What is the difference between fundamental deviation and tolerance zone?

The fundamental deviation defines the position of the tolerance zone relative to the zero line — it determines whether the dimension starts above or below the nominal dimension. For example, bore H always has a fundamental deviation of zero (lower deviation EI = 0). The tolerance zone describes the width of the permissible dimensional range, defined by the IT grade (e.g. IT7 = 25 µm at ⌀50). Together, fundamental deviation and IT grade form the complete tolerance zone: H7 at ⌀50 = 0 to +25 µm.

Why is the hole-basis system (H) more common in practice than the shaft-basis system?

Bores cannot be reworked as flexibly as shafts. Standard drilling tools (drills, reamers) always produce an H tolerance. The shaft, on the other hand, can be turned or ground to any fundamental deviation letter. The hole-basis system therefore significantly reduces tooling costs: one reamer for H7⌀25 covers all fit types; with the shaft-basis system, different drilling tools would be required for each application.

Which IT grade is suitable for rolling bearing applications?

Rolling bearings to DIN ISO 492 typically require IT5 or IT6 for the shaft seat, and IT6 or IT7 for the housing seat. IT5 (e.g. k5, m5) is used for precision applications and heavy loads; IT6 (k6, m6) is the standard for normal industrial gearboxes and motors. IT7 is sufficient for the housing outer diameter, as the outer ring normally does not carry circumferential load.

How do I calculate the maximum clearance of an H7/g6 fit at ⌀50?

For ⌀50 (nominal size range >30–50 mm): H7 bore = 0 to +25 µm (IT7=25); g6 shaft = −9 to −25 µm (es=−9, IT6=16, ei=−25). Maximum clearance = ES(bore) − ei(shaft) = +25 − (−25) = 50 µm. Minimum clearance = EI(bore) − es(shaft) = 0 − (−9) = 9 µm. The clearance therefore always lies between 9 and 50 µm.

What does a value of −1 µm in the clearance/interference range mean?

A value of −1 µm means a minimum interference of 1 µm: in the most unfavorable tolerance combination, the shaft is 1 µm larger than the bore. In practice this is barely noticeable, but technically it already constitutes a force-fit connection. This case occurs with transition fits that have a small overlap range and always requires an additional form-fit element (feather key).

Why does ISO 286 use uppercase letters for bores and lowercase letters for shafts?

The distinction stems from the international standardisation philosophy: bores (inside dimensions) are assigned uppercase letters (A–ZC), shafts (outside dimensions) lowercase letters (a–zc). This makes the fit designation in the drawing immediately readable: ⌀50 H7/k6 means without further annotation — bore H7, shaft k6. Before ISO 286 there were nationally different systems; the standardised letter convention enabled international harmonisation.

When should I use ISO 2768 instead of ISO 286 for a fit dimension?

ISO 2768 (general tolerances) applies to dimensions without individual tolerance entries in the drawing — it is a collective tolerance for non-critical free dimensions and corresponds roughly to IT12–IT14. ISO 286, on the other hand, is a deliberate, component-specific tolerance assignment for functional fits. ISO 286 is mandatory for every fit where clearance, transition or interference is to be set precisely. ISO 2768 is only suitable for free dimensions without a fitting function.

How does temperature affect a press fit made of steel and aluminium?

Steel has a thermal expansion coefficient of approx. 11–12 µm/(m·K), aluminium of approx. 23 µm/(m·K). With an H7/s6 interference fit (⌀50, interference approx. 18–59 µm) and +80 °C operating temperature, the aluminium hub expands by approx. 48 µm more than the steel shaft. This can completely eliminate the nominal minimum interference of 18 µm and loosen the joint. With a material mix, the interference must be thermally designed or the joint secured by form fit.

ISO Fits H7/H6: Tolerance Tables & Selection Guide

Before ISO 286 existed, manufacturers in different countries worked according to national tolerance standards, which differed in numerical values, letter designations and basic philosophy. ISO 286, adopted in Germany as DIN ISO 286, created a uniform language: the same specification - such as H7/k6 - means the same dimensional ratio worldwide.

