Much has been said and written about preventive and predictive maintenance programs and condition-monitoring techniques and applications. Many highly skilled, well-qualified and well-meaning engineers and maintenance managers diligently and consistently apply sound maintenance ideas to their plant and industrial machinery, yet fail miserably in their attempts to extend machinery life cycles or improve machine reliability. Why is this so?
One important reason is that plant management fails to consider the extreme operating conditions, to which the plant’s machinery may be subjected. This is a particular problem in plants where the management group has little or no technical training or experience, or ignores recommendations made by those who have.
Machine reliability problems associated with extreme conditions include such things as:
Each of these extreme conditions affects machinery life cycles and equipment reliability, and ultimately the efficiency and productivity of industry.
Figure 1. Shock Load
The relationship between life cycle costs and machinery design is a concept that is not always applied when the initial design of replacement machinery is considered. Put simply, life cycle costs include the total costs related to the design, manufacture, commissioning, operation and maintenance and finally, the disposal of any piece of machinery.
For example, if only 40 percent of the life cycle costs are applied to the design, manufacture and commissioning stages of a new piece of machinery, it stands to mathematical reason that most of the remaining 60 percent of the life cycle cost will be spent during its operational lifetime. Regardless of the level of maintenance or the techniques applied, this machine can experience failure after failure throughout its operational lifetime.
On the other hand, if 80 percent of the life cycle cost is spent during the design, manufacture and commissioning of a piece of machinery before it is put into operation, the machine can live a long, reliable and relatively problem-free life, provided it is operated for the purpose for which it was intended. No amount of maintenance can correct a poor design.
Machinery is modified every day in North American industrial plants without sufficient thought given to the extreme operational effects such modifications might cause. Consider the modification of a conveyor system in a crushing plant. To supply more material to the crushing equipment, the conveyor system was expanded to increase the supply of material. Wider, stronger belts were installed, along with an improved support roller and bearing system.
The gear drive mechanism which operated the conveyor failed catastrophically some six weeks later. Absolutely no thought had been given to whether or not the drive system (bearings, gears, lubricants, etc.) could support the increased capacity.
on Bearing Life
Using speed and load calculations, an engineer can determine expected rolling element bearing life with surprising accuracy. However, if a typical bearing load is doubled, the life cycle of the bearing may be reduced by as much as 90 percent. Doubling the rated speed, of a bearing can also reduce its life by as much as 50 percent.
Load X 2 = up to 90 percent reduction
Speed X2 = up to 50 percent reduction
These engineering rules of thumb must be kept in mind whenever production increases are demanded by an unknowing management, or if machine modifications are considered.
The obvious lesson is that every machine or mechanical drive system is only as strong as its weakest component. Remember that about 80 percent of bearing failures are usually a symptom of a much larger problem, such as excessive loads or speeds, extreme vibration conditions, poor lubrication practices, extreme temperatures and improper replacement (bearing selection and/or poor installation).
Vibration analysis experts suggest, and the statistics related to vibration problems confirm, that most machinery vibration problems are the direct result of component or drive system misalignment, component unbalance, mechanical looseness or machine resonance related to its design. Machinery wear or bolt loosening can cause mechanical looseness conditions. Misalignment can be caused by worn components, such as gear drives, but is caused primarily by poor initial installation and set up of drive shafts, couplings and belt drives.
Figure 2. Twisted Shaft Due to Overloading
Component unbalance can be caused by wear, but frequently it is caused by such conditions as dirt buildup on fan blades or pump impeller erosion. Resonance is the term applied to a machine’s inherent (natural) vibrating frequency. If the operating speed of the machine is such that its operational frequencies consistently come near to or continually pass through its resonant frequency, the operating frequencies can excite the resonant frequency so seriously that the machine will literally fly apart.
It is difficult to design a machine installation, including the base, piping, duct work, pedestals, etc., so that there are no natural frequencies coincident with any significant excitation generated by the machine’s operating frequencies. In addition, manufacturers are constantly reducing the machine’s mass and increasing capacities and loads in today’s equipment. As a result, resonance is a common and serious problem throughout industry.
Extreme temperature conditions include both extremely high or low temperatures. In the case of temperatures below 0ºF (-18ºC), industrial equipment requires the use of multiviscosity lubricants with high viscosity indexes. This will ensure that oil flows are sufficient to allow the formation of satisfactory oil film thickness that will prevent wear during machine startup. On the other hand, high temperature operation should be a greater concern in North American plants. Even a five-degree rise in temperature above those recommended can eventually cause severe damage.
