Fits in Practice
Bearing seats, keys and keyways, dowel pins, threads and plain bearings, with the Handbook tables and worked examples.
The fit theory on the other pages comes down to a few jobs that come up again and again: seating a bearing, fitting a key, locating with a dowel, and making a thread go together. Each has its own standard practice, and each is a fit with a particular purpose. This page gives the practical rules, with the Machinery’s Handbook tables transcribed where they fit and ISO limits computed for worked examples. It is part of the Fits & Tolerances guide.
Bearing seats: shafts and housings
A rolling bearing has a thin ring that is carried by a shaft (the inner ring) or a housing (the outer ring). If the ring is too loose on its seat it can creep, wear the seat and overheat; if it is too tight it loses its internal clearance and can seize. The rule, from the bearing makers’ standard (AFBMA, now ABMA Standard 7, and ISO 492), is: the ring that rotates relative to the load gets the tighter fit. The ring that stays stationary relative to the load can be a looser fit and is often axially free to move with thermal growth.
Shaft tolerance for the inner ring
The Handbook’s Table 14 selects the shaft tolerance class for metric radial ball and roller bearings of the ABEC-1 and RBEC-1 tolerance classes. When the inner ring is stationary relative to the load, use g6 if the ring has to slide easily and h6 if it does not. When the load direction rotates with the ring or is indeterminate, the class depends on the load and the shaft diameter:
| Load | Ball bearings | Cylindrical roller bearings | Spherical roller bearings |
|---|---|---|---|
| Light | to 18 mm: h5 over 18 mm: j6 | to 40: j6 · 40–140: k6 · 140–320: m6 · 320–500: n6 · over 500: p6 | to 40: j6 · 40–100: k6 · 100–320: m6 · 320–500: n6 · over 500: p6 |
| Normal | to 18 mm: j5 over 18 mm: k5 | to 40: k5 · 40–100: m5 · 100–140: m6 · 140–320: n6 · 320–500: p6 · over 500: r6 | to 40: k5 · 40–65: m5 · 65–100: m6 · 100–140: n6 · 140–280: p6 · 280–500: r6 · over 500: r7 |
| Heavy | 18–100 mm: k5 over 100 mm: m5 | to 40: m5 · 40–65: m6 · 65–140: n6 · 140–200: p6 · 200–500: r6 · over 500: r7 | to 40: m5 · 40–65: m6 · 65–100: n6 · 100–140: p6 · 140–200: r6 · over 200: r7 |
| Pure thrust | All diameters: j6 | Consult the bearing manufacturer | Consult the bearing manufacturer |
Diameters are the nominal shaft diameter in millimeters (the Handbook gives the same ranges in inches). Footnote: for solid steel shafts; hollow or nonferrous shafts may need tighter fits. Where greater accuracy is required use j5, k5 and m5 instead of j6, k6 and m6.
Housing tolerance for the outer ring
Table 15 does the same for the housing bore (metric ABEC-1 and RBEC-1 bearings), for cast-iron or steel housings; housings of nonferrous alloys may need tighter fits. Where wider tolerances are permissible, use P7, N7, M7, K7, J7 and H7 in place of P6, N6, M6, K6, J6 and H6.
| Outer ring | Loading and conditions | Housing tolerance |
|---|---|---|
| Stationary relative to the load; must be easily axially displaceable | Light, normal or heavy; heat input through the shaft | G7 |
| Light, normal or heavy; housing split axially | H7 | |
| Light, normal or heavy; housing not split axially | H6 | |
| Shock with temporary complete unloading; housing not split (transitional range: the ring may be tight or loose) | J6 | |
| Load direction indeterminate (transitional range) | Light and normal, or normal and heavy; housing not split | J6 |
| Heavy shock; split housing not recommended | K6 | |
| Rotating relative to the load; need not be axially displaceable | Light | M6 |
| Normal and heavy | N6 | |
| Heavy; thin-wall housing, not split | P6 |
Designing the seat
- Seats turned and ground to size. The Handbook quotes the bearing makers: bearing seats and locating shoulders are turned and ground to size with the shaft held on centers, so out-of-roundness and taper are small. Shoulders must give enough contact surface to locate the bearing face accurately.
