Drying andYankee cylinder

bearings and their lubrication

(Part 1)

SKF engineers working with paper mills get asked a lot of

As steam also heats journals and bearings, and because the

questions about bearing clearance, inner ring heat treatment and former will be hotter than the latter, bearing inner rings have to be

lubrication for drying and Yankee cylinder applications. In fact,

able to withstand radial expansion. This creates additional stresses

after issues relating to corrosion and contamination from process in inner rings on top of those created by mounting them with a tight

water in the wet section, these are probably the most common

things they have to deal with. As such, this issue of SKF Pulp &

Paper Practices will focus on them. It’s a broad subject so, in

order to cover this matter in sufficient depth, we will also

dedicate the next issue of our newsletter to it as well.

fit on the seat. Due to the temperature difference between the inner

and outer rings of the bearings, larger than normal radial internal

clearance is required to maintain some clearance and to avoid

preload.

The maximum internal clearance reduction and inner ring stress

are experienced during start up with a cold machine. The accompa-

nying drawings († fig. 2, 3, 5 and 6) show the calculated tempera-

ture distribution over the cross section of an arrangement during

start up. These results were tested against an actual drying cylinder

– on which temperatures were monitored – and confirmed for oil

flows of between one and two litres per minute with slight devia-

tions at high and low oil flow rates. The bearing in our simulation

model is a 23052 CCK/C4W33 spherical roller bearing with a rota-

tional speed of 130 rpm and an oil flow rate of 2 l/min. Our model

Bearing operating conditions

From the bearing perspective, drying andYankee cylinder applica-

tions are rather similar. Bearings don’t rotate at high speeds and

they aren’t heavily loaded. The steam used to heat such cylinders

does create some challenges however.

Steam passing through the bore of the journal on which the bearings

are mounted († fig. 1) causes radial and axial thermal expansion of assumes a steam temperature of 180 °C (356 °F) during start up

both the journal and the cylinder.

and 130 °C (266 °F) under normal operating conditions.

With the temperature distribution shown in fig. 2, the hoop stress

in the inner ring is increased by some 60% i.e. nearly 220 Mpa

instead of 136 Mpa after mounting.

Axial thermal expansion creates the need for a bearing or housing

that can accommodate relatively large axial expansion of the journal

relative to the machine frame on the front side.

Fig. 1 A typical drying cylinder bearing arrangement.

Drying cylinder

arrangement

1 Drying cylinder

2 Steam joint

3 Steam inlet

4 Siphon

5 Condensation

6 Journal insulation

7 Drive side bearing

8 Front side bearing

9 Gear drive

2.2.1 Nominal journal diameter

The dimension, Ba, is used as a basis when measuring a tapered

journal with a SKF 9205 taper gauge. It’s the distance from the cen-

tre of the bearing as finally mounted to the reference face of the

journal († figure 6).

Note:

After mounting, the centre of our SKF 241/600 ECAK30/

C083W33 bearing is 490 mm from the journal shoulder

reference face and the taper width, B_e, is 370 mm.

Knowing B_aand the bearing dimensions, it is possible to calculate

the nominal journal diameter, d_a, and its distance, B_d, from the

reference face.

The nominal journal diameter, d_a, is bigger than the bearing bore

diameter, d.

The bearing nominal bore, d, is the diameter of the inner ring

bore taper in the radial plane passing by the ring face. In reality,

between the ring face and the bore there is a chamfer and there’s no

contact between the journal and bearing. There is contact further up

the bore after the chamfer. If B_fis equal to the chamfer radius, then

contact occurs at a distance of B_ffrom the ring face († figure 6).

The diameter of the taper increases by a value equal to B_f/K.

Remember that K equals either 12 or 30 depending on the taper

angle. The default value for B_fcan be found in table 1. For

SKF 241/600 ECAK30/C083W33, B_f= 10 mm.

Fig. 4 Complete SKF 920512 taper gauge set.

Note that bearings are manufactured with tolerances for both the

nominal bore diameter and the taper deviation († figure 7).

B_c

M₁

d_b

d_a

B_e

B_d

B_f

90°

B_b

B_a

Fig. 5 Example of a plain press roll bearing arrangement from the

SKF Rolling bearings in paper machines handbook.

Fig. 6 The dimensions required for using the SKF 9205 taper gauge.