The current edition ISO 286-1:2010 defines the entire tolerance system for linear dimensions up to 3150 mm. Part 2 contains the standard tolerances and basic dimensions in tabular form. This guide provides the numerical valueswhich IT tolerance results in which dimension, which fit belongs to which application? Conceptual principles (fit types, assembly instructions) are described in the accompanying article Shafts and bearings: Understanding fits explained.

Abstract: H7/k6 is the standard transition fit for rolling bearing inner rings under normal load. H7/g6 is the typical clearance fit for plain bearings. H7/s6 stands for fixed interference fits (thermal joining). The exact µm values for your size can be found in the tables below.

Basics ISO 286: Four key terms

The ISO 286 system is based on four terms that build on each other:

Nominal dimension: The theoretically exact dimension from the drawing, e.g. ⌀50 mm. It is the reference point for all dimensions, not the actual manufactured dimension.
Fundamental deviation: Determines the position of the tolerance zone in relation to the zero line — whether the dimension starts above or below the nominal dimension. It is determined by the letter.
Tolerance grade (IT grade): Describes the width of the permissible dimensional range. IT1 is extremely narrow (precision metrology), IT18 is very wide (raw casting dimensions). The number in the abbreviation determines the IT grade.
Tolerance zone: The combination of fundamental deviation and IT grade. It is the zone between the largest and smallest permissible dimension.

The abbreviation clearly separates bore and shaft: Capital letter = Bore (inside dimension, A–ZC), Lowercase letter = Shaft (outside dimension, a–zc). The specification H7/k6 means: Bore with tolerance zone H, IT grade 7; shaft with tolerance zone k, IT grade 6.

Letters A to G (bores) / a to g (shafts): Fundamental deviation on the minus side of the zero line → create clearance. Letter H: lower deviation exactly on the zero line (EI = 0) → hole-basis. Letters K to ZC / k to zc: fundamental deviation on the plus side → create interference.

Basic tolerances IT01-IT18

The table shows the basic tolerance values in micrometers (µm) in accordance with ISO 286-1:2010 for four nominal dimension ranges. ⌀10 = range >6-10 mm, ⌀30 = >18-30 mm, ⌀50 = >30-50 mm, ⌀100 = >80-120 mm.

IT grade	Typical use	⌀10	⌀30	⌀50	⌀100
IT01	Gauge blocks, gauges of the highest quality	0.4	0.6	0.6	1
IT0	Gauge blocks, precision gauges	0.6	1	1	1.5
IT1	Precision gauges	1	1.5	1.5	2.5
IT2	Finest fits	1.5	2.5	2.5	4
IT3	Fine fits	2.5	4	4	6
IT4	Fine adjustments (e.g. rolling bearing P5)	4	6	7	10
IT5	Precise fits, roller bearings (k5, m5)	6	9	11	15
IT6	Fits in mechanical engineering Normal (k6)	9	13	16	22
IT7	Standard fits, H7 bore	15	21	25	35
IT8	Rough fits, H8 bore	22	33	39	54
IT9	General fits, h9 shafts	36	52	62	87
IT10	Rough tolerances	58	84	100	140
IT11	H11 bores, rough fits	90	130	160	220
IT12	Rough press fits	150	210	250	350
IT13	Sheet metal parts, stamped parts	220	330	390	540
IT14	General tolerances coarse	360	520	620	870
IT15	Free size tolerances	580	840	1 000	1 400
IT16	Clearance tolerances wide	900	1 300	1 600	2 200
IT17	Raw castings, forgings	1 500	2 100	2 500	3 500
IT18	Raw casting, widest tolerance	2 200	3 300	3 900	5 400

All values in µm. Bold indicates IT5–IT7, which are most relevant for fitting applications in mechanical engineering.

Hole-basis system (H) vs. shaft-basis system (h)

ISO 286 recognizes two system approaches: The hole-basis system keeps the bore tolerance constant (always H) and varies the shaft tolerances (g6, h6, k6, p6 …). The shaft-basis system keeps the shaft tolerance constant (always h) and varies the bore tolerances (F7, H7, K7, P7 …).