- Undercuts and radii. Where an undercut is needed for wheel runout in grinding, keep it as small as possible and finish it with a fillet, not a sharp corner, so it does not start a fatigue crack.
- Clamping nuts must cut threads square, and the thread should not run into the bearing seat. Use special bearing locknuts and tongued lock washers, and thread the shaft to a close class (the Handbook quotes American National Form, Class 3).
- No more than two bearings on a shaft where possible; more than two makes accurate alignment very difficult. A weak shaft deflects and misaligns the bearings.
- Housings need metal. Make bores in stiff sections with plenty of metal in the walls. Thin areas distort under the boring tool and under load. Soft and resilient housings (aluminum, magnesium, light sheet metal) lose their fit with differential thermal expansion: the outer ring may loosen and turn, and the only sure remedy is a steel liner or insert.
- Handle bearings gently. Bearings are precision parts: use proper tools and fixtures to press on the ring that carries the fit, never through the rolling elements, and keep them clean.
Keys and keyways
A parallel key carries torque between a shaft and a hub. The key fits into a keyway cut in the shaft and another in the hub, and the fit that matters is the width of the key in its two keyways. Metric keys (ANSI/ASME B18.25.3M, from ISO 773 practice) come in three fit classes, and the keyway tolerances are largely from the ISO system (ANSI B4.2):
| Fit class | Intended for | How it is made |
|---|---|---|
| Normal | Most keyed joints: the key slides into the shaft keyway and the hub goes over it | The key is tight in the shaft keyway (up to a small interference) and a little looser in the hub keyway |
| Close | Joints that are subject to reversing or heavy torque, with no play | The key is a tight fit in both keyways (interference at the tight end) |
| Free | Hubs that slide on the shaft (a gear or clutch that moves along the key) | Clearance in both keyways; the key must not rattle, but the hub must slide |
The Handbook’s Table 4 gives the keyway width tolerances and the resulting fits for each class and key size. For a key width of 20 to 28 mm (the group that includes 22 × 14 and 28 × 16 keys) the keyway width tolerances above the basic size, in millimeters, are:
| Fit class | Shaft keyway width | Hub keyway width |
|---|---|---|
| Normal | +0.085 / +0.035 | +0.110 / +0.060 |
| Close | +0.050 / +0.010 (shaft and hub) | +0.050 / +0.010 |
| Free | +0.135 / +0.085 | +0.200 / +0.110 |
Keyway depth tolerance is +0.2 / 0 for both the shaft and the hub keyway. The Handbook’s table also gives the maximum radius at the keyway corners (0.6 mm for the 22 mm width group, 1.6 mm and 2.5 mm for the wider keys) and the resulting maximum clearance and interference of each fit.
Key size by shaft diameter
The Handbook’s table of metric keyway sizes (it states it is for reference only and the selection of the proper size and type of key rests with the design authority) pairs the shaft diameter with a key width × height:
| Shaft | Key b × h | Shaft | Key b × h | Shaft | Key b × h | Shaft | Key b × h |
|---|---|---|---|---|---|---|---|
| 6–8 mm | 2 × 2 | 8–10 mm | 3 × 3 | 10–12 mm | 4 × 4 | 12–17 mm | 5 × 5 |
| 17–22 mm | 6 × 6 | 22–30 mm | 8 × 7 | 30–38 mm | 10 × 8 | 38–44 mm | 12 × 8 |
| 44–50 mm | 14 × 9 | 50–58 mm | 16 × 10 | 58–65 mm | 18 × 11 | 65–75 mm | 20 × 12 |
| 75–85 mm | 22 × 14 | 85–95 mm | 25 × 14 | 95–110 mm | 28 × 16 | 110–130 mm | 32 × 18 |
The shaft diameter ranges are over the first number up to and including the second. Larger shafts continue to 500 mm (keys to 100 × 50). The Handbook notes the same standard series applies to both square and rectangular parallel keys. Inch series keys follow ANSI B17.1, which gives key sizes and keyseat dimensions in the same way.
