Guide
Why do long turned shafts arrive with too much shaft runout, and what should the drawing and packing specify?
Shaft runout is the total movement of a dial indicator on a shaft surface while the shaft turns one full revolution about its datum axis, and on a long turned shaft it is usually caused less by the turning itself than by bow: stress in the bar, deflection during the cut, heat treatment, and how the shaft was supported, measured and packed. A shaft can leave the lathe within tolerance and still fail runout at receiving inspection.
By Saola Manufacturing engineering team · Published · Updated
The rejection usually reads the same way: “runout over tolerance at the middle journal” or “TIR out at the free end.” Before anyone argues about whose gauge is right, it helps to know whether the shaft is bent, whether a diameter was turned off center, or whether the two parties measured it differently. This guide works through that order: how to recognize the problem, what causes it, how to measure it so both sides get the same number, how to fix it, when straightening is acceptable, and what to write on the drawing, the RFQ and the packing instructions so the next lot arrives straight. The tolerance definitions themselves are covered in the circular vs total runout guide; this page is about the shafts that fail them.
On this page
How do you tell a bent shaft from a shaft with an off-center diameter?
Plot runout along the length: a bent shaft shows readings that rise toward the middle of the span or toward the free end, while an off-center diameter shows a reading that jumps at one step and stays constant along that diameter.
Read runout at several stations along the shaft, not only at the feature the drawing calls out, and note where the high point sits on each reading. The pattern points to the cause:
- Bow (bent shaft). Readings grow smoothly from the supports toward the middle, or toward the overhanging end, and the high points line up on the same side of the shaft. This is the classic long-shaft rejection.
- Eccentric diameter. One diameter reads high along its whole length while the neighboring diameters read low. That diameter was turned in a different setup, or after the part was re-gripped, from the datum journals.
- Out-of-round surface. The indicator moves two or three times per revolution instead of once. That is a form error, usually from clamping pressure or chatter, not a bent axis.
- Datum problem. The shaft reads well on centers but badly on V-blocks, or the reverse. The two methods are not establishing the same axis, which is a measurement disagreement rather than a bad part.
A useful rule from geometry: when a bowed shaft is turned on supports, the indicator sees the bow twice, once on the high side and once on the low side, so the indicator reading is twice the bow. The Precision Machined Products Association sets this out as TIR equal to twice the camber in its Craftsman’s Cribsheet on bar straightness, for a bar supported at the ends of the bowed span. As a rough check, halve a runout reading before comparing it with a camber or straightness value.
What causes long turned shafts to exceed their runout tolerance?
The usual causes are residual stress released from the bar stock, deflection of a slender shaft under the cutting force, distortion during heat treatment, features machined on one side only, and damage in handling and transit. Which one dominates depends on the shaft and the routing.
- Residual stress in the bar. Cold-drawn and straightened bar carries locked-in stress. Turning removes the outer layer unevenly, the remaining stress rebalances, and the shaft bows. A study of induction-hardened 1045 shafts by Dias and coauthors found that the cold drawing step had the largest influence on final bending distortion, ahead of hardening depth and grinding (Metallurgical and Materials Transactions A, 2017).
- Deflection during the cut. A long, thin shaft supported only at its ends bends away from the tool. The diameter comes out barrel- or taper-shaped and the shaft can spring into a slight bow when released. The ratio of length to diameter (L/D) decides which support a shaft needs; the table in the next section gives the usual bands.
- Heat treatment. Through-hardening, induction hardening and carburizing can all move a shaft, because the surface and the core change volume at different times. The same 2017 study describes bending that appears after induction hardening and is then corrected by straightening and/or grinding.
- One-sided features. A long keyway, a flat or a milled slot removes stressed material from one side only, and the shaft bows toward or away from it. The keyway guide covers the feature itself.
- Plating and coating. A coating is not usually the source of bow, but a thick or uneven build on a journal raises the runout reading at that diameter, and a post-plating bake is one more thermal step after which the shaft should be rechecked.
- Handling and transit. A long shaft stacked under other shafts, dropped, or loose in a carton can be bent by load or impact before it reaches you, and the damage looks the same at receiving as a machining problem.
How does shaft slenderness change what the supplier has to do?
