Carbon Guides vs. Rolling Element Bearings
There are many factors when deciding whether to pick a rolling element bearing or a carbon guide supported rotary joint. Both styles are self-supporting which are used when there is no external framework to mount a rotary joint, or the geometry and application do not require external support. For many applications, external ring bracket supported rotary joints such as the PTX® rotary joint or the CorrPro® rotary joint are the preferred option.
Carbon guide supported rotary joints, such as the SX® rotary joint or the ELS™ rotary joint, are best suited to low speed and high temperature applications. Carbon has a low coefficient of thermal expansion so it can tolerate a large temperature differential.
The nipple of the rotary joint will be about the same temperature as the fluid and heats up very quickly causing it to expand. The body of the rotary joint will heat more slowly, and is typically cooler due to being exposed to atmospheric conditions. Extra clearance is only needed on the ID of the guide to handle thermal expansion. In a bearing supported rotary joint, the ID and OD races are tightly fit and the thermal expansion needs to be taken up by the internal clearance of the bearing. This makes the setup and bearing selection temperature dependent.
Carbon guides also have some advantages in the larger sizes. Carbon is easily made to order whereas large roller element bearings can be more challenging to source, sometimes requiring a six-month lead time or more. Carbon guides do not require lubrication and only require service under normal conditions when the seal ring has worn and requires replacement Carbon-supported rotary joints range in size from ½” to 14” in flow diameter.
Roller element bearing supported joints, such as the RX® rotary union, are typically used in higher speed applications and when runout is critical. Rotary joints that operate at high speed use a balanced face seal which require precise alignment of the seal faces. In addition to face seals, the DuraSeal® technology multi-passage line of rotary joints requires control of runout that only roller elements can provide. Roller element bearings are also used in applications that require low drive torque. Some bearing applications require a lubrication maintenance schedule to be followed for optimal life expectancy. Roller element rotary joints also come in a wide variety of sizes, the larger sizes typically suited to custom applications.