A tapered roller bearing is the only common radial bearing that is deliberately built to take heavy radial and heavy axial load at the same time. The trick is the cone angle: the rollers are truncated cones that run on conical raceways, so the extended axes of all rolling elements meet at a single point on the shaft centreline. That geometry lets the bearing support a combined load without scrubbing, but it also means the bearing must be paired or adjusted, because the cone and the cup slide apart unless something holds them together.
This guide covers the part that buyers and fitters get wrong most often: reading the designation. It explains how the series digits encode the ISO 355 envelope, why a 32205 is not simply a wider 30205, how inch-series prefixes work, how to set axial clearance or preload on assembly, and how to decode a part number back to dimensions and capacity.
- The designation encodes an ISO 355 envelope. The first digit is the type, the second is the diameter series and the third is the width series, so 30205 and 32205 share a bore and set a different width.
- Cone and cup are a matched pair. A tapered roller bearing is supplied as an inner cone assembly and an outer cup, and mixing a cone from one series with a cup from another never fits correctly.
- Contact angle sets the axial share. A steeper, wider design carries more thrust for the same radial load; the metric series spans roughly 12 to 16 degrees of contact angle.
- Assembly setting decides life. Tapered rollers run correctly only when the axial clearance or preload is set to a value, typically a few tens of micrometres of end play or a defined starting torque.
Why the cone angle decides everything
Roll a cylinder along a flat surface and it travels in a straight line. Roll a cone and it tries to turn towards its apex, because the circumference is larger at the wide end than at the narrow end and both ends must complete the same number of revolutions in the same time. A tapered roller bearing uses exactly that behaviour: the apex of every roller points to the same spot on the shaft axis, so the rollers, the cone raceway and the cup raceway all turn about that common point without sliding against each other.
The angle of the cone therefore sets how much of the load is carried radially and how much axially. A shallow angle, like the 2 series in the table below, is close to a cylindrical roller in behaviour and takes most of its load radially. A steeper angle shifts a greater share of the load into the axial direction, which is why steep tapered bearings appear in wheel hubs and pinions where both load components are large. The often-quoted e factor in a catalogue is simply the load ratio at which the axial component starts to dominate the equivalent load calculation.
How to read a metric designation
Metric tapered roller bearings follow ISO 355, and the designation is a compact description of the envelope. Take 32205 and split it.
The series digits
The leading digit 3 is the bearing type for a single-row metric tapered roller bearing. The next digit 2 is the diameter series, which fixes the outside diameter for a given bore. The final digit 2 is the width series, which fixes how wide the assembly is and, because the cup is wider, how much load the design can carry. Only the last two digits, 05, are the bore code: multiply by 5 to get 25 mm. Used together, the digits name a standard envelope rather than a brand-specific part.
The width-diameter dimensions
A tapered roller bearing has more dimensions than a ball bearing because the cone and cup do not share a single width. The assembled width T, the cone width B and the cup width C are all recorded separately, and the small end of the cup is bored to a different diameter than the large end. Any cross-reference that lists only bore and outside diameter is incomplete, because the assembled width and the cup angles are what actually seat the bearing. The designation system itself is standardised by ISO 10317, and many European catalogues still list the older German standard DIN 720 alongside the ISO 355 envelope.
| Designation | Bore d | Outside D | Assembled width T | Typical C |
|---|---|---|---|---|
| 30205 | 25 mm | 52 mm | 16.25 mm | About 33 kN |
| 32205 | 25 mm | 52 mm | 19.25 mm | About 43 kN |
| 30305 | 25 mm | 62 mm | 18.25 mm | About 52 kN |
| 32305 | 25 mm | 62 mm | 25.25 mm | About 63 kN |
Read the table in two pairs. The 30205 and 32205 share a 25 mm bore and a 52 mm outside diameter, yet the 32205 is 3 mm wider and carries noticeably more load, because the wider assembly takes a longer roller. The 30305 and 32305 step up to a 62 mm outside diameter and follow the same logic. Our tapered roller range records the cone width, cup width and assembled width on every listing, and the designation system guide covers the bore and series rules in full.
