Selection guides

How to select a ball screw

A ball screw turns motor rotation into linear motion at about 90% efficiency. Selection is a loop: guess a lead and diameter, then check them against life, critical speed, buckling, DN limit and motor torque. Light axes are almost never limited by life; they fail on critical speed first.

Step 1. Write down the duty

• Moving mass m (table + workpiece + fixtures), orientation (horizontal or vertical).
• Maximum speed V_max, acceleration time t_a, stroke, and how many strokes per minute.
• External axial force (cutting, pressing), guide friction coefficient μ (about 0.003–0.01 for profile rail guides, 0.1–0.2 for plain slides).
• Positioning accuracy and required life in hours.

Step 2. Pick the lead

A shorter lead gives more thrust and resolution per motor rev, but a higher screw speed for the same table speed. Common leads: 2, 4, 5, 10, 16, 20, 25, 32 mm.

Step 3. Axial load in each phase of the move

Then the mean (cubic) load, weighted by the distance travelled in each phase:

Step 4. Rated life

C_a is the basic dynamic load rating from the catalogue. Load factor f_w: 1.0–1.2 smooth motion, 1.2–1.5 normal, 1.5–2.0 with shock and vibration. Typical targets: machine tools 20,000 h, industrial machines 10,000 h, automation and measuring equipment 15,000 h.

Step 5. Critical (whirling) speed

d_r is the root (minor) diameter and L_b the distance between supports. Keep the operating speed below N_c. If it fails, use a fixed–supported or fixed–fixed mounting, a larger diameter, a longer lead, or a rotating nut.

Step 6. Buckling

L_a is the distance from the nut (at its worst position) to the bearing that takes the thrust. Only compressive loads buckle the screw, but check the peak load anyway.

Fixed = a pair of angular contact bearings that take thrust both ways. Supported = a single deep groove bearing free to float axially.

Step 7. DN value and accuracy grade

Accuracy grade: rolled C7 (±0.05 mm per 300 mm) is fine for routers, printers and vending machines. Ground C5 or better for machine tools and metrology. Add preload (double nut or oversize balls) only when you need zero backlash; it adds friction torque and heat.

Step 8. Motor torque

The table mass is already in F_max through the m·a term, so do not add its reflected inertia twice. Size the motor so the total is below its torque at the operating speed with a margin of 1.5–2 for steppers.

The defaults are a 16 mm, 10 mm lead rolled screw on a 30 kg table. Note that it passes on life by a huge margin but sits right at its critical speed: shorten the span, go fixed–fixed, or move to a 20 mm screw. Take C_a and the root diameter from your supplier's catalogue; they vary between makers.

Bolts and nuts

A bolted joint works by clamping. The bolt is a spring stretched by tightening; the preload squeezes the parts together, and the joint carries load through that clamp force and the friction it creates. Get the preload right and most of the external load never reaches the bolt.

Property classes (ISO 898-1)

Values are for d ≤ 16 mm; 8.8 is slightly higher above M16. The class reads as UTS/100 and yield/UTS: 8.8 means 800 MPa UTS and yield at 0.8 × 800.

Metric coarse thread data

Preload and tightening torque

The nut factor K carries all the friction uncertainty: about 0.20 for plain dry steel, 0.15–0.18 for zinc plated or lightly oiled, 0.12 with moly grease. A torque wrench gives ±25% preload scatter at best, which is why the design checks below keep margin.

Tension joints: separation and bolt load

Only the fraction C of the external load P adds to the bolt; the rest unloads the clamped parts. This is why a well-preloaded bolt survives fatigue: it sees a small stress swing.

Shear joints: let friction carry it

Design machine joints so friction carries the shear. If the joint slips, the bolt bends and loosens. Where position matters, add dowel pins (H7/m6) and let the bolts only clamp.