- Key length and engagement. Torque is carried by the sides of the key; a longer hub engages more key, so a hub shorter than about 1.5 shaft diameters needs a larger key or a different joint.
- Fillet radii and chamfers. Keyway corners need a radius (and the key a chamfer) to limit stress concentration at the corner of the keyway, which is where shafts fail.
- Keys do not center. A key transmits torque but does not locate the hub; the hub is located by a fit (H7/k6, H7/p6, an LT or FN class) on the shaft.
Dowel pins
Hardened and ground dowel pins are made to a high degree of accuracy and are used to position and align machine components, as a positive stop, to add structural rigidity, or to create a part nest. When parts are assembled with dowels, the dowel is a very close fit with its mating part, so a tool or machine with dowel-pin construction can be taken apart for maintenance and put back together keeping its original positional accuracy. The Handbook’s rules for locating with dowels:
- Press fit in one part, slip fit in the other. A light press fit has 0.0002 to 0.0005 in (about 5 to 13 µm) of interference; a slip fit has 0.0002 to 0.0005 in of clearance.
- Position the dowels as far apart as possible, for rigidity and locational stability. Use two dowels; a third over-constrains the parts.
- Engage twice the dowel diameter in each component. A ¾-inch dowel should contact the hole for ¾ of its length in each component; a ¼-inch dowel for ½ inch.
- Avoid pressing a dowel into a blind hole. If there is no choice, use a pull dowel that has a threaded hole so it can be extracted from the blind hole. Avoid pressing a dowel into a hardened part. Provide a knockout hole.
- Relieve deep holes so only the locating length contacts the pin: the relief is about 0.03 in larger than the dowel (a stepped hole), and it is good practice to relieve any part of the hole not used for locating.
- Match the screws. When dowels are used with socket head cap screws, use the same size (a 3/8 in dowel with a 3/8 in screw). Making the dowel spacing different from the screw-to-dowel spacing prevents the parts being assembled backwards.
The Handbook shows four ways to install a dowel (Options A to D): press fit in the lower plate with a relieved knockout hole below; slip fit in the lower plate (blind hole) and press in the top plate; press fit in the lower plate with the slip fit in the top plate (because a dowel pressed in a blind hole is very hard to remove); or slip fit on both sides when through holes are not allowed.
Threads
Threads have their own fit system, built on the same ideas: a tolerance on each part and, for some classes, an allowance between them. For the Unified (inch) threads the Handbook describes the thread classes as follows. A numeral says how tight the tolerance is; the letter says external (A, a screw or bolt) or internal (B, a nut or tapped hole):
| Class | Applies to | What it is for |
|---|---|---|
| 2A and 2B | External and internal | The most commonly used for general applications, including production bolts, screws, nuts and similar fasteners. Class 2A has an allowance that minimizes galling and seizing in high-cycle wrench assembly and accommodates plating or coating; the maximum diameters of 2A threads are less than basic by the allowance. Class 2B has no allowance (its minimum diameters are basic) |
| 2AG | External | For rapid assembly with a lubricant, or for residual growth from high-temperature expansion, when a coated thread must keep its allowance: the class symbol is followed by G |
| 3A and 3B | External and internal | Closer tolerances than 2A and 2B, for when a tighter fit is wanted. Class 3A has no allowance (its maximum diameters are basic), and 3B minimum diameters are basic |
| 1A and 1B | External and internal | Ordnance and other special uses where quick and easy assembly is necessary, and where liberal allowances are required to permit ready assembly even with slightly bruised or dirty threads. Class 1A has an allowance |
- Allowance for coatings. Only Class 2A has a built-in allowance to take up plating. The standard says to keep threads inside their limits after plating: external threads should not exceed basic size after plating, and internal threads should not be below basic size after plating. To accept a plated 2A thread, the Handbook suggests a basic Class 3A GO gage and a Class 2A NOT-GO gage. Class 3A does not include an allowance, so reduce its limits by the 2A allowance before plating.
