load a group of apparently identical assemblies can endure for a rated life of one million inches — 25 400 m — of travel), Basic Static Thrust Capacity (the static thrust load producing a permanent ball track deformation of 0.0001 times the ball diameter), Equivalent Load (the mean load giving the same life as a combination of varying loads), Critical Speed (the rotational speed producing resonant vibration), Column Strength (the maximum compressive axial load without permanent structural deformation), and Drag Torque (the torque required to rotate the nut relative to the screw with no external load).
⚠️ Units note: the Standard regards U.S. customary values as standard and metric values as converted per ASME Guide SI-1. Specify one unit system for selection and acceptance; mixing values from the two systems is treated as inconsistent.
2. Accuracy Classes and Lead Error Requirements 🔧
Section 3 defines the accuracy class system for ball screws. The basic accuracy parameters include:
- Lead Error: the difference between measured and specified lead, expressed through Actual Linear Deviation (deviation from a reference line connecting the beginning and end points of measured lead error) and maximum linear deviation.
- Permissible error classes: Table I gives maximum permissible lead error and measurement intervals per class, with recommended lead-error measurement intervals in both inch and SI (metric) units.
- Lead Error Rate: the rate of change of lead error along the travel; acceptance templates (Fig. 1) define the template construction.
- Thread length vs. T factor: Table II gives the thread-length versus T-factor relationship for Classes 1, 2, 4, and 5.
- Full Indicator Movement: Table III gives FIM requirements for Classes 1 through 7.
| Accuracy Class | Primary Controlled Parameter | Typical Applications |
|---|---|---|
| Classes 1, 4 | Maximum permissible lead error (strict) | Precision jig borers, precision grinder feeds |
| Classes 2, 5 | Maximum linear deviation | CNC machine tools, machining-center feed axes |
| Classes 3, 6, 7 | Maximum lead error rate (template acceptance) | General machine tools, positioning mechanisms |
💡 Class-to-acceptance correspondence: Classes 3, 6, 7, and 8 use “maximum lead error rate plus acceptance template” (Fig. A1); Classes 2 and 5 use “maximum linear deviation” (Fig. A2); Classes 1 and 4 use “maximum permissible lead error” (Fig. A3). Judge each acceptance per the corresponding figure and template.
3. Recommended Diameter and Lead Combinations 📏
Sections 4 and 5 give two series of recommended “nominal screw diameter × nominal lead” combinations, in inch and metric units. Recommended combinations limit size proliferation, ease bearing and coupling selection, and span typical needs from high-resolution precision positioning to high-speed long-stroke motion:
| Series | Units | Typical Diameter Range | Typical Lead Range | Selection Principle |
|---|---|---|---|---|
| Inch series (Section 4) | Inches | Per Table IV | Per Table IV | Choose by load, stroke, and speed |
| Metric series (Section 5) | Millimeters | Per Table V | Per Table V | Metric machines and import substitution |
Design tradeoffs: a larger diameter carries more load and stiffness but more inertia; a smaller lead gives higher resolution and stronger self-locking tendency (though ball screws remain backdrivable); a larger lead gives higher speed but lower thrust and resolution. The recommended combinations keep the diameter-to-lead ratio in the empirically optimal range.
4. Performance Characteristics and Calculation Methods 📊
Section 7 provides the classic formula set for ball screw performance:
- Basic Load Rating (inch: 1 000 000-inch rated life; metric: 25 400 m) — computed from ball diameter, count, and contact angle, among other parameters.
- Basic Static Thrust Capacity — based on the criterion of 0.0001-times-ball-diameter permanent race deformation.
- Load rating at other than rated life — converting with the cube law between life and load.
- Equivalent load — averaging of varying load combinations.
- Appendix A1: axial torsion-deflection equations, approximate deflection, maximum drag torque, length-versus-pitch-diameter relationship, and the associated chart (Chart A1).
The most-used life formula is L = (C / P)³ × 10⁶ inches (travel), where C is the basic load rating and P the equivalent axial load. Long slender screws must also be checked for critical speed (to avoid resonance) and column strength (to avoid buckling); both depend strongly on screw length and end-support arrangement.
🚨 Checking warning: high-speed, long-stroke screws must be checked for critical speed and column strength. Critical speed depends on the support arrangement (fixed-supported, fixed-fixed, etc.); exceeding it causes violent resonance, and column buckling bends and ruins the screw. Both are mandatory checks in the selection phase.
5. Drawing Format and Engineering Practice Essentials 🛠️
The recommended drawing format in Section 6 requires the ball screw drawings to carry full information — rated loads, accuracy class, lead and thread length, end machining (journals, threads, coupling interfaces), lubrication and sealing requirements — with a specification sheet example (Fig. 2) and drawing formats (Figs. 3, 3A) so that design, manufacture, and acceptance stay consistent.
- 🔍 Selection workflow: define the axial load spectrum → preselect diameter and lead from the life target → check critical speed and column strength → select accuracy class → issue a specification sheet per the standard drawing format.
- 🔧 Preload and backlash: for high positioning accuracy use a preloaded nut (double nut or single nut with internal preload) to eliminate backlash; balance preload against drag torque and temperature rise.
- 📊 Lubrication and sealing: use end seals per the Standard to keep foreign matter out and retain lubricant; race and ball lubrication directly affects life.
- 📋 Acceptance measurement: measure lead error per the Standard, plot the error curve, and judge against the class; include full indicator movement and thread starts in acceptance.
⚠️ Errata note: the 1979 Errata to this Standard corrects two formula/symbol errors — in para. A1.2, “Symbols,” the term “helix angle, degrees” was changed to read “lead angle, degrees”; and the arctangent expression in para. A1.3, Axial Angle Deflection Equations, was corrected per the Errata sheet. Use the corrected content when citing formulas.
6. Frequently Asked Questions ❓
❓ Q: Why speak of a “ball screw assembly” rather than just a screw?
A: A ball screw works only together with the ball nut and the balls — the three form a functional unit (Ball Screw Assembly). The classes, loads, and life ratings defined by the Standard all refer to the assembly as a whole.
A: A ball screw works only together with the ball nut and the balls — the three form a functional unit (Ball Screw Assembly). The classes, loads, and life ratings defined by the Standard all refer to the assembly as a whole.
❓ Q: How do backlash and preload relate?
A: Backlash is the axial free travel between nut and screw. Preload (applying a preload force) eliminates backlash and increases axial stiffness, at the cost of higher drag torque and temperature rise. Positioning axes typically require low or zero backlash.
A: Backlash is the axial free travel between nut and screw. Preload (applying a preload force) eliminates backlash and increases axial stiffness, at the cost of higher drag torque and temperature rise. Positioning axes typically require low or zero backlash.
❓ Q: How does the basic load rating convert to actual service life?
A: By the life formula L = (C/P)³: when the equivalent load P equals the rating C, the rated life is one million inches (25 400 m); doubling the load cuts life to one eighth, so load margin is critical in selection.
A: By the life formula L = (C/P)³: when the equivalent load P equals the rating C, the rated life is one million inches (25 400 m); doubling the load cuts life to one eighth, so load margin is critical in selection.
❓ Q: Can inch and metric series be mixed?
A: Not directly. The two series use different units and recommended combinations; the Standard takes inch values as the basis and metric values as conversions. Unify on one unit system and state it explicitly in drawings and orders.
A: Not directly. The two series use different units and recommended combinations; the Standard takes inch values as the basis and metric values as conversions. Unify on one unit system and state it explicitly in drawings and orders.