Thread engagement, locking, and practice

• Engagement length: steel nut 0.8d; tapped steel 1.0–1.5d; cast iron 1.5d; aluminium 2d (or use a helicoil insert).
• Locking: correct preload is the main defence. Then wedge-locking washers or a medium threadlocker (e.g. Loctite 243) for vibration. Split spring washers do not stop vibration loosening.
• Use a hardened washer under the head on aluminium or plastic so the surface does not embed and lose preload.
• Long, thin bolts hold preload better than short fat ones: more stretch for the same embedding loss.
• In vending and food machines use A2 or A4 stainless, and avoid mixing stainless bolts in aluminium where wet (galvanic corrosion), or isolate them.

Bearing selection

Choose the type from the loads and space you have, then size it for life, then choose the fits, clearance, seals and lubrication.

Step 1. Choose the type

Step 2. Equivalent dynamic load

e and Y depend on f₀·F_a/C₀ (f₀ is the calculation factor in the catalogue, about 12–16):

Normal internal clearance. Other bearing types have their own X, Y in the catalogue.

Step 3. Basic rating life

Step 4. Static check

Step 5. Fits, arrangement, lubrication

• The ring that rotates relative to the load gets the interference fit. Rotating shaft: shaft k5/k6 (light loads j6, heavy m6), housing H7. Rotating housing (wheel hubs): housing N7/P7, shaft g6/h6.
• Locate one bearing axially on both rings (fixed end) and let the other float, so thermal growth does not load the bearings.
• Shaft shoulder radius must be smaller than the bearing chamfer; check the catalogue abutment diameters.
• Sealed and greased-for-life (2RS) suits most garage projects. Use ZZ shields where speed is high and dirt is low. Food machines need food-grade (NSF H1) grease and stainless bearings where washed down.
• Speed: keep below the catalogue limiting speed; for sealed bearings the seal speed limit is usually lower.

Defaults: a 6205 deep groove bearing (C 14.8 kN, C₀ 7.8 kN, f₀ 14). Check the ratings for the exact bearing in your supplier's catalogue.

Tolerances and GD&T

A tolerance is a promise about what the part must do. Tighten only the features that locate, seal, slide or carry load; give everything else a general tolerance. Each step tighter roughly doubles machining cost.

General tolerances

Put one note in the title block, such as ISO 2768-mK: medium linear tolerances (e.g. ±0.1 for 6–30 mm, ±0.2 for 30–120 mm, ±0.3 for 120–400 mm) and class K geometric tolerances. Then only dimension the critical features explicitly.

Fits (ISO 286, hole basis)

Hole basis means the hole is H (lower deviation zero) and you change the shaft letter. Reamers and boring tools come in H7, which keeps tooling simple.

The GD&T symbols

Concentricity and symmetry were removed from ASME Y14.5-2018. Use position or runout instead.

Datums: tell the inspector how the part sits

Datums A, B, C follow the 3-2-1 rule: the primary datum touches at three points (stops tilting), the secondary at two (stops rotating), the tertiary at one (stops sliding). Choose datums that are the surfaces the part actually mounts on in the assembly, not the ones easiest to measure.

Position and the bonus tolerance

The round zone has 57% more area than the square that fits inside it, so good parts are no longer rejected for being off on a diagonal. With the MMC modifier Ⓜ, the zone grows as the hole moves away from its smallest size: a Ø6.6 +0.1/0 hole with position Ø0.2 Ⓜ may be off by Ø0.3 when it measures Ø6.7.

Example: M6 bolt through Ø6.6 holes in two plates, floating: T = 6.6 − 6.0 = Ø0.6 for each plate. If one plate is tapped instead, each part gets Ø0.3.

Tolerance stack-up

Walk the loop of dimensions from one side of the gap to the other. Dimensions that increase the gap are +, those that reduce it are −. Worst case adds every tolerance and guarantees assembly; RSS (root sum square) assumes random variation and is realistic once there are four or more parts in production.

Default: a 50 mm housing with two 20 mm spacers and a 9.5 mm part inside. The gap must stay positive.