Many factors can affect temperature, but the most common are the use of lubricants of the wrong viscosity, excessive loads, speeds (or both), dirt and dust buildup on components (which has an insulating effect), or contaminated lubricants (which can cause viscosity changes, foaming or other conditions that will dramatically affect the oil’s ability to transmit heat). Table 1 lists some common mechanical systems and the recommended operating temperatures.
|Bearings||Do not exceed 160°F (71°C).|
|Hydraulic Systems||Bulk oil temperature (at exterior of reservoir) should not exceed 140°F (60°C).|
|Gear Drives||Operate best in a range of 120°F to 140°F (49°C to 60°C). Keep in mind that an operating temperature rise of 90°F (50°C) combined with an ambient temperature of 60°F (15.6°C) will result in a total “oil operating” temperatureof 150°F (66°C) in gear drives.|
temperatures should normally be in the range of
130°F to 160°F (54°C to 71°C).
Another serious problem is the overheated bearings lubricated with grease. It is a common practice to apply more grease to an already overheated bearing. The additional grease simply adds to the problem, due to frictional churning and because the lubricant acts as an insulator. The increased temperature reduces oil film thicknesses, resulting in stress on frictional surfaces that can lead to premature bearing failure. In these cases, consideration should be given to the application of oil lubrication using a centralized oil mist system. Oil mist systems can reduce bearing temperatures by 20 to 40 percent, because excessive lubricant is never allowed to remain in the bearing housing.
Excessive contamination is the cause of 75 percent of all failures related to lubrication in systems where close tolerances of machine parts are critical. This includes high-pressure hydraulic systems, precision bearing applications and engines.
It is a common mistake to conclude that if contaminants cannot be seen, they’re not present. Consider this; water contamination caused by condensation can be as high as 2000 parts per million (ppm), or 0.1 percent, yet may never be discovered without laboratory analysis. In some machinery where high lubricant flow rates are present, such as in a turbine, water contamination as low as 250 ppm can cause lubricant foaming. This can also occur in poorly designed piping systems where high flow rates can cause turbulence. In addition, very small amounts of condensation can contribute to serious damage in bearings in seasonally used machinery. The water may slowly evaporate (and condense on internal surfaces) after hot shut down, causing corrosive pitting and spalling of the protected bearing surface.
Now consider dirt and dust contamination. Poor air filter system design and the application of poor quality oil and air filters cause numerous machine failures in North America annually. Because careful failure analysis and particle counting are seldom carried out, these failures are filed under normal wear and tear. Dirt and dust particle sizes that cause the most damage are those that cannot be seen with the naked eye. These contaminants are in the size range of 4 to 40 micrometers and are referred to as silt contamination. This is why many equipment manufacturers and oil companies highly recommend that new lubricants be filtered prior to installation, and that contamination levels in lubricants in service should be monitored regularly using particle counting instruments.
In fact, the Society of Tribologists and Lubrication Engineers (STLE) states: The cleanliness of lubricants related to bearing surfaces should be as important to the bearing user as antiseptic measures are to the surgeon!1
An example of how controlling contamination can effectively increase equipment reliability is illustrated by the proactive maintenance actions taken by the Suhner Corporation of Switzerland. The company operates a plant in Sackingen, Germany in which it manufactures electronic and pneumatic motors, gears and machine tools. The manufacturing process requires the use of sophisticated machinery operated both electronically and hydraulically.
The company began to experience shut downs, stoppages and failures within some of its hydraulically operated manufacturing machinery. Careful failure analysis soon resulted in the discovery of a contamination problem in the plant’s hydraulic systems.
The maintenance group immediately implemented a program where particle analysis of hydraulic oils was carried out to monitor hydraulic oil condition. Improved filtration methods were then used wherever contamination levels exceed recommended ISO cleanliness codes (ISO 4406:1999). In addition, standards of plant cleanliness were reviewed and improved. Since these improvements were made, the reliability of the machines which had experienced problems has returned to 100 percent. The author visited the plant, and had the opportunity to see first hand, the extremely clean conditions. One could, quite literally, eat off the floor and every machine tool used in the manufacturing process was spotless.
This may be one of the most common causes of machine failure and in the author’s opinion, contributes directly to as much as a two to three percent reduction in plant productivity in North America.
Poor operating practices are the direct result of two conditions prevalent in our present society:
1. An uncaring attitude by workers.
2. A lack of appropriate training of workers.
In fact, these two conditions are directly related to each other, because a lack of training may be the cause of an uncaring attitude.
Extreme operating conditions can be even more fully understood and recurrences eliminated, if industry personnel properly and thoroughly carry out a root cause failure analysis every time a component or machine system fails. This root cause failure analysis must be carried out after every incident or failure, no matter how insignificant or unimportant it appeared at the time.
1. The Society of Tribologists and Lubrication Engineers. Interpreting Service Damage in Rolling Type Bearings.
This paper was presented by L. (Tex) Leugner at the International Maintenance Technology and Information Symposium Edmonton, Alberta, Canada in 1996.