The longer a shaft is relative to its diameter, the more support it needs while it is cut and the more operations it needs between roughing and finishing. A shaft beyond the tailstock band needs a steady rest or a changed design, and the price and inspection plan should reflect that.
There is no standard that sets these bands; they are shop rules of thumb and vary with material, depth of cut and machine. Use them to judge whether a shaft on your drawing is a routine turning job or a slender-shaft job before you set the runout tolerance.
| Typical L/D band | Typical workholding | Main runout risk | What to ask for |
|---|---|---|---|
| Up to about 3 × D | Chuck or collet only | Re-gripping between operations | Datum journals finished in one setup |
| About 3 to 10 × D | Chuck plus tailstock center, or between centers | Deflection at mid-span; center hole quality | Center holes allowed or specified; finish passes after roughing |
| Above about 10 × D | Steady rest or follow rest, plus tailstock | Bow from stress release and cutting force | Rough, stress relieve, then finish; runout checked on the datum journals |
| Very slender, above about 20 × D | Several rests, light cuts, often grinding to finish | Bow, and bending in handling and transit | Wider runout away from bearings; ground or precision bar stock; supported packing |
Bar straightness matters at every band, because stock that starts bent leaves less material to clean up. The PMPA cribsheet names ASTM A108 as the controlling standard for the straightness of cold-finished steel bar, and converts camber stated per 10 ft into the runout a machinist will see over a given span. The table below takes two of its columns. A bar with only 1/16 in of camber in 10 ft still reads about 0.031 in TIR over a 5 ft span, which is why the journals of a long shaft are turned or ground true rather than left on the as-drawn bar surface.
| Span between supports | TIR at 1/16 in camber in 10 ft | TIR at 1/4 in camber in 10 ft |
|---|---|---|
| 1 ft (305 mm) | 0.00125 in (0.032 mm) | 0.0050 in (0.127 mm) |
| 2 ft (610 mm) | 0.0050 in (0.127 mm) | 0.020 in (0.51 mm) |
| 5 ft (1524 mm) | 0.03125 in (0.79 mm) | 0.125 in (3.18 mm) |
How should shaft runout be measured so supplier and buyer get the same number?
Measure against the datum axis the drawing names, usually the two bearing journals, and write down how that axis is simulated: between centers, on V-blocks under the journals, or on a CMM. Many runout disputes on long shafts are two different setups giving two honest but different numbers.
- Datum journals, not centers, unless the drawing says so. Under ASME Y14.5 and ISO 1101, runout is read relative to the datum axis established from the datum features. Center holes are a convenient way to hold the shaft, but if the centers are not concentric with the journals the readings will differ from a V-block check. If centers are the functional datum, make them the datum on the drawing.
- Support on the datum journals. Gravity sag of a straight shaft points down at every angle of rotation, so it adds little to a runout reading taken while the shaft turns; what changes the number is supporting the shaft on diameters other than the datum features, which moves the axis. Sag does matter when straightness is checked with the shaft lying still on a surface plate or rollers: for a uniform bar, supports about 0.2232 of its length from each end give the minimum sag, with the middle sagging as much as the ends (see Airy points and related support points).
- Several stations. Read runout at each callout and at the middle of the longest span, and record the angle of each high point. Readings in phase along the shaft mean bow; readings out of phase mean eccentric diameters.
- Temperature and cleanliness. Let the shaft reach room temperature after machining or washing, and wipe burrs and chips from the journals and V-blocks.
- Same method at both ends of the shipment. State the method on the purchase order so the outgoing inspection and your incoming inspection use the same datum simulation.
What should you do with a lot of shafts that failed runout?
Hold the lot, re-measure a sample the way the drawing defines the datum, classify the failure from the reading pattern, then decide on use, rework or return before asking the supplier for a root cause tied to one step of the routing.
- Hold and re-measure. Quarantine the lot. Re-measure several rejected shafts on the drawing datum (V-blocks or a fixture under journals A and B, unless centers are the datum) and put your readings next to the supplier’s report for the same serial or sample numbers. If the numbers agree only when both sides use the same setup, the problem is the method, not the parts.
