Productivity Fundamentals: Kiln Lubrication - Part I

Our Mobil Serv experts in the field often get asked, what is the proper lubricant for a kiln open gear?

The answer is an open gear lubricant.

But did you know that there are other, not so obvious systems, on a kiln that require lubrication? There’s the kiln tire, the trunnion wheel, and trunnion bearings, just to name a few. This two-part tip series will focus on the “other” kiln systems and the lubricants required. Today’s tip focuses on trunnion wheel support bearings and the trunnion wheel/riding ring interface.

Trunnion Wheel Support Bearings

At the base of any kiln are the trunnion wheels (Figure 1). These rollers help carry the weight of the kiln and assist in rotation. The trunnion wheels are supported by the trunnion bearings which may be either plain or roller type. The trunnion wheel support bearings are typically lubricated by oil bath or pressurized lubrication.

Oil Bath

Oil bath is a method used in trunnion wheel support bearing lubrication. In this mode, the bearing is immersed in oil. As you can see from Figure 2, there are dipper buckets attached to the journal which scoop oil out of the sump and deliver it to the top of the journal, pouring it over the shaft. This oil cascades over the journal, down to the bearing to provide a complete oil film.

NOTE: As a word of caution, make sure there are at least 6 to 8 dipper buckets per bearing, as over time these tend to get broken off and not replaced.

Bath is an effective method of lubrication while the kiln is running, but has drawbacks during start-up. After a prolonged shutdown, the oil is absent of the bearing load zone, and without proper oil start-up procedures, the journal may rotate on a dry bearing shell, tearing the surface. To prevent this there are two manual procedures that must be performed:

  1. Pour oil by hand through the top of the bearing through the bearing cap, letting it flow down to the load zone.
  2. Use the hand pump mounted on the side of the bearing housing to pump high pressure oil under the shaft journal between the bearing and journal surfaces.

Pressurized Lubrication

Pressurized lubrication is a more modern method of lubricating the trunnion wheel support bearings. Under this regime, oil is circulated to the bearings from a reservoir, with a low pressure (LP) circuit to the top of the bearing and a high pressure (HP) circuit to the bottom. The HP circuit provides journal lift at the bearing load zone while the LP circuit coats the journal and other bearings surfaces with oil, helping with oil wedge formation when rotation is achieved. The advantages a pressurized lubrication system include:

  • Reduction of start-up wear
  • Maximizes the potential for hydrodynamic lubrication
  • Installed circulation system which will remove contaminants (filter) and cool the oil, maximizing oil and bearing life.

The proper lubricant used for either bath or pressure lubrication systems is a PAO (Polyalphaolephin) or PAG (Polyalkyleneglycol) synthetic with a viscosity between 680 and 1000 cSt @ 40⁰C. The most critical lubrication traits are maximum viscosity at operating temperature, resistance to oxidation and the ability to handle moisture.

The Trunnion Wheel and Riding Ring Interface

The trunnion wheel is supported by the trunnion bearings and contacts the riding ring, to support kiln rotation. The trunnion wheel and riding ring interface should be lubricated but is often overlooked by maintenance personnel. There are two ways to lubricate this combination;

  1. The first involves the use of a spray wand and graphite spray. In this procedure, the solubilized graphite is sprayed directly onto the roller wheel, the carrier evaporates and graphite is left behind as a boundary lubricant.
  2. The second method involves the use of a graphite block. This is held against the face of the trunnion wheel, slowly melting, to provide graphite for lubrication for both the wheel and the ring.

I hope this week’s tip was helpful – next week we’ll discuss the riding ring and the thrust roller. In the meantime, leave any questions below or “Like” this article if you found it useful!