- Combinations. The requirements depend on end use; they are met by specifying the proper combination of classes. For example, a Class 2A external thread may be used with a Class 1B, 2B or 3B internal thread.
- Length of engagement. The standard pitch diameter tolerances apply to a length of engagement up to 1½ diameters for the coarse and standard fine series; the extra-fine and special series are based on engagement of 9 pitches.
- Designation. Give the nominal size, threads per inch, series, class and gaging system: for example ¼-20 UNC-2A (21), or 10-32 UNF-2A (22).
- Metric threads have an equivalent system of tolerance classes (the ISO general-purpose pair is 6H for the nut and 6g for the bolt). Check the Handbook’s metric thread section and the standard for the limits.
Plain bearings and sliding fits
A plain (journal) bearing runs on a film of oil, and its fit is a clearance fit chosen for speed, load and temperature. The inch RC classes and the metric H8/f7 and H9/d9 fits were made for this. Use these as starting points:
| Duty | Inch class | Metric fit |
|---|---|---|
| Accurate location, no perceptible play, slow | RC 1, RC 2 | H7/g6 |
| Precision running at slow speed and light pressure | RC 3 | H7/f6 |
| Accurate machinery, moderate speed and journal pressure | RC 4 | H8/f7 |
| Higher speeds or heavy journal pressure | RC 5, RC 6 | H8/e7, H9/e8 |
| Large temperature variation, or accuracy not essential | RC 7 | H9/d9 |
| Wide commercial tolerances | RC 8, RC 9 | H11/c11 |
The inch classes are in Fits – Inch (ANSI B4.1) and the metric fits in Fits – Metric (ISO System). The pairing is approximate; check the clearance limits for your size. Running clearance must also allow for thermal expansion: see Temperature, Coatings and Fit Problems.
Worked examples
Example 1: a bearing seat on a 40 mm shaft
A deep-groove ball bearing is mounted on a 40 mm shaft; the inner ring rotates relative to the load and the load is normal. From the table: ball bearings over 18 mm, normal load: k5.
- IT5 for 30 to 50 mm is 11 µm. For k, the lower deviation is +2 µm, so k5 is +13 / +2 µm: the shaft is 40.002 to 40.013 mm.
- A normal-class bearing’s bore for this size is typically 40.000 down to 39.988 mm (ISO 492: 0 / −12 µm; check the catalog).
- Interference with the bore: smallest 2 µm, largest 25 µm. A light interference at the loose end keeps the ring from creeping; the tight end must be small enough that the ring does not lose its internal clearance.
Example 2: a housing bore for a 90 mm outer ring
The outer ring is stationary relative to the load, the load is normal and the housing is split axially, so Table 15 gives H7.
- IT7 for 80 to 120 mm is 35 µm, so the housing bore is 90.000 to 90.035 mm.
- A normal-class outer ring for this size is typically 90.000 down to 89.985 mm (0 / −15 µm).
- The ring is a clearance fit in the housing: from 0 to 50 µm. It can float axially to take up thermal growth, as the table intends.
Example 3: a key on a 30 mm shaft
A 30 mm shaft falls in the 22 to 30 mm range (over 22 up to and including 30), so the Handbook’s table gives a key of 8 × 7 mm: the keyways are 8 mm wide. For a normal fit the shaft keyway width is the basic 8 mm plus a tolerance above it, and the hub keyway a little wider; the exact values come from the Handbook’s Table 4 for the 8 mm width. Give the keyway depth with its +0.2 / 0 tolerance, and dimension the depth from the opposite side of the shaft or hub so that it can be measured.
Example 4: dowel hole sizes
A 3/8 in (0.3750) dowel is to locate two plates, press fit in the lower and slip fit in the upper, using the Handbook’s 0.0002 to 0.0005 in rule (ignoring the pin’s own diameter tolerance for this illustration).
