Gearbox Bearing

A gearbox bearing holds a shaft that the gear mesh pushes sideways and, with helical, bevel or worm gears, along its axis as well. Industrial gearboxes and speed reducers handle those loads with two bearing types: single row deep groove ball bearings from the 6200 and 6300 series, and tapered roller bearings from the 30200, 30300 and 32200 series. Which one sits in a given position depends mainly on how much axial load the gears produce. Below we cover the choice, the sizes by shaft diameter, clearance and lubrication, the usual failure causes, and what to send with an inquiry.

Which bearing goes where in a gearbox

The gear type sets the load on each shaft, and the load sets the bearing.

• Spur gears. The teeth are parallel to the shaft, so the mesh force is almost entirely radial. Deep groove ball bearings handle this well.
• Helical gears. The angled teeth create an axial force that grows with the torque and the helix angle. Deep groove ball bearings take moderate axial load in both directions. When the thrust is larger, the designer moves to tapered roller bearings in pairs.
• Bevel gears. The pinion shaft sees radial and axial load together and has to be held precisely so the gears mesh correctly. Tapered roller bearings in a pair are the normal choice.
• Worm gears. The worm shaft carries the reaction from the wheel as a large axial force. Tapered roller bearings are standard here, and steep-angle sizes such as the 31307 bearing are built for that kind of thrust. Where a slow shaft carries axial load only, a thrust ball bearing next to a radial bearing is another option.

Shaft position	Typical load	Common bearing choice
Input shaft, spur or helical	Light radial, some axial, highest speed	6200 or 6300 deep groove ball bearing, open
Intermediate shaft, helical	Radial plus axial from two gear meshes	Tapered roller pair (30200 or 32200), or deep groove if the thrust is small
Output shaft	Heavy radial, low speed	6300 deep groove, 32200 tapered pair, or a roller bearing in heavy units
Worm shaft	High axial	Tapered roller pair, often 30300 or steep-angle 31300
Bevel pinion shaft	Combined, needs precise location	Tapered roller pair, adjusted

Motors that drive gearboxes use the same 6200 and 6300 sizes; the electric motor bearing page covers them. Agricultural gearboxes for rotary tillers, mowers and feed mixers follow the same logic. They usually combine deep groove ball bearings on lighter shafts with tapered roller bearings where the bevel gears sit, and they work in dust, so the shaft seals matter as much as the bearings.

Deep groove vs tapered roller bearing in a gearbox

A deep groove ball bearing runs with low friction, suits high speed and needs no adjustment. One bearing can locate the shaft in both axial directions, which keeps the design simple. Its limit is load: the balls touch the raceways at points, so for the same bore it carries less than a roller bearing.

A tapered roller bearing carries radial and axial load at the same time, with a higher load rating for the same bore. Each bearing takes axial load in one direction only, so tapered bearings are mounted in opposed pairs, either back to back or face to face. The pair is adjusted during assembly with shims, spacers or a nut until the shaft has the endplay or preload the designer specified. The cup and cone come apart, which makes assembly easier but means the setting is in the hands of whoever builds or repairs the gearbox.

One detail that surprises buyers: for the same bore code, a 30200 tapered bearing has the same outside diameter as the 6200 ball bearing, and a 30300 matches the 6300. A housing bored for a 6206 bearing will accept the cup of a 30206 bearing. That does not make them interchangeable. The tapered bearing is wider, needs an opposed partner and needs a way to set the endplay. Swap types only on the gearbox maker’s drawing. The ball bearing vs roller bearing guide covers the trade-offs in more detail.

Gearbox bearing sizes by shaft diameter

The bore equals the shaft diameter at the bearing seat. The table lists the deep groove and tapered roller sizes FULI supplies for shafts from 20 mm to 60 mm. For the tapered bearings, T is the total width with cup and cone assembled.

Shaft d (mm)	Deep groove options	30200 tapered	d x D x T (mm)	Cr (kN)	32200 tapered	d x D x T (mm)	Cr (kN)
20	6204, 6304	30204	20 x 47 x 15.25	28.20	32204	20 x 47 x 19.25	30.30
25	6205, 6305	30205	25 x 52 x 16.25	32.20	32205	25 x 52 x 19.25	41.30
30	6206, 6306	30206	30 x 62 x 17.25	on request	32206	30 x 62 x 21.25	on request
35	6207, 6307	30207	35 x 72 x 18.25	54.20	32207	35 x 72 x 24.25	70.60
40	6208, 6308	30208	40 x 80 x 19.75	63.00	32208	40 x 80 x 24.75	77.90
45	6209, 6309	30209	45 x 85 x 20.75	67.90	32209	45 x 85 x 24.75	80.70
50	6210, 6310	30210	50 x 90 x 21.75	73.30	32210	50 x 90 x 24.75	82.80
55	6211, 6311	30211	55 x 100 x 22.75	90.80	32211	55 x 100 x 26.75	108.00
60	6212, 6312	30212	60 x 110 x 23.75	103.30	32212	60 x 110 x 29.75	132.80

Cr is the basic dynamic load rating. Values from FULI’s catalog; confirm for your application. The 30206 and 32206 load ratings are being checked against FULI’s catalog. Ask for them with the quote.

