Accuracy Design of Tapered Roller Bearing Fit

Time：2020.07.20  Source：Roller Bearings Suppliers

1. Tapered roller bearing system

Under normal circumstances, the tapered roller bearing will use the base hole system when fitting, but if it is a standard part, the matching nature of the parts that match it is determined by the standard part. As for the tapered roller bearing, since it is a standard part, the part matching the outer ring is made of base shaft, and the shaft matching the inner ring is made of base hole.The fit of the bearing inner ring and the shaft is made of base holes, although the tolerance bands of all tolerance grades of the tapered roller bearing inner ring are below the zero line and the upper deviation is zero. The main reason is the special requirements for bearing coordination. In most cases, the inner hole of the bearing should rotate with the shaft, and the fit between the two must have a certain interference.

2. Choice of tolerance grades for journals and bearing seats

The tolerance level of the hole and shaft matched with the tapered roller bearing is closely related to the tolerance level of the bearing. Generally, the shafts matched with /P6 and /PO bearings have a tolerance class of IT5~IT7, and a box hole of IT6~IT8.  WSBC 98400-98789D bearings online , if interested , pls contact us .

3. The choice of nature

The choice of the bearing matching nature is to determine the basic deviation code of the journal and bearing seat matched with the bearing.The basis for selecting the matching nature of the bearing is: the type of load on the inner and outer rings of the bearing, the size of the load on the bearing, the working conditions of the bearing, the material and loading and unloading requirements of the hole and shaft that cooperate with the bearing, etc.

(1) Load type

Local load: The synthetic radial load acting on the bearing is relatively stationary with the ferrule, that is, the load direction always acts on the local area of the ferrule raceway. Usually a small clearance fit or transition fit is used.

Cyclic load: The synthetic radial load acting on the bearing rotates relative to the ferrule, that is, the synthetic radial load acts sequentially on the entire circumference of the ferrule. Usually an interference or tight transition fit is used.

Oscillating load: The synthetic radial load acting on the bearing and the bearing ring are relatively oscillated in a certain area, that is, the synthetic radial load often acts on the part of the ring raceway less than 180°.

(2) The size of the load

Under the effect of load, the bearing ring will be deformed, so that the force of the mating surface is uneven, causing looseness. Therefore, the coordination should be tighter when under heavy load, and looser when under light load. Generally, the load is classified as follows:

Light load: P≤0.07C;

Normal load: 0.07C

Heavy load: P>0.15C.

(3) Working temperature

When the bearing rotates, the temperature of the ring is often higher than the temperature of the adjacent parts. The inner ring of the bearing may loosen the fit due to thermal expansion; the outer ring will tighten the fit due to thermal expansion. The influence of temperature should be considered when choosing the fit.

(4) Rotation accuracy and rotation speed

When there is a high requirement for the rotation accuracy of the bearing, in order to eliminate the influence of elastic deformation and vibration, the fit with clearance should be avoided, but it should not be too tight. The higher the bearing speed, the tighter the fit. 

4. Determination of geometric tolerance and surface roughness

In order to ensure the normal operation of the bearing, in addition to the correct selection of the tolerance level and coordination of the bearing, journal and box hole, the geometric tolerance and surface roughness of the journal and box hole should also be required.

Shape tolerance: mainly the surface cylindricity requirements of journal and box hole.

Position tolerance: mainly the runout tolerance of the shaft shoulder end face.

Surface roughness: The value of the surface roughness directly affects the quality of the fit and the connection strength. Therefore, any surface that cooperates with the inner and outer rings of the bearing usually puts higher requirements on the surface roughness.

5. Examples of matching selection and drawing annotation of tapered roller bearings

(1) Marking on the assembly drawing

On the assembly drawing, it is not necessary to mark the tolerance class code of the bearing, but only the tolerance class code of the bearing seat and the journal matched with it.

(2) Marking on the part drawing

On the part drawing, the following parameters should be marked:

A. Dimensional tolerance

B. Shape tolerance

C. Position tolerance

D. Surface roughness