- Map each failing shaft. Record runout at every callout plus the middle of the longest span, with the angle of each high point. Sort the shafts into bow, eccentric diameter, out-of-round surface or datum disagreement using the patterns above.
- Check function before scrapping. Ask design engineering whether the failing reading is on a diameter that carries a bearing, seal or gear. A reading over tolerance on a clearance diameter may be a candidate for a documented use-as-is decision; a bearing journal usually is not.
- Choose the rework, if any. Straightening only where the drawing or purchase order allows it, followed by the agreed stress relief and a full re-inspection; regrinding or re-turning the journals only where diameter tolerance leaves room. Shafts that cannot be brought back go back to the supplier.
- Ask for a cause tied to a step. The corrective action should name the step that moved the shaft (bar, roughing, heat treatment, keyway, straightening or packing) and show runout maps from the next lot, not just a statement that inspection was tightened. The 8D report guide covers the format, and a first article inspection on the next lot is the place to confirm the fix.
How do you fix or prevent excessive runout on long shafts?
Match the fix to the pattern: bow calls for stress control and support, an eccentric diameter calls for finishing the datum journals and the related diameters in one setup, and runout that appears only after receipt calls for better packing.
| Symptom | Most likely cause | Process fix | Drawing, RFQ or packing fix |
|---|---|---|---|
| Runout grows toward mid-span, high points in phase | Bow from bar stress or cutting deflection | Rough, stress relieve, then finish; steady rest; lighter finish cuts | Name material condition; allow a stress-relief step in the routing |
| One diameter reads high along its length | Diameter turned in a separate setup from the datum journals | Finish journals and related diameters in one setup or between the same centers | Keep runout callouts to features that share one datum axis |
| Shaft was good before heat treatment, bent after | Hardening distortion | Leave grind stock; finish after hardening; controlled straightening then temper or stress relief | State whether straightening is allowed after hardening and what must follow it |
| Bow toward a long keyway or flat | One-sided stress release | Mill the keyway before the final finish pass or grind on the journals; balance roughing | Move or shorten the keyway if function allows |
| Passed at the supplier, fails on receipt | Impact or load in transit; different datum setup | Supported packing; same measuring method at both ends | Write packing and measurement method on the PO |
| Lot-to-lot variation with the same program | Different bar heat or condition | Record bar heat per lot; buy from the same condition | Name grade, condition and straightness of the stock |
Two design changes are often cheaper than any process fix. First, tighten runout only where a bearing, seal or gear actually runs, and give the rest of the shaft a wider value; total runout along a whole shaft is rarely what the function needs. Second, if a shaft is so slender that it cannot be held straight in transit, ask whether it can be split, made hollow from tube, or given a larger diameter between the bearings. The stress relieving guide explains where a stress-relief step belongs in the routing.
Straightening a bent shaft: when it is acceptable and what must follow it
Straightening is a normal step for long shafts, especially after hardening, but it bends the shaft past yield and leaves new residual stress behind. Allow it where the function tolerates that, and require a temper or stress relief afterward where fatigue or later movement matters.
Press straightening pushes the high side of the bow until the shaft yields back toward straight. The shaft reads straight on the press, but the stresses introduced can relax again when the shaft is later machined, ground, plated and baked, or put into service, and the yielded zone can change local fatigue behavior. Points to settle with the supplier:
- Whether straightening is allowed at all. For rotating shafts under cyclic load, some buyers prohibit straightening after final heat treatment unless approved case by case.
- What follows it. A temper or a stress relief below the tempering temperature after straightening is commonly specified to reduce the chance that the shaft moves later.
- When it happens. Straightening before finish grinding or finish turning lets the final cut remove the stressed surface; straightening a finished, coated shaft can crack the coating.
- How it is recorded. Ask that straightened parts be identified on the inspection record, so a later failure can be traced.
What should the drawing, RFQ and packing instructions specify?
The drawing should name the datum journals and put tight runout only on functional diameters; the RFQ should state material condition, heat treatment and whether straightening is allowed; and the purchase order should state the measuring method and how long shafts are packed.
Drawing checklist for a long shaft:
- Datums. Use the two bearing journals as a common datum axis (A–B) where the shaft runs in two bearings, rather than one short journal that cannot establish a stable axis.