A 32200 bearing fits the same housing bore as the 30200 bearing with the same bore code but is wider, so it has longer rollers and a higher load rating. The full dimension set for each size is on its model page in the tapered roller bearing range; the ball bearing sizes are in the deep groove ball bearing range. If the marking has worn off, measure the bore, the outside diameter and the width, and check them in the bearing size finder.

Older gearboxes, and many built in North America, use inch tapered bearings where the cone and cup carry separate numbers. Read both. The Timken bearing cross reference lists the inch cone and cup numbers on this site.

Clearance and endplay in gearbox bearings

Deep groove ball bearings. Internal clearance shrinks when the inner ring is pressed onto the shaft and again when the gearbox warms up, because the shaft and inner ring run hotter than the housing. In a gearbox with interference fits and oil temperatures well above ambient, C3 is common. Keep the clearance class of the original bearing and write it in the model, for example 6308 C3. Our guide to bearing internal clearance explains how the groups work.

Tapered roller bearings. They have no C-class. Their running clearance is created during assembly by the axial position of one bearing against the other. Too tight, and the bearings run hot and wear quickly. Too loose, and the shaft moves under load, the gear contact pattern shifts and the teeth and bearings both suffer. Measure the endplay with a dial indicator after assembly and set it to the gearbox maker’s value. If you only replace one bearing of a pair, check the setting again, because a new cone and cup will not sit exactly where the worn ones did.

Lubrication: oil-bath gearbox bearings are usually open

In most gearboxes the bearings share the gear oil, either sitting in the bath or catching the splash. They need to be open so the oil can reach the rolling elements. A 2RS bearing in an oil bath keeps the oil out and runs on whatever grease it was filled with. Order a shielded or sealed bearing only where the original had one, for example a vertical shaft position fed with grease or a bearing with a shield to keep debris out.

Tapered roller bearings normally leave the factory with rust protection only and are lubricated when they are installed. Our factory follows the same practice: tapered bearings are rarely pre-greased, because the gearbox oil or the grease specified for the machine goes in at assembly.

The oil also carries wear particles from the gear teeth. Those particles get rolled into the raceways and leave dents that later grow into spalling. Changing the oil on the gearbox maker’s schedule and checking a magnetic drain plug does more for bearing life than any bearing upgrade. For grease-lubricated positions, the bearing lubrication basics article covers fill quantities.

Why gearbox bearings fail early

When a bearing fails early in a gearbox that has run well before, look outside the bearing first. These are the common patterns:

1. Contaminated oil. Fine dents across the raceway, then flaking. Look at the oil and the drain plug before you order.
2. Wrong endplay on tapered pairs. Set too tight, the bearings overheat and the oil darkens. Set too loose, you hear noise and see uneven gear wear.
3. Worn housing or shaft seats. If the outer ring has turned in the housing, the bore is polished or scored. A new bearing in a worn seat fails the same way.
4. Low oil level or wrong viscosity. A bearing high up in the housing may run nearly dry if the level is low.
5. Damage during fitting. Driving a bearing on by hitting the outer ring, or the cage, marks the raceways before the gearbox ever turns. Press on the ring that has the interference fit.
6. A lower grade than the gearbox was designed for. A bearing with wider tolerances or a noisier grade in a precision reducer runs rough and loud from the start. Match the precision class and any noise grade of the original.

If a bearing fails twice in the same position, check the seat, the endplay and the oil before you change the bearing size or the supplier.

How to write a gearbox bearing inquiry

An inquiry with these items can usually be priced in one round:

1. The full number with prefixes and suffixes, for example 6308 C3, 6207-Z or 32210.
2. For tapered bearings, whether you need the complete bearing, the cone only or the cup only. For inch sizes, both numbers.
3. The quantity for each model.
4. The gearbox type and the position: input, intermediate, output, worm or pinion shaft.
5. How the bearing is lubricated: oil bath, splash or grease.
6. A photo of the old bearing and its markings, or the drawing if you build the gearbox.
7. Any marking or packaging requirement.

Send your model numbers and quantities to sales@fuli-bearings.com or through the contact page. The quotation request template has a copy-ready format. We check the numbers against the standard designations, mark any item we cannot meet as an exception on the specification sheet rather than substituting it, and send the quote. Rings and rolling elements are GCr15 chrome bearing steel, and the steel maker’s material certificate is available on request for OEM and industrial orders. An inspection report can cover dimensions, radial clearance, rotation noise, vibration, appearance, material certificate and batch traceability. Export documents are the commercial invoice, packing list and CCPIT certificate of origin, plus an FTA Form COO where the destination country qualifies.

Backed by 20+ years of bearing export experience in Linqing, Shandong. FULI ships to 50+ countries & regions.