The Case of the Mysterious X-Dimension

PrintAll Kadant Johnson PTX® rotary joints have an X-dimension. The X-dimension is the setup dimension used for the initial rotary joint installation and found on the assembly drawing for the specific rotary joint. This same X-dimension can also used to measure and monitor seal ring wear. As the seal ring wears, the X-dimension increases. Once the X-dimension increases to about one inch, the seal ring is considered worn and replacement of the seal ring should be scheduled. Replacing the seal ring and elastomeric seals inside the rotary joint when the X-dimension is reached prevents damage to metal parts that normally don’t need to be replaced when the rotary joint is rebuilt.

The X-dimension can be used as a troubleshooting tool as well. When your machine is equipped with a Kadant Johnson PTX rotary joint, the X-dimension is preset at the time of installation. The X-dimension is based on stack-up dimensions calculated during the design process. The X-dimension should be in a range of 0.3” to 0.5”, plus or minus 0.25”. The 0.25” variable is there because the journal lengths may vary. This is especially true on Beloit machines and older paper machines.

The rotary joint’s X-dimension increases over time as the seal ring wears. So, what happens if all of a sudden the “X” increases or changes dramatically in a short period and/or the rotary joint has been performing well and all of a sudden it starts to leak?

If this phenomenon occurs, it is most the result of a drive-side bearing (held bearing) failure. In many cases, it is difficult to determine the mode of bearing failure because the locking ring that holds the bearing on to the dryer’s shaft has come loose and allowed the dryer to shift towards the tending side. When the locking ring becomes loose, vibration equipment, used to monitor bearing frequencies, will not pick this up because the bearing has simply come loose on the shaft. Typical vibrations caused by a cracked inner or outer races or failed rollers will go unnoticed, leaving the vibration tech to believe the bearing is fine.

To further investigate the drive-side bearing, take a look at the tending-side bearing housing. In most cases, the tending side bearing housing will be farther away from the machine frame than the other dryers next to the suspect dryer. In other cases, the tending side bearing housing may have fallen off its rockers too. Some machines are equipped with tending side CARB® bearing. If equipped with a CARB bearing, the end of the journal to the bearing cover dimension will need to be measured. This dimension will have to be checked at the same location on other dryers next to the suspect dryer. If the dimension varies more than 0.5”, this suggests the drive-side bearing has come loose.

To correct the mysterious X-dimension issue, the drive side bearing should be replaced.

CARB is a registered trademark of Aktiebolaget SKF.