In practice, the hole-basis system dominates: standard drilling tools (drills, reamers) automatically provide an H tolerance. A single reamer for H7⌀30 covers all fit types — from clearance fit with g6 to interference fit with s6 — because the adjustment is made exclusively on the shaft by turning or grinding.

The shaft-basis system is recommended if a shaft comes unchanged from the trade (e.g. drawn round steel h11) and several mating partners (bearing, pulley, gear) require different fit types. Then you can vary with different bore tolerances without changing the shaft.

Fitting types

Clearance fit

The tolerance zone of the bore is completely above that of the shaft — there is always positive clearance. Typical pairings: H7/g6 (plain bearings, guide pins), H7/f7 (running shaft in oil bearing), H8/f7 (loosely running hub). The clearance enables relative movement; sufficient lubrication is a prerequisite.

Transition fit

The tolerance zones overlap. Depending on the actual individual dimension, there is either a slight clearance or a slight oversize. Typical pairings: H7/k6 (gear wheel, roller bearing inner ring normal), H7/m6 (clutch hub), H7/n6 (pin connection). Transition fits are always combined with a feather key or pin, as the frictional connection alone is not sufficient.

Press fit

The tolerance zone of the shaft is completely above that of the bore - there is always interference and therefore a force-fit connection. Typical pairings: H7/p6 (bearing inner ring with rotating load, slight interference fit), H7/r6 (bushing tight fit, medium oversize), H7/s6 (fixed hub, large oversize - thermal joining often necessary). Press fits transmit torque without additional shaping elements.

Top 15 standard fits: Clearance and interference in µm

All fits in the hole-basis system. Format: Minimum … Maximum in µm. Positive = clearance (bore > shaft), negative = interference (shaft > bore).

¹ H7/p6 at ⌀10: Maximum +2 µm clearance possible - standard-compliant, to be designed as a press fit.

Fit	Type	⌀10	⌀30	⌀50	⌀100	Typical application
H7/g6	Clearance	+5 … +29	+7 … +41	+9 … +50	+12 … +69	Plain bearing, guide pin
H7/h6	Clearance	0 … +24	0 … +34	0 … +41	0 … +57	Standard shaft, detachable key hub
H7/f7	Clearance	+13 … +43	+20 … +62	+25 … +75	+36 … +106	Running fit, oil bearing
H8/f7	Clearance	+13 … +50	+20 … +74	+25 … +89	+36 … +125	Loose running shaft
H8/h9	Clearance	0 … +58	0 … +85	0 … +101	0 … +141	Loose standard fit, spacer sleeve
H9/h9	Clearance	0 … +72	0 … +104	0 … +124	0 … +174	Rough fit, covers
H11/h11	Clearance	0 … +180	0 … +260	0 … +320	0 … +440	Very rough assembly fit
H7/js6	Transition	−5 … +19	−7 … +27	−8 … +33	−11 … +46	Easily detachable hub, symmetrical transition
H6/k5	Transition	−7 … +8	−11 … +11	−13 … +14	−18 … +19	Precision rolling bearings (P5)
H7/k6	Transition	−10 … +14	−15 … +19	−18 … +23	−25 … +32	Gear, rolling bearing Normal (standard)
H7/m6	Transition	−13 … +11	−21 … +13	−25 … +16	−35 … +22	Clutch hub, fixed hub with spring
H7/n6	Transition	−19 … +5	−28 … +6	−33 … +8	−45 … +12	Pin connection, ring fit
H7/p6	Press	−22 … +2 ¹	−35 … −1	−42 … −1	−59 … −2	Bearing inner ring, rotating load
H7/r6	Press	−25 … −1	−41 … −7	−50 … −9	−76 … −19	Bushing tight fit, medium oversize
H7/s6	Press	−32 … −8	−48 … −14	−59 … −18	−93 … −36	Hub solid, thermal joining

Selection matrix: Application → recommended fit

The following matrix translates common design tasks directly into a fitting recommendation (standard bore system).