The cone-and-cup pairing convention
A tapered roller bearing is a pair of components, not one unit. The inner assembly is the cone: the inner ring, the roller set and the cage pressed together as a single replaceable item. The outer ring is the cup. They are manufactured to a matched set, and the pairing is what gives the bearing its running accuracy.
Why 32205 is not a wider 30205
Although both designations describe a 25 mm by 52 mm envelope, the cup angles differ. The 32205 cup is a different part with a different large-end diameter and width. Pressing a 30205 cone into a 32205 cup does not produce a 32205: it produces a bearing with the wrong contact angle, wrong clearance and no guarantee of life. Always order and fit the cone and cup as the complete designation, and if only one component is worn, replace both as a matched pair.
Inch-series exceptions
The inch series is a parallel universe with its own prefixes. Designations such as LM, HM, JM and the many four- and five-digit numbers are dimensioned in inches and are not covered by the metric ISO 355 envelope. They remain common in automotive, agricultural and older American machinery, so a cross-reference request that arrives with an LM prefix must be answered from inch tables and not by converting to millimetres. The converted figure will not match the tolerance bands, which are defined on the original inch units. Our interchange lookup handles both families and flags where a metric part is not a valid substitute.
Contact angle versus axial capacity
The single most useful ratio when comparing tapered designs is the balance between radial and axial capacity. As the contact angle steepens, the bearing trades radial capacity for axial capacity at roughly the rate shown below. These are representative figures for the 25 mm bore class and vary by manufacturer.
| Designation | Contact angle (approx.) | Typical C | Axial share |
|---|---|---|---|
| 30205 | 13 degrees | About 33 kN | Light thrust, mainly radial |
| 32205 | 15 degrees | About 43 kN | Moderate thrust, balanced |
| 30305 | 12 degrees | About 52 kN | Light thrust, heavy radial |
| 32305 | 15 degrees | About 63 kN | Substantial thrust both ways when paired |
Modern catalogue ratings have also been revised upwards by improved steel cleanliness and heat treatment. A bearing that used to be rated at a given C may now carry 10 to 20 percent more in a current catalogue, which is one reason a cross-reference must use the current ratings from the chosen brand rather than an old handbook. The life rating guide shows how the rating feeds into a calculated L10 life.
Setting axial clearance or preload
Tapered rollers only run correctly when the axial setting is right, because the cone, cup and rollers form a wedging system. Left loose, the rollers slide instead of rolling and the bearing fails early. Left too tight, the rollers are squeezed and the bearing runs hot. The correct setting sits between the two and is a measured value, not a feel.
End play and torque methods
Two methods dominate assembly. In the end play method, the axial float of the shaft is measured with a dial indicator while the adjusting nut or shim is varied, and a target of a few tens of micrometres of float is held for a running position. In the torque method, the nut is tightened until the shaft can just be turned against a specified starting torque, which sets a controlled preload. The end play method is simpler and standard for gearbox and axle positions; the torque method is used where stiffness matters and a small preload is desirable. Either way, the value must be set cold and then rechecked warm, because the shaft grows faster than the housing and the setting changes with temperature.
Mounting in pairs: DB, DF and DT
Because a single tapered roller takes thrust in one direction only, most applications use two bearings in one of three arrangements. The letters describe how the cone apexes point relative to each other.
| Arrangement | Axial behaviour | Typical use |
|---|---|---|
| Back-to-back (DB, O layout) | Wide support base, accepts both directions, tolerates moderate misalignment | Gearbox shafts, wheel hubs |
| Face-to-face (DF, X layout) | Narrow base, less tolerant of misalignment but easier to preload | Short rigid shafts, some pump positions |
| Tandem (DT) | Same direction only, doubles one-direction axial capacity | Very high thrust in one direction |
The back-to-back layout is the most common because the effective spread between the two bearing centres is large, which holds the shaft squarely and limits deflection. Face-to-face packages have a narrower spread and are chosen when the shaft is short or when the housing must be compact. Tandem pairs are used where the thrust is high and one-directional, and they must be paired with a separate bearing that handles the reverse direction.