- Press fit hole (lower plate): 0.3745 to 0.3748 in (0.0002 to 0.0005 in interference).
- Slip fit hole (upper plate): 0.3752 to 0.3755 in (0.0002 to 0.0005 in clearance).
- Engagement: twice the diameter = 0.75 in in each component, so the plates must be at least that thick at the dowel, or the hole must be relieved around the pin.
Troubleshooting
| Symptom | Likely cause | Try |
|---|---|---|
| Bearing runs hot soon after assembly | Fit too tight, or seat out of round, so the ring lost its internal clearance | Check the seat roundness and size; use the next looser class; check the bearing’s internal clearance group |
| Inner ring creeps (a polished or fretted shaft seat) | Fit too loose for the load, or the shaft seat is undersized | Use a tighter class (k, m, n); restore the seat; lock axially |
| Outer ring spins in the housing | Soft or resilient housing lost its fit as it expanded | Fit a steel liner; use a tighter housing class; check thermal growth |
| A key rattles or the hub moves on the shaft | Free fit used where a normal or close fit was needed, or keyway worn | Use a normal or close fit; add an interference fit on the shaft for centering |
| Dowel will not come out of a blind hole | Press fit in a blind hole with no knockout or pull feature | Use a pull dowel with a threaded hole, or provide a knockout hole |
| A plated thread will not assemble | Plating thickness used up the 2A allowance | Allow for plating in the limits before coating; gage as the Handbook suggests |
Related
- Fits & Tolerances: the overview and quick reference.
- How Fits Work: the three fit types.
- Fits – Metric (ISO System): computing the limits for any size.
- Press, Shrink and Expansion Fits: interference fits for hubs and sleeves.
- Temperature, Coatings and Fit Problems: why fits change with temperature.
- Tap Drill Size & Thread Pitch Lookup: tap drills for common threads.
FAQ
Why does the rotating ring get the tighter fit?
A ring that rotates relative to the load sees the load move around its circumference, so it is flexed and can creep on its seat if it is loose. The stationary ring sees a steady load in one place and does not creep, so it can be looser and axially free.
Can I use the ISO shaft classes for inch bearings?
Yes. The Handbook’s Table 14 gives the same classes for metric and inch bore bearings (in separate diameter columns), and the AFBMA/ABMA tables give the numerical limits for both.
Which keyway fit is the usual choice?
The normal fit for most joints. Use the close fit for reversing or heavy torque and the free fit only when the hub has to slide along the key.
Should I use two dowels or three?
Two, as far apart as practical. A third dowel over-constrains the assembly and makes it hard to put together as the parts are made, and it is the screws that clamp the parts, not the dowels.
What do the A and B in thread classes mean?
A is an external thread (a bolt or screw) and B is an internal thread (a nut or tapped hole). The numeral 1, 2 or 3 sets the tolerance from loosest to tightest. Class 2A/2B is the usual general-purpose pair.
Do I need to change the fit for a plated thread?
Class 2A has an allowance built in for coatings. The other classes do not, so adjust the limits before plating, as the standard describes.
References
- Machinery’s Handbook, 31st edition, Industrial Press: “Ball and Roller Bearings” (Tables 14 and 15; AFBMA/ABMA 7), “Mounting Bearings”, “Metric Keys and Keyways” (ANSI/ASME B18.25.3M), “Dowel Pins” (assembly with dowel pins) and “Unified Screw Threads” (thread classes).
- ABMA Std. 7, Shaft and Housing Fits for Metric Radial Ball and Roller Bearings; ISO 492, Rolling bearings: radial bearings: tolerances.
- ANSI/ASME B1.1, Unified Inch Screw Threads; ANSI/ASME B17.1, Keys and Keyseats.
Last reviewed: October 2026. The bearing-seat tables are transcribed from the Handbook; for a critical application, confirm with the bearing manufacturer’s catalog.