This week’s tip goes back to a question that was recently asked by a member of the Mobil SHC Club: “What is the cause of high temperature in a slide shoe bearing on a ball mill?”
While the answer is very specific, it got me thinking, how many of us know what a slide shoe is, where it is used, what it does, and how is it lubricated? Probably not too many!
So, this week we’ll discuss mill shoe bearings and how to enhance their reliability and productivity.
Cement Ball Mills
A cement ball mill is designed to grind clinker, gypsum and for the drying of cement additives. It is a component in the cement making process. Today’s Ball Mills are typically designed with a slide shoe bearing.
These bearings have replaced the more traditional trunnion bearings, as they are less expensive to purchase and maintain. The function of the slide shoe bearing is to keep the mill aligned to the mechanical process while allowing for mill rotation. The advantages provided by the slide shoe bearing includes:
Lighter mill weight (less energy to operate)
More compact mill while maintaining capacity
Reduced mechanical stress
Design and Lubrication
Slide shoe bearings are hydrodynamic bearings cast of a steel with a babbit overlay applied. They are fixed in relation to the sliding ring and the shoe itself rests on a ball to allow for lift, flexibility and alignment.
During mill start-up, the slide shoe bearings requires pre-lubrication. This is provided by a high pressure oil pump which delivers oil between the bearing and journal. The oil, injected under pressure, lifts the mill and helps to reduce the friction between the shoe and ring, providing alignment. The minimum oil viscosity for most mills is 44 cSt at 100°C and typically requires an oil with an ISO 460 Cst viscosity.
A mineral based gear or R&O oil (and oil specifically formulated to better combat rust and oxidation) may be used to lubricate the slide shoe. Mobilgear XP 460 has been used successfully in many instances. However, the use of Mobil SHC 600 Series synthetics should be considered. Specifically Mobil SHC 632 or 634. These are PAO synthetics which offer:
Improved pumpability at start-up due to lower pour points and the lower fluid friction provided by a synthetic oil. This allows for enhanced oil flow to the shoe bearings. This is particularly important in cold climates where fluid flow is a concern.
And improved filterability over gear oil, providing longer lubricant life, improved filter longevity, and again better oil flow to the bearings.
If high bearing operating temperatures cause the mill to trip offline, consider increasing the trip set points. Consult your OEM when doing this, making sure to evaluate the maximum operating temperature for the bearing materials used. Remember, just because the oil can handle the operating temperature, doesn’t mean the bearing can. Other causes of high heat at the shoe bearings include:
Lack of oil flow
Oil ports clogged
Filters plugged - remember these system may (should) have fine filtration
Pump malfunction - many of these mills will have a high pressure (lift) and low pressure (system) pumps; does the high pressure pump de-energize or turn off when the mill runs?
Low ambient temperatures - this will inhibit oil flow.
Mill process temperature too high
Mill cooling temperatures inefficient or not effective.
Once again, thanks for the opportunity to provide a little bit of knowledge. I hope you enjoyed this week’s tip and let me know if you have any suggestions for future tips in the section below!
Excellent information, Rick, thank you for sharing on the maintenance of the cement mill footing, you could prepare some tips on the maintenance of clinker refrigerators, high Temperatures, low speeds and other factors, so that they can use them, and on what the proper lubricant is.
Jose - Clinker cooler have two lubrication requirements that I am aware. They include a grease for the cooling fans (any Mobil hi-temp grease with an ISO 220 oil) and an oil for the conveyor drives and grinding wheels. The oil would most likely be SHC 634 (right angle shaft) or SHC 630 (parallel shaft). The coulpings and electric motors should be sealed. Let me know if I am missing any application.
I do not think it's not, those are all the applications it has, thanks for your answer, Rick, greetings.
Jose - I may have missed some applications as there are many different varieties of clinker coolers. Other applications I can think of include damper bearings (any Mobil hi-temp grease with an ISO 220 oil) and possible air over oil hydraulics (DTE 25 / Hyd AW 46). If you have specifics, let me know.
Temperature Control in the bearings
In order to ensure an uninterrupted and high quality clinker production, the clinker cooler must continually receive a sufficient amount of Refrigeration air. The TCT compact temperature sensor detects the increase in the temperature of the bearings in the aeration Blower Fan, which is usually an indication that there are damage or wear on the bearing.
Proper maintenance of a blower fan:
* is too make sure the motor is of the correct HP for the Fan,
* that the fan impellor is of the air handling type,
* that the bearings are correct, properly installed and aligned,
* and that the bearings are greased properly - every couple of weeks with a 2 grade grease with an ISO 220 base oil.
For the sensor to indicate a high temperature in the bearings is usually the result of one of the things I just mentioned not being favorable.
Good information about the temperature in the bearings, for sharing the important points to consider that the Bearings are in good condition.
Two years ago, we had a case where the lubricant for a cement mill shoe bearing dropped dramatically in a few months. Luckily, Mobil Serv Lubricant Analysis was in use. The oil in use was a Mobil SHC 630 and viscosity dropped to almost 80cSt @40°C in a couple of months.
The weird thing was that this only happened at one of the lubrication systems. Each bearing had it's own system.
We drained the oil and we saw the cause immediately. The oil heater tubes were covered with a tar like substance.
We removed one of the heaters and tested it. It went up to temperatures of almost 400°C. At these temperatures even PAO oil start cracking and have a viscosity drop. There was a bug in the PLC program controlling the temperature for the oil reservoir.
I have nice pictures and a report on this case.
Again, a great proof of the value of Used Oil Analysis.
Bram - Thank you for your comments. I wouldvery much like to see any pictures you have as well as oil analysis results. What you experienced was thermal crack of the oil leading to viscosity degradation. I am glad you were able to isolate the cause and repair. Your case is a good example of using oil analysis to isolate and correct a problem before it become a system failure. Well done.