Once the grease is put into use, the inspection work will become more difficult. Although there are relevant grease analysis methods, such as ASTMD7918 (Standard Test Method for Measuring the flow Characteristics of Grease by Die Head Extrusion and Evaluating its wear, Contaminants and Oxidation Properties) or dedicated abrasive particle detection, these methods have not been widely applied. Because the application scenarios of grease are mostly in "invisible" places such as bearings, problems are easily overlooked before major malfunctions occur. Proactive sampling and monitoring of grease should be part of a high-quality preventive maintenance plan, but this is still an exceptional case rather than an industry norm at present.
Replacing grease is no small matter. Before doing so, it is essential to first understand the actual situation within the system. The following types of scenarios are often the beginning of problems:
Due to personnel taking the wrong grease injection gun, different types of grease were accidentally mixed.
Due to operational adjustments, the equipment's rotational speed increased, but it was not evaluated whether the existing grease could meet the requirements.
The operating temperature rises, causing the pressure on the grease to exceed its rated tolerance range.
Changes in storage conditions, such as an increase in warehouse temperature or open-air storage, can lead to the degradation of grease.
The performance of most greases is not solely determined by their chemical properties. The operation mode, filling method and operating conditions also play a key role. Next, we will sort out the most common grease problems one by one, as well as the corresponding solutions and preventive measures.
1. The grease fails prematurely or is exhausted
The rapid deterioration of grease is mostly caused by high temperature, bearing speed, mechanical shock or chemical contamination.
Solution: Reduce the operating temperature as much as possible and strengthen pollution control; Select lubricating grease with higher base oil viscosity, more viscosity enhancers or higher NLGI grade to enhance mechanical stability.
2. Premature wear of the bearing
Premature damage to bearings may be due to mechanical issues such as overload, poor alignment, incorrect dimensions, or improper lubrication (such as using grease with a base oil viscosity that is too low).
Solution approach: Conduct a comprehensive failure investigation. First, check the load, alignment, grease selection, and grease replenishment method. Most problems are caused by multiple factors superimposed rather than a single reason.
3. The grease turns black
Blackening of grease is usually a signal of oxidation, contamination by bearing debris or overheating. In extreme cases, stray voltage or electrostatic discharge inside the bearing can cause arc damage, manifested as surface grooves or pitting corrosion.
Solution: Smell for any burnt odor, check the grounding of the equipment, and inspect the bearings for any arc marks. When necessary, send the samples to the laboratory for pollutant analysis.
4. The grease turns milky white
Grease turning milky white is usually caused by water pollution. Some thickeners are prone to absorbing water, while others have stronger water resistance. Mild contamination does not necessarily directly damage grease, but it will shorten its service life.
Solution: Eliminate water sources; Select hydrophobic type (such as complex aluminum-based grease containing polymer additives) or high-efficiency emulsifying type (such as complex calcium sulfonate-based grease) grease.
5. Grease leakage (seepage)
When in use or storage, if oil seeps out of the grease, it is called "oil leakage". High temperature, vibration or gravity may all cause the base oil to separate from the thickener.
Solution: Replace the grease with a more stable thickener, such as upgrading the common lithium-based grease to complex lithium-based grease or complex calcium sulfonate-based grease, which can enhance mechanical stability and reduce oil output.
6. Grease clumps
The drying up or hardening of grease is often caused by excessive oil separation, evaporation, or incompatibility of thickeners. Excessive grease injection into sealed bearing boxes can also lead to this.
Solution: Select synthetic base oil grease (with better anti-evaporation ability than mineral oil); Adjust the preventive maintenance (PM) cycle. For open systems, shorten the cleaning cycle; for sealed systems, extend the grease injection interval.
Also, be sure to be vigilant against the issue of incompatibility of lubricating grease! Mixing incompatible greases may have no obvious effect at best, but at worst, it can cause the grease to become thinner, leak or harden and form lumps. The main cause of incompatibility is the thickener component. Base oil and additives may also cause problems. If compatibility is not certain, it is assumed to be incompatible by default. When replacing, try to remove the old grease as much as possible and closely monitor the equipment temperature. Even for the same type of thickener (such as complex lithium-based grease), there may be differences with different formulations.
7. Grease cannot flow in the centralized lubrication system
If the grease is too cold, too thick or too viscous, it may clog the pipelines and fail to reach the key components. The ideal grease for centralized lubrication systems is NLGI grade 0 or 1 (relatively soft) with less tackifying agent type. The pumpability test data in the technical data sheet can be referred to.
Solution: Check if there is any accumulated oil in the storage container (indicating oil separation); When necessary, switch to a grease with a lower NLGI grade or a lower base oil viscosity to ensure that the product matches the system design specifications.
8. The grease was washed away by water
Bearings with dust covers or equipment exposed to flushing, rainwater or process water are prone to the problem of grease being washed away by water.
Solution: Switch to moisture-resistant grease. Calcium sulfonate-based compound grease has the best water resistance, followed by calcium-based compound grease and aluminum-based compound grease. The water resistance of the compound lithium-based grease is acceptable, but it performs poorly in high-humidity environments. It is necessary to give priority to choosing products with a "hydrolyzed stable" formula.
9. The bearing is overheating
The most common cause of overheating of bearings is excessive grease injection. Excess grease will increase internal pressure, damage the bearing seal, and lead to a rise in temperature. Although insufficient lubrication is less common, it may also cause overheating.
Solution: Inject a small amount of fat each time and monitor the temperature changes; If the temperature drops, the frequency of lipid injection can be slightly increased. If the temperature rises, the excess grease should be removed and the amount of grease injected should be reduced. High-speed bearings should be lubricated with grease of lower base oil viscosity.
10. 其他加注问题
Low temperatures or improper storage can cause cavitation in the grease tubes and poor flow in the buckets. The oil drum not using the push plate will cause product waste.
Solution: Use a new type of grease cylinder with a cut seal to prevent air lock. Large oil drums need to be equipped with compliant push plates to achieve a product utilization rate of at least 98%.
The reason why grease problems are so troublesome is that most of them occur deep inside the machine and are hard to detect. However, as long as one pays attention to subtle warning signs such as temperature changes, grease consistency, color or leakage, problems can be detected early.
It is essential to maintain cooperation with reliable grease suppliers and Oems to select the appropriate grease for the actual application scenarios. Before replacing the product, compatibility must be verified through controlled tests, and the usage effect should be closely monitored. Sometimes, using thicker grease or base oil with higher viscosity may mask potential mechanical failures. However, if the root cause is not addressed, the malfunction will eventually recur. In lubrication management, those invisible hidden dangers are often the key to causing damage.
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