Four-point contact ball bearings – QJ bearings for axial loads in both directions

What are four-point contact ball bearings and what are they used for?

A four-point contact ball bearing is a single-row radial ball bearing with specially shaped raceways. The raceways of the inner and outer rings are formed by two arcs, so under load each ball contacts each ring at two points.
This geometry allows axial loads in both directions to be transmitted by a single bearing. The inner ring is split into two halves, which allows a larger number of balls to be fitted and therefore provides a higher load carrying capacity.
Four-point contact ball bearings are used mainly:

• as an axial bearing in combination with a roller bearing that takes up the radial load,
• in gearboxes and industrial gear units,
• in compressors and pumps,
• in drives for ships, wind turbines and heavy machinery,
• in bearing positions with limited space in the axial direction,
• in thin-section designs for robotics, medical and measuring equipment.

Main features of four-point contact ball bearings

Four-point contact bearings combine high axial load carrying capacity with a small mounting width.
Key features include:

• transmission of axial loads in both directions with a single bearing,
• a contact angle of 35°, which provides high axial load carrying capacity,
• a small mounting width compared with a pair of angular contact bearings,
• a split inner ring that allows a larger number of balls,
• precise axial location of the shaft,
• designs with slots that prevent the outer ring from rotating,
• brass or polyamide cages.

Four-point contact bearings are designed mainly for axial loads. When a predominantly radial load acts on the bearing, the balls contact the rings at several points simultaneously, which increases friction and heat generation. For this reason, they are combined with a roller bearing where radial loads are higher.

Four-point contact bearings in the M&M Intercom range

The range consists mainly of bearings with the designation QJ, which for most manufacturers is the standard designation for a four-point contact ball bearing.
Bearings are available in three dimension series:

Series	Example	Feature
QJ 2	QJ 212, QJ 216	medium cross-section, the most common design
QJ 3	QJ 310, QJ 318	larger cross-section and higher load carrying capacity
QJ 10	QJ 1012, QJ 1040	smaller cross-section for limited space

The vast majority of products in the category are made by FAG and also carry the suffix XL for the X-life design. Individual bearings from other manufacturers are also available, including the older designations Q 206, Q 213 and Q 220.

QJ bearing suffixes

The suffixes in a bearing designation define the design of the cage, the outer ring and the clearance. When replacing a bearing, it is important to keep the same suffixes as on the original bearing.
The most common suffixes on FAG bearings are:

• MPA – solid brass cage, guided on the outer ring,
• TVP – cage made of glass-fibre reinforced polyamide,
• N2 – two slots in the outer ring that prevent the ring from rotating in the housing,
• C3 and C4 – increased bearing clearance,
• XL – X-life design.

For example, the designation QJ 216 XL-N2-MPA-C3 denotes a four-point contact bearing of series 2 with an 80 mm bore, two slots in the outer ring, a brass cage and increased clearance.

Anti-rotation slots – N2 design

When a four-point contact bearing is used as a pure axial bearing, the outer ring is often mounted in the housing with radial clearance. In this way, the bearing does not take up radial loads, which are carried by the roller bearing.
To prevent the outer ring from rotating in this case, the N2 design has two slots on its face. Pins or a dowel in the housing engage in these slots and hold the ring in place.

Combination with a roller bearing

The most common use of a four-point contact bearing is in combination with a cylindrical roller bearing. The roller bearing takes up the radial load, while the four-point contact bearing provides axial location of the shaft in both directions.
This combination is used in gearboxes, compressors and drives where radial loads are high and axial location must be precise. You will find suitable roller bearings in the cylindrical roller bearings category.

Thin-section four-point contact bearings

The range also includes thin-section four-point contact bearings, for example designs with the designations CSXA, CSXB or SA. These bearings have the same cross-section regardless of the bore size, which enables very light and compact designs.
Thin-section bearings are used in robotics, measuring devices, medical equipment and other applications where low weight and minimal space are important. For larger rotating connections, slewing rings are also available.