Use case	Recommended fit	Type	Note
Rolling bearing inner ring, light/normal rotating load	H7/k6 or H6/k5	Transition	Circumferential load → Seat must remain firm; k5 for P5 bearing
Rolling bearing inner ring, heavy/impact load	H7/m6 or H7/p6	Transition / Press	Larger oversize prevents the inner ring from moving
Rolling bearing inner ring, vertical (point load)	H7/h6 or H7/g6	Game	Can be dismantled; inner ring does not rotate
Pulley, detachable (maintenance)	H7/h6 or H7/js6	Game / Transition	With feather key, washer must be removable
Pulley, fixed (no removal)	H7/k6 or H7/m6	Transition	With feather key; washer sits without play
Gear wheel on shaft, backlash-free	H7/k6 or H7/m6	Transition	Torque transmission via feather key DIN 6885
Clutch hub (detachable clutch)	H7/k6	Transition	Standard for detachable clutches; with feather key
Cylindrical pin (DIN 7)	H7/p6 or H7/r6	Press	Dowel pin can only be released by pressing or driving out
Sliding bushing, replaceable	H7/f7 or H7/g6	Game	Easy to insert and replace, no tools required

Bearing fits according to SKF/INA convention

Rolling bearing manufacturers distinguish between two load cases. Circumferential load (circumferential ring): The ring rotates relative to the load - it must have a tight fit, otherwise it will "wander" and widen the seat. Point load (stationary ring): The load always engages at the same point - slight play is permissible and even desirable so that the ring rotates slowly and the wear is evenly distributed.

Component / load case	Shaft tol. (inner ring)	Housing tol. (outer ring)
Inner ring, normal circumferential load	k5, k6, m5, m6	H7, J7
Inner ring, heavy / impact-type circumferential load	n6, p6	H7
Inner ring, point load (stationary)	g6, h5, h6, js5, js6	H7, H8
Outer ring, normal point load (located in the housing)	–	H7, J7
Outer ring, circumferential load (rotates in the housing)	–	K7, M7, N7

Detailed calculations and installation instructions can be found in the article Shafts and bearings: Understanding fits.

Common errors with ISO 286 fits

Error 1: Tolerance and surface roughness mixed up

IT6 at ⌀50 is 16 µm. If the surface roughness R_z = 16 µm (Ra ≈ 3.2 µm), roughness peaks fill the entire tolerance zone - the fit does not work. Rule of thumb: R_z ≤ 25 % of the IT grade. For IT6 (16 µm) therefore R_z ≤ 4 µm (Ra ≤ 0.8 µm). Also note for press fits: Roughness peaks are smoothed during joining - the effective interference decreases by approx. 0.6 × R_z.

Error 2: Temperature influence on press bracing ignored

Steel expands by approx. 11-12 µm/(m-K), aluminum by approx. 23 µm/(m-K). With a steel shaft in an aluminum hub (H7/s6, ⌀50) and +80 °C operating temperature, the hub expands by approx. 48 µm more than the shaft. This exceeds the nominal minimum interference of 18 µm and can loosen the connection. In the case of a material mix, design the oversize thermally or secure it positively.

Error 3: ISO 2768 instead of ISO 286 for fitting dimensions

ISO 2768 (general tolerances) roughly corresponds to IT12-IT14. If you do not enter an explicit tolerance for a rolling bearing pairing and rely on "ISO 2768 medium", you will get around IT13 ≈ 390 µm at ⌀50 - 15 times as much as IT7. Each mating surface requires an explicit ISO 286 tolerance in the drawing entry.

Error 4: Transition fit without form fit

H7/k6 at ⌀30 has a possible play of up to +19 µm. If the most unfavorable tolerance combination occurs, the hub is seated with minimal frictional connection - or with play. A transition fit must never be designed as the sole means of torque transmission. A feather key, clamping sleeve or clamping element are always necessary.

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