- Runout by function. Circular runout on seal and gear diameters, total runout only where a full-length surface matters. The concentricity vs runout guide explains why runout is usually the right control for a rotating shaft.
- Straightness separately, if needed. If the shaft must pass through a bore, add straightness of the axis; the straightness guide covers the choice.
- Material and condition. Grade, delivery condition (cold drawn, turned and polished, stress relieved) and hardness after heat treatment.
- Center holes. State whether center holes are allowed, required or must be removed.
RFQ and purchase order notes that prevent repeat rejections:
- Function line. One sentence on what the shaft does and which diameters carry bearings or seals.
- Straightening rule. Allowed, allowed only before final finishing, or not allowed after hardening; and what stress relief must follow.
- Measuring method. Datum simulation (V-blocks under A and B, or centers), stations to read and how the report records them.
- Packing. Each shaft supported along its length or at defined points in a rigid box or crate, separated so shafts do not touch, protected against corrosion on bare journals, and stored horizontal and supported, not leaning. ASTM D3951 sets general minimum requirements for commercial packaging; name it if you want a baseline, and spell out the shaft supports separately.
How is a long shaft handled on a Saola order?
Shafts are turned at our lead CNC plant, T.K Viet Nam Precision Company Limited, on CNC lathes, with milled features such as keyways cut on its machining centers. Heat treatment, plating and other finishes go to partner plants we manage, and the quote names each site and the country of origin.
When the RFQ arrives, our engineers look at the length-to-diameter ratio, the material condition and the runout callouts together; ask for the quote to state how the shaft will be supported, whether a stress-relief step is planned and how runout will be checked. Turning and milling are described on the CNC turning and CNC milling pages; plating and other finishes are covered on the surface finishing page. Inspection equipment listed for the lead CNC plant includes a 2D vision measuring machine, a height gauge and a hardness tester; if you need a runout report between centers, on V-blocks or from a CMM, name it on the RFQ and the quote will state how it is verified. Write your straightening rule and packing requirement on the RFQ as well, so they are priced in from the start. ISO 9001 certification is in progress, not held. Send the drawing, with one line on what the shaft does, through the quote form.
Frequently asked
Is runout the same as bow or straightness?
No. Runout is an indicator reading while the shaft turns about a datum axis, so it combines bow, eccentricity and out-of-roundness. On a bowed shaft read on supports, the runout reading is about twice the bow.
Should a long shaft be measured on centers or on V-blocks?
Measure on whatever simulates the datum the drawing names. If the datums are the bearing journals, V-blocks or a CMM under those journals match the drawing; centers match it only if the center holes are the datum or are proven concentric with the journals.
Can a bent shaft be straightened and still accepted?
Often, if the drawing or purchase order allows it. For shafts under cyclic load, specify whether straightening is allowed after hardening and require a temper or stress relief afterward, and ask for straightened parts to be identified on the inspection record.
Why did shafts pass at the supplier and fail at our receiving inspection?
Check three things in order: the datum setup used at each end, which diameters the shaft was supported on during measurement, and how the shafts were packed. A different datum simulation or damage in transit is the usual explanation.
Does buying ground and polished bar solve runout on long shafts?
It helps, because the stock starts straighter and with more uniform stress, but it does not remove bow from heat treatment, one-sided features or transit. The routing, straightening rule and packing still need to be specified.
Sources
- ASME Y14.5-2018, Dimensioning and Tolerancing, ASME, 2018
- ISO 1101:2017, GPS — Geometrical tolerancing — Tolerances of form, orientation, location and run-out, ISO, 2017
- ASTM A108, Standard Specification for Steel Bar, Carbon and Alloy, Cold-Finished, ASTM International, 2024
- Craftsman’s Cribsheet 78: Straightness of Cold Finished Steel Bars (M. Free), Precision Machined Products Association, 2019
- Bending Distortion Analysis of a Steel Shaft Manufacturing Chain from Cold Drawing to Grinding (V. W. Dias et al.), Metallurgical and Materials Transactions A 48(4), 2017
- Airy points (Airy, Bessel and minimum-sag support points), Wikipedia, accessed October 2026
- ASTM D3951-18(2023), Standard Practice for Commercial Packaging, ASTM International, 2023