Mounting four-point contact bearings

The inner ring of a four-point contact bearing is split into two halves. During mounting, care must be taken that the halves stay together in the correct order and are not mixed up with parts of another bearing.
The outer ring with the split inner ring and the balls can be mounted separately, which makes mounting easier. Suitable bearing tools are used for mounting and dismounting, and bearing locknuts for axial fastening.

Lubrication of four-point contact bearings

Four-point contact bearings are most commonly oil-lubricated, particularly in gearboxes and gear units. In grease-lubricated applications, it is important that the bearings are relubricated regularly in accordance with the machine manufacturer's instructions.
Suitable greases are available in the greases and pastes category.

How to choose the right four-point contact ball bearing?

When selecting a bearing, first determine whether it will carry mainly axial loads or also part of the radial load. Then determine the dimensions and design.
Important selection data include:

• bore diameter, outside diameter and bearing width,
• magnitude of the axial and radial loads,
• rotational speed,
• mounting arrangement: on its own or in combination with a roller bearing,
• need for N2 anti-rotation slots,
• cage material,
• required clearance,
• lubrication method and operating temperature.

When replacing a bearing, the most reliable information is the full designation of the existing bearing, including the suffixes for the cage, slots and clearance. Where higher radial load carrying capacity is required together with axial location, an angular contact ball bearing is more suitable. For purely axial loads in one direction, thrust ball bearings are also an option.

Four-point contact ball bearings from M&M Intercom

At M&M Intercom we offer 190+ four-point contact ball bearings for gearboxes, compressors, pumps and heavy-duty drives. The range includes bearings in the QJ 2, QJ 3 and QJ 10 series with brass or polyamide cages, with or without N2 slots and with increased clearance, as well as thin-section designs.
Bearings from the manufacturer FAG INA and other manufacturers are available. For a complete bearing arrangement, the range also includes cylindrical roller bearings, bearing housings and bearing accessories.
If you need help selecting or replacing a four-point contact bearing, send us the designation of the existing bearing or details of the application. We will help you choose the right design.

 

Frequently asked questions about four-point contact ball bearings

1. What is a four-point contact ball bearing?
A four-point contact ball bearing is a single-row ball bearing in which each ball contacts the rings at four points. As a result, it carries high axial loads in both directions.
2. What does the designation QJ mean?
For most manufacturers, the designation QJ denotes a four-point contact ball bearing with a split inner ring. The numbers after the designation define the dimension series and the bore of the bearing.
3. What is the difference between a four-point contact bearing and a pair of angular contact bearings?
A four-point contact bearing carries axial loads in both directions with a single bearing and is narrower than a pair of angular contact bearings. A pair of angular contact bearings, on the other hand, is better at carrying combined radial and axial loads.
4. Does a four-point contact bearing carry radial loads?
It can also carry radial loads, but it is designed mainly for axial loads. Where radial loads are higher, it is combined with a cylindrical roller bearing.
5. What does the suffix N2 mean?
N2 means that the outer ring has two anti-rotation slots. The slots allow the outer ring to be secured when the bearing is mounted in the housing with radial clearance.
6. What does the suffix MPA mean?
On FAG bearings, the suffix MPA denotes a solid brass cage guided on the outer ring. This type of cage is suitable for more demanding operating conditions.
7. Why is the inner ring split?
A split inner ring allows a larger number of balls to be fitted, which increases the load carrying capacity of the bearing. The two halves of the inner ring must stay together and in the correct order during mounting.
8. Where are four-point contact bearings used?
They are used in gearboxes, compressors, pumps, heavy machinery drives and wherever axial location of the shaft in both directions is required in a limited space.
9. What are thin-section four-point contact bearings?
Thin-section bearings have the same cross-section regardless of the bore size. They are used in robotics, measuring and medical equipment and other lightweight, compact designs.
10. How do I choose the right four-point contact ball bearing?
Consider the dimensions, axial and radial loads, mounting arrangement, need for N2 slots, cage and clearance. When replacing a bearing, the full designation of the existing bearing is the most reliable information. At M&M Intercom we will help you choose the right design based on the designation or application details.