Step-by-step selection by load case
The table below maps a load case to the most economical family, and the numbered sequence that follows turns it into a decision you can defend.
| Load case | Recommended family | Why |
|---|---|---|
| Heavy radial, no thrust | Cylindrical roller (NU or NJ) | Higher radial capacity, no axial restraint needed |
| Radial plus one-direction thrust | Single tapered roller | Cone angle carries both components |
| Radial plus thrust both directions | Two tapered rollers (DB or DF) | Each bearing takes one direction |
| Very high one-direction thrust | Tandem pair plus locator | Two bearings share one direction |
| Shock load with misalignment | Spherical roller | Self-aligning geometry absorbs deflection |
- Estimate the radial and axial components of the load at the bearing position, including the axial forces generated by the cone angle itself, and combine them into an equivalent load.
- Match the family to the load case using the table above, then choose the arrangement (DB, DF or DT) that restrains the shaft correctly.
- Decode the designation to the ISO 355 envelope and confirm the bore, outside diameter and assembled width against the drawing and the seat width in the housing.
- Set the axial clearance or preload from the machine builder's figure, or from a first-principles calculation, and verify it cold and warm.
- Check lubrication at the rib contact and confirm the grease or oil supplies the film thickness the speed demands, using the grease selection guide for the thickener and base oil choice.
Common mistakes with tapered roller bearings
A tapered roller bearing is easy to specify and easy to fit incorrectly. These are the recurring errors.
- Mixing cone and cup from different series. A 30205 cone in a 32205 cup is a wrong-angle bearing, not a substitute. Order the complete designation and keep the pair together.
- Ignoring the seat width. The assembled width T is greater than the sum of the nominal ring widths, and a housing machined for a narrower cup will not seat the wider assembly.
- Setting no end play. Fitting a tapered roller with the nut simply tightened to feel leaves the rollers sliding and flat-spotting within hours. Measure the end play or the starting torque.
- Assuming a metric substitute for an inch part. An LM or HM designation is dimensioned in inches; converting to millimetres fails the tolerance bands. Use the inch tables.
- Using an old rating. Current catalogues rate improved designs higher; cross-referencing on a previous C value understates or overstates life. Use the chosen brand's current figure.
- Neglecting shaft and housing squareness. The cone and cup must sit square to the thrust line, and a cocked cup edge-loads the rollers even when the end play is correct.
If the number alone is all you have, the bearing selector resolves designations to dimensions, and the TIMKEN and FAG listings show how the metric and inch families are presented across brands. Where the duty is too severe for a tapered roller, the spherical roller guide and the cylindrical roller guide cover the alternatives.
Frequently asked questions
How do I read a tapered roller bearing number such as 30205?
The 3 is the type for a single-row metric tapered roller bearing, the 2 is the diameter series, the 0 is the width series and the final 05 is the bore code. Multiply the bore code by 5 for a 25 mm bore. The digits together define an ISO 355 envelope, not a brand-specific size.
What is the difference between 30205 and 32205?
They share a 25 mm bore and a 52 mm outside diameter but differ in width series. The 32205 is about 3 mm wider, takes a longer roller and carries more load. The cup angles differ too, so a 30205 cone cannot be fitted into a 32205 cup as a substitute.
Are tapered roller bearings sold as a pair?
A single tapered roller bearing is supplied as a matched cone assembly and cup. They are manufactured as a set and must be fitted together. If one component is worn, replace both as the complete designation, because mixing cones and cups from different sets destroys the contact angle and clearance.
How do I set the axial clearance on a tapered roller bearing?
Either measure the axial end play with a dial indicator and hold a target of a few tens of micrometres of float, or tighten the adjusting nut to a specified starting torque that sets a small preload. Set the value cold, then recheck it warm, because the shaft expands faster than the housing.
Why does a tapered roller bearing need two bearings?
A single tapered roller takes thrust in one direction only, because the cone and cup would separate under reverse load. Most applications use two bearings back-to-back, face-to-face or in tandem, so that each direction of axial load is carried and the shaft is located.
What are LM and HM prefixes on tapered roller bearings?
They mark the inch series, which has its own designation system and is dimensioned in inches rather than millimetres. LM, HM and similar prefixes are common in automotive and agricultural machinery. Convert them using inch tables, not by converting the number to millimetres, because the tolerance bands are defined on inches.